Method and circuit for detecting conduction time of secondary side of switching power supply

Abstract

The invention provides a method and a circuit for detecting conduction time of a secondary side of a switching power supply. The detection method comprises the following steps: detecting a conduction starting moment of a secondary side of the switching power supply; delaying a first preset time after the starting time to obtain a delayed starting time; detecting a moment when the voltage VS of the auxiliary winding of the switching power supply drops to a preset reference voltage, and taking the moment as a conduction ending moment of the secondary side of the switching power supply; and taking the time interval between the delay starting moment and the conduction ending moment as the conduction time of the secondary side of the switching power supply. According to the invention, by introducing a detection mode of delaying the starting time, the measurement of the conduction time is ensured to be more accurate and consistent. The method simplifies the design of a detection circuit, improves the stability and reliability of measurement, facilitates the improvement of the precision of constant current control of the switching power supply and the overall performance of a system, and is suitable for various switching power supply application scenes.

Description

Technical Field

[0001] This invention relates to the field of switching power supply technology, and in particular to a method and circuit for detecting the conduction time of the secondary side of a switching power supply. Background Technology

[0002] The output current of a flyback converter is closely related to the secondary-side conduction time, and accurate detection of the secondary-side conduction time is crucial for achieving constant current control of the system. Existing technologies commonly use knee sampling and zero-crossing sampling for conduction time detection. Knee sampling determines the conduction end time by detecting the slope change of the auxiliary winding voltage. However, as transformer size decreases and leakage inductance increases, the voltage slope change becomes smoother, resulting in an indistinct detection signal and affecting the accuracy and stability of the detection. Zero-crossing sampling determines the conduction end time by detecting the auxiliary winding voltage dropping to zero. Although the circuit structure is relatively simple and easy to implement, the actual time when the voltage drops to zero is delayed compared to the true conduction end time, causing a deviation in the measured conduction time. These issues affect the accuracy of conduction time detection, and consequently, the performance of constant current control in the flyback converter system. Therefore, improving the accuracy and reliability of conduction time detection is a key technical challenge for achieving high-performance flyback switching power supply control. Summary of the Invention

[0003] The purpose of this invention is to provide a method and circuit for detecting the conduction time of the secondary side of a switching power supply, so as to improve the accuracy and reliability of conduction time detection and solve the measurement error caused by the insignificant signal change or the deviation of the detection time in the existing detection method.

[0004] To achieve the above objectives, the present invention discloses the following technical solution: The first aspect of this invention provides a method for detecting the conduction time of the secondary side of a switching power supply, the method comprising: Detect the start time of conduction on the secondary side of the switching power supply; The delayed start time is obtained by delaying the start time by a first preset time after the start time. The moment when the voltage VS of the auxiliary winding of the switching power supply drops to a preset reference voltage is used as the end time of the conduction of the secondary side of the switching power supply. The time interval between the delay start time and the conduction end time is taken as the conduction time of the secondary side of the switching power supply.

[0005] Optionally, it also includes: Obtain the oscillation period of the primary circuit of the switching power supply; One-quarter of the oscillation period is taken as the first preset time.

[0006] Optionally, the formula for the first preset time is: Among them, L p C is the equivalent inductance value of the primary-side circuit. d This is the resonant capacitance value of the primary circuit.

[0007] Optionally, the specific steps for obtaining the delayed start time by delaying the start time by a first preset time after the start time include: After detecting the start of the conduction of the secondary side of the switching power supply, the constant current source is controlled to charge the delay capacitor. When the voltage of the delay capacitor reaches a preset first threshold voltage, a delay start signal is output, and the time when the delay start signal is generated is the delay start time.

[0008] Optionally, the charging current of the constant current source is: Wherein, V1 is the first threshold voltage, and C is the capacitance of the delay capacitor.

[0009] In a second aspect, the present invention provides a detection circuit for the secondary conduction time of a switching power supply, including a delay module, a first comparator CMP1, a first inverter A1, a D flip-flop Drff1, and an SR flip-flop SRL1. The input terminal of the delay module is connected to the conduction start signal of the secondary side of the switching power supply, and the output terminal is connected to the set terminal of the SR flip-flop SRL1. It is used to perform a first preset time delay processing on the conduction start signal and output the delayed start signal after the delay. The non-inverting input of the first comparator CMP1 is connected to a preset reference voltage Vref_zx, the inverting input of the first comparator CMP1 is connected to the voltage VS of the auxiliary winding of the switching power supply, and the output of the first comparator CMP1 is connected to the clock input of the D flip-flop Drff1. The input terminal of the first inverter A1 is connected to the switching control signal SW of the switching power supply, and the output terminal of the first inverter A1 is connected to the reset terminal of the D flip-flop Drff1. The data terminal of the D flip-flop Drff1 is connected to the power supply voltage VDD, and the output terminal of the D flip-flop Drff1 is connected to the reset terminal of the SR flip-flop SRL1. The output terminal of the SR trigger SRL1 outputs a secondary conduction time control signal.

[0010] Optionally, the delay module includes a second inverter A2, a constant current source I, a first switching transistor M1, a second switching transistor M2, and a first capacitor C1; The input terminal of the second inverter A2 is connected to the conduction start signal, and the output terminal of the second inverter A2 is connected to the control terminals of the first switch M1 and the second switch M2. The first terminal of the first switch transistor M1 is connected to the constant current source I. The second terminal of the first switch transistor M1 is connected to the first terminal of the second switch transistor M2 and the first terminal of the first capacitor C1. The second terminal of the second switch transistor M2 and the second terminal of the first capacitor C1 are connected and grounded.

[0011] Optionally, the delay module further includes a Schmitt trigger SMT and a third inverter A3; The input terminal of the Schmitt trigger SMT is connected to the first terminal of the first capacitor C1, and the output terminal of the Schmitt trigger SMT is connected to the input terminal of the third inverter A3. The third inverter A3 outputs the delayed start signal.

[0012] Optionally, the delay module further includes a second comparator CMP2; The non-inverting input of the second comparator CMP2 is connected to the first terminal of the first capacitor C1, the inverting input of the second comparator CMP2 is connected to the comparison threshold voltage Vref_dly, and the output of the second comparator CMP2 outputs the delay start signal.

[0013] Optionally, the first threshold voltage is the threshold voltage of the Schmitt trigger (SMT) or the comparison threshold voltage Vref_dly.

[0014] According to the present invention, by delaying the secondary-side conduction start time by a first preset time and then determining the conduction end time by combining the moment when the auxiliary winding voltage drops to a preset reference voltage, a conduction end time closer to the true secondary-side conduction time can be obtained. The technical solution of the present invention can effectively correct the time deviation caused by the delayed detection time in zero-crossing sampling, improve the accuracy of conduction time measurement, ensure the accuracy of constant current control parameter calculation, and improve the system's response performance under load changes.

[0015] Furthermore, by incorporating a delay module consisting of a constant current source, a delay capacitor, and switching devices into the detection circuit, and combining it with the logic control of a comparator and a flip-flop, accurate delay of the first preset time and precise capture of the conduction end time are achieved, thus ensuring stable detection of the conduction time. This circuit structure can directly calculate the delay time based on the primary-side oscillation period, with clearly defined parameters, making it easy to apply in different switching power supplies.

[0016] The above description is merely an overview of the technical solution of the present invention. In order to better understand the technical means of the present invention and to implement it in accordance with the contents of the specification, the preferred embodiments of the present invention are described in detail below. Attached Figure Description

[0017] Figure 1 A schematic flowchart of a method for detecting the secondary conduction time of a switching power supply according to an embodiment of the present invention is shown; Figure 2 The circuit topology diagram of a flyback switching power supply is shown. Figure 3 A signal waveform diagram according to an embodiment of the present invention is shown; Figure 4 A circuit topology diagram for detecting the on-time of the secondary side of a switching power supply according to an embodiment of the present invention is shown. Figure 5 A circuit topology diagram of a delay module according to an embodiment of the present invention is shown; Figure 6 A circuit topology diagram of a delay module according to another embodiment of the present invention is shown. Detailed Implementation

[0018] To make the above-mentioned objectives, features, and advantages of this application more apparent and understandable, the specific embodiments of this application will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the scope of this application. Furthermore, it should be noted that, for ease of description, only the parts relevant to this application are shown in the accompanying drawings, not the entire structure. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without inventive effort are within the scope of protection of this application.

[0019] The terms “comprising” and “having”, and any variations thereof, used in this application are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or apparatus that includes a series of steps or units is not limited to the steps or units listed, but may optionally include steps or units not listed, or may optionally include other steps or units inherent to such process, method, product, or apparatus.

[0020] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0021] Figure 1 A schematic flowchart illustrating a method for detecting the secondary-side conduction time of a switching power supply according to an embodiment of the present invention is shown. Figure 1As shown, the method for detecting the secondary side conduction time of this switching power supply includes the following specific steps: Step S100: Detect the start time of conduction on the secondary side of the switching power supply.

[0022] In a flyback switching power supply, when the primary-side switch is turned off, the energy stored in the transformer begins to be released to the load through the secondary winding. The freewheeling diode transitions from the off state to the on state, and the secondary current builds up from zero and quickly rises to the normal conduction level. The conduction start moment refers to the instant the freewheeling diode just begins to conduct. This moment can be determined by detecting sudden changes in the auxiliary winding voltage or other relevant signals. Accurately capturing this start moment helps ensure the precision of the starting point for subsequent conduction time measurements, thereby improving overall detection accuracy.

[0023] Step S200: Delay the start time by a first preset time to obtain the delayed start time.

[0024] Step S300: Detect the moment when the voltage VS of the auxiliary winding of the switching power supply drops to a preset reference voltage, and use this as the end time of the switching power supply's secondary side conduction.

[0025] In the above system, the auxiliary winding of the switching power supply senses the voltage of the secondary winding. The voltage VS signal of the auxiliary winding synchronously reflects the voltage change of the secondary winding. The switching power supply control chip can know the voltage change trend of the secondary winding by monitoring the voltage VS. The preset reference voltage can be set to 0V or close to 0V. When the secondary winding finishes discharging, the voltage VS of the auxiliary winding will drop rapidly. The moment when VS drops to about 0V is taken as the end of the conduction of the secondary side of the switching power supply.

[0026] Steps S200 and S300 jointly determine the start and end points of the secondary-side conduction time. In step S200, after detecting the start time of secondary-side conduction, the present invention does not directly use it as the timing start point, but delays it by a preset time that matches the subsequent end detection mechanism to obtain the delayed start time. Subsequently, step S300 detects the moment when the auxiliary winding voltage VS drops to a preset reference voltage and uses this moment as the end time of secondary-side conduction. Since the zero-crossing of the auxiliary winding voltage is used as the end criterion, the end time detection itself has a certain delay. By introducing a corresponding compensation delay at the start time, the definition of the start and end times is kept consistent, thereby making the measured conduction time more accurately reflect the actual secondary-side demagnetization process. This collaborative design effectively improves the accuracy of conduction time measurement and provides more reliable time parameters for constant current control.

[0027] Step S400: The time interval between the delay start time and the conduction end time is used as the conduction time of the secondary side of the switching power supply.

[0028] According to the above embodiments, by delaying the secondary-side conduction start time by a first preset time and then determining the conduction end time by combining the moment when the auxiliary winding voltage drops to a preset reference voltage, a more accurate secondary-side conduction time can be obtained. The technical solution of this invention can effectively correct the time deviation caused by the delayed detection time in zero-crossing sampling, improve the accuracy of conduction time measurement, ensure the accuracy of constant current control parameter calculation, and improve the system's response performance under load changes.

[0029] In one embodiment, the first preset time is one-quarter of the oscillation period of the primary circuit of the switching power supply.

[0030] In one embodiment, the formula for the first preset time is: Among them, L p C is the equivalent inductance value of the primary circuit. d This is the resonant capacitance value of the primary circuit.

[0031] In this embodiment, after the secondary winding demagnetizes, the system is in a dead zone, meaning the primary, secondary, and auxiliary windings are all non-conducting. The transformer winding begins to resonate. The oscillation period of the primary circuit is determined by its equivalent inductance and resonant capacitance. This oscillation reflects the changing pattern of the drain voltage of the switching transistor. A quarter of the oscillation period corresponds to the time it takes for the resonant voltage to gradually decrease from its highest point to the reference level, which precisely reflects the process of the auxiliary winding voltage reaching the preset reference voltage after it begins to decrease. Using this time as a delay can effectively compensate for the time deviation caused by the auxiliary winding voltage detection, making the start time of the delay more accurately correspond to the actual start of secondary winding conduction, thereby improving the accuracy of conduction time measurement.

[0032] Figure 2 The topology diagram of a flyback switching power supply circuit is shown, as follows: Figure 2 As shown, when the secondary circuit of the switching power supply enters the conduction stage, the secondary winding begins to discharge. During this process, the energy decreases over time, and the current in the freewheeling diode gradually drops to zero, marking the end of the demagnetization process of the secondary winding. Simultaneously, the voltage Vd at the drain node of the primary-side switching transistor Q1 enters a resonant state after demagnetization. This resonance is generated by a resonant circuit formed by the equivalent inductance Lp of the primary side of the switching power supply and the capacitance Cd between the drain of the switching transistor and ground. Its center voltage is the input voltage Vin, and the resonant period is Tosc. During the resonance process, when the Vd voltage rises to Vin, the voltage difference between the primary and secondary sides of the switching power supply drops to zero. At this time, the auxiliary winding voltage VS also synchronously drops to zero. It is worth noting that this moment is delayed compared to the end of the secondary demagnetization, and this delay time is equal to one-quarter of the resonant period, i.e., Tdly = Tosc / 4.

[0033] Figure 3A signal waveform diagram according to an embodiment of the present invention is shown. Figure 3 As shown, in the process of detecting the secondary-side conduction time, the originally detected conduction start time signal Tson is first delayed, that is, after Tson begins to rise, a delay of Tdly is applied, and the delayed moment is used as the new timing start point. The signal obtained after this delay processing is defined as the delayed start time signal Tson_d. The end time of Tson_d is triggered by the instant the auxiliary winding voltage VS drops to 0V, thus obtaining the calibrated secondary-side conduction time. Since the start and end points of the delayed start time signal Tson_d are both delayed and corrected, the secondary-side demagnetization time it reflects is closer to the actual situation. Replacing the original start time signal Tson with the delayed start time signal Tson for constant current mode system parameter calculation and design can significantly improve the accuracy of conduction time measurement and constant current control.

[0034] In one embodiment, the specific steps of delaying the start time by a first preset time after the start time in step S200 include: Step S210: After detecting the start time of conduction of the secondary side of the switching power supply, control the constant current source to charge the delay capacitor.

[0035] Step S220: When the voltage of the delay capacitor reaches the preset first threshold voltage, a delay start signal is output. The time when the delay start signal is generated is the delay start time.

[0036] In this embodiment, after detecting the start of the secondary side conduction of the switching power supply, a constant current source is controlled to charge the delay capacitor, causing the capacitor voltage to rise at a stable rate. When the capacitor voltage reaches a preset first threshold, a delay start signal is output, which serves as the delay start time. This process achieves a precise and controllable time delay through constant current charging, ensuring that the timing of the delay start signal is stable and reliable. This provides an accurate starting point for subsequent conduction time measurement, improving the overall detection accuracy and consistency.

[0037] In one embodiment, according to the aforementioned first preset time T dly The formula, combined with the capacitor charging relationship The charging current of the constant current source can be obtained as: Where V1 is the first threshold voltage and C is the capacitance of the delay capacitor.

[0038] Figure 4 A circuit topology diagram for detecting the secondary-side conduction time of a switching power supply according to an embodiment of the present invention is shown. Figure 4As shown, the detection circuit includes a delay module, a first comparator CMP1, a first inverter A1, a D flip-flop Drff1, and an SR flip-flop SRL1. The input of the delay module is connected to the on-start signal of the secondary side of the switching power supply, and its output is connected to the set terminal of the SR flip-flop SRL1. It is used to delay the on-start signal for a first preset time and output the delayed start signal. The non-inverting input of the first comparator CMP1 is connected to a preset reference voltage Vref_zx, and the inverting input is connected to the voltage VS of the auxiliary winding of the switching power supply. The output of the first comparator CMP1 is connected to the clock input of the D flip-flop Drff1. The input of the first inverter A1 is connected to the switching control signal SW of the switching power supply, and its output is connected to the reset terminal of the D flip-flop Drff1. The data terminal of the D flip-flop Drff1 is connected to the power supply voltage VDD, and its output is connected to the reset terminal of the SR flip-flop SRL1. The output of the SR flip-flop SRL1 outputs the secondary side on-time control signal.

[0039] In this embodiment, precise measurement of the secondary-side conduction time is achieved through collaborative operation. During circuit operation, the delay module receives the secondary-side conduction start signal Tson_st, generates a brief high level, delays it for a preset time, and outputs a delayed start signal Tson_stdly. This delayed start signal Tson_stdly triggers the SR flip-flop SRL1 to set, causing the secondary-side conduction time control signal Tson_d to output a high level, marking the start of the conduction time countdown. Subsequently, the auxiliary winding voltage VS gradually decreases. The first comparator CMP1 continuously monitors the relationship between the auxiliary winding voltage VS and the reference voltage Vref_zx. When it reaches the preset reference voltage Vref_zx, the first comparator CMP1 outputs a signal that triggers the rising edge of the D flip-flop Drff1. The first inverter A1 receives the switch control signal SW and sends its inverted signal to the reset terminal of the D flip-flop Drff1 to reset Drff1 when the switch is closed. Because the D terminal of D flip-flop Drff1 is connected to the power supply voltage VDD and the switching control signal SW is at a low level, the output of D flip-flop Drff1 is high, resetting SR flip-flop SRL1 and pulling the secondary-side conduction time control signal Tson_d low, indicating the end of the conduction time. When the primary-side switch is turned on again, i.e., when the switching control signal SW is high, the signal is reset to the reset terminal of D flip-flop Drff1 through the first inverter A1, ensuring correct signal acquisition for the next conduction cycle. This cycle repeats, achieving a stable output of the accurate secondary-side conduction time signal Tson_d based on the delayed start time and the detection of the zero-crossing voltage of the auxiliary winding.

[0040] Figure 5 A circuit topology diagram of a delay module according to an embodiment of the present invention is shown. Figure 5 As shown, the delay module includes a second inverter A2, a constant current source I, a first switch M1, a second switch M2, and a first capacitor C1. The input terminal of the second inverter A2 is connected to a conduction start signal, and the output terminal of the second inverter A2 is connected to the control terminals of the first switch M1 and the second switch M2. The first terminal of the first switch M1 is connected to the constant current source I, and the second terminal of the first switch M1 is connected to the first terminal of the second switch M2 and the first terminal of the first capacitor C1. The second terminal of the second switch M2 and the second terminal of the first capacitor C1 are connected and grounded.

[0041] In the delay module of this embodiment, a constant current source I provides a stable charging current to the first capacitor C1, achieving a precise timing delay for the input turn-on start signal. This first capacitor C1 is the aforementioned delay capacitor. When the turn-on start signal Tson_st is input to the second inverter A2, its output controls the first switch M1 to turn on and the second switch M2 to turn off. The current from the constant current source I flows through the first switch M1 to the first capacitor C1, linearly charging it. As the voltage of the first capacitor C1 gradually increases, when it reaches a set threshold, it triggers the subsequent delay start signal output. This structure utilizes a constant current charging method, giving the delay process good linearity and controllability, thereby ensuring the accuracy and stability of the delay time and meeting the precise delay requirement for the turn-on start moment.

[0042] In one embodiment, reference Figure 5 The delay module also includes a Schmitt trigger (SMT) and a third inverter (A3). The input of the Schmitt trigger (SMT) is connected to the first terminal of the first capacitor C1, and the output of the Schmitt trigger (SMT) is connected to the input of the third inverter (A3). The third inverter (A3) outputs a delay start signal.

[0043] In this embodiment, the delay module employs a combination of a Schmitt trigger (SMT) and a third inverter (A3). The input of the Schmitt trigger (SMT) is connected to the first terminal of the first capacitor C1, used to detect the voltage change of the first capacitor C1 after being charged by the constant current source I. Its hysteresis characteristic effectively suppresses noise and signal jitter, ensuring more stable signal switching. The output of the Schmitt trigger (SMT) is connected to the third inverter (A3), which further shapes and inverts the signal, outputting the final delayed start signal, thereby achieving accurate delay processing of the conduction start moment.

[0044] Figure 6 A circuit topology diagram of a delay module according to another embodiment of the present invention is shown. Figure 6As shown, the delay module also includes a second comparator CMP2. The non-inverting input of the second comparator CMP2 is connected to the current output of the first switching transistor M1, the inverting input of the second comparator CMP2 is connected to the comparison threshold voltage Vref_dly, and the output of the second comparator CMP2 outputs a delay start signal.

[0045] In this embodiment, the delay module uses a second comparator CMP2 to generate the delay start signal. The non-inverting input of the second comparator CMP2 is connected to the current output of the first switching transistor M1, and the inverting input of the second comparator CMP2 is connected to a preset comparison threshold voltage Vref_dly. When the input voltage reaches the comparison threshold voltage Vref_dly, the second comparator CMP2 outputs a high level, triggering the delay start signal. This scheme has a simple structure, fast response speed, is easy to implement, and can flexibly adjust the threshold voltage to meet different delay requirements.

[0046] The embodiments of the above delay module can be selected according to specific application requirements. If a combination structure of a Schmitt trigger (SMT) and a third inverter (A3) is used, the first threshold voltage is the threshold voltage of the Schmitt trigger (SMT), which has stronger anti-interference capability and signal stability, and is suitable for occasions with complex noise environments. If a comparator scheme is used, the first threshold voltage is the comparison threshold voltage Vref_dly, which has a simpler structure and faster response speed, and is suitable for scenarios with high requirements for delay accuracy and circuit complexity. Both can achieve precise delay of the secondary side conduction start time, improving the detection accuracy and the constant current control performance of the system.

[0047] According to the above embodiments, by setting a delay module consisting of a constant current source, a delay capacitor, and switching devices in the detection circuit, and combining it with the logic control of comparators and flip-flops, accurate delay of the first preset time and precise capture of the conduction end time are achieved, thereby ensuring stable detection of the conduction time. This circuit structure can directly calculate the delay time based on the primary-side oscillation period, with clearly defined parameters, facilitating application in different switching power supplies. Furthermore, this solution supports the flexible selection of Schmitt triggers (SMTs) or comparators as signal detection elements, allowing users to choose the most suitable solution based on specific noise environments and design requirements, thus improving the system's adaptability and reliability.

[0048] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0049] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A method for detecting the conduction time of the secondary side of a switching power supply, characterized in that, The method includes: Detect the start time of conduction on the secondary side of the switching power supply; The delayed start time is obtained by delaying the start time by a first preset time after the start time. The moment when the voltage VS of the auxiliary winding of the switching power supply drops to a preset reference voltage is used as the end time of the conduction of the secondary side of the switching power supply. The time interval between the delay start time and the conduction end time is taken as the conduction time of the secondary side of the switching power supply.

2. The detection method according to claim 1, characterized in that, The first preset time is one-quarter of the oscillation period of the primary circuit of the switching power supply.

3. The detection method according to claim 2, characterized in that, The formula for the first preset time is: Among them, L p C is the equivalent inductance value of the primary-side circuit. d This is the resonant capacitance value of the primary circuit.

4. The detection method according to claim 3, characterized in that, The specific steps for obtaining the delayed start time by delaying the start time by a first preset time after the start time include: After detecting the start of the conduction of the secondary side of the switching power supply, the constant current source is controlled to charge the delay capacitor. When the voltage of the delay capacitor reaches a preset first threshold voltage, a delay start signal is output, and the time when the delay start signal is generated is the delay start time.

5. The detection method according to claim 4, characterized in that, The charging current of the constant current source is: Wherein, V1 is the first threshold voltage, and C is the capacitance of the delay capacitor.

6. A detection circuit for the secondary-side conduction time of a switching power supply, characterized in that, It includes a delay module, a first comparator CMP1, a first inverter A1, a D flip-flop Drff1, and an SR flip-flop SRL1; The input terminal of the delay module is connected to the conduction start signal of the secondary side of the switching power supply, and the output terminal is connected to the set terminal of the SR flip-flop SRL1. It is used to perform a first preset time delay processing on the conduction start signal and output the delayed start signal after the delay. The non-inverting input of the first comparator CMP1 is connected to the preset reference voltage Vref_zx, the inverting input of the first comparator CMP1 is connected to the voltage VS of the auxiliary winding of the switching power supply, and the output of the first comparator CMP1 is connected to the clock input of the D flip-flop Drff1. The input terminal of the first inverter A1 is connected to the switching control signal SW of the switching power supply, and the output terminal of the first inverter A1 is connected to the reset terminal of the D flip-flop Drff1. The data terminal of the D flip-flop Drff1 is connected to the power supply voltage VDD, and the output terminal of the D flip-flop Drff1 is connected to the reset terminal of the SR flip-flop SRL1. The output terminal of the SR trigger SRL1 outputs a secondary conduction time control signal.

7. The detection circuit according to claim 6, characterized in that, The delay module includes a second inverter A2, a constant current source I, a first switching transistor M1, a second switching transistor M2, and a first capacitor C1; The input terminal of the second inverter A2 is connected to the conduction start signal, and the output terminal of the second inverter A2 is connected to the control terminals of the first switch M1 and the second switch M2. The first terminal of the first switch transistor M1 is connected to the constant current source I. The second terminal of the first switch transistor M1 is connected to the first terminal of the second switch transistor M2 and the first terminal of the first capacitor C1. The second terminal of the second switch transistor M2 and the second terminal of the first capacitor C1 are connected and grounded.

8. The detection circuit according to claim 7, characterized in that, The delay module also includes a Schmitt trigger SMT and a third inverter A3; The input terminal of the Schmitt trigger SMT is connected to the first terminal of the first capacitor C1, and the output terminal of the Schmitt trigger SMT is connected to the input terminal of the third inverter A3. The third inverter A3 outputs the delayed start signal.

9. The detection circuit according to claim 7, characterized in that, The delay module also includes a second comparator CMP2; The non-inverting input of the second comparator CMP2 is connected to the first terminal of the first capacitor C1, the inverting input of the second comparator CMP2 is connected to the comparison threshold voltage Vref_dly, and the output of the second comparator CMP2 outputs the delay start signal.

10. The detection circuit according to claim 8 or 9, characterized in that, The first threshold voltage is the threshold voltage of the Schmitt trigger (SMT) or the comparison threshold voltage Vref_dly.

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