Switching power supply control method and system for preventing original and auxiliary side intercommunication, switching power supply

Abstract

The application belongs to a switching power supply control method, and aims at solving the technical problem that the switching power supply structure with a secondary side feedback will be mis-opened by interference after the driving signal is closed, and if it cannot be closed in time, the primary side and the secondary side will be interconnected, when the primary side driving is closed again, the VDS stress will be out of limit, and even the machine will be exploded, and provides a switching power supply control method and system for preventing the primary side and the secondary side from being interconnected, and a switching power supply, when the driving signal in the switching power supply is low, a low pulse signal with a pulse width of T1 time is used to shield the driving signal in the switching power supply, so that the power tube in the switching power supply is continuously closed in T1 time after being closed, and can be blanked, and it is ensured that the driving will not be continuously opened to cause the interconnection and the explosion of the machine in a short time, and the method is applied to a specific switching power supply, and only a pulse circuit and an AND gate circuit need to be added in the existing power supply circuit.

Description

Technical Field

[0001] This invention pertains to switching power supply control methods, specifically relating to a switching power supply control method and system for preventing interconnection between the primary and secondary sides, and a switching power supply. Background Technology

[0002] Currently, most switching power supply structures with secondary-side feedback on the market control the drive signal through the FB state and clock signal, or by using zero-crossing detection. When the FB is interfered with, or when the zero-crossing detection signal is abnormal, false turn-on can easily occur. When the drive signal is turned off, if it encounters interference and falsely turns on, and the turn-on time is close to the turn-off time, the synchronous rectification will be in the blanking state. If it cannot be turned off in time, a connection between the primary and secondary sides will occur. In addition, since demagnetization has just begun and the inductor current is large, when the primary-side drive turns off again, it will cause VDS stress to exceed the limit, or even cause the power supply to explode. Summary of the Invention

[0003] This invention addresses the technical problem of current secondary-side feedback switching power supply structures where, after the drive signal is turned off, interference causes accidental turn-on, and if not turned off in time, a primary-secondary side interconnection occurs. When the primary-side drive is turned off again, it can cause VDS stress to exceed the limit, or even cause the power supply to explode. This invention provides a switching power supply control method and system, as well as a switching power supply, to prevent primary-secondary side interconnection.

[0004] To achieve the above objectives, the present invention employs the following technical solution:

[0005] A switching power supply control method for preventing interconnection between primary and secondary sides, characterized by the following steps:

[0006] When the internal drive signal of the switching power supply goes low, a low pulse signal with a pulse width of T1 is generated to shield the internal drive signal of the switching power supply, so that the internal power transistor of the switching power supply remains in the off state for a period of T1 after it is turned off.

[0007] Furthermore, the specific method of generating a low pulse signal with a pulse width of T1 when the internal drive signal of the switching power supply becomes low is as follows: when the internal drive signal of the switching power supply is high, a high level is input to the drive input terminal of the power supply circuit; when the internal drive signal of the switching power supply is low, a low level is first input to the drive input terminal of the power supply circuit, and after a delay of T1 time, a high level is input, thus forming a low pulse signal with a pulse width of T1 time.

[0008] Furthermore, the determination of whether the internal drive signal of the switching power supply is low is specifically made in the following way:

[0009] When the drive output terminal of the power supply circuit is in the off state, it indicates that the internal drive signal of the switching power supply is low.

[0010] Furthermore, when the drive signal inside the switching power supply goes low, a low pulse signal with a pulse width of T1 is generated, specifically:

[0011] This is achieved by setting up a low-pulse signal circuit with a pulse width of T1 between the PWM master control and drive of the power supply circuit;

[0012] Alternatively, it can be achieved by connecting an external circuit between the power supply and the drive in the power supply circuit.

[0013] The present invention also provides a switching power supply control system for preventing primary-secondary switching power supply interconnection, which can realize the above-mentioned switching power supply control method for preventing primary-secondary switching power supply interconnection. Its special feature is that it includes a pulse module and a first logic judgment module:

[0014] The first logic judgment module is used to determine whether the internal drive of the switching power supply is low;

[0015] The pulse module is used to generate a low pulse signal with a pulse width of T1 when the internal drive signal of the switching power supply becomes low. The low pulse signal is used to shield the internal drive signal of the switching power supply, so that the internal power transistor of the switching power supply remains in the off state for a period of T1 after it is turned off.

[0016] Furthermore, the pulse module includes a second logic judgment module (nand), a capacitor (C11), and a resistor (R11);

[0017] The first input terminal of the second logic judgment module NAND is used to connect to the PWM main control of the power supply circuit; one end of the resistor R11 is connected to the PWM main control of the power supply circuit, and the other end is connected to the second input terminal of the second logic judgment module NAND; one end of the capacitor C11 is connected between one end of R11 and the second input terminal of the second logic judgment module NAND, and the other end is grounded.

[0018] The second logic judgment module NAND is used to output a low level when both the first and second input terminals are high, and to output a high level when at least one of the first and second input terminals is low.

[0019] Correspondingly, the present invention provides a switching power supply for preventing interconnection between the primary and secondary sides, including a power supply circuit; its special feature is that it also includes the aforementioned switching power supply control system for preventing interconnection between the primary and secondary sides.

[0020] The first input terminal of the first logic judgment module is connected to the PWM main control of the power supply circuit, the second input terminal is connected to the output terminal of the pulse circuit, and the output terminal is connected to the drive input terminal of the power supply circuit.

[0021] The input terminal of the pulse module is connected to the PWM main control of the power supply circuit.

[0022] The present invention also provides another switching power supply control system for preventing primary and secondary side interconnection, which can realize the above-mentioned switching power supply control method for preventing primary and secondary side interconnection. Its special feature is that it includes a release module, and a rectifier module, a charging and discharging module and a switching module connected in sequence; the input terminal of the rectifier module is connected to the power supply of the power supply circuit, and the output terminal of the switching module is connected to the drive output terminal of the power supply circuit.

[0023] The rectifier module is used to rectify the power supply voltage of the power circuit when the switching power supply is demagnetized; and to terminate the rectification of the power supply voltage of the power circuit after the demagnetization of the switching power supply is completed.

[0024] The charging and discharging module is used to start charging with the voltage rectified by the rectifier module when the switching power supply is demagnetized, so that the charging and discharging module is in a short-circuit state, and in an open-circuit state after charging is completed.

[0025] The switching module is used to make itself in a conducting state when the charging and discharging module is in a short-circuit state, and to make itself in a disconnected state when the charging and discharging module is in an open-circuit state.

[0026] The release module is connected to the charge-discharge module and is used to discharge the charge-discharge module when the charge-discharge module is in an open-circuit state.

[0027] Furthermore, the release module is a resistor R3, the rectifier module is a diode D2, the charging and discharging module is a capacitor C4, and the switching module includes a resistor R4, a resistor R5, and a transistor VT1;

[0028] The positive terminal of diode D2 is connected to the power supply of the power supply circuit; one end of resistor R3 is connected to the negative terminal of diode D2, and the other end is connected to one end of resistor R4; one end of capacitor C4 is connected to the negative terminal of diode D2, and the other end is connected to one end of resistor R4; the other end of resistor R4 is connected to the base of transistor VT1, the emitter of transistor VT1 is grounded, and the collector is connected to the output terminal of the power switch drive.

[0029] Accordingly, the present invention also provides a second type of switching power supply for preventing primary and secondary side interconnection, including a power supply circuit; its special feature is that it also includes the above-mentioned second type of switching power supply control system for preventing primary and secondary side interconnection.

[0030] The first input terminal of the first logic judgment module is connected to the PWM main control of the power supply circuit, the second input terminal is connected to the output terminal of the pulse circuit, and the output terminal is connected to the drive input terminal of the power supply circuit.

[0031] The input terminal of the pulse module is connected to the PWM main control of the power supply circuit.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] 1. This invention provides a switching power supply control method to prevent interconnection between the primary and secondary sides. When the internal drive signal of the switching power supply goes low, a low pulse signal with a pulse width of T1 is used to shield the internal drive signal of the switching power supply. This causes the internal power transistor of the switching power supply to remain off for a period of T1 after being turned off, ensuring that the synchronous rectifier tube on the secondary side of the switching power supply transformer and the power transistor on the primary side will not conduct simultaneously, thereby preventing a short circuit in the transformer and avoiding the phenomenon of VDS stress exceeding the limit and burning out the power transistor due to interconnection between the primary and secondary sides of the transformer.

[0034] 2. In this invention, the determination of whether the internal drive signal of the switching power supply is high or low can be made based on the state of the drive output terminal of the power supply circuit. When the drive output terminal is in the off state, the internal drive signal of the switching power supply is low; when the drive output terminal is in the on state, the internal drive signal of the switching power supply is low. The judgment standard is more direct, making the control response faster.

[0035] 3. The control method of the present invention can be implemented either by connecting a corresponding circuit inside the power supply circuit or by connecting a circuit outside the power supply circuit, making the method of the present invention more applicable and adaptable to different product design requirements.

[0036] 4. This invention also provides two corresponding control systems for implementing the above control method, one inside the power supply circuit and the other outside the power supply circuit. The internal control system adds a pulse module and a first logic judgment module to the existing power supply circuit. By judging the gate's closed state, it enables the timing delay, ensuring that the timing delay drive enable signal is shielded, thereby achieving drive delay blanking after shutdown. The internal control system has a lower cost. The pulse module and the first logic judgment module implement delay control inside the switching power supply and are packaged inside the switch during use, without increasing the size of the switching power supply chip. The external control system provides delay control outside the switching power supply through diode rectification and capacitor charging and discharging. The delay time can also be changed by adjusting the capacitance value. The external control system is more convenient, flexible, and easier to maintain. It ensures that during operation, the transformer secondary synchronous rectifier and primary power transistor will not conduct simultaneously for a certain period of time, thus preventing a transformer short circuit and avoiding the phenomenon of VDS stress exceeding the standard and burning out the power transistor due to interconnection between the primary and secondary sides of the transformer.

[0037] 5. This invention also proposes two switching power supplies to prevent interconnection between the primary and secondary sides. The above control method can be applied to specific switching power supplies. One method only requires adding a pulse module and a first logic judgment module to the existing power supply circuit. The other method only requires connecting an external circuit structure consisting of a release module, a rectification module, a charging and discharging module, and a switching module between the power supply and the drive output terminal to realize the control method of the switching power supply. This ensures that when the switching power supply is working, the synchronous rectifier tube on the secondary side of the transformer and the power tube on the primary side will not conduct simultaneously within a certain period of time, thereby preventing a short circuit in the transformer and avoiding the phenomenon of VDS stress exceeding the standard and burning out the power tube due to interconnection between the primary and secondary sides of the transformer.

[0038] 6. The pulse module and the first logic judgment module in the switching power supply of the present invention, as a switching power supply control system to prevent interconnection between the primary and secondary sides, can not only be used in this application, but also form a general delay module that can be used for driving delay blanking of various driving signals. It has versatility and can achieve delay effect in various circuits. Attached Figure Description

[0039] Figure 1 This is a circuit diagram of a first embodiment of the switching power supply for preventing interconnection between the primary and secondary sides of the present invention;

[0040] Figure 2 This is a circuit diagram of the pulse circuit in Embodiment 1 of the present invention;

[0041] Figure 3 A circuit diagram of a second embodiment of the switching power supply for preventing primary and secondary circuitry according to the present invention.

[0042] Figure 4 This is a schematic diagram of the operating waveforms of an existing switching power supply with the primary and secondary sides interconnected.

[0043] Figure 5 This is a schematic diagram of the working waveform of a switching power supply embodiment 1 of the present invention for preventing interconnection between the primary and secondary sides. Detailed Implementation

[0044] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0045] In existing technologies, the drive switch is controlled based on zero-crossing detection and FB status, without any blanking processing of the drive signal. This limits the upper limit of the operating frequency to some extent. In practical applications, many switching power supplies have experienced failures due to continuous accidental turn-on, with the continuous turn-on time being approximately 1µs, seriously affecting the safety and reliability of the switching power supply.

[0046] To address the aforementioned problems, this invention proposes a control method for the interconnection between the primary and secondary sides of a switching power supply. The method samples from the Gate or CS terminal of the internal drive output of the switching power supply to detect the drive turn-off moment. Then, by adjusting the delay, it achieves drive-on shielding, i.e., enabling the blanking function, ensuring that the drive will not be continuously turned on within a short period, preventing interconnection and power failure. Specifically, when the internal drive signal of the switching power supply goes low, a low pulse signal with a pulse width of T1 is used to shield the internal drive signal, ensuring that the internal power transistor remains continuously off for the time T1 after being turned off. The specific duration of T1 can be adjusted according to actual needs. In one embodiment of this invention, the preferred range of T1 is 500 ns to 5 µs.

[0047] Example 1

[0048] To enable the switching power supply to implement the above control method, a pulse module and a first logic judgment module can be added to the power supply circuit of the switching power supply. The following is a specific embodiment of the switching power supply for preventing primary and secondary side interconnection according to the present invention. Figure 1 As shown, the system includes a power supply circuit, a pulse module, and a first logic judgment module. The power supply circuit includes a power source, resistors R1 and R2, diode D1, a zero-crossing detection module, a PWM main controller, a driver, a polarized capacitor C2, a power transistor M0, a synchronous rectifier transistor M1, a transformer T, a synchronous rectifier chip, and a polarized capacitor C3. One end of the power source is grounded, and the other end is connected to the anode of diode D1. The positive terminal of polarized capacitor C2 is connected to the cathode of diode D1, and its negative terminal is grounded. One end of resistor R1 is connected to the anode of diode D1, and the other end is connected to one end of resistor R2. The other end of resistor R2 is grounded. One end of resistor R2 is connected to the input terminal of the zero-crossing detection module. The output terminal of the zero-crossing detection module is connected to the first input terminal of the PWM main controller. The positive terminal of polarized capacitor C2 is connected to the second input terminal of the PWM main controller. The output terminal of the PWM main controller outputs the internal clock drive signal gdk. Figure 1 In the diagram, Zcd is the IC pin name, whose main function is zero-crossing detection, and Vaux is the auxiliary winding output voltage.

[0049] The pulse module is mainly used to generate a low pulse signal with a pulse width of T1 when the internal drive signal of the switching power supply goes low. This low pulse signal is used to shield the internal drive signal of the switching power supply, ensuring that the internal power transistor remains off for the T1 time period after it is turned off. Figure 2 As shown, as an embodiment of the pulse module, the pulse module includes a second logic judgment module Nand and a capacitor C11, as well as inverters inv1, inv2, resistor R11, and inverter inv3 connected in sequence. In this embodiment, the second logic judgment module Nand can be a NAND gate. The first input terminal of the NAND gate is connected to the output terminal of inverter inv3, the second input terminal is connected to the output terminal of inverter inv1, and the output terminal of the NAND gate is connected to the second input terminal of the AND gate. One end of capacitor C11 is connected to the input terminal of inverter inv3, and the other end is grounded. The first input terminal of the AND gate is connected to the PWM main control of the power supply circuit, the second input terminal is connected to the output terminal of the NAND gate, and the output terminal of the AND gate is connected to the drive input terminal of the power supply circuit. The drive output terminal outputs the internal clock signal Gate. The gate of power transistor M0 is connected to the output terminal of the drive, the source is connected to one end of resistor Rcs, the other end of resistor Rcs is grounded, and one end of resistor Rcs is connected to the third input terminal of the PWM main control. The drain of power transistor M0 is connected to one end of the primary winding of transformer T. The other end of the primary winding of transformer T is connected to the positive terminal of polarized capacitor C1, and the negative terminal of polarized capacitor C1 is grounded. A diode assembly is also connected to the primary winding of transformer T, and the output terminal of the diode assembly is connected between the positive and negative terminals of polarized capacitor C1. The gate of synchronous rectifier transistor M1 is connected to the synchronous rectifier chip, the positive terminal of polarized capacitor C3 is connected to the synchronous rectifier chip, the source of synchronous rectifier transistor M1 is connected to one input terminal of the output module, the other input terminal of the output module is connected to one end of the secondary winding of transformer T, and the other end of the secondary winding of transformer T is connected to the drain of synchronous rectifier transistor M1. The synchronous rectifier chip is connected to the drain of synchronous rectifier transistor M1.

[0050] In addition, in this embodiment, the first logic judgment module can be an AND gate. The first input terminal is connected to the PWM main control of the power supply circuit, the second input terminal is connected to the output terminal of the Nand of the second logic judgment module of the pulse module, and the output terminal of the AND gate is connected to the drive input terminal of the power supply circuit, which can be used to determine whether the internal drive of the switching power supply is low.

[0051] The working principle of the pulse module and the first logic judgment module is as follows: When gdk goes high, the output t1_b of the pulse module immediately goes high; when gdk goes low, the output of the first logic judgment module goes low. Through the delay circuit formed by resistor R11 and capacitor C11, t1_b goes low first, and then goes high after a delay of T1 time. Therefore, when gdk goes low, t1_b forms a low-level pulse signal with a pulse width of T1 time. Therefore, in practical applications, by determining that the Gate is in the off state, a timing delay of T1 is applied to ensure that the drive turn-on signal is shielded within the timing delay, thereby achieving drive delay blanking after turn-off, and achieving the function that the power transistor MO cannot be turned on again within T1 time after it is turned off.

[0052] For a switching power supply, when the internal clock drive signal gdk output from the PWM main control output terminal is detected to be low, the timing delay of the pulse module is enabled. While the t1_b signal is in a low pulse state, the drive signal gdk and zero-crossing detection are disabled. When the T1 delay ends, the t1_b signal goes high, and the masking ends. Therefore, from the moment the power transistor MO is turned off, it cannot be turned on again within the T1 time period. Otherwise, the synchronous rectifier diode located on the secondary side of transformer T may turn on simultaneously with the power transistor MO located on the primary side, causing a short circuit in transformer T and further leading to the burnout of power transistor M0. Therefore, to prevent power failure, after power transistor MO is turned off, it is prevented from turning on again within the T1 time period, also avoiding the phenomenon of simultaneous conduction of the primary and secondary windings of the transformer. The pulse module and the first logic judgment module implement delay control internally within the switching power supply and are packaged inside the switching power supply during use, without increasing the size of the switching power supply chip.

[0053] Example 2

[0054] like Figure 3The diagram shows another specific embodiment of the switching power supply for preventing primary and secondary circuitry from interconnecting, according to the present invention. It includes a resistor R3, a diode D2, a capacitor C4, resistors R4 and R5, and a transistor VT1. The anode of diode D2 is connected to the power supply of the power circuit. One end of resistor R3 is connected to the cathode of diode D2, and the other end is connected to one end of resistor R4. One end of capacitor C4 is connected to the cathode of diode D2, and the other end is connected to one end of resistor R4. The other end of resistor R4 is connected to the base of transistor VT1. The emitter of transistor VT1 is grounded, and its collector is connected to the output terminal of the power switch driver. Diode D2 forms a rectifier module, used to rectify the power supply voltage during demagnetization and to terminate rectification after demagnetization. Capacitor C4 forms a charge / discharge module, used to begin charging using the rectified voltage during demagnetization, keeping the charge / discharge module in a short-circuit state, and to open-circuit state after charging is complete. Resistors R4 and R5, along with transistor VT1, form a switching module. This module keeps itself in a conducting state when the charging / discharging module is short-circuited and in a disconnected state when the module is open-circuited. Resistor R3 forms a release module, used to discharge the charging / discharging module when it is open-circuited. The switching power supply's delay time can be adjusted by changing the capacitor value, making the control system more convenient, flexible, and easier to maintain. This ensures that during operation, the transformer's secondary synchronous rectifier and primary power transistors will not conduct simultaneously for a certain period, preventing a transformer short circuit and avoiding the VDS stress exceeding limits and burning out the power transistors due to interconnection between the primary and secondary sides.

[0055] The working principle of the second specific embodiment is as follows: Diode D2 is connected to the power supply. When the power supply circuit begins to demagnetize, diode D2 starts to rectify, and capacitor C4 starts to charge. The charging of capacitor C4 provides a corresponding delay. During the charging process, capacitor C4 is equivalent to a short circuit. Through the voltage division of resistors R4 and R5, it provides the base current and voltage of transistor VT1, making transistor VT1 conduct. Therefore, when capacitor C4 is charging, transistor VT1 is in the conducting state, pulling down the voltage at the drive output terminal of the power supply circuit. By adjusting the capacitance value of capacitor C4, the delay time can be changed. When capacitor C4 is fully charged, it becomes an open circuit. The voltage division is achieved through resistor R3 connected in parallel with C4. Resistor R3 can be in the 100K range, while resistors R4 and R5 can be in the K range, differing by two orders of magnitude. Therefore, the voltage division by resistors R4 and R5 is very low, failing to meet the turn-on condition of transistor VT1. Transistor VT1 remains in a blocking state, unable to pull down the output voltage of the power supply circuit, allowing the power supply circuit to operate normally. Simultaneously, after the power supply circuit demagnetizes, diode D2 ceases rectification, effectively stopping the charging of capacitor C4. The energy of capacitor C4 is released through resistor R3 connected in parallel, and it waits for the next charging time, followed by another delay. The circuit structures in the two embodiments described above, except for the power supply control system preventing primary-secondary circuitry, are merely specific examples of the switching power supply. In other embodiments of this invention, the specific structure of the power supply circuit can also be different, and the switching power supply control method and control system for preventing primary-secondary circuitry of this invention are equally applicable.

[0056] like Figure 3 As shown, in a switching power supply without an added pulse module and a first logic judgment module, at the turn-off time t4 of the power transistor MO, the synchronous rectifier transistor M1 on the secondary side is turned on. When the synchronous rectifier chip controls the synchronous rectifier transistor M1 to turn on, it generates a minimum on-time Ts_min. Figure 3 The time period from t4 to t7 can be considered as such. If an interference occurs in the system at time t5, causing GDK to mistakenly trigger the power transistor M0 to turn on, and this mistaken turn-on occurs within the minimum conduction time of the synchronous rectifier transistor M1, then during the time period from t4 to t5, both the primary switching power transistor M0 and the synchronous rectifier transistor M1 will be conducting simultaneously. During this time, the transformer T will be in a short-circuit state, with its inductance almost zero. Therefore, the power transistor M0 will operate in the linear region with a very large current. After the power transistor M0 turns off, at time t6, a very large spike voltage will be generated at the drain terminal (Vds) of the power transistor M0, which may very well break down the power transistor M0. The demagnetization time is the length from t4 to t8. Figure 3 In the diagram, Drain represents the drain waveform of the power transistor MO, and Vd represents the zero-crossing detection signal.

[0057] Correspondingly, such as Figure 4 The diagram shown is a working waveform diagram corresponding to an embodiment of the present invention. Here, t1_b is a 2us low pulse shielding signal and a misleading pass signal generated by gdk at time t2. If no drive signal is generated during the shielding time of t1_b, and the drain terminal generates a zero crossing at time t3, then Valley does not generate a zero crossing signal during the shielding time of t1_b.

[0058] Figure 3 and Figure 4 In the waveform diagram, the horizontal axis represents the working time, and the vertical axis represents the voltage.

[0059] In addition, the pulse module and the first logic judgment module mentioned above can also be used as delay modules for driving delay blanking of various driving signals.

[0060] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A switching power supply control system for preventing interconnection between primary and secondary sides, characterized in that: Includes a pulse module and a first logic judgment module: The first logic judgment module is used to determine whether the internal drive of the switching power supply is low. The pulse module is used to generate a low pulse signal with a pulse width of T1 when the internal drive signal of the switching power supply becomes low. The low pulse signal is used to shield the internal drive signal of the switching power supply, so that the internal power transistor of the switching power supply remains in the off state for a period of T1 after it is turned off. The pulse module includes a second logic judgment module nand, a capacitor C11, and a resistor R11; The first input terminal of the second logic judgment module NAND is used to connect to the PWM main control of the power supply circuit; one end of the resistor R11 is connected to the PWM main control of the power supply circuit, and the other end is connected to the second input terminal of the second logic judgment module NAND; one end of the capacitor C11 is connected between one end of R11 and the second input terminal of the second logic judgment module NAND, and the other end is grounded. The second logic judgment module NAND is used to output a low level when both the first and second input terminals are high, and to output a high level when at least one of the first and second input terminals is low.

2. A switching power supply for preventing interconnection between primary and secondary sides, comprising a power supply circuit; characterized in that: It also includes the switching power supply control system for preventing interconnection between the primary and secondary sides as described in claim 1; The first input terminal of the first logic judgment module is connected to the PWM main control of the power supply circuit, the second input terminal is connected to the output terminal of the pulse circuit, and the output terminal is connected to the drive input terminal of the power supply circuit. The input terminal of the pulse module is connected to the PWM main control of the power supply circuit.

3. A switching power supply control system for preventing interconnection between primary and secondary sides, characterized in that: It includes a release module, and a rectifier module, a charge / discharge module, and a switching module that are connected in sequence. The rectifier module is used to rectify the power supply voltage of the power circuit when the switching power supply is demagnetized; and to terminate the rectification of the power supply voltage of the power circuit after the demagnetization of the switching power supply is completed. The charging and discharging module is used to start charging with the voltage rectified by the rectifier module when the switching power supply is demagnetized, so that the charging and discharging module is in a short-circuit state, and in an open-circuit state after charging is completed. The switching module is used to make itself in a conducting state when the charging and discharging module is in a short-circuit state, and to make itself in a disconnected state when the charging and discharging module is in an open-circuit state. The release module is connected to the charge-discharge module and is used to discharge the charge-discharge module when the charge-discharge module is in an open circuit state. The release module is a resistor R3, the rectifier module is a diode D2, the charging and discharging module is a capacitor C4, and the switching module includes a resistor R4, a resistor R5, and a transistor VT1; The positive terminal of diode D2 is used to connect to the power supply of the power supply circuit; one end of resistor R3 is connected to the negative terminal of diode D2, and the other end is connected to one end of resistor R4; one end of capacitor C4 is connected to the negative terminal of diode D2, and the other end is connected to one end of resistor R4; the other end of resistor R4 is connected to the base of transistor VT1, the emitter of transistor VT1 is grounded, and the collector is used to connect to the drive output terminal of the power supply circuit; one end of resistor R5 is connected to the base of transistor VT1, and the other end is grounded.

4. A switching power supply for preventing interconnection between primary and secondary sides, comprising a power supply circuit; characterized in that: It also includes the switching power supply control system for preventing interconnection between the primary and secondary sides as described in claim 3; The positive terminal of diode D2 is connected to the power supply of the power supply circuit, and the collector of transistor VT1 is connected to the drive output terminal of the power supply circuit.

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