Voltage conversion circuit, primary side control circuit and primary side switch control method

Abstract

The present invention discloses a voltage conversion circuit, a primary control circuit and a primary switch control method, wherein the primary control circuit includes a primary switch control circuit, and the primary switch control circuit is used to couple a primary switch; when the primary switch control circuit detects that the primary current of the optocoupler is less than a set first threshold, the primary switch control circuit sends a control signal to the primary switch that can control its conduction; when the primary switch control circuit detects that the peak value of the current flowing through the primary switch reaches a set second threshold, the primary switch control circuit sends a control signal to the primary switch that can control its shutdown. The voltage conversion circuit, primary control circuit and primary switch control method proposed in the present invention do not require a frequency oscillator, have a simple structure, and can reduce the power consumption of the circuit in the standby state.

Description

Technical Field

[0001] The present invention belongs to the field of electronic technology and relates to a voltage conversion circuit, and in particular to a primary side control circuit and a primary side switch control method. Background Art

[0002] Figure 1 The figure shows a typical secondary side feedback (SSR) control topology of a flyback voltage converter. The SSR control of the flyback voltage converter usually transmits the secondary side signal through an optocoupler, and the control circuit on the primary side controls the conduction state of the primary side switch tube Q through the signal FB detected by the optocoupler to control the secondary side output signal Vout.

[0003] In the prior art, the primary side control circuit of the voltage converter generally includes a frequency oscillator for generating high frequency pulses and turning on the primary side switch Q based on the pulses generated by the oscillator. Figure 1 When the load is reduced and the device enters the standby state, the output voltage Vout rises, the voltage at point A drops, the secondary current Is of the optocoupler rises, the primary current Ip of the optocoupler also rises, and the signal FB obtained from the primary optocoupler also rises accordingly; the primary control circuit intermittently shields the pulse according to the detected signal FB to reduce the output voltage Vout, such as Figure 2 shown.

[0004] In standby mode, the primary side intermittently outputs pulses to control the primary side switch tube Q. The control signal has both high-frequency and low-frequency components, resulting in uneven output and poor control balance.

[0005] In addition, in the prior art, the primary side optocoupler is usually connected to a resistor to generate a voltage signal FB. In the standby state, since the primary side current Ip rises higher, a large power loss will be caused on the resistor, so the standby loss is large and the system efficiency is not high. At the same time, in order to make the control stable, it is necessary to obtain the average value of the output signal Vout, so it is often necessary to add a filter circuit composed of a capacitor C1 and a resistor R1 in the secondary side feedback loop.

[0006] In view of this, there is an urgent need to design a new voltage conversion circuit to overcome at least part of the above-mentioned defects of the existing voltage conversion circuit. Summary of the invention

[0007] The present invention provides a voltage conversion circuit, a primary side control circuit and a primary side switch control method, which can reduce the power consumption of the circuit in a standby state.

[0008] To solve the above technical problem, according to one aspect of the present invention, the following technical solution is adopted:

[0009] A primary side control circuit, the primary side control circuit comprising a primary side switch control circuit, the primary side switch control circuit being used for coupling a primary side switch;

[0010] When the primary switch control circuit detects that the primary current of the optocoupler is less than a set first threshold, the primary switch control circuit sends a control signal to the primary switch that can control the primary switch to be turned on;

[0011] When the primary switch control circuit detects that the peak value of the current flowing through the primary switch reaches a set second threshold, the primary switch control circuit sends a control signal to the primary switch that can control the primary switch to be turned off.

[0012] As an implementation mode of the present invention, the primary switch control circuit includes:

[0013] A conduction control circuit, used to output a control signal according to the magnitude of the primary current of the optocoupler, and when the primary current of the optocoupler is less than a set first threshold, the conduction control circuit outputs a valid level;

[0014] a shutdown control circuit, configured to output a control signal according to the magnitude of the primary current flowing through the primary switch, wherein when the primary current flowing through the primary switch reaches a set second threshold, the shutdown control circuit outputs a valid level; and

[0015] A trigger circuit, wherein a first input terminal is coupled to the output terminal of the conduction control circuit, a second input terminal of the trigger circuit is coupled to the output terminal of the shutdown control circuit, and an output terminal of the trigger circuit is coupled to the primary switch; when the conduction control circuit outputs a valid level, the trigger circuit outputs a valid level, and the primary switch is turned on; when the shutdown control circuit outputs a valid level, the trigger circuit outputs an invalid level, and the primary switch is turned off.

[0016] As an implementation manner of the present invention, the conduction control circuit includes a first comparator, a non-inverting input terminal of the first comparator is coupled to a first threshold signal, and an inverting input terminal of the first comparator is coupled to a primary current of an optocoupler.

[0017] As an embodiment of the present invention, the conduction control circuit further includes an AND gate, a first input terminal of the AND gate is coupled to the output terminal of the first comparator, a second input terminal of the AND gate is coupled to the first reference signal, and an output terminal of the AND gate is coupled to the first input terminal of the trigger circuit.

[0018] As an embodiment of the present invention, the conduction control circuit further comprises:

[0019] a current detection circuit, coupled to the optical coupler, for detecting the current in the optical coupler to obtain a detection signal; and

[0020] A current source is coupled to the optocoupler, and the optocoupler is driven by the current source.

[0021] As an implementation mode of the present invention, the shutdown control circuit includes:

[0022] A second comparator, a non-inverting input terminal of which is coupled to the primary current of the primary switch, and an inverting input terminal of which is coupled to the second threshold signal; and

[0023] An OR gate has a first input terminal coupled to the second reference signal, a second input terminal coupled to the output terminal of the second comparator, and an output terminal coupled to the second input terminal of the trigger circuit.

[0024] As an implementation manner of the present invention, the primary side control circuit further includes a primary side switch.

[0025] According to another aspect of the present invention, the following technical solution is adopted: a voltage conversion circuit, wherein the voltage conversion circuit includes the above-mentioned primary side control circuit.

[0026] As an embodiment of the present invention, the voltage conversion circuit also includes a primary winding, a secondary winding and an optocoupler, wherein the optocoupler includes a primary optocoupler and a secondary optocoupler; the primary control circuit couples the primary winding and the primary optocoupler to obtain the optocoupler primary current.

[0027] According to another aspect of the present invention, the following technical solution is adopted: a primary switch control method, the primary switch control method comprising:

[0028] When it is detected that the primary current of the optocoupler is less than a set first threshold value, a control signal capable of controlling the primary switch to be turned on is sent to the primary switch;

[0029] When it is detected that the peak value of the current flowing through the primary switch reaches a set second threshold, a control signal capable of controlling the primary switch to be turned off is sent to the primary switch.

[0030] As an implementation manner of the present invention, the primary switch control method includes:

[0031] Step a, outputting a signal to a trigger circuit according to the magnitude of the primary current of the optocoupler, and outputting a valid level to the first input terminal of the trigger circuit when the primary current of the optocoupler is less than a set first threshold;

[0032] Step b, outputting a signal to the trigger circuit according to the primary current flowing through the primary switch, and outputting a valid level to the second input terminal of the trigger circuit when the primary current flowing through the primary switch reaches a set second threshold; and

[0033] Step c: when the first input terminal of the trigger circuit receives a valid level, the trigger circuit outputs a valid level and the primary switch is turned on; when the second input terminal of the trigger circuit receives a valid level, the trigger circuit outputs an invalid level and the primary switch is turned off.

[0034] As an embodiment of the present invention, step a includes: comparing the primary current of the optocoupler with a first threshold signal, and if the primary current of the optocoupler is less than the set first threshold and the first reference signal is a valid level, outputting a valid level to the first input terminal of the trigger circuit.

[0035] As an embodiment of the present invention, step b includes: comparing the primary current of the primary switch with a second threshold signal, and if the primary current of the primary switch reaches the second threshold signal or the second reference signal is a valid level, outputting a valid level to the second input terminal of the trigger circuit.

[0036] As an implementation manner of the present invention, the primary switch control method further includes coupling a current source with a primary optocoupler to generate an optocoupler primary current.

[0037] The beneficial effects of the present invention are as follows: the voltage conversion circuit, primary side control circuit and primary side switch control method proposed in the present invention do not require a frequency oscillator, have a simpler structure, and can reduce the power consumption of the circuit in the standby state.

[0038] Since the primary optocoupler is driven by the current source Iup, the primary current Ip has a maximum pull-up current Iup, which can reduce the loss of the primary circuit of the system in standby state. The current detection circuit can use a current mirror, which has lower power loss in standby state. Since the switch control of the above logic is adopted, there is no need to detect the average value of the output signal, so the secondary side does not need a filtering circuit, and the feedback loop of C1 and R1 can be omitted. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 The figure is a circuit diagram of an existing flyback voltage converter.

[0040] Figure 2 The figure is a pulse waveform diagram of a primary side control circuit of an existing flyback voltage converter.

[0041] Figure 3 FIG. 4 is a circuit diagram of a voltage conversion circuit in one embodiment of the present invention.

[0042] Figure 4 FIG. 4 is a circuit diagram of a primary side control circuit in an embodiment of the present invention.

[0043] Figure 5 FIG. 4 is a waveform diagram of related signals of the primary side control circuit in one embodiment of the present invention. DETAILED DESCRIPTION

[0044] The preferred embodiments of the present invention are described in detail below with reference to the accompanying drawings.

[0045] In order to further understand the present invention, preferred embodiments of the present invention are described below in conjunction with examples. However, it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, rather than limiting the claims of the present invention.

[0046] The description in this section is only for several typical embodiments, and the present invention is not limited to the scope of the embodiments. The same or similar prior art means and some technical features in the embodiments are mutually replaced within the scope of the present invention.

[0047] The term "coupled" or "connected" in the specification includes both direct connection and indirect connection. Indirect connection is a connection through an intermediate medium, such as a connection through an electrically conductive medium, which may have parasitic inductance or parasitic capacitance; indirect connection may also include connection through other active or passive devices on the basis of achieving the same or similar functional purpose, such as connection through circuits or components such as switches and follower circuits.

[0048] The present invention discloses a primary control circuit, which includes a primary switch control circuit, and the primary switch control circuit is used to couple a primary switch. When the primary switch control circuit detects that the primary current of the optocoupler is less than a set first threshold, the primary switch control circuit sends a control signal to the primary switch that can control the primary switch to be turned on; when the primary switch control circuit detects that the peak value of the current flowing through the primary switch reaches a set second threshold, the primary switch control circuit sends a control signal to the primary switch that can control the primary switch to be turned off.

[0049] Figure 4 is a circuit diagram of a primary side control circuit in an embodiment of the present invention; please refer to Figure 4In one embodiment of the present invention, the primary control circuit includes a primary switch control circuit, and the primary switch control circuit includes: a conduction control circuit 41, a shutdown control circuit 42 and a trigger circuit 43. In one embodiment, the primary control circuit further includes a primary switch. The conduction control circuit 41 is used to output a control signal according to the magnitude of the primary current of the optocoupler. When the primary current of the optocoupler is less than a set first threshold, the conduction control circuit 41 outputs a valid level. The shutdown control circuit 42 is used to output a control signal according to the magnitude of the primary current flowing through the primary switch. When the primary current flowing through the primary switch reaches a set second threshold, the shutdown control circuit 42 outputs a valid level. The first input end of the trigger circuit 43 is coupled to the output end of the conduction control circuit 41, the second input end of the trigger circuit 43 is coupled to the output end of the shutdown control circuit 42, and the output end of the trigger circuit 43 is coupled to the primary switch. When the conduction control circuit 41 outputs a valid level, the trigger circuit 43 outputs a valid level and the primary switch is turned on; when the shutdown control circuit 42 outputs a valid level, the trigger circuit 43 outputs an invalid level and the primary switch is turned off.

[0050] In one embodiment of the present invention, Figure 4 As shown, when the optical coupler detection current is less than the preset first threshold, that is, Figure 4 When the detection signal VFB in is less than the first threshold signal Vth_FB, the conduction control circuit 41 outputs a valid level, the trigger circuit 43 is set, the PWM signal is a valid level, and the primary switch Q is turned on. When the primary current of the current flowing through the primary switch Q reaches the set second threshold, that is, Figure 4 When the primary detection current signal CS_sense is greater than the second threshold signal Vth_OCP, the shutdown control circuit 42 outputs a valid level, the trigger circuit 43 is reset, the PWM signal is an invalid level, and the primary switch Q is turned off.

[0051] Please continue reading Figure 4 In one embodiment of the present invention, the conduction control circuit 41 includes a first comparator 412; a non-inverting input terminal of the first comparator 412 is coupled to the first threshold signal Vth_FB, and an inverting input terminal of the first comparator 412 is coupled to the optocoupler primary current detection signal VFB.

[0052] In one embodiment of the present invention, the conduction control circuit 41 may further include an AND gate 413; a first input terminal of the AND gate 413 is coupled to the output terminal of the first comparator 412, a second input terminal of the AND gate 413 is coupled to the first reference signal (fs_max, and an output terminal of the AND gate 413 is coupled to the first input terminal of the trigger circuit 43. The first reference signal fs_max may be a signal associated with the falling edge of the PWM signal, and becomes a valid level after a preset time from the falling edge of the PWM signal, and is used to perform system maximum frequency control to ensure that the shutdown time is not too short, and to reduce switch power consumption.

[0053] In one embodiment, the conduction control circuit 41 may further include a current detection circuit 411 and a current source I0. The current detection circuit 411 is coupled to the optocoupler OC to detect the current in the optocoupler OC to obtain a detection signal VFB; the current source I0 is coupled to the optocoupler OC, and the optocoupler OC is driven by the current source I0. In one embodiment, the upper limit of the current flowing through the optocoupler is I0, and VFB also has a corresponding upper limit. In one embodiment, the current detection circuit 411 may include a current mirror. The current detection circuit 411 may also use any other suitable circuit to generate a voltage signal VFB proportional to the primary current of the optocoupler. Since the current source has an upper limit, the power consumption of the system in the standby state has an upper limit value, thereby improving the efficiency of the system. The current detection circuit 411 does not include a detection resistor but includes a current mirror, which can also be used to further reduce the power consumption of the system.

[0054] Please continue reading Figure 4 In one embodiment of the present invention, the shutdown control circuit 42 includes a second comparator 421 and an OR gate 422. The non-inverting input terminal of the second comparator 421 is coupled to the primary detection current CS_sense of the primary switch, and the inverting input terminal of the second comparator 421 is coupled to the second threshold signal Vth_OCP. The first input terminal of the OR gate 422 is coupled to the second reference signal Ton_max, the second input terminal of the OR gate 422 is coupled to the output terminal of the second comparator 421, and the output terminal of the OR gate 422 is coupled to the second input terminal of the trigger circuit 43. The second reference signal Ton_max is used to control the maximum width of the high-level pulse, control the on-time of the primary switch to be less than the maximum on-time threshold, and stabilize the system control.

[0055] Figure 4 The primary side control circuit shown eliminates the use of a frequency oscillator and can be used to minimize power consumption in standby mode. Figure 5 FIG. 1 is a waveform diagram of related signals of the primary side control circuit in one embodiment of the present invention; please refer to FIG. Figure 5In one embodiment of the present invention, before time t1, the system works normally; after time t1, the load becomes lighter, VFB becomes larger, and the OFF time of the PWM signal becomes longer; thereby achieving the automatic reduction of the on-duty cycle of the primary switch Q. The PWM high-level pulses are evenly dispersed, and the output is relatively balanced.

[0056] Figure 3 is a circuit diagram of a voltage conversion circuit in one embodiment of the present invention; please refer to Figure 3 The present invention discloses a voltage conversion circuit, wherein the voltage conversion circuit includes the above-mentioned primary side control circuit. Figure 3 In the embodiment shown, the primary side control circuit is fabricated in a semiconductor chip IC. The primary side control circuit IC comprises a primary side switch control circuit 4, a current source Iup and a primary side switch Q.

[0057] In one embodiment of the present invention, the voltage conversion circuit further includes a primary winding L1, a secondary winding L2 and an optocoupler, wherein the optocoupler includes a secondary optocoupler and a primary optocoupler, the secondary optocoupler is located at the secondary side of the voltage conversion circuit as a transmitting end, and the primary optocoupler is located at the primary side of the voltage conversion circuit as a receiving end. The primary control circuit IC is coupled to the primary winding L1; the primary control circuit IC is coupled to the receiving end of the optocoupler to obtain the optocoupler primary current Ip; the optocoupler primary current Ip corresponds to the secondary current Is of the optocoupler transmitting end, and when the optocoupler secondary current Is increases, the optocoupler primary current Ip increases accordingly.

[0058] In one embodiment, the secondary output voltage Vout is transmitted to the primary side through the optocoupler, which is reflected as the optocoupler primary current Ip; wherein the primary optocoupler is coupled to the current source Iup and driven by the current source Iup. The feedback signal FB is obtained by detecting the optocoupler primary current Ip. When the output voltage rises, the voltage at point B rises, the voltage at point A drops, the optocoupler secondary current Is rises, and the optocoupler primary current Ip rises; when the output voltage drops, the voltage at point B drops, the voltage at point A rises, the optocoupler secondary current Is drops, and the optocoupler primary current Ip drops. The primary switch control circuit 4 realizes the switch control of the primary switch Q according to the sampled optocoupler primary current, and the control logic is: when the detection signal VFB of the optocoupler primary current Ip is less than the set first threshold, the primary switch Q is turned on; when the peak value of the current flowing through the primary switch Q reaches the set second threshold, the primary switch Q is turned off.

[0059] In another embodiment of the present invention, the current source Iup may also be used as a part of the primary switch control circuit 4 .

[0060] An embodiment of the present invention discloses a primary switch control method, which includes: when it is detected that the primary current of the optocoupler is less than a set first threshold, sending a control signal to the primary switch that can control its conduction; when it is detected that the peak value of the current flowing through the primary switch reaches a set second threshold, sending a control signal to the primary switch that can control its shutdown.

[0061] In one embodiment of the present invention, the primary switch control method includes (in one embodiment, the above steps a, b, and c may not be in the following order):

[0062] Step a, outputting a signal to a trigger circuit according to the magnitude of the primary current of the optocoupler, and outputting a valid level to the first input terminal of the trigger circuit when the primary current of the optocoupler is less than a set first threshold;

[0063] Step b, outputting a signal to the trigger circuit according to the primary current flowing through the primary switch, and outputting a valid level to the second input terminal of the trigger circuit when the primary current flowing through the primary switch reaches a set second threshold; and

[0064] Step c: when the first input terminal of the trigger circuit receives a valid level, the trigger circuit outputs a valid level and the primary switch is turned on; when the second input terminal of the trigger circuit receives a valid level, the trigger circuit outputs an invalid level and the primary switch is turned off.

[0065] In one embodiment, step a includes: comparing the primary current of the optocoupler with a first threshold signal, and if the primary current of the optocoupler is less than the set first threshold and the first reference signal is at a valid level, outputting a valid level to the first input terminal of the trigger circuit.

[0066] In one embodiment of the present invention, step b includes: comparing the primary current of the primary switch with a second threshold signal, and if the primary current of the primary switch reaches the second threshold signal or the second reference signal is a valid level, outputting a valid level to the second input terminal of the trigger circuit.

[0067] In one embodiment of the present invention, the method further includes coupling a current source with a primary optocoupler to generate an optocoupler primary current, thereby limiting an upper limit of the optocoupler primary current and reducing power consumption of the system.

[0068] The above briefly introduces the process of the primary side switch control method. For specific details, please refer to the above description of the primary side control circuit, which will not be repeated here.

[0069] In summary, the voltage conversion circuit, primary side control circuit and primary side switch control method proposed in the present invention do not require a frequency oscillator, have a simpler structure, and can reduce the power consumption of the circuit in the standby state.

[0070] Since the primary optocoupler is driven by the current source Iup, the primary current Ip has a maximum pull-up current Iup, which can reduce the loss of the primary circuit of the system in standby state. The current detection circuit can use a current mirror, which has lower power loss in standby state. Since the switch control of the above logic is adopted, there is no need to detect the average value of the output signal, so the secondary side does not need a filtering circuit, and the feedback loop of C1 and R1 can be omitted.

[0071] The description and application of the present invention here are illustrative, and it is not intended to limit the scope of the present invention to the above-mentioned embodiments. The effects or advantages involved in the embodiments may not be embodied in the experimental examples due to interference from various factors, and the description of the effects or advantages is not used to limit the embodiments. The deformation and changes of the embodiments disclosed here are possible, and the replacement of the embodiments and the various equivalent components are well known to those of ordinary skill in the art. It should be clear to those skilled in the art that the present invention can be implemented in other forms, structures, arrangements, proportions, and with other components, materials and parts without departing from the spirit or essential features of the present invention. Other deformations and changes can be made to the embodiments disclosed here without departing from the scope and spirit of the present invention.

Claims

1. A primary side control circuit, It is characterized in that The primary side control circuit comprises a primary side switch control circuit, and the primary side switch control circuit is used to couple the primary side switch; When the primary switch control circuit detects that the primary current of the optocoupler is less than a set first threshold, the primary switch control circuit sends a control signal capable of controlling the conduction of the primary switch to the primary switch, wherein the optocoupler includes a secondary optocoupler and a primary optocoupler, the secondary optocoupler is located at the secondary side of the voltage conversion circuit as a transmitting end, and the primary optocoupler is located at the primary side of the voltage conversion circuit as a receiving end, and the secondary output voltage is transmitted to the primary side through the optocoupler, which is reflected as the primary current of the optocoupler; When the primary switch control circuit detects that the peak value of the current flowing through the primary switch reaches a set second threshold, the primary switch control circuit sends a control signal to the primary switch that can control the primary switch to be turned off.

2. The primary side control circuit according to claim 1, Features: The primary switch control circuit comprises: A conduction control circuit, used to output a control signal according to the magnitude of the primary current of the optocoupler, and when the primary current of the optocoupler is less than a set first threshold, the conduction control circuit outputs a valid level; a shutdown control circuit, configured to output a control signal according to the magnitude of the primary current flowing through the primary switch, wherein when the primary current flowing through the primary switch reaches a set second threshold, the shutdown control circuit outputs a valid level; and A trigger circuit, wherein a first input terminal is coupled to the output terminal of the conduction control circuit, a second input terminal of the trigger circuit is coupled to the output terminal of the shutdown control circuit, and an output terminal of the trigger circuit is coupled to the primary switch; when the conduction control circuit outputs a valid level, the trigger circuit outputs a valid level, and the primary switch is turned on; when the shutdown control circuit outputs a valid level, the trigger circuit outputs an invalid level, and the primary switch is turned off.

3. The primary side control circuit according to claim 2, Features: The conduction control circuit includes a first comparator, a non-inverting input terminal of the first comparator is coupled to a first threshold signal, and an inverting input terminal of the first comparator is coupled to a primary current of an optocoupler.

4. The primary side control circuit according to claim 3, Features: The conduction control circuit further includes an AND gate, a first input terminal of the AND gate is coupled to the output terminal of the first comparator, a second input terminal of the AND gate is coupled to a first reference signal, and an output terminal of the AND gate is coupled to a first input terminal of the trigger circuit, wherein the first reference signal is a signal associated with a falling edge of the output signal of the trigger circuit, and is used for performing system maximum frequency control.

5. The primary side control circuit according to claim 3 or 4, Features: The conduction control circuit further comprises: a current detection circuit, coupled to the optical coupler, for detecting the current in the optical coupler to obtain a detection signal; and A current source is coupled to the optocoupler, and the optocoupler is driven by the current source.

6. The primary side control circuit according to claim 2, Features: The shutdown control circuit comprises: A second comparator, a non-inverting input terminal of which is coupled to the primary current of the primary switch, and an inverting input terminal of which is coupled to the second threshold signal; and An OR gate, whose first input terminal is coupled to a second reference signal, whose second input terminal is coupled to an output terminal of the second comparator, and whose output terminal is coupled to a second input terminal of the trigger circuit, wherein the second reference signal is used to control a maximum width of a high-level pulse.

7. The primary side control circuit according to any one of claims 1 to 4, Features: The primary side control circuit further includes a primary side switch.

8. A voltage conversion circuit, It is characterized in that The voltage conversion circuit comprises the primary side control circuit as described in any one of claims 1 to 7.

9. The voltage conversion circuit according to claim 8, Features: The voltage conversion circuit also includes a primary winding, a secondary winding and an optocoupler, wherein the optocoupler includes a primary optocoupler and a secondary optocoupler; the primary control circuit is coupled to the primary winding and the primary optocoupler to obtain the optocoupler primary current.

10. A primary switch control method, It is characterized in that The primary switch control method comprises: When it is detected that the primary current of the optocoupler is less than a set first threshold value, a control signal capable of controlling the conduction of the primary switch is sent to the primary switch, wherein the optocoupler includes a secondary optocoupler and a primary optocoupler, the secondary optocoupler is located at the secondary side of the voltage conversion circuit as a transmitting end, and the primary optocoupler is located at the primary side of the voltage conversion circuit as a receiving end, and the secondary output voltage is transmitted to the primary side through the optocoupler, which is reflected as the primary current of the optocoupler; When it is detected that the peak value of the current flowing through the primary switch reaches a set second threshold, a control signal capable of controlling the primary switch to be turned off is sent to the primary switch.

11. The primary switch control method according to claim 10, Features: The primary switch control method comprises: Step a, outputting a signal to a trigger circuit according to the magnitude of the primary current of the optocoupler, and outputting a valid level to the first input terminal of the trigger circuit when the primary current of the optocoupler is less than a set first threshold; Step b, outputting a signal to the trigger circuit according to the primary current flowing through the primary switch, and outputting a valid level to the second input terminal of the trigger circuit when the primary current flowing through the primary switch reaches a set second threshold; and Step c: when the first input terminal of the trigger circuit receives a valid level, the trigger circuit outputs a valid level and the primary switch is turned on; when the second input terminal of the trigger circuit receives a valid level, the trigger circuit outputs an invalid level and the primary switch is turned off.

12. The primary switch control method according to claim 11, Features: The step a includes: comparing the primary current of the optocoupler with a first threshold signal; if the primary current of the optocoupler is less than the set first threshold and the first reference signal is at a valid level, outputting a valid level to the first input terminal of the trigger circuit, wherein the first reference signal is a signal associated with the falling edge of the output signal of the trigger circuit, and is used for controlling the maximum frequency of the system.

13. The primary switch control method according to claim 11, Features: The step b includes: comparing the primary current of the primary switch with a second threshold signal, and if the primary current of the primary switch reaches the second threshold signal or the second reference signal is at a valid level, outputting a valid level to the second input terminal of the trigger circuit, wherein the second reference signal is used to control the maximum width of the high-level pulse.

14. The primary switch control method according to claim 10, Features: The primary switch control method further includes coupling a current source to a primary optocoupler to generate an optocoupler primary current.

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