Flyback voltage conversion circuit and control circuit and start control method thereof

Abstract

The invention provides a flyback voltage conversion circuit and a control circuit and a start control method thereof. The control circuit comprises a conduction control circuit which provides a conduction control signal for conducting the primary side switch, and the conduction control circuit is used for controlling the frequency of the primary side switch; the turn-off control circuit comprises a soft start circuit and a peak current control circuit, and the turn-off control circuit provides a turn-off control signal for turning off the primary side switch; the trigger circuit is coupled with the turn-on control circuit and the turn-off control circuit, and the output end of the trigger circuit is coupled with the control end of the primary side switch; wherein the conduction control circuit controls the primary side switch to perform switching action at a first frequency in a first stage of the starting process of the flyback voltage conversion circuit and perform switching action at a second frequency in a second stage of the starting process of the flyback voltage conversion circuit, and the first stage is before the second stage; the maximum value of the first frequency is lower than the maximum value of the second frequency. According to the flyback voltage conversion circuit, the control circuit thereof and the start control method, the problem of start abnormity caused by valley bottom detection abnormity in the soft start process of the quasi-resonance flyback voltage conversion circuit can be solved.

Description

Technical Field

[0001] The present invention relates to the field of electronics, and specifically but not limited to a flyback voltage conversion circuit, a control circuit and a startup control method thereof. Background Art

[0002] In applications of small and medium power switching power supplies that require electrical isolation between input and output, a flyback converter circuit is the most commonly used topology. Figure 1 The flyback voltage conversion circuit is shown. The flyback voltage conversion circuit has the advantages of simplicity, reliability, low cost and easy implementation.

[0003] Figure 2 The diagram illustrates the operation of a flyback voltage converter circuit. Taking discontinuous mode operation as an example, during the flyback voltage converter circuit's on-phase (TON), the control signal PWM is high, turning on primary switch Q1 (the main power switch), bringing the voltage Vds across it close to 0V. The secondary freewheeling diode D7 is reverse-blocked, and the transformer's primary winding voltage equals its input voltage Vin. The transformer's magnetizing inductor stores energy, and the primary current Ip increases linearly. During the off-phase (TOFF), the control signal PWM is low, turning off primary switch Q1 and conducting secondary diode D7. The voltage across the transformer's secondary winding is Vo + Vf, and the voltage across primary switch Q1 is Vin + Nps * (Vo + Vf), where Nps is the turns ratio between the primary and secondary windings, Vo is the flyback voltage converter circuit's output voltage, and Vf is the forward voltage drop across diode D7. The transformer's magnetizing inductor releases energy to the load, and the transformer's secondary current Is decreases linearly. When the secondary current Is drops to zero, diode D7 turns off, and the drain-source voltage Vds of the primary switch Q1 decreases due to the resonance of the primary magnetizing inductance and the output junction capacitance. During this process, if the system operates in fixed-frequency mode, the frequency signal initiates the next switching cycle. If operating in quasi-resonant mode, the chip initiates the next switching cycle at the valley of Vds, turning on the primary switch Q1.

[0004] When a flyback voltage conversion system starts up, a soft start function is usually required to prevent the output voltage Vo from overshooting during the startup process. Figure 3 The figure shows a flyback voltage conversion circuit with soft start function. The output voltage Vo can be obtained through the feedback signal FB. Figure 3The voltage DEM on the auxiliary winding Na shown in the figure is used to detect the valley position of the voltage Vds across the primary switch Q1. Primary switch Q1 is turned on at the valley position of Vds. Typically, the voltage DEM is compared with the reference voltage Vref to detect the valley position of Vds. However, during startup, due to the low output voltage Vo, the voltage signal DEM also falls below the reference voltage Vref due to inductive coupling, resulting in the system being unable to effectively detect the valley position, which can cause system startup anomalies.

[0005] In view of this, it is necessary to provide a new structure or control method to solve at least part of the above problems. Summary of the Invention

[0006] To address at least one or more problems in the background technology, the present invention proposes a flyback voltage conversion circuit, a control circuit thereof, and a startup control method.

[0007] According to one aspect of the present invention, a control circuit for controlling a primary switch in a flyback voltage conversion circuit includes: a conduction control circuit, which provides a conduction control signal for turning on the primary switch, and the conduction control circuit is used to control the frequency of the primary switch; a shutdown control circuit, which includes a soft start circuit and a peak current control circuit, and the shutdown control circuit provides a shutdown control signal for turning off the primary switch; and a trigger circuit, wherein a first input terminal of the trigger circuit is coupled to the conduction control circuit, a second input terminal of the trigger circuit is coupled to the shutdown control circuit, and an output terminal of the trigger circuit is coupled to a control terminal of the primary switch; wherein the conduction control circuit controls the primary switch to perform switching action at a first frequency in a first stage of the flyback voltage conversion circuit startup process, and to perform switching action at a second frequency in a second stage of the flyback voltage conversion circuit startup process, wherein the maximum value of the first frequency in the first stage and before the second stage is lower than the maximum value of the second frequency.

[0008] Optionally, the conduction control circuit includes: a valley detection circuit, which detects the valley position of the voltage across the primary switch and controls the primary switch to allow conduction when the voltage across the primary switch drops to the valley bottom; and a frequency limiting circuit, which controls the switching frequency of the primary switch to be lower than a first limiting frequency in the first stage and lower than a second limiting frequency in the second stage, wherein the first limiting frequency is lower than the second limiting frequency.

[0009] Optionally, the conduction control circuit includes: a comparison circuit, coupled to the auxiliary winding of the flyback voltage conversion circuit for obtaining a voltage detection signal, the comparison circuit compares the voltage detection signal with a threshold signal; a valley detection circuit, coupled to the auxiliary winding of the flyback voltage conversion circuit for obtaining a voltage detection signal, the valley detection circuit obtains a valley detection signal based on the voltage detection signal; when the voltage detection signal is less than the threshold signal, it is the first stage, the conduction control circuit controls the primary switch to perform switching at a fixed bottoming frequency, and when the voltage detection signal is greater than the threshold signal, it enters the second stage, and the conduction control circuit controls the primary switch to conduct at the valley based on the valley detection signal.

[0010] Optionally, the conduction control circuit includes a valley detection circuit, which is coupled to an auxiliary winding of the flyback voltage conversion circuit for obtaining a voltage detection signal, and the valley detection circuit obtains a valley detection signal based on the voltage detection signal; the conduction control circuit controls the primary switch to perform switching at a fixed bottoming frequency in the first N switching cycles, and controls the primary switch to conduct at the valley based on the valley detection signal in subsequent switching cycles.

[0011] According to another aspect of the present invention, a flyback voltage conversion circuit is provided, comprising a primary switch according to any one of the above embodiments and a control circuit for controlling the primary switch.

[0012] According to another aspect of the present invention, a startup control method for a flyback voltage conversion circuit is proposed, wherein the flyback voltage conversion circuit has soft start control, and the control method includes: in a first stage of the startup process, controlling the primary switch in the flyback voltage conversion circuit to perform a switching action at a first frequency; in a second stage of the startup process, controlling the primary switch in the flyback voltage conversion circuit to perform a switching action at a second frequency, wherein the maximum value of the first frequency in the first stage and before the second stage is lower than the maximum value of the second frequency.

[0013] Optionally, the control method further includes controlling the primary switch to turn on when it is detected that the voltage across the primary switch drops to the bottom and is lower than the upper clamp frequency, wherein in the first stage the upper clamp frequency is a first limiting frequency, and in the second stage the upper clamp frequency is a second limiting frequency, and the first limiting frequency is lower than the second limiting frequency.

[0014] Optionally, the first stage includes the first N switching cycles of the primary switch after startup, where N is a positive integer, and the second stage is the subsequent stage.

[0015] Optionally, in the first stage, the primary switch in the flyback voltage conversion circuit is controlled to perform switching at a fixed bottoming frequency. In the second stage, when it is detected that the voltage across the primary switch drops to the bottom, the primary switch is controlled to turn on.

[0016] Optionally, the control method further includes: measuring the voltage on the auxiliary winding to obtain a voltage detection signal, wherein the auxiliary winding is coupled with the primary winding and the secondary winding in the flyback voltage conversion circuit; and comparing the voltage detection signal with a reference signal to obtain a valley detection signal.

[0017] Optionally, when the voltage detection signal is less than the threshold signal, it is the first stage, and when the voltage detection signal is greater than the threshold signal, it enters the second stage, enabling the valley detection function of the primary side switch.

[0018] Optionally, the first stage includes the first N switching cycles of the primary switch after startup, where N is a positive integer.

[0019] Optionally, the first stage is a time period after startup in which the timing is less than a preset time.

[0020] The flyback voltage conversion circuit, control circuit and startup control method proposed in the present invention can solve the startup abnormality problem caused by abnormal valley bottom detection during the soft startup process of the quasi-resonant flyback voltage conversion circuit. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The accompanying drawings are used to provide a further understanding of the present invention and, together with the description, to explain the embodiments of the present invention, but do not constitute a limitation of the present invention. In the accompanying drawings:

[0022] Figure 1 A flyback voltage conversion circuit is shown;

[0023] Figure 2 The working mode of the flyback voltage conversion circuit is shown;

[0024] Figure 3 A flyback voltage conversion circuit with a soft start function is shown;

[0025] Figure 4 A control circuit for a flyback voltage conversion circuit with an optimized soft start function according to an embodiment of the present invention is shown;

[0026] Figure 5 It shows an embodiment of the present invention can be used for Figure 4 Schematic diagram of control waveform of flyback voltage conversion circuit;

[0027] Figure 6 shows a control circuit according to another embodiment of the present invention;

[0028] Figure 7 A control circuit according to yet another embodiment of the present invention is shown. DETAILED DESCRIPTION

[0029] 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.

[0030] The description in this section focuses on a few typical embodiments only, and the present invention is not limited to the scope of the embodiments described. Combinations of different embodiments, replacement of certain technical features in different embodiments, and replacement of certain technical features in the embodiments with the same or similar prior art methods are also within the scope of the present invention.

[0031] "Coupled" or "connected" in this specification encompasses both direct and indirect connections. An indirect connection is a connection through an intermediate medium, such as a connection through an electrically conductive medium like a conductor, which may contain parasitic inductance or capacitance, or a connection through an intermediate circuit or component as described in the embodiments of this specification. An indirect connection may also include a connection through other active or passive devices that achieve the same or similar functions, such as a connection through circuits or components such as switches, signal amplifiers, and follower circuits. "Multiple" or "many" refers to two or more.

[0032] Figure 4 The present invention shows a control circuit for controlling the primary switch in a flyback voltage conversion circuit according to an embodiment of the present invention. The control circuit includes a conduction control circuit 11, a shutdown control circuit 12, and a trigger circuit 13. The conduction control circuit 11 provides a conduction control signal ON for turning on the primary switch. When the conduction control signal ON switches from an invalid state to a valid state, the control circuit provides a valid switch control signal PWM, such as a high-level signal for turning on the primary switch. The conduction control circuit 11 includes a frequency control circuit for controlling the frequency of the primary switch. The conduction control circuit 11 includes a valley detection circuit 111. In one embodiment, the valley detection circuit 111 is coupled to the auxiliary winding La (such as Figure 3As shown) is used to obtain a voltage detection signal DEM. The valley detection circuit 111 obtains a valley detection signal based on the voltage detection signal, which indicates whether the voltage Vds across the primary switch Q1 has dropped to the valley bottom. The conduction control circuit 11 can control the primary switch Q1 to be turned on when the voltage Vds across Q1 drops to the valley bottom. However, due to the control of soft start, in the early stage of the startup phase, the output voltage Vo is low and the voltage detection signal DEM is also low, which causes the detection of the valley detection circuit 111 to be abnormal, which may cause the frequency of the primary switch to be too high, resulting in excessive stress and loss of components such as the rectifier circuit in the flyback voltage conversion circuit, affecting the normal startup of the system. The shutdown control circuit 12 provides a shutdown control signal OFF for shutting down the primary switch. The shutdown control circuit 12 includes a soft start circuit, which is used to control the system to achieve soft start during the startup phase to prevent the output voltage from overshooting. In one embodiment, the soft start circuit is used to control the output voltage to rise slowly during the startup phase. The shutdown control circuit 12 also includes a peak current control circuit for shutting off the primary switch when a current detection signal CS representing the current flowing through the primary switch (primary current) reaches a reference current value. The shutdown control circuit may further include other circuits, which are not relevant to the present invention and are not described here in detail. A first input terminal of the trigger circuit 13 is coupled to the conduction control circuit 11, a second input terminal of the trigger circuit 13 is coupled to the shutdown control circuit 12, and an output terminal of the trigger circuit 13 is coupled to the control terminal of the primary switch. The trigger circuit 13 controls the conduction and shutdown of the primary switch based on a conduction control signal ON and a shutdown control signal OFF. In an embodiment of the present invention, the conduction control circuit 11 controls the primary switch Q1 to switch at a lower first frequency during the first, i.e., earlier, phase of the flyback voltage conversion circuit startup process, and to switch at a second frequency during the second phase of the flyback voltage conversion circuit startup process. The maximum value of the first frequency in the first phase and before the second phase is lower than the maximum value of the second frequency. The above-described startup control method for a flyback voltage converter circuit with soft-start control is used to prevent problems such as excessive primary-side switching frequency, excessive device stress, and excessive losses caused by abnormal detection during startup. The control circuit may further include other circuits, such as a protection circuit providing a protection signal (PG), which are not relevant to the present invention and are not described here.

[0033] FIG5 shows an example of a method for Figure 4 Schematic diagram of control waveform of flyback voltage conversion circuit. This embodiment sets internal frequency limit by frequency limit circuit. Figure 4 It is used to illustrate the working mode of this embodiment. Figure 4, the conduction control circuit includes a valley detection circuit 111 and a frequency limiting circuit 112. The valley detection circuit 111 is used to detect whether the voltage difference across the primary switch reaches the valley position, and controls the primary switch to turn on when the voltage difference drops to the valley. In one embodiment, the primary switch is a MOSFET (metal oxide semiconductor field effect transistor), and the voltage difference across the primary switch is its drain-source voltage Vds. In one embodiment, the valley detection circuit 111 is coupled to the auxiliary winding of the flyback converter circuit to obtain a voltage detection signal DEM, and the valley detection circuit 111 further obtains a valley detection signal representing the valley position of Vds based on the voltage detection signal DEM. In one embodiment, as Figure 3 As shown, the voltage detection signal DEM is proportional to the voltage on the auxiliary winding La. The auxiliary winding La is coupled to the primary winding Lp and the secondary winding Ls in the flyback voltage conversion circuit. When the primary switch is off, the secondary diode D7 is on, the voltage across the secondary side of the transformer is Vo + Vf, and the voltage across the primary switch Q1 is Vin + Nps * (Vo + Vf). The voltage detection signal DEM on the auxiliary winding La is proportional to the voltages Vo + Vf on the primary winding Lp and the secondary winding Ls, and can be used to reflect changes in the voltage across the primary switch and the output voltage Vo. The voltage detection signal DEM is compared with a reference signal Vref to obtain a valley detection signal indicating whether the voltage across the primary switch has dropped to a valley level. In one embodiment, the valley detection signal is valid when the voltage detection signal DEM is less than the reference signal and a predetermined delay has elapsed. In other embodiments, the valley detection signal can be obtained using other conventional techniques. At the same time, the frequency limiting circuit 112 controls the primary switch to operate below the upper clamp frequency. When the valley detection signal is valid and the corresponding switching frequency is higher than the upper clamp frequency, the primary switch is not allowed to turn on. Instead, it is controlled to turn on in a subsequent oscillation cycle when the voltage across the primary switch drops to a valley and the corresponding switching frequency is lower than the upper clamp frequency. The conduction control circuit 11 may further include other circuits, such as an amplitude-frequency curve control circuit based on the feedback signal FB, for frequency modulation during normal operation, such as controlling conduction at the Mth valley based on the feedback signal or load size. These circuits are not further described here.

[0034] exist Figure 5In the illustrated embodiment, the upper clamp frequency Fs_limit is a floating value. During the first phase T1 of the startup process Tstart, the upper clamp frequency Fs_limit provided by the frequency limiting circuit 112 is a first limiting frequency, such that during the first phase T1, the switching frequency of the primary switch is lower than the first limiting frequency. In the illustrated embodiment, the first limiting frequency is a fixed preset value Fs_limit1. In other embodiments, the first limiting frequency may also be a floating value. During the second phase T2 of the startup phase Tstart, the upper clamp frequency is a second limiting frequency, and the frequency limiting circuit 112 controls the switching frequency to be lower than the second limiting frequency. In the illustrated embodiment, the second limiting frequency is a floating value that increases linearly until it reaches a maximum value Fs_limit2 upon exiting the startup mode and entering the normal operating mode. As can be seen from the figure, the first limiting frequency Fs_limit1 during the first phase T1 is lower than the second limiting frequency during the second phase T2. In this way, during the first few cycles of startup, the switching frequency is limited to a lower frequency. At this time, if the valley detection circuit of the chip system works abnormally, the primary side switch will not be turned on at an abnormally high frequency due to the low frequency limit. It needs to be lower than the limit frequency before it can be turned on, avoiding system problems caused by abnormal valley detection during startup.

[0035] Figure 6 FIG2 shows a control circuit according to another embodiment of the present invention. The conduction control circuit 21 includes a valley detection circuit 211, a comparison circuit 212, and a bottom frequency signal generating circuit 213. The valley detection circuit 211 is coupled to the auxiliary winding of the flyback converter circuit to obtain a voltage detection signal DEM, and based on the voltage detection signal DEM, obtains a valley detection signal indicating that the voltage across the primary switch reaches the valley bottom. Figure 3As shown. The comparison circuit 212 compares the voltage detection signal DEM with the threshold signal Lolim. The bottom frequency signal generation circuit 213 sets the bottom frequency. When the voltage detection signal DEM is less than the threshold signal Lolim, it is the first stage of the startup process. The conduction control circuit 21 controls the primary switch to switch at a fixed bottom frequency. When the voltage detection signal DEM is greater than the threshold signal Lolim, it enters the second stage of the startup process. The conduction control circuit 21 controls the primary switch to conduct at the bottom position of the voltage difference between its two ends based on the valley detection signal. In this way, when the system is turned on, there is no need to adjust the frequency based on valley detection. By detecting the voltage detection signal DEM on the auxiliary winding, when the voltage detection signal DEM is lower than the minimum limit voltage Lolim, the control circuit shields the detection function of the valley detection circuit 211, and the primary switch operates at a fixed lower bottom frequency, avoiding frequency abnormality problems caused by abnormal valley detection. When the output voltage of the flyback voltage conversion circuit rises, the voltage detection signal DEM rises as the output voltage rises. When the voltage detection signal DEM is higher than the threshold signal Lolim, the control circuit enables the valley detection circuit 211, and controls the primary switch to be turned on when the voltage difference across the primary switch reaches the valley position. At the same time, the corresponding valley conduction can be achieved according to the valley number set by the feedback signal FB of the output voltage. For example, in the second stage of the startup process, when the output voltage is low, the valley number is low and the frequency is high. When the output voltage is high, the valley number increases and the frequency decreases. In the second stage of the startup process, the method of turning on the primary switch by valley detection avoids the system frequency being affected by the following factors: Figure 4 The underfrequency phenomenon caused by the soft-start frequency limit shown in the figure optimizes the slow startup problem. Thus, when the voltage detection signal DEM is below the minimum limit voltage Lolim, the first stage of the startup process begins, and the primary switch operates at a lower support frequency. When the voltage detection signal DEM exceeds the minimum limit voltage Lolim, the second stage of the startup process begins, and the primary switch operates in a quasi-resonant state. The switching frequency in this quasi-resonant state is higher than the support frequency.

[0036] Figure 7 FIG. 4 shows a control circuit according to an embodiment of the present invention. In this embodiment, Figure 6 The embodiment shown differs in that the conduction control circuit, rather than determining the output voltage, controls the primary switch to switch at a fixed bottoming frequency for the first N switching cycles. In subsequent switching cycles, the primary switch is then turned on at the valley bottom based on a valley detection signal, operating in quasi-resonant mode. Specifically, the first phase of the startup process is the first N switching cycles after startup, and the second phase is N switching cycles later, where N is a positive integer. This solution is simple and easy to implement, and effectively avoids defects caused by abnormal valley detection during startup.

[0037] In another embodiment, the conduction control circuit includes a timing circuit, which controls the primary switch to perform switching at a fixed bottoming frequency within a preset time after startup, and controls the primary switch to conduct at the valley bottom based on the valley detection signal after the timing overflows, that is, after the preset time, to operate in a quasi-resonant mode, that is, the first stage of the startup process is the time period after startup when the timing is less than the preset time, and the second stage is after the preset time.

[0038] Those skilled in the art should know that the logic controls such as "high level" and "low level", "set" and "reset", "AND gate" and "OR gate", "non-inverting input terminal" and "inverting input terminal" in the logic controls involved in the specification or drawings can be interchanged or changed with each other, and the same function or purpose as the above embodiment can be achieved by adjusting the subsequent logic controls.

[0039] The description and application of the present invention here are illustrative and are not intended to limit the scope of the present invention to the above-mentioned embodiments. The relevant descriptions of the effects or advantages involved in the specification may not be reflected in the actual experimental examples due to the uncertainty of specific condition parameters or other factors, and the relevant descriptions of the effects or advantages are not used to limit the scope of the invention. Variations and changes to the embodiments disclosed here are possible, and the replacement of the embodiments and 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, without departing from the spirit or essential characteristics of the present invention, the present invention can be implemented in other forms, structures, arrangements, proportions, and with other components, materials and parts. Without departing from the scope and spirit of the present invention, other variations and changes can be made to the embodiments disclosed here.

Claims

1. A control circuit for controlling a primary switch in a flyback voltage conversion circuit, comprising: A conduction control circuit, providing a conduction control signal for turning on the primary switch, and the conduction control circuit is used to control the frequency of the primary switch; A shutdown control circuit, including a soft start circuit and a peak current control circuit, provides a shutdown control signal for shutting down the primary switch; as well as a trigger circuit, wherein a first input terminal of the trigger circuit is coupled to the on-control circuit, a second input terminal of the trigger circuit is coupled to the off-control circuit, and an output terminal of the trigger circuit is coupled to the control terminal of the primary switch; The conduction control circuit controls the primary switch to perform switching at a first frequency in a first stage of a flyback voltage conversion circuit startup process, and to perform switching at a second frequency in a second stage of a flyback voltage conversion circuit startup process, wherein the maximum value of the first frequency in the first stage and before the second stage is lower than the maximum value of the second frequency.

2. The control circuit according to claim 1 , wherein the conduction control circuit comprises: The valley detection circuit detects the valley position of the voltage across the primary switch and controls the primary switch to be turned on when the voltage across the primary switch drops to the valley bottom. as well as The frequency limiting circuit controls the switching frequency of the primary switch to be lower than a first limiting frequency in the first stage and lower than a second limiting frequency in the second stage, wherein the first limiting frequency is lower than the second limiting frequency.

3. The control circuit according to claim 1 , wherein the conduction control circuit comprises: a comparison circuit coupled to the auxiliary winding of the flyback voltage conversion circuit for obtaining a voltage detection signal, wherein the comparison circuit compares the voltage detection signal with a threshold signal; as well as A valley detection circuit is coupled to the auxiliary winding of the flyback voltage conversion circuit to obtain a voltage detection signal. The valley detection circuit obtains a valley detection signal based on the voltage detection signal to indicate whether the voltage across the primary switch has dropped to a valley bottom. When the voltage detection signal is less than the threshold signal, it is the first stage. The conduction control circuit controls the primary switch to switch at a fixed bottoming frequency. When the voltage detection signal is greater than the threshold signal, it enters the second stage. The conduction control circuit controls the primary switch to conduct at the valley bottom based on the valley detection signal.

4. The control circuit of claim 1 , wherein the conduction control circuit includes a valley detection circuit coupled to an auxiliary winding of the flyback voltage conversion circuit for obtaining a voltage detection signal, the valley detection circuit obtaining a valley detection signal based on the voltage detection signal, wherein the conduction control circuit controls the primary switch to switch at a fixed bottoming frequency during the first N switching cycles, and controls the primary switch to conduct when the voltage across the primary switch drops to a valley position based on the valley detection signal during subsequent switching cycles.

5. A flyback voltage conversion circuit, comprising a primary switch and a control circuit according to any one of claims 1 to 4.

6. A startup control method for a flyback voltage conversion circuit, wherein the flyback voltage conversion circuit has soft start control, the control method comprising: In the first stage of the startup process, the primary switch in the flyback voltage conversion circuit is controlled to perform a switching action at a first frequency; In the second stage of the startup process, the primary switch in the flyback voltage conversion circuit is controlled to perform switching at a second frequency, wherein the maximum value of the first frequency in the first stage before the second stage is lower than the maximum value of the second frequency.

7. The control method according to claim 6, further comprising controlling the primary switch to be turned on when it is detected that the voltage across the primary switch drops to a valley bottom and is lower than an upper clamping frequency, wherein the upper clamping frequency is a first limiting frequency in the first stage and a second limiting frequency in the second stage, and the first limiting frequency is lower than the second limiting frequency. 8 . The control method of claim 7 , wherein the first phase comprises the first N switching cycles of the primary switch after startup, wherein N is a positive integer.

9. The control method of claim 6, wherein in a first stage, the primary switch in the flyback voltage conversion circuit is controlled to switch at a fixed bottoming frequency, and in a second stage, the primary switch is controlled to be turned on when it is detected that the voltage across the primary switch drops to a valley bottom.

10. The control method according to claim 9, further comprising: detecting a voltage on an auxiliary winding to obtain a voltage detection signal, wherein the auxiliary winding is coupled to a primary winding and a secondary winding in a flyback voltage conversion circuit; as well as The voltage detection signal is compared with a reference signal to obtain a valley detection signal.

11. The control method according to claim 10, wherein when the voltage detection signal is less than the threshold signal, it is the first stage, and when the voltage detection signal is greater than the threshold signal, it enters the second stage to enable the valley detection function of the primary side switch.

12. The control method of claim 9, wherein the first phase includes the first N switching cycles of the primary switch after startup, where N is a positive integer.

13. The control method according to claim 9, wherein the first stage is a time period after startup that is less than a preset time.

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