CN103929048B - A kind of zero cross detection circuit of Switching Power Supply - Google Patents

Abstract

The invention provides a zero-crossing detection circuit of a switching power supply, which at least includes a voltage input module; a voltage output module connected to the voltage input module; an inductor for controlling the output voltage through charging and discharging; a switch tube for controlling charging and discharging of the inductor; The resonant signal between the drain of the switch tube is coupled to the coupling circuit of the switch tube grid; it is used to take out the current proportional to the negative voltage of the output signal of the coupling circuit and convert it into a resonant sampling module for voltage output; according to the The voltage output by the resonant sampling module generates a comparison control module that generates a control signal, and a control and drive module that generates the switch gate voltage. The zero-crossing detection circuit of the switching power supply of the present invention can greatly reduce the chip area of the pulse width modulation power supply, and enhance product competitiveness.

Description

A zero-crossing detection circuit of switching power supply

technical field

The invention relates to the field of microelectronics, in particular to a zero-crossing detection circuit of a switching power supply.

Background technique

A switching power supply uses a power semiconductor device as a switch to maintain a stable output voltage or current by controlling the on and off of the switch. Compared with linear power supply, switching power supply is smaller, more efficient, more energy-saving and environmentally friendly than linear power supply. In some fields, it has completely replaced the linear power supply. Especially in the current trend of decreasing size of electrical equipment, decreasing voltage, and widespread use of mobile devices, the application of switching power supplies is becoming more and more extensive. At present, switching power supplies are developing in the direction of high frequency and miniaturization.

Due to the continuous development and innovation of electronic technology, new forms of switching power supply chip control technology are also emerging. Regardless of the control method, the internal circuit of the chip needs to sample the output voltage or current every moment, and adjust itself by sampling the change of the output. According to the size of the output power, the structure of the system and the feedback control technology used are worked out.

During the working process of the switching power supply, if the current in the inductor is not completely released during the switching process, it belongs to the continuous current mode (CCM); if the current in the inductor is completely released and recharged after a period of time, it belongs to the discontinuous mode; If the current in the inductor is fully discharged and then charged immediately, it belongs to Boundary Mode (BCM). Whether it is discontinuous mode or critical mode, the current in the inductor must be completely released. According to the characteristic that the current in the inductor is zero, different sampling techniques and control circuits are born.

As shown in FIG. 1 , a source-driven zero-crossing detection circuit 1 commonly used at present is shown. The circuit includes a voltage input module 11, a voltage output module 12, an inductor Lm, a diode D, a coupling circuit 13, a switch tube M1, a switch tube M2, a resonant sampling module 14, a comparator 15, a trigger 16 and a sampling resistor Rcs, wherein the voltage The input module 11, the voltage output module 12, the inductor Lm, the diode D, the coupling circuit 13, the switch tube M1 and the sampling resistor Rcs are off-chip devices, and the switch tube M2, the resonant sampling module 14, the comparator 15 and the trigger 16 are on-chip components. device. The coupling circuit 13 couples the resonance signal between the inductor Lm and the switching tube M1 to the drain terminal of the switching tube M2 on-chip, and then the resonance sampling module 14 samples this signal, and simultaneously the The comparator 15 compares the sampling signal at the drain end of the switching tube M2 with a reference voltage, the flip-flop 16 receives the output signals of the resonant sampling module 14 and the comparator 15, and outputs and controls the switching tube M2 On and off control signals. In this sampling method, the current in the inductor Lm must all flow through the switch tube M2 inside the chip. The switch tube M2 requires a large area, and the larger the current in the inductor Lm, the larger the area of the switch tube M2 inside the chip. The larger the size, the larger the corresponding chip area. Based on this, the manufacturing cost of the chip will be greatly increased, and the competitiveness of the product will be reduced accordingly.

How to increase the cost of the zero-crossing detection chip of the switching power supply and improve product competitiveness is an urgent problem to be solved by those skilled in the art.

Contents of the invention

In view of the above-mentioned shortcomings of the prior art, the purpose of the present invention is to provide a zero-crossing detection circuit of a switching power supply, which is used to solve the problems of high cost and low product competitiveness of the zero-crossing detection chip of a switching power supply in the prior art .

In order to achieve the above purpose and other related purposes, the present invention provides a zero-crossing detection circuit of a switching power supply. The zero-crossing detection circuit of a switching power supply at least includes:

Voltage input module, voltage output module, inductor, switch tube, coupling circuit, resonant sampling module, comparison control module and control drive module;

The voltage output module is connected to the voltage input module for outputting voltage;

The inductance is connected to the voltage output module, and the output voltage is adjusted through charging and discharging of the inductance to stabilize it at a set value;

The switching tube is connected to the inductor, and the charging and discharging of the inductor is controlled by turning on and off the switching tube. When the switching tube is turned on, the inductor is charged, and when the switching tube is turned off, the inductor is charged. When off, the inductor discharges;

The input terminal of the coupling circuit is connected between the inductor and the drain of the switch tube, and the output terminal of the coupling circuit is connected to the gate of the switch tube for connecting the inductor and the switch tube The resonant signal between the drains of the tubes is coupled to the gate of the switch tube;

The resonant sampling module is connected to the coupling circuit, and is used to take out a current proportional to the negative voltage of the output signal of the coupling circuit, and convert it into a voltage output;

The comparison control module is connected to the resonance sampling module, and generates a control signal according to the voltage output by the resonance sampling module;

The control drive module is connected to the comparison control module and the output terminal of the switch tube, and is used to generate a gate voltage for controlling the switch tube to be turned on and off.

Preferably, the zero-crossing detection circuit of the switching power supply is suitable for pulse width modulation power supplies in discontinuous mode or critical conduction mode.

Preferably, the voltage input module includes a power supply and a filter capacitor connected to the power supply.

Preferably, the voltage output module includes an output capacitor and a load connected in parallel to the output capacitor.

Preferably, the switch tube is an NMOSFET.

Preferably, the coupling circuit is a capacitor or a circuit composed of a capacitor and a resistor connected in series.

Preferably, the comparison control module includes a first comparator, a first flip-flop and a buffer; the input terminals of the first comparator are respectively connected to the resonant sampling module and the first reference voltage, and the resonant sampling module The output signal is compared with the first reference voltage; the first flip-flop is connected to the first comparator, and when the first comparator outputs a high-level signal, the first flip-flop outputs a high voltage flat pulse; the buffer is connected to the first flip-flop for buffering the output signal.

More preferably, the first flip-flop is a D flip-flop.

Preferably, the control drive circuit includes a second comparator and a second flip-flop, the second comparator compares the voltage signal corresponding to the output current of the switching tube with a second reference voltage, and compares the comparison result output to the second flip-flop, and the second flip-flop is used to output the gate voltage for driving the switching tube.

More preferably, the second flip-flop may be a D flip-flop or an RS flip-flop.

Preferably, a diode is further included, the anode of the diode is connected between the inductor and the drain of the switch tube, the cathode of the diode is connected to the voltage input module, when the switch tube is turned on, The diode is turned off; when the switch tube is turned off, the diode is turned on to provide a path for the inductor and the voltage output module.

Preferably, a sampling resistor is further included, one end of the sampling resistor is connected to the output end of the switching tube, and the other end is grounded, for collecting the voltage corresponding to the output current of the switching tube.

As mentioned above, the zero-crossing detection circuit of the switching power supply of the present invention has the following beneficial effects:

The zero-crossing detection circuit of the switching power supply of the present invention optimizes the circuit structure, combines the switch tube inside the chip and the switch tube outside the chip into a switch, and places it outside the chip, which can greatly reduce the area of the chip and improve the product quality. competitiveness.

Description of drawings

FIG. 1 is a schematic diagram of a zero-crossing detection circuit of a switching power supply in the prior art.

FIG. 2 is a schematic diagram of the zero-crossing detection circuit of the switching power supply of the present invention.

FIG. 3 is a schematic circuit diagram of a resonant sampling module of the zero-crossing detection circuit of the switching power supply of the present invention.

FIG. 4( a ) is a schematic diagram showing the waveforms of the charging and discharging of the inductor in the discontinuous mode of the zero-crossing detection circuit of the switching power supply of the present invention.

FIG. 4( b ) is a schematic diagram of the waveform of the Lx node in the discontinuous mode of the zero-crossing detection circuit of the switching power supply of the present invention.

FIG. 4( c ) is a schematic diagram of the waveform of the node Gs in the discontinuous mode of the zero-crossing detection circuit of the switching power supply of the present invention.

FIG. 5( a ) is a schematic diagram of the waveforms of the charging and discharging of the inductor in the critical mode of the zero-crossing detection circuit of the switching power supply of the present invention.

FIG. 5( b ) is a schematic diagram of the waveform of the Lx node in the critical mode of the zero-crossing detection circuit of the switching power supply of the present invention. FIG. 5( c ) is a schematic diagram of the waveform of the node Gs in the critical mode of the zero-crossing detection circuit of the switching power supply of the present invention. Component designation description

1 Zero-crossing detection circuit of switching power supply

11 voltage input module

12 voltage output modules

13 coupling circuit

14 Resonant Sampling Modules

15 comparators

16RS flip-flop

2 Zero-crossing detection circuit of switching power supply

21 voltage input module

22 voltage output modules

221 load

23 coupling circuit

24 resonant sampling modules

25 comparison control circuit

251 first comparator

252 first trigger

253 buffer

26 control drive module

261 second comparator

262 second trigger

CT filter capacitor

Cout output capacitance

Lm inductance

M switch tube

M1 switch tube

M2 switch tube

D diode

Rcs sampling resistance

detailed description

Embodiments of the present invention are described below through specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific implementation modes, and various modifications or changes can be made to the details in this specification based on different viewpoints and applications without departing from the spirit of the present invention.

Please refer to Figure 2 to Figure 5(c). It should be noted that the diagrams provided in this embodiment are only schematically illustrating the basic idea of the present invention, and only the components related to the present invention are shown in the diagrams rather than the number, shape and shape of the components in actual implementation. Dimensional drawing, the type, quantity and proportion of each component can be changed arbitrarily during actual implementation, and the component layout type may also be more complicated.

As shown in Figure 2, the present invention provides a zero-crossing detection circuit 2 of a switching power supply, the zero-crossing detection circuit 2 of the switching power supply at least includes:

Voltage input module 21, voltage output module 22, inductor Lm, switch tube M, coupling circuit 23, resonant sampling module 24, comparison control module 25, control driving module 26, diode D and sampling resistor Rcs.

The voltage input module 21 is used to provide an input voltage Vin.

As shown in FIG. 2 , the voltage input module 21 includes a power supply Vin and a filter capacitor CT, one end of the filter capacitor CT is connected to the output end of the voltage input module 21, and the other end is grounded, and is used for controlling the voltage input module. The voltage Vin output by 21 is filtered.

The voltage output module 22 is connected to the voltage input module 21 for outputting a stable voltage.

As shown in FIG. 2 , the voltage output module 22 includes an output capacitor Cout and a load 221 connected in parallel to the output capacitor Cout. The output capacitor Cout is used to store charge and supply power to the load 221 .

The inductance Lm is connected to the voltage output module 22, and the output voltage is adjusted through charging and discharging of the inductance Lm to stabilize it at a set value.

As shown in FIG. 2 , one end of the inductor Lm is connected to the output capacitor Cout and the load 221 , and the other end is connected to the drain end of the switch M.

The switching tube M is connected to the inductor Lm, and the charging and discharging of the inductor Lm is controlled by turning on and off the switching tube M. When the switching tube M is turned on, the input voltage Vin is given to the The inductor Lm is charged, and when the switching tube M is turned off, the inductor Lm is discharged to provide power to the voltage output module 22 . In this embodiment, the switching tube M is preferably an NMOSFET.

The anode of the diode D is connected between the inductor Lm and the switch tube M, the cathode of the diode D is connected to the voltage input module 21, when the switch tube M is turned off, the diode D A path is provided for the inductor Lm and the voltage output module 22 .

As shown in Figure 2, the drain of the switch M is connected to the inductor Lm, when the switch M is turned on, the input voltage Vin charges the inductor Lm; when the switch M is turned off When off, the charging path is disconnected, and the inductance Lm provides power to the loop formed by the diode D and the output module 22 .

One end of the sampling resistor Rcs is connected to the source of the switching tube M, and the other end is grounded for collecting the voltage corresponding to the output current of the switching tube M.

The input end of the coupling circuit 23 is connected between the inductor Lm and the drain of the switch M, and the output end of the coupling circuit 23 is connected to the gate of the switch M. The coupling circuit 23 is a capacitor or a circuit composed of a capacitor and a resistor connected in series. In this embodiment, the coupling circuit 23 is a capacitor.

As shown in FIG. 2, the input end of the coupling circuit 23 is connected between the inductor Lm and the drain of the switch M, and the output end of the coupling circuit 23 is connected to the gate of the switch M. , when the inductance Lm crosses zero, the oscillation starts, and the capacitor 23 blocks DC and AC, and couples the resonant signal to the gate of the switching tube M.

The resonant sampling module 24 is connected to the coupling circuit 23, and is used for taking out a current proportional to the negative voltage of the output signal of the coupling circuit 24, and converting the current into a voltage output.

As shown in Figure 3, it is an embodiment of the resonant sampling module 24, one end of the resistance Rs is connected to the output terminal Gs of the coupling circuit 23, and the MOS transistor Mn1 and the MOS transistor Mn2 form a current mirror structure, because the source of the MOS transistor Mn2 Pole b is grounded, and the source a point potential of MOS transistor Mn1 is also 0. When the voltage V _GS at point Gs is less than zero, a current flows through the path formed by the resistor Rs, the MOS transistor Mn1 , the MOS transistor Mp1 and the MOS transistor Mp2 , and the current I _RS =|V _GS |/Rs. After the mirror image between Mp1, Mp2 and Mp5, Mp6, a sampling current is generated: Isense=K×I _RS , where K is the proportional factor of the mirror image, and the current is proportional to the negative voltage of the voltage V _GS at point Gs. It is converted into a voltage output through the resistor Rsense, and the voltage Vsense=Isense×Rsense. The greater the resonance zero-crossing, that is, the smaller the voltage V _GS at the Gs point is, the greater the I _RS is, and the greater the corresponding Isense and Vsense are. If the voltage V _GS at the Gs point is greater than or equal to 0, Vsense is also equal to zero.

The comparison control module 25 is connected to the resonance sampling module 24 and generates a pulse signal Dpulse according to the voltage output by the resonance sampling module 24 .

As shown in Figure 2, in this embodiment, the comparison control module 25 includes a first comparator 251; a first flip-flop 252, in this embodiment, the first flip-flop is preferably a D flip-flop; and Buffer 253 . The input end of the first comparator 251 is connected to the resonant sampling module 24 and the first reference voltage Vref1 respectively, as shown in FIG. 2 , the positive input end of the first comparator 251 is connected to the resonant sampling module 24, the inverting input terminal of the first comparator 251 is connected to the first reference voltage Vref1, and the first comparator 251 compares the output signal of the resonant sampling module 24 with the first reference voltage Vref1 In comparison, when the signal output by the resonant sampling module 24 is greater than the first reference voltage Vref1, a high-level signal is output, otherwise, a low-level signal is output. The first flip-flop 252 is connected to the first comparator 251, and when the first comparator 251 outputs a high-level signal, the first flip-flop 252 generates a high-level pulse signal. The buffer 253 is connected to the output end of the first flip-flop 252 to act as a buffer to ensure synchronous transmission of data or clock, and the output signal of the buffer 253 is a pulse signal Dpulse.

The control drive module 26 is connected to the comparison control module 25 and the output end of the switch M, and is used to generate a gate voltage V _GS for controlling the switch M to turn on and off.

As shown in FIG. 2, in this embodiment, the control driving circuit 26 includes a second comparator 261 and a second flip-flop 262, and the second flip-flop 262 can be a D flip-flop or an RS flip-flop. In this embodiment In an embodiment, the second flip-flop 262 is an RS flip-flop. The second comparator 261 compares the voltage signal Vcs corresponding to the output current of the switching tube M with the second reference voltage Vref2, and outputs the comparison result to the RS flip-flop, and the RS flip-flop is used to output Drive the gate voltage V _GS of the switching tube M. When the signal Dpulse output by the buffer 253 is at a high level and the signal Vcs output by the second comparator 261 is at a low level, the gate voltage V _GS is High level, otherwise it is low level.

The working principle of the zero-crossing detection circuit 2 of the switching power supply is as follows:

When the switch tube M is turned on, the voltage input module 21, the voltage output module 22, the inductor Lm, the switch tube M and the sampling resistor Rcs form a loop, and the input point voltage Vin is given by The inductance Lm is charged; when the switching tube M is turned off, the loop is disconnected, and the output module 22, the inductance Lm and the diode D form a new loop. At this time, the inductance Lm is discharged, which is The output module 22 provides energy.

The zero-crossing detection circuit 2 of the switching power supply is suitable for pulse width modulation power supplies in discontinuous mode or critical conduction mode.

In discontinuous mode, when the discharge current in the inductor Lm slowly drops to 0, the current in the inductor Lm begins to resonate, as shown in Figure 4(a), the current in the inductor Lm is at Resonance occurs at 0 o'clock. The voltage of the node Lx between the inductor Lm and the drain of the switch M also resonates accordingly, as shown in FIG. 4( b ). The resonance signal is coupled to the gate voltage V _GS of the switching tube M through the coupling circuit 23, as shown in FIG. 4(c), since the switching tube M is in an off state when the inductance Lm is discharged, In this embodiment, the switching tube is an NMOSFET, so the part of the gate voltage V _GS at a low level is coupled with a resonant signal, which in this embodiment represents a voltage of 0V.

Resonance of the gate voltage V _GS at the 0V voltage position will produce a voltage lower than 0V. When the voltage lower than 0V is collected by the resonance sampling module 24, a current proportional to the negative voltage will be generated, and the voltage will be proportional to the voltage form output, and generate a high-level signal through the comparison control module 25; at the same time, the first comparator 261 compares the sampling voltage Vcs with the reference voltage Vref2, and since the inductor Lm is in a discharge state, the switching tube M turn off, the voltage Vcs corresponding to the output current of the switching tube M is less than the reference voltage Vref2, and the second comparator 261 outputs a low-level signal; the high-level signal generated by the comparison control module 25 is connected to The R terminal of the RS flip-flop 262, the low-level signal output by the second comparator 261 is connected to the S terminal of the RS flip-flop 262, and the RS flip-flop 262 generates the gate voltage of the switching tube M V _GS , the gate voltage V _GS is a high-level pulse, acting on the gate of the switching tube M, the switching tube M is turned on, and the inductor Lm enters a charging mode.

The inductor Lm starts to charge, and the current value gradually increases, as shown in FIG. 4( a ). The voltage of the node Lx between the inductor Lm and the switch tube M is 0V, as shown in FIG. 4( b ). The gate voltage V _GS of the switching tube M is a high-level signal, and when the gate voltage V _GS is greater than or equal to 0V, the comparison control module 25 generates a low-level signal. The output current of the switching tube M gradually increases, and when the voltage Vcs corresponding to the output current of the switching tube M is greater than the reference voltage Vref1, the RS flip-flop 262 jumps to a low level, and the switching tube M turn off, the inductor Lm re-enters the discharge mode.

By controlling the gate voltage V _GS of the grid of the switching tube M to control the switching on and off of the switching tube M, further controlling the charging and discharging of the inductor Lm, and adjusting the output voltage of the voltage output module 22, make it stable at a certain set value.

The principle is the same in the critical conduction mode, so I won’t repeat them one by one here. The difference is that in this mode, after the current in the inductor Lm is completely released, it will be charged again immediately. There is only one zero-crossing point, and resonance will not occur for a long time. . Figure 5(a) is a schematic diagram of the charging and discharging waveforms of the inductor Lm in the critical mode. Figure 5(b) shows a schematic diagram of the waveform of the Lx node in critical mode. Figure 5(c) shows a schematic diagram of the waveform of the Gs node in critical mode.

In the zero-crossing detection circuit 2 of the switching power supply of the present invention, the voltage input module 21, the voltage output module 22, the inductor Lm, the diode D, the coupling circuit 23, the switching tube M and the sampling resistor Rcs are off-chip devices; Module 24, comparator 15 and flip-flop 16 are on-chip devices. The zero-crossing detection circuit of the switching power supply of the present invention combines the large-area MOS tubes on the chip with the MOS tubes off the chip on the basis of optimizing the circuit to ensure stable output voltage, greatly reducing the chip area, effectively reducing costs, and improving product competition force.

In summary, the zero-crossing detection circuit of the switching power supply of the present invention at least includes a voltage input module that provides power; a voltage output module that is connected to the voltage input module to output voltage; an inductance that controls the output voltage through charging and discharging; A switch tube for controlling the charge and discharge of the inductor; a coupling circuit for coupling the resonance signal to the grid of the switch tube; the resonant sampling module for collecting the gate voltage zero-crossing signal of the switch tube; a comparison control module for generating a pulse signal and generating the switch The control drive module of the gate voltage. When the inductor current is zero, resonance occurs between the switch tube and the inductor, and the resonance signal is coupled to the gate of the switch tube through the coupling circuit. The resonant signal is coupled to the gate of the switch tube as a zero-crossing oscillating signal. The resonant sampling circuit extracts a resonant current proportional to the negative voltage of the resonant signal, the resonant current is converted into a voltage and output to the comparison control circuit, and then the control drive module generates a control signal to control the conduction of the switch tube. The zero-crossing detection circuit of the switching power supply of the present invention can greatly reduce the chip area of the pulse width modulation power supply. Therefore, the present invention effectively overcomes various shortcomings in the prior art and has high industrial application value.

The above-mentioned embodiments only illustrate the principles and effects of the present invention, but are not intended to limit the present invention. Anyone skilled in the art can modify or change the above-mentioned embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or changes made by those skilled in the art without departing from the spirit and technical ideas disclosed in the present invention should still be covered by the claims of the present invention.

Claims (12)

1. a zero cross detection circuit for Switching Power Supply, is characterized in that, the zero cross detection circuit of described Switching Power Supply at least comprises:

Voltage input module, voltage output module, inductance, switching tube, coupling circuit, resonance sampling module, compare control module and control driver module;

Described voltage output module is connected to described voltage input module, for output voltage;

Described inductance is connected to described voltage output module, carrys out regulation output voltage by the charge and discharge of described inductance, makes it be stabilized in set point;

Described switching tube is connected to described inductance, controls described inductance charge and discharge by the turn-on and turn-off of described switching tube, when described switching tube conducting, and described induction charging, when described switching tube turns off, described inductive discharge;

The input of described coupling circuit is connected between the drain electrode of described inductance and described switching tube, the output of described coupling circuit is connected to the grid of described switching tube, for the resonance signal between described inductance and the drain electrode of described switching tube being coupled to the grid of described switching tube;

Described resonance sampling module is connected to described coupling circuit, for taking out the proportional electric current of the negative voltage that outputs signal with described coupling circuit, and is converted into voltage and exports;

The described control module that compares is connected to described resonance sampling module, produces control signal according to the voltage that described resonance sampling module exports;

Described control driver module is connected to the described output comparing control module and described switching tube, for generation of the gate voltage controlling described switching tube turn-on and turn-off.

2. the zero cross detection circuit of Switching Power Supply according to claim 1, is characterized in that: the zero cross detection circuit of described Switching Power Supply is applicable to the pulse width modulated power supply of discontinuous mode or critical conduction mode.

3. the zero cross detection circuit of Switching Power Supply according to claim 1, is characterized in that: described voltage input module comprises power supply and is connected to the filter capacitor of described power supply.

4. the zero cross detection circuit of Switching Power Supply according to claim 1, is characterized in that: described voltage output module comprises output capacitance and is parallel to the load of described output capacitance.

5. the zero cross detection circuit of Switching Power Supply according to claim 1, is characterized in that: described switching tube is NMOSFET.

6. the zero cross detection circuit of Switching Power Supply according to claim 1, is characterized in that: described coupling circuit is electric capacity or by the electric capacity circuit formed in series with a resistor.

7. the zero cross detection circuit of Switching Power Supply according to claim 1, is characterized in that: the described control module that compares comprises the first comparator, the first trigger and buffer; The input of described first comparator connects described resonance sampling module and the first reference voltage respectively, the output signal of described resonance sampling module and described first reference voltage is compared; Described first trigger is connected to described first comparator, and described in when described first comparator exports high level signal, the first trigger exports high level pulse; Described buffer is connected to described first trigger, for buffer output signal.

8. the zero cross detection circuit of Switching Power Supply according to claim 7, is characterized in that: described first trigger is d type flip flop.

9. the zero cross detection circuit of Switching Power Supply according to claim 1, it is characterized in that: described control drive circuit comprises the second comparator and the second trigger, the voltage signal corresponding with described switching tube output current and the second reference voltage are made comparisons by described second comparator, and comparative result being exported to described second trigger, described second trigger drives the gate voltage of described switching tube for exporting.

10. the zero cross detection circuit of Switching Power Supply according to claim 9, is characterized in that: described second trigger is d type flip flop or rest-set flip-flop.

The zero cross detection circuit of 11. Switching Power Supplies according to claim 1, it is characterized in that: also comprise diode, the anode of described diode is connected between the drain electrode of described inductance and described switching tube, the negative electrode of described diode is connected to described voltage input module, when described switching tube conducting, described diode cut-off; When described switching tube turns off, described diode current flow, provides path to described inductance and described voltage output module.

The zero cross detection circuit of 12. Switching Power Supplies according to claim 1, it is characterized in that: also comprise sampling resistor, one end of described sampling resistor is connected to the output of described switching tube, other end ground connection, for gathering the voltage corresponding with described switching tube output current.