TWI888210B - Flyback switching power supply and its control circuit - Google Patents

Abstract

A flyback switching power supply and a control circuit thereof are provided, wherein the flyback switching power supply includes a transformer and a transistor connected to a primary winding of the transformer, and the control circuit for the flyback switching power supply is configured to: generate a charge switch control signal and a discharge switch control signal based on an output feedback indication signal related to an output voltage of the flyback switching power supply; control the charging and discharging of a first capacitor based on the charge switch control signal and the discharge switch control signal; generate a turn-off threshold signal for controlling the transistor to change from a conducting state to an off state based on a voltage on the first capacitor; and generate a pulse width modulation control signal for controlling the conduction and shutoff of the transistor based on the turn-off threshold signal and the output feedback indication signal.

Description

Flyback switching power supply and its control circuit

The present invention relates to the field of circuits, and more specifically to a flyback switching power supply and its control circuit.

A switching power supply, also known as an alternating current power supply or a switching converter, is a type of power supply. The function of a switching power supply is to convert a reference voltage into the voltage or current required by the user through different forms of architecture (for example, fly-back architecture, buck architecture, or boost architecture, etc.).

According to a control circuit for a flyback switching power supply according to an embodiment of the present invention, the flyback switching power supply includes a transformer and a transistor connected to the primary winding of the transformer, and the control circuit is configured to: generate a charge switch control signal and a discharge switch control signal based on an output feedback indication signal related to the output voltage of the flyback switching power supply; control the charging and discharging of the first capacitor based on the charge switch control signal and the discharge switch control signal; generate a turn-off threshold signal for controlling the transistor to change from the on state to the off state based on the voltage on the first capacitor; and generate a pulse width modulation control signal for controlling the on and off of the transistor based on the turn-off threshold signal and the output feedback indication signal.

The flyback switching power supply according to the embodiment of the present invention includes the above-mentioned control circuit.

△i1: frequency-jittering current

1,2: Comparator

100: Flyback switching power supply

102: AC rectifier module

104: Voltage-dividing feedback module

106: Isolation/non-isolation feedback signal transmission module

108: Feedback signal receiving and identification module

110,110-1,110-2,110-3,110-4: Threshold voltage/current generation module

112: Primary current sampling module

114: Switch control logic module

C1,C2,C3,Cf,Cf1,C1i,C1n:capacitors

Cin: input capacitance

Clk_J: frequency-jittering current control signal

Clock_h: high frequency discharge control signal

Cout1: first output capacitor

CS: primary side current flow detection signal

CS_pk,CS_pk1,CS_pk2,CS_pk1’,CS_pk2’: turn off threshold signal

CS_SS1, CS_SS2: shutdown threshold voltage

CV1: System output voltage

D1: Secondary side-flow diode

FB_ob: Charging switch anti-positive signal

FB_oh: Feedback narrow pulse signal

FB_ohd, FB_up: charging switch control signal

FB_os: discharge switch reset signal

FB_osb, FB_dw: discharge switch control signal

FB_pulse: output feedback indication signal

FB_rec: Output feedback characteristic signal

FB_req: output voltage feedback signal

FB_s: Threshold voltage sampling signal

FB1: Output voltage characteristic signal

i2,i3,I3’: current

Ipk: primary current

Np: Number of turns of primary coil

Ns1: Number of turns of the first coil on the secondary side

PWM: Pulse Width Modulation

Q1: Transistor

R1, Rf, Rf1: resistance

S0,S1,S2,S4,S5,S6,S7,S8: switch

S0_SEL: basic switch selection signal

S11_SEL, S1n_SEL: Additional switch selection signal

T_pulse: Feedback sampling enable signal

T1: Transformer

Tri_off: Falling edge trigger signal

V11: Clamping voltage

Va1, Va2, Va4, Vramp, Vramp1, Vramp2: voltage

Vin_ac: AC input voltage

Vin_rec: DC input voltage

Vref1: reference voltage

The present invention can be better understood from the following description of the specific implementation of the present invention in conjunction with the drawings, wherein:

FIG1 shows a schematic circuit diagram of a flyback switching power supply according to an embodiment of the present invention.

FIG2 shows a schematic circuit diagram of an example implementation of the threshold voltage/current generation module shown in FIG1 .

FIG3 shows the working waveforms of multiple signals when the threshold voltage/current generation module shown in FIG2 uses a cycle-by-cycle sampling method to generate a threshold voltage sampling signal.

FIG4 shows the working waveforms of multiple signals when the threshold voltage/current generation module shown in FIG2 uses a multi-cycle sampling method to generate a threshold voltage sampling signal.

FIG5 shows a schematic circuit diagram of another example implementation of the threshold voltage/current generation module shown in FIG1 .

FIG6 shows the operating waveforms of multiple signals in the threshold voltage/current generation module shown in FIG5.

FIG7 shows a schematic circuit diagram of another example implementation of the threshold voltage/current generating module shown in FIG1.

FIG8 shows a waveform diagram of a turn-off threshold signal generated when the frequency-jitter switch control circuit shown in FIG7 uses an additional switch selection signal whose signal envelope is a low-frequency triangular wave to gate the capacitor.

FIG9 shows a waveform diagram of a turn-off threshold signal generated when the frequency-jittering switch control circuit shown in FIG7 uses an additional switch selection signal generated in a pseudo-random manner to gate the capacitor.

FIG10 shows a schematic circuit diagram of another example implementation of the threshold voltage/current generation module shown in FIG1 .

FIG11 shows a waveform diagram of the turn-off threshold signal generated when the threshold voltage/current generation module shown in FIG10 uses a low-frequency triangular wave frequency-jittering current.

FIG12 shows a waveform diagram of the shutdown threshold signal generated when the threshold voltage/current generation module shown in FIG10 uses a pseudo-random frequency-jittering current.

The features and exemplary embodiments of various aspects of the present invention are described in detail below. In the detailed description below, many specific details are proposed in order to provide a comprehensive understanding of the present invention. However, it is obvious to those skilled in the art that the present invention can be implemented without some of these specific details. The following description of the embodiments is only for providing a better understanding of the present invention by illustrating examples of the present invention. The present 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 present invention. In the drawings and the following description, known structures and techniques are not shown in order to avoid unnecessary ambiguity of the present invention. In addition, it should be noted that the term "A is connected to B" used here can mean "A is directly connected to B" or "A is indirectly connected to B via one or more other components".

With the development of switching power supply technology, various switching power supply topologies have emerged. Among them, the switching power supply topology that controls the on and off of the transistor on the primary side of the transformer based on the feedback information from the secondary side of the transformer (i.e., the flyback architecture) is becoming more and more widely used due to its simple control circuit and good output dynamic characteristics.

FIG1 shows a schematic circuit diagram of a flyback switching power supply according to an embodiment of the present invention. In the flyback switching power supply 100 shown in FIG1 , the AC rectifier module 102 and the input capacitor Cin rectify and filter the AC input voltage Vin_ac to obtain the DC input voltage Vin_rec; the primary winding of the transformer T1 stores energy when the transistor Q1 is in the on state, and transmits energy to the secondary winding of the transformer T1 through the mutual inductance effect when the transistor Q1 is in the off state; the voltage-dividing feedback module 104 divides the system output voltage CV1 of the flyback switching power supply 100 to obtain The output voltage characteristic signal FB1 is obtained by the comparator 1; the comparator 1 compares the output voltage characteristic signal FB1 with the reference voltage Vref1 to obtain the output voltage feedback signal FB_req; the isolated/non-isolated feedback signal transmission module 106 generates the output feedback characteristic signal FB_rec based on the output voltage feedback signal FB_req and transmits the output feedback characteristic signal FB_rec to the primary side of the transformer T1; the feedback signal receiving and identifying module 108 generates the output feedback characteristic signal FB_rec based on the output feedback characteristic signal FB_rec. The output feedback indication signal FB_pulse is generated (the output feedback indication signal FB_pulse is related to the system output voltage CV1); the threshold voltage/current generation module 110 generates a turn-off threshold signal CS_pk for controlling the transistor Q1 to change from the on state to the off state based on the output feedback indication signal FB_pulse (the turn-off threshold signal CS_pk can be a current signal or a voltage signal); the primary current sampling module 112 generates a current signal based on the current flowing through the transformer T1. The primary current Ipk of the primary winding generates a primary current sensing signal CS; the comparator 2 compares the primary current sensing signal CS with the turn-off threshold signal CS_pk to obtain a falling edge trigger signal Tri_off for controlling the transistor Q1 to change from an on state to an off state; the switch control logic module 114 generates a pulse width modulation (PWM) control signal for controlling the on and off of the transistor Q1 based on the output feedback indication signal FB_pulse and the falling edge trigger signal Tri_off. It should be noted that the feedback signal receiving and identifying module 108, the threshold voltage/current generating module 110, the primary current sampling module 112, the switch control logic module 114, and the comparator 2 can be located in the control circuit of the flyback switching power supply 100.

FIG. 2 shows a schematic circuit diagram of an example implementation of the threshold voltage/current generation module shown in FIG. 1 . In the threshold voltage/current generation module 110-1 shown in FIG2 , the narrow pulse generation unit generates the feedback narrow pulse signal FB_oh by performing narrow pulse interception on the output feedback indication signal FB_pulse when the output feedback indication signal FB_pulse is at a high level; the multi-cycle counting unit generates the feedback sampling enable signal T_pulse by performing cycle counting on the output feedback indication signal FB_pulse (for example, whenever the cycle count of the output feedback indication signal FB_pulse reaches a predetermined count value, a high-level feedback sampling enable signal T_pulse is generated, and the output feedback indication signal FB_pulse is cycle counted again); the sampling unit generates the feedback sampling enable signal T_pulse by performing cycle counting on the output feedback indication signal FB_pulse; The threshold voltage sampling signal FB_s is generated by sampling and enabling the feedback narrow pulse signal FB_oh based on the feedback sampling enable signal T_pulse; the delay unit generates the charging switch control signal FB_ohd by delaying the feedback narrow pulse signal FB_oh; the high-frequency pulse generation unit generates the high-frequency discharge control signal Clock_h by counting the high-frequency clock when the output feedback indication signal FB_pulse is at a low level, wherein the frequency of the high-frequency discharge control signal Clock_h is a multiple of the frequency of the output feedback indication signal FB_pulse; the logic or unit generates the charging switch control signal FB_ohd and the high-frequency discharge control signal Clock_h by The discharge switch reset signal FB_os is generated by performing a logical OR operation; the gate generates the discharge switch control signal FB_osb by performing a logical inverse operation on the discharge switch reset signal FB_os; when the charge switch control signal FB_ohd is at a high level, switches S1 and S2 are in the on state, Va1=Va2, capacitor C1 is quickly charged to V1, and the buffer buffers the voltage Va2 on capacitor C1 to obtain a voltage Vramp1=Va2; when the charge switch control signal FB_ohd is at a low level, switch S4 switches between the on state and the off state at a high speed under the control of the high-frequency discharge control signal Clock_h, and the voltage Va2 on capacitor C1 passes through resistor R1 The voltage is periodically discharged to capacitor C2 until the output feedback indication signal FB_pulse changes from a low level to a high level. When the threshold voltage sampling signal FB_s is at a high level, switch S5 is in the on state, and the RC filter circuit composed of resistor Rf and capacitor Cf filters the voltage Vramp1 to obtain the turn-off threshold voltage CS_SS. 1, and keep the off threshold voltage CS_SS1 on the capacitor Cf; when the threshold voltage sampling signal FB_s is at a low level, the switch S5 is in the off state, the off threshold voltage CS_SS1 on the capacitor Cf remains unchanged, and the proportional current/voltage conversion unit generates the off threshold signal CS_pk1 based on the off threshold voltage CS_SS1. It should be noted that the capacitance of capacitor C1 is usually tens of times that of capacitor C2, so that the voltage Va2 on capacitor C1 can be slowly and periodically released to capacitor C2, thereby changing the voltage Va4 on capacitor C2 with the periodic changes of the output feedback indication signal FB_pulse.

FIG3 shows a working waveform diagram of multiple signals when the threshold voltage/current generation module shown in FIG2 adopts a cycle-by-cycle sampling method to generate a threshold voltage sampling signal FB_s. As shown in Figure 3, the longer the period T1...Tn of the output feedback indication signal FB_pulse is, the lower the lowest point of the voltage Vramp1 is; before the voltage Vramp1 is reset to V1 each time, the threshold voltage sampling signal FB_s changes from a low level to a high level, the switch S5 changes from an off state to an on state, and the RC filter circuit composed of a resistor Rf and a capacitor Cf filters the voltage Vramp1 to obtain a turn-off threshold voltage CS_SS1 which is held on the capacitor Cf; when the threshold voltage sampling signal FB_s is at a low level, the switch S5 is in a turn-off state, and the turn-off threshold voltage CS_SS1 on the capacitor Cf remains basically unchanged.

FIG4 shows the working waveforms of multiple signals when the threshold voltage/current generation module shown in FIG2 adopts a multi-cycle sampling method to generate the threshold voltage sampling signal FB_s. As shown in FIG4, every m (m≧2) cycles of the output feedback indication signal FB_pulse, the threshold voltage sampling signal FB_s changes from a low level to a high level under the control of the feedback sampling enable signal T_pulse, and the shutdown threshold voltage CS_SS1 is updated every m cycles under the control of the threshold voltage sampling signal FB_s; when the threshold voltage sampling signal FB_s is at a low level, the switch S5 is in the off state, and the shutdown threshold voltage CS_SS1 on the capacitor Cf remains basically unchanged.

FIG5 shows a schematic circuit diagram of another example implementation of the threshold voltage/current generation module shown in FIG1. In the threshold voltage/current generation module 110-2 shown in FIG5, the multi-cycle counting unit generates the feedback sampling enable signal T_pulse by counting the output feedback indication signal FB_pulse periodically (for example, whenever the cycle count of the output feedback indication signal FB_pulse reaches a predetermined count value, a high-level feedback sampling enable signal T_pulse is generated). ulse, and re-count the cycle of the output feedback indication signal FB_pulse); the sampling unit generates a threshold voltage sampling signal FB_s by enabling the output feedback indication signal FB_pulse based on the feedback sampling enable signal T_pulse; the narrow pulse generation and delay unit generates a threshold voltage sampling signal FB_s by enabling the output feedback indication signal FB_pulse when the output feedback indication signal FB_pulse is During the high level period, the output feedback indication signal FB_pulse is intercepted by a narrow pulse and the intercepted narrow pulse is delayed to generate the charging switch control signal FB_up; the high-frequency pulse generating unit generates the high-frequency discharge control signal Clock_h by counting the high-frequency clock when the output feedback indication signal FB_pulse is at a low level, wherein the high-frequency discharge The frequency of the control signal Clock_h is a multiple (e.g., several dozen times) of the frequency of the output feedback indication signal FB_pulse; the gate generates a charge switch inversion signal FB_ob by performing a logical inversion operation on the charge switch control signal FB_up; the logic or unit generates a charge switch inversion signal FB_ob by performing a logical inversion operation on the charge switch control signal FB_up and the high-frequency discharge control signal Clock_h. The discharge switch control signal FB_dw is generated by performing a logical OR operation; when the charge switch control signal FB_up is at a high level, the switch S6 is in an on state, the current i2 charges the capacitor C3 through the switch S6, and the voltage Vramp on the capacitor C3 rises rapidly to the clamping voltage V11; when the charge switch control signal FB_up changes from a high level to a low level, the discharge switch control signal FB_dw changes from a low level to a high level, the switch S7 changes from an off state to an on state, and the pull-down current i3 discharges the capacitor C3. Since the discharge switch control signal FB_dw is a high-frequency discharge control signal Clock_h when the charge switch control signal FB_up is at a low level, the voltage Vramp on the capacitor C3 rises rapidly to the clamping voltage V11. When the signal FB_up is at a low level, the voltage Vramp on the capacitor C3 is slowly decreased; the buffer buffers the voltage Vramp on the capacitor C3 to obtain the voltage Vramp2=Vramp; when the threshold voltage sampling signal FB_s is at a high level, the switch S8 is in the on state, and the RC filter circuit composed of the resistor Rf1 and the capacitor Cf1 filters the voltage Vramp2 to obtain the turn-off threshold voltage. The voltage CS_SS2 is generated and the turn-off threshold voltage CS_SS2 is maintained on the capacitor Cf1; when the threshold voltage sampling signal FB_s is at a low level, the switch S8 is in the off state, the turn-off threshold voltage CS_SS2 on the capacitor Cf1 remains unchanged, and the proportional current/voltage conversion unit generates the turn-off threshold signal CS_pk2 based on the turn-off threshold voltage CS_SS2.

FIG. 6 shows operating waveforms of multiple signals in the threshold voltage/current generating module shown in FIG. 5 . As shown in Figure 6, the longer the period T1...Tn of the output feedback indication signal FB_pulse is, the lower the lowest point of the voltage Vramp2 is; before the voltage Vramp2 is reset to V11 each time, the threshold voltage sampling signal FB_s changes from a low level to a high level, the switch S8 changes from an off state to an on state, and the RC filter circuit composed of the resistor Rf1 and the capacitor Cf1 filters the voltage Vramp2 to obtain the off threshold voltage CS_SS2 which is maintained on the capacitor Cf1; when the threshold voltage sampling signal FB_s is at a low level, the switch S8 is in an off state, and the off threshold voltage CS_SS2 on the capacitor Cf1 remains basically unchanged.

Since the flyback switching power supply will have high-frequency noise signals in actual applications and these signals will radiate a large amount of electromagnetic waves through the transformer and some inductive components, in order to reduce the energy of electromagnetic waves radiated to the outside, frequency jitter control can be added to the flyback switching power supply to reduce electromagnetic radiation, so that it meets the electromagnetic radiation standards of various electronic products.

Fig. 7 shows a schematic circuit diagram of another exemplary implementation of the threshold voltage/current generating module shown in Fig. 1. The difference between the threshold voltage/current generating module 110-3 shown in Fig. 7 and the threshold voltage/current generating module 110-1 shown in Fig. 2 is that a frequency jitter control unit is added. As shown in FIG7 , the frequency jittering control unit includes a switch S0, a frequency jittering switch control circuit, and a frequency jittering switch capacitor array. The frequency jittering switch control circuit is configured to generate a basic switch selection signal S0_SEL and additional switch selection signals S11_SEL to S1n_SEL. The basic switch selection signal S0_SEL is used to control the on and off of the switch S0, and the additional switch selection signals S11_SEL to S1n_SEL are used to select one of the n capacitors in the frequency jittering switch capacitor array. When the basic switch selection signal S0_SEL is at a low level and any one of the additional switch selection signals S11_SEL to S1n_SEL, S1i_SEL, is at a high level, the capacitor C1 is connected in series with the capacitor C1i (not shown in the figure) selected in the frequency jitter switch capacitor array to form a combined capacitor. The combined capacitor formed by the series connection of the capacitor C1 and the capacitor C1i is charged under the control of the charging switch control signal FB_ohd, and discharged under the control of the discharging switch control signal FB_osb.

FIG8 shows a waveform diagram of the turn-off threshold signal generated when the frequency-jitter switch control circuit shown in FIG7 uses an additional switch selection signal with a signal envelope frequency of a low-frequency triangular wave to gate the capacitor. As shown in FIG8, when the frequency-jitter switch control circuit shown in FIG7 uses additional switch selection signals S11_SEL to S1n_SEL with a signal envelope frequency of a low-frequency triangular wave to gate the capacitors C1i to C1n in the frequency-jitter switch capacitor array, the waveform of the turn-off threshold signal CS_pk1’ generated is a regularly fluctuating low-frequency triangular wave.

FIG9 shows a waveform diagram of a turn-off threshold signal generated when the frequency-jittering switch control circuit shown in FIG7 uses an additional switch selection signal generated in a pseudo-random manner to gate the capacitor. As shown in FIG9, when the frequency-jittering switch control circuit shown in FIG7 uses additional switch selection signals S11_SEL to S1n_SEL generated in a pseudo-random manner to gate the capacitor in the frequency-jittering switch capacitor array, the waveform of the turn-off threshold signal CS_pk1' generated in the end is jittered in a pseudo-random manner.

FIG10 shows a schematic circuit diagram of another example implementation of the threshold voltage/current generation module shown in FIG1. The difference between the threshold voltage/current generation module 110-4 shown in FIG10 and the threshold voltage/current generation module 110-2 shown in FIG5 is that a frequency jitter control unit is added, wherein the frequency jitter control unit includes a low-frequency clock encoding circuit and a frequency jitter current generation circuit, the low-frequency clock encoding circuit is configured to generate a frequency jitter current control signal Clk_J, and the frequency jitter current generation circuit is configured to generate a frequency jitter current △i1 based on the frequency jitter current control signal Clk_J. When switch S6 is in the off state and switch S7 is in the on state, the current I3’=I3+△i1 discharges the capacitor C3.

FIG11 shows a waveform diagram of the shutdown threshold signal generated when the threshold voltage/current generation module shown in FIG10 adopts a low-frequency triangular wave frequency-dithering current. FIG12 shows a waveform diagram of the shutdown threshold signal generated when the threshold voltage/current generation module shown in FIG10 adopts a pseudo-random frequency-dithering current. As shown in FIG11, when a low-frequency triangular wave frequency-dithering current is adopted, the waveform of the shutdown threshold signal CS_pk2' is also a low-frequency triangular wave. As shown in FIG11, when a pseudo-random frequency-dithering current is adopted, the waveform of the shutdown threshold signal CS_pk2' is pseudo-randomly jittered.

In summary, by changing the operating frequency of the flyback switching power supply 100 cycle by cycle through the turn-off threshold signal generated by the low-frequency triangle wave or pseudo-random frequency jitter, the harmonic component of the system can be increased and the electromagnetic radiation energy of the system to the outside can be reduced.

The present invention is not limited to a flyback switching power supply with a single output on the secondary side, but can also be applied to a flyback switching power supply with multiple outputs, or implemented in a specific form of other topologies without departing from its spirit and essential characteristics. For example, the algorithm described in a specific embodiment can be modified, and the system architecture does not deviate from the basic spirit of the present invention. Therefore, the present embodiments are regarded as exemplary rather than restrictive in all aspects, and the scope of the present invention is defined by the attached claims rather than the above description, and all changes that fall within the meaning and scope of equivalents of the claims are therefore included in the scope of the present invention.

1,2: Comparator

100: Flyback switching power supply

102: AC rectifier module

104: Voltage-dividing feedback module

106: Isolation/non-isolation feedback signal transmission module

108: Feedback signal receiving and identification module

110: Threshold voltage/current generation module

112: Primary current sampling module

114: Switch control logic module

Cin: input capacitance

Cout1: first output capacitor

CS: primary side current flow detection signal

CS_pk: Shutdown threshold signal (can be a current signal or a voltage signal)

CV1: System output voltage

D1: Secondary side-flow diode

FB_pulse: output feedback indication signal

FB_rec: Output feedback characteristic signal

FB_req: output voltage feedback signal

FB1: Output voltage characteristic signal

Ipk: primary current

Np: Number of turns of primary coil

Ns1: Number of turns of the first coil on the secondary side

PWM: Pulse Width Modulation

Q1: Transistor

T1: Transformer

Tri_off: Falling edge trigger signal

Vin_ac: AC input voltage

Vin_rec: DC input voltage

Vref1: reference voltage

Claims (12)

A control circuit for a flyback switching power supply, wherein the flyback switching power supply includes a transformer and a transistor connected to a primary winding of the transformer, and the control circuit is configured to: generate a charge switch control signal and a discharge switch control signal based on an output feedback indication signal related to an output voltage of the flyback switching power supply; control the charging and discharging of a first capacitor based on the charge switch control signal and the discharge switch control signal; and generate a control signal for controlling the discharge of the first capacitor based on a voltage on the first capacitor. A turn-off threshold signal for the transistor to change from the on state to the off state; and based on the turn-off threshold signal and the output feedback indication signal, a pulse width modulation control signal for controlling the on and off of the transistor is generated; a buffer voltage is generated by buffering the voltage on the first capacitor; a turn-off threshold voltage for controlling the transistor to change from the on state to the off state is generated by filtering the buffer voltage; and the turn-off threshold signal is generated by converting the turn-off threshold voltage. The control circuit as described in claim 1 is further configured to: control the first capacitor to discharge to the second capacitor based on the discharge switch control signal. The control circuit as described in claim 1 is further configured to: generate a feedback narrow pulse signal by performing narrow pulse interception on the output feedback indication signal when the output feedback indication signal is at a high level; and generate the charging switch control signal by delaying the feedback narrow pulse signal. The control circuit as described in claim 3 is further configured to: generate a high-frequency discharge control signal by performing a high-frequency clock count during the period when the output feedback indication signal is at a low level, wherein the frequency of the high-frequency discharge control signal is a multiple of the frequency of the output feedback indication signal; generate a discharge switch reset signal by performing a logical or operation on the charge switch control signal and the high-frequency discharge control signal; and generate the discharge switch control signal by performing a logical inverse operation on the discharge switch reset signal. The control circuit as described in claim 1 is further configured to: generate a basic switch control signal and an additional switch control signal; based on the basic switch control signal, control the connection and disconnection between the first capacitor and the circuit reference ground; and based on the additional switch control signal, select any one capacitor in the frequency-jittering switch capacitor array to be connected in series with the first capacitor. The control circuit as described in claim 1 is further configured to: generate a feedback narrow pulse signal by performing narrow pulse interception on the output feedback indication signal during the period when the output feedback indication signal is at a high level; generate a feedback sampling enable signal by performing cycle counting on the output feedback indication signal; generate a threshold voltage sampling signal by performing sampling enable on the feedback narrow pulse signal based on the feedback sampling enable signal; and generate the shutdown threshold voltage by performing sampling filtering on the buffer voltage during the period when the threshold voltage sampling signal is at a high level. The control circuit as described in claim 1 is further configured to: control the first current to discharge the first capacitor based on the discharge switch control signal. The control circuit as described in claim 1 is further configured to generate the charging switch control signal by intercepting a narrow pulse of the output feedback indication signal and delaying the intercepted narrow pulse when the output feedback indication signal is at a high level. The control circuit as described in claim 8 is further configured to: generate a high-frequency discharge control signal by performing a high-frequency clock count during the period when the output feedback indication signal is at a low level, wherein the frequency of the high-frequency discharge control signal is a multiple of the frequency of the output feedback indication signal; generate a charge switch inversion signal by performing a logical inverse operation on the charge switch control signal; and generate the discharge switch control signal by performing a logical or operation on the charge switch inversion signal and the high-frequency discharge control signal. The control circuit as described in claim 1 is further configured to: generate a feedback sampling enable signal by counting the cycles of the output feedback indication signal; generate a threshold voltage sampling signal by enabling sampling of the output feedback indication signal based on the feedback sampling enable signal; and generate the shutdown threshold voltage by sampling and filtering the buffer voltage during the period when the threshold voltage sampling signal is at a high level. The control circuit as described in claim 7 is further configured to: generate a frequency-jittering current control signal; generate a frequency-jittering current based on the frequency-jittering current control signal; and control the first current and the frequency-jittering current to discharge the first capacitor simultaneously based on the discharge switch control signal. A flyback switching power supply, comprising a control circuit as described in any one of claims 1 to 11.