CN101997413B - Power converter with synchronous rectifier and control method for synchronous rectifier - Google Patents

Abstract

The invention discloses a power converter with a synchronous rectifier and a control method of the synchronous rectifier, wherein the power converter comprises a switching circuit; a transformer having a primary winding and a secondary winding, the primary winding being connected to the switching circuit; the main control circuit is connected with the switching circuit and controls the switching circuit to operate; a synchronous rectifier connected in series with the secondary winding; a current transformer connected in series with the synchronous rectifier and outputting a detection signal according to a current flowing through the synchronous rectifier; the synchronous rectification control circuit is connected with the control end of the synchronous rectifier, the control end of the current transformer and the control end of the switching circuit to receive the detection signal and the main control signal and is used for controlling the synchronous rectifier to operate; when the main control circuit controls the switching circuit to be conducted, the synchronous rectification control circuit controls the synchronous rectifier to be conducted, and the synchronous rectification control circuit controls the synchronous rectifier to be cut off according to the detection signal. The synchronous rectifier is not easy to burn out, and the overall efficiency is relatively improved.

Description

Power converter with synchronous rectifier and control method for synchronous rectifier

technical field

The invention relates to a power converter, in particular to a power converter with a synchronous rectifier which uses a current transformer to detect the cut-off time of the synchronous rectifier and a control method for the synchronous rectifier.

Background technique

In various power converter circuits, such as resonant power converters, etc., there are usually switching circuits, transformers and rectifier diodes. Wherein the switching circuit is connected with the primary winding of the transformer, and is controlled by a control circuit to be turned on or off. The transformer receives electric energy through the primary winding, and transmits the electric energy of the primary winding to the secondary winding of the transformer by electromagnetic induction when the switching circuit is switched on or off. As for the rectifier diode, it is connected with the secondary winding of the transformer, and is used for rectifying the AC voltage induced by the secondary winding into a DC voltage for supplying to the load.

However, since the forward conduction voltage drop of the rectifier diode will cause considerable conduction loss of the rectifier diode, the synchronous rectifier composed of transistors gradually replaces the rectifier diode and is applied in the power converter. Compared with the traditional power converter architecture using rectifier diodes, power converters using synchronous rectifiers for rectification can reduce power loss.

Although using a synchronous rectifier for rectification can indeed reduce the power loss of the power converter, since the synchronous rectifier is composed of transistors, the synchronous rectifier needs to be precisely controlled to switch on or off. The current control method of the synchronous rectifier is to directly control the conduction of the synchronous rectifier by a control integrated circuit. In addition, the control integrated circuit will sample the voltage difference between the drain and the source of the synchronous rectifier, and then calculate the current The current through the synchronous rectifier is used to control the synchronous rectifier to cut off.

However, the control method of the above-mentioned synchronous rectifier will be affected by the leakage inductance on the line of the power converter, so that the control integrated circuit cannot accurately sample the voltage difference between the drain and the source of the synchronous rectifier, and thus the control integrated circuit cannot accurately measure To control the operation of the synchronous rectifier, for example, the control integrated circuit may control the synchronous rectifier to cut off early, so that the synchronous rectifier is easy to burn out, and the overall efficiency of the power converter is also relatively poor.

Therefore, how to develop a power converter with a synchronous rectifier and a control method for the synchronous rectifier that can improve the above-mentioned defects of the known technology is an urgent problem to be solved at present.

Contents of the invention

The main purpose of the present invention is to provide a power converter with a synchronous rectifier and a control method for the synchronous rectifier, to solve the problem that the known power converter directly uses the control integrated circuit to control the operation of the synchronous rectifier, so the control integrated circuit is affected by the known power conversion. When affected by the leakage inductance on the circuit of the synchronous rectifier, the action of the synchronous rectifier cannot be accurately controlled, resulting in defects such as possible burnout of the synchronous rectifier and poor overall efficiency of the conventional power converter.

In order to achieve the above object, a broad implementation of the present invention provides a power converter, which includes a switching circuit for receiving an input voltage; a transformer with a primary winding and a secondary winding, the primary winding and the power output end of the switching circuit and The first common contact connection; the main control circuit, connected to the control terminal of the switching circuit, is used to generate at least one main control signal to control the operation of the switching circuit, so that the energy of the input voltage is selectively transmitted to the primary winding through the switching circuit; at least one a synchronous rectifier, connected in series with the secondary winding of the transformer and the second common point; at least one current transformer, connected in series with the synchronous rectifier, for outputting a detection signal according to the current flowing through the synchronous rectifier; and at least one synchronous rectification control circuit, It is connected with the control terminal of the synchronous rectifier, the current transformer and the control terminal of the switching circuit, which receives the corresponding detection signal and the corresponding main control signal, and controls the operation of the synchronous rectifier; when the main control circuit controls the switching circuit to be turned on, the synchronous rectification The control circuit controls the synchronous rectifier to be turned on, and the synchronous rectification control circuit controls the synchronous rectifier to be turned off according to the detection signal.

To achieve the above purpose, another broad implementation of the present invention is to provide a synchronous rectifier control method for controlling at least one synchronous rectifier of a power converter, wherein the power converter further includes a switching circuit, a main control circuit, a transformer , at least one current transformer and at least one synchronous rectification control circuit, the switching circuit is connected to the primary winding of the transformer, the main control circuit is connected to the control terminal of the switching circuit, the current transformer and the synchronous rectifier are connected in series with the secondary winding of the transformer, and the synchronous rectification control circuit The control terminal of the switching circuit is connected to the control terminal of the synchronous rectification controller, and the control method of the synchronous rectification control circuit includes: making the transformer perform energy conversion: (a) generating a main control signal to the switching circuit and the synchronous rectification control circuit through the main control circuit , make the switching circuit operate so that the energy of the input voltage is selectively transmitted to the transformer through the switching circuit; (b) the synchronous rectification control circuit controls the synchronous rectifier to conduct according to the main control signal; (c) the current transformer detects the synchronous rectifier and transmits the detection signal to the synchronous rectification control circuit; and (d) the synchronous rectification control circuit controls the synchronous rectifier to cut off according to the detection signal.

The present invention provides a power converter with a synchronous rectifier and a control method for the synchronous rectifier, which quickly detects the reverse flow of the current flowing through the synchronous rectifier through the current transformer, so that the synchronous rectification control circuit can be real-time and accurate The control of the synchronous rectifier cuts off, so the synchronous rectifier of the present invention is not easy to burn out, and the overall efficiency of the power converter of the present invention is relatively improved.

Description of drawings

FIG. 1 : It is a circuit block diagram of a power converter according to a preferred embodiment of the present invention.

FIG. 2 : It is a schematic diagram of a detailed circuit structure of the first synchronous rectification control circuit shown in FIG. 1 .

Fig. 3: It is a schematic diagram of the current, voltage and timing states of the power converter shown in Fig. 1 .

Fig. 4: It is another modification example of the comparing circuit shown in Fig. 2 .

Fig. 5: It is a modification example of the start-up circuit shown in Fig. 2 .

FIG. 6 is a schematic diagram of the circuit structure of the first synchronous rectification control circuit according to another preferred embodiment of the present invention.

FIG. 7 is a schematic diagram of the circuit structure of the first synchronous rectification control circuit according to another preferred embodiment of the present invention.

FIG. 8 : It is another variation example of the reset circuit shown in FIG. 5 .

Fig. 9: It is a flow chart of the control method of the first synchronous rectifier shown in Fig. 1 .

Fig. 10: It is a sub-step of step S93 shown in Fig. 9 .

The reference numerals in the above-mentioned accompanying drawings are explained as follows:

1: Power converter

11: switch circuit

12: Main control circuit

13a~13b: first~second synchronous rectification control circuits

14a to 14b: first to second synchronous rectifiers

15: Resonant circuit

16: filter circuit

20: Start circuit

201: Totem Pole Circuit

21: Comparison circuit

210: Comparison unit

50: Sustain circuit

51: Reset circuit

311: Voltage source

OP: Comparator

CT ₁ ~ CT ₂ : First to second current transformers

COM ₁ ~ COM ₂ : first to second common contact

S ₁ ~ S ₂ : The first ~ second main control signals

S ₃ ～S ₄ : first to second synchronous rectification control signals

I _D1 ~ I _D2 : 1st ~ 2nd current

Q ₁ to Q ₂ : First to second main switching elements

Q ₃ , Q ₆ , Q ₇ : first to third auxiliary switching elements

Q ₄ , Q ₅ , Q ₈ : first to third reset circuit switching elements

Q _1a , Q _2a , Q _3a , Q _4a , Q _5a , Q _8a : first current conducting terminals

Q _1b , Q _2b , Q _3b , Q _4b , Q _5b , Q _8b : second current conducting terminals

T: Transformer

N _p : primary winding

N _s : Secondary winding

R ₁ : Starting circuit resistance

R ₂ : Comparator circuit resistance

R ₃ , R ₄ : 1st to 2nd sustain circuit resistors

R ₅ , R ₈ : first to second reset circuit resistors

R ₆ ~ R ₇ : first to second comparison unit resistance

D ₁ to D ₂ : first to second diodes

V _t1 to V _t2 : First to second detection signals

B ₁ : NPN bipolar junction transistor

B ₂ : PNP bipolar junction transistor

T ₁ ~ T ₄ : time

V _in : input voltage

V _CC : auxiliary voltage

V _o : output voltage

V _ref : Reference voltage

L _o : load

D _sc : Schottky diode

C: holding capacitor

C _s : power supply capacitor

C _f : filter capacitor

C _r1 , C _r2 : first to second resonance capacitors

L _r : Resonant inductance

L _m : Magnetizing inductance

S90-93: Flow steps of the control method of the first synchronous controller

S930: Startup steps

S931: comparison step

S932: maintenance step

S933: Reset step

Detailed ways

Some typical embodiments embodying the features and advantages of the present invention will be described in detail in the description in the following paragraphs. It should be understood that the present invention is capable of various changes in different ways without departing from the scope of the present invention, and that the description and drawings therein are illustrative in nature rather than limiting the present invention.

Please refer to FIG. 1 , which is a circuit block diagram of a power converter according to a preferred embodiment of the present invention. As shown in FIG. 1 , the power converter 1 of this embodiment is connected to a load L _o for converting an input voltage V _in into an output voltage V _o for supplying the load L _o . The power converter 1 includes a switching circuit 11, a transformer T, a main control circuit 12, a resonant circuit 15, a filter circuit 16, and at least one synchronous rectification control circuit, such as the first synchronous rectification control circuit shown in FIG. 1 13a and a second synchronous rectification control circuit 13b, at least one synchronous rectifier, such as the first synchronous rectifier 14a and the second synchronous rectifier 14b shown in Figure 1, and at least one current transformer, such as the first current transformer CT shown in Figure 1 ₁ and the second current transformer CT ₂ .

The switching circuit 11 receives the input voltage V _in , and the switching circuit 11 may be but not limited to be composed of a first main switching element _Q1 and a second main switching element _Q2 , wherein the first current conduction of the first main switching element _Q1 The terminal _Q1a is connected to the second current conducting terminal _Q2b of the second main switching element _Q2 , and the first current conducting terminal _Q2a of the second main switching element _Q2 is connected to the first common point _COM1 . And in this embodiment, the first main switch element _Q1 and the second main switch element _Q2 are alternately turned on or off.

The resonant circuit 15 is connected between the switching circuit 11 and the primary winding _Np of the transformer T. As shown in FIG. In this embodiment, the resonant circuit 15 includes at least one resonant capacitor, such as the first resonant capacitor C _r1 and the second resonant capacitor C _r2 shown in FIG. 1 , and a resonant inductor L _r , wherein the first resonant capacitor C _r1 One end is connected to the second current conducting end _Q1b of the first main switching element _Q1 , the other end of the first resonant capacitor C _r1 is connected to one end of the second resonant capacitor C _r2 , and the other end of the second resonant capacitor C _r2 Then it is connected to the first current conducting terminal _Q2a of the second main switching element _Q2 and the first common contact _COM1 . One end of the resonant inductor L _r is connected between the first current conducting end _Q1a of the first main switching element _Q1 and the second current conducting end _Q2b of the second main switching element _Q2 , and the other end of the resonant inductor L _r is It is connected with the primary winding N _p of the transformer T.

The transformer T has a primary winding N _p and a secondary winding N _s , wherein one end of the primary winding N _p is connected to the power output end of the switching circuit 11, that is, the first current conducting end Q _1a of the first main switching element Q ₁ and the second main The second current conducting terminal Q _2b of the switch element Q ₂ is connected, the other end of the primary winding N _p is connected to the first common point COM ₁ , and the secondary winding N _s of the transformer T has a center tap and is connected to the load L _o . In addition, in this embodiment, the primary winding _Np may include, but is not limited to, an equivalent magnetizing inductance _Lm , which is connected in parallel to the primary winding _Np to excite the transformer T when the equivalent primary winding _Np is running. resulting inductance characteristics.

The filter circuit 16 is connected in parallel with the load L _o , which is used to filter the electric energy transmitted to the load L _o , and in this embodiment, the filter circuit 16 may be but not limited to be composed of a filter capacitor C _f , the filter capacitor C _f One end of is connected between the center tap of the secondary winding N _s of the transformer T and the load L _o , and the other end is connected with the second common point COM ₂ .

The main control circuit 12 is connected with the control end of the switching circuit 11, that is, the main control circuit 12 is connected with the control end of the first main switching element _Q1 and the control end of the second main switching element _Q2 , and the main control circuit 12 is used to generate A first main control signal S ₁ and a second main control signal S ₂ are sent to the control terminal of the first main switching element Q ₁ and the control terminal of the second main switching element Q ₂ to respectively pass the first main control signal S ₁ and the second main control signal _S2 to control the operation of the first main switching element _Q1 and the second main switching element _Q2 , so that the energy of the input voltage V _in can be selectively transmitted to the primary of the transformer T through the switching circuit 11 winding N _p , so that the secondary winding N _s of the transformer T can generate an induced AC voltage due to electromagnetic induction.

The first synchronous rectifier 14a is connected in series between one end of the secondary winding _Ns of the transformer T and the second common node COM ₂ , and the second two synchronous rectifiers 14b are connected in series between the other end of the secondary winding Ns and the second node Between the COM ₂ , the first synchronous rectifier 14 a and the second synchronous rectifier 14 b are used to rectify the induced AC voltage generated by the secondary winding N _s of the transformer T into a DC voltage.

The first current transformer CT ₁ and the second current transformer CT ₂ are respectively connected in series to both ends of the secondary winding N _s of the transformer T, and the first current transformer CT ₁ is further connected to the first synchronous rectification control circuit 13a and the second common point COM ₂ is connected, and the second current transformer CT ₂ is further connected with the second synchronous rectification control circuit 13b and the second common contact point COM _2. The first current transformer CT ₁ is used for the first current I flowing through the first synchronous rectifier 14a _D1 outputs the first detection signal V _t1 , and the second current transformer CT ₂ is used to output the second detection signal V _t2 according to the second current _ID2 flowing through the second synchronous rectifier 14 b.

The first synchronous rectification control circuit 13a is connected to the control terminal of the first synchronous rectifier 14a and the first current transformer _CT1 , and then receives the first detection signal V _t1 , and the first synchronous rectification control circuit 13a is further connected to the first main switching element Q ₁ to receive the first main control signal S 1 output from the main control circuit 12 to the first main switching element Q ₁ , the first synchronous rectification control circuit _13a is used to receive the first main control signal S ₁ and the first detection The signal V _t1 outputs a first synchronous rectification control signal _S3 to the control terminal of the first synchronous rectifier 14 a to control the first synchronous rectifier 14 a to be turned on or off. The second synchronous rectification control circuit 13b is connected to the control terminal of the second synchronous rectifier 14b and the second current transformer _CT2 , and then receives the second detection signal V _t2 , and the second synchronous rectification control circuit 13b is further connected to the second main switching element The control terminal of Q ₂ is connected to receive the second main control signal S 2 output from the main control circuit 12 to the second main switching element Q ₂ , and the second synchronous rectification control circuit _13b is used for according to the second main control signal S ₂ and the second The signal V _t2 is detected to output a second synchronous rectification control signal _S4 to the control terminal of the second synchronous rectifier 14b to control the second synchronous rectifier 14b to be turned on or off.

In this embodiment, when the main control circuit 12 controls the first main switching element _Q1 of the switching circuit 11 to conduct through the first main control signal _S1 , the first synchronous rectification control circuit 13a also controls the first main control signal S1 according to the first main control signal S1. ₁ to control the first synchronous rectifier 14a to be turned on, and the first synchronous rectification control circuit 13a further controls the first synchronous rectifier 14a to be turned off according to the first detection signal V _t1 output from the first current transformer _CT1 . Similarly, when the main control circuit 12 controls the second main switching element _Q2 of the switching circuit 11 to conduct through the second main control signal _S2 , the second synchronous rectification control circuit 13b is also controlled according to the second main control signal _S2 The second synchronous rectifier 14b is turned on, and the second synchronous rectification control circuit 13b further controls the second synchronous rectifier 14b to be turned off according to the second detection signal V _t2 output from the second current transformer CT ₂ .

The internal circuit structure and connection relationship of the first synchronous rectification control circuit 13 a will be described below with reference to FIG. 2 . In addition, since some circuit components in FIG. 1 and FIG. 2 are connected to each other, the same English symbols will be marked in FIG. 1 and FIG. 2 to represent their mutual connection.

Please refer to FIG. 2 , which is a schematic diagram of a detailed circuit structure of the first synchronous rectification control circuit shown in FIG. 1 . As shown in the figure, the first synchronous rectification control circuit 13a mainly includes a start-up circuit 20 and a comparison circuit 21, wherein the input end of the start-up circuit 20 is connected to the control end of the first main switching element Q1 of the switch circuit ₁₁ to receive the The first main control signal S ₁ output by the main control circuit 12 , the output terminal of the starting circuit 20 is connected to the control terminal of the first synchronous rectifier 14a, and the starting circuit 20 outputs a first main control signal S ₁ according to the first main control signal S 1 The synchronous rectification control signal _S3 is sent to the control terminal of the first synchronous rectifier 14a. Therefore, when the main control circuit 12 controls the first main switching element _Q1 to be turned on through the first main control signal _S1 , the startup circuit 20 is also based on the first main control signal S1. Control the signal _S1 to output the first synchronous rectification control signal _S3 of enable level to the control terminal of the first synchronous rectifier 14a, so as to drive the first synchronous rectifier 14a to conduct. In addition, the startup circuit 20 can further prevent the voltage level of the first main control signal _S1 from following the voltage level of the first synchronous rectification control signal _S3 when the voltage level of the first synchronous rectification control signal _S3 is pulled down. Ping was pulled low while being pulled low. The comparison circuit 21 is connected to the first current transformer CT ₁ , the control terminal of the first synchronous rectifier 14a and the output terminal of the start-up circuit 20, it receives the first detection signal V _t1 output by the first current transformer CT ₁ , and when the first When the detection signal V _t1 is greater than a reference voltage in the comparison circuit 21, the voltage level of the first synchronous rectification control signal _S3 output from the start-up circuit 20 to the control terminal of the first synchronous rectifier 14a is pulled down, so that the first synchronous rectification control The signal _S3 is at a disable level, thereby driving the first synchronous rectifier 14a to be turned off.

In the above embodiment, the starting circuit 20 mainly includes a starting circuit resistor R ₁ , one end of the starting circuit resistor R ₁ is connected to the control terminal of the first main switching element Q 1 , and the other end of the starting circuit resistor R ₁ is connected to the first main switching element Q ₁ . The control terminal of a synchronous rectifier 14a is connected, when the value of the first detection signal V _t1 is greater than the value of the reference voltage of the comparison circuit 21, the comparison circuit 21 will start the voltage level of the first synchronous rectification control signal _S3 output by the circuit 20 When pulled low, the startup circuit 20 can prevent the level of the first main control signal S ₁ from being pulled low as the voltage level of the first synchronous rectification control signal S ₃ is pulled low through the startup circuit resistor R ₁ . The comparison circuit 21 includes a comparison circuit resistor R ₂ , a comparison unit 210 and a first diode D ₁ , wherein one end of the comparison circuit resistor R ₂ is connected to the first current transformer CT ₁ to receive the first detection signal V _t1 , The other end of the comparison circuit resistor _R2 is connected to the input terminal of the comparison unit 210, so the input terminal of the comparison unit 210 receives the first detection signal V _t1 through the comparison circuit resistance _R2 , so the comparison unit 210 is based on the first detection signal Whether the value of V _t1 is greater than the value of the conduction voltage of the comparison unit 210 itself and is turned on or off. The output terminal of the comparison unit 210 is connected with the output terminal of the starting circuit 20 and the control terminal of the first synchronous rectifier 14a, and the ground terminal of the comparison unit 210 is connected with the second common contact point COM ₂ , when the value of the first detection signal V _t1 is greater than When the comparison unit 210 is turned on by the conduction voltage of the comparison unit 210 , the voltage level of the first synchronous rectification control signal S ₃ is pulled down by the comparison circuit 21 , thereby driving the first synchronous rectifier 14 a to be turned off. It can be seen from this that the conduction voltage of the comparison unit 210 is set as a reference voltage used by the comparison circuit 21 to compare with the first detection signal V _t1 .

In this embodiment, the comparison unit 210 can be realized by but not limited to the first auxiliary switching element _Q3 , so the conduction voltage of the first auxiliary switching element _Q3 is used by the comparison circuit 21 to compare with the first detection signal V The reference voltage to which _t1 is compared. The control end of the first auxiliary switching element _Q3 is connected to the other end of the comparison circuit resistor _R2 through the input end of the comparison unit 210, and receives the first detection signal V _t1 , the second current of the first auxiliary switching element _Q3 The conducting terminal Q _3b is connected to the other end of the starting circuit resistor R ₁ of the starting circuit 20 and the control terminal of the first synchronous rectifier 14a through the output terminal of the comparison unit 210, and the first current conducting terminal of the first auxiliary switching element Q ₃ Q _3a is connected to the second common point COM ₂ via the ground terminal of the comparison unit 210 . The cathode end of the first diode D1 is connected between the other end of the comparator circuit resistor _R2 and the control end of the first auxiliary switching element _Q3 , and the anode end of the first diode _D1 is connected to the second common point COM ₂ is connected, and the first diode _D1 has a clamping function, which is used to clamp the voltage between the control terminal of the first auxiliary switching element _Q3 and the first current conducting terminal _Q3a , preventing the first detection signal V _t1 from When the value is negative, the reverse current is too large and the first auxiliary switching element _Q3 is burned.

In this embodiment, the internal circuit structure of the second synchronous rectification control circuit 13b is similar to the internal circuit structure of the first synchronous rectification control circuit 13a, and the internal circuit elements of the second synchronous rectification control circuit 13b and the second main switching element Q _2. The connection relationship between the second current transformer CT ₂ and the second synchronous rectifier 14b is also similar to the internal circuit elements of the first synchronous rectification control circuit 13a and the first main switching element Q ₁ , the first current transformer CT ₁ and the first synchronous rectifier. The connection relationship between a synchronous rectifier 14a, so the internal circuit structure of the second synchronous rectification control circuit 13b and the connection with the second main switching element Q ₂ , the second current transformer CT ₂ and the second synchronous rectifier 14b will not be repeated below. The connection relationship among them, and the various possible implementation modes of the second synchronous rectification control circuit 13b will not be repeated, and only the first synchronous rectification control circuit 13a is taken as an example to illustrate the technology of the present invention and various possible implementation modes .

In the above embodiments, the first main switching element Q ₁ , the second main switching element Q ₂ , the first auxiliary switching element Q ₃ , the first synchronous rectifier 14a and the second synchronous rectifier 14b can all be metal oxide semiconductor field effect Transistor (MOSFET), but not limited thereto, and the first main switching element Q ₁ , the second main switching element Q ₂ , the first auxiliary switching element Q ₃ , the first synchronous rectifier 14a and the second synchronous rectifier 14b can further use N-type or P-type metal-oxide-semiconductor field effect transistors according to actual circuit requirements.

The operation principle of the power converter of the present invention will be described below. Please refer to FIG. 3 , together with FIG. 1 and FIG. 2 , wherein FIG. 3 is a schematic diagram of the current, voltage and timing states of the power converter shown in FIG. 1 . As shown in FIG. 3, when, for example, at time _T1 , the main control circuit 12 will output the first main control signal _S1 at the enable level to the control terminal of the first main switching element Q1 of the switching circuit ₁₁ , so that The first main switching element _Q1 starts to conduct, so _the first synchronous rectification control circuit 13a outputs the first synchronous rectification control signal _S3 at an enable level to the first synchronous rectifier 14a according to the first main control signal S1 control terminal, and then control the first synchronous rectifier 14a to conduct, so that the first current _ID1 flowing through the first synchronous rectifier 14a starts to rise, so the first detection signal V output by the first current transformer _CT1 Then _t1 is negative corresponding to the first current _ID1 . In addition, since the first synchronous rectifier 14a is composed of MOSFETs, the first current _ID1 will rise from zero due to the body diode of the first synchronous rectifier 14a before the time _T1 . .

When a period of time passes until time _T2 , at this time, the first current _ID1 flowing through the first synchronous rectifier 14a has dropped to zero and is about to change its flow direction, the first detection signal V output by the first current transformer _{CT1 t1} will change from a negative value to a positive value corresponding to the redirection of the first current _ID1 , and form a pulse (pulse). At this time, the first detection signal V _t1 will be converted by the comparison circuit resistor _R2 in the comparison circuit 21. sent to the control terminal of the first auxiliary switching element _Q3 , and driving the first auxiliary switching element _Q3 to conduct when the value of the first detection signal V _t1 is greater than the conduction voltage value of the first auxiliary switching element _Q3 , so that Then, the comparison circuit 21 will pull down the voltage level of the first synchronous rectification control signal _S3 , so that the first synchronous rectification control signal _S3 changes from the enable level to the disable level, so the first synchronous rectifier 14a will end. As can be seen from the above, when the first current _ID1 flowing through the first synchronous rectifier 14a of the present invention is about to flow in reverse, the first current transformer _CT1 can quickly detect this situation, and notify the first The synchronous rectification control circuit 13a makes the first synchronous rectifier 14a controlled by the first synchronous rectification control circuit 13a cut off immediately.

Please refer to FIG. 4 together with FIG. 3 , wherein FIG. 4 is another modification example of the comparing circuit shown in FIG. 2 . As shown in Figure 4, compared with the comparison circuit 21 shown in Figure 2, the comparison circuit 21 of the present embodiment only has a comparison circuit resistance _R2 and a comparison unit 210, and in this embodiment, the comparison unit 210 is replaced by a comparison device OP and a voltage source 311. Wherein, one end of the resistor R ₂ of the comparison circuit is connected to the first current transformer CT ₁ to receive the first detection signal V _t1 , and the other end of the resistor R ₂ of the comparison circuit is inverted with that of the comparator OP via the input terminal of the comparison unit 210 The input terminal is connected, and the voltage source 311 is connected in series between the non-inverting input terminal of the comparator OP and the ground terminal of the comparison unit 210, and is connected to the second common contact point COM ₂ through the ground terminal of the comparison unit 210, which is used for A reference voltage V _ref is provided to the non-inverting input terminal of the comparator OP, and the output terminal of the comparator OP is synchronized with the other end of the starting circuit resistor R ₁ of the starting circuit 20 and the first terminal through the output terminal of the comparing unit 210 The control terminal of the rectifier 14a is connected to the comparator OP for comparing the first detection signal V _t1 and the reference voltage V _ref , and when the value of the first detection signal V _t1 is greater than the value of the reference voltage V _ref , the output terminal outputs a low voltage level signal to pull down the voltage level of the first synchronous rectification control signal _S3 output by the startup circuit 20, so that the first synchronous rectification control signal _S3 is at a disabled level, and then drives the first synchronous rectifier 14a to be turned off .

Therefore, as shown in FIG. 3, when, for example, at time _T1 , the first synchronous rectifier 14a receives the first synchronous rectification control signal _S3 at the enable level and assumes a conduction state, and after a period of time, the flow through When the first current _ID1 of the first synchronous rectifier 14a begins to gradually _decrease , the first detection signal V t1 output by the first current transformer CT ₁ will correspondingly start to gradually increase, and when the value of the first detection signal V _t1 is substantially is greater than the value of the reference voltage _Vref , for example, at time _T2 , the comparator OP will output a low-level signal to pull down the voltage level of the first synchronous rectification control signal _S3 output by the start-up circuit 20, The first synchronous rectification control signal _S3 is turned into a disabled level, so that the first synchronous rectifier 14a can be driven to be turned off. Therefore, the first synchronous rectifier control circuit 13 a using the comparison circuit 211 of this embodiment can also control the first synchronous rectifier 14 a to be cut off in real time according to the detection result of the first current transformer CT ₁ .

As for the action modes of the second main switching element Q ₂ , the second current transformer CT ₂ , the second synchronous rectification control circuit 13b and the second synchronous rectifier 14b are respectively the same as the first main switching element Q ₁ , the first current transformer CT ₁ , The operation modes of the first synchronous rectification control circuit 13a and the first synchronous rectifier 14a are similar, so the details will not be repeated here.

Since the current transformer has the characteristic of rapidly detecting current changes, the power converter 1 of the present invention can quickly detect the current flowing through the first synchronous rectifier 14a through the first current transformer _CT1 and the second current transformer _CT2 respectively. The first current _ID1 and the second current _ID2 of the second synchronous rectifier 14b have a situation of reverse flow, so that the first synchronous rectification control circuit 13a and the second synchronous rectification control circuit 13b can control the first synchronous rectification immediately and accurately. The rectifier 14a and the second synchronous rectifier 14b are turned off, so that the first synchronous rectifier 14a and the second synchronous rectifier 14b are not easy to burn out, and the overall efficiency of the power converter 1 of the present invention is relatively improved.

Other possible implementations of the first synchronous rectification control circuit 13a and the second synchronous rectification control circuit 13b of the present invention will be further described below. Since the second synchronous rectification control circuit 13 a is similar to the first synchronous rectification control circuit 13 b in terms of circuit structure and connection relationship, the following only takes the first synchronous rectification control circuit 13 a as an example for exemplary illustration. In addition, since some circuit components in the following figures are connected to some circuit components in FIG. 1 , the same English symbols will be marked in FIG. 1 and the following figures to represent their interconnection.

In some embodiments, in order to enhance the driving capability of the internal circuit of the power converter 1, as shown in FIG. The column circuit 201 is connected to the control terminal of the switching circuit 11, that is, between the control terminal of the first main switching element _Q1 and the starting circuit resistor _R1 of the starting circuit 20, and includes an NPN bipolar junction transistor _B1 and a PNP bipolar junction transistor _B2 , wherein the base of NPN bipolar junction transistor _B1 is connected to the base of PNP bipolar junction transistor _B2 and the control terminal of the first main switching element _Q1 , and the NPN bipolar junction The collector of the type transistor B ₁ receives an auxiliary voltage V _cc , the emitter of the NPN bipolar junction transistor B ₁ is connected to the starting circuit resistor R ₁ of the starting circuit 20, and the emitter of the PNP bipolar junction transistor B ₂ is connected to Between the emitter of the NPN bipolar junction transistor _B1 and the startup circuit resistor _R1 , the collector of the PNP bipolar junction transistor _B2 is connected to the second common point _COM2 . In this embodiment, the NPN bipolar junction transistor _B1 and the PNP bipolar junction transistor _B2 are respectively controlled by the first main control signal _S1 to turn on or off, so that the totem pole circuit 201 can strengthen the power supply The drive capability of the internal circuit of converter 1. Certainly, the totem pole circuit 201 is not limited to be used in the start-up circuit 20 of the first synchronous rectification control circuit 13a shown in FIG. In the startup circuit of the synchronous rectification control circuit.

In other embodiments, when the first synchronous rectifier 14a is turned off, the leakage inductance on the line of the power converter 1 and the stray capacitance of the first synchronous rectifier 14a may oscillate, causing the first current transformer _CT1 to The output first detection signal V _t1 also changes continuously, so that the first synchronous rectifier 14a will be repeatedly turned on or off and cannot be kept in the off state, so in order to avoid the above possible situation, a hold can be set The circuit 50 forms the circuit structure shown in FIG. 6 in the comparison circuit 21 of the first synchronous rectification control 13a shown in FIG. 2 . The holding circuit 50 is connected between the comparison circuit resistor R ₂ and the control terminal of the comparison unit 210, the holding circuit 50 is used to receive the first detection signal V _t1 through the comparison circuit resistance R ₂ , and make the first detection signal V _t1 in The pulse formed by changing from a negative value to a positive value lasts for a specific time. Therefore, as shown in FIG. 3, at time _T2 , the first current _ID1 flowing through the first synchronous rectifier 14a drops from a positive value to zero and intends to flow in the reverse direction, and the first detection signal V _t1 changes from a negative value. When a pulse is formed as a positive value, the sustain circuit 50 will maintain the pulse state and level of the first detection signal V _t1 for a specific time, so that the first synchronous rectifier 14a is corresponding to the first detection signal V _t1 And maintain the cut-off state at the specific time, so the first synchronous rectifier 14a is not affected by the leakage inductance on the line of the power converter 1 and the stray capacitance oscillation of the first synchronous rectifier 14a, and cannot be maintained in the cut-off state. Condition.

In the above embodiment, the sustain circuit 50 may include a second diode D ₂ , a sustain capacitor C, a first sustain circuit resistor R ₃ and a second sustain circuit resistor R ₄ , wherein the second diode D The anode end of ₂ is connected with the other end of the comparison circuit resistor _R2 of the comparison circuit 21 and the cathode end of the first diode _D1 , and the cathode end of the second diode _D2 is connected with one end of the first maintenance circuit resistor _R3 And one end of the sustaining capacitor C is connected, the other end of the first sustaining circuit resistor _R3 is connected to the control terminal of the first auxiliary switching element _Q3 and one end of the second sustaining circuit resistor _R4 , and the other end of the second sustaining circuit resistor _R4 One end and the other end of the holding capacitor C are connected to the second common contact point COM ₂ , and in this embodiment, the resistance value of the second holding circuit resistor _R4 can be but not limited to be greater than the resistance of the first holding circuit resistor _R3 value.

The operation of the sustain circuit 50 shown in FIG. 6 will be briefly described below. Please refer to FIG. 3 and FIG. 6, when at time _T2 , the first current _ID1 flowing through the first synchronous rectifier 14a drops from a positive value to zero and wants to flow in the reverse direction, the first current ID of the first current transformer _CT1 The detection signal V _t1 will change from a negative value to a positive value to form a pulse, and charge the holding capacitor C through the comparison circuit resistor R ₂ and the second diode D ₂ . And when the leakage inductance on the line of the power converter 1 and the stray capacitance of the first synchronous rectifier 14a oscillate, causing the first detection signal V _t1 output by the first current transformer _CT1 to continuously fluctuate, for example, drop to At zero time, the first detection signal V _t1 will stop charging the holding capacitor C. At this time, due to the impedance relationship between the first holding circuit resistor _R3 and the second holding circuit resistor _R4 , the holding capacitor C cannot be discharged, so that The voltage of the holding capacitor C will be maintained at a high level state continuously, so the first auxiliary switching element _Q3 will still be maintained in a conducting state, so that the first synchronous rectifier 14a will not be affected by the power converter 1 Influenced by the leakage inductance on the line and the stray capacitance oscillation of the first synchronous rectifier 14a, the cut-off state is continuously maintained.

In addition, since the maintaining circuit 50 will keep the first synchronous rectifier 14a in the cut-off state, in order to make the first synchronous rectifier 14a normally change from the cut-off state to the conduction state in the next switching cycle of the first main switching element _Q1 State, instead of being maintained in the cut-off state, in some embodiments, as shown in FIG. 50 and the control terminal of the first main switching element _Q1 of the switching circuit 11 are connected to reset the holding circuit 50, so that the first synchronous rectifier 14 can be switched in the next switching cycle of the first main switching element _Q1 Normal transition from off state to on state.

The reset circuit 51 may include a first reset circuit switching element Q ₄ , a second reset circuit switching element Q ₅ and a first reset circuit resistor R ₅ , wherein the control terminal of the first reset circuit switching element Q ₄ is connected to the switching circuit 11 The control end of the first main switching element Q ₁ is connected to receive the first main control signal S ₁ output by the main control circuit 12, and the first current conducting end Q _4a of the first reset circuit switching element Q ₄ is connected to the second The common terminal COM ₂ , the second current conducting terminal _Q4b of the switching element _Q4 of the first reset circuit is connected with one end of the resistor _R5 of the first reset circuit and the control terminal of the switching element _Q5 of the second reset circuit, and the second reset circuit The first current conduction terminal _Q5a of the switch element _Q5 is connected to the second common contact point _COM2 , and the second current conduction terminal _Q5b of the switch element _Q5 of the second reset circuit is connected to the first auxiliary switch element _Q3 of the comparison circuit 21. The control terminal, the other end of the first holding circuit resistor R ₃ and one end of the second holding circuit resistor R ₄ are connected, and the other end of the first reset circuit resistor R ₅ receives the auxiliary voltage source V _CC . In addition, in some embodiments, the first reset circuit switch element _Q4 and the second reset circuit switch element _Q5 may be, but not limited to, composed of metal oxide semiconductor field effect transistors, and the first reset circuit switch element _Q4 And the switching element _Q5 of the second reset circuit can be an N-type or P-type metal oxide semiconductor field effect transistor according to the actual needs of the circuit.

The operation of the reset circuit 51 shown in FIG. 6 will be briefly described below. Please refer to FIG. 3 and FIG. 6, when from time _T1 to time _T2 , the first main control signal _S1 is maintained at the enable level, and the first detection signal V _t1 will change from a negative value to a positive value and A pulse is formed, and the pulse is maintained for a specific time from time _T2 by the sustain circuit 50 . When the time _T3 is reached, the first main control signal _S1 will change from the enabling level to the disabling level, and at this time the switching element _Q4 of the first reset circuit will be cut off corresponding to the first main control signal _S1 , so the auxiliary voltage V _CC will drive the second reset circuit switch element _Q5 to conduct through the first reset circuit resistor _R5 , so the holding capacitor C will discharge the electric energy through the conduction of the second reset circuit switch element _Q5 In this way, the first auxiliary switching element _Q3 will be turned off, that is, the sustaining circuit 50 is reset by the reset circuit 51, so at time _T4 , the first main control signal _S1 changes from the disabled level to the enabled level again. When the level is enabled, the startup circuit 20 will output the first synchronous rectification control signal _S3 of the enable level to the control terminal of the first synchronous rectifier 14a according to the first main control signal _S1 , so the first synchronous rectifier 14a can be During the next switching cycle of the first main switching element Q ₁ , it normally changes from the off state to the on state.

Please refer to FIG. 7 , which is a schematic diagram of the circuit structure of the first synchronous rectification control circuit according to another preferred embodiment of the present invention. As shown in the figure, the partial circuit structure of the first synchronous rectification control circuit of this embodiment is similar to the partial circuit structure of the synchronous rectification control circuit shown in FIG. The method will not be repeated here. Compared with Fig. 6, the comparison unit 210 in the comparison circuit 21 of this embodiment is changed to a second auxiliary switching element Q ₆ , a third auxiliary switching element Q ₇ , a first comparison unit resistance R ₆ and a second The comparing unit resistor _R7 is formed, whereby the comparing unit 210 and the maintaining circuit 50 will form an interlock circuit. Wherein the second auxiliary switching element _Q6 can be but not limited to be a PNP bipolar junction transistor, and the base of the second auxiliary switching element _Q6 is connected to one end of the second comparison unit resistor _R7 and the third auxiliary switching element _Q7 is connected to the collector, and is connected to the control terminal of the first synchronous rectifier 14a and the output terminal of the start-up circuit 20 through the output terminal of the comparison unit 210, and the emitter of the second auxiliary switching element _Q6 is connected to the first comparison unit resistor _R6 One end of the second auxiliary switch element _Q6 is connected to the output end of the holding circuit 50 through the input end of the comparison unit 210, that is, the other end of the first holding circuit resistor _R3 and one end of the second holding circuit resistor _R4 The third auxiliary switching element _Q7 can be but not limited to be an NPN bipolar junction transistor, and the base of the third auxiliary switching element _Q7 is connected to the input end of the comparison unit 210 and the output end of the holding circuit 50, that is, the first The other end of a sustaining circuit resistor _R3 is connected to one end of the second sustaining circuit resistor _R4 , and the collector of the third auxiliary switching element _Q7 is connected to one end of the second comparison unit resistor _R7 and the end of the second auxiliary switching element _Q6 . The base is connected to the control terminal of the first synchronous rectifier 14a through the output terminal of the comparison unit 210, and the emitter of the third auxiliary switching element _Q7 is connected to the second common point COM ₂ through the ground terminal of the comparison unit 210. , the other end of the first comparison unit resistor R ₆ and the other end of the second comparison unit resistor R ₇ are connected to each other to receive the auxiliary voltage V _CC .

In addition, compared with FIG. 6, the sustaining capacitor C of the sustaining circuit 50 of this embodiment is connected in parallel with the second sustaining circuit resistor _R4 , and one end of the sustaining capacitor C is connected to the other end of the first sustaining circuit resistor _R3 , the second One end of the sustaining circuit resistor _R4 is connected to the input terminal of the comparison unit 210, and the other end of the sustaining capacitor C is connected to the other end of the second sustaining circuit resistor _R4 and the second common contact point _COM2 , and the sustaining circuit of this embodiment The circuit 50 can also achieve the same effect as the maintaining circuit 50 shown in FIG. 6 .

The operation of the synchronous rectification control circuit 13a shown in FIG. 7 will be briefly described below. Referring to FIG. 3 and FIG. 7, when, for example, at time _T1 , the first synchronous rectifier 14a receives the first synchronous rectification control signal _S3 at the enable level and presents a conduction state, and after a period of time, the current When the first current _ID1 passing through the first synchronous rectifier 14a begins to gradually decrease, the first detection signal V _t1 output by the first current transformer CT ₁ will correspondingly begin to gradually increase, and when the value of the first detection signal V _t1 When the conduction voltage of the third auxiliary switching element _Q7 is substantially greater than, for example, at time _T2 , the third auxiliary switching element _Q7 will be turned on, and at this time, the base of the second auxiliary switching element _Q6 will be turned on. pulled low, causing the second auxiliary switching element _Q6 to be turned on, so the base of the third auxiliary switching element _Q7 will receive the auxiliary voltage V _CC through the first comparison unit resistor _R6 , resulting in the third auxiliary switching element _Q7 The first synchronous rectification control signal _S3 is kept low, and due to the function of the maintenance circuit 50, the first synchronous rectification control signal _S3 is continuously pulled low, causing the first synchronous rectifier 14a It is not affected by the leakage inductance on the line of the power converter 1 and the stray capacitance oscillation of the first synchronous rectifier 14a, and the cut-off state is continuously maintained. It can be known from the above that the comparison circuit 21 will pull down the first synchronous control signal _S3 when the first detection signal V _t1 is greater than the conduction voltage of the third auxiliary switching element _Q7 , so the conduction of the third auxiliary switching element _Q7 The voltage is the reference voltage used by the comparison circuit 21 to compare with the first detection signal V _t1 .

In some embodiments, the holding circuit 50 shown in FIG. 7 can also be applied to the comparing circuit 21 shown in FIG. 4 to form the circuit structure shown in FIG. 8 . In the illustrated embodiment, the hold circuit 50 is connected between the comparison circuit resistor R ₂ and the input terminal of the comparison unit 210 .

Of course, in other embodiments, the resistance of the first comparison unit R ₆ and the second comparison unit resistance R ₇ are not limited to receiving the auxiliary voltage V _cc as shown in FIG. 7 . In other embodiments, the resistance of the first comparison unit The other end of R ₆ and the second comparison unit resistor R ₇ can also be connected to the control end of the switching circuit 11, that is, the control end of the first main switching element Q ₁ to receive the first main control signal S ₁ , and this embodiment Likewise, the holding circuit 50 can achieve the same effect. In addition, in some embodiments, the hold circuit 50 can be, but not limited to, be formed by an RS flip-flop.

Certainly, the internal circuit structure of the reset circuit is not limited to the embodiment shown in FIG. 6 , and may also have different changes as shown in FIGS. 7 to 8 . Please refer to FIG. 7, in this embodiment, the reset circuit 51 is composed of a Schottky diode D _sc , the cathode terminal of the Schottky diode D _sc is connected to the control terminal of the first main switching element _Q1 to receive the second A main control signal S ₁ , the anode terminal of the Schottky diode D _sc is connected to the first sustaining circuit resistor R ₃ and the second sustaining circuit resistor R ₄ . And when the first main control signal _S1 is at the disabled level, the reset circuit 51 can reset the sustain circuit 50 through the Schottky diode D _sc , so that the sustain capacitor C discharges the stored electric energy.

Please refer to FIG. 8 , which is another variation example of the reset circuit shown in FIG. 6 . As shown in the figure, in this embodiment, the reset circuit 51 includes a second reset circuit resistor R ₈ and a third reset circuit switch element Q ₈ , wherein one end of the second reset circuit resistor R ₈ is connected to the control end of the switching circuit 11 , that is, the control terminal of the first main switching element _Q1 is connected to receive the first main control signal _S1 , and the other end of the second reset circuit resistor _R8 is connected to the control terminal of the third reset circuit switching element _Q8 . The third reset circuit switch element _Q8 can be formed by a P-type metal oxide semiconductor field effect transistor, but not limited thereto, and can also be formed by a PNP bipolar junction field effect transistor. The third reset circuit switch element The first current conduction terminal _Q8a of _Q8 is connected to the second common contact point _COM2 , and the second current conduction terminal _Q8b of the switching element _Q8 of the third reset circuit is connected to the other end of the first maintenance circuit resistor _R3 and the second maintenance circuit. One end of the circuit resistor _R4 is connected. Therefore, through the above circuit structure, the reset circuit 51 can reset the sustain circuit 50 when the first main control signal _S1 is at a disabled level, so that the sustain capacitor C discharges the stored electric energy.

Of course, in other embodiments, one end of the resistor R8 of the second reset circuit can be changed to be connected to the other control end of the switch circuit 11, that is, the control end of the second main switching element _Q2 to receive the second main control signal _S2 , The switch element Q ₈ of the third reset circuit needs to be correspondingly changed to an NMOS field effect transistor, and this embodiment can also make the reset circuit 51 achieve the same effect.

Of course, the comparison circuit 21 shown in FIG. 2, the comparison circuit 21 shown in FIG. 4 and the comparison circuit 21 shown in FIG. 7 can be replaced with each other, and the comparison unit 210 shown in FIG. Replace each other, the maintenance circuit 50 shown in Figure 6 and the maintenance circuit 50 shown in Figure 7 and Figure 8 can be replaced with each other, the reset circuit 51 of Figure 6, the reset circuit 51 of Figure 7 and the reset circuit 51 of Figure 8 can be replaced with each other , but not limited to this.

The flow steps of the method for controlling the first synchronous rectifier shown in FIG. 1 will be further described below. Please refer to FIG. 9 , together with FIG. 2 , wherein FIG. 9 is a flowchart of a control method of the first synchronous rectifier 14 a shown in FIG. 1 . As shown in Figure 9, first, as shown in step S90, the main control circuit 12 generates the first main control signal _S1 to the switching circuit 11 and the first synchronous rectification control circuit 13a, so that the switching circuit 11 operates to drive the transformer T Carry out energy conversion. Next, as shown in step S91 , the first detection signal V _t1 is transmitted to the first synchronous rectification control circuit 13 a through the current transformer CT. Then, as shown in step S92, the first synchronous rectification control circuit 13a controls the first synchronous rectifier 14a to conduct according to the first main control signal _S1 . Finally, as shown in step S93 , the first synchronous rectification control circuit 13 a controls the first synchronous rectifier 14 a to be turned off according to the first detection signal V _t1 .

In addition, in some embodiments, as shown in FIG. 10 , there are four sub-steps in the step S93 , which are the starting step S930 , the comparing step S931 , the maintaining step S932 and the resetting step S932 . Firstly, the start-up step S930 is performed, that is, _{the start-up circuit 20 receives the first main control signal S1, and outputs a first synchronous rectification control signal S3 to the} control terminal of the first synchronous rectifier 14a according to the first main control signal S1 , and prevent the voltage level of the first main control signal _S1 from being pulled down as the voltage level of the first synchronous rectification control signal _S3 is pulled down. Next, a comparison step S931 is performed, that is, the comparison unit 210 compares the detection signal V _t with the reference voltage, so as to drive the first synchronous rectifier 14 a to be turned off when the level of the detection signal V _t is greater than the level of the reference voltage. Then, the sustaining step S932 is performed, that is, the sustaining circuit 50 is used to maintain the pulse formed by the detection signal V _t changing from a negative value to a positive value for a specific time. Finally, perform the reset step S933 , that is, reset the sustaining circuit 50 through the reset circuit 51 , so that the first synchronous rectifier 14 a can normally change from the off state to the on state in the next switching cycle of the switching circuit 11 .

The control method of the second synchronous rectifier 14b shown in FIG. 1 is similar to that of the first synchronous rectifier 14a, so it will not be repeated here. Since the power converter 1 of the present invention can instantly and accurately control the cut-off of the first synchronous rectifier 14a and the second synchronous rectifier 14b through the first current transformer _CT1 and the second current transformer _CT2 , the first synchronous rectifier 14a and the second synchronous rectifier 14a The synchronous rectifier 14b is not easy to burn out, and the overall efficiency of the power converter 1 of the present invention is relatively improved.

To sum up, the present invention provides a power converter with a synchronous rectifier and a control method for the synchronous rectifier, which quickly detects the reverse flow of the current flowing through the synchronous rectifier through the current transformer, so that the synchronous rectifier control The circuit can immediately and accurately control the cut-off of the synchronous rectifier, so the synchronous rectifier of the present invention is not easy to burn out, and the overall efficiency of the power converter of the present invention is relatively improved. The present invention can be modified in various ways by those skilled in the art without departing from the protection scope of the appended claims.

Claims (21)

1. A power converter comprising: a switching circuit receiving an input voltage; A transformer has a primary winding and a secondary winding, the primary winding is connected to the power output end of the switching circuit and a first common contact; A main control circuit, connected to at least one control terminal of the switching circuit, used to generate at least one main control signal to control the operation of the switching circuit, so that the energy of the input voltage is selectively transmitted to the primary winding through the switching circuit; at least one synchronous rectifier connected in series with the secondary winding of the transformer and a second common point; At least one current transformer, connected in series with the synchronous rectifier, is used to output a detection signal according to the current flowing through the synchronous rectifier; and At least one synchronous rectification control circuit connected to the control terminal of the synchronous rectifier, the current transformer and the control terminal of the switching circuit, which receives the corresponding detection signal and the corresponding main control signal, and outputs a synchronous rectification control signal controlling the operation of the synchronous rectifier; Wherein when the main control circuit controls the switch circuit to be turned on, the synchronous rectification control circuit controls the synchronous rectifier to be turned on, and the synchronous rectification control circuit controls the synchronous rectifier to be turned off according to the detection signal. 2. The power converter as claimed in claim 1, wherein the synchronous rectification control circuit further comprises a comparison circuit having a comparison circuit resistance and a comparison unit, wherein one end of the comparison circuit resistance is connected to the current transformer to receive the detection signal, and the other end of the resistance of the comparison circuit is connected to the input end of the comparison unit, and the output end of the comparison unit is connected to the control end of the synchronous rectifier. The comparison circuit is used when the level of the detection signal is greater than a reference voltage When the level of the synchronous rectifier is lowered, the level of the synchronous rectification control signal is lowered to a disabled level, so as to drive the synchronous rectifier to be cut off. 3. The power converter as claimed in claim 2, wherein the comparing unit comprises: A comparator, the inverting input end of the comparator is connected with the other end of the resistance of the comparison circuit through the input end of the comparison unit, the output end of the comparator is connected with the control of the synchronous rectifier through the output end of the comparison unit terminal connections; and A voltage source is connected in series between the non-inverting input terminal of the comparator and the ground terminal of the comparison unit, and connected to the second common point through the ground terminal of the comparison circuit to provide the reference voltage. 4. The power converter as claimed in claim 2, wherein the comparing unit comprises: A first auxiliary switch element, the control end of the first auxiliary switch element is connected to the other end of the comparison circuit resistor through the input end of the comparison unit, and a first current conducting end of the first auxiliary switch element is connected to the other end of the comparison circuit resistor through the comparison unit. The ground end of the unit is connected to the second common point, a second current conduction end of the first auxiliary switching element is connected to the control end of the synchronous rectifier through the output end of the comparison unit, and the conduction of the first auxiliary switching element voltage is the reference voltage. 5. The power converter as claimed in claim 2, wherein the comparing unit comprises: A second auxiliary switching element, the base of the second auxiliary switching element is connected to the control end of the synchronous rectifier through the output end of the comparison unit, and the collector of the second auxiliary switching element is connected through the input end of the comparison unit and connected to the other end of the comparison circuit resistance; A third auxiliary switching element, the base of the third auxiliary switching element is connected to the other end of the resistor of the comparison circuit and the collector of the second auxiliary switching element, and the collector of the third auxiliary switching element is connected to the The output terminal is connected to the control terminal of the synchronous rectifier, and the emitter of the third auxiliary switching element is connected to the second common point through the ground terminal of the comparison unit; A first comparison unit resistor, one end of the first comparison unit resistor is connected to the emitter of the second auxiliary switching element, and the other end of the first comparison unit resistor is connected to the control terminal of the switching circuit to receive the main control signal ;as well as A second comparison unit resistance, one end of the second comparison unit resistance is connected to the base of the second auxiliary switching element, the collector of the third auxiliary switching element and the control terminal of the synchronous rectifier, the second comparison unit resistance The other end of the switch circuit is connected to the control end to receive the main control signal; Wherein, the conduction voltage of the third auxiliary switching element is the reference voltage. 6. The power converter as claimed in claim 2, wherein the comparing unit comprises: A second auxiliary switching element, the base of the second auxiliary switching element is connected to the control end of the synchronous rectifier through the output end of the comparison unit, and the collector of the second auxiliary switching element is connected through the input end of the comparison unit and connected to the other end of the comparison circuit resistance; A third auxiliary switching element, the base of the third auxiliary switching element is connected to the other end of the resistor of the comparison circuit and the collector of the second auxiliary switching element, and the collector of the third auxiliary switching element is connected to the The output terminal is connected to the control terminal of the synchronous rectifier, and the emitter of the third auxiliary switching element is connected to the second common point through the ground terminal of the comparison unit; a first comparison unit resistance, one end of the first comparison unit resistance is connected to the emitter of the second auxiliary switching element, and the other end of the first comparison unit resistance receives an auxiliary voltage; and A second comparison unit resistance, one end of the second comparison unit resistance is connected to the base of the second auxiliary switching element, the collector of the third auxiliary switching element and the control terminal of the synchronous rectifier, the second comparison unit resistance The other end of the resistor is connected to the other end of the first comparison unit to receive the auxiliary voltage; Wherein, the conduction voltage of the third auxiliary switching element is the reference voltage. 7. The power converter as claimed in claim 6, wherein the synchronous rectification control circuit further comprises a holding circuit, the holding circuit is connected to the resistance of the comparison circuit to receive the detection signal, so that the detection signal is changed from a negative value to The pulse formed by transitioning to a positive value lasts for a specified time. 8. The power converter as claimed in claim 7, wherein the maintaining circuit comprises: a second diode, the anode of the second diode is connected to the other end of the comparison circuit resistor; a first sustain circuit resistor, one end of the first sustain circuit resistor is connected to the cathode terminal of the second diode; a holding capacitor, one end of the holding capacitor is connected to the other end of the first holding circuit resistor, and the other end of the holding capacitor is connected to the second common contact; A second sustaining circuit resistor connected in parallel with the sustaining capacitor, one end of the second sustaining circuit resistor is connected to the other end of the first sustaining circuit resistor, and the other end of the second sustaining circuit resistor is connected to the second common point . 9. The power converter as claimed in claim 7, wherein the maintaining circuit comprises: a second diode, the anode of the second diode is connected to the other end of the comparison circuit resistor; a holding capacitor, one end of the holding capacitor is connected to the cathode end of the second diode, and the other end of the holding capacitor is connected to the second common point; a first sustain circuit resistor, one end of the first sustain circuit resistor is connected to one end of the sustain capacitor; and A second sustain circuit resistor, one end of the second sustain circuit resistor is connected to the other end of the first sustain circuit resistor, and the other end of the second sustain circuit resistor is connected to the second common point. 10. The power converter as claimed in claim 9, wherein the synchronous rectification control circuit further comprises a reset circuit connected to the output end of the sustain circuit for resetting the sustain circuit so that the synchronous rectifier is in the switching circuit In the next switching cycle, it can normally change from the off state to the on state. 11. The power converter as claimed in claim 10, wherein the reset circuit comprises: A first reset circuit switching element, the control end of the first reset circuit switching element is connected to the control end of the switching circuit to receive the main control signal, a first current conducting end of the first reset circuit switching element is connected to the The second common terminal is connected; A second reset circuit switch element, the control end of the second reset circuit switch element is connected to a second current conduction end of the first reset circuit switch element, and a first current conduction end of the second reset circuit switch element is connected to The second common contact is connected, and a second current conducting end of the switching element of the second reset circuit is connected to the output end of the holding circuit; and A first reset circuit resistor, one end of the first reset circuit resistor is connected to the first current conducting end of the first reset circuit switch element, and the other end of the first reset circuit resistor receives the auxiliary voltage. 12. The power converter as claimed in claim 10, wherein the reset circuit comprises: A second reset circuit resistor, one end of the second reset circuit resistor is connected to the control terminal of the switching circuit to receive the main control signal; and A third reset circuit switching element, the control end of the third reset circuit switching element is connected to the other end of the second reset circuit resistor, a first current conducting end of the third reset circuit switching element is connected to the second common point connected, a second current conducting end of the switching element of the third reset circuit is connected to the output end of the sustaining circuit, and the switching element of the third reset circuit is a P-type metal oxide semiconductor field effect transistor. 13. The power converter as claimed in claim 10, wherein the reset circuit comprises: a second reset circuit resistor, one end of the second reset circuit resistor is connected to the other control end of the switching circuit to receive the corresponding main control signal; and A third reset circuit switching element, the control end of the third reset circuit switching element is connected to the other end of the second reset circuit resistor, a first current conducting end of the third reset circuit switching element is connected to the second common point connected, a second current conduction terminal of the switch element of the third reset circuit is connected with the output terminal of the sustain circuit, and the switch element of the third reset circuit is an N-type metal oxide semiconductor field effect transistor. 14. The power converter according to any one of claims 2-13, wherein the synchronous rectification control circuit further comprises a startup circuit connected to the control terminal of the switching circuit and the control terminal of the synchronous rectifier for receiving the main control signal and output the synchronous rectification control signal to the control terminal of the synchronous rectifier according to the main control signal, and when the voltage level of the synchronous rectification control signal is pulled down, prevent the voltage level of the main control signal from following The voltage level of the synchronous rectification control signal is pulled low to be pulled low. 15. The power converter as claimed in claim 14, wherein the start-up circuit comprises a start-up circuit resistor. 16. The power converter as claimed in claim 15, wherein the start-up circuit further comprises a totem-pole circuit, the totem-pole circuit is connected between the control terminal of the switching circuit and the start-up circuit resistor to enhance the power conversion drive capability of the device. 17. A method for controlling a synchronous rectifier, used to control at least one synchronous rectifier of a power converter, wherein the power converter further comprises a switching circuit, a main control circuit, a transformer, at least one current transformer and at least one synchronous A rectification control circuit, the switching circuit is connected to a primary winding of the transformer, the main control circuit is connected to the control terminal of the switching circuit, the current transformer and the synchronous rectifier are connected in series with the primary winding of the transformer, the synchronous rectification The control circuit is connected to the control terminal of the switching circuit and the control terminal of the synchronous rectification controller, and the control method of the synchronous rectification control circuit includes: (a) Generate a main control signal to the switching circuit and the synchronous rectification control circuit through the main control circuit, so that the switching circuit operates and the energy of an input voltage is selectively transmitted to the transformer through the switching circuit; (b) The synchronous rectification control circuit controls the conduction of the synchronous rectifier according to the main control signal; (c) the current transformer detects the synchronous rectifier and sends a detection signal to the synchronous rectification control circuit; and (d) The synchronous rectification control circuit controls the synchronous rectifier to cut off according to the detection signal. 18. The control method of the synchronous rectifier as claimed in claim 17, wherein step (d) also includes a comparison step, is to compare the detection signal with a reference voltage by a comparison unit, so that when the level of the detection signal is greater than When the level of the reference voltage is low, the synchronous rectifier is driven to be cut off. 19. The method for controlling a synchronous rectifier as claimed in claim 18, wherein step (d) further includes a maintaining step, which is to maintain a specific pulse formed by changing the detection signal from a negative value to a positive value through a maintaining circuit. time. 20. The control method of a synchronous rectifier as claimed in claim 19, wherein step (d) further comprises a reset step, and the maintenance circuit is reset by a reset circuit, so that the synchronous rectifier can be normal in the next switching cycle of the switching circuit The ground transitions from an off state to an on state. 21. The control method of a synchronous rectifier as claimed in any one of claims 17-20, wherein step (d) further comprises a starting step, receiving a main control signal output by the main control circuit through a starting circuit, and according to the main control signal to output a synchronous rectification control signal to the control terminal of the synchronous rectifier, and prevent the voltage level of the main control signal from being pulled down as the voltage level of the synchronous rectification control signal is pulled down.

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