TW201840108A - Flyback power supply system and control method thereof - Google Patents

Abstract

The invention provides a flyback power supply system and a control method thereof. The flyback power supply system comprises a transformer, a power switch connected to a primary side winding of the transformer and a system control module used for making the power switch be in a cut-off state from a conduction state or be in the conduction state from the cut-off state. The system control module makes the power switch be in the cut-off state from the conduction state based on a current sampling voltage on a current sampling resistor connected between the power switch and ground, and makes the power switch be in the conduction state from the cut-off state based on a feedback voltage and a feedback current of an auxiliary winding of a transformer. The feedback voltage represents a system output voltage of the flyback power supply system and the feedback current represents a system input voltage of the flyback power supply system.

Description

   Flyback power system and control method thereof   

The invention relates to the field of circuits, in particular to a flyback power supply system and a control method thereof.

Generally, the flyback power system isolates the primary input and secondary output through a transformer, and feeds back the sampling information of the output voltage to the control chip located on the primary side through an isolation element such as an optocoupler, so that the control chip can Output voltage sampling information to adjust the output voltage. However, isolation components such as optocouplers will not only increase the cost of the flyback power supply system, but will also become one of the limiting factors of the service life of the flyback power supply system due to its very limited service life.

In view of this, a flyback power system based on primary feedback is proposed. The flyback power system does not need any isolation element to feed back the sampling information of the output voltage to the control chip located on the primary side, but directly based on the secondary side. Sampling the voltage / current information to adjust the output voltage.

In view of one or more of the problems described above, the present invention provides a flyback power supply system and a control method thereof capable of sampling a voltage based on a current from a primary side of a transformer, and a feedback voltage and a feedback current from an auxiliary winding of the transformer. To control the state switching of the power switch, thereby controlling the system output current.

A flyback power supply system according to an embodiment of the present invention includes a transformer, a power switch connected to a primary winding of the transformer, and a system control module for controlling the power switch from an on state to an off state or from an off state to an on state. Group, where: the system control module controls the power switch from the on state to the off state based on the current sampling voltage on the current sampling resistor connected between the power switch and ground, and is based on the feedback voltage and feedback current from the auxiliary winding of the transformer Control the power switch from the off state to the on state, the feedback voltage represents the system output voltage of the flyback power system, and the feedback current represents the system input voltage of the flyback power system.

In some embodiments, the system control module determines the on-time of the secondary winding of the transformer based on the feedback voltage, and controls the power switch from turning off based on the magnitude relationship between the feedback current and the first threshold current, and the on-time of the secondary winding of the transformer. The state becomes the on state, so that the ratio between the secondary winding on-time of the transformer and the switching cycle of the power switch is the first value when the feedback current is less than the first threshold current and the first value when the feedback current is greater than the first threshold current. Binary.

In some embodiments, when the current sampling voltage is greater than the first threshold voltage, the system control module controls the power switch from an on state to an off state.

In some embodiments, the system control module includes a second on-time sensing circuit, and the second on-time sensing circuit generates whether the secondary winding of the transformer is in a conducting state based on the magnitude relationship between the feedback voltage and the second threshold voltage. On-state indication signal.

In some embodiments, the system control module includes a line voltage-line current conversion circuit and a first comparator. The line voltage-line current conversion circuit converts the feedback current into a line current according to a predetermined conversion relationship. The first comparator is based on The magnitude relationship between the line current and the second threshold current generates a first comparison result indication signal, and the second threshold current is proportional to the first threshold current.

In some embodiments, the system control module includes a variable ratio control module, and the variable ratio control module includes first to third current mirrors, a capacitor, and a second comparator, wherein the first current mirror is in the first The comparison result indicates that the capacitor is charged under the control of the indication signal; the second current mirror charges the capacitor under the control of the on-state indication signal; the third current mirror discharges the capacitor under the control of the on-state indication signal, and The two comparators generate a second comparison result indication signal based on the magnitude relationship between the voltage on the capacitor and the third threshold voltage, and are used to control the power switch from the off state to the on state.

In some embodiments, the system control module further includes a third comparator and an RS trigger. The third comparator generates a third comparison result indication signal based on a magnitude relationship between the current sampling voltage and the first threshold voltage, and RS (Reset -Set) The trigger generates a pulse frequency modulation signal for controlling the power switch from the on state to the off state or from the off state to the on state based on the second comparison result indication signal and the third comparison result indication signal.

In some embodiments, the feedback voltage is obtained by dividing the voltage on the auxiliary winding of the transformer when the power switch is in an on state.

According to a control method for a flyback power supply system according to an embodiment of the present invention, the flyback power supply system includes a transformer and a power switch connected to a primary winding of the transformer, and the control method includes: The current sampling voltage on the current sampling resistor controls the power switch from the on state to the off state, and controls the power switch from the off state to the on state based on the feedback voltage from the auxiliary winding of the transformer and the feedback current, where the feedback voltage represents the flyback The output voltage and feedback current of the power supply system represent the system input voltage of the flyback power supply system.

In some embodiments, the on-time of the secondary winding of the transformer is determined based on the feedback voltage, and the power switch is controlled to change from the off state to the on-state based on the magnitude relationship between the feedback current and the first threshold current and the on-time of the secondary winding of the transformer State so that the ratio between the on-time of the secondary winding of the transformer and the switching cycle of the power switch is a first value when the feedback current is less than the first threshold current and a second value when the feedback current is greater than the first threshold current.

VFB‧‧‧Output Characterization Voltage

V _AC ‧‧‧ AC input voltage

VR‧‧‧Threshold Voltage

Vbulk‧‧‧line voltage

Vramp‧‧‧Voltage on capacitor C

Vaux‧‧‧Voltage

CC_ctrl‧‧‧status control signal

Naux‧‧‧T1's auxiliary winding turns

PFM‧‧‧ Pulse Frequency Modulation Signal

The number of turns of the primary winding of Np‧‧‧T1

Q1‧‧‧Power Switch

V _FB ‧‧‧Feedback voltage

T1‧‧‧Transformer

I _FB ‧‧‧Feedback Current

Ics‧‧‧Transformer primary current

Vout‧‧‧System output voltage

Rs‧‧‧Current sampling resistor

CC1, CC2‧‧‧ system output current

Vcs‧‧‧Current sampling voltage

K1, K2‧‧‧‧ ratio (Tons / Ts)

Isec‧‧‧Transformer secondary current

204-2‧‧‧line voltage-line current conversion circuit

Ro‧‧‧Load

204-4‧‧‧Second on-time sensing circuit

Iout‧‧‧Current flowing through Ro

204-6‧‧‧Variable ratio control circuit

Tons‧‧‧second on-time

204-8‧‧‧Switch driving circuit

Ts‧‧‧second on-time

Iline‧‧‧line current

202‧‧‧ Rectification Filter Module

I1, I2, I0‧‧‧ current mirror

204‧‧‧System Control Module

C‧‧‧Capacitor

R1, R2‧‧‧Feedback resistor

State_Tons‧‧‧Continuity state indication signal

L‧‧‧ primary winding inductance

PFM‧‧‧ Pulse Frequency Modulation Signal

D1, D2‧‧‧‧rectified diode

Nsec‧‧‧T1 transformer secondary winding turns

VCC‧‧‧ Controller supply voltage

Req‧‧‧ Equivalent Resistance

Vth, Vth_bulk‧‧‧threshold voltage

BD, CS, FB‧‧‧ terminals

Iovp_1, Iovp_2‧‧‧ Overvoltage protection threshold current

Line_high, OCP‧‧‧ Comparison result indication signal

cmp1, cmp2, cmp3, cmp4‧‧‧ comparator

The present invention can be better understood from the following description of specific embodiments of the present invention with reference to the accompanying drawings, in which: FIG. 1 shows a circuit diagram of a conventional flyback power system based on a primary feedback; FIG. 2 shows A circuit diagram of a flyback power supply system according to an embodiment of the present invention; FIG. 3 illustrates an example circuit diagram of the secondary on-time sensing module in FIG. 2; and FIG. 4 illustrates a variable in FIG. 2 Example circuit diagram of the ratio control module; FIG. 5 shows that the secondary on-time sensing module and the variable ratio control module in FIG. 2 are implemented as the example circuits shown in FIGS. 3 and 4 respectively. Timing diagram of the output voltage VFB, the on-state indication signal State_Tons, the voltage Vramp on the capacitor C, the state control signal CC_ctrl, and the PFM (Pulse Frequency Modulation) signal; The schematic diagram of the system output current of the flyback power system as shown in the figure varies with the system input voltage.

Features and exemplary embodiments of various aspects of the invention will be described in detail below. In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it is obvious to a person skilled in the art that the present invention can be implemented without the need for some of these specific details. The following description of the embodiments is merely for providing a better understanding of the present invention by showing examples of the present invention. The invention is by no means limited to any specific configuration and algorithm proposed below, but covers any modification, replacement and improvement of elements, components and algorithms without departing from the spirit of the invention. In the drawings and the following description, well-known structures and techniques are not shown in order to avoid unnecessarily obscuring the present invention.

FIG. 1 shows a circuit diagram of a conventional flyback power system based on a single feedback. In the case where the flyback power system shown in Figure 1 works in the Discontinuous Conduction Mode (DCM): When the power switch Q1 is in the on state, the transformer T1 stores energy and flows through the primary winding of the transformer T1 The primary current Ics of the transformer rises linearly, and the current sampling voltage Vcs on the current sampling resistor Rs rises linearly; when the current sampling voltage Vcs on the current sampling resistor Rs reaches the threshold voltage Vth, the power switch Q1 changes from the on state to the off state; when When the power switch Q1 is in the off state, the transformer T1 releases energy, and the secondary current Isec of the transformer flowing through the secondary winding of the transformer T1 decreases linearly.

According to the one-time principle, the system output current of the flyback power supply system shown in Figure 1, that is, the current Iout flowing through the load Ro can be expressed as:

Among them, N is the turns ratio of the primary winding to the secondary winding of the transformer T1, Ip is the primary peak current of the transformer (that is, the peak value of the transformer primary current Ics) flowing through the primary winding of the transformer T1, and Tons is the secondary of the transformer T1 On-time, Ts is the switching period of the power switch Q.

According to Equation 1 and Equation 2, as long as the ratio of the secondary on-time Tons of the transformer T1 to the switching period Ts of the power switch Q1 and the primary peak current Ip of the transformer flowing through the primary winding of the transformer T1 are fixed, The system output current of the flyback power supply system shown in FIG. 1 can be kept constant. In this case, the system output current remains constant regardless of whether the system input voltage of the flyback power system shown in Figure 1 is constant.

In some countries or regions with underdeveloped power grids, such as India, the AC input voltage is quite unstable. If the flyback power system shown in Figure 1 still keeps the system output current constant when the AC input voltage is too high, it will be caused by The system is overheated and damaged.

In view of the above problems, a novel flyback power supply system is proposed, which can control the state switching of the power switch based on both the current sampling voltage from the primary side of the transformer and the output characterization voltage from the secondary side of the transformer, thereby controlling the system Output current.

FIG. 2 shows a circuit diagram of a flyback power supply system according to an embodiment of the present invention. As shown in FIG. 2, the flyback power supply system according to the embodiment of the present invention includes a transformer T1, a rectification and filtering module 202 located on the primary side of the transformer T1, a system control module 204, a power switch Q1, and a current sampling resistor Rs. And the feedback voltage dividing resistors R1 and R2 on the secondary side of the transformer T1, wherein the first terminal and the second terminal of the rectifying and filtering module 202 are respectively connected to the positive and negative poles of the AC power supply, and the rectifying and filtering module 202 The third terminal is connected to the first terminal of the primary winding inductance L of the transformer T1, and the fourth terminal of the rectification and filtering module 202 is grounded; the BD terminal of the system control module 204 is connected to the base of the power switch Q1, and the system control module The CS terminal of 204 is connected to the first terminal of the current sampling resistor Rs, the FB terminal of the system control module 204 is connected to the connection node between the feedback voltage dividing resistors R1 and R2; the collector of the power switch Q1 is connected to the primary winding of the transformer T1 The second terminal of the inductor L is connected, and the emitter of the power switch Q1 is connected to the first terminal of the current sampling resistor Rs; the second terminal of the current sampling resistor Rs is grounded; and the feedback voltage dividing resistors R1 and R2 are connected in series Connected between the dotted end of secondary winding of the transformer T1 and the ground.

In the flyback power supply system shown in FIG. 2, the rectification and filtering module 202 generates a line voltage Vbulk by rectifying and filtering the AC input voltage V _AC . When the power switch Q1 is in an on state, the auxiliary winding of the transformer T1 is The voltage Vaux is negative and its magnitude is proportional to the line voltage Vbulk, as follows:

Among them, Naux is the number of turns of the auxiliary winding of the transformer T1, and Np is the number of turns of the primary winding of the transformer T1.

When the power switch Q1 is in the on state, the voltage Vaux on the auxiliary winding of the transformer T1 can represent the line voltage Vbulk (ie, the system input voltage), so the feedback current I _FB from the auxiliary winding of the transformer T1 can also represent the line voltage Vbulk. When the power switch Q1 is in the off state, the voltage Vaux on the auxiliary winding of the transformer T1 is proportional to the system output voltage, that is, the voltage Vout on the load Ro, so the feedback voltage V _FB from the auxiliary winding of the transformer T1 can characterize the system output Voltage Vout.

In the flyback power supply system shown in FIG. 2, the system control module 204 may determine the current sampling voltage Vcs on the current sampling resistor Rs and the feedback voltage V _FB and the feedback current I _FB from the auxiliary winding of the transformer T1. The power switch Q1 is controlled to change from the on state to the off state or from the off state to the on state, thereby controlling the system output current, that is, the current Iout flowing through the load Ro.

Specifically, when the power switch Q1 is on, the transformer T1 stores energy, and the primary current Ics of the transformer flowing through the primary winding of the transformer T1 rises linearly, and the current sampling voltage Vcs on the current sampling resistor Rs increases linearly; when the current sampling resistor Rs When the current sampling voltage Vcs reaches the threshold voltage Vth, the system control module 204 controls the power switch Q1 from the on state to the off state. When the power switch Q1 is in the off state, the system control module 204 can determine when the secondary winding of the transformer T1 starts to demagnetize and when to end the demagnetization according to the feedback voltage V _FB from the auxiliary winding of the transformer T1 (ie, the sense of the transformer T1 Secondary conduction time Tons), and control the power switch Q1 from the off state according to the magnitude relationship between the secondary conduction time Tons of the transformer T1 and the feedback current I _FB from the auxiliary winding of the transformer T1 and the overvoltage protection threshold current Iovp_1 Into a conducting state so that the ratio (Tons / Ts) between the secondary conduction time Tons of the transformer T1 and the switching period Ts of the power switch Q1 is less than the overvoltage protection threshold current from the feedback current I _FB from the auxiliary winding of the transformer T1 K1 at Iovp_1 and K2 when the feedback current I _FB from the transformer T1 is greater than the overvoltage protection threshold current Iovp_1.

In some embodiments, the system control module 204 may further include a line voltage-line current conversion circuit 204-2, a second on-time sensing circuit 204-4, a variable ratio control circuit 204-6, an RS trigger, a switch The driving circuit 204-8 and the comparators cmp1 and cmp2, among which: the line voltage-line current conversion circuit 204-2 can sample the line voltage Vbulk and convert it into a line current Iline when the power switch Q1 is on. The current Iline is output to the comparator cmp1. For example, the line voltage-line current conversion circuit 204-2 may convert the line voltage Vbulk into a line current Iline according to the following equation and combining Equation 3:

Here, A represents a conversion coefficient and is constant.

The comparator cmp1 can compare the line current Iline with the overvoltage protection threshold current Iovp_2, and output the comparison result indication signal Line_high to the variable ratio control circuit 204-6. Here, when the line current Iline is less than the overvoltage protection threshold current Iovp_2, the comparison result indication signal Line_high output by the comparator cmp1 is at a low level; when the line current Iline is greater than the overvoltage protection threshold current Iovp_2, the comparison result output by the comparator cmp1 indicates The signal Line_high is high. Here, Iovp_1 = Iovp_2 × A.

The secondary on-time sensing circuit 204-4 can determine when the secondary winding of the transformer T1 starts to demagnetize and when to end the demagnetization according to the feedback voltage V _FB from the auxiliary winding of the transformer T1, that is, to sense the secondary on-time Tons of the transformer T1 And provides the secondary on-time Tons of the transformer T1 to the variable ratio control circuit 204-6.

FIG. 3 shows an example circuit diagram of the secondary on-time sensing module in FIG. 2. As shown in FIG. 3, the comparator cmp4 determines whether the demagnetization of the secondary winding of the transformer T1 starts and ends by comparing whether the feedback voltage V _FB from the auxiliary winding of the transformer T1 is higher than, for example, 0.1V. When the feedback voltage V _FB from the auxiliary winding of the transformer T1 is higher than 0.1V, it indicates that the secondary winding of the transformer T1 starts to demagnetize, that is, the secondary winding of the transformer T1 is in a conducting state; when the feedback voltage V from the auxiliary winding of the transformer T1 _{When FB is} lower than 0.1V, it indicates that the secondary winding of the transformer T1 is demagnetized, that is, the secondary winding of the transformer T1 is in a cut-off state.

The output signal of the secondary on-time sensing module shown in FIG. 3 is a conduction state indication signal State_Tons that indicates whether the secondary winding of the transformer T1 is in a conducting state; when the secondary winding of the transformer T1 is in a conducting state, the conducting state The indication signal State_Tons is at a high level; when the secondary winding of the transformer T1 is in an off state, the on-state indication signal state_Tons is at a low level.

The variable ratio control circuit 204-6 may determine the switching cycle of the power switch Q1 based on Tons / Ts = K1 when the comparison result indicates that the signal Line_high is at a low level, based on Tons / Ts = K1, and based on the determined power switch Q1 Switching cycle controls the power switch Q1 from the off state to the on state, so that the system output current ; When the comparison result indicates that the signal Line_high is at a high level, the secondary conduction time Tons of the transformer T1 is used to determine the switching cycle of the power switch Q1 based on Tons / Ts = K2, and the power switch Q1 is controlled based on the determined switching cycle of the power switch Q1 The off state becomes the on state, so that the system output current .

FIG. 4 shows an example circuit diagram of the variable ratio control module in FIG. 2. As shown in Figure 4, when the on-state indication signal State_Tons output by the second on-time sensing module is at a high level, the current mirror I2 discharges the capacitor C, and the voltage Vramp on the capacitor C decreases linearly. When the on-state indication signal State_Tons output by the test module is at a low level, the current mirror I1 or I1 + I0 charges the capacitor C, and the voltage Vramp on the capacitor C rises linearly; when the voltage Vramp on the capacitor C is higher than the threshold voltage VR, The state control signal CC_ctrl output by the comparator cmp3 is at a high level; when the voltage Vramp on the capacitor C is lower than VR, the state control signal CC_ctrl output by the comparator cmp3 is at a low level.

The comparator cmp2 can compare the current sampling voltage Vcs on the current sampling resistor Rs with the threshold voltage Vth, and output the comparison result indication signal OCP to the RS flip-flop. Here, when the current sampling voltage Vcs on the current sampling resistor Rs is larger than the threshold voltage Vth, the comparison result indicated by the comparator cmp2 indicates that the signal OCP is at a high level; when the current sampling voltage Vcs on the current sampling resistor Rs is smaller than the threshold voltage Vth, The comparison result output by the comparator cmp2 indicates that the signal OCP is at a low level.

The RS trigger can generate a pulse frequency modulation (PFM) signal based on the comparison result indication signal OCP from the comparator cmp2 and the state control signal CC_ctrl from the variable ratio control module 204-6.

The switch driving module 204-8 may generate a driving signal for the power switch Q1 based on the PFM signal from the RS trigger.

Figure 5 shows the secondary winding from the transformer T1 when the secondary on-time sensing module and the variable ratio control module in Figure 2 are implemented as the example circuits shown in Figures 3 and 4 respectively. Timing diagram of the feedback voltage V _FB , the on-state indication signal State_Tons, the voltage Vramp on the capacitor C, the state control signal CC_ctrl, and the PFM signal.

From the analysis above, it can be known that when the line current Iline is less than the overvoltage protection threshold current Iovp_2, the comparison result output by the comparator cmp1 indicates that the signal Line_high is low, and the switching cycle of the power switch Q1 Transformer primary peak current of transformer T1 , System output current ; When the line current Iline is greater than the overvoltage protection threshold current Iovp_2, the comparison result output signal of the comparator cmp1 indicates that the signal Line_high is at a high level, and the switching cycle of the power switch Q1 Transformer primary peak current of transformer T1 , System output current

Fig. 6 is a schematic diagram showing the system output current of the flyback power supply system shown in Fig. 2 as a function of the system input voltage. As shown in FIG. 6, when the line voltage Vbulk is less than the threshold voltage Vth_bulk, the system output current is CC1, and when the line voltage Vbulk voltage is greater than the threshold voltage Vth_bulk, the system output current is CC2. It can be seen that when the system input voltage is too high, the system output current becomes proportionally smaller, which helps to avoid overheating of the system.

It needs to be clear that the invention is not limited to the specific configurations and processes described above and shown in the figures. And, for the sake of brevity, detailed descriptions of known method technologies are omitted here. In the above embodiments, several specific steps have been described and shown as examples. However, the method of the present invention is not limited to the specific steps described and shown, and those skilled in the art can make various changes, modifications, and additions after understanding the spirit of the present invention.

The present invention may be implemented in other specific forms without departing from the spirit and essential characteristics thereof. For example, the algorithms described in particular embodiments may be modified without the system architecture departing from the basic spirit of the invention. Therefore, the current embodiment is considered in all aspects as exemplary rather than limiting, the scope of the present invention is defined by the scope of the attached patent application rather than the above description, and the meanings and equivalents falling within the scope of the patent application All changes within the scope of the substance are thus included in the scope of the present invention.

Claims (10)

    A flyback power supply system includes a transformer, a power switch connected to a primary winding of the transformer, and a system control module for controlling the power switch from an on state to an off state or from an off state to an on state. Wherein, the system control module controls the power switch from an on state to an off state based on a current sampling voltage on a current sampling resistor connected between the power switch and ground, and is based on assistance from the transformer The feedback voltage and feedback current of the winding control the power switch from the off state to the on state. The feedback voltage represents the system output voltage of the flyback power system, and the feedback current represents the System input voltage.         The flyback power supply system according to item 1 of the scope of patent application, wherein the system control module determines the on-time of the secondary winding of the transformer based on the feedback voltage, and based on the feedback current and a first threshold The magnitude relationship between the currents and the secondary winding on-time of the transformer controls the power switch from an off state to an on state, so that the secondary winding on-time of the transformer is different from the switching cycle of the power switch. The ratio between them is a first value when the feedback current is less than the first threshold current and a second value when the feedback current is greater than the first threshold current.         The flyback power supply system according to item 1 of the patent application scope, wherein when the current sampling voltage is greater than a first threshold voltage, the system control module controls the power switch from an on state to an off state.         The flyback power supply system according to item 2 of the scope of patent application, wherein the system control module includes a second on-time sensing circuit based on the feedback voltage and the second on-time sensing circuit. The magnitude relationship between the threshold voltages generates a conduction state indication signal that indicates whether the secondary winding of the transformer is in a conduction state.         The flyback power supply system according to item 2 or 4 of the scope of patent application, wherein the system control module includes a line voltage-line current conversion circuit and a first comparator, and the line voltage-line current conversion circuit is based on A predetermined conversion relationship converts the feedback current into a line current, and the first comparator generates a first comparison result indication signal based on a magnitude relationship between the line current and a second threshold current, and the second threshold current It is proportional to the first threshold voltage.         The flyback power supply system according to item 5 of the scope of patent application, wherein the system control module includes a variable ratio control module, and the variable ratio control module includes first to third current mirrors and capacitors. And a second comparator, wherein the first current mirror charges the capacitor under the control of the first comparison result indication signal, and the second current mirror is under the control of the on-state indication signal To charge the capacitor, the third current mirror discharges the capacitor under the control of the on-state indication signal, and the second comparator is based on the voltage on the capacitor and a third threshold voltage The magnitude relationship between them generates a second comparison result indication signal for controlling the power switch from an off state to an on state.         The flyback power supply system according to item 6 of the patent application scope, wherein the system control module further includes a third comparator and an RS trigger, and the third comparator is based on the current sampling voltage and the The magnitude relationship between the first threshold voltages generates a third comparison result indication signal, and the RS trigger generates a control signal for controlling the power switch from turning on based on the second comparison result indication signal and the third comparison result indication signal. A pulse frequency modulation signal whose state changes to an off state or from an off state to an on state.         The flyback power supply system according to item 1 of the scope of patent application, wherein the feedback voltage is obtained by dividing the voltage on the auxiliary winding of the transformer when the power switch is in an on state.         A control method for a flyback power system includes a transformer and a power switch connected to a primary winding of the transformer, and the control method includes: based on being connected between the power switch and ground The current sampling voltage on the current sampling resistor controls the power switch from the on state to the off state, and controls the power switch from the off state to the on state based on the feedback voltage and the feedback current from the auxiliary winding of the transformer, The feedback voltage represents a system output voltage of the flyback power system, and the feedback current represents a system input voltage of the flyback power system.         The control method according to item 9 of the scope of patent application, wherein the on-time of the secondary winding of the transformer is determined based on the feedback voltage, and based on a magnitude relationship between the feedback current and a first threshold current, and The on time of the secondary winding of the transformer controls the power switch from the off state to the on state, so that the ratio between the on time of the secondary winding of the transformer and the switching cycle of the power switch is less than the feedback current. The first threshold current is a first value when the feedback current is greater than the first threshold current and is a second value.