Synchronous rectification control circuit, synchronous rectification control method and switching power supply

Abstract

Disclosed are a synchronous rectification control circuit, a synchronous rectification control method and a switching power supply, the synchronous rectification control circuit comprising: a volt-second setting unit for processing a volt-second product reference value into at least any two of a first volt-second product threshold to a third volt-second product threshold; the first control driving unit is used for sending a first conduction signal to conduct the synchronous rectifier tube and controlling the pulse width of the first conduction signal according to a first volt-second product threshold value; the second control driving unit is used for calculating a first volt-second product of voltages at two ends of the secondary winding within first time after the first conduction signal is generated, and controlling the synchronous rectifier tube to be conducted for follow current after the first volt-second product is greater than a second volt-second product threshold value; and the voltage judgment unit is used for calculating a second volt-second product of voltages at two ends of the secondary winding within a second time after the first conduction signal is generated, and judging the state of the input voltage according to the second volt-second product and a third volt-second product threshold value. Through the arrangement, the volt-second setting pin of the synchronous rectification control circuit can be reused to execute various functions.

Description

Technical Field

[0001] The present application relates to the field of power electronics technology, and in particular to a synchronous rectification control circuit, a synchronous rectification control method and a switching power supply. Background Art

[0002] With the rapid development of electronic systems, people have an increasingly higher demand for high power density and high efficiency switching power converters. Flyback switching power supplies, forward switching power supplies, double-clamped ZVS converters, etc. have been widely studied and applied due to their excellent characteristics such as high efficiency, full-range soft switching, and adaptability to high switching frequencies.

[0003] Figure 1 A schematic circuit diagram of a flyback switching power supply is shown in FIG. Figure 1 As shown, the common flyback switching power supply topology includes a main power tube Q1, a transformer and a synchronous rectifier tube Q2. The transformer includes a primary winding Np and a secondary winding Ns. The main power tube Q1 and the sampling resistor Rcs are connected to the primary winding Np, and the main power tube Q1 is turned on and off by the primary control chip 1. The synchronous rectifier tube Q2 is connected to the secondary winding Ns, and the synchronous rectifier tube Q2 is turned on and off by the secondary control chip 2. The flyback switching power supply also includes an auxiliary winding Na and resistors R1 and R2 connected to the auxiliary winding Na. The middle node of the resistors R1 and R2 is connected to the VS pin of the chip 1. The VS pin is grounded through the resistor R3 and the switch S1, so that the current Ivs of the VS pin can be obtained. The current Ivs can be used to characterize the size of the input voltage Vin. When Ivs is greater than a certain threshold, it indicates that the input voltage has reached the turn-on voltage. When Ivs is less than another threshold, it indicates that the input voltage has reached the turn-off voltage, that is, the brown in and brown out functions of the circuit are realized.

[0004] In addition, the secondary control chip 2 can obtain the volt-second product at both ends of the secondary winding and compare it with the volt-second product threshold to control the freewheeling of the synchronous rectifier Q2. The secondary control chip 2 can also control the synchronous rectifier Q2 to be turned on for a period of time before the main power tube Q1 is turned on, so that the main power tube Q1 can be soft-started. The realization of these two functions requires relying on separate pins, for example, the VT pin and the VT2 pin are required to realize these two functions respectively.

[0005] Therefore, the current flyback switching power supply requires multiple pins to implement the above-mentioned multiple functions, the circuit occupies a large area and the system cost is high. Summary of the invention

[0006] In order to solve the above technical problems, the present application provides a synchronous rectification control circuit, a synchronous rectification control method and a switching power supply.

[0007] According to one aspect of the present invention, a synchronous rectification control circuit is provided for controlling the conduction of a synchronous rectifier tube in a flyback switching power supply, wherein the flyback switching power supply comprises a transformer, wherein the transformer comprises a primary winding and a secondary winding, and wherein the flyback switching power supply further comprises a main power tube connected to the primary winding and a synchronous rectifier tube connected to the secondary winding, wherein the synchronous rectification control circuit comprises:

[0008] A volt-second setting unit obtains a volt-second product reference value from a volt-second setting pin of a synchronous rectification control circuit, and processes the set volt-second product reference value into at least any two of a first volt-second product threshold value, a second volt-second product threshold value, and a third volt-second product threshold value;

[0009] a first control driving unit, sending a first conduction signal to the synchronous rectifier to turn on the synchronous rectifier; starting to calculate a third volt-second product generated by the output voltage of the flyback switching power supply at the time when the first conduction signal is generated, and controlling a pulse width of the first conduction signal according to the third volt-second product and the first volt-second product threshold;

[0010] A second control driving unit calculates a first volt-second product of the voltage across the secondary winding within a first time after the first conduction signal is generated, and after the first volt-second product is greater than the second volt-second product threshold, the synchronous rectifier can be controlled to conduct for freewheeling;

[0011] The voltage judgment unit calculates a second volt-second product of the voltage across the secondary winding within a second time after the first conduction signal is generated, and judges the state of the input voltage of the flyback switching power supply according to the magnitude of the second volt-second product and the third volt-second product threshold.

[0012] Optionally, the first control driving unit includes a shutdown control unit, and the shutdown control unit controls the synchronous rectifier to be shut down when the third volt-second product is greater than the first volt-second product threshold.

[0013] Optionally, when the third volt-second product is greater than the first volt-second product threshold, it indicates that during the period when the first turn-on signal controls the conduction of the synchronous rectifier, the negative current generated on the main power tube reaches a current threshold, and based on the current threshold, the main power tube can achieve zero voltage turn-on.

[0014] Optionally, the first control driving unit includes a conduction control unit, which generates a switch conduction signal based on a compensation signal generated by an output voltage of the flyback switching power supply, and generates the first conduction signal based on the switch conduction signal.

[0015] Optionally, the second control driving unit includes a switch control unit,

[0016] The switch control unit controls the synchronous rectifier to be turned on when the first volt-second product is greater than the second volt-second product threshold and the drain-source voltage of the synchronous rectifier decreases below a first threshold voltage.

[0017] Optionally, the second control driving unit further includes a volt-second product calculation unit, and the volt-second product calculation unit includes:

[0018] a volt-second integration unit, which integrates and calculates the first volt-second product according to the drain voltage of the synchronous rectifier tube and the output voltage of the flyback switching power supply within the first time;

[0019] a first time setting unit, which controls the volt-second integration unit to start working at the start time of the first time, and controls the volt-second integration unit to reset at the end time of the first time; and

[0020] The volt-second determination unit compares the first volt-second product with the second volt-second product threshold.

[0021] Optionally, the starting moment of the first time is any moment between the moment when the first conduction signal is generated and the moment when the main power tube is turned on.

[0022] Optionally, the end moment of the first time is any moment between the turn-off moment of the main power tube and the moment when the drain voltage of the synchronous rectifier tube is less than the output voltage of the flyback switching power supply.

[0023] Optionally, the voltage judgment unit generates a start control signal when the second volt-second product is greater than the third volt-second product threshold, and the start control signal indicates that the input voltage reaches the start-up voltage.

[0024] Optionally, the voltage judgment unit includes:

[0025] A second time setting unit, which starts timing the second time from the start time of the second time;

[0026] an integrating unit, calculating the second volt-second product within the second time, and clearing the second volt-second product to zero at the end of the second time; and

[0027] The comparison unit compares the second volt-second product with the third volt-second product threshold, and generates a start control signal according to the comparison result.

[0028] Optionally, the time period in which the second time falls is a time period in which the drain voltage of the synchronous rectifier tube is greater than the output voltage of the flyback switching power supply after the first conduction signal is generated.

[0029] Optionally, the first volt-second product threshold, the second volt-second product threshold and the third volt-second product threshold are all proportional to the volt-second product reference value.

[0030] Optionally, the volt-second setting pin of the synchronous rectification control circuit is connected to a volt-second setting resistor, and the volt-second setting resistor is connected to a current source inside the synchronous rectification control circuit to generate the volt-second product reference value, and the size of the volt-second product reference value is adjusted by adjusting the current source or the volt-second setting resistor.

[0031] The present invention also provides a synchronous rectification control method for controlling the conduction of a synchronous rectifier tube in a flyback switching power supply, wherein the flyback switching power supply includes a transformer, the transformer includes a primary winding and a secondary winding, the flyback switching power supply also includes a main power tube connected to the primary winding and a synchronous rectifier tube connected to the secondary winding, wherein the synchronous rectification control method includes:

[0032] Processing the set volt-second product reference value into at least any two of a first volt-second product threshold value, a second volt-second product threshold value, and a third volt-second product threshold value;

[0033] Sending a first conduction signal to the synchronous rectifier to turn on the synchronous rectifier, starting to calculate a third volt-second product generated by the output voltage of the flyback switching power supply at the time when the first conduction signal is generated, and controlling the pulse width of the first conduction signal according to the third volt-second product and the first volt-second product threshold;

[0034] Calculating a first volt-second product of the voltage across the secondary winding within a first time after the first conduction signal is generated, and controlling the synchronous rectifier to conduct for freewheeling after the first volt-second product is greater than the second volt-second product threshold;

[0035] A second volt-second product of the voltage across the secondary winding is calculated within a second time after the first conduction signal is generated, and a state of the input voltage of the flyback switching power supply is determined according to the second volt-second product and the third volt-second product threshold.

[0036] Optionally, the synchronous rectifier is controlled to be turned off when it is detected that the third volt-second product is greater than the first volt-second product threshold;

[0037] Among them, when the third volt-second product is greater than the first volt-second product threshold, it indicates that during the period when the first conduction signal controls the conduction of the synchronous rectifier tube, the negative current generated on the main power tube reaches the current threshold, and the current threshold enables the main power tube to achieve zero voltage turn-on.

[0038] Optionally, after the first volt-second product is greater than the second volt-second product threshold and the drain-source voltage of the synchronous rectifier decreases below a first threshold voltage, the synchronous rectifier is controlled to be turned on.

[0039] Optionally, a start control signal is generated when the second volt-second product is greater than the third volt-second product threshold, and the start control signal indicates that the input voltage reaches the start-up voltage.

[0040] Optionally, the first volt-second product threshold, the second volt-second product threshold and the third volt-second product threshold are all proportional to the volt-second product reference value.

[0041] The present invention also provides a switching power supply, comprising a transformer, wherein the transformer comprises a primary winding and a secondary winding, the switching power supply further comprises a main power tube connected to the primary winding and a synchronous rectifier tube connected to the secondary winding, wherein the switching power supply further comprises:

[0042] According to the synchronous rectification control circuit described above, the synchronous rectification control circuit is used to control the working state of the synchronous rectification tube; and

[0043] A volt-second setting resistor is connected to a volt-second setting pin of the synchronous rectification control circuit, and the volt-second product reference value is set by adjusting a current flowing through the volt-second setting resistor or adjusting a resistance value of the volt-second setting resistor.

[0044] The embodiments of the present invention have at least the following beneficial effects:

[0045] The synchronous rectification control circuit, synchronous rectification control method and switching power supply provided by the present invention are based on the volt-second product of the output voltage and the drain voltage of the synchronous rectifier tube in the first time as the control condition for turning on or off the synchronous rectifier tube, and the volt-second product of the voltage at both ends of the secondary winding in the second time is used to judge the input voltage of the primary winding. Therefore, all or at least any two of the functions of the secondary side freewheeling conduction, brown in and brown out detection, and the secondary side early conduction to enable the main power tube to realize ZVS (Zero Voltage Switch) can be concentrated in the synchronous rectification control circuit of the secondary side to realize, so that the synchronous rectification control circuit becomes the main control circuit, simplifying the circuit structure and control method. In addition, the execution of the above three functions is converted to rely on the volt-second product for judgment, and different volt-second product thresholds can be uniformly set by the synchronous rectification control circuit, and part of the circuit structure can be reused to calculate multiple volt-second products, further simplifying the circuit design, improving the control performance, and reducing the occupied space of the primary control circuit.

[0046] Furthermore, the volt-second setting unit receives the volt-second product reference value from the outside of the synchronous rectification control circuit and converts it into at least two different volt-second product thresholds, so that the volt-second setting pin of the synchronous rectification control circuit can be reused to achieve multiple different function judgments, reduce the use of pins, and reduce chip manufacturing costs. In addition, a certain proportional relationship is formed between the multiple volt-second product thresholds, so that multiple different volt-second product thresholds can be quickly obtained according to the set volt-second product reference value, and circuit optimization is achieved while ensuring the circuit function.

[0047] Furthermore, before the main power tube is turned on, the first control driving unit generates a first conduction signal to control the conduction of the synchronous rectifier tube, so that a negative current is generated on the main power tube. The moment when the third volt-second product generated by the integration of the output voltage in the first time is greater than the first volt-second product threshold is used as the turn-off moment of the synchronous rectifier tube, so that the pulse width of the first conduction signal can be adjusted, so that the appropriate negative current can be set, so that the main power tube can achieve ZVS turn-on.

[0048] Furthermore, the second volt-second product generated by the voltage at both ends of the secondary winding is calculated within the second time after the first conduction signal is generated, and then the state of the input voltage of the flyback switching power supply is judged according to the second volt-second product and the third volt-second product threshold, and whether the input voltage reaches the start-up voltage is judged. The size of the third volt-second product threshold can be indirectly adjusted by adjusting the size of the volt-second product reference value, thereby adjusting the judgment line of whether the start-up voltage is reached, so that the application range of the switching power supply is wider.

[0049] It should be noted that the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 A schematic circuit diagram of a flyback switching power supply is shown;

[0051] Figure 2 A schematic circuit diagram of a switching power supply according to an embodiment of the present invention is shown;

[0052] Figure 3 and Figure 4 A schematic block diagram and a schematic circuit diagram of a synchronous rectification control circuit according to an embodiment of the present invention are respectively shown;

[0053] Figure 5 shows a schematic waveform diagram of various signals of a switching power supply according to an embodiment of the present invention when it is in normal operation;

[0054] Figure 6 A schematic flow chart of a synchronous rectification control method provided according to an embodiment of the present invention is shown. DETAILED DESCRIPTION

[0055] In order to facilitate the understanding of the present invention, the present invention will be described more fully below with reference to the relevant drawings. The preferred embodiments of the present invention are shown in the drawings. However, the present invention can be implemented in different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the understanding of the disclosure of the present invention more thorough and comprehensive.

[0056] Figure 2 The schematic circuit diagram of the switching power supply according to the embodiment of the present invention is shown. The present invention mainly controls the on and off of the synchronous rectifier tube on the secondary side of the flyback switching power supply, and determines the state of the input voltage according to the secondary winding, so the switching power supply is preferably a flyback switching power supply.

[0057] like Figure 2 As shown, the flyback switching power supply of this embodiment includes a main circuit 10, a primary control circuit 100 and a synchronous rectification control circuit 200. The main circuit 10 includes a primary main power tube Q1, a secondary synchronous rectifier Q2 and a transformer T, and the transformer T includes a primary winding Np and a secondary winding Ns coupled to each other. The main power tube Q1 and the synchronous rectifier Q2 can be, for example, MOS tubes, or GaN tubes, SiC tubes, etc. The source of the main power tube Q1 is grounded, for example, through a sampling resistor Rcs, the drain is connected to the primary winding Np, and the gate is connected to the primary control circuit 100, and the main power tube Q1 is turned on and off according to the primary drive signal VDR. The gate of the synchronous rectifier Q2 is connected to the synchronous rectification control circuit 200, and the synchronous rectifier Q2 is turned on and off according to the drive signal VGT or VGTZ. The source of the synchronous rectifier Q2 is grounded (the source grounding of Q2 is only one example here, and of course other connections can be made, for example, connected to the ground through a resistor), and the drain is connected to the secondary winding Ns. A diode D1 and a capacitor C0 are connected between the drain of the primary main power tube Q1 and the input power supply Vin, and a resistor Ri is connected in parallel at both ends of the capacitor C0, wherein D1, C0 and Ri do not represent limitations on the flyback converter structure in the present invention, and other structures can also be used to achieve the same function; a filter capacitor Ci is also connected between the input power supply and the ground. The input voltage Vin generated by the alternating current source AC forms a bus voltage Vbus after passing through the rectifier bridge BD. A filter capacitor Co is connected between the source of the synchronous rectifier Q2 and the opposite end of the secondary winding Ns on the secondary side. Figure 2In the embodiment shown, a first node voltage Vsw, which can also be referred to as the voltage on the drain end of the synchronous rectifier, is obtained from the common node of the secondary winding Ns and the synchronous rectifier Q2, and an output voltage Vo is obtained from the output end. The difference between the first node voltage Vsw and the output voltage Vo represents the voltage across the secondary winding. During the switching cycle of the switching power supply, when the main power tube Q1 is turned on, the input voltage Vin excites the transformer T via the main power tube Q1, the synchronous rectifier Q2 is turned off, and the output filter capacitor Co supports the load RL. When the main power tube Q1 is turned off, the synchronous rectifier Q2 is turned on, and the electric energy stored on the transformer T is transmitted to the secondary side to supply power to the load RL and charge the capacitor Co.

[0058] In this embodiment, the primary control circuit 100 includes a plurality of pins, such as a DRV pin and a CS pin. The CS pin is connected to the common node of the sampling resistor Rcs and the main power tube Q1, and is used to obtain the voltage on the sampling resistor Rcs, so as to determine the current size on the main power tube Q1. If the direction shown by the Ip current in the figure is the positive direction, the current opposite to its current direction is called the reverse current. The DRV pin is connected to the gate of the main power tube Q1, and controls the working state of the main power tube Q1 according to the primary drive signal VDR. The synchronous rectification control circuit 200 also includes a plurality of pins, such as a GT pin, a SW pin, a VT pin and a VO pin, etc. The VO pin receives the output voltage Vo, the SW pin receives the first node voltage Vsw, and the VT pin can provide a volt-second product reference value through an external volt-second setting resistor Rvt, and then provide it to the internal circuit to perform the corresponding function. The GT pin is used to output a drive signal VGT or VGTZ.

[0059] exist Figure 2In the embodiment, the synchronous rectification control circuit 200 obtains the volt-second product reference value VTref from the VT pin (which can be regarded as the volt-second setting pin), and converts it into a plurality of different volt-second product thresholds, for example, it is converted into at least two of the first volt-second product threshold value ZVS-VTth, the second volt-second product threshold value SR-VTth and the third volt-second product threshold value BR-VTth, and it can also be converted into the first volt-second product threshold value to the third volt-second product threshold value, or even more volt-second product threshold values. The synchronous rectification control circuit 200 is connected to the external volt-second setting resistor Rvt through the VT pin, and the volt-second setting resistor Rvt is connected to the current source inside the synchronous rectification control circuit 200 to generate the volt-second product reference value VTref on the volt-second product quasi-resistor Rvt according to the action of the current source. Moreover, the size of the volt-second product reference value VTref can be adjusted by adjusting the current size generated by the current source or replacing different volt-second setting resistors Rvt. After the circuit is started, the synchronous rectification circuit 200 can send a first conduction signal VGTZ to the synchronous rectification tube according to the compensation signal Vcom to turn on the synchronous rectification tube Q2 when both the main power tube Q1 and the synchronous rectification tube Q2 are turned off, so that a negative current is generated on the main power tube Q1. The compensation signal Vcom represents the error between the output feedback signal and the reference signal of the flyback switching power supply. The output feedback signal can be obtained based on the output voltage, output power or output current. In this embodiment, for example, the compensation signal Vcom can be obtained based on the output voltage Vo. The direction of the negative current generated on the main power tube Q1 is the same as Figure 1 The direction of the Ip current is opposite. The negative current releases the charge on the parasitic capacitors at both ends of the main power tube Q1, and pulls the source-drain voltage Vds of the main power tube Q1 down to zero, so that the main power tube Q1 can be turned on at zero voltage (ZVS), achieving soft start, reducing the switching loss of the switching power supply to improve the efficiency of the switching power supply. The primary control circuit 100 characterizes the current on the main power tube Q1 by detecting the voltage signal Vcs on the sampling resistor Rcs. After detecting that the voltage signal Vcs is less than the first reference voltage Vcsref1 of the negative voltage, it is considered that the main power tube Q1 has generated a corresponding negative current, and then the main power tube Q1 can be controlled to be turned on. The primary control circuit 100 also controls the main power tube Q1 to be turned off when it detects that the voltage signal Vcs on the sampling resistor Rcs is greater than the second reference voltage Vcsref2 of the positive voltage.

[0060] Furthermore, the synchronous rectification control circuit 200 integrates the output voltage of the flyback switching power supply from the moment the first conduction signal VGTZ is generated to generate a third volt-second product VT3. When the third volt-second product VT3 is greater than the first volt-second product threshold value ZVS-VTth, it indicates that the negative current on the main power tube Q1 has reached the required threshold value, and at this time, the synchronous rectifier tube Q2 is controlled to be turned off.

[0061] In addition, the synchronous rectification control circuit 200 also calculates the first volt-second product VT1 of the voltage across the secondary winding Ns within the first time after the first conduction signal VGTZ is generated, and compares the first volt-second product VT1 with the second volt-second product threshold SR-VTth to determine the working state of the main power tube Q1. The first time may be greater than the conduction time of the main power tube Q1, or the first time may include the conduction time of the main power tube Q1. When the synchronous rectification control circuit 200 determines that the first volt-second product VT1 is greater than the second volt-second product threshold SR-VTth, it indicates that the main power tube Q1 is working normally, so that the synchronous rectifier tube Q2 can be allowed to continue current thereafter. Further, the synchronous rectification control circuit 200 can provide the second conduction signal VGT to the synchronous rectifier Q2 after the first volt-second product VT1 is greater than the second volt-second product threshold SR-VTth, and after detecting that the drain-source voltage of the synchronous rectifier Q2 decreases below the first threshold voltage or the slope of the change of the drain-source voltage of the synchronous rectifier Q2 meets the condition, so that the synchronous rectifier Q2 is turned on, and the first conduction signal VGTZ is continuously output in the next switching cycle. The alternating conduction of the main power tube Q1 and the synchronous rectifier Q2 can realize the transmission of electric energy. Moreover, the synchronous rectification control circuit 200 can also control the synchronous rectifier Q2 to turn off when it is detected that the drain-source voltage of the synchronous rectifier Q2 increases to above the second threshold voltage. That is, when it is detected that the first volt-second product VT1 is greater than the second volt-second product threshold SR-VTth, the synchronous rectifier Q2 will not be turned on immediately, and the condition that the drain-source voltage of the synchronous rectifier Q2 decreases below the first threshold voltage needs to be met before the synchronous rectifier Q2 is turned on.

[0062] Furthermore, the synchronous rectification control circuit 200 can also start timing the second time at a certain moment after the first conduction signal VGTZ is generated, for example, after the first conduction signal VGTZ is generated and the drain voltage of the synchronous rectifier Q2 (the first node voltage Vsw) is greater than the output voltage Vo of the flyback switching power supply, and calculate the second volt-second product VT2 of the voltage across the secondary winding (the difference between the first node voltage Vsw and the output voltage Vo) within the second time, and generate a start control signal when the second volt-second product VT2 is greater than the third volt-second product threshold BR-VTth, and the start control signal indicates that the input voltage has reached the start voltage; within the second time, when the second volt-second product VT2 is less than the third volt-second product threshold BR-VTth, a shutdown control signal is generated, and the shutdown control signal indicates that the input voltage has not reached the start voltage (shutdown voltage). Thus, the judgment function of browning and browning out is realized; of course, a corresponding hysteresis can also be set between the start voltage and the shutdown voltage. Among them, the time period of the second time is the time period in which the drain voltage of the synchronous rectifier tube is greater than the output voltage of the flyback switching power supply after the first conduction signal is generated. Therefore, the start time and end time of the second time can be set according to actual applications.

[0063] Since the flyback switching power supply reuses the VT pin to set different volt-second product thresholds, it can realize at least two of the above three functions. By setting different multiple volt-second product thresholds, it can also realize the judgment and control of multiple different functions, so that the circuit structure can be concentrated inside the synchronous rectification control circuit 200, so that Figure 1 On the basis of the auxiliary winding Na and the resistors R1-R3 connected to the auxiliary winding Na are omitted, pin settings are saved, the circuit structure is simplified, the space occupation is reduced, and the circuit cost is reduced. The following introduces the synchronous rectification control circuit and the synchronous rectification control method of the present application in combination with a specific circuit.

[0064] Figure 3 and Figure 4 A schematic block diagram and a schematic circuit diagram of a synchronous rectification control circuit according to an embodiment of the present invention are respectively shown.

[0065] like Figure 3 and Figure 4As shown, the synchronous rectification control circuit 200 of this embodiment mainly includes a volt-second setting unit 210, a first control driving unit 240, a second control driving unit 270, a voltage judgment unit 250 and a logic unit 260. The second control driving unit 270 also includes a volt-second product calculation unit 220 and a switch control unit 230. Among them, the volt-second setting unit 210 processes the set volt-second product reference value VTref into at least any two of the first volt-second product threshold value ZVS-VTth, the second volt-second product threshold value SR-VTth and the third volt-second product threshold value BR-VTth (these three volt-second product threshold values ​​are set at the same time in this embodiment for illustration). The first control driving unit 240 is used to send a first conduction signal VGTZ to the synchronous rectifier tube Q2 to turn on the synchronous rectifier tube Q2, and is connected to the volt-second setting unit 210, and controls the pulse width of the first conduction signal VGTZ according to the first volt-second product threshold value ZVS-VTth. The second control driving unit 270 is connected to the volt-second setting unit 210, and is used to calculate the first volt-second product VT1 of the voltage (Vsw-Vo) across the secondary winding Ns within the first time T1 after the first conduction signal VGTZ is generated. After the first volt-second product VT1 is greater than the second volt-second product threshold SR-VTth, it indicates that the main power tube Q1 is working normally, so that the synchronous rectifier Q2 can be allowed to continue current, and the second conduction signal VGT is sent to the synchronous rectifier Q2. After the first volt-second product VT1 is greater than the second volt-second product threshold SR-VTth, it can be delayed for a period of time or wait for a certain condition to be met before controlling the synchronous rectifier Q2 to conduct. The voltage judgment unit 250 is also connected to the volt-second setting unit 210. The voltage judgment unit 250 calculates the second volt-second product VT2 of the voltage across the secondary winding within the second time T2 after the first conduction signal VGTZ is generated, and judges the state of the input voltage Vin of the flyback switching power supply according to the second volt-second product VT2 and the third volt-second product threshold BR-VTth. For example, the voltage determination unit 250 generates a start control signal when the second volt-second product VT2 is greater than the third volt-second product threshold value BR-VTth, and generates a shutdown control signal when the second volt-second product VT2 is less than the third volt-second product threshold value BR-VTth. The start control signal indicates that the input voltage Vin has reached the turn-on voltage (brown in), and the shutdown control signal indicates that the input voltage Vin has not reached the turn-on voltage (brown out).

[0066] Specifically, when the flyback switching power supply is working, the maximum peak current allowed when the main power tube Q1 is turned on is Icsmax, and the maximum peak current Icsmax is reflected on the sampling resistor Rcs as a voltage signal Vcsmax. In order to take into account the efficiency under light load, it is necessary to control the minimum peak current when the main power tube Q1 is turned on to reach a certain proportion of Icsmax, such as 1 / 5-1 / 3. Of course, the proportion value can also be set to other values ​​according to actual applications; the current or voltage on the main power tube Q1 is proportional to the current or voltage on the secondary winding. The secondary side determines whether the main power tube Q1 is turned on normally by judging whether the volt-second product on the secondary winding reaches a certain threshold during the time period when the main power tube Q1 may be turned on, thereby judging whether the synchronous rectifier tube needs to be turned on. The integral of the voltage over a certain period of time is the volt-second product, and according to the volt-second conservation law, I*Ls=V*t, so obtaining the volt-second product under the condition that the inductance Ls of the secondary winding is constant is equivalent to obtaining the current value. Therefore, it is possible to determine whether the current value on the primary winding has reached the required value by judging whether the volt-second product of the secondary winding has reached the set threshold within a certain period of time. The voltage signal Vcs on the sampling resistor Rcs represents the current on the main power tube Q1. The volt-second product generated by the voltage across the secondary winding during the time period when the current of the main power tube Q1 reaches the maximum allowable current Icsmax from zero is the fourth volt-second product VT4. A proportional value K1 below 1 / 5-1 / 3 is selected. For example, K1 is set to 16%. When the volt-second product of the voltage across the secondary winding reaches this proportional value within the first period of time, it is considered that the main power tube Q1 is normally turned on, and the synchronous rectifier Q2 on the secondary side can be turned on for continuous current. Based on this idea, the following formula 1 is obtained:

[0067] SR―VTth=K1×VT4

[0068] In formula 1, a proportional relationship between the second volt-second product threshold SR-VTth and the fourth volt-second product VT4 is established, or in other words, a relationship between the second volt-second product threshold SR-VTth and Icsmax is established. K1 may be 16%, for example.

[0069] Furthermore, when the synchronous rectifier receives the first conduction signal VGTZ and conducts, a negative current will be generated on the main power tube Q1. The negative current is used to release the parasitic capacitance at both ends of the main power tube Q1. The negative current can release the parasitic capacitance to near zero. At this time, the negative current is approximately a certain proportion of Icsmax, and the proportion is about 5%. Based on similar reasons, the first volt-second product threshold ZVS-VTth that controls the pulse width of the first conduction signal VGTZ can be set to a certain proportion of the fourth volt-second product VT4. Thus, the following formula 2 can be obtained:

[0070] ZVS―VTth=K2×VT4

[0071] In formula 2, a proportional relationship between the first volt-second product threshold value ZVS-VTth and the fourth volt-second product VT4 is established, or the relationship between the first volt-second product threshold value ZVS-VTth and Icsmax is established. K2 can be 8%, for example. When the third volt-second product VT3 reaches the first volt-second product threshold value ZVS-VTth, the negative current mapped on the primary winding by the current generated on the secondary winding is sufficient to enable the main power tube Q1 to achieve ZVS. Then according to the relationship between K1 and K2, the first volt-second product threshold value ZVS-VTth and the second volt-second product threshold value SR-VTth can be in a proportional relationship, for example, the second volt-second product threshold value SR-VTth is twice the first volt-second product threshold value ZVS-VTth. For example, K2 can be selected from 5%-8%, and K1 can be selected from 10%-16% accordingly. The ratio can also be set according to actual conditions, and only an example is given here.

[0072] Furthermore, since the primary winding Np and the secondary winding Ns have a turns ratio of N, the current and voltage of the primary and secondary sides are in a certain proportion. When Q1 is turned on, the bus voltage Vbus is proportional to the voltage across the secondary winding (Vsw=Vbus / N+Vo), and the bus voltage Vbus is proportional to the input voltage Vin. Then, by detecting the magnitude of the first node voltage Vsw and the output voltage Vo, the magnitude of the input voltage Vin can actually be reflected. The turn-on condition (brown in) is reached when the input voltage is greater than the turn-on voltage. The detected (Vsw-Vo) can be compared with the set reference value Vref to determine the state of the input voltage. When Vref=(Vsw-Vo), it means that the judgment line is reached, and Vbus=N*(Vsw-Vo)=N*Vref. When the size of the judgment line needs to be adjusted, it is not convenient to adjust Vref as a fixed value, and N needs to be adjusted, resulting in major changes in the circuit structure and the switching power supply chip. Therefore, in this embodiment, the volt-second product of the voltage on the auxiliary winding in the second time is selected to represent the input voltage, and the third volt-second product threshold BR-VTth is set to have a certain proportional relationship with the fourth volt-second product VT4, so that the following formula 3 can be obtained:

[0073] BR―VTth=K3×VT4

[0074] In formula 3, K3 is a proportional coefficient, thereby establishing a proportional relationship between the third volt-second product threshold BR-VTth and the fourth volt-second product VT4, or establishing a proportional relationship between the third volt-second product threshold BR-VTth and Icsmax. K3 can be, for example, 6%. The ratio can also be set according to actual conditions, and this is just an example.

[0075] Therefore, there is a certain proportional relationship between the first volt-second product threshold value ZVS-VTth, the second volt-second product threshold value SR-VTth and the third volt-second product threshold value BR-VTth. For example, the first volt-second product threshold value ZVS-VTth is 5%-8% of the fourth volt-second product VT4, the second volt-second product threshold value SR-VTth is 10%-16% of the fourth volt-second product VT4, and the third volt-second product threshold value BR-VTth is 5%-16% of the fourth volt-second product VT4. Moreover, the fourth volt-second product VT4 can be proportional to the volt-second product reference value VTref, so that the fourth volt-second product VT4 can be characterized by the volt-second product reference value VTref, and then the first volt-second product threshold to the third volt-second product threshold can be set; wherein the proportional coefficient between the volt-second product reference value VTref and the fourth volt-second product VT4 is set to 1 in the above formulas 1 to 3. Of course, the proportional coefficient can also be set to other values, and the fourth volt-second product VT4 can be characterized by the volt-second product reference value VTref. For the convenience of setting, the first volt-second product threshold value ZVS-VTth can be set to be equal to the volt-second product reference value VTref, and the first volt-second product threshold value ZVS-VTth can also be set to be proportional to the volt-second product reference value VTref. The second volt-second product threshold value SR-VTth is set to 2 times the first volt-second product threshold value ZVS-VTth. Of course, the above proportional relationship is only an example, and the first volt-second product threshold value to the third volt-second product threshold value can also have other proportional relationships with each other.

[0076] See again Figure 3In this embodiment, the volt-second product calculation unit 220 is used to calculate the first volt-second product VT1 generated by the voltage (Vsw-Vo) across the secondary winding Ns within the first time T1 after the first conduction signal VGTZ is generated. Among them, the starting time of the first time can be any time between the generation time of the first conduction signal and the conduction time of the main power tube. For example, the starting time of the first time T1 can be the generation time of the first conduction signal VGTZ, or the end time of the first conduction signal VGTZ, or a certain time before the main power tube Q1 starts to conduct. Of course, this is only an example, and other moments can also be used as the starting time of the first time in other embodiments. The end time of the first time can be any time between the turn-off time of the main power tube and the moment when the drain voltage of the synchronous rectifier tube is less than the output voltage of the flyback switching power supply. For example, the moment when the drain voltage of the synchronous rectifier tube is less than the output voltage of the flyback switching power supply can be used as the end time of the first time, and of course, other moments can also be set as the end time of the first time according to actual applications. After calculating the first volt-second product VT1, the volt-second product calculation unit 220 also needs to compare the first volt-second product VT1 with the second volt-second product threshold SR-VTth. Since the first volt-second product VT1 varies at different starting times of the first time T1, different volt-second product compensation values ​​can be matched to the first volt-second product VT1 calculated at different starting times of the first time T1, which can be specifically set according to actual applications.

[0077] In addition, the voltage determination unit 250 calculates the second volt-second product VT2 generated by the voltage at both ends of the secondary winding within the second time T2 after the first conduction signal VGTZ is generated. The starting time of the second time T2 is, for example, after the first conduction signal VGTZ is generated and the drain voltage Vsw (first node voltage) of the synchronous rectifier Q2 is greater than the output voltage Vo of the flyback switching power supply, or when the main power tube Q1 starts to conduct, or at a certain time after the drain voltage (first node voltage Vsw) of the synchronous rectifier Q2 reaches the maximum value.

[0078] like Figure 4As shown, further, the first control driving unit 240 includes a compensation unit 241, a conduction control unit 242, a shutdown control unit 243 and an RS trigger U2. The compensation unit 241 is connected to the conduction control unit 242 to transmit the compensation signal Vcom to the conduction control unit 242. The set end S of the RS trigger U2 is connected to the output end of the conduction control unit 242 to receive the switch conduction signal Von, the reset end R of the RS trigger U2 is connected to the output end of the shutdown control unit 243 to receive the switch off signal Voff, and the output end Q outputs the first conduction signal VGTZ. The conduction control unit 242 sends the switch conduction signal Von based on the compensation signal Vcom to control the conduction of the synchronous rectifier Q2. Specifically, the compensation unit 241 is used to provide a compensation signal Vcom that characterizes the error between the output feedback signal and the reference signal of the flyback switching power supply. The output feedback signal can be obtained according to the output voltage, output power or output current, etc. In this embodiment, the compensation unit 241 can, for example, generate the compensation signal Vcom according to the output voltage Vout. For example, the compensation unit 241 includes a voltage-dividing resistor and an error amplifier EA connected in series. A voltage-dividing resistor is connected in series between the output terminal of the flyback switching power supply and the ground, and a feedback voltage Vfb representing the output voltage is provided at the middle node of the voltage-dividing resistor. One input terminal of the error amplifier EA receives the feedback voltage Vfb, the other output terminal receives the reference voltage Vref, and the output terminal outputs the compensation signal Vcom. Of course, the compensation unit 241 can also use other circuit structures to generate the compensation signal Vcom, and is not limited to this method.

[0079] The shutdown control unit 243 includes, for example, a comparator COM3, a positive input terminal receiving a third volt-second product VTvo (VT3), and a reverse input terminal receiving a first volt-second product threshold value ZVS-VTth output by the volt-second setting unit 210. When the third volt-second product VTvo is greater than the first volt-second product threshold value ZVS-VTth, a switch shutdown signal Voff is generated to control the synchronous rectifier Q2 to be turned off. The third volt-second product VTvo is a volt-second product generated by integrating the output voltage Vo of the flyback switching power supply from the moment when the first conduction signal VGTZ is generated. When the third volt-second product VTvo is greater than the first volt-second product threshold value ZVS-VTth, it indicates that during the period when the first conduction signal controls the synchronous rectifier Q2 to be turned on, the negative current generated on the main power tube Q1 reaches the current threshold.

[0080] Further, the volt-second product calculation unit 220 of the second control driving unit 270 includes a volt-second integration unit 222, a first time setting unit 221 and a volt-second judgment unit 225. The volt-second integration unit 222 integrates and calculates the first volt-second product VT1 according to the first node voltage Vsw and the output voltage Vo of the flyback switching power supply within the first time T1; the first time setting unit 221 controls the volt-second integration unit 222 to start working at the start time of the first time T1, for example, the generation time of the first conduction signal VGTZ can be set as the start time of the first time T1, and the time when the drain terminal voltage of the synchronous rectifier Q2 is less than the output voltage Vo of the flyback switching power supply is set as the end time of the first time T1, and controls the volt-second integration unit 222 to reset at the end time; of course, the start time and end time of the first time T1 can also be set in other ways, and the circuit is correspondingly deformed accordingly, and only one example is given here. The volt-second judgment unit 225 compares the first volt-second product VT1 with the second volt-second product threshold SR-VTth. The switch control unit 230 is connected to the volt-second judgment unit 225, and after the first volt-second product VT1 is greater than the second volt-second product threshold SR-VTth and the drain-source voltage of the synchronous rectifier Q2 decreases below the first threshold voltage, the switch control unit 230 sends a second turn-on signal VGT to the synchronous rectifier Q2 to turn on the synchronous rectifier Q2. When the drain-source voltage of the synchronous rectifier Q2 increases to above the second threshold voltage, the synchronous rectifier Q2 is controlled to be turned off.

[0081] like Figure 4In one embodiment, the first time setting unit 221 includes a comparator COM1 and an RS trigger U1. The positive input terminal and the reverse input terminal of the comparator COM1 receive the first node voltage Vsw and the output voltage Vo respectively, and the output terminal outputs the first comparison signal. The set terminal of the RS trigger U1 receives the rising edge of the first conduction signal VGTZ, and the reset terminal R is connected to the output terminal of the comparator COM1, and resets at the time when the falling edge FP of the first comparison signal is generated. That is, the comparator COM1 resets the RS trigger U1 when the first node voltage Vsw is less than the output voltage Vo. Further, the volt-second integration unit 222 may include a first integration unit 223 and a second integration unit 224, and the volt-second judgment unit 225 may include a comparator COM2 and an operation unit 226. The second integration unit 224 integrates the first node voltage Vsw at the connection node between the secondary winding and the synchronous rectifier Q2 within the first time T1 to obtain the integrated voltage VTsw. The first integration unit 223 integrates the output voltage Vo within the first time T1 to obtain the integrated voltage VTvo. The operation unit 226 is used to calculate the sum of the integrated voltage VTvo and the second volt-second product threshold SR-VTth, or to calculate the difference between the integrated voltage VTsw and the integrated voltage VTvo, and compare the difference with the second volt-second product threshold SR-VTth. For example, the second volt-second product threshold SR-VTth is greater than the volt-second product generated by the synchronous rectifier Q2 during the parasitic decay oscillation, and is less than the volt-second product generated on the secondary winding during the conduction period of the main power tube Q1.

[0082] For example, the second integration unit 224 includes a first voltage-controlled current source A1 and a first capacitor C1 connected in series, a first transistor M1 is connected across the first capacitor C1, and a control terminal of the first transistor M1 is connected to the output inverting terminal of the RS trigger U1. The first integration unit 223 includes a second voltage-controlled current source A2 and a second capacitor C2 connected in series, a second transistor M2 is connected at both ends of the second capacitor C2, and a control end of the second transistor M2 is connected to the output inverting end of the RS trigger. Then, at the time of the rising edge RP of the first conduction signal VGTZ, the setting terminal S is at a high level, the transistors M1 and M2 are turned off, the first node voltage Vsw charges the first capacitor C1 and integrates it to generate an integrated voltage VTsw, and the output voltage Vo charges the second capacitor C2 and integrates it to generate an integrated voltage VTvo. When Vsw begins to be less than Vo, the reset terminal R is at a high level, the transistors M1 and M2 are turned on, the first capacitor C1 and the second capacitor C2 are discharged, and the integral is reset. The transistors M1 and M2 are, for example, NMOS tubes, and the first capacitor C1 and the second capacitor C2 are the same.

[0083] In one possible embodiment, the operation unit 226 may be an adder, which calculates the sum of the integrated voltage VTvo and the second volt-second product threshold SR-VTth. The output of the adder is connected to the inverting input of the comparator COM2, and the output of the second integration unit 224 is connected to the positive input of the comparator COM2. In another embodiment, the operation unit 226 may be a subtractor, which is connected between the second integration unit 224 and the positive input of the comparator COM2, and is used to calculate the difference between the integrated voltage VTsw and the second volt-second product threshold SR-VTth, and the output of the first integration unit 223 is connected to the inverting input of the comparator COM2. In another embodiment, the operation unit 226 may be a subtractor or a differential amplifier, and the two inputs are respectively connected to the second integration unit 224 and the first integration unit 223, and the difference VT1 between the integrated voltage VTsw and the integrated voltage VTvo is calculated, and the output of the operation unit 226 is connected to the positive input of the comparator COM2, and the inverting input of the comparator COM2 receives the second volt-second product threshold SR-VTth. When (VTsw-VTvo)>SR-VTth, the comparator COM2 outputs a VTOK signal.

[0084] In another possible embodiment, the capacitor C2 and the transistor M2 are removed, and after the output of the voltage-controlled current source A1 is subtracted from the output of the voltage-controlled current source A2, the capacitor C1 is charged, and the volt-second product (Vsw-Vo) is calculated and output to the positive input terminal of the comparator COM2, and the reverse input terminal of the comparator COM2 receives the second volt-second product threshold SR-VTth. In another embodiment, the capacitor C2, the transistor M2 and the voltage-controlled current source A2 are removed, and the control terminal voltage of the voltage-controlled current source A1 is replaced by (Vsw-Vo). The above embodiments can all realize the volt-second product calculation, and can all be used as possible implementation methods of the volt-second product calculation unit 220, which is not limited here.

[0085] Further, in the present embodiment, the voltage judgment unit 250 includes a second time setting unit 251, an integration unit 252 and a comparison unit 253. The second time setting unit 251 starts timing the second time T2 from the starting moment of the above-mentioned second time T2 after the first conduction signal VGTZ is generated, for example, the moment when the drain terminal voltage of the synchronous rectifier Q2 is greater than the output voltage Vo of the flyback switching power supply is taken as the starting moment of the second time T2. The integration unit 252 calculates the second volt-second product VT2 generated by the voltage across the secondary winding within the second time T2, and clears the second volt-second product VT2 at the end moment of the second time T2. The comparison unit 253 compares the second volt-second product VT2 with the third volt-second product threshold BR-VTth, and generates a start control signal BROK and a shutdown control signal BRNOK according to the comparison result. For example, as Figure 4As shown, the integration unit 252 includes a voltage-controlled current source A3, a transistor M3 and a capacitor C3, and the connection method of the three can be the same as the first integration unit 223 and the second integration unit 224, and the working principle is the same. The specific connection method is not described in detail. The integration unit 252 is used to receive the voltage information of Vsw-Vo to generate a second volt-second product VT2. The comparison unit 253 can include a comparator or a hysteresis comparator.

[0086] The second time setting unit 251 may include a plurality of logic gates, for example, an RS trigger U3, an AND gate U4, a delay unit 254, a NOT gate U5, an AND gate U6, and a NOT gate U7. The set end S of the RS trigger U3 receives the rising edge of the first conduction signal VGTZ, and is set to start working accordingly. The reset end R receives the VTOK signal or may also receive the rising edge of VTOK, and the output end Q is connected to an input end of the AND gate U4. The other input end of the AND gate U4 is connected to the output end of the comparator COM1. When the first node voltage Vsw is greater than the output voltage Vo, the comparator COM1 outputs a high level. When the rising edge of the first conduction signal VGTZ comes, the RS trigger outputs a high level, and does not output a low level until the VTOK signal is received. The output end of the AND gate U4 is connected to an input end of the AND gate U6 and the input end of the delay unit 254, and the output end of the delay unit 254 is connected to the other input end of the AND gate U6 through the NOT gate U5. The output end of the AND gate U6 is connected to the control end of the transistor M3 through the NOT gate U7. During the generation of the first conduction signal VGTZ, the output of the comparator COM1 is all low level. At this time, the AND gate U4 and the AND gate U6 both output low level, and the NOT gate U7 outputs high level, so that M3 is turned on and the integral is cleared. When Vsw is greater than Vo, after the comparator COM1 outputs high level, the AND gate U4 outputs high level, the NOT gate U5 outputs high level, the AND gate U6 outputs high level, the NOT gate U7 outputs low level, the transistor M3 is turned off, and the integration starts. After the delay unit 254 delays the second time T2, the NOT gate U5 outputs low level, the AND gate U6 outputs low level, the NOT gate U7 outputs high level, and M3 is turned on again to clear the integral. Therefore, the integration is performed within the delay time generated by the delay unit 254, and the delay time is the second time T2. The first time T1 can be greater than the second time T2, and the time period of the second time T2 can be within the time period of the first time T1.

[0087] Furthermore, if Figure 4As shown, the first conduction signal VGTZ generated by the first control driving unit 240 and the second conduction signal VGT generated by the second control driving unit 270 are both sent from the GT pin to the synchronous rectifier Q2, so a logic unit 260 can be added between the first control driving unit 240 and the second control driving unit 270. The logic unit 260 can include, for example, an OR gate U8, which is used to select the first conduction signal VGTZ or the second conduction signal VGT to be output.

[0088] Above Figure 3 and Figure 4 The circuit diagram is only one possible implementation mode of the present invention and is not intended to limit the synchronous rectification control circuit of the present invention. The circuit may also be deformed accordingly to achieve the same function.

[0089] Figure 5 A schematic waveform diagram of various signals of a switching power supply provided according to an embodiment of the present invention when operating normally is shown.

[0090] like Figure 5 As shown, the horizontal axis represents time, Vds represents the drain-source voltage across the main power tube Q1, Vsw represents the first node voltage, VGTZ represents the first conduction signal, VGT represents the second conduction signal, Vcs represents the voltage signal across the sampling resistor, VDR represents the primary drive signal, VTsw represents the integration signal generated by the first integration unit 224, VT2 represents the integration signal generated by the integration unit 252, VTvo represents the integration signal generated by the second integration unit 223, T1 represents the first time, and T2 represents the second time.

[0091] The waveform diagram of this embodiment can be, for example, Figure 2-Figure 4 Based on the circuit and synchronous rectification control method, see Figure 5 , a first turn-on signal VGTZ is generated at time t0, and a falling edge of the first turn-on signal VGTZ is generated at time t1, so that the voltage signal Vcs reaches the first reference voltage Vcsref1. In the time period t1-t2, the drain-source voltage Vds of the main power tube Q1 drops, and at time t2, the drain-source voltage Vds of the main power tube Q1 drops to the threshold voltage. The threshold voltage is, for example, zero voltage. At this time, the main power tube Q1 is turned on, and zero voltage turn-on of the main power tube can be achieved. In the time period t2-t4, the main power tube Q1 is continuously turned on. At time t4, the voltage signal Vcs reaches the second reference voltage Vcsref2, and the main power tube Q1 is turned off. The first time T1 is counted from time t0. At time t5, the Vsw voltage is less than the Vo voltage, which is the end time of the first time T1. The volt-second product generated by Vsw and Vo can be calculated separately within the first time T1, and can be calculated according to Figure 4The circuit calculates VTsw and VTvo respectively. In the first time T1, the output voltage Vo is integrated to generate the third volt-second product VTvo. At time t1, the third volt-second product VTto begins to be greater than the first volt-second product threshold value ZVS-VTth. This is the end time of the first conduction signal VGTZ, that is, the first conduction signal VGTZ generates a falling edge at time t1. In the first time T1, the size of the integrated voltage VTsw and VTvo is determined. When the difference between VTsw and VTvo is greater than the second volt-second product threshold value SR-VTth, a VTOK signal is generated. Figure 5 At time t12, which is equivalent to time t5, it can be seen that the difference between VTsw and VTvo is the first volt-second product VT1, and the first volt-second product VT1 is greater than the second volt-second product threshold SR-VTth. Similarly, at time t5, the first volt-second product VT1 is also greater than the second volt-second product threshold SR-VTth; here, the first volt-second product VT1 is greater than the second volt-second product threshold SR-VTth as shown at times t5 and t12. In fact, before times t5 and t12, the first volt-second product VT1 may have reached the second volt-second product threshold SR-VTth, and the VTOK signal can be generated at this moment; therefore, at time t5, when the drain voltage of the synchronous rectifier Q2 drops below the first threshold voltage (for example, zero voltage), the synchronous rectifier Q2 is controlled to be turned on and the continuous current begins. At the same time, at time t5, the Vsw voltage is less than the Vo voltage, and the integral values ​​of VTsw and VTvo are cleared. At time t6, the drain voltage of the synchronous rectifier Q2 rises above the second threshold voltage (e.g., a negative voltage), and the synchronous rectifier Q2 is controlled to be turned off. After the first conduction signal VGTZ is generated, that is, after time t0, at time t1, the Vsw voltage is greater than the Vo voltage, and the comparator COM1 outputs a high level. At this time, the second time T2 is started. Within the second time T2, it is determined whether the second volt-second product VT2 generated by the difference between the first node voltage Vsw and the output voltage Vo exceeds the third volt-second product threshold BR-VTth. The t1-t3 time period is, for example, the second time T2. At time t3, it can be seen that the volt-second product VT2 generated by the difference between the first node voltage VTsw and the output voltage Vo is greater than the third volt-second product threshold BR-VTth, so the output is the start control signal BROK.

[0092] Further, after the moment t6, the resonance phase or the parasitic oscillation phase is entered, and the time period t7-t8 is half an oscillation cycle. At the moment t9, the first control driving unit 240 generates the first conduction signal VGTZ again according to the compensation signal Vcom, and a new switching cycle begins again. At the moment t10, the third volt-second product is greater than the first volt-second product threshold, which is the end moment of the first conduction signal VGTZ. After the moment t10, the main power tube Q1 is turned on. The time period t9-t12 is the first time T1, and the time period t10-t11 is the second time T2. The first volt-second product VT1 is calculated within the first time T1 and compared with the second volt-second product threshold. At the moment t12, the first volt-second product is greater than the second volt-second product threshold, and the drain voltage of the synchronous rectifier Q2 drops below the first threshold voltage (for example, zero voltage), and the synchronous rectifier Q2 is turned on. In the second time T2, the volt-second product VT2 generated by the voltage (Vsw-Vo) across the secondary winding is greater than the third volt-second product threshold BR-VTth, and the start control signal BROK is output. Then, a new switching cycle begins again from time t13, and the cycle repeats. The changes of the signals in this waveform are similar to those mentioned above. Figure 3-Figure 4 The circuit provided is consistent, so the synchronous rectification control circuit and the switching power supply of this embodiment can use one volt-second setting pin VT to set three or even more volt-second product thresholds to perform corresponding functions.

[0093] It should be noted that in Figure 3-Figure 4 In the embodiment of the present invention, the reset time of the third volt-second product is the same as the reset time of the first volt-second product. The present invention is not limited thereto. A volt-second integration unit may be separately provided to obtain the third volt-second product. In this case, the reset time of the third volt-second product may be set to other times. For example, after the third volt-second product is greater than the third volt-second product threshold, the third volt-second product may be reset. In addition, Figure 3-Figure 4 This is one of the embodiments provided by the present invention, and other modifications may also be made to achieve the same function.

[0094] Figure 6 A schematic flow chart of a synchronous rectification control method according to an embodiment of the present invention is shown.

[0095] The synchronous rectification control method is applied to the above-mentioned switching power supply and synchronous rectification control circuit to control the conduction of the synchronous rectification tube Q2 in the switching power supply, see Figure 6 The synchronous rectification control method of this embodiment includes the following steps:

[0096] In step S101 , the set volt-second product reference value is processed into at least any two of a first volt-second product threshold value, a second volt-second product threshold value, and a third volt-second product threshold value.

[0097] In step S102, a first conduction signal is sent to the synchronous rectifier to turn on the synchronous rectifier, and the third volt-second product generated by the output voltage of the flyback switching power supply is calculated at the time when the first conduction signal is generated, and the pulse width of the first conduction signal is controlled according to the third volt-second product and the first volt-second product threshold.

[0098] In step S103, a first volt-second product of the voltage across the secondary winding is calculated within a first time after the first conduction signal is generated, and after the first volt-second product is greater than a second volt-second product threshold, the synchronous rectifier can be controlled to conduct for freewheeling.

[0099] In step S104, a second volt-second product of the voltage across the secondary winding is calculated within a second time after the first conduction signal is generated, and a state of the input voltage of the flyback switching power supply is determined according to the second volt-second product and a third volt-second product threshold.

[0100] Further, in step S101, the first volt-second product threshold, the second volt-second product threshold and the third volt-second product threshold are, for example, all in a certain proportional relationship with the volt-second product reference value, so that different volt-second product thresholds can be set according to appropriate proportions.

[0101] Furthermore, the synchronous rectifier tube is controlled to be turned off when it is detected that the third volt-second product is greater than the first volt-second product threshold; wherein, when the third volt-second product is greater than the first volt-second product threshold, it indicates that during the period when the first conduction signal controls the conduction of the synchronous rectifier tube, the negative current generated on the main power tube reaches the current threshold, and the current threshold enables the main power tube to achieve zero voltage turn-on.

[0102] Further, after the first volt-second product is greater than the second volt-second product threshold and the drain-source voltage of the synchronous rectifier decreases below the first threshold voltage, the synchronous rectifier is controlled to be turned on.

[0103] Further, when the second volt-second product is greater than a third volt-second product threshold, a start control signal is generated, and the start control signal indicates that the input voltage reaches the start voltage.

[0104] The above-mentioned synchronous rectification control method is implemented based on the synchronous rectification control circuit. The specific steps have been reflected in the description of the above-mentioned circuit structure and will not be repeated here. The synchronous rectification control method of this embodiment can set at least two of the above-mentioned three volt-second product thresholds according to the set volt-second product reference value, which is used for the on and off control of the synchronous rectifier tube and the determination of the input voltage size. When the volt-second product is required to determine other functions, a fourth, fifth or even more volt-second product thresholds with a proportional relationship to each other can be set. Multiple different volt-second product thresholds are set through a VT pin to perform different functions, thereby simplifying the circuit structure, saving chip area, and saving costs.

[0105] The synchronous rectification control circuit, synchronous rectification control method and switching power supply provided by the present invention use the volt-second product of the output voltage and the first node voltage in the first time as the control condition for turning on or off the synchronous rectifier tube, and use the volt-second product of the voltage at both ends of the secondary winding in the second time to judge the input voltage of the primary winding. Thus, any two or three of the functions of the secondary side freewheeling conduction, brown in and brown out detection, and the secondary side early conduction to enable the main power tube to realize ZVS (Zero Voltage Switch) can be realized in the synchronous rectification control circuit of the secondary side, so that the synchronous rectification control circuit becomes the main control circuit, simplifying the circuit structure and control method. In addition, the execution of the above three functions is converted to rely on the volt-second product for judgment, and different volt-second product thresholds can be uniformly set by the synchronous rectification control circuit, and part of the circuit structure can be reused to calculate multiple volt-second products, further simplifying the circuit design, improving the control performance, and reducing the occupied space of the primary control circuit.

[0106] Furthermore, the volt-second setting unit receives the volt-second product reference value from the outside of the synchronous rectification control circuit and converts it into at least two different volt-second product thresholds, so that the volt-second setting pin of the synchronous rectification control circuit can be reused to achieve multiple different function judgments, reduce the use of pins, and reduce chip manufacturing costs. In addition, a certain proportional relationship is formed between the multiple volt-second product thresholds, so that multiple different volt-second product thresholds can be quickly obtained according to the set volt-second product reference value, and circuit optimization is achieved while ensuring the circuit function.

[0107] Furthermore, before the main power tube is turned on, the first control driving unit generates a first conduction signal to control the conduction of the synchronous rectifier tube, so that a negative current is generated on the main power tube. The moment when the third volt-second product generated by the integration of the output voltage in the first time is greater than the first volt-second product threshold is used as the turn-off moment of the synchronous rectifier tube, so that the pulse width of the first conduction signal can be adjusted, so that the appropriate negative current can be set, so that the main power tube can achieve ZVS turn-on.

[0108] Furthermore, the second volt-second product generated by the voltage at both ends of the secondary winding is calculated within the second time after the first conduction signal is generated, and then the state of the input voltage of the flyback switching power supply is judged according to the second volt-second product and the third volt-second product threshold, and whether the input voltage reaches the start-up voltage is judged. The size of the third volt-second product threshold can be indirectly adjusted by adjusting the size of the volt-second product reference value, thereby adjusting the judgment line of whether the start-up voltage is reached, so that the application range of the switching power supply is wider.

[0109] It should be noted that the numerical values ​​in this article are only used for exemplary description. In other embodiments of the present invention, other numerical values ​​can also be sampled to implement this solution. The specific settings should be reasonable according to the actual situation, and the present invention does not limit this.

[0110] Finally, it should be noted that: Obviously, the above embodiments are only examples for clearly explaining the present invention, and are not intended to limit the implementation methods. For ordinary technicians in the relevant field, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived from this are still within the scope of protection of the present invention.

[0111] It should also be understood that the terms and expressions used herein are for description only, and one or more embodiments of this specification should not be limited to these terms and expressions. The use of these terms and expressions does not mean to exclude any equivalent features of the illustrations and descriptions (or parts thereof), and it should be recognized that various modifications that may exist should also be included in the scope of the claims. Other modifications, changes and substitutions may also exist. Accordingly, the claims should be deemed to cover all such equivalents.

Claims

1. A synchronous rectification control circuit for controlling the conduction of a synchronous rectifier tube in a flyback switching power supply, wherein the flyback switching power supply comprises a transformer, the transformer comprises a primary winding and a secondary winding, the flyback switching power supply further comprises a main power tube connected to the primary winding and a synchronous rectifier tube connected to the secondary winding, wherein: The synchronous rectification control circuit comprises: a volt-second setting unit, which obtains a volt-second product reference value from a volt-second setting pin of the synchronous rectification control circuit, and processes the volt-second product reference value into at least any two of a first volt-second product threshold, a second volt-second product threshold, and a third volt-second product threshold; a first control driving unit, sending a first conduction signal to the synchronous rectifier to turn on the synchronous rectifier; starting to calculate a third volt-second product generated by the output voltage of the flyback switching power supply at the time when the first conduction signal is generated, and controlling a pulse width of the first conduction signal according to the third volt-second product and the first volt-second product threshold; A second control driving unit calculates a first volt-second product of the voltage across the secondary winding within a first time after the first conduction signal is generated, and after the first volt-second product is greater than the second volt-second product threshold, the synchronous rectifier can be controlled to conduct for freewheeling; The voltage judgment unit calculates a second volt-second product of the voltage across the secondary winding within a second time after the first conduction signal is generated, and judges the state of the input voltage of the flyback switching power supply according to the magnitude of the second volt-second product and the third volt-second product threshold.

2. The synchronous rectification control circuit according to claim 1, wherein: The first control driving unit includes a shutdown control unit, and the shutdown control unit controls the synchronous rectifier to be shut down when the third volt-second product is greater than the first volt-second product threshold.

3. The synchronous rectification control circuit according to claim 1, wherein: When the third volt-second product is greater than the first volt-second product threshold, it indicates that during the period when the first conduction signal controls the conduction of the synchronous rectifier, the negative current generated on the main power tube reaches the current threshold, and based on the current threshold, the main power tube can achieve zero voltage turn-on.

4. The synchronous rectification control circuit according to claim 1 or 2, wherein: The first control driving unit includes a conduction control unit, which generates a switch conduction signal based on a compensation signal generated by an output voltage of the flyback switching power supply, and generates the first conduction signal based on the switch conduction signal.

5. The synchronous rectification control circuit according to claim 1, wherein: The second control driving unit includes a switch control unit, The switch control unit controls the synchronous rectifier to be turned on when the first volt-second product is greater than the second volt-second product threshold and the drain-source voltage of the synchronous rectifier decreases below a first threshold voltage.

6. The synchronous rectification control circuit according to claim 1, wherein: The second control driving unit further includes a volt-second product calculation unit, and the volt-second product calculation unit includes: a volt-second integration unit, which integrates and calculates the first volt-second product according to the drain voltage of the synchronous rectifier tube and the output voltage of the flyback switching power supply within the first time; a first time setting unit, which controls the volt-second integration unit to start working at the start time of the first time, and controls the volt-second integration unit to reset at the end time of the first time; and The volt-second determination unit compares the first volt-second product with the second volt-second product threshold.

7. The synchronous rectification control circuit according to claim 6, wherein: The starting time of the first time is any time between the generation time of the first conduction signal and the conduction time of the main power tube.

8. The synchronous rectification control circuit according to claim 6, wherein: The end moment of the first time is any moment between the turn-off moment of the main power tube and the moment when the drain terminal voltage of the synchronous rectifier tube is less than the output voltage of the flyback switching power supply.

9. The synchronous rectification control circuit according to claim 1, wherein: The voltage determination unit generates a start control signal when the second volt-second product is greater than the third volt-second product threshold, and the start control signal indicates that the input voltage reaches the start voltage.

10. The synchronous rectification control circuit according to claim 9, wherein: The voltage determination unit comprises: A second time setting unit, which starts timing the second time from the start moment of the second time; an integrating unit, calculating the second volt-second product within the second time, and clearing the second volt-second product to zero at the end of the second time; and The comparison unit compares the second volt-second product with the third volt-second product threshold, and generates a start control signal according to the comparison result.

11. The synchronous rectification control circuit according to claim 10, wherein: The time period in which the second time falls is a time period in which the drain voltage of the synchronous rectifier tube is greater than the output voltage of the flyback switching power supply after the first conduction signal is generated.

12. The synchronous rectification control circuit according to claim 1, wherein: The first volt-second product threshold, the second volt-second product threshold, and the third volt-second product threshold are all proportional to the volt-second product reference value.

13. The synchronous rectification control circuit according to claim 1, wherein: The volt-second setting pin of the synchronous rectification control circuit is connected to a volt-second setting resistor, and the volt-second setting resistor is connected to a current source inside the synchronous rectification control circuit to generate the volt-second product reference value, and the size of the volt-second product reference value is adjusted by adjusting the current source or the volt-second setting resistor.

14. A synchronous rectification control method for controlling the conduction of a synchronous rectifier tube in a flyback switching power supply, wherein the flyback switching power supply comprises a transformer, the transformer comprises a primary winding and a secondary winding, the flyback switching power supply further comprises a main power tube connected to the primary winding and a synchronous rectifier tube connected to the secondary winding, wherein: The synchronous rectification control method comprises: Processing the set volt-second product reference value into at least any two of a first volt-second product threshold value, a second volt-second product threshold value, and a third volt-second product threshold value; Sending a first conduction signal to the synchronous rectifier to turn on the synchronous rectifier, starting to calculate a third volt-second product generated by the output voltage of the flyback switching power supply at the time when the first conduction signal is generated, and controlling the pulse width of the first conduction signal according to the third volt-second product and the first volt-second product threshold; Calculating a first volt-second product of the voltage across the secondary winding within a first time after the first conduction signal is generated, and controlling the synchronous rectifier to conduct for freewheeling after the first volt-second product is greater than the second volt-second product threshold; A second volt-second product of the voltage across the secondary winding is calculated within a second time after the first conduction signal is generated, and a state of the input voltage of the flyback switching power supply is determined according to the second volt-second product and the third volt-second product threshold.

15. The synchronous rectification control method according to claim 14, wherein: Controlling the synchronous rectifier to turn off when detecting that the third volt-second product is greater than the first volt-second product threshold; Among them, when the third volt-second product is greater than the first volt-second product threshold, it indicates that during the period when the first conduction signal controls the conduction of the synchronous rectifier tube, the negative current generated on the main power tube reaches the current threshold, and the current threshold enables the main power tube to achieve zero voltage turn-on.

16. The synchronous rectification control method according to claim 14, wherein: After the first volt-second product is greater than the second volt-second product threshold and the drain-source voltage of the synchronous rectifier decreases below a first threshold voltage, the synchronous rectifier is controlled to be turned on.

17. The synchronous rectification control method according to claim 14, wherein: When the second volt-second product is greater than the third volt-second product threshold, a start-up control signal is generated, wherein the start-up control signal indicates that the input voltage reaches a start-up voltage.

18. The synchronous rectification control method according to claim 14, wherein: The first volt-second product threshold, the second volt-second product threshold, and the third volt-second product threshold are all proportional to the volt-second product reference value.

19. A switching power supply, comprising a transformer, the transformer comprising a primary winding and a secondary winding, the switching power supply further comprising a main power tube connected to the primary winding and a synchronous rectifier tube connected to the secondary winding, wherein: The switching power supply further comprises: The synchronous rectification control circuit according to any one of claims 1 to 13, wherein the synchronous rectification control circuit is used to control the working state of the synchronous rectifier tube; and A volt-second setting resistor is connected to a volt-second setting pin of the synchronous rectification control circuit, and the volt-second product reference value is set by adjusting a current flowing through the volt-second setting resistor or adjusting a resistance value of the volt-second setting resistor.

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