Switching power supply control circuit, switching power supply and control method thereof

Abstract

The present application discloses a control circuit for a switching power supply, a switching power supply, and a control method thereof. The control circuit includes: a short-circuit protection circuit that generates a detection signal based on a sampling voltage provided by a sampling resistor used to sample the current of a switching transistor in the switching power supply; if the rising slope of the sampling voltage is less than a preset slope value, the detection signal provides a pulse signal; and a drive circuit that provides a switching signal and increases the duty cycle of the switching signal used to control the switching transistor based on the pulse signal to increase the switch-off time of the switching transistor. This circuit effectively protects the switching power supply against short circuits in the current sampling resistor under full-voltage AC input.

Description

Technical Field

[0001] The present invention relates to power electronics technology, and more particularly to a control circuit of a switching power supply, a switching power supply and a control method thereof. Background Art

[0002] With the increasing demand for switching power supplies, reducing production costs, increasing application functions and optimizing performance have become basic requirements for the power supply part of general consumer electronic products.

[0003] During the production, testing, and use of switching power supplies, many factors may cause damage or short circuits to certain components in the system, posing a safety hazard to the normal operation of the switching power supply. Therefore, the switching power supply is required to be able to self-protect in the event of abnormalities in certain components in the system, thereby further improving the safety of the switching power supply. For example, in a switching power supply application system, if the sampling resistor is damaged or short-circuited due to poor welding during the production process, the current flowing through the switching tube will be very large, which may exceed the safe upper limit of the switching tube current or cause the transformer core to be severely saturated, thereby causing the components in the switching power supply to burn out, a phenomenon known as "burning the machine", which poses a significant safety hazard to the production and use of the switching power supply.

[0004] Therefore, there is a need to improve the existing technology to enhance the safety and reliability of the switching power supply. Summary of the Invention

[0005] In view of the above problems, the object of the present invention is to provide a control circuit of a switching power supply, a switching power supply and a control method thereof, so as to provide short-circuit protection for the switching power supply under full voltage AC input, thereby avoiding damage or failure of the switching power supply due to abnormal sampling resistor or short circuit.

[0006] According to a first aspect of the present invention, a control circuit for a switching power supply is provided, comprising: a short-circuit protection circuit for generating a detection signal based on a sampling voltage, wherein the sampling voltage is provided by a sampling resistor in the switching power supply for sampling the current of a switching tube, and if the rising slope of the sampling voltage is less than a preset slope value, the detection signal provides a pulse signal; and a drive circuit for providing a switching signal for controlling the switching tube, and increasing the duty cycle of the switching signal of the switching tube according to the pulse signal to increase the off time of the switching tube.

[0007] Preferably, the driving circuit responds to each of the pulse signals to increase the duty cycle and the off time of the switching tube under the control of each of the pulse signals.

[0008] Preferably, the driving circuit responds to a group of the pulse signals, each group of the pulse signals includes a plurality of the pulse signals that appear in sequence, so that: every certain number of the pulse signals, the driving circuit adjusts the switching signal so that the duty cycle and the turn-off time of the switching tube are increased accordingly.

[0009] Preferably, each of the pulse signals has a rising edge and a falling edge, and the driving circuit changes the switch signal from a valid level to an invalid level when triggered by the falling edge of the pulse signal, so that the switch tube changes from an on state to an off state.

[0010] Preferably, the short-circuit protection circuit includes: a comparison module, which compares the sampled voltage and the reference voltage when the switch tube is in the on state, and outputs a result signal, wherein the result signal indicates whether the rising slope of the sampled voltage is less than the preset slope value; and when the switch tube is in the off state, the result signal is reset to a low level; a timing module, which provides a timing pulse when the on-duration time of the switch signal reaches the detection time; a logic module, which provides the detection signal according to the timing pulse and the result signal, and when the timing pulse starts, if the sampled voltage is less than the reference voltage, the detection signal provides the pulse signal; if the sampled voltage is greater than or equal to the reference voltage, the detection signal is maintained at a high level; and when the timing pulse ends, the detection signal is reset to the high level.

[0011] Preferably, the short-circuit protection circuit further includes a voltage generating module, configured to provide the constant reference voltage according to the preset slope value.

[0012] Preferably, the comparison module is implemented by a comparator, the non-inverting input terminal of the comparator receives the reference voltage, the inverting input terminal of the comparator receives the sampling voltage, the output terminal of the comparator provides the result signal, the timing pulse is a high-level pulse, when the conduction duration of the switch tube reaches the detection time, the timing pulse has a rising edge, and after the switching cycle ends, the timing pulse has a falling edge, and the logic module performs logical operations on the timing pulse and the result signal to generate the detection signal.

[0013] Preferably, the reference voltage is less than (ULmin*Rcs)*t / Lp, where ULmin represents the minimum DC high voltage value of the input voltage when the switching power supply is working normally, Rcs represents the resistance of the sampling resistor, Lp represents the inductance of the primary winding, and t represents the detection time.

[0014] Preferably, the control circuit also includes: a counter, which receives the detection signal and counts the pulse signal to obtain a count value. When the count value is equal to a set value, the counter provides an effective protection signal to the drive circuit, so that the switch tube is turned off, the switching power supply is restarted, and the count value is reset.

[0015] Preferably, the driving circuit includes: an oscillator, which provides square wave pulses, and the oscillator adjusts the pulse width of the square wave pulses according to the detection signal; and a switch driving module, which generates the switching signal according to the square wave pulses, and the pulse width of each square wave pulse corresponds to the working cycle or invalid level time of the switching signal.

[0016] Preferably, the control circuit also includes: a feedback circuit for providing a feedback signal based on the output current, and / or for providing the feedback signal based on the sampling voltage or the output voltage, and the driving circuit adjusts the switching signal according to the feedback signal; and a power supply circuit for providing a power supply voltage to the power supply end of the driving circuit according to the input current.

[0017] Preferably, the preset slope value is smaller than the minimum change rate of the sampling voltage when the switching power supply is operating normally.

[0018] According to a second aspect of the present invention, a switching power supply is provided, comprising: a primary winding and a secondary winding, the primary winding generating an input current based on an input voltage, and the secondary winding generating an output current based on the input current; a switching tube and a sampling resistor, the switching tube, the sampling resistor and the primary winding being connected in series, the sampling resistor sampling the current flowing through the switching tube to obtain a sampling voltage, and a switching signal controlling the on-time and off-time of the switching tube; a short-circuit protection circuit generating a detection signal based on the sampling voltage, wherein if the rising slope of the sampling voltage is less than a preset slope value, the detection signal provides a pulse signal; and a driving circuit for providing the switching signal and increasing the duty cycle of the switching signal based on the pulse signal to increase the off-time of the switching tube.

[0019] Preferably, the driving circuit responds to each of the pulse signals to increase the duty cycle and the off time of the switching tube under the control of each of the pulse signals.

[0020] Preferably, the driving circuit responds to a group of the pulse signals, each group of the pulse signals includes a plurality of the pulse signals that appear in sequence, so that: every certain number of the pulse signals, the driving circuit adjusts the switching signal so that the duty cycle and the turn-off time of the switching tube are increased accordingly.

[0021] Preferably, each of the pulse signals has a rising edge and a falling edge, and the driving circuit changes the switch signal from a valid level to an invalid level when triggered by the falling edge of the pulse signal, so that the switch tube changes from an on state to an off state.

[0022] Preferably, the short-circuit protection circuit includes: a comparison module, which compares the sampled voltage and the reference voltage when the switch tube is in the on state, and outputs a result signal, wherein the result signal indicates whether the rising slope of the sampled voltage is less than the preset slope value; and when the switch tube is in the off state, the result signal is reset to a low level; a timing module, which provides a timing pulse when the on-duration time of the switch signal reaches the detection time; a logic module, which provides the detection signal according to the timing pulse and the result signal, and when the timing pulse starts, if the sampled voltage is less than the reference voltage, the detection signal provides the pulse signal; if the sampled voltage is greater than or equal to the reference voltage, the detection signal is maintained at a high level; and when the timing pulse ends, the detection signal is reset to the high level.

[0023] Preferably, the short-circuit protection circuit further includes a voltage generating module, configured to provide the constant reference voltage according to the preset slope value.

[0024] Preferably, the comparison module is implemented by a comparator, the non-inverting input terminal of the comparator receives the reference voltage, the inverting input terminal of the comparator receives the sampling voltage, the output terminal of the comparator provides the result signal, the timing pulse is a high-level pulse, when the conduction duration of the switch tube reaches the detection time, the timing pulse has a rising edge, and after the switching cycle ends, the timing pulse has a falling edge, and the logic module performs logical operations on the timing pulse and the result signal to generate the detection signal.

[0025] Preferably, the switching power supply further comprises: a counter, which receives the detection signal and counts the pulse signal to obtain a count value. When the count value is equal to a set value, the counter provides a valid protection signal to the drive circuit, so that the switch tube is turned off, the switching power supply is restarted, and the count value is reset.

[0026] Preferably, the driving circuit includes: an oscillator, which provides square wave pulses, and the oscillator adjusts the pulse width of the square wave pulses according to the detection signal; and a switch driving module, which generates the switching signal according to the square wave pulses, and the pulse width of each square wave pulse corresponds to the working cycle or invalid level time of the switching signal.

[0027] Preferably, the switching power supply further includes: a feedback circuit, configured to provide a feedback signal based on the output current, and / or configured to provide the feedback signal based on the sampling voltage or the output voltage, wherein the driving circuit adjusts the switching signal based on the feedback signal; and a power supply circuit, configured to provide a power supply voltage to the power supply end of the driving circuit based on the input current.

[0028] Preferably, the preset slope value is smaller than the minimum change rate of the sampling voltage when the switching power supply is operating normally.

[0029] Preferably, the reference voltage is less than (ULmin*Rcs)*t / Lp, where ULmin represents the minimum DC high voltage value of the input voltage when the switching power supply is working normally, Rcs represents the resistance of the sampling resistor, Lp represents the inductance of the primary winding, and t represents the detection time.

[0030] According to a third aspect of the present invention, a method for controlling a switching power supply is provided, comprising: generating a detection signal based on a sampling voltage, wherein the sampling voltage is provided by a sampling resistor in the switching power supply for sampling the current of a switching tube, and providing a pulse signal if a rising slope of the sampling voltage is less than a preset slope value; and increasing a duty cycle of a switching signal for controlling the switching tube based on the pulse signal to increase an off time of the switching tube.

[0031] Preferably, the control method further comprises: reducing the on-time of the switch tube according to the pulse signal.

[0032] Preferably, the pulse signal has a rising edge and a falling edge, and when triggered by the falling edge of the pulse signal, the switch tube changes from an on state to an off state.

[0033] Preferably, the step of generating a detection signal according to the sampling voltage includes: providing a reference voltage; and when the on-time of the switching signal reaches a detection time, if the sampling voltage is less than the reference voltage, providing the pulse signal.

[0034] Preferably, when the on-duration time of the switching signal reaches the detection time, if the sampling voltage is less than the reference voltage, the step of providing the pulse signal includes: providing a timing pulse, when the on-duration time of the switching signal reaches the detection time, the timing pulse flips from a low level to a high level, and is reset to a low level after the switching cycle ends; after the timing pulse starts, if the sampling voltage is less than the reference voltage, the falling edge of the pulse signal is provided, and the rising edge of the pulse signal is provided after the switching tube is turned off.

[0035] Preferably, the step of providing the reference voltage includes: generating the reference voltage according to the detection time.

[0036] Preferably, the detection time is set according to the magnetic flux density of the primary winding to prevent the primary winding from reaching saturation.

[0037] Preferably, the control method further comprises: counting the pulse signal to obtain a count value; and when the count value is equal to a set value, turning off the switch tube, restarting the switching power supply, and resetting the count value.

[0038] Preferably, the step of increasing the duty cycle of the switching signal according to the pulse signal to increase the turn-off time of the switching tube includes: providing square wave pulses, the pulse width of each square wave pulse corresponding to the duty cycle or invalid level time of the switching signal; and adjusting the pulse width of the square wave pulse according to the detection signal.

[0039] Preferably, the step of increasing the duty cycle of the switching signal according to the pulse signal to increase the off time of the switching tube further includes: increasing the duty cycle and the off time of the switching tube under the control of each of the pulse signals.

[0040] Preferably, the step of increasing the duty cycle of the switching signal according to the pulse signal to increase the off time of the switching tube also includes: adjusting the switching signal every at least one pulse signal so that the duty cycle and off time of the switching tube are increased accordingly.

[0041] Preferably, the preset slope value is smaller than the minimum change rate of the sampling voltage when the switching power supply is operating normally.

[0042] Preferably, the reference voltage is smaller than a first current limit step value of the switching power supply during a soft start process.

[0043] The control circuit, switching power supply, and control method thereof provided by embodiments of the present invention detect whether a sampling resistor is abnormal based on whether the rising slope of a sampled voltage provided by the sampling resistor is less than a preset slope value. When the rising slope of the sampled voltage is less than the preset slope value, indicating an abnormality in the sampling resistor, a pulse signal is generated to reduce the operating frequency of the switching power supply, thereby increasing the off-time of the switch. Adjustment of the off-time is not limited by the switching cycle, allowing the current in the primary winding of the transformer to be fully released, preventing excessive current accumulation in the primary winding when the sampling resistor is abnormal. This protects the switching power supply under full AC input voltage conditions, even in the event of a short circuit or abnormality in the sampling resistor.

[0044] In a preferred embodiment, when the rising slope of the sampling voltage is less than the preset slope value, the conduction time of the switching tube can be reduced while reducing the operating frequency of the switching power supply, so that the current accumulation time of the primary winding is shortened, further alleviating the current accumulation of the primary current.

[0045] In a preferred embodiment, it is also possible to determine whether the switching power supply needs to be restarted by counting the pulse signal. When the number of times the pulse signal appears reaches a preset upper limit, it is determined that the switching power supply needs to be restored by restarting, thereby protecting the switching power supply under full voltage AC input conditions and avoiding abnormalities or even short circuits in the sampling resistor that may cause damage to the switching power supply.

[0046] In a preferred embodiment, by reasonably setting the preset slope value (the preset slope value is slightly smaller than the rising slope of the sampling voltage at the minimum input voltage when the switching power supply is operating normally), the short-circuit protection mechanism of the switching power supply can be prevented from being falsely triggered when the sampling resistor is operating normally, thereby ensuring that the switching power supply can operate continuously without interruption under normal operating voltage conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] The above and other objects, features and advantages of the present invention will become more apparent through the following description of the embodiments of the present invention with reference to the accompanying drawings, in which:

[0048] Figure 1 FIG. 1 is a schematic structural diagram of a switching power supply according to a first embodiment of the present invention.

[0049] Figure 2 FIG. 1 is a schematic structural diagram of a short-circuit protection circuit according to a first embodiment of the present invention.

[0050] Figure 3a Schematic diagram showing various signal waveforms of the short-circuit protection circuit of the first embodiment of the present invention under normal circumstances. Figure 3b Schematic diagram showing various signal waveforms of the short-circuit protection circuit in the first embodiment of the present invention under short-circuit protection conditions.

[0051] Figure 4 FIG. 1 is a schematic structural diagram of a driving circuit according to a first embodiment of the present invention.

[0052] Figure 5 FIG. 1 is a schematic diagram showing the waveform of the input current in one working cycle according to the first embodiment of the present invention.

[0053] Figure 6 FIG. 1 is a schematic structural diagram of a switching power supply according to a second embodiment of the present invention.

[0054] Figure 7 FIG. 1 is a schematic structural diagram of a switching power supply according to a third embodiment of the present invention.

[0055] Figure 8 A schematic diagram showing the waveform of the input current provided by the primary winding in a switching power supply.

[0056] Figure 9 A control method for a switching power supply according to a fourth embodiment of the present invention is shown. DETAILED DESCRIPTION

[0057] Various embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. In each of the accompanying drawings, identical elements are represented by identical or similar reference numerals. For the sake of clarity, the various parts in the accompanying drawings are not drawn to scale. In addition, some well-known parts may not be shown.

[0058] Many specific details of the present invention are described below, such as device structures, materials, dimensions, processing techniques, or technologies, to facilitate a clearer understanding of the present invention. However, as will be appreciated by those skilled in the art, the present invention may be practiced without following these specific details.

[0059] The present invention may be embodied in various forms, some examples of which are described below.

[0060] Figure 1 FIG. 1 is a schematic structural diagram of a switching power supply according to a first embodiment of the present invention.

[0061] like Figure 1 As shown, the switching power supply 100 includes a rectifier bridge 110, a control circuit 1000, a transformer, a switch tube M1, a sampling resistor Rcs, a freewheeling diode D0, an input capacitor Cin, and an output capacitor Cout. The rectifier bridge 110 is used to convert an AC input signal into an input voltage Vin, and the input capacitor Cin is used to stabilize and filter the input voltage Vin. The transformer includes a primary winding Lp and a secondary winding Ls. The primary winding is connected in series with the switch tube M1 and the sampling resistor Rcs and generates an input current Iin based on the input voltage Vin. The secondary winding Ls is connected in series with the freewheeling diode D0 and generates an output current based on the input current Iin. The output capacitor Cout stabilizes and filters the output voltage Vout. The sampling resistor Rcs samples the current flowing through the switch tube M1 to obtain a sampled voltage Vcs, which is used, for example, to detect the operating state of the switching power supply or implement feedback control.

[0062] As a specific embodiment, Figure 1 As shown, the switch tube M1 is connected in series between the primary winding Lp and the sampling resistor Rcs. The first end of the sampling resistor Rcs connected to the switch tube M1 provides the sampling voltage Vcs, and the other end is connected to the reference ground potential. It should be noted that Figure 1Only one structure of the switching power supply is shown. The switching power supply of the embodiment of the present invention can also be composed of components such as the switching tube M1, the transformer, and the freewheeling diode D0 in other connection modes, so that the sampling resistor Rcs can provide the sampling voltage Vcs according to the current flowing through the switching tube M1.

[0063] Control circuit 1000 is configured to provide a switching signal sw based on a sampled voltage Vcs and to place the switching power supply into a short-circuit warning mode when the sampled voltage Vcs is abnormal. The control terminal of switch M1 receives the switching signal sw, which controls the on-time ton and off-time toff of switch M1.

[0064] Specifically, the control circuit 1000 includes a drive circuit 130 and a short-circuit protection circuit 120. The short-circuit protection circuit 120 is used to detect whether the switching power supply is operating in an abnormal state (for example, the sampling resistor Rcs is abnormal or even short-circuited, resulting in excessive current accumulation in the primary winding Lp) and generate a detection signal Vsen based on the detection result. Under the control of the detection signal Vsen, the drive circuit 130 provides a switching signal sw to the switching transistor M1. When the detection signal Vsen indicates a normal state, the drive circuit 130 drives the switching power supply to continue normal operation. When the detection signal Vsen indicates an abnormal state, the drive circuit 130 drives the switching power supply to enter a short-circuit warning mode to protect the switching power supply.

[0065] In this embodiment, the short-circuit protection circuit 120 determines whether the switching power supply is operating abnormally based on the rising slope of the sampled voltage Vcs. When the rising slope of the sampled voltage Vcs is less than a preset slope value, the detection signal Vsen provides a pulse signal. Upon receiving the pulse signal, the driver circuit 130 increases the duty cycle of the switching signal sw, thereby reducing the operating frequency of the switching power supply. This increases the off-time toff of the switching transistor M1 without being restricted by the normal operating cycle, allowing the current accumulated in the primary winding Lp to be fully discharged within the off-time.

[0066] In a preferred embodiment, the driver circuit 130 can further reduce the on-time ton of the switch M1 upon receiving the pulse signal. In a specific preferred embodiment, the driver circuit 130 can turn off the switch M1 upon the falling edge of the pulse signal. At this time, the on-time of the switch M1 is reduced to a fixed value, for example, to the detection time tj.

[0067] It should be noted that in some embodiments, the duty cycle of the switching signal can be increased under the control of each pulse signal, that is, the drive circuit 130 responds to each pulse signal. In some alternative embodiments, the amount of current accumulated in the primary winding is limited, so the switching signal can be adjusted after at least one pulse signal, so that the duty cycle and the off-time of the switch tube are correspondingly increased. In other words, the drive circuit responds to a group of pulse signals, each group of pulse signals including multiple pulse signals that appear in sequence, so that: after a certain number of pulse signals, the drive circuit adjusts the switching signal, so that the duty cycle and off-time of the switch tube are correspondingly increased.

[0068] Figure 2 FIG. 1 is a schematic structural diagram of a short-circuit protection circuit according to a first embodiment of the present invention. Figure 3a A schematic diagram showing waveforms of various signals of the short-circuit protection circuit according to the first embodiment of the present invention under normal circumstances is shown. Figure 3b A schematic diagram showing waveforms of various signals of the short-circuit protection circuit of the first embodiment of the present invention under a short-circuit protection condition is shown.

[0069] like Figure 2 As shown, the short circuit protection circuit 120 includes a comparison module 121 , a timing module 122 , a logic module 123 and a voltage generation module 124 .

[0070] The comparison module 121 is configured to compare the sample voltage Vcs with the reference voltage Vref and output a result signal Vcomp. The result signal Vcomp indicates whether the rising slope of the sample voltage Vcs is less than a preset slope value.

[0071] The timing module 122 is used to provide a timing pulse Vj. There is a fixed detection time between the start of a switching cycle and the rising edge of the timing pulse within the switching cycle.

[0072] The voltage generation module 124 is used to provide a reference voltage Vref. The reference voltage Vref can be a fixed voltage or a ramp voltage that varies with the input voltage. By properly setting the reference voltage Vref, the comparison result between the reference voltage Vref and the sampled voltage Vcs can indicate whether the rising slope of the sampled voltage Vcs is less than a preset slope value.

[0073] The logic module 123 is configured to provide a detection signal Vsen based on the timing pulse Vj and the result signal Vcomp. During the detection time, no comparison is performed between the sampled voltage and the reference voltage, and the detection signal is in an initial state (e.g., a high level). When the rising edge of the timing pulse Vj begins, if the sampled voltage Vcs is less than the reference voltage Vref, this indicates that the rising slope of the sampled voltage Vcs is less than a preset slope value (i.e., the rising rate of the current flowing through the switch M1 has not reached a reasonable value), indicating that the switching power supply may have an abnormal condition and needs to enter a short-circuit warning mode. In this case, the detection signal Vsen provides a pulse signal. Conversely, when the rising edge of the timing pulse Vj begins, if the sampled voltage Vcs is greater than or equal to the reference voltage Vref, this indicates that the rising slope of the sampled voltage Vcs is greater than or equal to the preset slope value (i.e., the rising slope of the current flowing through the switch M1 has reached a reasonable value), indicating that the switching power supply has no abnormal condition and does not need to enter the short-circuit warning mode. In this case, the detection signal Vsen maintains the initial high level. Furthermore, the detection signal Vsen can be reset to a high level after the switch is turned off.

[0074] As a specific embodiment, Figure 2 、 Figure 3a and Figure 3b As shown, the comparison module 121 is implemented by a comparator, the non-inverting input terminal of the comparator receives the reference voltage Vref, the inverting input terminal of the comparator receives the sampling voltage Vcs, the output terminal of the comparator provides a result signal Vcomp, the timing pulse Vj is a square wave pulse, and the logic module 123 performs a logic operation (for example, a NAND operation) on the timing pulse Vj and the result signal Vcomp to generate a detection signal Vsen, thereby providing a pulse signal when the switching power supply is abnormal.

[0075] When the switching power supply is operating normally, the input voltage may be equal to a smaller value within the normal input voltage Vin range. At this time, the rising rate of the sampled voltage Vcs is very slow, so it is necessary to reasonably set the preset slope value to prevent false triggering of the short-circuit warning mode. Specifically, the preset slope value can be calculated based on the minimum voltage value after the input voltage Vin stabilizes (that is, the minimum value of the input voltage when the switching power supply is operating normally), so that the preset slope value is less than or equal to the minimum change rate of the input current provided by the switching power supply during normal operation. Based on this, the reference signal Vref satisfies, for example:

[0076]

[0077] Wherein, ULmin represents the minimum value of the input DC high voltage when the switching power supply is working normally, Rcs represents the resistance of the sampling resistor, Lp represents the inductance of the primary winding, and tj represents the detection time.

[0078] On the other hand, in addition to preventing false triggering, we must also be aware of the problem of the switching power supply failing to trigger the short-circuit protection when starting up if the sampling resistor is short-circuited. Therefore, the reference voltage should be set to be lower than the first current limit step value during the switching power supply's soft start process.

[0079] In addition, in order to prevent the components in the switching power supply from being burned due to excessive current on the primary winding when the sampling resistor Rcs is short-circuited, during the first working cycle after the switching power supply enters the short-circuit warning mode, the on-time of the switch tube is equal to the detection time, and the peak current Ipk on the primary winding should meet the following requirements:

[0080]

[0081]

[0082] Where Bm represents the transformer's maximum magnetic flux density, Np represents the number of turns on the transformer's primary side, Ae represents the effective cross-sectional area of the transformer's magnetic core, UL represents the input DC high voltage (generally, because current rises very rapidly under high voltage, the calculation of Ipk and Bm here should be based on the maximum input voltage within the system's normal operating voltage range), Lp represents the inductance of the primary winding, t represents the activation time of the short-circuit protection, and Iv represents the initial current. It should be noted that the boundary value of the magnetic flux density is typically 0.35, but those skilled in the art may also set other boundary values based on actual needs.

[0083] Figure 4 FIG. 1 is a schematic structural diagram of a driving circuit according to a first embodiment of the present invention. Figure 5 FIG. 1 is a schematic diagram showing the waveform of the input current in one working cycle according to the first embodiment of the present invention.

[0084] like Figure 4 As shown, the driving circuit 130 includes a switch driving module 131 and an oscillator 132. The switch driving module 131 is used to generate a switching signal sw; the oscillator 132 is used to provide square wave pulses to the switch driving module 131, each of the square wave pulses corresponding to the duty cycle or inactive level time of the switching signal, so that the oscillator 132 can adjust the duty cycle or inactive level time of the switching signal sw output by the switch driving module 131. Specifically, when the oscillator 132 receives a pulse signal, the oscillator 132 increases the pulse width of the square wave pulse, so that the switch driving module 131 reduces the operating frequency of the switch tube, thereby increasing the off time toff of the switch tube, so that the current accumulated on the primary winding can be fully released.

[0085] When multiple pulse signals are continuously generated in the switching power supply, in order to ensure that the primary current does not accumulate seriously, it is necessary to reasonably set the switch tube off time toff so that the increase ΔI1 of the input current during the conduction process in each working cycle is close to or the same as the decrease ΔI2 of the input current during the off process (e.g. Figure 5 As shown), that is, it is necessary to meet:

[0086]

[0087] Where ULmax represents the maximum DC voltage of the input voltage Vin under normal conditions, ton represents the on-time of the switch M1, toff represents the off-time of the switch M1, n represents the turns ratio between the primary and secondary windings, and Vs represents the voltage across the secondary winding. It can be seen that when the switching power supply drives a load with a high voltage after the sampling resistor is short-circuited, Vs is minimum (for example, 0.5V), and the required off-time toff is the longest.

[0088] Figure 6 FIG. 1 is a schematic structural diagram of a switching power supply according to a second embodiment of the present invention.

[0089] like Figure 6 As shown, the switching power supply 200 of the second embodiment of the present invention is basically the same as the switching power supply 100 of the first embodiment described above, except that: the switching power supply 200 can also control the drive circuit from the short-circuit warning mode to the short-circuit protection mode according to the number of pulse signals, so that the switching power supply is restarted or directly shut down to prevent the components in the switching power supply from being subjected to excessive current and being burned when the sampling resistor is short-circuited or abnormal.

[0090] Specifically, based on the above first embodiment, Figure 6 As shown, the control circuit 1000 of the switching power supply 200 further includes a counter 140 , which receives the detection signal Vsen, and is capable of counting the pulse signal to obtain a count value.

[0091] When the count value equals the set value, counter 140 provides a valid protection signal Vrst to driver circuit 130, causing driver circuit 130 to enter short-circuit protection mode. In short-circuit protection mode, switch M1 is turned off, allowing the entire switching power supply to be shut down / restarted, and the count value is reset to 0, ready to re-count the pulse signal after the switching power supply is restarted.

[0092] In some embodiments, the count value obtained by the counter may also be used to control the driving circuit so that the driving circuit does not respond to every pulse signal but only responds to a pulse signal separated by at least one pulse signal.

[0093] Figure 7FIG. 1 is a schematic structural diagram of a switching power supply according to a third embodiment of the present invention.

[0094] like Figure 7 As shown, the switching power supply 300 of the third embodiment of the present invention is substantially the same as the switching power supplies of the first and second embodiments described above, wherein the control circuit 1000 may be the same as the control circuit of the first embodiment or the control circuit of the second embodiment described above. The difference is that the switching power supply 300 further includes a feedback circuit 360 and / or a power supply circuit 350.

[0095] The feedback circuit 360 is used to provide a feedback signal Vfb based on the sampled voltage Vcs and / or the output voltage Vout (or sampled voltage) generated by the output current. The driving circuit in the control circuit 1000 adjusts the switching signal sw according to the feedback signal Vfb, thereby performing feedback control on the switching power supply.

[0096] The power supply circuit 350 is used to provide a power supply voltage Vcc to the control circuit 1000 according to the input current Iin. The power supply voltage Vcc can be used as the power supply voltage for the driving circuit and the short-circuit protection circuit in the control circuit.

[0097] Figure 8 A schematic diagram showing the waveform of the input current provided by the primary winding in a switching power supply.

[0098] like Figure 1 、 Figure 6 、 Figure 7 and Figure 8 As shown, when the switching power supply is operating normally (e.g., corresponding to time period t0-t1), the drive circuit 130 operates in a normal operating mode, and the switch M1 is turned on and off by the switching signal sw to ensure the stability of the output voltage Vout and the constancy of the duty cycle T. When the switching signal sw is valid, the switch M1 is turned on, and the input current Iin provided by the primary winding Lp gradually increases over time. In this case, the rising slope of the input current Iin is greater than or equal to a preset slope value. When the switching signal sw is invalid, the switch M1 is turned off, and the input current Iin gradually decreases over time.

[0099] When the switching power supply operates in an abnormal state (e.g., corresponding to the time period t1-t2), such as when the sampling resistor Rs is short-circuited, if there is no short-circuit protection circuit 120, the voltage value of the sampling voltage Vcs cannot be detected, and the switch M1 will operate at the maximum duty cycle. At this time, the on-time of the switch M1 is very long and continuously greater than the off-time, which will cause the primary current of the transformer to continue to accumulate and cannot be fully discharged in each working cycle T. When the input current causes the magnetic core of the primary winding to be severely saturated, the switching power supply will burn out under the action of the high current.

[0100] In the embodiment of the present application, when the switching power supply is operating in an abnormal state (e.g., corresponding to the time period t2-t3), such as when the sampling resistor Rs is short-circuited, the short-circuit protection circuit 120 determines the rising rate of the sampling voltage Vcs. If the rising rate of the sampling voltage Vcs is less than a reasonable preset slope value, a pulse signal is generated, causing the driver circuit 130 to enter a short-circuit warning mode. In the short-circuit warning mode, the duty cycle T of the switch tube M1 is increased to increase the off time toff of the switch tube M1, thereby directly increasing the time it takes for the primary winding to release current.

[0101] In a preferred embodiment, the on-time of the switch tube M1 can also be reduced (for example, reduced to a fixed value) so that the time for the primary winding to accumulate current can also be shortened, further preventing the input current on the primary winding from being too large.

[0102] In a preferred embodiment, Figure 8 As shown, when the number of pulse signals reaches a set value (for example, after 3 moments), the driving circuit 130 can turn off the switch tube M1 to shut down / restart the switching power supply, so that the input current on the primary winding continues to decrease.

[0103] Figure 9 A short-circuit protection method for a switching power supply according to a fourth embodiment of the present invention is shown.

[0104] The switching power supply of this embodiment can be the switching power supply in the above embodiments, including: a primary winding and a secondary winding, the primary winding generates an input current according to the input voltage, and the secondary winding generates an output current according to the input current; a switching tube and a sampling resistor, the switching tube, the sampling resistor and the primary winding are connected in series, the sampling resistor samples the current flowing through the switching tube to obtain a sampling voltage, and the switching signal controls the on time and off time of the switching tube.

[0105] like Figure 9 As shown, the short circuit protection method includes the following steps S110 to S140, wherein steps S120 and S140 can be omitted.

[0106] Step S110: Generate a detection signal based on the sampled voltage. If the rising slope of the sampled voltage is less than a preset slope value, the detection signal provides a pulse signal. This pulse signal is used to put the switching power supply into a short-circuit warning mode. The pulse signal has a rising edge and a falling edge.

[0107] As an embodiment, the sampling voltage and the reference voltage can be compared to detect whether the rising slope of the sampling voltage is less than a preset value. When the on-time of the switch signal reaches the detection time, if the sampling voltage is less than the reference voltage, a pulse signal is provided.

[0108] The specific steps of providing a pulse signal include: when the conduction duration of the switching signal reaches the detection time, starting to provide a timing pulse (at this time, the timing pulse, for example, has a rising edge, and is not reset to a low level until the end of the switching cycle); if the sampling voltage is detected to be less than the reference voltage at this time, providing a falling edge of the pulse signal; when the switching tube is turned off, the timing pulse can be ended, and the rising edge of the pulse signal can be provided or the detection signal can be reset to the initial level.

[0109] As a preferred embodiment, the detection time can be set based on the magnetic flux density of the primary winding to prevent the primary winding from reaching saturation. A reference voltage can also be provided based on the detection time. The reference voltage can be a constant voltage or a ramp voltage. To ensure that the short-circuit warning mode is not falsely triggered during normal operation (when the input voltage is low), the preset slope value is slightly smaller than the rate of change of the input current at the minimum input voltage during normal operation of the switching power supply.

[0110] Step S120, determining whether the number of occurrences of the pulse signal reaches a set value. If yes, proceed to step S140; if not, proceed to step S130.

[0111] In step S130 , the duty cycle of the switch signal is increased according to the pulse signal to increase the off time of the switch tube.

[0112] As an embodiment, a square wave pulse representing the duty cycle / off time of the switching signal can be provided, and the pulse width of each square wave pulse corresponds to the duty cycle or invalid level time of the switching signal, for example. The pulse width of the square wave pulse is then adjusted according to the detection signal, so that the duty cycle and off time of the switching tube in the short-circuit warning mode are lengthened.

[0113] In a preferred embodiment, step S130 further includes reducing the on-time of the switch according to the pulse signal, thereby further alleviating current accumulation in the primary winding. Specifically, the switch can be triggered by the falling edge of the pulse signal to change from the on state to the off state, thereby shutting down the switch before the end of the normal on-time.

[0114] In step S140, when the number of pulse signals reaches a set value, the switch is turned off, causing the switching power supply to enter short-circuit protection mode from short-circuit warning mode. The specific steps include: counting the pulse signals to obtain a count value; when the count value equals the set value, turning off the switch, restarting the switching power supply, and resetting the count value.

[0115] In summary, the control circuit, switching power supply, and control method thereof provided by embodiments of the present invention detect whether a sampling resistor is abnormal based on whether the rising slope of the sampled voltage provided by the sampling resistor is less than a preset slope value. When the rising slope of the sampled voltage is less than the preset slope value, indicating an abnormality in the sampling resistor, a pulse signal is generated to reduce the operating frequency of the switching power supply, thereby increasing the off-time of the switching transistor. Adjustment of the off-time is not limited by the switching cycle, allowing the current in the primary winding of the transformer to be fully released, preventing excessive current accumulation in the primary winding when the sampling resistor is abnormal, thereby protecting the switching power supply in the event of a short circuit or abnormality in the sampling resistor.

[0116] In a preferred embodiment, when the rising slope of the sampling voltage is less than the preset slope value, the conduction time of the switching tube can be reduced while reducing the operating frequency of the switching power supply, so that the current accumulation time of the primary winding is shortened, further alleviating the current accumulation of the primary current.

[0117] In a preferred embodiment, it is also possible to determine whether the switching power supply needs to be restarted by counting the pulse signal. When the number of times the pulse signal appears reaches a preset upper limit, it is determined that the switching power supply needs to be restored by restarting, thereby protecting the switching power supply under full voltage AC input conditions and avoiding abnormalities or even short circuits in the sampling resistor that may cause damage to the switching power supply.

[0118] In a preferred embodiment, by reasonably setting the preset slope value (the preset slope value is slightly smaller than the rising slope of the sampling voltage at the minimum input voltage when the switching power supply is operating normally), the short-circuit protection mechanism of the switching power supply can be prevented from being falsely triggered when the sampling resistor is operating normally, thereby ensuring that the switching power supply can operate continuously without interruption under normal circumstances.

[0119] The embodiments of the present invention are described above, but these embodiments do not describe all details in detail, nor do they limit the invention to specific embodiments. Obviously, many modifications and variations can be made based on the above description. This specification selects and specifically describes these embodiments in order to better explain the principles and practical applications of the present invention, so that those skilled in the art can make good use of the present invention and modify and use it based on the present invention. The scope of protection of the present invention shall be based on the scope defined by the claims of the present invention.

Claims

1. A control circuit of a switching power supply, characterized in that: include: A short-circuit protection circuit generates a detection signal according to a sampling voltage, wherein the sampling voltage is provided by a sampling resistor in the switching power supply for sampling the current of the switching tube, and if the rising slope of the sampling voltage is less than a preset slope value, the detection signal provides a pulse signal; as well as The driving circuit is used to provide a switching signal for controlling the switching tube, and reduce the operating frequency of the switching power supply according to the pulse signal, increase the duty cycle of the switching signal of the switching tube, so as to increase the off time of the switching tube, and the adjustment of the off time is not limited by the switching cycle. Wherein, the driving circuit includes an oscillator, providing square wave pulses, and the oscillator adjusts the pulse width of the square wave pulses according to the detection signal; and A switch driving module generates the switch signal according to the square wave pulse, wherein the pulse width of each square wave pulse corresponds to the duty cycle of the switch signal. When the switching power supply operates in a normal operating mode, the switch tube is turned on and off under the action of the switching signal to ensure the stability of the output voltage and the constancy of the working cycle. When the switching power supply operates in an abnormal state, the short-circuit protection circuit generates the pulse signal, causing the drive circuit to enter a short-circuit warning mode, and in the short-circuit warning mode, the on-time of the switch tube is reduced while the duty cycle is increased, so that the off-time of the switch tube is increased.

2. The control circuit according to claim 1, wherein: The driving circuit responds to each of the pulse signals to increase the duty cycle and the off time of the switch tube under the control of each of the pulse signals.

3. The control circuit according to claim 1, wherein: The driving circuit responds to a group of the pulse signals, each group of the pulse signals comprising a plurality of the pulse signals appearing in sequence, such that: At intervals of a certain number of the pulse signals, the driving circuit adjusts the switching signal so that the duty cycle and the off time of the switching tube are increased accordingly.

4. The control circuit according to claim 1, wherein: Each of the pulse signals has a rising edge and a falling edge. The driving circuit changes the switch signal from a valid level to an invalid level when triggered by the falling edge of the pulse signal, so that the switch tube changes from an on state to an off state.

5. The control circuit according to claim 1, wherein: The short-circuit protection circuit comprises: a comparison module, which compares the sampled voltage with a reference voltage when the switch tube is in the on state and outputs a result signal, wherein the result signal indicates whether the rising slope of the sampled voltage is less than the preset slope value; and resets the result signal to a low level when the switch tube is in the off state; a timing module, which provides a timing pulse when the on-time duration of the switch signal reaches the detection time; a logic module, providing the detection signal according to the timing pulse and the result signal, When the timing pulse starts, if the sampling voltage is less than the reference voltage, the detection signal provides the pulse signal; if the sampling voltage is greater than or equal to the reference voltage, the detection signal is maintained at a high level. When the timing pulse ends, the detection signal is reset to the high level.

6. The control circuit according to claim 5, wherein: The short-circuit protection circuit further includes a voltage generating module, configured to provide the constant reference voltage according to the preset slope value.

7. The control circuit according to claim 5, wherein: The comparison module is implemented by a comparator, the non-inverting input terminal of the comparator receives the reference voltage, the inverting input terminal of the comparator receives the sampled voltage, and the output terminal of the comparator provides the result signal. The timing pulse is a high-level pulse. When the on-time duration of the switch tube reaches the detection time, the timing pulse has a rising edge. After the switching cycle ends, the timing pulse has a falling edge. The logic module performs a logic operation on the timing pulse and the result signal to generate the detection signal.

8. The control circuit according to claim 5, wherein: The reference voltage is less than (ULmin*Rcs)*t / Lp, Wherein, ULmin represents the minimum DC high voltage input voltage when the switching power supply is working normally, Rcs represents the resistance of the sampling resistor, Lp represents the inductance of the primary winding, and t represents the detection time.

9. The control circuit according to claim 1, wherein: Also includes: a counter that receives the detection signal and counts the pulse signal to obtain a count value, When the count value is equal to the set value, the counter provides a valid protection signal to the driving circuit, so that the switch tube is turned off, the switching power supply is restarted, and the count value is reset.

10. The control circuit according to claim 1, wherein: Also includes: a feedback circuit, configured to provide a feedback signal according to the output current, and / or to provide the feedback signal according to the sampled voltage or the output voltage, wherein the drive circuit adjusts the switching signal according to the feedback signal; as well as The power supply circuit is used to provide a power supply voltage to the power supply terminal of the driving circuit according to the input current.

11. The control circuit according to claim 1, wherein: The preset slope value is smaller than the minimum change rate of the sampling voltage when the switching power supply is operating normally.

12. A switching power supply, characterized in that: include: a primary winding and a secondary winding, wherein the primary winding generates an input current according to an input voltage, and the secondary winding generates an output current according to the input current; A switching tube and a sampling resistor, wherein the switching tube, the sampling resistor and the primary winding are connected in series, the sampling resistor samples the current flowing through the switching tube to obtain a sampling voltage, and a switching signal controls the on-time and off-time of the switching tube; a short-circuit protection circuit, generating a detection signal according to the sampled voltage, wherein if the rising slope of the sampled voltage is less than a preset slope value, the detection signal provides a pulse signal; as well as The driving circuit is used to provide the switching signal and reduce the operating frequency of the switching power supply according to the pulse signal, thereby increasing the duty cycle of the switching signal to increase the off time of the switching tube, and the adjustment of the off time is not limited by the switching cycle. Wherein, the driving circuit includes an oscillator, providing square wave pulses, and the oscillator adjusts the pulse width of the square wave pulses according to the detection signal; and A switch driving module generates the switch signal according to the square wave pulse, wherein the pulse width of each square wave pulse corresponds to the duty cycle or invalid level time of the switch signal. When the switching power supply operates in a normal operating mode, the switch tube is turned on and off under the action of the switching signal to ensure the stability of the output voltage and the constancy of the working cycle. When the switching power supply operates in an abnormal state, the short-circuit protection circuit generates the pulse signal, causing the drive circuit to enter a short-circuit warning mode, and in the short-circuit warning mode, the on-time of the switch tube is reduced while the duty cycle is increased, so that the off-time of the switch tube is increased.

13. The switching power supply according to claim 12, wherein: The driving circuit responds to each of the pulse signals to increase the duty cycle and the off time of the switch tube under the control of each of the pulse signals.

14. The switching power supply according to claim 12, wherein: The driving circuit responds to a group of the pulse signals, each group of the pulse signals comprising a plurality of the pulse signals appearing in sequence, such that: At intervals of a certain number of the pulse signals, the driving circuit adjusts the switching signal so that the duty cycle and the off time of the switching tube are increased accordingly.

15. The switching power supply according to claim 12, wherein: Each of the pulse signals has a rising edge and a falling edge. The driving circuit changes the switch signal from a valid level to an invalid level when triggered by the falling edge of the pulse signal, so that the switch tube changes from an on state to an off state.

16. The switching power supply according to claim 12, wherein: The short-circuit protection circuit comprises: a comparison module, which compares the sampled voltage with a reference voltage when the switch tube is in the on state and outputs a result signal, wherein the result signal indicates whether the rising slope of the sampled voltage is less than the preset slope value; and resets the result signal to a low level when the switch tube is in the off state; a timing module, which provides a timing pulse when the on-time duration of the switch signal reaches the detection time; a logic module, providing the detection signal according to the timing pulse and the result signal, When the timing pulse starts, if the sampling voltage is less than the reference voltage, the detection signal provides the pulse signal; if the sampling voltage is greater than or equal to the reference voltage, the detection signal is maintained at a high level. When the timing pulse ends, the detection signal is reset to the high level.

17. The switching power supply according to claim 16, wherein: The short-circuit protection circuit further includes a voltage generating module, configured to provide the constant reference voltage according to the preset slope value.

18. The switching power supply according to claim 16, wherein: The comparison module is implemented by a comparator, the non-inverting input terminal of the comparator receives the reference voltage, the inverting input terminal of the comparator receives the sampled voltage, and the output terminal of the comparator provides the result signal. The timing pulse is a high-level pulse. When the on-time duration of the switch tube reaches the detection time, the timing pulse has a rising edge. After the switching cycle ends, the timing pulse has a falling edge. The logic module performs a logic operation on the timing pulse and the result signal to generate the detection signal.

19. The switching power supply according to claim 12, wherein: Also includes: a counter that receives the detection signal and counts the pulse signal to obtain a count value, When the count value is equal to the set value, the counter provides a valid protection signal to the driving circuit, so that the switch tube is turned off, the switching power supply is restarted, and the count value is reset.

20. The switching power supply according to claim 12, wherein: Also includes: a feedback circuit, configured to provide a feedback signal according to the output current, and / or to provide the feedback signal according to the sampled voltage or the output voltage, wherein the drive circuit adjusts the switching signal according to the feedback signal; A power supply circuit is used to provide a power supply voltage to a power supply terminal of the driving circuit according to the input current.

21. The switching power supply according to claim 12, wherein: The preset slope value is smaller than the minimum change rate of the sampling voltage when the switching power supply is operating normally.

22. The switching power supply according to claim 16, wherein: The reference voltage is less than (ULmin*Rcs)*t / Lp, Wherein, ULmin represents the minimum DC high voltage value of the input voltage when the switching power supply operates normally, Rcs represents the resistance of the sampling resistor, Lp represents the inductance of the primary winding, and t represents the detection time.

23. A method for controlling a switching power supply, characterized in that: include: generating a detection signal according to a sampling voltage, wherein the sampling voltage is provided by a sampling resistor in the switching power supply for sampling the current of the switching tube, and providing a pulse signal if the rising slope of the sampling voltage is less than a preset slope value; as well as The operating frequency of the switching power supply is reduced according to the pulse signal, and the duty cycle of the switching signal used to control the switching tube is increased to increase the off time of the switching tube, and the adjustment of the off time is not limited by the switching cycle. The step of reducing the operating frequency of the switching power supply according to the pulse signal and increasing the duty cycle of the switching signal to increase the off time of the switching tube includes: Providing square wave pulses, wherein the pulse width of each of the square wave pulses corresponds to the duty cycle or inactive level time of the switching signal; and adjusting the pulse width of the square wave pulse according to the detection signal, The control method further includes: when the switching power supply operates in a normal operating mode, the switching tube is turned on and off under the action of the switching signal to ensure the stability of the output voltage and the constancy of the working cycle. When the switching power supply operates in an abnormal state, the pulse signal is generated, causing the switching power supply to enter a short-circuit warning mode, and in the short-circuit warning mode, the on-time of the switching tube is reduced while the duty cycle is increased, so that the off-time of the switching tube is increased.

24. The control method according to claim 23, wherein: Also includes: The on-time of the switch tube is reduced according to the pulse signal.

25. The control method according to claim 24, wherein: The pulse signal has a rising edge and a falling edge. When triggered by the falling edge of the pulse signal, the switch tube changes from an on state to an off state.

26. The control method according to claim 25, wherein: The step of generating a detection signal according to the sampled voltage comprises: providing a reference voltage; and When the on-time duration of the switch signal reaches the detection time, if the sampling voltage is less than the reference voltage, the pulse signal is provided.

27. The control method according to claim 26, wherein: When the on-time of the switch signal reaches the detection time, if the sampling voltage is less than the reference voltage, the step of providing the pulse signal includes: Providing a timing pulse, when the on-time of the switching signal reaches the detection time, the timing pulse flips from a low level to a high level, and then resets to a low level after the switching cycle ends; After the timing pulse starts, if the sampling voltage is less than the reference voltage, a falling edge of the pulse signal is provided, and after the switch tube is turned off, a rising edge of the pulse signal is provided.

28. The control method according to claim 26, wherein: The step of providing the reference voltage includes: The reference voltage is generated according to the detection time.

29. The control method according to claim 26, wherein: The detection time is set according to the magnetic flux density of the primary winding to prevent the primary winding from reaching saturation.

30. The control method according to claim 23, wherein: Also includes: Counting the pulse signal to obtain a count value; as well as When the count value is equal to the set value, the switch tube is turned off, the switching power supply is restarted, and the count value is reset.

31. The control method according to claim 23, wherein: The step of increasing the duty cycle of the switching signal according to the pulse signal to increase the off time of the switching tube further includes: Under the control of each pulse signal, the duty cycle and the off time of the switch tube are increased.

32. The control method according to claim 23, wherein: The step of increasing the duty cycle of the switching signal according to the pulse signal to increase the off time of the switching tube further includes: The switching signal is adjusted every at least one pulse signal, so that the duty cycle and the off time of the switching tube are increased accordingly.

33. The control method according to claim 23, wherein: The preset slope value is smaller than the minimum change rate of the sampling voltage when the switching power supply is operating normally.

34. The control method according to claim 26, wherein: The reference voltage is smaller than a first current limiting step value of the switching power supply during a soft start process.

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