Control method for changing minimum on-time of synchronous rectifying switch and controller thereof

Abstract

The present invention provides a control method for changing a minimum on-time of a synchronous rectification switch and a controller thereof. In one embodiment, a synchronous rectification controller and the synchronous rectification switch are applied to a power supply for supplying power to a load. The control method comprises: providing a minimum on-time; turning on the synchronous rectification switch according to a signal of one end of the synchronous rectification switch; making an on-time of the synchronous rectification switch not less than the minimum on-time; providing a detection result associated with the load; and changing the minimum on-time according to the detection result.

Description

Technical Field

[0001] This invention generally relates to power supplies, and more particularly to a control method and related synchronous rectification controller that can change the minimum on-time of the synchronous rectification switch of the power supply. Background Technology

[0002] In switching power supplies, diodes function as rectifiers to provide DC power. However, for power supplies providing high current, the forward bias voltage of the rectifier diode often leads to considerable power loss. To improve the power conversion efficiency of switching power supplies, synchronous rectification introduces a synchronous rectifier switch and related controller to replace the rectifier diode. When the rectifier diode should be conducting, the synchronous rectifier switch should be on, providing a very small on-resistance to transfer current; when the rectifier diode should be off, the synchronous rectifier switch should be off to prevent reverse current.

[0003] Besides considerations of power conversion efficiency, the trend towards thinner and smaller electronic products necessitates miniaturization of switching power supplies. However, this reduction in size also presents several technical challenges. For example, using smaller inductors often requires switching power supplies to operate at higher switching frequencies. Enabling synchronous rectifier switches to operate stably at these higher frequencies remains a key industry objective. Summary of the Invention

[0004] This invention provides a control method for a synchronous rectifier switch, applicable to a synchronous rectifier controller. The synchronous rectifier controller and the synchronous rectifier switch are used in a power supply to supply power to a load. The control method includes: providing a minimum on-time; turning on the synchronous rectifier switch based on a signal from one end of the synchronous rectifier switch; ensuring that an on-time of the synchronous rectifier switch is not less than the minimum on-time; providing a detection result associated with the load; and changing the minimum on-time based on the detection result.

[0005] This invention provides a synchronous rectification controller for controlling a synchronous rectification switch, suitable for a power supply supplying power to a load. The synchronous rectification controller includes a trigger condition detector, a load estimator, a minimum turn-on time generator, and a logic gate. The trigger condition detector turns on the synchronous rectification switch based on the voltage at one end of the switch. The load estimator provides a detection result associated with the load. Based on the detection result, the minimum turn-on time generator provides a minimum turn-on time. The logic gate ensures that the turn-on time of the synchronous rectification switch is not less than the minimum turn-on time. Attached Figure Description

[0006] Figure 1 This invention shows a flyback power supply 10 implemented according to the present invention.

[0007] Figure 2 This displays a control method 100 applicable to the synchronous rectifier controller 14.

[0008] Figure 3A For example, the synchronous rectifier controller 200a is shown.

[0009] Figure 3B Display used for Figure 3A Control method 100a in the middle.

[0010] Figure 4A This displays some signal waveforms of the flyback power supply 10 and the synchronous rectifier controller 200a when the flyback power supply 10 is operating in DCM.

[0011] Figure 4B This displays some signal waveforms of the flyback power supply 10 and the synchronous rectifier controller 200a when the flyback power supply 10 is operating in CCM.

[0012] Figure 5 This illustrates another control method 100b implemented according to the present invention.

[0013] [Symbol Explanation]

[0014] 10 flyback power supplies

[0015] 12-bridge rectifier

[0016] 13 Power Controller

[0017] 14 Synchronous Rectifier Controller

[0018] 16 load

[0019] 17 Output Capacitors

[0020] 26 Input grounding wire

[0021] Control methods for 100, 100a, and 100b

[0022] Steps 102, 104, 106, 108, 110, 112, 114, 116, 118

[0023] 200A Synchronous Rectifier Controller

[0024] 202 Load Estimator

[0025] 204 Minimum On-Time Generator

[0026] 206 Trigger Condition Detector

[0027] 208 logic gates

[0028] 209 Synchronous Switch Driver

[0029] 220 comparator

[0030] DF1, DF2, DF3 D flip-flops

[0031] DTC work cycle

[0032] IN Input power line

[0033] LA auxiliary winding

[0034] LP main winding

[0035] LS secondary winding

[0036] MOD Detection Results

[0037] N1 Main Power Switch

[0038] RA and RB resistors

[0039] RED signal

[0040] SMIN pulse

[0041] SON activation signal

[0042] S PRI Pulse width modulation signal

[0043] S SEC Signal

[0044] SWSR Synchronous Rectifier Switch

[0045] t00, t01, t10, t11, tC, tCCM time points

[0046] T DMG Discharge time

[0047] TF Transformer

[0048] T MIN-B T MIN-S time

[0049] T SR-OFF Closing Time

[0050] T SR-ON Opening time

[0051] T SR-ON-MIN Minimum opening time

[0052] V AUX Trans-pressure

[0053] V D Drain signal

[0054] V IN Input voltage

[0055] V OUT Output voltage

[0056] V REF Reference signal Detailed Implementation

[0057] In this specification, some identical symbols are used to represent elements having the same or similar structure, function, or principle, which can be inferred by those skilled in the art based on the teachings of this specification. For the sake of brevity, elements with the same symbols will not be repeated.

[0058] One embodiment of the present invention is a flyback power supply that converts electrical energy from the primary side to the secondary side to supply power to a load. The flyback power supply has a synchronous rectifier switch and a synchronous rectifier controller on the secondary side. The synchronous rectifier controller triggers the synchronous rectifier switch to open based on a signal from one end of the synchronous rectifier switch, initiating an on-time for the synchronous rectifier switch. The synchronous rectifier controller ensures that the on-time is not less than a minimum on-time. The synchronous rectifier controller also provides a detection result that is approximately correlated with the load. Based on the detection result, the synchronous rectifier controller adjusts the minimum on-time.

[0059] In one embodiment, when the detection result confirms that the flyback power supply is operating in a discontinuous conduction mode (DCM), the synchronous rectification controller sets the minimum turn-on time to a first time. When the detection result does not confirm that the flyback power supply is operating in DCM, the synchronous rectification controller sets the minimum turn-on time to a second time shorter than the first time. When it is determined that the power supply is operating in DCM, the load should be lightly loaded or unloaded; when it is uncertain whether the power supply is operating in DCM, the load should be medium-loaded or heavily loaded.

[0060] In one embodiment, the synchronous rectification controller calculates the number of times the terminal signal crosses a reference signal and uses this number to determine whether the flyback power supply is operating in a discontinuous conduction (DCM) mode. If the number of times the terminal signal crosses the reference signal exceeds 2, it means that the flyback power supply is definitely operating in DCM; if the number does not exceed 2, it means that the flyback power supply may be operating in either DCM or CCM. Heavier loads are more likely to operate in CCM. Therefore, this number can be roughly correlated with the load.

[0061] When the flyback power supply may operate in CCM, making the minimum on-time a relatively short second time can prevent the short-circuit risk that the primary power switch and the secondary synchronous rectifier switch of the flyback power supply will both remain in the on state when the switching frequency of the flyback power supply is very high.

[0062] When the flyback power supply is determined to be operating in DCM, setting the minimum on-time to a relatively long first time allows the power controller on the primary side of the flyback power supply more time to detect the output voltage supplied to the load, thus avoiding obtaining incorrect output voltage information.

[0063] Figure 1 The present invention illustrates a flyback power supply 10, which includes a bridge rectifier 12, a transformer TF, a main power switch N1, a power controller 13, an output capacitor 17, a synchronous rectifier switch SWSR, and a synchronous rectifier controller 14. The transformer TF includes a main winding LP and an auxiliary winding LA on the primary side, and a secondary winding LS on the secondary side, which are inductively coupled to each other.

[0064] The flyback power supply 10 is an AC-to-DC power supply that converts the AC voltage from the primary side of the mains into a DC output voltage V on the secondary side. OUT This is used to power load 16. Bridge rectifier 12 provides full-wave rectification, converting the AC voltage into the input voltage V on the input power line IN. IN This is connected to the input ground (the 0 voltage reference potential on the primary side) on input ground line 26. Power controller 13 controls the main power switch N1. When the main power switch N1 is open, providing a short-circuit path, transformer TF begins to store energy; when the main power switch N1 is closed, changing the previous short-circuit path into an open-circuit path, transformer TF begins to release energy to establish the output voltage V on output capacitor 17. OUT Power is supplied to the load 16.

[0065] The synchronous rectifier controller 14 can control the drain signal V on the drain of the synchronous rectifier switch SWSR. DThe synchronous rectifier controller 14 determines whether the transformer TF is releasing energy. When the synchronous rectifier controller 14 determines that the transformer TF is releasing energy, it turns on the synchronous rectifier switch SWSR, providing a very low on-resistance connection between the drain and source of the synchronous rectifier switch SWSR. When the synchronous rectifier controller 14 determines that the transformer TF is not releasing energy, it turns off the synchronous rectifier switch SWSR, essentially isolating the drain and source of the synchronous rectifier switch SWSR. In this way, the purpose of synchronous rectification is achieved.

[0066] When the synchronous rectifier switch SWSR is turned on, the drain and source are approximately short-circuited, which is the turn-on time T of the synchronous rectifier switch SWSR. SR-ON When the synchronous rectifier switch SWSR is off, the drain and source are approximately open circuits, which is the off time T of the synchronous rectifier switch SWSR. SR-OFF .

[0067] The power controller 13 detects the voltage V across the auxiliary winding LA via resistors RA and RB connected in series. AUX When the transformer TF releases energy, the output voltage V located on the secondary side can be indirectly detected. OUT Therefore, the power controller 13 can change the pulse width modulation signal S PRI The switching frequency and / or duty cycle are used to regulate the output voltage V. OUT .

[0068] Figure 2 This displays a control method 100 applicable to the synchronous rectifier controller 14, used to control the synchronous rectifier switch SWSR.

[0069] In step 102, the synchronous rectifier controller 14 turns on the synchronous rectifier switch SWSR, so the turn-on time T of the synchronous rectifier switch SWSR is... SR-ON start.

[0070] In step 104, the synchronous rectifier controller 14 provides a detection result associated with the load 16, and determines the minimum turn-on time T based on this detection result. SR-ON-MIN .

[0071] In step 106, the synchronous rectifier controller 14 enables the turn-on time T SR-ON Not less than the minimum opening time T SR-ON-MIN Minimum opening time T SR-ON-MIN That is, the start time T SR-ON The minimum value of T can improve switching efficiency and avoid excessively short and ineffective on-time. SR-ON .

[0072] In step 108, the synchronous rectifier controller 14 determines the power supply based on the drain signal V. DThis is used to update the detection result. This detection result is associated with load 16. An example will be provided later.

[0073] Figure 3A For example, the synchronous rectification controller 200a is shown. In one embodiment, it can be used in... Figure 1 In the flyback power supply 10, the synchronous rectifier controller 14 is replaced. The synchronous rectifier controller 200a includes, but is not limited to, a load estimator 202, a trigger condition detector 206, a minimum on-time generator 204, a logic gate 208, and a synchronous switch driver 209.

[0074] Trigger condition detector 206 detects the drain signal V of synchronous rectifier switch SWSR. D This is used to determine whether the transformer TF is releasing energy. For example, the trigger condition detector 206 detects the drain signal V. D The value of the drain signal and the slope of the signal change. When the drain signal V... D When the voltage drops below -0.4V and the slope of the change is large enough, the trigger condition detector 206 determines that the transformer TF has started to release energy. Therefore, it provides an enable signal SON, which, through logic gate 208, triggers the synchronous switch driver 209 to start releasing energy with signal S. SEC Turn on the synchronous rectifier switch SWSR.

[0075] The minimum on-time generator 204, in response to the on-signal SON, sends a pulse SMIN to logic gate 208. The pulse width of pulse SMIN represents the minimum on-time T. SR-ON-MIN The MOD result determines the minimum startup time T. SR-ON-MIN When the detection result MOD is logically 1, the minimum start time T is... SR-ON-MIN For a relatively long time T MIN-B When the detection result MOD is logically 0, the minimum start time T is... SR-ON-MIN For a relatively short time T MIN-S In other words, time T MIN-B Greater than time T MIN-S .

[0076] Trigger condition detector 206 also detects the drain signal V of synchronous rectifier switch SWSR. D This is used to determine whether the transformer TF has finished releasing energy. For example, when the drain signal V... D When the voltage is greater than 0V, the transformer TF is considered to have finished releasing energy. The synchronous rectifier switch SWSR is then turned off via the enable signal SON, logic gate 208, and synchronous switch driver 209. Enable time T SR-ON Finish.

[0077] Logic gate 208 controls the on-time T of the synchronous rectifier switch SWSR. SR-ON The length is not less than the minimum opening time T SR-ON-MIN .

[0078] Please also refer to Figure 3B Its display Figure 3A The control method used is 100a.

[0079] Figure 3B In step 102, the trigger condition detector 206 starts to open at time T. SR-ON .

[0080] Figure 3B Step 104 in the process involves identifying the drain signal V. D Was it at the previous closing time T? SR-OFF Internal crossing of reference signal V REF Twice. Only during the previous closing time T SR-OFF Internal, drain signal V D Crossing the reference signal V REF More than twice, Figure 3A The two rising edges output by comparator 220 will cause D flip-flops DF1 and DF2 to pass a logical 1, causing the signal RED to be on during the turn-on time T. SR-ON The initial value is logically 1. Therefore, the drain signal V can be identified from the logical value of the RED signal. D Is it at a closing time T? SR-OFF Internal crossing of reference signal V REF twice.

[0081] if Figure 3B If the answer in step 104 is yes, then step 110 is executed, setting the detection result MOD to a logical 1 to determine that the flyback power supply 10 is operating in DCM; if it is no, then step 106 is executed, setting the detection result MOD to a logical 0 to determine that the flyback power supply 10 may be operating in CCM. Figure 3A In the middle. The rising edge of the enable signal SON causes the D flip-flop DF3 in the load estimator 202 to record the current signal RED, making the detection result MOD equal to the signal RED. If the flyback power supply 10 operates in DCM, the load 16 should be lightly loaded or unloaded; if the flyback power supply 10 operates in CCM, the load 16 should be medium-loaded or heavily loaded.

[0082] In steps 112 and 108, the detection result MOD determines the minimum opening time T. SR-ON-MIN When the detection result MOD is logically 1, the minimum start time T is... SR-ON-MIN For a relatively long time T MIN-BWhen the detection result MOD is logically 0, the minimum start time T is... SR-ON-MIN For a relatively short time T MIN-S In other words, time T MIN-B Greater than time T MIN-S .

[0083] Step 114 is executed by logic gate 208, which causes the synchronous rectifier switch SWSR to turn on for a time T. SR-ON The length is not less than the minimum opening time T SR-ON-MIN .

[0084] At the start time T SR-ON Inside, Figure 3A The D flip-flops DF1 and DF2 in the circuit are reset by the enable signal SON, and their outputs are both logically 0.

[0085] Step 116 ended the start time T SR-ON And the shutdown time T began. SR-OFF .

[0086] Step 118 Calculate the closing time T SR-OFF Internal, drain signal V D Crossing the reference signal V REF The number of times this is performed is determined by comparator 220, D flip-flops DF1 and DF2 together. The signal RED will be logically 1 only when the number of times is greater than or equal to 2; otherwise, the signal RED will be logically 0.

[0087] Please see Figure 4A This displays some signal waveforms of the flyback power supply 10 and the synchronous rectifier controller 200a when the flyback power supply 10 is operating in DCM.

[0088] At time point t00, the pulse width modulation signal S PRI After the main power switch N1 is turned off, the discharge time T DMG start.

[0089] The initial value of signal RED at time t00 only affects the current switching cycle starting from time t00, and will not affect the next switching cycle. Here, it is assumed that signal RED is logically 0 at time t00.

[0090] as Figure 4A As shown, the trigger condition detector 206 is activated when the drain signal V... D When the value is negative, the enable signal SON is provided, turning on the synchronous rectifier switch SWSR. Enable time T SR-ON This refers to the period when the SON signal is set to logic 1.

[0091] Opening time T SR-ONInitially, the RED signal is used to update the detection result MOD, so the detection result MOD is logically 1, resulting in the minimum on-time T provided by the pulse SMIN. SR-ON-MIN For a relatively long time T MIN-B At the same time, D flip-flops DF1 and DF2 are reset, so the signal RED becomes logically 0.

[0092] At the start time T SR-ON After it ends, the closing time T SR-OFF Initially, the drain signal V D It also begins to oscillate. At time point tC, the drain signal V D The second time it exceeded the reference signal V REF Therefore, the RED signal changes to a logical 1. The off time T SR-OFF It will end at time point t01, which is the next drain signal V. D When it turns negative, it is approximately at the next discharge time T. DMG At the beginning.

[0093] from Figure 4A It can be seen that if the drain signal V D There were two instances where the reference signal V was crossed. REF The flyback power supply 10 must be operating in DCM, with a minimum on-time T. SR-ON-MIN Set to a relatively long time T MIN-B .

[0094] akin, Figure 4B This displays some signal waveforms of the flyback power supply 10 and the synchronous rectifier controller 200a when the flyback power supply 10 is operating in CCM. Figure 4B and Figure 4A Similarities or resemblances can be understood from the previous explanations and will not be repeated here.

[0095] Figure 4B In this context, assume that the signal RED is logically 0 at time t10. Therefore, the on-time T... SR-ON Initially, the RED signal updates the detection result MOD, so the detection result MOD is logically 0, causing the minimum on-time T provided by the pulse SMIN to be limited. SR-ON-MIN For a relatively short time T MIN-S At the same time, D flip-flops DF1 and DF2 are reset, so the RED signal remains logically 0.

[0096] exist Figure 4B At time point tCCM, at the drain signal V D When it is still negative, the pulse width modulation signal S PRIThe main power switch N1 is turned on, forcing the drain signal V high. D This makes the discharge time T DMG and opening time T SR-ON The end. Therefore, Figure 4B The flyback power supply 10 is shown operating in the CCM.

[0097] At time point t11, the next start time T SR-ON Begin. From Figure 4B As can be seen from this, during the switching cycle from time point t01 to time point t11, the drain signal V D Only once, at time point tCCM, it crossed the reference signal V. REF Therefore, the signal RED will remain at logical 0 at time t11.

[0098] from Figure 4B It can be seen that if the drain signal V D The reference signal V was crossed only once. REF The flyback power supply 10 may be operating in CCM, with a minimum on-time T. SR-ON-MIN Set to a relatively short time T MIN-S .

[0099] When operating in CCM, load 16 should be medium or heavy load, with a minimum start time T. SR-ON-MIN Relatively short (T) MIN-S This prevents short-circuit ride-through, meaning that when the primary power switch N1 is turned on, the secondary synchronous rectifier switch SWSR is turned on for a minimum time T. SR-ON-MIN Forcing it to remain in the on state creates a dangerous situation similar to a short circuit in the power cord.

[0100] When operating in DCM, load 16 should be lightly loaded or unloaded, with a minimum start time T. SR-ON-MIN Relatively long (T) MIN-B This allows the primary-side power controller 13 a relatively long time to detect the secondary-side output voltage V. OUT .if Figure 1 The flyback power supply 10 uses primary-side regulation (PSR), and the power controller 13 needs to be connected to RB through resistors RA during the turn-on time T. SR-ON Detect the trans-voltage V of the auxiliary winding LA AUX This is used to indirectly detect the output voltage V. OUT Once load 16 is unloaded, if the minimum start time T... SR-ON-MIN Too short, start time T SR-ONIt may end very soon, and the power controller 13 is very likely not detecting the correct output voltage V. OUT This leads to an output voltage V OUT The instability. Therefore, a relatively long minimum on time T. SR-ON-MIN This allows the power controller 13 to obtain the correct output voltage V when there is no load. OUT It has a relatively good voltage stabilization effect.

[0101] Figure 3A The load estimator 202 in the example is merely one instance. Taking the determination by the load estimator 202 of whether the flyback power supply 10 is definitely operating in DCM mode as an example, it detects the state of the load 16 and sets the minimum on-time T accordingly. SR-ON-MIN When load 16 may be unloaded, the minimum start time T is... SR-ON-MIN Relatively long; when load 16 is likely to be heavily loaded, the minimum on-time T is... SR-ON-MIN The time is relatively short. However, the invention is not limited to this. In other embodiments of the invention, other signals can be detected to approximate the state of load 16, thereby setting the minimum on-time T. SR-ON-MIN .

[0102] Figure 5 This invention shows another control method 100b implemented according to the present invention, which can be applied to the synchronous rectifier controller 14 in the flyback power supply 10. Figure 5 Control method 100b and Figure 3B The control method 100a is the same as or similar to the previous description, and will not be repeated here. Control method 100b can be based on the enable signal SON or the signal S. SEC The duty cycle DTC is determined by the start time T. SR-ON Divide by one switching cycle, where one switching cycle is one on-time T. SR-ON With a closing time T SR-OFF If the duty cycle DTC is low, that is, less than a preset value, it can be considered that load 16 may be a light load, so control method 100b sets the minimum start time T. SR-ON-MIN For a relatively large time T MIN-B If the duty cycle DTC is high, that is, greater than the preset value, it can be considered that load 16 may be overloaded. Therefore, control method 100b sets the minimum start time T. SR-ON-MIN For a relatively small time T MIN-S .

[0103] In other embodiments of the present invention, the switching period (on-time T) of the synchronous rectifier switch SWSR can be detected. SR-ON With a closing time T SR-OFFThe combined values ​​(of the loads) are used to roughly estimate the state of load 16. If the switching cycle is relatively long, load 16 can be considered to be under light load, so a minimum on-time T is set. SR-ON-MIN For a relatively large time T MIN-B Conversely, if load 16 is considered to be under heavy load, then a minimum start time T should be set. SR-ON-MIN For a relatively small time T MIN-S .

[0104] In other embodiments of the present invention, detection can be performed at the on-time T. SR-ON The drain signal V just ended D The rising slope is used to roughly estimate the state of load 16. If the start time T... SR-ON After completion, the drain signal V D The rapid ascent with an incline greater than a preset value suggests CCM operation; a load of 16 may indicate heavy load. The minimum start time T should be set. SR-ON-MIN For a relatively small time T MIN-S Conversely, if the start time T SR-ON After completion, the drain signal V D A slow ascent with a slope less than the preset value suggests operation is occurring in DCM mode. A load of 16 may indicate no load. Set the minimum on-time T. SR-ON-MIN For a relatively long time T MIN-B .

[0105] The above description is only a preferred embodiment of the present invention. All equivalent changes and modifications made in accordance with the claims of the present invention shall be within the scope of the present invention.

Claims

1. A control method for a synchronous rectifier switch, applicable to a synchronous rectifier controller, wherein, The synchronous rectifier controller and the synchronous rectifier switch are suitable for power supplies that supply power to loads. The control method includes: Provide minimum opening time; The synchronous rectifier switch is turned on based on the signal from one end of the synchronous rectifier switch; The opening time of the synchronous rectifier switch shall not be less than the minimum opening time; Provide test results, wherein the test results are associated with whether the load is no load, light load, medium load, or heavy load; and Based on the test results, the minimum opening time is changed by varying the duration of the test.

2. The control method as described in claim 1, wherein, When the detection result is the first result, the load is a light first load, and the minimum turn-on time is a long first time; when the detection result is the second result, the load is a second load greater than the first load, and the minimum turn-on time is a second time less than the first time.

3. The control method as described in claim 2, comprising: Calculate the number of times the signal at this end crosses the reference signal; When the number of occurrences exceeds a preset value, the load is considered light, and the minimum on-time is set to the longest possible first time; and When the number of times is less than the preset value, the minimum opening time is set to the second time.

4. The control method as described in claim 3, comprising: The number of times is reset when the synchronous rectifier switch is turned on; and The number of times is calculated when the synchronous rectifier switch is closed.

5. The control method as described in claim 3, further comprising: When the detection result indicates that the power supply is operating in discontinuous conduction mode, the minimum on-time is set to a first time; and When the test result does not indicate that the power supply is operating in continuous conduction mode, the minimum turn-on time is set to a second time, which is less than the first time.

6. The control method as described in claim 1, wherein, The test result represents the operating cycle of the synchronous rectifier switch.

7. A synchronous rectification controller for controlling a synchronous rectification switch, suitable for a power supply supplying power to a load, the synchronous rectification controller comprising: The trigger condition detector turns on the synchronous rectifier switch based on the terminal voltage of the synchronous rectifier switch. The load estimator provides detection results, in which... The test result is related to whether the load is no load, light load, medium load, or heavy load. A minimum opening time generator provides a minimum opening time based on the detection result, wherein the minimum opening time can be changed by varying the duration of the time based on the detection result; as well as A logic gate is used to ensure that the turn-on time of the synchronous rectifier switch is not less than the minimum turn-on time.

8. The synchronous rectification controller as described in claim 7, wherein, When the power supply operates in discontinuous conduction mode, the minimum turn-on time generator sets the minimum turn-on time to a first time. When the power supply operates in continuous conduction mode, the minimum turn-on time generator sets the minimum turn-on time to a second time, which is shorter than the first time.

9. The synchronous rectification controller as described in claim 7, wherein, The load estimator calculates the number of times the signal at one end of the synchronous rectifier switch crosses the reference signal. When the number of times is greater than a preset value, the load is identified as light load, and the minimum opening time is set to a first time. When the number of times is less than the preset value, the minimum opening time is set to a second time, which is less than the first time.

10. The synchronous rectification controller as described in claim 9, wherein, The load estimator resets the number of times when the synchronous rectifier switch is turned on, and calculates the number of times when the synchronous rectifier switch is turned off.

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