Synchronous rectification control method and synchronous rectification controller

Abstract

A synchronous rectification control method and a synchronous rectification controller are suitable for a power supply having a primary side and a secondary side isolated from each other. When the control signal of the control end of the power supply is larger than the critical voltage, the synchronous rectification switch on the secondary side is turned on, and a short-circuit channel is provided; when the control signal is between the critical voltage and the ground reference voltage, the synchronous rectification switch is turned off to open the channel. The method comprises the following steps: providing a detection signal according to one end of a channel; the detection signal is compared with a preset reflection level, the preset reflection level is higher than the ground reference voltage, and the time when the detection signal is larger than the preset reflection level is the energy storage time of the transformer; comparing the energy storage time of the transformer with a preset energy storage time to generate a sufficient signal; the detection signal is compared with a starting reference level, a clockwise bias signal is generated, and the starting reference level is lower than the ground reference voltage; the control end is pre-charged according to the sufficient signal and the forward bias voltage signal, so that the control signal is a subcritical voltage between the critical voltage and the ground reference voltage.

Description

Technical Field

[0001] The present invention generally relates to synchronous rectification, and more particularly to a synchronous rectification controller and control method capable of precharging a synchronous rectification switch. Background Art

[0002] Synchronous rectification is a technology that replaces diodes with actively controlled electronic switches to improve rectification efficiency. The electronic switches used are mostly power MOSFETs or bipolar transistors. The voltage drop of a general semiconductor diode is fixed, about 0.5 to 1V. Within the operating current range, such a voltage drop does not change significantly with the current; the voltage drop of an active synchronous rectification switch is more like a resistor, and the voltage drop is very small at low currents.

[0003] The synchronous rectifier switch needs to be carefully controlled at the time of opening and closing. If it is turned on too early, reverse current may occur, which will greatly reduce the rectification efficiency. On the contrary, if it is turned on too late, most of the current may flow through the diode, losing the main effect of synchronous rectification. Summary of the invention

[0004] An embodiment of the present invention provides a synchronous rectification control method, which is applicable to a power supply. The power supply has a primary side and a secondary side that are isolated from each other. The power supply has a synchronous rectification switch on the secondary side, which has a control end. When a control signal on the control end is greater than a critical voltage, the synchronous rectification switch can be turned on to provide a substantially short-circuited channel; when the control signal is between the critical voltage and a ground reference voltage, the synchronous rectification switch can be turned off to substantially make the channel present an open circuit. The synchronous rectification control method includes: providing a detection signal according to one end of the channel; comparing the detection signal with a preset reflection level, wherein the preset reflection level is higher than the ground reference voltage, and the time when the detection signal is greater than the preset reflection level is a transformer energy storage time; comparing the transformer energy storage time and a preset energy storage time to generate a sufficient signal; comparing the detection signal with a start reference level to generate a forward bias signal, wherein the start reference level is lower than the ground reference voltage; and, according to the sufficient signal and the forward bias signal, precharging the control end so that the control signal is a critical voltage between the critical voltage and the ground reference voltage.

[0005] The embodiment of the present invention also provides a synchronous rectification controller, which is suitable for a power supply. The power supply has a primary side and a secondary side that are isolated from each other. The power supply has a synchronous rectification switch on the secondary side. The synchronous rectification controller provides a control signal to a control end of the synchronous rectification switch. When the control signal is greater than a critical voltage, the synchronous rectification switch can be turned on to provide a substantially short-circuited channel; when the control signal is between the critical voltage and a ground reference voltage, the synchronous rectification switch can be turned off to substantially make the channel present an open circuit. A detection signal is provided at one end of the channel. The synchronous rectification controller includes a primary side start-up judge, a forward bias detector, and a precharge circuit. The primary side start-up judge compares a detection signal on one end of the channel with a preset reflection level. The preset reflection level is higher than the ground reference voltage, and the time when the detection signal is greater than the preset reflection level is a transformer energy storage time. When the transformer energy storage time continues to exceed a preset energy storage time, the primary side start-up judge generates a sufficient signal. The forward bias detector compares the detection signal with a start reference level, which is lower than the ground reference voltage, to generate a forward bias signal. According to the sufficient signal and the forward bias signal, the precharge circuit precharges the control terminal to a primary critical voltage, which is between the critical voltage and the ground reference voltage. BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Figure 1 A flyback power supply 10 is implemented according to the present invention.

[0007] Figure 2 show Figure 1 The synchronous rectification controller 14 in FIG.

[0008] Figure 3 Display used in Figure 2 A control method 800 of the synchronous rectifier controller 14 is provided.

[0009] Figure 4 Displays the time from point t 10 At time t 1E During the switching cycle TCYC1, Figure 1 and Figure 2 Some signal waveforms in .

[0010] Figure 5 The switching cycles TCYC2 and TCYC3 are displayed, as well as some signal waveforms therein.

[0011]

Explanation of symbols

[0012] 10Flyback Power Supply

[0013] 12 Bridge Rectifier

[0014] 13 Power Controller

[0015] 14Synchronous Rectification Controller

[0016] 16 Load

[0017] 17 Output Capacitor

[0018] 26 Input ground wire

[0019] 28 Output ground wire

[0020] 102 Primary side start judgement

[0021] 104 Forward bias detector

[0022] 106 Precharge Circuit

[0023] 108 Pull down the controller

[0024] 112 Comparator

[0025] 114 Debounce Circuit

[0026] 116 Recorder

[0027] 120, 122 switch

[0028] 130 Control circuit

[0029] 126 Current Source

[0030] 800 Control Method

[0031] Steps 802 to 820

[0032] DET detection terminal

[0033] DRV control terminal

[0034] IN Input power line

[0035] I SEC Induced current

[0036] LP main winding

[0037] LS Secondary Winding

[0038] MI, MII drive circuit

[0039] M1_ON turn on signal

[0040] N1 Power switch

[0041] PR_EN precharge signal

[0042] PRM primary side

[0043] SEC Secondary Side

[0044] SH pull high switch

[0045] SL Pull-Down Switch

[0046] S SEC Control Signal

[0047] S PRI PWM signal

[0048] SR_FL fully open signal

[0049] SR_FW forward bias signal

[0050] SR_PRE sufficient signal

[0051] SWSR Synchronous Rectification Switch

[0052] t 10 ,t 11 ,t 12 ,t 13 ,t 14 ,t 15 ,t 16 ,t 1E ,t 20 ,t 21 ,t 22 ,t 30 ,t 31 ,t 32 ,t 3E

[0053] Time

[0054] TCYC1, TCYC2, TCYC3 switching cycle

[0055] TF Transformer

[0056] T PL 、T PL1 、T PL2 Transformer energy storage time

[0057] T PRE1 Precharge time

[0058] T QR Preset storage time

[0059] TR-_GD confirms the signal

[0060] V AC AC Power

[0061] V DD Fixed turn-on voltage

[0062] V DET Heartbeat

[0063] V H High voltage level

[0064] V IN Input Power

[0065] V L Low voltage level

[0066] V OUT Output Power

[0067] V PL Preset reflection levels

[0068] V REF_OP Preset reference levels

[0069] V SR_ON Turn on reference level

[0070] V SR_TH Critical voltage

[0071] V SUB_T Subthreshold Voltage

[0072] ZM Block DETAILED DESCRIPTION

[0073] In this specification, there are some identical symbols, which represent elements with the same or similar structure, function, and principle, and can be inferred by those skilled in the art according to the teaching of this specification. For the sake of brevity of the specification, elements with the same symbols will not be repeated.

[0074] Figure 1 The flyback power supply 10 implemented according to the present invention is used as an isolated AC to DC power converter to convert the AC power V AC , which is converted into a DC output power V at the secondary side SEC OUT The flyback power supply 10 is only an embodiment of the present invention and is not intended to limit the present invention. For example, the present invention may also be an LLC converter, an active-clamp flyback power converter (ACF), or an asymmetric half-bridge (AHB) power supply.

[0075] exist Figure 1 In the flyback power supply 10, the transformer TF divides the flyback power supply 10 into a primary side PRM and a secondary side SEC. In the primary side PRM, the bridge rectifier 12 converts the AC power V ACRectification provides input power V on the input power line IN IN , providing an input ground reference voltage on the input ground line 26. The power controller 13 provides a pulse width modulation (PWM) signal S PRI , controls the power switch N1, and controls the energy storage and release of the transformer TF.

[0076] As the transformer TF stores and releases energy, an induced voltage is generated at both ends of the secondary winding LS, which also causes an induced current I flowing through the secondary winding LS. SEC The synchronous rectification switch SWSR is connected between the output ground line 28 and the secondary winding LS to rectify the inductive current I SEC , roughly maintaining the induced current I SEC Not less than 0A. Induction current I SEC The output capacitor 17 is charged, and a DC output power supply V is established on the output capacitor 17. OUT , supplying power to load 16.

[0077] The synchronous rectification controller 14 provides a control signal S through the control terminal DRV. SEC , to control the synchronous rectification switch SWSR. The synchronous rectification switch SWSR can provide a channel, one end of which is connected to the output ground line 28; the other end is connected to the secondary side winding LS and provides a detection signal V DET , is sent to the synchronous rectification controller 14 through the detection terminal DET. In the following, the voltage level on the secondary side SEC is expressed as a relative voltage with the ground reference voltage of the output ground line 28 being 0V. For example, if the detection signal V DET is 15V, indicating that the detection signal V DET , which is 15V more than the ground reference voltage on the output ground line 28.

[0078] as Figure 1 The example shows that the synchronous rectification switch SWSR is an enhancement-mode NMOS transistor, and its device characteristics have a threshold voltage V SR_TH When the control signal S SEC Greater than the critical voltage V SR_TH When the synchronous rectification switch SWSR provides a substantially short-circuited path, connecting the output ground line 28 and the secondary winding LS; the control signal S SEC Between 0V and threshold voltage V SR_TH When , that channel becomes roughly an open circuit, isolating the output ground line 28 from the secondary winding LS.

[0079] In an embodiment of the present invention, according to the detection signal V DET, the synchronous rectification controller 14 can confirm two possible conditions: 1) the power switch N1 is turned on for a long enough time; and 2) the channel forward bias voltage of the synchronous rectification switch SWSR is large enough. As long as one of the two conditions occurs, the synchronous rectification controller 14 pre-charges the control terminal DRV to the sub-threshold voltage V SUB_T , which is between the critical voltage V SR_TH and 0V. In this way, the synchronous rectification switch SWSR can be based on the subsequent detection signal V DET changes and is turned on quickly to increase the efficiency of synchronous rectification.

[0080] Figure 2 show Figure 1 The synchronous rectification controller 14 includes a primary-side start-up determiner 102, a forward bias detector 104, a pre-charge circuit 106, drive circuits MI and MII, a pull-down controller 108, a pull-down switch SL, and some logic controls.

[0081] In the primary side start-up determination device 102, the comparator 112 compares the detection signal V DET With preset reflection level V PL For example, the preset reflection level V PL It can be 15V. Here, when the detection signal V DET Above the preset reflection level V PL The time is called the transformer energy storage time T PL The debounce circuit 114 checks the transformer energy storage time T PL Whether it has continued to exceed the preset energy storage time T QR If the transformer energy storage time T PL The preset energy storage time T is exceeded continuously QR , the determination signal TR-_GD will change from logical "0" to "1", otherwise it remains logical "0". When the rising edge of the determination signal TR-_GD changes from logical "0" to "1" occurs, the sufficient signal SR_PRE becomes "1". Figure 2 In the example, the recorder 116 is an SR trigger. At the falling edge of the start signal M1_ON, the recorder 116 is reset, and the sufficient signal SR_PRE is "0". In short, when the transformer storage time T PL The preset energy storage time T is exceeded continuously QR , the sufficient signal SR_PRE output by the recorder 116 will be "1".

[0082] The forward bias detector 104 compares the detection signal V DET With the turn-on reference level V SR_ON For example, turn on the reference level V SR_ONis -0.4V. When the detection signal V DET Lower than the turn-on reference level V SR_ON When the forward bias voltage detector 104 sets the forward bias voltage signal SR_FW to "1", it means that there is a sufficiently large forward bias voltage at both ends of the channel of the synchronous rectification switch SWSR. Otherwise, the forward bias voltage detector 104 sets the forward bias voltage signal SR_FW to "0".

[0083] The pre-charge circuit 106 is used to increase the control signal S on the control terminal DRV according to the sufficient signal SR_PRE and the forward bias signal SR_FW. SEC Precharge to sub-threshold voltage V SUB_T , which is between the critical voltage V SR_TH For example, the threshold voltage V SR_TH is 5V, and the subthreshold voltage V SUB_T is 2.5V. Figure 2 As shown, as long as either the forward bias signal SR_FW or the sufficient signal SR_PRE is logically "1", the current source 126 can charge the control terminal DRV through the switches 120 and 122, and quickly pull up the control signal S SEC To subthreshold voltage V SUB_T until.

[0084] The driving circuit MI has a pull-up switch SH and a control circuit 130. The pull-up switch SH is a PMOS transistor. When the pull-up switch SH is turned on, the control signal S SEC To the fixed turn-on voltage V DD For example, the fixed turn-on voltage V DD is an operating power supply voltage. The forward bias signal SR_FW and the sufficient signal SR_PRE can trigger the control circuit 130 to turn on the pull-up switch SH through an AND gate. For example, the control circuit 130 can be a pulse generator. Figure 2 It can be seen from the analysis that when the forward bias signal SR_FW and the sufficient signal SR_PRE are both logically "1", the pulse generator of the control circuit 130 generates a pulse with a preset pulse width to turn on the pull-up switch SH for a preset period of time.

[0085] Figure 2 The display driving circuit MII is an operational amplifier, comparing the detection signal V DET With the preset reference level V REF_OP , used to drive the control signal S SEC , so as to adjust the detection signal V DET The preset reference level V REF_OP In one embodiment, the preset reference level V REF_OP Follow the start reference level VSR_ON In another embodiment, the preset reference level V REF_OP is -0.5V, while the reference level V SR_ON When the sufficient signal SR_PRE is logically "1", the driving circuit MII is disabled and its output becomes high impedance.

[0086] The pull-down controller 108 can turn on the pull-down switch SL and quickly turn the control signal S SEC Pull down to 0V to turn off the synchronous rectification switch SWSR and avoid the inductive current I SEC For example, the controller 108 compares the detection signal V DET and 0V (the ground reference voltage on the output ground line 28). DET When it is higher than 0V, the controller 108 is pulled down to open the pull-down switch SL, so that the control signal S SEC is approximately 0 V. When the sufficient signal SR_PRE is logically "1", the pull-down controller 108 is disabled, closing the pull-down switch SL.

[0087] Figure 3 Display used in Figure 2 A control method 800 of the synchronous rectification controller 14 is provided. Figure 4 Displays the time from point t 10 At time t 1E During the switching cycle TCYC1, Figure 1 and Figure 2 Some signal waveforms in Figure 4 In the figure, from top to bottom, they are the PWM signal S that controls the power switch N1. PRI , detection signal V DET , control signal S of synchronous rectification switch SWSR SEC , the determination signal TR-_GD output by the de-jitter circuit 114, the sufficient signal SR_PRE output by the recorder 116, the pre-charge signal PR_EN in the pre-charge circuit 106, the forward bias signal SR_FW output by the forward bias detector 104, and the full-open signal SR_FL input to the control circuit 130 in the drive circuit MI.

[0088] Figure 3 Step 802 checks the transformer energy storage time T PL Whether it has continued to exceed the preset energy storage time T QR . See Figure 4 , at time point t 10 , PWM signal S PRI The power switch N1 starts to be turned on. Because of the inductive coupling of the transformer TR, the detection signal VDET At time point t 10 rises to a high voltage level V H , exceeds the preset reflection level V PL At time point t 10 , Figure 2 The comparator 112 in the transformer informs the transformer of the energy storage time T PL1 Start, that is, detect signal V DET Starts to exceed the preset reflection level V PL At time point t 11 , Figure 2 The de-jitter circuit 114 in the transformer finds the transformer energy storage time T PL1 The preset energy storage time T is exceeded continuously QR Therefore, the determination signal TR-_GD will change from logical "0" to "1", which also causes the sufficient signal SR_PRE and the pre-charge signal PR_EN to change to logical "1", just like Figure 4 shown.

[0089] When the sufficient signal SR_PRE is "1", Figure 2 The middle driving circuit MII and the pull-down controller 108 are both disabled and do not clamp the control signal S SEC is 0V.

[0090] when Figure 3 When the result of the check in step 802 is yes, step 804 then sets the control signal S SEC Precharged to sub-threshold voltage V SUB_T . Figure 4 Time point t 11 to 13 , defined as the pre-charge time T PRE1 , is the transformer energy storage time T PL1 Exceeding the preset energy storage time T QR part. Figure 4 Time point t 11 , the precharge signal PR_EN turns to logic "1", so the precharge circuit 106 turns the control signal S of the control terminal DRV SEC Precharge to sub-threshold voltage V SUB_T , which is between the critical voltage V SR_TH Between 0V and Figure 4 As shown. During the pre-charge time T PRE1 The drive circuit MII is disabled, and the control signal S SEC is precharged to the subthreshold voltage V SUB_T .

[0091] Figure 3 Step 806 is a continuation of step 804, where the bias detector 104 compares the detection signal V DETWith the turn-on reference level V SR_ON Once the detection signal V DET Lower than the turn-on reference level V SR_ON , step 808 starts to make the driving circuit MI turn on the synchronous rectification switch SWSR. Figure 4 Time point t 12 , PWM signal S PRI The power switch N1 starts to be turned off, so the detection signal V DET From the high voltage level V H Starts to drop rapidly, transformer energy storage time T PL1 Finish. Figure 4 Mid-time point t 13 , detection signal V DET has fallen below the turn-on reference level V SR_ON , the full-open signal SR_FL turns to logic "1", the driving circuit MI turns on the pull-up switch SH with the control circuit 130, and quickly pulls up the control signal S SEC To the fixed turn-on voltage V DD .

[0092] After the driving circuit MI turns on the synchronous rectification switch SWSR for a period of time, step 810 disables the driving circuit MI and starts to enable the driving circuit MII. 14 The pull-up switch SH is turned off and the recorder 116 is reset, so that both the sufficient signal SR_PRE and the full-open signal SR_FL are turned to "0". Therefore, the driving circuit MI is disabled, the pull-up switch SH is turned off, and the control signal S is no longer pulled up. SEC To the fixed turn-on voltage V DD The driving circuit MII is enabled and starts to regulate the detection signal V DET To the preset reference level V REF_OP , so the control signal S SEC Starting from a fixed turn-on voltage V DD Begins to slide down, like Figure 4 shown.

[0093] In step 812, when the detection signal V DET When the voltage is greater than 0V or other time limits are met, the controller 108 is pulled down to open the pull-down switch SL, so that the control signal S SEC Fixed to 0V. Figure 4 Time point t 15 , detection signal V DET Starts above the turn-on reference level V SR_ON , the pre-charge circuit 106 stops responding to the control signal S SEC clamped, and the control signal S SEC It continues to be driven by the driving circuit MII and decreases. At time point t16 , detection signal V DET The pull-down switch SL is turned on and the control signal S SEC is 0V.

[0094] from Figure 4 It can be seen that during the transformer energy storage time T PL1 Exceeding the preset energy storage time T QR After that, it can be regarded that the synchronous rectification controller 14 confirms that the power switch N1 on the primary side PRM has been turned on long enough, and the synchronous rectification switch SWSR is ready to meet the large current rectification, so the control signal S SEC It is pre-charged to the sub-threshold voltage V SUB_T In this way, at a subsequent time point t 13 When the control signal S SEC can quickly rise from the subthreshold voltage V SUB_T The synchronous rectifier switch SWSR starts to rise, allowing the synchronous rectifier switch SWSR to be turned on quickly, providing a channel with small enough impedance to reduce the resistance loss of current flowing through this channel. At time point t13, the driving circuit MI is used to turn on the synchronous rectifier switch SWSR to quickly convert the control signal S SEC Pull up to the fixed start voltage V DD , and the channel impedance of the synchronous rectification switch SWSR can also be reduced.

[0095] Figure 5 Displays the time from point t 20 At time t 30 The switching cycle TCYC2, and the time point t 30 At time t 3E The switching cycle TCYC3 and some of the signal waveforms therein. Figure 5 and Figure 4 The same and similar parts can be found in the previous Figure 4 This can be learned from the explanation, so I will not repeat it here.

[0096] as Figure 5 As shown, the preset reflection level V PL At high voltage level V H With low voltage level V L The high voltage level V H When the power switch N1 is turned on, the detection signal V DET The voltage level reached. Low voltage level V L It is approximately the transformer TF at time t- 23 After the discharge is completed, the oscillation can reach the highest point. For example, the low voltage level V L Approximately DC output power V OUT Twice the voltage.

[0097] In the switching cycle TCYC2, the detection signal V DET Continuously exceeds the preset reflection level V PL Transformer energy storage time T PL2 , than the preset energy storage time T QR Therefore, Figure 2 The de-jitter circuit 114 in the embodiment will fix the determination signal TR-_GD to be logically "0", which also results in that the sufficient signal SR_PRE and the full-on signal SR_FL are both "0" in the entire switching cycle TCYC2. This means that the driving circuit MI will not be used to turn on the synchronous rectification switch SWSR. Figure 3 In the control method 800, the result of step 802 is no, so step 814 checks the detection signal V DET Is it lower than the turn-on reference level V SR_ON .

[0098] At the time point t of the switching cycle TCYC2 21 , detection signal V DET has fallen below the turn-on reference level V SR_ON , causing the pre-charge circuit 106 to quickly control the signal S SEC Precharge to sub-threshold voltage V SUB_T (Step 816), and the driving circuit MII then generates a signal according to the detection signal V DET , slowly increase or decrease the control signal S SEC (Step 818), roughly maintaining the control signal S SEC At the critical voltage V SR_TH above.

[0099] At time point t 22 , detection signal V DET Can no longer maintain the turn-on reference level V SR_ON , control signal S SEC is pulled down, below the critical voltage V SR_TH , the forward bias signal SR_FW and the pre-charge signal PR_EN both turn into logical "0".

[0100] At time point t 23 , detection signal V DET The pull-down switch SL is turned on and the control signal S SEC is 0V (step 820).

[0101] The switching cycle TCYC3 is similar to the switching cycle TCYC2. The same or similar points can be found in the previous description and will not be repeated here. 31 , for some reason, the detection signal VDET briefly drops below the turn-on reference level V SR_ON Therefore, the forward bias signal SR_FW and the pre-charge signal PR_EN both briefly turn into logic "1".

[0102] Figure 5 Block ZM in the figure zooms in on the area near time point t 31 The control signal S SEC As the precharge signal PR_EN changes to “1”, the control signal S SEC At time point t 31 is quickly precharged to the subthreshold voltage V SUB_T At this time, the driving circuit MII also responds to the detection signal V DET , slowly change the control signal S SEC . With the detection signal V DET At time point t 32 rises above 0V, the pull-down switch SL is turned on and the fixed control signal S SEC is 0V.

[0103] In one embodiment, the synchronous rectification controller 14 can detect the control signal S in a timely manner. SEC To determine the critical voltage V of the synchronous rectifier switch SWSR SR_TH For example, in Figure 4 and 5 Time point t 15 or 22 , at the falling edge of the bias signal SR_FW, the synchronous rectification controller 14 samples the control signal S SEC , as the critical voltage V SR_TH .

[0104] The synchronous rectification controller 14 can be configured to provide a voltage according to the critical voltage V SR_TH The subthreshold voltage V SUB_T In one embodiment, the subthreshold voltage V SUB_T Proportional to the critical voltage V SR_TH , or 0.5 times the critical voltage V SR_TH .

[0105] The above descriptions are only preferred embodiments of the present invention. All equivalent changes and modifications made according to the claims of the present invention should fall within the scope of the present invention.

Claims

1. A synchronous rectification control method, applicable to a power supply, the power supply having a primary side and a secondary side isolated from each other, the power supply having a synchronous rectification switch on the secondary side, the synchronous rectification switch having a control end, when a control signal on the control end is greater than a critical voltage, the synchronous rectification switch is turned on to provide a short-circuited channel, when the control signal is between the critical voltage and a ground reference voltage, the synchronous rectification switch is turned off to make the channel open, the synchronous rectification control method comprising: providing a detection signal according to one end of the channel; Comparing the detection signal with a preset reflection level, wherein the preset reflection level is higher than the ground reference voltage, and the time during which the detection signal is greater than the preset reflection level is the transformer energy storage time; Comparing the transformer energy storage time with a preset energy storage time to generate a sufficient signal; comparing the detection signal with a turn-on reference level to generate a forward bias signal, wherein the turn-on reference level is lower than the ground reference voltage; and According to the sufficient signal and the forward bias signal, the control terminal is precharged to make the control signal a sub-threshold voltage between the threshold voltage and the ground reference voltage.

2. The synchronous rectification control method according to claim 1, further comprising: An operational amplifier is provided to drive the control terminal according to the detection signal so as to adjust the detection signal to a preset reference level lower than the ground reference voltage.

3. The synchronous rectification control method according to claim 2, further comprising: Provide a pull-high switch; and After the transformer has stored energy for a period of time, when the detection signal is lower than the start reference level, the pull-up switch is turned on, so that the control signal is a fixed start voltage greater than the critical voltage, and the synchronous rectification switch is turned on.

4. The synchronous rectification control method according to claim 3, further comprising: After the pull-up switch is turned on for a predetermined period of time, the pull-up switch is turned off, leaving the operational amplifier to regulate the detection signal at the preset reference level.

5. The synchronous rectification control method according to claim 4, wherein the portion of the transformer energy storage time that exceeds the preset energy storage time is the pre-charging time, further comprising: During the pre-charging time, pre-charging the control terminal so that the control signal is at the sub-threshold voltage; During the precharge time, disabling the operational amplifier; and After closing the pull-up switch, the operational amplifier is enabled.

6. The synchronous rectification control method according to claim 2, further comprising: Provide a pull-down switch; comparing the detection signal with the ground reference voltage; and When the detection signal is higher than the ground reference voltage, the pull-down switch is turned on to make the control signal the ground reference voltage.

7. A synchronous rectification controller, suitable for a power supply, the power supply having a primary side and a secondary side isolated from each other, the power supply having a synchronous rectification switch on the secondary side, the synchronous rectification controller providing a control signal to a control end of the synchronous rectification switch, when the control signal is greater than a critical voltage, turning on the synchronous rectification switch to provide a short-circuited channel, when the control signal is between the critical voltage and a ground reference voltage, turning off the synchronous rectification switch to make the channel open, one end of the channel providing a detection signal, the synchronous rectification controller comprising: The primary side starts the judgement to compare the detection signal on one end of the channel with the preset reflection level, wherein, The preset reflection level is higher than the ground reference voltage, and the time when the detection signal is greater than the preset reflection level is the transformer energy storage time. When the transformer energy storage time continues to exceed the preset energy storage time, the primary side start-up judgement device generates a sufficient signal; a forward bias voltage detector, comparing the detection signal with a turn-on reference level, which is lower than the ground reference voltage, to generate a forward bias voltage signal; as well as The pre-charging circuit is used for pre-charging the control terminal to a sub-threshold voltage between the threshold voltage and the ground reference voltage according to the sufficient signal and the forward bias signal.

8. The synchronous rectification controller according to claim 7, wherein: The primary side opening judgement device comprises: A comparator, for comparing the detection signal with the preset reflection level, for detecting the transformer energy storage time during which the detection signal is greater than the preset reflection level; A de-jitter circuit, which provides a confirmation signal when the transformer energy storage time is greater than a preset energy storage time; as well as A recorder is used for recording the determination signal to generate the sufficient signal.

9. The synchronous rectification controller according to claim 7, further comprising: The driving circuit is used to pull up the control signal, including: A pull-up switch, when turned on, is used to make the control signal a fixed turn-on voltage greater than the critical voltage, turning on the synchronous rectification switch; and The control circuit turns on the pull-up switch according to the sufficient signal and the forward bias signal.

10. The synchronous rectification controller according to claim 9, wherein: The control circuit turns on the pull-up switch for a preset time.

11. The synchronous rectification controller according to claim 7, further comprising: The operational amplifier drives the control terminal according to the detection signal to adjust the detection signal to a preset reference level lower than the ground reference voltage.

12. The synchronous rectification controller according to claim 11, wherein: The part of the transformer energy storage time that continues to exceed the preset energy storage time is the pre-charging time. During the pre-charging time, the pre-charging circuit pre-charges the control end to a sub-threshold voltage, and the operational amplifier is disabled.

13. The synchronous rectification controller according to claim 11, further comprising: Pull the switch low; and Pull down the controller and compare the detection signal with the ground reference voltage; in, When the detection signal is higher than the ground reference voltage, the pull-down switch is turned on, so that the control signal is the ground reference voltage.

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