Synchronous rectification control circuit, DRV adjustment circuit and switching power supply

Abstract

The present invention discloses a synchronous rectification control circuit, a DRV adjustment circuit and a switching power supply, relating to the technical field of electronic circuits. It includes a conduction control circuit, a turn-off control circuit, a driving circuit and a DRV adjustment circuit. The conduction control circuit is used to issue a conduction signal; the turn-off control circuit is used to issue a turn-off signal; the driving circuit is used to turn on the MOSFET according to the conduction signal or turn off the MOSFET according to the turn-off signal; the DRV adjustment circuit is used to reduce the gate driving voltage of the MOSFET before turn-off; the DRV adjustment circuit includes a first control circuit, a second control circuit, a pull-down current source circuit and N discharge circuits, where N is not less than 2; any discharge circuit can conduct to discharge, so as to reduce the DRV voltage value. The present invention reduces the time for DRV to decrease from a high level to V REG and prevents the DRV from decreasing excessively, thereby not only accelerating the turn-off speed of the MOSFET but also improving the stability.

Description

Technical Field

[0001] The present invention relates to the technical field of electronic circuits, and particularly to a synchronous rectification control circuit, a DRV adjustment circuit, and a switching power supply. Background Art

[0002] A switching power supply usually uses a synchronous rectification control circuit to drive a metal-oxide-semiconductor field effect transistor (MOSFET) to complete the rectification function. During the conduction period of the MOSFET, the synchronous rectification control circuit samples the voltage (VDS) across the source-drain terminals of the MOSFET. When VDS is higher than the MOSFET turn-off threshold, the MOSFET will turn off after a turn-off delay (td2).

[0003] Before the MOSFET starts to conduct and VDS reaches the MOSFET turn-off threshold (VTH_OFF), the synchronous rectification control circuit needs to adjust the gate drive voltage (DRV) to the required target value (V REG ) within a short time. By reducing DRV, the gate voltage of the MOSFET can be gradually reduced from a high-level voltage so that the MOSFET can enter the off state at any time from a lower voltage. When the voltage across the MOSFET is quite low, the above VDS is adjusted within a certain range, thus accelerating the turn-off speed of the MOSFET.

[0004] In some scenarios, the synchronous rectification control circuit is externally connected to a power supply VCC. The magnitude of DRV when the MOSFET starts to conduct generally depends on the magnitude of VCC. In a typical application scenario, the power supply VCC is 5V. When the DRV voltage (gate drive voltage) adjustment circuit works, it is necessary to adjust the DRV voltage (gate drive voltage) from the power supply voltage VCC (5V in the above typical application scenario) to the required target value V REG so that the subsequent MOSFET can enter the off state at any time from a lower voltage.

[0005] In some other application scenarios, the power supply voltage VCC is raised to a higher voltage level, that is, the power supply voltage VCC is higher than 5 volts. Thus, when the DRV voltage (gate drive voltage) adjustment circuit adjusts the DRV voltage (gate drive voltage), it needs to pull down from a higher voltage to the required target value V REG , resulting in a longer adjustment time; at the same time, only one normally-on discharge circuit is designed for discharging in the traditional synchronous rectification control circuit. To improve the pull-down speed, it is necessary to consider using a large pull-down current. However, if a large pull-down current is used, it is easy to pull down DRV excessively. Summary of the Invention

[0006] The purpose of the embodiments of the present invention is to provide a synchronous rectification control circuit, a DRV adjustment circuit, and a switching power supply to reduce the time for DRV to decrease from a high level to V REG and prevent DRV from decreasing excessively, thereby accelerating the turn-off speed of the MOSFET and improving the stability.

[0007] To achieve the above object, the embodiments of the present invention provide the following solutions:

[0008] A synchronous rectification control circuit includes:

[0009] A conduction control circuit for sending a conduction signal;

[0010] A turn-off control circuit for sending a turn-off signal;

[0011] A drive circuit is respectively connected to the conduction control circuit, the turn-off control circuit, and the MOSFET, and is used to turn on the MOSFET according to the conduction signal or turn off the MOSFET according to the turn-off signal;

[0012] A gate drive voltage DRV adjustment circuit is connected to the MOSFET and is used to reduce the gate drive voltage before the MOSFET is turned off; the DRV adjustment circuit includes a first control circuit, a second control circuit, and N discharge circuits, where N is not less than 2;

[0013] When DRV is higher than the reference voltage, the first control circuit is in a conducting state to reduce the voltage value of DRV; when DRV is less than or equal to the reference voltage or the drain voltage of the MOSFET is greater than or equal to the second threshold voltage, the first control circuit is in a disconnected state;

[0014] The N discharge circuits are all connected to the MOSFET; when any one of the discharge circuits is conducting, it can discharge to reduce the voltage value of DRV; any one of the discharge circuits is a constantly conducting discharge circuit or a controlled circuit controlled by the second control circuit;

[0015] The second control circuit is used for: when DRV has not dropped to the required target value V REG and is greater than the third threshold voltage, controlling the controlled circuit to conduct, and when DRV is less than or equal to the third threshold voltage, controlling the controlled circuit to disconnect;

[0016] The first control circuit includes:

[0017] A pull-down current source circuit;

[0018] The first controller is configured to control the pull - down current source circuit to conduct when DRV is higher than the reference voltage, so as to reduce the voltage value of DRV to the reference voltage; and to control the pull - down current source circuit to disconnect when the voltage value of DRV is equal to the reference voltage;

[0019] The second control circuit includes:

[0020] A fourth comparator, the positive input terminal of the fourth comparator is used to input DRV, and the negative input terminal is used to input the third threshold voltage;

[0021] A second switch, the control terminal of the second switch is connected to the output terminal of the fourth comparator, and the second switch is used to disconnect or close the controlled circuit;

[0022] An amplifier, the positive input terminal of the amplifier is used to input the drain voltage, the negative input terminal is used to input the second threshold voltage, and the output terminal is connected to the discharge circuit.

[0023] Optionally, the first controller is specifically configured to:

[0024] Compare the magnitudes of DRV and the reference voltage, the magnitudes of the drain voltage and the first threshold voltage, and the magnitudes of the drain voltage and the second threshold voltage; the first threshold voltage is less than the second threshold voltage;

[0025] If DRV is greater than the reference voltage, the drain voltage is greater than the first threshold voltage and less than the second threshold voltage, control the pull - down current source circuit to close;

[0026] If DRV is less than or equal to the reference voltage and the drain voltage is greater than the second threshold voltage, control the pull - down current source circuit to disconnect.

[0027] Optionally, the first controller includes:

[0028] A first comparator, the negative input terminal of the first comparator is used to input DRV, and the positive input terminal is used to input the reference voltage;

[0029] A second comparator, the positive input terminal of the second comparator is used to input the first threshold voltage, and the negative input terminal is used to input the drain voltage;

[0030] A NOR gate, the first input terminal of the NOR gate is connected to the output terminal of the first comparator, and the second input terminal of the NOR gate is connected to the output terminal of the second comparator;

[0031] A third comparator, the positive input terminal of the third comparator is used to input the drain voltage, and the negative input terminal is used to input the second threshold voltage;

[0032] An inverter, the input end of the inverter is connected to the output end of the third comparator;

[0033] A first switch;

[0034] A NAND gate, the first input end of the NAND gate is connected to the output end of the NOR gate, and the second input end of the NAND gate is connected to the output end of the inverter; the output end of the NAND gate is used to output a control signal for controlling the first switch.

[0035] Optionally, the second control circuit is specifically configured to:

[0036] When DRV is greater than or equal to the third threshold voltage and the drain voltage is greater than or equal to the second threshold voltage, control the controlled circuit to conduct;

[0037] When DRV is less than the third threshold voltage and the drain voltage is greater than or equal to the second threshold voltage, control the controlled circuit to turn off.

[0038] The present invention also provides a DRV adjustment circuit, which is applied to a synchronous rectification control circuit, and the DRV adjustment circuit is connected to the MOSFET for reducing the gate drive voltage before the MOSFET is turned off;

[0039] The DRV adjustment circuit includes: a first control circuit, a second control circuit, and N discharge circuits, where N is not less than 2; wherein:

[0040] When DRV is higher than the reference voltage, the first control circuit is in a conducting state to reduce the voltage value of DRV; when DRV is less than or equal to the reference voltage or the drain voltage of the MOSFET is greater than or equal to the second threshold voltage, the first control circuit is in a disconnected state;

[0041] The N discharge circuits are all connected to the MOSFET; any one of the discharge circuits can discharge when it is conducting to reduce the voltage value of DRV; any one of the discharge circuits is a normally-on discharge circuit or a controlled circuit controlled by the second control circuit;

[0042] The second control circuit is used to: when DRV has not dropped to the required target value VREG and is greater than the third threshold voltage, control the controlled circuit to conduct, and when DRV is less than or equal to the third threshold voltage, control the controlled circuit to turn off.

[0043] Optionally, the controlled circuit includes a first NMOS transistor; the normally-on discharge circuit includes a second NMOS transistor;

[0044] The output terminal of the amplifier is respectively connected to the gate of the first NMOS transistor and the gate of the second NMOS transistor; the drain of the first NMOS transistor is connected to the gate of the MOSFET through the second switch;

[0045] the drain of the second NMOS transistor is connected to the gate of the MOSFET;

[0046] the sources and substrates of the first NMOS transistor and the second NMOS transistor are grounded.

[0047] Optionally, the first NMOS transistor and the second NMOS transistor have different parameters.

[0048] The present invention also provides a switching power supply applying the synchronous rectification control circuit, including: the synchronous rectification control circuit, a capacitor, a transformer, a positive electrode interface and a negative electrode interface;

[0049] The capacitor is respectively connected to the positive electrode interface and the negative electrode interface, and the capacitor is used for storing electrical energy;

[0050] The negative electrode interface is grounded;

[0051] The positive electrode interface is connected to one end of the transformer; the other end of the transformer is connected to the drain end of the MOSFET; the source and substrate of the MOSFET are grounded;

[0052] The synchronous rectification control circuit is connected to the MOSFET.

[0053] According to the specific embodiments provided by the present invention, the following technical effects are disclosed:

[0054] The embodiment of the present invention provides a synchronous rectification control circuit, a DRV adjustment circuit and a switching power supply. The synchronous rectification control circuit includes a conduction control circuit, a turn-off control circuit, a drive circuit and a DRV adjustment circuit. The cooperation of each circuit is as follows: the conduction control circuit is used for sending out a conduction signal; the turn-off control circuit is used for sending out a turn-off signal; the drive circuit is used for turning on the MOSFET according to the conduction signal or turning off the MOSFET according to the turn-off signal; the DRV adjustment circuit is used for reducing the gate drive voltage of the MOSFET before turn-off.

[0055] The above DRV adjustment circuit includes a first control circuit, a second control circuit, a pull-down current source circuit and N discharge circuits (N is not less than 2). The above N discharge circuits are all connected to the MOSFET; any one of the discharge circuits is a normally-on discharge circuit or a controlled circuit controlled by the second control circuit. The controlled circuit is controlled by the second control circuit and conducts when the DRV has not dropped to the required target value V REG and is greater than the third threshold voltage, otherwise it is disconnected.

[0056] In different situations, the adjustment processes of the DRV adjustment circuit are as follows:

[0057] For the situation where VCC is higher than the reference voltage (which can be called the second situation), since the initial voltage of DRV when the MOSFET is turned on is determined by VCC, the initial voltage of DRV is higher than the reference voltage. The first control circuit will be in the conducting state to reduce the voltage value of DRV. At the same time, the second control circuit and N discharge circuits will also function to pull down DRV.

[0058] After DRV is reduced from the initial voltage to the reference voltage or when the drain voltage of the MOSFET is greater than or equal to the second threshold voltage, the first control circuit will enter the off state. Thereafter, the second control circuit and N discharge circuits will continue to function.

[0059] In the situation where VCC is equal to the reference voltage (which can be called the first situation), the first control circuit will always be in the off state after the MOSFET is turned on. At the same time, the second control circuit and N discharge circuits will function to pull down DRV.

[0060] Compared with the first situation, in the second situation, the first control circuit will participate in the adjustment of DRV, thereby shortening the time for DRV to be pulled down to V REG so that the required adjustment time in the second situation is the same as that in the first situation.

[0061] Regardless of which situation, after the MOSFET is turned on, the functions (working processes) of the second control circuit and N discharge circuits are as follows: When DRV has not been reduced to the required target value V REG and is greater than the third threshold voltage, the second control circuit controls the above-mentioned controlled circuit to be in the conducting state. At this time, N discharge circuits discharge quickly together to reduce the voltage value of DRV (if in the second situation, the first control circuit also participates in pulling down DRV together). Compared with the design of setting a constantly conducting discharge circuit in the traditional synchronous rectification control circuit, its discharge speed is faster, that is, DRV is reduced to the required target value V REG faster, so the turn-off speed of the MOSFET is improved.

[0062] When DRV is less than or equal to the third threshold voltage, the second control circuit controls the controlled circuit to be turned off. Thereafter, the constantly conducting discharge circuit is still working. Compared with discharging together with N discharge circuits, the falling speed of DRV can be slowed down to avoid excessive pulling down of DRV. Therefore, the DRV adjustment circuit is more likely to enter the stable adjustment state, improving the stability of the synchronous rectification control circuit and also improving the stability of the switching power supply applying the synchronous rectification control circuit.

[0063] In summary, the technical solution provided by the embodiment of the present invention can reduce the time for DRV to be reduced from the high level to V REGThe time, and prevent the DRV from decreasing excessively, thereby not only accelerating the turn-off speed of the MOSFET but also improving the stability. Brief Description of the Drawings

[0064] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0065] Figure 1 Structural schematic diagram of a synchronous rectification control circuit provided by an embodiment of the present invention;

[0066] Figure 2 Partial node change curve diagram of DRV of a synchronous rectification control circuit provided by an embodiment of the present invention;

[0067] Figure 3 Another partial node change curve diagram of DRV of a synchronous rectification control circuit provided by an embodiment of the present invention;

[0068] Figure 4 Structural schematic diagram of a switching power supply applying a synchronous rectification control circuit provided by an embodiment of the present invention.

[0069] Symbol Description:

[0070] Conduction control circuit - 1, turn-off control circuit - 2, drive circuit - 3, gate drive voltage DRV adjustment circuit - 4, first comparator - 41, second comparator - 42, NOR gate - 43, third comparator - 44, inverter - 45, first switch - 46, NAND gate - 47, pull-down current source circuit - 48, fourth comparator 49, second switch - 410, amplifier - 411, always-on discharge circuit - 412, controlled circuit - 413, capacitor - 5, transformer - 6, positive interface - 7, negative interface - 8. Detailed Embodiments

[0071] The structures and scenarios described in the embodiments of the present application are for more clearly illustrating the technical solutions of the embodiments of the present application, and do not constitute a limitation on the technical solutions provided by the embodiments of the present application. Those of ordinary skill in the art know that with the emergence of new scenarios, the technical solutions provided by the embodiments of the present application are also applicable to similar technical problems.

[0072] It should be noted that in this application, words such as "exemplary" or "for example" are used to represent examples, illustrations, or explanations. Any embodiment or design solution described as "exemplary" or "for example" in this application should not be construed as being more preferred or having more advantages than other embodiments or design solutions. Rather, the use of words such as "exemplary" or "for example" is intended to present relevant concepts in a specific manner.

[0073] The purpose of the embodiment of the present invention is to provide a synchronous rectification control circuit, a DRV adjustment circuit, and a switching power supply, which reduce the time for DRV to decrease from a high level to V REG and prevent DRV from decreasing excessively, thereby both accelerating the turn-off speed of the MOSFET and improving the stability.

[0074] The above switching power supply uses a synchronous rectification control circuit to drive the MOSFET to complete the rectification function.

[0075] Figure 1 An exemplary structure of the above synchronous rectification control circuit is shown, including a turn-on control circuit 1, a turn-off control circuit 2, a drive circuit 3, and a gate drive voltage DRV adjustment circuit 4. The functions of each part are introduced below:

[0076] The turn-on control circuit 1, the turn-off control circuit 2, and the drive circuit 3 are introduced in detail below.

[0077] Still referring to Figure 1 , the turn-on control circuit 1 includes: a first input terminal, a second input terminal, and a first output terminal.

[0078] In one example, the drain voltage (VDET) is input to the first input terminal, the turn-on reference voltage (Vth_on) is input to the second input terminal, the first output terminal is connected to the drive circuit 3, and the turn-on signal (Turn_on) is output from the first output terminal. VDET is used to detect and reflect the magnitude of the source-drain voltage VDS of the NMOS transistor (MN0). When the secondary side freewheeling starts, the MOSFET is in the off state, and the secondary side current flows through the parasitic diode D0 of the MOSFET to achieve freewheeling, and at the same time, a negative VDS voltage is formed across the parasitic diode, that is, VDET drops from a positive voltage to a negative voltage. When the turn-on control circuit 1 detects that the negative voltage of VDET is lower than the reference voltage Vth_on, the Turn_on signal is output, and the MOSFET will turn on after a delay td1.

[0079] The turn-off control circuit 2 includes: a third input terminal, a fourth input terminal, and a second output terminal.

[0080] In one example, the third input terminal inputs the drain voltage (VDET), the fourth input terminal inputs the turn-off reference voltage (Vth_off), the second output terminal is connected to the drive circuit 3, and the second output terminal is used to output the turn-off signal (Turn_off). VDET is used to detect and reflect the source-drain voltage VDS of the NMOS tube (MN0). During the conduction period of the MOSFET, as the freewheeling current decreases, the source-drain voltage VDET of the NMOS tube MN0 gradually increases (gradually changes from a negative voltage to a positive voltage). When VDET rises to the turn-off threshold Vth_off, the MOSFET will be turned off after a delay of td2. The reference voltage Vth_off is greater than the reference voltage Vth_on.

[0081] The driving circuit 3 is connected to the on-control circuit 1 , the off-control circuit 2 and the MOSFET respectively. The driving circuit 3 is used to turn on the MOSFET according to the on-signal or turn off the MOSFET according to the off-signal.

[0082] The input end of the driving circuit 3 is connected to the signal turn_on and the turn_off signal respectively, and the output end outputs DRV, the high level of DRV is VCC, and the low level is the ground voltage. The output DRV drives the MOSFET to achieve the purpose of turning the MOSFET on and off.

[0083] The gate drive voltage DRV adjustment circuit 4 is connected to the MOSFET, and the DRV adjustment circuit is used to reduce the gate drive voltage before the MOSFET is turned off; the DRV adjustment circuit includes a first control circuit, a second control circuit, and N discharge circuits, where N is not less than 2. Those skilled in the art can flexibly design the number of discharge circuits, such as 2, 3, 5, etc., which will not be described in detail here.

[0084] See also Figure 1 , for example, the MOSFET may include an NMOS tube (in Figure 1 MN0 in the figure) and the diode (in Figure 1 The gate of the MOSFET is input (or connected) to DRV, and the source and substrate are grounded. The diode is a parasitic diode of the NMOS tube.

[0085] The first control circuit is described in detail below.

[0086] When DRV is higher than the reference voltage, the first control circuit is in a conducting state to reduce the voltage value of DRV.

[0087] When DRV is less than or equal to the reference voltage or the drain voltage of the MOSFET is greater than or equal to the second threshold voltage, the first control circuit is in a disconnected state.

[0088] Exemplarily, the reference voltage may be the internal power supply VCCI, or those skilled in the art can fix the reference voltage to a fixed value. Those skilled in the art can flexibly design the magnitude of VCCI or the above fixed value, such as 4 volts, 5 volts, 6 volts, etc., which will not be elaborated here. When the power supply voltage VCC is greater than the minimum allowable operating voltage of the DRV adjustment circuit 4 and not higher than 5 volts, the internal power supply VCCI is equal to the power supply voltage VCC.

[0089] The first control circuit can be implemented using a variety of circuit structures. In one example, the first control circuit may include a pull-down current source circuit 48 and a first controller.

[0090] Specifically, the first controller is used for:

[0091] Comparing the magnitudes of DRV and the reference voltage, the drain voltage and the first threshold voltage Vth1, and the drain voltage and the second threshold voltage Vth2; the first threshold voltage Vth1 is less than the second threshold voltage Vth2.

[0092] When DRV is higher than the reference voltage or the drain voltage of the MOSFET is greater than the first threshold voltage and less than the second threshold voltage, the first controller controls the pull-down current source circuit 48 to conduct, and the first control circuit is in the conducting state, reducing the voltage value of DRV to the reference voltage. When the voltage value of DRV is less than or equal to the reference voltage and the drain voltage of the MOSFET is greater than or equal to the second threshold voltage, the first controller controls the pull-down current source circuit 48 to disconnect, the first control circuit is in the disconnected state, and the voltage value of DRV stops decreasing.

[0093] It should be noted that when the MOSFET starts to conduct, the magnitude of DRV generally depends on the magnitude of VCC. Then when the MOSFET starts to conduct, since DRV is equal to VCCI, the aforementioned comparison of the magnitudes of DRV and the reference voltage is also equivalent to comparing the magnitudes of VCCI and VCC when the MOSFET starts to conduct.

[0094] At the same time, there is also a relationship between VCCI and VCC - VCCI can be generated by VCC and VCCI is less than or equal to VCC. In design, generally, the power supply voltage of at least the output driving part of the devices is VCC, and the power supply of the remaining devices is the internal power supply VCCI. For example, the power supplies of the aforementioned conduction control circuit 1, the turn-off control circuit 2, and the drive circuit 3 can be VCC, and the power supplies of the remaining circuits can be the internal power supply VCCI. The synchronous rectification control circuit can include multiple internal power supplies VCCI.

[0095] The voltage variation range of the power supply VCC is relatively large. In a typical application, VCCI is equal to 5V. When the power supply VCC is greater than 5V, the internal power supply VCCI is equal to 5V. When the power supply VCC is greater than the minimum allowable operating voltage and not higher than 5V, the internal power supply VCCI is equal to the power supply VCC.

[0096] In one example, the first threshold voltage may be negative 80 mV or negative 70 mV, and the second threshold voltage may be negative 40 mV or negative 30 mV.

[0097] Of course, the function of the first controller can be realized by using a circuit structure. Figure 1 , exemplarily, the first controller at least includes: a first comparator 41, a second comparator 42, a NOR gate 43, a third comparator 44, an inverter 45, a first switch 46 and a NAND gate 47. Wherein:

[0098] The inverting input terminal of the first comparator 41 is used to input DRV, and the positive input terminal of the first comparator 41 is used to input a reference voltage (eg VCCI).

[0099] The positive input terminal of the second comparator 42 is used to input the first threshold voltage Vth1 , and the negative input terminal of the second comparator 42 is used to input the drain voltage.

[0100] A first input terminal of the NOR gate 43 is connected to the output terminal of the first comparator 41 , and a second input terminal of the NOR gate 43 is connected to the output terminal of the second comparator 42 .

[0101] The positive input terminal of the third comparator 44 is used to input the drain voltage, and the negative input terminal is used to input the second threshold voltage Vth2.

[0102] An input terminal of the inverter 45 is connected to an output terminal of the third comparator 44 .

[0103] A first input terminal of the NAND gate 47 is connected to the output terminal of the NOR gate 43 , and a second input terminal of the NAND gate 47 is connected to the output terminal of the inverter 45 ; and an output terminal of the NAND gate 47 is used to output a control signal for controlling the first switch 46 .

[0104] The first switch 46 is used to open or close the pull-down current source circuit 48 according to the control signal. The pull-down current source circuit 48 is used to reduce DRV from VCC to VCCI.

[0105] In an example, the pull-down current source circuit 48 may be a conventional current source.

[0106] The functions of the above components are as follows:

[0107] The first comparator 41 is used to detect the magnitudes of DRV and the internal power supply VCCI. When DRV is greater than the internal power supply VCCI, the first comparator 41 outputs a low level. The low level output by the first comparator 41 is one of the conditions for the first switch 46 to close.

[0108] The first comparator 41 is used to detect the magnitudes of the drain voltage (VDET) and the first threshold voltage (Vth1). When the drain voltage is greater than the first threshold voltage Vth1, the second comparator 42 outputs a low level. The low level output by the second comparator 42 is the second condition for the first switch 46 to close.

[0109] Both the first input terminal and the second input terminal of the NOR gate 43 receive low levels, and the NOR gate 43 outputs a high level.

[0110] When the drain voltage is less than the second threshold voltage Vth2, the third comparator 44 outputs a high level.

[0111] The third comparator 44 outputs a high level. The output terminal of the third comparator 44 is connected to the input terminal of the inverter 45, and the inverter 45 outputs a low level.

[0112] The NOR gate 43 outputs a high level, the inverter 45 outputs a low level, and the NAND gate 47 outputs a high level. At this time, the first switch closes; otherwise, the first switch 46 opens. Specifically, when DRV is greater than the internal power supply VCCI and the drain voltage is greater than the first threshold voltage, that is, when both conditions for the first switch 46 to close are satisfied, the first switch 46 closes, the pull - down current source circuit 48 conducts, DRV discharges, and the voltage decreases. One end of the pull - down current source circuit 48 is connected to the first switch 46, and the other end is grounded. When DRV is not higher than the internal power supply VCCI, the first switch 46 is disconnected. When the drain voltage is greater than the second threshold voltage, the first switch 46 is forcibly disconnected, that is, when one of the two conditions for the first switch 46 to close is not satisfied, the first switch 46 is disconnected.

[0113] In one example, the first switch 46 can specifically be an NMOS switch tube or a transmission gate circuit.

[0114] The following details the second control circuit.

[0115] When DRV has not dropped to the required target value V REG and is greater than the third threshold voltage Vth3, the second control circuit controls the controlled circuit 413 to conduct.

[0116] When DRV is less than or equal to the third threshold voltage Vth3, the second control circuit controls the controlled circuit 413 to disconnect.

[0117] Still referring to Figure 1, Exemplarily, the second control circuit includes at least N discharge circuits, a fourth comparator 49, a second switch 410, and an amplifier 411. The N discharge circuits include a normally-on discharge circuit 412 and a controlled circuit 413.

[0118] The second control circuit can be implemented using various circuit structures. In one example, the second control circuit includes at least N discharge circuits and a second controller.

[0119] The second controller is specifically configured to:

[0120] Compare the magnitudes of DRV and the third threshold voltage Vth3, and the magnitudes of the drain voltage and the second threshold voltage Vth2.

[0121] When DRV is greater than or equal to the third threshold voltage and the drain voltage is greater than or equal to the second threshold voltage (Vth2), the second controller controls the controlled circuit 413 to conduct. When DRV is less than the third threshold voltage and the drain voltage is greater than or equal to the second threshold voltage, the second controller controls the controlled circuit 413 to turn off. The N discharge circuits are all connected to the MOSFET; both the normally-on discharge circuit 412 and the controlled circuit 413 can discharge when conducting, so as to reduce the voltage value of DRV.

[0122] In one example, the simultaneous discharge of the normally-on discharge circuit 412 and the controlled circuit 413 is faster than the discharge of only the normally-on discharge circuit 412.

[0123] The controlled circuit 413 includes a first NMOS transistor; the normally-on discharge circuit 412 includes a second NMOS transistor.

[0124] In one example, the normally-on discharge circuit 412 can include one second NMOS transistor. The controlled circuit 413 can include one or more first NMOS transistors. Those skilled in the art can flexibly design the number of first NMOS transistors, such as 1, 2, 3, etc., which will not be elaborated here.

[0125] The parameters of the first NMOS transistor and the second NMOS transistor are different, and the different parameters can specifically be different in size.

[0126] In one example, when the second switch 410 is closed, the first NMOS transistor and the second NMOS transistor jointly adjust DRV, and the discharge speed is faster; when the second switch 410 is open, the first NMOS transistor no longer adjusts DRV, there is no discharge current, and only the second NMOS transistor adjusts DRV, so the discharge speed becomes slower. The first NMOS transistor serves as a fast adjustment transistor for DRV; the second NMOS transistor serves as an adjustment transistor for DRV.

[0127] The second controller can specifically be a chip.

[0128] Of course, the function of the second controller can also be implemented by a circuit structure. Exemplarily, still referring to Figure 1 , the second controller at least includes: a fourth comparator 49, a second switch 410, and an amplifier 411. Among them:

[0129] The positive terminal of the fourth comparator 49 is used to input DRV, and the negative input terminal of the fourth comparator 49 is used to input the third threshold voltage Vth3.

[0130] The second switch 410 is connected to the output terminal of the fourth comparator 49, and the second switch 410 is used to disconnect or close the controlled circuit 413.

[0131] The positive input terminal of the amplifier 411 is used to input the drain voltage, and the negative input terminal of the amplifier 411 is used to input the second threshold voltage Vth2. The output terminal of the amplifier 411 is respectively connected to the gates of the first NMOS transistor (MN1) and the second NMOS transistor (MN2). The drain of the first NMOS transistor is connected to the MOSFET through the second switch 410; the drain of the second NMOS transistor is connected to DRV; the sources and substrates of the first NMOS transistor and the second NMOS transistor are grounded.

[0132] In one example, when the drain voltage is greater than the second threshold voltage Vth2, the amplifier 411 outputs a high level. The output terminal of the amplifier 411 is respectively connected to the drains of the first NMOS transistor and the second NMOS transistor. The drain of the first NMOS transistor is connected to the MOSFET through the second switch 410. When the amplifier 411 outputs a high level and the second switch 410 is closed, the first NMOS transistor and the second NMOS transistor start to discharge. If DRV is greater than the third threshold voltage Vth3, the second switch 410 is closed. After the second switch 410 is closed, the first NMOS transistor MN1 and the second NMOS transistor MN2 discharge together, entering the fast discharge stage. The other end of the second switch 410 is connected to DRV. If the DRV voltage is equal to the third threshold voltage Vth3, the second switch 410 is opened, and the first NMOS transistor MN1 stops discharging. Only the second NMOS transistor MN2 discharges DRV more slowly, which can avoid over-discharging of DRV and make the DRV adjustment circuit easier to enter the stable adjustment state.

[0133] The functions of the above-mentioned devices are as follows:

[0134] The fourth comparator 49 is used to compare the magnitudes of DRV and the third threshold voltage Vth3. When DRV is greater than the third threshold voltage Vth3, the fourth comparator 49 outputs a high level. The high level output by the fourth comparator 49 is used as the condition for closing the second switch 410.

[0135] In one example, the third threshold voltage Vth3 can specifically be -10 mV or -5 mV.

[0136] The amplifier 411 is used to compare the magnitude of the drain voltage with the second threshold voltage Vth2. When the drain voltage is greater than the second threshold voltage Vth2, the amplifier 411 outputs a high level.

[0137] Typical application scenarios of the synchronous rectification control circuit can include two types:

[0138] Application scenario one Figure 2 is a partial node voltage curve diagram of the synchronous rectification control circuit in the embodiment of the present invention under application scenario one. As Figure 2 shown, when the secondary side freewheeling starts, the MOSFET is in the off state, and the secondary side current flows through the parasitic diode D0 of the MOSFET to achieve freewheeling. At the same time, a negative source-drain voltage (VDS) is formed across the parasitic diode D0, that is, VDET drops from a positive voltage to a negative voltage. When VDET drops to the conduction threshold (Vth_on), the MOSFET will turn on after a delay (td1), and DRV changes from low to high. Under this application condition, the power supply VCC is 5V, the internal power supply VCCI is set to 5V, and the first switch 46 is not closed. As the freewheeling current decreases, VDET gradually increases (changes from a negative voltage to a positive voltage gradually). When VDET rises to the second threshold voltage Vth2, the second switch 410 closes, and DRV is quickly discharged during tc1, so that the DRV adjustment circuit can enter the stable adjustment state faster. When DRV quickly drops to the third threshold voltage Vth3, the second switch 410 opens, and the discharge speed of DRV slows down, which can avoid over-discharging of DRV and make it easier for the DRV adjustment circuit to enter the stable adjustment state.

[0139] After passing through the tc2 time, DRV drops to V REG and enters the stable adjustment state. Subsequently, DRV continues to gradually decrease from V REG Through this mechanism, when the current passing through the MOSFET is quite low, VDS is adjusted near the second threshold voltage Vth2. As the freewheeling current continues to decrease, when VDET rises to the turn-off threshold voltage (Vth_off), the MOSFET turns off after a delay of td2 time. At this time, DRV is already very low, which can accelerate the turn-off speed of the MOSFET.

[0140] Application scenario two Figure 3 is a partial node voltage curve diagram of the synchronous rectification control circuit in the embodiment of the present invention under application scenario two. As Figure 3As shown, when the secondary side freewheeling starts, the MOSFET is in the off state. The secondary side current flows through the parasitic diode D0 of the MOSFET to achieve freewheeling. At the same time, a negative VDS voltage is formed across the parasitic diode, that is, VDET drops from a positive voltage to a negative voltage. When VDET drops to the conduction threshold Vth_on, the MOSFET will conduct after a delay td1, and DRV changes from a low voltage to a high voltage. Under this application condition, the power supply VCC is higher than 5V, and the internal power supply VCCI is set to 5V. As the freewheeling current decreases, the VDET voltage gradually increases (changes from a negative voltage to a positive voltage direction). When VDET rises to the first threshold voltage Vth1, the first switch 46 closes. After discharging DRV to the VCCI voltage, the first switch 46 is turned off. The subsequent DRV adjustment process is the same as that in Figure 2 the application scenario 1, which will not be elaborated here.

[0141] It can be seen by comparison that in the second application scenario, there is an additional process of discharging DRV from VCC to VCCI by the pull-down current source circuit 48, making the process of DRV decreasing from VCCI to V REG independent of VCC, so that the adjustment time required in the second scenario is the same as that required in the first scenario.

[0142] In summary, the synchronous rectification control circuit reduces the time for DRV to decrease from a high level to V REG and prevents DRV from decreasing excessively, thus not only accelerating the turn-off speed of the MOSFET but also improving the stability.

[0143] Please refer to Figure 4 , a switching power supply applying a synchronous rectification control circuit. Exemplarily, a switching power supply applying a synchronous rectification control circuit at least includes: a synchronous rectification control circuit, a capacitor 5, a transformer 6, a positive electrode interface 7, and a negative electrode interface 8.

[0144] In one example, the capacitor 5 is respectively connected to the positive electrode interface 7 and the negative electrode interface 8, and the capacitor 5 is used for storing electrical energy.

[0145] The negative electrode interface 8 is grounded.

[0146] The positive electrode interface 7 is connected to one end of the transformer 6, and the other end of the transformer 6 is connected to the drain of the MOSFET; the source and substrate of the MOSFET are grounded. The transformer 6 includes a primary side inductor and a secondary side inductor. Exemplarily, the left side is the primary side inductor and the right side is the secondary side inductor. Secondary side freewheeling means that there is current flowing through "ground terminal (GND) - MOSFET - secondary side inductor - positive electrode interface 7 (VOUT+)" on the secondary side.

[0147] The synchronous rectification control circuit is connected to the MOSFET.

[0148] Figure 4It is a switching power supply applying a synchronous rectification control circuit. As Figure 4 shown, the VDET terminal of the synchronous rectification control circuit detects the VDS voltage of the power MOSFET (MN0), and outputs DRV to control the turn-on and turn-off of the power transistor MN0. The gate of MN0 is connected to DRV, the source and the substrate are grounded, and the drain is connected to one end of the transformer 6. When the secondary side starts to freewheel and the MOSFET is not turned on, freewheeling is performed through the parasitic diode D0. The other end of the transformer 6 is the positive interface 7 (VOUT+), and the negative interface 8 (VOUT-) is the ground terminal. The capacitor 5 is connected between the positive interface 7 and the negative interface 8.

[0149] The embodiments of the present invention also claim to protect the DRV adjustment circuit in all the above embodiments.

[0150] In this specification, the various embodiments are described in a progressive manner. Each embodiment focuses on the differences from other embodiments. For the same and similar parts among the various embodiments, reference may be made to each other.

[0151] Specific examples are used in this article to elaborate on the principles and implementation manners of the embodiments of the present invention. The descriptions of the above embodiments are only used to help understand the core idea of the embodiments of the present invention; at the same time, for those of ordinary skill in the art, based on the idea of the embodiments of the present invention, there will be changes in the specific implementation manners and application scopes. In summary, the content of this specification should not be construed as a limitation on the embodiments of the present invention.

Claims

1. A synchronous rectification control circuit, characterized in that Comprising: A conduction control circuit for generating a conduction signal; A turn-off control circuit for generating a turn-off signal; A drive circuit, respectively connected to the conduction control circuit, the turn-off control circuit and the MOSFET, for turning on the MOSFET according to the conduction signal or turning off the MOSFET according to the turn-off signal; A gate drive voltage DRV adjustment circuit, connected to the MOSFET, for reducing the gate drive voltage before the MOSFET is turned off; the DRV adjustment circuit includes a first control circuit, a second control circuit, and N discharge circuits, where N is not less than 2; When DRV is higher than the reference voltage, the first control circuit is in the conduction state to reduce the voltage value of DRV; when DRV is less than or equal to the reference voltage or the drain voltage of the MOSFET is greater than or equal to the second threshold voltage, the first control circuit is in the off state; The N discharge circuits are all connected to the MOSFET; when any one of the discharge circuits is turned on, it can discharge to reduce the voltage value of DRV; Any one of the discharge circuits is a normally-on discharge circuit or a controlled circuit controlled by the second control circuit; The second control circuit is configured to: when DRV has not dropped to the required target value V REG and is greater than the third threshold voltage, control the controlled circuit to turn on, and when DRV is less than or equal to the third threshold voltage, control the controlled circuit to turn off; The first control circuit includes: A pull-down current source circuit; A first controller for controlling the pull-down current source circuit to conduct when DRV is higher than the reference voltage to reduce the voltage value of DRV to the reference voltage; and for controlling the pull-down current source circuit to disconnect when the voltage value of DRV is equal to the reference voltage; The second control circuit includes: A fourth comparator, the positive input terminal of the fourth comparator is used to input DRV, and the negative input terminal is used to input the third threshold voltage; A second switch, the control terminal of the second switch is connected to the output terminal of the fourth comparator, and the second switch is used to disconnect or close the controlled circuit; An amplifier, the positive input terminal of the amplifier is used to input the drain voltage, the negative input terminal is used to input the second threshold voltage, and the output terminal is connected to the discharge circuit.

2. The synchronous rectification control circuit according to claim 1, wherein The first controller is specifically used for: Comparing the magnitudes of DRV and the reference voltage, the magnitudes of the drain voltage and the first threshold voltage, and the magnitudes of the drain voltage and the second threshold voltage; the first threshold voltage is less than the second threshold voltage; If DRV is greater than the reference voltage, the drain voltage is greater than the first threshold voltage and less than the second threshold voltage, controlling the pull-down current source circuit to close; If DRV is less than or equal to the reference voltage and the drain voltage is greater than the second threshold voltage, controlling the pull-down current source circuit to disconnect.

3. The synchronous rectification control circuit according to claim 2, wherein The first controller includes: A first comparator, the negative input terminal of the first comparator is used to input DRV, and the positive input terminal is used to input the reference voltage; A second comparator, the positive input terminal of the second comparator is used to input the first threshold voltage, and the negative input terminal is used to input the drain voltage; A NOR gate, wherein a first input end of the NOR gate is connected to an output end of the first comparator, and a second input end of the NOR gate is connected to an output end of the second comparator; A third comparator, wherein a positive input end of the third comparator is configured to input the drain voltage, and a negative input end of the third comparator is configured to input the second threshold voltage; An inverter, wherein an input end of the inverter is connected to an output end of the third comparator; A first switch; A NAND gate, wherein a first input end of the NAND gate is connected to an output end of the NOR gate, and a second input end of the NAND gate is connected to an output end of the inverter; an output end of the NAND gate is configured to output a control signal for controlling the first switch.

4. The synchronous rectification control circuit according to claim 1, wherein the second control circuit is specifically configured to: when DRV is greater than or equal to the third threshold voltage and the drain voltage is greater than or equal to the second threshold voltage, control the controlled circuit to conduct; when DRV is less than the third threshold voltage and the drain voltage is greater than or equal to the second threshold voltage, control the controlled circuit to turn off.

5. The synchronous rectification control circuit according to claim 1, wherein The controlled circuit includes a first NMOS transistor; the always-on discharge circuit includes a second NMOS transistor; an output end of the amplifier is respectively connected to a gate of the first NMOS transistor and a gate of the second NMOS transistor; a drain of the first NMOS transistor is connected to a gate of the MOSFET through the second switch; a drain of the second NMOS transistor is connected to the gate of the MOSFET; sources and substrates of the first NMOS transistor and the second NMOS transistor are grounded.

6. The synchronous rectification control circuit according to claim 5, wherein Parameters of the first NMOS transistor and the second NMOS transistor are different.

7. A switching power supply applying a synchronous rectification control circuit, characterized in that including: the synchronous rectification control circuit according to any one of claims 1-6, and a capacitor, a transformer, a positive electrode interface, and a negative electrode interface; the capacitor is respectively connected to the positive electrode interface and the negative electrode interface, and the capacitor is configured to store electric energy; the negative electrode interface is grounded; the positive electrode interface is connected to one end of the transformer; the other end of the transformer is connected to a drain end of the MOSFET; a source and a substrate of the MOSFET are grounded; the synchronous rectification control circuit is connected to the MOSFET.

8. A DRV adjustment circuit, characterized in that, Applied to the synchronous rectification control circuit according to any one of claims 1-6, the DRV adjustment circuit is connected to the MOSFET and is configured to reduce a gate drive voltage of the MOSFET before turn-off; the DRV adjustment circuit includes: a first control circuit, a second control circuit, and N discharge circuits, where N is not less than 2; wherein: when DRV is higher than a reference voltage, the first control circuit is in a conducting state to reduce a voltage value of DRV; when DRV is less than or equal to the reference voltage or a drain voltage of the MOSFET is greater than or equal to the second threshold voltage, the first control circuit is in a non-conducting state; the N discharge circuits are all connected to the MOSFET; any one of the discharge circuits can discharge when conducting to reduce the voltage value of DRV; any one of the discharge circuits is an always-on discharge circuit or a controlled circuit controlled by the second control circuit; The second control circuit is configured to: control the controlled circuit to turn on when DRV has not dropped to the required target value VREG and is greater than the third threshold voltage, and control the controlled circuit to turn off when DRV is less than or equal to the third threshold voltage.

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