Flyback switching power supply and primary and secondary control circuit

Abstract

The application provides a flyback switching power supply and a primary side and secondary side control circuit thereof. The flyback switching power supply comprises a transformer, a primary side control circuit connected to a primary side winding of the transformer, and a universal serial bus (USB) interface connected to a secondary side winding of the transformer. The secondary side control circuit used in the flyback switching power supply is connected to the secondary side winding of the transformer, and is configured to, in the case of being in a normal working state: enter a zero-power standby state when detecting that the USB interface is not electrically connected with an electronic device, and send a first predetermined code through the transformer to inform the primary side control chip to also enter the zero-power standby state.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of circuit, more particularly to a flyback switching power supply and its primary side and secondary side control circuit. BACKGROUND

[0002] In recent years, with the rapid increase of people's dependence on electronic devices, the output power of electronic devices is getting larger and larger, the charging time is getting shorter and shorter, and the power conversion efficiency is getting higher and higher. However, most of the time, the electronic devices are in standby state without charging, and the charger is often not powered off and in "no-load" standby state. Although the standby power consumption indicator is not large, the total power consumption accumulated by long time standby is considerable. How to reduce this part of power consumption is a topic that the industry is increasingly concerned about. SUMMARY

[0003] According to the embodiment of the present application, the secondary side control circuit used in the flyback switching power supply includes a transformer, a primary side control circuit connected to the primary side winding of the transformer, and a universal serial bus (USB) interface connected to the secondary side winding of the transformer. The secondary side control circuit is connected to the secondary side winding of the transformer and is configured to, in the case of being in a normal working state: enter a zero-power standby state when detecting that the USB interface is not electrically connected with the electronic device, and send a first predetermined code through the transformer to inform the primary side control chip to also enter the zero-power standby state.

[0004] According to the embodiment of the present application, the primary side control circuit used in the flyback switching power supply includes a transformer and a secondary side control circuit connected to the secondary side winding of the transformer. The primary side control circuit is connected to the primary side winding of the transformer and is configured to, in the case of being in a normal working state: judge whether the secondary side winding of the transformer ends demagnetization based on a demagnetization detection signal representing the demagnetization condition of the secondary side winding of the transformer; and in the case of judging that the secondary side winding of the transformer ends demagnetization, start recognizing a predetermined code from the secondary side control circuit when detecting that a primary side current detection signal representing the current flowing through the primary side winding of the transformer is a negative voltage, the amplitude of the primary side current detection signal is lower than a second predetermined voltage, and the duration of the primary side current detection signal being a negative voltage reaches a third predetermined time length.

[0005] According to the embodiment of the present application, the flyback switching power supply includes: a transformer; the above-mentioned secondary side control circuit connected to the secondary side winding of the transformer; and the above-mentioned primary side control circuit connected to the primary side winding of the transformer. BRIEF DESCRIPTION OF DRAWINGS

[0006] The present application can be better understood from the following description of specific embodiments thereof, given by way of example and with reference to the accompanying drawings, in which:

[0007] Figure 1 A structure diagram of a flyback switching power supply according to an embodiment of the present application is shown.

[0008] Figure 2 A structure diagram of a flyback switching power supply according to an embodiment of the present application is shown. Figure 1 A waveform diagram of a plurality of signals of the flyback switching power supply shown in the normal working state is shown.

[0009] Figure 3 A structure diagram of a flyback switching power supply according to an embodiment of the present application is shown. Figure 1 An example flow chart of the working state conversion process of the flyback switching power supply shown is shown.

[0010] Figure 4 A structure diagram of a flyback switching power supply according to an embodiment of the present application is shown. Figure 1 A structure diagram of a flyback switching power supply according to an embodiment of the present application is shown.

[0011] Figure 5 A structure diagram of a flyback switching power supply according to an embodiment of the present application is shown. Figure 1 A waveform diagram of a plurality of signals of the flyback switching power supply shown in the normal working state is shown.

[0012] Figure 6 A structure diagram of a flyback switching power supply according to an embodiment of the present application is shown. Figure 1 A waveform diagram of a plurality of signals of the flyback switching power supply shown in the normal working state is shown.

[0013] Figure 7 A structure diagram of a flyback switching power supply according to an embodiment of the present application is shown. Figure 1 A waveform diagram of a plurality of signals of the flyback switching power supply shown in the normal working state is shown. DETAILED DESCRIPTION

[0014] The features and exemplary embodiments of the various aspects of the present application 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 application. However, it will be apparent to one of ordinary skill in the art that the present application can be practiced without some or all of these specific details. The description of the embodiments is merely illustrative of the present application and is not intended to limit the present application to any particular configuration or algorithm unless otherwise specified. The present application covers any and all modifications, variations, and alternatives that fall within the spirit and scope of the present application. In the drawings and the description below, like numerals indicate like elements in a figure, and like elements across different figures are indicated by like numerals. In addition, the use of the term "connected" herein can mean "directly connected," "indirectly connected," or "connected via one or more other elements."

[0015] According to the energy standard, when the standby power consumption is less than 5 milliwatts, it can be considered as zero standby power consumption. Based on this, the flyback switching power supply according to the embodiment of the present application can achieve ultra-low standby power consumption without charging the electronic device, thereby greatly reducing energy waste.

[0016] Figure 1 The structure diagram of the flyback switching power supply according to the embodiment of the present application is shown. As shown in the figure, Figure 1 The flyback switching power supply 100 includes a transformer T1 (including a primary winding Np, a secondary winding Ns, and an auxiliary winding Naux), a primary control chip 102 with a built-in power switch tube Q1, a secondary control chip 104 with a built-in synchronous rectification switch tube Q2, and a universal serial bus (USB) / TYPE-C interface.

[0017] As shown in the figure, Figure 1 In the case that the flyback switching power supply 100 is in a normal working state, when the secondary control chip 104 detects that there is no electrical connection between the USB / TYPE-C interface and the electronic device through the DP / DN or CC1 / CC2 signal line, it enters a zero-power standby state, and sends a predetermined code 1 through the transformer T1 to inform the primary control chip 102 to also enter a zero-power standby state. At this time, most of the functional modules in the primary control chip 102 and the secondary control chip 104 stop working, only a few functional modules are in working state, and the entire circuit system enters a zero-power standby state. In the case that the flyback switching power supply 100 is in a zero-power standby state, when the secondary control chip 104 detects that there is an electrical connection between the USB / TYPE-C interface and the electronic device through the DP / DN or CC1 / CC2 signal line, it exits the zero-power standby state, and sends a predetermined code 2 through the transformer T1 to inform the primary control chip 102 to also exit the zero-power standby state. At this time, the entire circuit system exits the zero-power standby state within less than 150 ms.

[0018] As shown in the figure, Figure 1As shown, in the flyback switching power supply 100, when the AC input voltage is connected, the chip supply capacitor Cdd connected to the chip supply pin VDD of the primary side control chip 102 is charged through the electromagnetic interference (EMI) filter circuit, the AC high voltage starting circuit, and the power supply module (PS) inside the primary side control chip 102; when the voltage on the chip supply capacitor Cdd (i.e., the chip supply voltage of the primary side control chip 102) is higher than the under voltage lock out (UVLO) voltage of the primary side control chip 102, the primary side control chip 102 starts to work; when the system output voltage Vo is low, the body diode D2 of the power switch Q2 inside the secondary side control chip 104 is in the on state, and the system output voltage Vo gradually rises; when the system output voltage Vo is higher than the UVLO voltage of the secondary side control chip 104, the secondary side control chip 104 starts to work.

[0019] As shown in FIG. 1, the flyback switching power supply 100 includes the following components: Figure 1 As shown, the working process of the flyback switching power supply 100 in the normal working state includes the following stages:

[0020] In the first stage, when the power switch Q1 is in the on state, the primary winding Np of the transformer T1 stores energy, and the secondary output capacitor Co provides energy for the secondary side control chip 104 and the output load.

[0021] In the second stage, when the power switch Q1 changes from the on state to the off state, the synchronous rectification switch Q2 changes from the off state to the on state; during the period when the power switch Q1 is in the off state and the synchronous rectification switch Q2 is in the on state, the energy stored in the primary winding Np of the transformer T1 is released to the secondary winding Ns of the transformer T1, providing energy for the output load and charging the secondary output capacitor Co; the demagnetization detection signal Vdemg representing the demagnetization condition of the secondary winding Ns of the transformer T1 is generated by the voltage on the auxiliary winding Naux of the transformer T1 after being divided by the voltage dividing resistors R1 and R2.

[0022] In the third stage, after the synchronous rectification switch Q2 changes from the on state to the off state, the inductance of the primary winding Np of the transformer T1 and the output capacitor of the power switch Q1 resonate, and according to different output loads, the primary side control chip 102 can control the power switch Q1 to change from the off state to the on state at different resonance valley bottoms, and the above three stages are repeatedly cycled, finally providing the desired output voltage, output current, and / or protection function through the USB / TYPE-C interface to the electronic device.

[0023] Figure 2 As shown in FIG. 1, the flyback switching power supply 100 includes the following components: Figure 1The diagram shows waveforms of multiple signals for a flyback switching power supply under normal operating conditions. Here, Vgate represents the switching control signal used to control the turn-on and turn-off of power switch Q1; Vdemg represents the demagnetization detection signal characterizing the demagnetization of the secondary winding Ns of transformer T1; Vcs represents the primary current detection signal characterizing the current flowing through the primary winding Np of transformer T1; SR Vdrain represents the drain voltage of synchronous rectifier switch Q2; SR gate represents the synchronous rectification control signal used to control the turn-on and turn-off of synchronous rectifier switch Q2; SR Isk represents the current flowing through synchronous rectifier switch Q2 (i.e., the current flowing through the secondary winding Ns of transformer T1, also called secondary current); Tdemg represents the demagnetization time of the secondary winding Ns of transformer T1; and Tring represents the resonant time of the inductance of the primary winding Np of transformer T1 and the output capacitance of power switch Q1.

[0024] Figure 3 It shows Figure 1 The diagram shows an example flowchart of the operating state transition process of a flyback switching power supply. Figure 3 As shown, the operating state transition process of the flyback switching power supply 100 includes: S102, when the flyback switching power supply 100 starts working, it first enters the normal operating state; S104, the flyback switching power supply 100 detects whether the USB / TYPE-C interface is electrically connected to the electronic device. If it is, it returns to S102 (i.e., continues to be in the normal operating state), otherwise it returns to S106; S106, the flyback switching power supply 100 enters the zero-power standby state (for example, first enters the no-load standby state, and then enters the zero-power standby state); S108, the flyback switching power supply 100 detects whether the USB / TYPE-C interface is electrically connected to the electronic device. If it is, it returns to S102 (for example, first enters the no-load standby state, and then enters the normal operating state), otherwise it returns to S106 (i.e., continues to be in the zero-power standby state).

[0025] Figure 4 It shows Figure 1 The diagram shows a schematic of the circuit modules in the primary and secondary control chips related to entering and exiting the zero-power standby state. (See diagram for example.) Figure 4As shown, in the secondary side control chip 104, the DP / DM input / output (I / O) module or the CC1 / CC2 I / O module generates and sends a control signal to the digital control module to notify the digital control module of the absence or presence of the electrical connection between the USB / TYPE-C interface and the electronic device when detecting the absence or presence of the electrical connection between the USB / TYPE-C interface and the electronic device; the digital control module generates and sends a zero-power standby enable signal for enabling or disabling the zero-power standby state to the zero-power standby control module when receiving the control signal; the zero-power standby control module controls the relevant modules in the secondary side control chip 104 to stop or restart work based on the zero-power standby enable signal, generates and sends a zero-power standby notification signal for notifying the exit or entry of the zero-power standby state to the logic control module; the synchronous rectification control module generates and sends a synchronous rectification control signal to the logic control module based on the drain voltage of the synchronous rectification switch tube Q2; the logic control module generates a predetermined code 1 or 2 based on the zero-power standby notification signal and the synchronous rectification control signal; and the driver module drives the synchronous rectification switch tube Q2 to switch between the on state and the off state based on the predetermined code 1 or 2. Correspondingly, in the primary side control chip 102, the decoding module identifies the predetermined code 1 or 2 based on the primary side current detection signal Vcs after determining that the demagnetization of the secondary side winding Ns of the transformer T1 is completed based on the demagnetization detection signal Vdem, generates and sends a decoding result to the zero-power standby control module; the zero-power standby control module controls the relevant modules in the primary side control chip 102 to stop or restart work and generates a switch tube control signal for controlling the on and off of the power switch tube Q1 based on the decoding result; and the driver module drives the power switch tube Q to switch between the on state and the off state based on the switch tube control signal.

[0026] The working processes of the flyback switching power supply 100 entering and exiting the zero-power standby state are described below in combination with the drawings.

[0027] 1) Working process of exiting the zero-power standby state

[0028] In some embodiments, the secondary side control circuit 104 can send the predetermined code 1 by controlling the turn-off time of the synchronous rectification switch tube Q2 from the on state to the off state. In order to avoid mis-detection, the predetermined code 1 can include multiple flag bits, each of which corresponds to one encoding period or one normal working period of the flyback switching power supply 100. For example, the flag bit 1 can correspond to one encoding period of the flyback switching power supply 100, and the flag bit 0 can correspond to one normal working period of the flyback switching power supply 100. The secondary side control circuit 104 can control the synchronous rectification switch tube Q2 to delay a predetermined time length (for example, 5us) at the turn-off time in the encoding period compared with the turn-off time in the normal working period.

[0029] Figure 5 The waveforms of the signals of the flyback switching power supply are shown in the encoding period and the normal working period. As shown in Figure 1 In the encoding period (i.e., the period corresponding to the flag bit 1), the synchronous rectification switch Q2 changes from the on state to the off state with a delay of a predetermined time (e.g., 5us) relative to the normal working period (i.e., the period corresponding to the flag bit 0), the current SR Isk flowing through the synchronous rectification switch Q2 continues to increase negatively to Isk1 after zero crossing, the system output voltage Vo reversely excites the secondary winding Ns of the transformer T1, the voltage on the auxiliary winding Naux of the transformer T1 is clamped at Vaux = (Vo*Naux) / Ns, and the demagnetization detection signal Vdemg maintains a plateau voltage; after the synchronous rectification switch Q2 changes from the on state to the off state, the secondary winding Ns of the transformer T1 transfers the energy stored by the reverse excitation to the primary winding Np of the transformer T1, the primary winding Np of the transformer T1 is reversely demagnetized, and the reverse demagnetization energy flows back to the primary bulk capacitor Cbulk through the primary winding Np of the transformer T1 and the primary current detection resistor Rs. Figure 5 As shown in

[0030] In the encoding period, since the synchronous rectification switch Q2 changes from the on state to the off state with a delay of a predetermined time relative to the normal working period, the primary current detection signal Vcs is a negative voltage, which is obviously different from the positive voltage in the normal working period. Therefore, the primary control chip 102 can start to identify the predetermined encoding 1 when it is determined that the secondary winding Ns of the transformer T1 ends demagnetization based on the demagnetization detection signal Vdemg, and the primary current detection signal Vcs is detected as a negative voltage, the amplitude of the primary current detection signal Vcs is lower than a predetermined voltage (e.g., -100mV), and the duration of the primary current detection signal Vcs as a negative voltage reaches a predetermined time (e.g., 150ns). Figure 5 In the case that the flyback switching power supply 100 adopts the fast charging mode and the system output voltage Vo exceeds 5V, according to the charger fast charging protocol, once the USB / TYPE-C interface is electrically disconnected from the electronic device, the system output voltage Vo quickly returns to 5V through the discharging circuit, and the switch M on the output bus changes from the on state to the off state.

[0031]

[0032] The waveforms of the signals of the flyback switching power supply entering the zero power standby state are shown. As shown in Figure 6 In the encoding period (i.e., the period corresponding to the flag bit 1), the synchronous rectification switch Q2 changes from the on state to the off state with a delay of a predetermined time (e.g., 5us) relative to the normal working period (i.e., the period corresponding to the flag bit 0), the current SR Isk flowing through the synchronous rectification switch Q2 continues to increase negatively to Isk1 after zero crossing, the system output voltage Vo reversely excites the secondary winding Ns of the transformer T1, the voltage on the auxiliary winding Naux of the transformer T1 is clamped at Vaux = (Vo*Naux) / Ns, and the demagnetization detection signal Vdemg maintains a plateau voltage; after the synchronous rectification switch Q2 changes from the on state to the off state, the secondary winding Ns of the transformer T1 transfers the energy stored by the reverse excitation to the primary winding Np of the transformer T1, the primary winding Np of the transformer T1 is reversely demagnetized, and the reverse demagnetization energy flows back to the primary bulk capacitor Cbulk through the primary winding Np of the transformer T1 and the primary current detection resistor Rs. Figure 1 Figure 6 ​As shown, the secondary side control chip 104 can use 1010 as the predetermined code 1 to inform the primary side control chip 104 to enter the zero standby power state, and after the zero standby power enable signal changes from the logic low level to the logic high level and delays for a period of time, the secondary side control chip 104 enters the zero standby power state, turns off most of the functional modules, and only keeps a few detection modules to continue to work, so as to reduce the working current of the secondary side control chip 104 while maintaining the secondary side control chip 104 without power-off. In addition, the primary side control chip 102 enters the zero standby power state after identifying the predetermined code 1, and turns off multiple functional modules to reduce the working current of the primary side control chip 102. In this way, the primary side control chip 102 and the secondary side control chip 104 can achieve ultra-low working current, and the entire system can achieve zero standby power consumption.

[0033] After entering the zero standby power state, in order to maintain the system output without power-off, the primary side control chip 102 and the secondary side control chip 104 have the following power supply states:

[0034] When the primary side control chip 102 detects that the voltage on the chip power supply capacitor Cdd is lower than the sum of the UVLO voltage of the primary side control chip 102 and a predetermined voltage (for example, 0.5V), the primary side control chip 102 controls the power switch tube Q1 to change from the off state to the on state, and when the power switch tube Q1 is in the on state for a predetermined duration, the primary side control chip 102 controls the power switch tube Q1 to change from the on state to the off state. In other words, when the primary side control chip 102 detects that the voltage on the chip power supply capacitor Cdd reaches or is lower than the sum of the UVLO voltage of the primary side control chip 102 and 0.5V, the primary side control chip 102 controls the power switch tube Q1 to be on for a minimum Tonmin time to maintain the primary side control chip 102 without power-off.

[0035] When the secondary side control chip 104 detects that the system output voltage Vo is lower than the sum of the UVLO voltage of the secondary side control chip 104 and a predetermined voltage (for example, 0.5V), the secondary side control chip 104 controls the synchronous rectification switch tube Q2 to change from the off state to the on state, and when the synchronous rectification switch tube Q2 is in the on state for a predetermined duration, the secondary side control chip 104 controls the synchronous rectification switch tube Q2 to change from the on state to the off state. In other words, once the secondary side control chip 104 detects that the system output voltage Vo is lower than the sum of the UVLO voltage of the secondary side control chip 104 and 0.5V, the secondary side control chip 104 controls the synchronous rectification switch tube Q2 to be on for a short period of time. When the demagnetization detection pin DEMG of the primary side control chip 102 couples out a negative small pulse, the primary side control chip 102 detects a negative small pulse with an amplitude lower than a predetermined threshold (for example, -125mV) and a pulse width greater than a predetermined duration (for example, 150ns) via the DEMG pin, and no second similar negative small pulse is received within a predetermined time, the primary side control chip 102 temporarily turns on part of the functional modules, controls the power switch tube Q1 to be on for a minimum Tonmin time, and maintains the secondary side control chip 104 without power-off.

[0036] 2) The process of exiting zero-power standby mode

[0037] In some embodiments, the secondary-side control chip 104 can transmit a predetermined code 2 by controlling the synchronous rectifier switch Q2 to switch between an on state and an off state. For example, the secondary-side control chip 104 can transmit the predetermined code 2 by controlling the synchronous rectifier auxiliary switch Q3 (which is weakly turned on by limiting the current I1) to switch between an on state and an off state during multiple synchronous rectifier weak conduction cycles at a fixed frequency.

[0038] Figure 7 It shows Figure 1 The diagram shows the waveforms of multiple signals when a flyback switching power supply exits zero-power standby mode. (Example:) Figure 7 As shown, the predetermined code 2 consists of multiple (e.g., 8) fixed-frequency synchronous rectification weak conduction cycles, which couple out negative small pulses at the demagnetization detection pin DEMG of the primary-side control chip 102. When the primary-side control chip 102 detects multiple negative small pulses with amplitudes less than a predetermined threshold (e.g., -125mV) and pulse widths of a predetermined duration (e.g., 150ns) within a predetermined duration (e.g., 500us) via the demagnetization detection pin DEMG, it controls the power switch Q1 to conduct for a short period of time, causing the system output voltage Vo to rise back to 5V and exit the zero-power standby state (e.g., enter the no-load standby state).

[0039] The flyback switching power supply according to embodiments of the present invention achieves ultra-low standby power consumption by entering a zero-power standby state when there is no electrical connection between the USB / TYPE-C interface and the electronic device, thereby greatly reducing energy waste. Furthermore, the flyback switching power supply according to embodiments of the present invention requires no additional components, resulting in low system cost and a simple and reliable implementation.

[0040] This invention can be implemented in other specific forms without departing from its spirit and essential characteristics. For example, the algorithm described in a particular embodiment can be modified without departing from the basic spirit of the invention. Therefore, the present embodiments are to be regarded as exemplary rather than limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description, and all changes falling within the meaning and scope of the claims and their equivalents are thus included within the scope of the invention.

Claims

1. A secondary-side control circuit for use in a flyback switching power supply, the flyback switching power supply including a transformer, a primary-side control circuit connected to the primary winding of the transformer, and a Universal Serial Bus (USB) interface connected to the secondary winding of the transformer, the secondary-side control circuit being connected to the secondary winding of the transformer and configured to, under normal operating conditions: When it detects that there is no electrical connection between the USB interface and the electronic device, it enters a zero-power standby state and sends a first predetermined code through the transformer to notify the primary-side control chip to also enter the zero-power standby state. in, The flyback switching power supply also includes a synchronous rectifier switch connected to the secondary winding of the transformer, and the secondary control circuit is further configured to send the first predetermined code by controlling the off-time of the synchronous rectifier switch from an on state to an off state.

2. The secondary-side control circuit according to claim 1, wherein, The first predetermined code includes a plurality of flag bits, each of which corresponds to a coding cycle or a normal operating cycle of the flyback switching power supply, and the secondary-side control circuit is further configured to control the turn-off time of the synchronous rectifier switch in the coding cycle to be delayed by a first predetermined duration compared to the turn-off time in the normal operating cycle.

3. The secondary-side control circuit according to claim 2, wherein, The plurality of flag bits include a start flag bit, which corresponds to one encoding cycle of the flyback switching power supply.

4. The secondary-side control circuit according to claim 3, wherein, The secondary-side control circuit is further configured to, when in the zero-power standby state, control the synchronous rectifier switch to change from a turn-off state to a turn-on state when it detects that the system output voltage of the flyback switching power supply is lower than the sum of the undervoltage lockout voltage of the secondary-side control circuit and a first predetermined voltage, and control the synchronous rectifier switch to change from a turn-on state to a turn-off state when the duration of the synchronous rectifier switch being in the turn-on state reaches a second predetermined duration.

5. The secondary-side control circuit according to claim 1 is further configured to, when in the zero-power standby state: When an electrical connection is detected between the USB interface and the electronic device, the system exits the zero-power standby state and sends a second predetermined code through the transformer to notify the primary-side control chip to also exit the zero-power standby state.

6. The secondary-side control circuit according to claim 5, wherein, The flyback switching power supply also includes a synchronous rectifier auxiliary switch connected to the secondary winding of the transformer, and the secondary control circuit is further configured to send the second predetermined code by controlling the synchronous rectifier auxiliary switch to switch between an on state and an off state.

7. The secondary-side control circuit according to claim 6, wherein, The secondary-side control circuit is further configured to transmit the second predetermined code by controlling the synchronous rectification auxiliary switch to switch between an on state and an off state during multiple synchronous rectification weak conduction cycles at a fixed frequency.

8. A primary-side control circuit for use in a flyback switching power supply, the flyback switching power supply including a transformer and a secondary-side control circuit connected to a secondary winding of the transformer, the primary-side control circuit being connected to the primary winding of the transformer and configured to, under normal operating conditions: Based on the demagnetization detection signal characterizing the demagnetization status of the secondary winding of the transformer, it is determined whether the demagnetization of the secondary winding of the transformer has ended; When it is determined that the secondary winding of the transformer has finished demagnetizing, when the primary current detection signal, which characterizes the current flowing through the primary winding of the transformer, is detected as a negative voltage and the amplitude of the primary current detection signal is lower than a second predetermined voltage and the duration of the negative voltage reaches a third predetermined duration, the predetermined code from the secondary control circuit is identified. as well as When the predetermined code is identified as a predetermined code used to notify the primary-side control circuit to enter the zero-power standby state, the system enters the zero-power standby state.

9. The primary-side control circuit according to claim 8, wherein, The flyback switching power supply also includes a power switching transistor connected between the primary winding of the transformer and ground, and the primary control circuit is further configured to operate in the zero-power standby state as follows: When the supply voltage for the primary-side control circuit is detected to be lower than the sum of the undervoltage lockout voltage of the primary-side control circuit and the third predetermined voltage, the power switch is controlled to change from the off state to the on state, and when the power switch is in the on state for a duration of a fourth predetermined time, the power switch is controlled to change from the on state to the off state.

10. The primary-side control circuit according to claim 8 is further configured to, when in the zero-power standby state: When the demagnetization detection signal, which characterizes the demagnetization of the secondary winding of the transformer, detects multiple negative small pulses with a minimum voltage value lower than the fourth predetermined voltage and a pulse width reaching the sixth predetermined duration within the fifth predetermined duration, the zero-power standby state is exited.

11. A flyback switching power supply, comprising: transformer; The secondary control circuit according to any one of claims 1 to 7 is connected to the secondary winding of the transformer; as well as The primary-side control circuit according to any one of claims 8 to 10 is connected to the primary winding of the transformer.

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