HK1072508B

HK1072508B - A self-excited switching power supply circuit

Info

Publication number: HK1072508B
Application number: HK05104965.8A
Authority: HK
Inventors: 饴井俊裕
Original assignee: Smk株式会社
Priority date: 2003-06-16
Filing date: 2005-06-14
Publication date: 2012-05-25

Description

Self-excited switching power supply circuit

Technical Field

The present invention relates to a self-excited switching power supply circuit, and more particularly to a flyback self-excited switching power supply circuit that discharges energy stored in a transformer from a secondary output winding when an exciting current of a primary winding of the transformer is stopped.

Background

As a stable power supply, a switching power supply circuit is used in a battery charger, an AC adapter, or the like. Driving methods (switching methods) of the switching element are roughly classified into a self-excited oscillation method and an independent-excited oscillation method, and the self-excited oscillation method performs an oscillation operation by feeding back a voltage generated in a feedback winding of an inductance component such as a transformer as a driving signal to a control terminal of the switching element.

As such a self-excited switching power supply circuit, a circuit as shown in fig. 4 is known (for example, see japanese patent laid-open No. 2002-51546).

The conventional self-excited switching power supply circuit 100 will be described below with reference to fig. 4 to 6. In the figure, 1 denotes an unstable dc power supply whose voltage may fluctuate, 1a denotes a high-voltage-side terminal thereof, and 1b denotes a low-voltage-side terminal thereof. Further, 2a denotes a primary winding of the transformer 2, 2b denotes a feedback winding provided on a primary side of the transformer 2, 2c denotes a secondary output winding of the transformer 2, and 3 denotes a field effect transistor for oscillation (hereinafter, denoted by FET). 21 denotes a circuit for supplying a forward bias voltage (in other words, at a threshold voltage V) to the gate of the FET3 at the time of circuit startup_THThe above gate voltage), the resistor 25 connected in series with the starting resistor 21 has a resistance value smaller than that of the starting resistor 21, and divides the voltage of the dc power supply 1 to output a low dc voltageWhen the circuit is not started, the circuit is not started.

6 a zener diode to prevent excessive input to the gate; reference numeral 12 denotes a feedback capacitor connected in series between the feedback winding 2b and the gate of the FET3, which constitutes a conduction control circuit together with the feedback resistor 23; 24 represents a resistance to prevent excessive input to the gate; reference numeral 5 denotes an off control transistor element having a collector connected to the gate and an emitter connected to the low voltage side terminal 1 b. Further, 22 denotes a control resistor which constitutes an oscillation stabilizing circuit together with the off control capacitor 11; the connection point with the off-control capacitor 11 is connected to the base of the off-control transistor element 5.

The secondary output winding 2c side is shown with 4 and 13 respectively showing a rectifying diode and a smoothing capacitor constituting a rectifying and smoothing circuit for rectifying and smoothing the output of the secondary output winding 2c and outputting the rectified output to between the high-voltage side output line 20a and the low-voltage side output line 20 b.

In the self-excited switching power supply circuit 100 configured as described above, when a dc voltage is applied to the high-voltage side terminal 1a and the low-voltage side terminal 1b of the power supply 1 at the beginning, the feedback capacitor 12 is charged via the starting resistor 21 (the lower electrode of the capacitor is + and the upper electrode thereof is-), and the charging voltage of the feedback capacitor 12 gradually increases.

When the charging voltage of the feedback capacitor 12 reaches the threshold voltage V_THWhen the gate of FET3 is forward biased, FET3 is turned on (drain-source conduction).

Referring to fig. 5 and 6, the self-oscillation operation after the FET3 is turned on will be described.

Fig. 5 and 6 show: in the conventional self-excited switching power supply circuit 100 shown in fig. 4, a 200V dc power supply 1 is applied as a power supply voltage, the resistance values of the starting resistor 21 and the resistor 25 are set to 1.5M Ω and 100k Ω, respectively, and the capacitance of the feedback capacitor 12 and the resistance value of the feedback resistor 23 are set to 0.01 μ F and 100 Ω, respectively, and the operating waveforms of the respective portions shown in (1) to (6) of fig. 4 are in a self-excited oscillation state.

When the FET3 is turned on and the field current from the dc power supply 1 starts flowing through the primary winding 2a connected in series, an induced electromotive force is generated in each winding of the transformer 2 (see t in fig. 6)₁₂To t₁₀Voltage waveform of the feedback winding 2b indicated by (5) in between), and excitation energy is accumulated in the transformer 2. At this time, the voltage generated as the drive signal in the feedback winding 2b charges the off-control capacitor 11 via the control resistor 22, and the base voltage of the off-control transistor 5 rises (t in fig. 5 (a))₁₂To t₁₀)。

In addition, from t₁₂To t₁₀In the on period of the FET3 shown in (b), the induced voltage generated in the feedback winding 2b ((5) in fig. 6) and the charging voltage of the feedback capacitor 12 ((6) in fig. 6) overlap each other, and the gate voltage of the FET3 ((2) in fig. 6) is maintained at the threshold voltage V_THThe above voltages. At this time, an excessive input to the gate is prevented by the zener diode 6.

When the off-control capacitor 11 is charged and the charging voltage (the base voltage of the off-control transistor 5) becomes equal to or higher than a predetermined bias voltage (t in fig. 5 (a))₁₀) Then, a base current flows through the off control transistor 5, and the collector-emitter is turned on. As a result, the gate of the FET3 is actually in a short-circuit state with the low-voltage-side terminal 1b by turning off the control transistor 5 ((b) of fig. 5 and (2) of fig. 6), and the FET3 is turned off.

When the FET3 is thus turned off and the current flowing through the transformer is substantially interrupted, a flyback voltage (induced back electromotive force) is generated in each winding (t in fig. 5 (d))₁₀To t₁₁). At this time, the flyback voltage generated in the secondary output winding 2c is rectified and smoothed by the smoothing rectifier circuit formed by the rectifying diode 4 and the capacitor 13, and is output as power to be supplied to a load connected between the output lines 20a and 20 b.

On the other hand, the flyback voltage generated in the feedback winding 2b is proportional to the flyback voltage generated in the secondary winding 2c by the load connected to the output side, and is based on the flyback voltage generated in the feedback winding 2b (t in fig. 6)₁₀To t₁₁In between (5)), the feedback capacitor 12 is charged (t of fig. 6)₁₀To t₁₁(6) in between, the lower electrode is + and the upper electrode is- "in fig. 4).

At this time, the zener diode 6 applies a reverse bias to the gate of the FET3, and functions as a path of a charging current for charging the feedback capacitor 12 from the low-voltage terminal 1b side.

When the release of the electric energy accumulated in the secondary output winding 2c by the induced back electromotive force is completed (fig. 5(d), t of fig. 6)₁₁Time), the flyback voltage of the feedback winding 2b acting as a reverse bias to the gate decreases (t of fig. 6)₁₁To t₁₂(5) in between), therefore, since the charging voltage of the feedback capacitance 12 is always maintained ((6) of fig. 6), the gate voltage of the FET3 exceeds the threshold voltage V_TH(t in FIG. 5(b) and FIG. 6 (2)₁₂) The FET3 is turned on again, thus repeating a series of oscillation actions.

In the conventional self-excited switching power supply circuit 100, the time constant of the conduction control circuit composed of the feedback capacitor 12 and the feedback resistor 23 is determined so that the feedback capacitor 12 uses the flyback voltage (t in fig. 6) generated in the feedback winding 2b₁₀To t₁₁(5)) between charges.

That is, t of the energy accumulated in the transformer discharged from the secondary output winding 2c₁₁Before that time, the time constant of the turn-on control circuit is determined so that the feedback capacitor 12 substantially reaches the charging voltage (flyback voltage), and thus, when the energy of the transformer is discharged and the flyback voltage drops, the FET3 quickly shifts to the next turn-on action.

As shown in fig. 5 d, the drain (the primary winding 2a side) of the FET3 is turned on and the voltage is increased from t₁₂The power supply voltage of 200V is changed to 0V, and the excitation current starts to flow from the dc power supply 1.

On the other hand, in the primary winding 2a or the FET3, there is stray capacitance between the windings or parasitic capacitance between the drain and the source, and these capacitances are charged by the flyback voltage whose voltage on the primary winding 2a side is on the high voltage side in the off period, so if the voltage is turned on in a state before and after 200V, in which the voltage does not sufficiently drop at the drain (on the primary winding 2a side) of the FET3, rapid discharge occurs.

As a result, a discharge current having a magnitude shown in a of fig. 5(c) occurs, and the loss of the switching element such as the FET3 increases, which causes noise.

Disclosure of Invention

In view of the above problems, the present invention aims to provide a self-excited switching power supply circuit: when the oscillation field effect transistor is turned on, the discharge current generated is reduced, and as a result, the energy loss at the time of switching is small and noise is not generated.

The self-excited switch power supply circuit of the invention is provided with: a transformer provided with a primary winding, a secondary output winding and a feedback winding; is connected in series with the primary winding to a DC power supply and reaches a threshold voltage V at the gate voltage_THA field effect transistor for oscillation which is turned on; a starting resistor connected between a high-voltage side terminal of the direct-current power supply and a gate of the field effect transistor for oscillation; a conduction control circuit composed of a feedback capacitor and a feedback resistor connected in series between the feedback winding and the gate of the field effect transistor for oscillation; and an off control transistor connected between the gate of the oscillating field effect transistor and the low-voltage side terminal of the DC power supply, and configured to turn on the gate and the low-voltage side terminal after a predetermined time has elapsed after the oscillating field effect transistor is turned on, and to turn off the oscillating field effect transistor. After the oscillating field effect transistor is turned off, the gate voltage is raised to a threshold voltage V by a charging voltage of a feedback capacitor charged by a flyback voltage generated on a feedback winding_THThe field effect transistor for oscillation is turned on again. In such a self-excited switching power supply circuit, the time constant of the conduction control circuit is set so that the energy stored in the transformer is discharged from the secondary output winding and the gate voltage exceeds the threshold voltage V in a state where the polarity of the voltage of at least the feedback winding is reversed_TH。

When the energy stored in the transformer is discharged, free oscillation about the power supply voltage starts from the parasitic capacitance of the oscillating field effect transistor, the stray capacitance between the primary windings, and the inductance of the primary winding, and the voltage polarity of the feedback winding, which is proportional to the primary winding voltage, is also inverted.

In a state where the voltage polarity of the feedback winding is inverted, the voltage of the primary winding becomes proportional to or lower than the power supply voltage, and the electric charges accumulated in the stray capacitance between the primary windings or the parasitic capacitance of the oscillating field effect transistor start to be slowly discharged. Further, since the primary winding voltage is equal to or lower than the power supply voltage, the voltage between the drain and the source of the oscillating field effect transistor is also decreased.

Therefore, at this timing, by making the gate voltage exceed the threshold voltage V_THAnd the oscillation field effect transistor is turned on, so that the discharge current generated during the turn-on is reduced, the energy loss in the oscillation field effect transistor is reduced, and the noise is hard to generate.

A self-excited switching power supply circuit of another aspect of the present invention sets the time constant of the conduction control circuit by: when the energy stored in the transformer is discharged from the secondary output winding and the voltage of the feedback winding reaches the initial maximum value, the gate voltage exceeds the threshold voltage V_TH。

Since free oscillation gradually attenuates with energy loss, the voltage of the primary winding oscillating around the power supply voltage becomes the minimum voltage in a state where the voltage of the feedback winding reaches the first maximum value.

Therefore, at this timing, the gate voltage is made to exceedOver-threshold voltage V_THAnd the field effect transistor for oscillation is turned on, thereby reducing the discharge current generated when the conduction effect is optimal.

Drawings

Fig. 1 is a circuit diagram of a self-excited switching power supply circuit 10 associated with an embodiment of the present invention.

Fig. 2 is a waveform diagram of each part of the self-excited switching power supply circuit 10 that performs a self-oscillation operation, in which (a) shows a base voltage waveform (1) of the off control transistor 5; (b) represents the gate voltage waveform (2) of FET 3; (c) a waveform (3) representing the leakage current of the FET 3; (d) the drain voltage waveform (4) of FET3 is shown.

Fig. 3 is an enlarged waveform diagram showing the gate voltage waveform (2) of the FET3 of the self-excited switching power supply circuit 10 performing the self-oscillation operation, the voltage waveform (5) of the first feedback winding 2 b-side terminal of the feedback capacitor 12, and the charging voltage waveform (6) of the feedback capacitor 12.

Fig. 4 is a circuit diagram of a conventional self-excited switching power supply circuit 100.

Fig. 5 is a waveform diagram of each part of a conventional self-excited switching power supply circuit 100 that performs a self-oscillation operation, in which (a) shows a base voltage waveform (1) of an off control transistor 5; (b) represents the gate voltage waveform (2) of FET 3; (c) a waveform (3) representing the leakage current of the FET 3; (d) the drain voltage waveform (4) of FET3 is shown.

Fig. 6 is an enlarged waveform diagram showing the gate voltage waveform (2) of the FET3, the voltage waveform (5) of the first feedback winding 2 b-side terminal of the feedback capacitor 12, and the charging voltage waveform (6) of the feedback capacitor 12 of the conventional self-excited switching power supply circuit 100 that performs the self-oscillation operation.

(symbol description)

1, a direct current power supply; 1a high voltage side terminal; 1b a low-voltage side terminal; 2, a transformer; 2a primary winding; 2b feedback winding (first feedback winding); 2c a secondary output winding; 3 a field effect transistor for oscillation; 5 turning off the control transistor; 10 self-excited switching power supply circuit; 12 a feedback capacitor; 21 a starting resistor; 23 feedback resistance.

Detailed Description

Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. Fig. 1 is a circuit diagram showing a configuration of a self-excited switching power supply circuit 10 according to an embodiment of the present invention. The self-excited switching power supply circuit 10 of the present embodiment shares the same main circuits and circuit elements with the conventional self-excited switching power supply circuit 100 shown in fig. 4, and the same components are denoted by the same reference numerals, and the description thereof is omitted.

As shown in fig. 1, the transformer 2 includes a primary winding 2a, a first feedback winding 2b wound in the same direction as the primary winding 2a, and a second feedback winding 2d wound in the opposite direction to the primary winding 2a on the primary side, and a secondary output winding 2c on the secondary side.

The primary winding 2a is connected in series with an oscillating field effect transistor (hereinafter referred to as FET)3 to the dc power supply 1, and on/off of a current flowing through the primary winding 2a is controlled by on/off operation of the FET 3.

The FET3 is a MOSFET in this example, and has a drain connected to the primary winding 2a and a source connected to the low-voltage-side terminal 1b of the dc power supply 1 via the primary current detection resistor 51.

The gate is connected to a connection point J1 between the starting resistor 21 and the resistor 25 connected in series to the dc power supply 1 via a resistor 24 that prevents an excessive input to the gate. The resistance values of the starting resistor 21 and the resistor 25 are 1.5M Ω and 100k Ω, respectively, as in the circuit shown in fig. 4, and therefore, when the power supply voltage of the dc power supply 1 unstable at about 200V is significantly reduced, the gate voltage of the FET3 does not reach the threshold voltage V_THAnd does not oscillate.

Between the connection point J1 between the starting resistor 21 and the resistor 25 and the first feedback winding 2b, the feedback capacitor 12 and the feedback resistor 23 constituting the conduction control circuit are connected in series, and the other side of the first feedback winding 2b is connected to the low-voltage-side terminal 1b of the dc power supply 1.

In this example, the capacitance of the feedback capacitor 12 and the resistance of the feedback resistor 23 are set to 1000pF and 4.7k Ω, and the time constant is 4.7 times as long as that of the conventional self-excited switching power supply circuit 100 shown in fig. 4.

Between a connection point J1 between the starting resistor 21 and the resistor 25 and the low voltage side terminal 1b, an off control transistor 5 that reduces the gate voltage of the FET3 and performs off control is disposed. In this example, the off control transistor 5 is an NPN transistor having a collector connected to the connection point J1 and an emitter connected to the low voltage side terminal 1 b.

One side of the second feedback winding 2d is connected to the low-voltage-side terminal 1b of the dc power supply 1 via a rectifying diode 54 and a driving capacitor 55 connected in series, and the other side is directly connected to the low-voltage-side terminal 1b of the dc power supply 1, thereby forming a closed loop.

The rectifying diode 54 is arranged with the charging direction of the driving capacitor 55 being the positive direction, and with this configuration, the driving capacitor 55 is charged with the flyback voltage occurring in the second feedback winding 2 d.

A connection point J2 between the rectifier diode 54 and the driving capacitor 55 is connected to the low-voltage-side terminal 1b via the photo-coupling light-receiving element 39 and the off-control capacitor 53, and a series connection point J3 between the photo-coupling light-receiving element 39 and the off-control capacitor 53 is connected to the base of the off-control transistor 5.

The series connection point J3, i.e., the base of the off control transistor 5 is also connected to the connection point J4 between the FET3 and the primary current detection resistor 51 via the resistor 52, and when the voltage drop caused by the primary current detection resistor 51 becomes equal to or greater than a certain value, the base voltage rises, and the off control transistor 5 performs an on operation.

The photo-coupling light receiving element 39 performs a photo-coupling operation with the photo-coupling light emitting element 35 on the secondary side of the transformer 2, and when receiving light from the photo-coupling light emitting element 35, a current proportional to the amount of received light flows from the connection points J2 to J3.

A rectifying diode 4 is connected in series to the secondary output winding 2c of the transformer, and a smoothing capacitor 13 is connected in parallel to constitute an output-side rectifying/smoothing circuit.

The self-excited switching power supply circuit 10 includes: and a circuit for monitoring the voltage between the output lines 20a and 20b and stabilizing the output voltage between the output lines 20a and 20 b.

That is, voltage dividing resistors 30 and 31 are connected in series between a high-voltage side output line 20a and a low-voltage side output line 20b of the rectifying and smoothing circuit, a center tap 32 thereof is connected to an inverting input terminal of an error amplifier 33, and an output detection voltage which is a divided voltage of an output voltage is input to the inverting input terminal. A reference power supply 34 is connected between the non-inverting input terminal of the error amplifier 33 and the low-voltage side output line 20b, and a reference voltage for comparison with the output detection voltage is input to the non-inverting input terminal.

The output side of the error amplifier 33 is connected to the photo-coupling light emitting element 35, the photo-coupling light emitting element 35 is connected to the high-voltage side output line 20a via the resistor 36, and the photo-coupling light emitting element 35 is photo-coupled to the photo-coupling light receiving element 39 on the primary side as described above in response to the turning-off of the output value of the error amplifier 33.

Ac negative feedback elements 37 and 38 are connected between the intermediate tap 32 and the non-inverting input terminal of the error amplifier 33 and between the output of the error amplifier 33.

The operation of the self-excited switching power supply circuit 10 configured as described above will be described below with reference to fig. 1 to 3. Fig. 2 and 3 show waveforms of respective parts in the self-oscillation operation, corresponding to fig. 5 and 6, respectively, and fig. 2(a) shows a voltage at the series connection point J3, that is, a base voltage waveform (1) of the off control transistor 5; fig. 2(b) shows a gate voltage waveform (2) of the FET 3; fig. 2(c) shows a primary winding current waveform (3) flowing through the primary winding 2a, which is a leakage current of the FET 3; fig. 2(d) shows the drain voltage waveform (4) of the FET 3.

The voltage waveforms shown in (2), (5), and (6) of fig. 3 are the gate voltage waveform (2) of the FET3, the voltage waveform (5) of the first feedback winding 2 b-side terminal of the feedback capacitor 12, and the charging voltage waveform (6) of the feedback capacitor 12 with the voltage of the first feedback winding 2 b-side terminal as a reference, respectively.

Initially, when a dc voltage of about 200V is generated between the high-voltage side terminal 1a and the low-voltage side terminal 1b of the dc power supply 1, the feedback capacitor 12 is charged via the starting resistor 21 and the feedback resistor 23 by dividing the power supply voltage into 1/16 voltage by the starting resistor 21 and the resistor 25 (the lower electrode is + and the upper electrode is-).

The charging voltage of the feedback capacitor 12 to be charged gradually rises and reaches the threshold voltage V of the FET3_THThen the gate of FET3 is forward biased so that FET3 turns on and the drain-source is turned on.

When the FET3 turns on and the excitation current from the dc power supply 1 starts to flow through the primary winding 2a connected in series, induced electromotive force is generated in each winding of the transformer 2, and energy is accumulated in the transformer 2. Induced voltage (t of fig. 3) occurring in the feedback winding 2b₂To t₀The voltage (5) between the gate and the gate of the FET3 is overlapped with the charging voltage (fig. 3 (6)) of the feedback capacitor 12, and the gate voltage (fig. 2(b) and fig. 3 (2)) is maintained at the threshold voltage V_THThe above voltage (on voltage).

At this time, the off control capacitor 53 is charged via the resistor 52 by a voltage generated at the FET3 side of the primary current detection resistor 51, that is, at the connection point J4, based on the current flowing through the primary winding 2 a. The current flowing through the primary winding 2a increases substantially linearly with the time after the on state, and the charging voltage of the off control capacitor 53 also increases.

When the bias voltage of the off control transistor 5 is reached, the collector-emitter gap is in an on state, and the gate of the FET3 is substantially in a short circuit state (here, the potential of the low-voltage side terminal 1b is, for example, 0V) by turning off the control transistor 5, and the FET3 is turned off.

When the FET3 is turned off and the current flowing through the transformer 2 is substantially interrupted, a flyback voltage (induced back electromotive force) is generated in each winding (t in fig. 2(d))₀To t₁). At this time, the flyback voltage generated in the secondary output winding 2c is rectified and smoothed by the smoothing rectifier circuit formed by the rectifying diode 4 and the capacitor 13, and is output as a voltage supplied to a load connected between the output lines 20a and 20 b.

On the other hand, the flyback voltage generated in the first feedback winding 2b is proportional to the flyback voltage generated in the secondary winding 2c by the load connected to the output side, and passes through the flyback voltage generated in the first feedback winding 2b (t of fig. 3)₀To t₁In (5)), the feedback capacitor 12 is charged (t of fig. 3)₀To t₁And (6) the lower electrode is + and the upper electrode is-in fig. 1-directing the next FET3 to turn on.

In a state where the output voltage between the high-voltage side output line 20a and the low-voltage side output line 20b, which rectifies and smoothes the flyback voltage generated in the secondary winding 2c, does not reach a set voltage determined by the reference power supply of the reference power supply 34 (hereinafter, referred to as a transient state), the photo-coupling photo-receiving element 39 does not operate as described later, and therefore, the base voltage of the off control transistor 5 becomes equal to or lower than the bias voltage. However, the base and collector of the turn-off control transistor 5 function as an equivalent diode, and the feedback capacitor 12 is charged from the first feedback winding 2b through the primary current detection resistor 51 to the resistor 52, the base to collector of the turn-off control transistor 5, and the feedback resistor 23 as a path of a charging current.

As shown in fig. 2(d), when the electric energy accumulated in the secondary output winding 2c by the induced back electromotive force is discharged at t₁When this is completed, the voltage waveform (4) on the FET3 side of the primary winding 2a starts free oscillation around the power supply voltage 200V due to the parasitic capacitance of the FET3, the stray capacitance between the primary windings 2a, and the inductance of the primary winding 2a, as indicated by the broken line connected to the solid line in the figure, and the polarity is inverted with the voltage drop.

As shown in (5) of FIG. 3, forThe voltage on the feedback capacitor 12 side of the first feedback winding 2b, which oscillates in proportion to the free oscillation of the primary winding voltage, also disappears at t when the flyback voltage acting in reverse bias on the gate electrode disappears₁Thereafter, the polarity is increased and reversed, acting as a forward bias to the gate of FET 3. Then, the charging voltage of the feedback capacitor 12 which is always charged is applied ((6) of fig. 3), and the gate voltage of the FET3 exceeds the threshold voltage V_THThe FET3 is turned on again, thereby repeating a series of self-oscillation actions.

The energy accumulated in the transformer 2 in one oscillation cycle is approximately proportional to the square of the on time of the FET3, i.e., the time until the base voltage of the off control transistor 5 reaches the bias voltage after being turned on, and in a transient state in which the secondary-side output voltage does not reach the set voltage, the photocouplers 35 and 39 are not operated, and therefore, the energy does not participate in the increase of the base voltage, and is operated for the maximum on time determined by the resistance value of the primary current detection resistor 51. As a result, the output voltage rises every repetition of oscillation until reaching the set voltage, and if exceeding the set voltage, the output voltage shifts to the following normal oscillation operation controlled by a circuit for stabilizing the output.

When the output voltage between the high-voltage side output line 20a and the low-voltage side output line 20b exceeds the set voltage, the divided voltage of the center tap 32 input to the inverting input terminal of the error amplifier 33 also rises, and the potential difference with the reference voltage of the reference power supply 34 is amplified in reverse phase and becomes a potential exceeding the light emission threshold of the photo-coupled light emitting element 35.

As a result, the photo-coupling light emitting element 35 emits light, and the photo-coupling light receiving element 39 receives light, and a current proportional to the amount of received light flows from the connection point J2 to the connection point J3 (the base of the off control transistor 5).

While the FET3 is on, the induced electromotive force generated in the second feedback winding 2d acts in reverse direction on the rectifier diode 54 and is not transmitted to the base of the off control transistor 5, but the flyback voltage generated in the second feedback winding 2d during the off operation of the FET3 before charges the driving capacitor 55, and a discharge current flows from the driving capacitor 55 to the connection point J3 to charge the off control capacitor 53, and a voltage generated in the primary current detection resistor 51 due to the flow of the primary winding current is applied to the base of the off control transistor 5 via the resistor 52, thereby accelerating the increase of the base voltage.

Accordingly, the off control transistor 5 is turned on rapidly and the FET3 is turned off, so that the on time is shortened and the output voltage is reduced. On the other hand, when the output voltage drops below the set voltage, the photo-coupling light emitting element 35 does not emit light, and therefore, the current from the photo-coupling light receiving element 39 is interrupted, and the off control capacitor 53 is charged only by the voltage drop of the primary current detection resistor 51. As a result, the on of the off control transistor 5 is delayed, the on time (on duty) of the duty cycle of the FET3 increases, and the output voltage rises, and the constant voltage control of the output voltage is performed through the above process.

As shown in fig. 2(a), in the normal oscillation operation, the off control transistor 5 is turned off at t at which the FET3 is turned off₀At this time, the base voltage reaches the bias of 0.6V, and both the collector and the emitter are turned on to be substantially at the ground potential, but the base voltage is also maintained at a voltage equal to or higher than the bias voltage during the period in which the output voltage on the secondary side exceeds the set voltage during the off operation of the FET 3.

That is, even during the off operation period of the FET3, the photo coupling light emitting element 35 consumes the energy stored in the transformer 2 by the load connected between the output lines 20a and 20b, and is actually turned on until the output voltage becomes lower than the set voltage. Therefore, the flyback voltage generated in the second feedback winding 2d is charged into the off control capacitor 53 by turning on the photo-coupling photo-receiving element 39, and the charged voltage raises the base voltage to the bias voltage or higher.

Even in the off operation period of the FET3, since the collector and the emitter are turned on while the base voltage of the off control transistor 5 is biased, the feedback capacitor 12 is charged by the flyback voltage generated in the first feedback winding 2b through a path of a charging current from the emitter to the collector of the off control transistor 5 and the feedback resistor 23 (the lower electrode is + the upper electrode is-in fig. 1).

In the self-excited switching power supply circuit 10 of the present embodiment, in the normal oscillation operation, the time constant of the feedback capacitor 12 and the feedback resistor 23 constituting the conduction control circuit is 4.7 times as long as that of the conventional self-excited switching power supply circuit 100 as described above, and therefore, the flyback voltage (t in fig. 3) generated in the first feedback winding 2b passes through₀To t₁Middle (5)), the feedback capacitor 12 is slowly charged (t of fig. 3)₀To t₁(6) above).

That is, the time constants of the feedback capacitor 12 and the feedback resistor 23 are determined so that the energy accumulated in the transformer is at t₁When the time is released from the secondary output winding 2c, the voltage on the feedback capacitor 12 side of the first feedback winding 2b ((5) of fig. 3) starts to oscillate freely and reverses its polarity to reach the initial maximum value t₂At this time, the gate voltage ((2) of fig. 3) of the FET3 to which the charging voltage ((6) of fig. 3) of the feedback capacitor 12 is applied exceeds the threshold voltage V_TH. In fig. 3, the gate voltage of the FET3 does not become the sum of the voltage on the first feedback winding 2b side and the charging voltage of the feedback capacitor 12, and is due to the voltage drop of the feedback resistor 23.

The first feedback winding 2b and the primary winding 2a freely oscillate with an amplitude proportional to the turn ratio thereof, and the amplitude gradually attenuates, and therefore, t, which reaches the initial maximum value on the + side of the first feedback winding 2b₂At this time, the voltage on the FET3 side (drain of FET 3) of the primary winding 2a becomes the minimum value. That is, when the gate voltage of the FET3 (fig. 2(b)) exceeds the threshold voltage V_THT is turned on₂At this time, the drain voltage of the FET3 (fig. 2(d)) is the minimum voltage around 120V, and when turned on, it becomes 0V around 120V, and the field current starts to flow through the primary winding 2 a.

Therefore, the charges accumulated in the primary winding 2a or the stray capacitance between the windings of the FET3 or the parasitic capacitance between the drain and the source due to the flyback voltage start to be discharged at the time when the polarity of the primary winding 2a is inverted due to the free oscillation, and thereafter, the charges are discharged at one timeThe voltage on the low-voltage side of the secondary winding drops to the lowest t₂At time point, FET3 turns on and short-circuits with low-voltage-side terminal 1b of dc power supply 1, and therefore, a slow discharge current flows.

As a result, as shown in B of fig. 2(c), only a small amount of discharge current appears in the primary winding current immediately after the on state, and the loss in the switching element such as the FET3 is small, and noise is not generated.

In the transient state, the feedback capacitor 12 is charged according to the time constant determined by adding the resistance values of the primary current detection resistor 51, the resistor 52, and the feedback resistor 23 and the capacitance value of the feedback capacitor 12, and therefore, the charging is performed later than the charging voltage waveform (6) of the feedback capacitor 12 shown in fig. 3, and the charging reaches t, which is the first maximum value, on the + side of the first feedback winding 2b₂At that time, the gate voltage is not necessarily charged to the threshold voltage V_THAnd, even in the normal oscillation operation, the flyback voltage varies depending on the magnitude of the load connected to the secondary side, t₂The charging voltage at that time varies, so to reliably turn on, the time constant of the turn-on control circuit can be set as follows: the gate voltage is made to exceed the threshold voltage V before the polarity of the first feedback winding 2b reaches the initial maximum value after the polarity inversion_TH。

According to the present embodiment, a large discharge current does not occur at the time of turning on, and the charging of the feedback capacitor 12 for turning on can be performed via the off-control transistor 5 without using a path for charging like the zener diode 6 provided in the conventional circuit.

Effects of the invention

As described above, according to the present invention, when the field effect transistor for oscillation is turned on, the discharge current generated is reduced, the energy loss at the time of switching is reduced, and noise is not generated.

In addition, another aspect of the present invention is that the oscillating field effect transistor is turned on at a timing at which the primary winding voltage that freely oscillates about the power supply voltage becomes minimum, and therefore, the discharge current generated at the time of turning on can be reduced most effectively.

Claims

1. A self-excited switching power supply circuit, characterized by:

therein is provided with

A transformer (2) having a primary winding (2a), a secondary output winding (2c) and a feedback winding (2 b);

an oscillating field effect transistor (3) connected in series with the primary winding (2a) via a primary current detection resistor (51) to a DC power supply (1) when the gate voltage reaches a threshold voltage V_THIs conducted;

a starting resistor (21) connected between a high-voltage-side terminal (1a) of the DC power supply (1) and the gate of the oscillating field effect transistor (3);

an on-state control circuit including a feedback capacitor (12) and a feedback resistor (23) connected in series between the other end of a feedback winding (2b) connected at one end thereof to a low-voltage-side terminal (1b) of a direct-current power supply (1) and the gate of an oscillation field-effect transistor (3); and

an off control transistor (5) having a collector and an emitter connected between the gate of the oscillation field effect transistor (3) and the low-voltage-side terminal (1b) of the direct current power supply (1), a base connected to the low-voltage-side terminal (1b) via a resistor (52) and a primary current detection resistor (51), and turning off the oscillation field effect transistor (3) by turning on the gate and the low-voltage-side terminal (1b) after a predetermined time has elapsed since the oscillation field effect transistor (3) was turned on;

after the oscillating field effect transistor (3) is turned off, the gate voltage is raised to a threshold voltage V by the charging voltage of a feedback capacitor (12) charged by a flyback voltage generated in a feedback winding (2b)_THIn a self-excited switching power supply circuit in which the field effect transistor (3) for oscillation is again on-controlled,

charging a feedback capacitor (12) with a flyback voltage generated in a feedback winding (2b) via an off control transistor (5) which is connected on the gate side of the oscillating field effect transistor (3) and which is turned on between the collector and the emitter during the off period of the oscillating field effect transistor (3) and which functions as an equivalent diode between the collector or the emitter and the base,

the time constant of the conduction control circuit is set so that the energy stored in the transformer (2) is discharged from the secondary output winding (2c) and the gate voltage exceeds the threshold voltage V at least in the state where the polarity of the voltage in the feedback winding (2b) is reversed_TH。

2. A self-excited switching power supply circuit according to claim 1, wherein:

the time constant of the conduction control circuit is set so that when the energy stored in the transformer (2) is discharged from the secondary output winding (2c) and the voltage of the feedback winding (2b) reaches the initial maximum value, the gate voltage exceeds the threshold voltage V_TH。