CN115882701A - Synchronous rectification control circuit, DRV (digital Drive Voltage regulator) circuit and switching power supply - Google Patents

Abstract

The invention discloses a synchronous rectification control circuit, a DRV (digital Drift voltage regulator) circuit and a switching power supply, and relates to the technical field of electronic circuits. The device comprises a turn-on control circuit, a turn-off control circuit, a driving circuit and a DRV adjusting circuit. The conduction control circuit is used for sending out a conduction signal; the turn-off control circuit is used for sending a turn-off signal; the driving circuit is used for switching on the MOSFET according to the switching-on signal or switching off the MOSFET according to the switching-off signal; the DRV adjusting circuit is used for reducing the grid driving voltage before the MOSFET is turned off; the DRV adjusting circuit comprises a first control circuit, a second control circuit, a pull-down current source circuit and N discharging circuits, wherein N is not less than 2; any discharge circuit is conducted to discharge so as to reduce the DRV voltage value. The invention reduces DRV from high level to V _REG And preventing excessive drop of DRV, thereby increasing both the turn-off speed of the MOSFET and the stability.

Description

Synchronous rectification control circuit, DRV (digital Drive Voltage regulator) circuit and switching power supply

Technical Field

The invention relates to the technical field of electronic circuits, in particular to a synchronous rectification control circuit, a DRV (dry Drv) adjusting circuit and a switching power supply.

Background

Switching power supplies typically use synchronous rectification control circuits to drive metal-oxide semiconductor field effect transistors (MOSFETs ) to perform the rectification function. During the on period of the MOSFET, the synchronous rectification control circuit samples the Voltage (VDS) at the two ends of the source-drain of the MOSFET, and when the VDS is higher than the turn-off threshold value of the MOSFET, the MOSFET is turned off after turn-off delay (td 2).

The synchronous rectification control circuit needs to adjust the gate drive voltage (DRV) to a desired target value (V) in a short time before the MOSFET starts to conduct to VDS and reaches the MOSFET turn-OFF threshold (VTH _ OFF) _REG ). The gate voltage of the MOSFET can be gradually decreased from the high level voltage by decreasing DRV so that the MOSFET is brought into an off state from a lower voltage at any time. When the voltage across the MOSFET is relatively low, VDS is adjusted to within a certain range, which speeds up the turn-off of the MOSFET.

Under some scenes, the synchronous rectification control circuit is externally connected with a power supply VCC. The amount of DRV that the MOSFET begins to conduct is generally determined by the amount of VCC. In a typical application scenario, when the power supply VCC is 5V, and the DRV voltage (gate driving voltage) adjustment circuit operates, the DRV voltage (gate driving voltage) needs to be adjusted from the power supply VCC (5V in the above typical application scenario) to the required target value V in a short time _REG So that the subsequent MOSFET goes from a lower voltage to an off state at any time.

In some other applications the supply voltage VCC is raised to a higher voltage level, i.e. the supply voltage VCC is higher than 5 volts. Thus, when the DRV voltage (gate driving voltage) adjustment circuit adjusts the DRV voltage (gate driving voltage), it is necessary to pull down the DRV voltage from a higher voltage to the desired target value V _REG So that the adjustment time becomes long; meanwhile, only one normally-on discharge circuit is designed in the traditional synchronous rectification control circuit for discharging, and in order to improve the pull-down speed, the use of large pull-down current needs to be considered, however, if the large pull-down current is used, DRV is easy to be excessiveAnd (4) pulling down.

Disclosure of Invention

An object of an embodiment of the present invention is to provide a synchronous rectification control circuit, a DRV adjustment circuit and a switching power supply, so as to reduce DRV from high level to V _REG And prevent the DRV from being excessively lowered, thereby increasing the turn-off speed of the MOSFET and improving the stability.

In order to achieve the above object, the embodiment of the present invention provides the following solutions:

a synchronous rectification control circuit comprising:

the conduction control circuit is used for sending out a conduction signal;

the turn-off control circuit is used for sending a turn-off signal;

the driving circuit is respectively connected with the turn-on control circuit, the turn-off control circuit and the MOSFET and is used for turning on the MOSFET according to the turn-on signal or turning off the MOSFET according to the turn-off signal;

the grid driving voltage DRV adjusting circuit is connected with the MOSFET and is used for reducing the grid driving voltage before the MOSFET is turned off; the DRV adjusting circuit comprises a first control circuit, a second control circuit and N discharging circuits, wherein N is not less than 2;

when the DRV is higher than the reference voltage, the first control circuit is in a conducting state to reduce the voltage value of the 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 a second threshold voltage, the first control circuit is in an off state;

the N discharge circuits are connected with the MOSFET; when any discharge circuit is conducted, the discharge circuit can discharge to reduce the voltage value of DRV; any discharge circuit is a normally-on discharge circuit or a controlled circuit controlled by the second control circuit;

the second control circuit is used for controlling the DRV not to fall to the required target value V _REG And when the DRV is less than or equal to the third threshold value, the controlled circuit is controlled to be switched on, and when the DRV is less than or equal to the third threshold value, the controlled circuit is controlled to be switched off.

Optionally, the first control circuit comprises:

a pull-down current source circuit;

the first controller is used for controlling the pull-down current source circuit to be conducted when the DRV is higher than the reference voltage so as to reduce the voltage value of the DRV to the reference voltage; and controlling the pull-down current source circuit to be switched off when the voltage value of the DRV is equal to the reference voltage.

Optionally, the first controller is specifically configured to:

comparing the DRV with the reference voltage, the drain voltage with a first threshold voltage, and the drain voltage with a second threshold voltage; the first threshold voltage is less than the second threshold voltage;

if the DRV is larger than the reference voltage, and the drain voltage is larger than a first threshold voltage and smaller than a second threshold voltage, controlling the pull-down current source circuit to be closed;

and if the 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 be disconnected.

Optionally, the first controller comprises:

a first comparator, wherein the inverting input terminal of the first comparator is used for inputting the DRV, and the forward input terminal of the first comparator is used for inputting the reference voltage;

a second comparator, wherein a positive input terminal of the second comparator is used for inputting the first threshold voltage, and a negative input terminal of the second comparator is used for inputting the drain voltage;

a first input end of the NOR gate is connected with the output end of the first comparator, and a second input end of the NOR gate is connected with the output end of the second comparator;

a positive input end of the third comparator is used for inputting the drain voltage, and a negative input end of the third comparator is used for inputting the second threshold voltage;

the input end of the inverter is connected with the output end of the third comparator;

a first switch;

the first input end of the NAND gate is connected with the output end of the NOR gate, and the second input end of the NAND gate is connected with the output end of the inverter; and the output end of the NAND gate is used for outputting a control signal for controlling the first switch.

Optionally, the second control circuit is specifically configured to:

if DRV is larger than or equal to the third threshold voltage and the drain voltage is larger than or equal to the second threshold voltage, controlling the controlled circuit to be conducted;

and if the DRV is less than the third threshold voltage and the drain voltage is greater than or equal to the second threshold voltage, controlling the controlled circuit to be switched off.

Optionally, the second control circuit comprises:

a fourth comparator, a positive input end of which is used for inputting the DRV, and a negative input end of which is used for inputting the third threshold voltage;

a control end of the second switch is connected with an output end of the fourth comparator, and the second switch is used for opening or closing the controlled circuit;

and the positive input end of the amplifier is used for inputting the drain voltage, the negative input end of the amplifier is used for inputting the second threshold voltage, and the output end of the amplifier is connected with the discharge circuit.

The invention also provides a DRV adjusting circuit which is applied to the synchronous rectification control circuit, is connected with the MOSFET and is used for reducing the grid driving voltage before the MOSFET is turned off;

the DRV adjustment circuit includes: the device comprises a first control circuit, a second control circuit and N discharge circuits, wherein N is not less than 2; wherein:

when the DRV is higher than the reference voltage, the first control circuit is in a conducting state to reduce the voltage value of the 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 a second threshold voltage, the first control circuit is in an off state;

the N discharge circuits are connected with the MOSFET; any discharge circuit can discharge when being conducted so as to reduce the voltage value of DRV; any discharge circuit is a normally-on discharge circuit or a controlled circuit controlled by the second control circuit;

the second control circuit is configured to: and controlling the controlled circuit to be switched on when the DRV does not fall to the required target value VREG and is greater than a third threshold value, and controlling the controlled circuit to be switched off when the DRV is less than or equal to the third threshold value.

Optionally, the controlled circuit comprises a first NMOS transistor; the normally-on discharge circuit comprises a second NMOS tube;

the output end of the amplifier is respectively connected with the grid electrode of the first NMOS tube and the grid electrode of the second NMOS tube; the drain electrode of the first NMOS tube is connected with the grid electrode of the MOSFET through the second switch;

the drain electrode of the second NMOS tube is connected with the grid electrode of the MOSFET;

the source electrodes and the substrate of the first NMOS tube and the second NMOS tube are grounded.

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

The invention also provides a switching power supply applying the synchronous rectification control circuit, which comprises: the synchronous rectification control circuit, the capacitor, the transformer, the positive electrode interface and the negative electrode interface;

the capacitor is respectively connected with the positive electrode interface and the negative electrode interface and is used for storing electric energy;

the negative electrode interface is grounded;

the positive electrode interface is connected with one end of the transformer; the other end of the transformer is connected with the drain end of the MOSFET; the source end and the substrate of the MOSFET are grounded;

the synchronous rectification control circuit is connected with the MOSFET.

According to the specific embodiment provided by the invention, the following technical effects are disclosed:

the embodiment of the invention provides a synchronous rectification control circuit, a DRV (digital Drift regulator) circuit and a switching power supply. The circuits cooperate as follows: the conduction control circuit is used for sending out a conduction signal; the turn-off control circuit is used for sending a turn-off signal; the driving circuit is used for switching on the MOSFET according to the switching-on signal or switching off the MOSFET according to the switching-off signal; the DRV adjustment circuit is used for reducing the grid driving voltage before the MOSFET is turned off.

The DRV adjusting circuit comprises a first control circuit, a second control circuit, a pull-down current source circuit and N discharging circuits (N is not less than 2). The N discharge circuits are connected with 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 the DRV does not drop to the required target value V _REG And is switched on when the voltage is larger than the third threshold value, and is switched off when the voltage is not larger than the third threshold value.

Under different occasions, the adjustment processes of the DRV adjustment circuit are respectively as follows:

for the occasion (which can be called as a second occasion) that VCC is higher than the reference voltage, because the initial voltage of DRV is determined by VCC when the MOSFET is conducted, the initial voltage of DRV is higher than the reference voltage, the first control circuit will be in a conducting state to reduce the voltage value of DRV; meanwhile, the second control circuit and the N discharge circuits also play a role in pulling down the DRV.

After the DRV is reduced to the reference voltage from the initial voltage or when the drain voltage of the MOSFET is greater than or equal to the second threshold voltage, the first control circuit enters an off state, and then the second control circuit and the N discharge circuits continue to work.

And VCC is equal to the reference voltage (which may be referred to as the first case), the first control circuit is always in the off state after the MOSFET is turned on. Meanwhile, the second control circuit and the N discharge circuits will act to pull down the DRV.

In the second instance, the first control circuit participates in the adjustment of the DRV, thereby shortening the pull-down of the DRV to V _REG The adjustment time required in the second occasion is made to be consistent with the adjustment time required in the first occasion.

In any case, after the MOSFET is turned on, the second control circuit and the N discharge circuits functionThe function (working process) is as follows: when DRV is not reduced to the required target value V _REG And when the voltage value is larger than the third threshold value, the second control circuit controls the controlled circuit to be in a conducting state, at the moment, the N discharge circuits carry out quick discharge together to reduce the voltage value of the DRV (if in a second occasion, the first control circuit participates in DRV pull-down together), compared with the design that a normally-on discharge circuit is arranged in the traditional synchronous rectification control circuit, the discharge speed is higher, namely the DRV is reduced to a required target value V _REG Faster and therefore increases the turn-off speed of the MOSFET.

And when the DRV is less than or equal to a third threshold value, the second control circuit controls the controlled circuit to be disconnected, and then the normally-on discharge circuit still works. Compared with the N discharge circuits for discharging together, the DRV control circuit can slow down the drop speed of the DRV and avoid excessive pull-down of the DRV, so that the DRV control circuit can enter a stable regulation state more easily, the stability of the synchronous rectification control circuit is improved, and the stability of a switching power supply applying the synchronous rectification control circuit is also improved.

In summary, the technical solution provided by the embodiment of the present invention can reduce DRV from high level to V _REG And prevent the DRV from being excessively lowered, thereby increasing the turn-off speed of the MOSFET and improving the stability.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required to be used in the embodiments will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings based on these drawings without inventive labor.

Fig. 1 is a schematic structural diagram of a synchronous rectification control circuit according to an embodiment of the present invention;

fig. 2 is a partial node variation graph of a synchronous rectification control circuit DRV according to an embodiment of the present invention;

fig. 3 is another partial node variation graph of a synchronous rectification control circuit DRV according to an embodiment of the present invention;

fig. 4 is a schematic structural diagram of a switching power supply using a synchronous rectification control circuit according to an embodiment of the present invention.

Description of the symbols:

the circuit comprises a turn-on control circuit-1, a turn-off control circuit-2, a driving circuit-3, a gate driving voltage DRV adjusting circuit-4, a first comparator-41, a second comparator-42, a NOR gate-43, a third comparator-44, an inverter-45, a first switch-46, a NAND gate-47, a pull-down current source circuit-48, a fourth comparator 49, a second switch-410, an amplifier-411, a normally-on discharge circuit-412, a controlled circuit-413, a capacitor-5, a transformer-6, a positive electrode interface-7 and a negative electrode interface-8.

Detailed Description

The structure and scenario described in the embodiment of the present application are for more clearly illustrating the technical solution of the embodiment of the present application, and do not form a limitation on the technical solution provided in the embodiment of the present application, and it is known by a person skilled in the art that with the occurrence of a new scenario, the technical solution provided in the embodiment of the present application is also applicable to similar technical problems.

It should be noted that in this application, words such as "exemplary" or "for example" are used to indicate examples, illustrations or descriptions. Any embodiment or design described herein as "exemplary" or "e.g.," is not necessarily to be construed as preferred or advantageous over other embodiments or designs. Rather, use of the word "exemplary" or "such as" is intended to present relevant concepts in a concrete fashion.

Embodiments of the present invention provide a synchronous rectification control circuit, a DRV adjustment circuit and a switching power supply, which reduce DRV from high level to V _REG And prevent the DRV from being excessively lowered, thereby increasing the turn-off speed of the MOSFET and improving the stability.

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

Fig. 1 shows an exemplary structure of the synchronous rectification control circuit described above, 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 function of each part is described as follows:

the on control circuit 1, the off control circuit 2, and the drive circuit 3 will be described in detail below.

Still referring to fig. 1, the conduction control circuit 1 includes: a first input, a second input and a first output.

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 driving circuit 3, and the Turn-on signal (Turn _ on) is output to the first output terminal. VDET is used for detecting and reflecting the magnitude of the VDS of the source-drain terminal of the NMOS tube (MN 0). When the secondary side freewheeling starts, the MOSFET is in a closed state, the secondary side current flows through the parasitic diode D0 of the MOSFET to realize freewheeling, and a negative VDS voltage is formed across the parasitic diode, i.e., VDET is decreased from a positive voltage to a negative voltage. When the Turn-on control circuit 1 detects that the negative voltage of the VDET is lower than the reference voltage Vth _ on, a Turn _ on signal is output, and the MOSFET is turned on after a delay td 1.

The shutdown control circuit 2 includes: a third input, a fourth input, and a second output.

In one example, the drain Voltage (VDET) is input to the third input terminal, the off-reference voltage (Vth _ off) is input to the fourth input terminal, and the second output terminal is connected to the driving circuit 3 and outputs the off-signal (Turn _ off). VDET is used for detecting and reflecting the magnitude of the voltage VDS between the source and the drain of the NMOS tube (MN 0). During the on period of the MOSFET, the source-drain terminal voltage VDET of the NMOS transistor MN0 gradually increases (gradually changes from a negative voltage to a positive voltage) as the freewheeling current decreases. When VDET rises to the turn-off threshold Vth _ off, the MOSFET will turn off after a delay td 2. The reference voltage Vth _ off is greater than the reference voltage Vth _ on.

The driving circuit 3 is respectively connected with the on-control circuit 1, the off-control circuit 2 and the MOSFET, and the driving circuit 3 is used for turning on the MOSFET according to an on-signal or turning off the MOSFET according to an off-signal.

The input end of the driving circuit 3 is respectively connected with a signal Turn-on signal and a signal Turn-off signal, and the output end outputs DRV, wherein the high level of the DRV is VCC and the low level is the grounding voltage. The output DRV drives the MOSFET to achieve the purpose of opening and closing the MOSFET.

The gate driving voltage DRV adjusting circuit 4 is connected with the MOSFET and is used for reducing the gate driving voltage before the MOSFET is turned off; the DRV adjusting circuit comprises a first control circuit, a second control circuit and N discharging circuits, wherein N is not less than 2. Those skilled in the art can flexibly design the number of the discharge circuits, such as 2, 3, 5, etc., which will not be described herein.

Referring to fig. 1, the MOSFET may include an NMOS transistor (denoted by MN0 in fig. 1) and a diode (denoted by D0 in fig. 1), for example. The gate of the MOSFET is input (or connected) to DRV, and the source and substrate terminals are grounded. The diode is a parasitic diode of the NMOS tube.

The first control circuit is described in detail below.

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

For example, the reference voltage may be the internal power supply VCCI, or a person skilled in the art may fix the reference voltage to a fixed value, and the person skilled in the art may flexibly design the VCCI or the fixed value, for example, 4 volts, 5 volts, 6 volts, and so on, which are not described herein again. When the power supply voltage VCC is greater than the DRV adjustment circuit 4 minimum allowable operating voltage and not higher than 5 volts, the internal power supply VCCI is equal to the power supply voltage VCC.

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

The first controller is specifically configured to:

comparing the DRV with a reference voltage, the drain voltage with a first threshold voltage Vth1, and the drain voltage with a second threshold voltage Vth 2; the first threshold voltage Vth1 is smaller than the second threshold voltage Vth2.

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 be turned on, and the first control circuit is in a turned-on state, reducing the voltage value of DRV to the reference voltage. When the voltage value of the 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 be turned off, the first control circuit is in an off state, and the voltage value of the DRV stops decreasing.

It should be noted that the size of DRV is generally determined by the size of VCC when the MOSFET starts to conduct. Then when the MOSFET starts to conduct, since DRV is equal to VCCI, the aforementioned comparison of DRV with the reference voltage also corresponds to the comparison of VCCI with VCC when the MOSFET starts to conduct.

Meanwhile, 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, at least the power supply voltage of the output driving part of the device is generally designed to be VCC, and the power supply of the other devices is designed to be an internal power supply VCCI. For example, the power supply of the aforementioned on-control circuit 1, off-control circuit 2 and driving circuit 3 may be VCC, and the power supply of the rest of the circuits may be an internal power supply VCCI. The synchronous rectification control circuit may include a plurality of internal power supplies VCCI.

The voltage variation range of the power supply VCC is large, and in a typical application, VCCI is equal to 5V; when the power supply VCC is larger than 5V, the internal power supply VCCI is equal to 5V; and when the power supply is higher than the minimum allowable operating voltage and not higher than 5V, the internal power supply VCCI is equal to the power supply VCC.

In one example, the first threshold voltage may specifically be negative 80 millivolts or negative 70 millivolts. The second threshold voltage may specifically be negative 40 millivolts or negative 30 millivolts.

Of course, the function of the first controller may be implemented using a circuit configuration. Still referring to fig. 1, illustratively, the first controller includes at least: 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:

the inverting input of the first comparator 41 is used for inputting DRV, and the positive input of the first comparator 41 is used for inputting a reference voltage (e.g., VCCI).

A positive input terminal of the second comparator 42 is used for inputting the first threshold voltage Vth1, and a negative input terminal of the second comparator 42 is used for inputting the drain voltage.

A first input of the nor gate 43 is connected to the output of the first comparator 41 and a second input of the nor gate 43 is connected to the output of the second comparator 42.

The third comparator 44 has a positive input terminal for inputting the drain voltage and a negative input terminal for inputting the second threshold voltage Vth2.

An input of the inverter 45 is connected to an output of the third comparator 44.

A first input end of the NAND gate 47 is connected with the output end of the NOR gate 43, and a second input end of the NAND gate 47 is connected with the output end of the inverter 45; the output of the nand-gate 47 is used to output a control signal for controlling the first switch 46.

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

In one example, the pull-down current source circuit 48 may be embodied as a conventional current source.

The functions of the above devices are as follows:

the first comparator 41 is used to detect the magnitude of DRV and the internal power supply VCCI, and when DRV is greater than the internal power supply VCCI, the first comparator 41 outputs a low level. The first comparator 41 outputs a low level as one of the conditions for the first switch 46 to be closed.

The first comparator 41 is used for detecting the magnitude of the drain Voltage (VDET) and the first threshold voltage (Vth 1), and when the drain voltage is greater than the first threshold voltage Vth1, the second comparator 42 outputs a low level. The second comparator 42 outputs a low level as the second condition for the first switch 46 to be closed.

Both the first input terminal and the second input terminal of the nor gate 43 input a low level, and the nor gate 43 outputs a high level.

The drain voltage is less than the second threshold voltage Vth2, and the third comparator 44 outputs a high level.

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.

The nor gate 43 outputs a high level, the inverter 45 outputs a low level, and the nand gate 47 outputs a high level, when the first switch is closed, otherwise the first switch 46 is opened. Specifically, when DRV is greater than the internal power supply VCCI and the drain voltage is greater than the first threshold voltage, that is, both conditions of the first switch 46 being closed are satisfied, the first switch 46 is closed, the pull-down current source circuit 48 is turned on, DRV is discharged, and the voltage is decreased. One end of the pull-down current source circuit 48 is connected to the first switch 46, and the other end is grounded. The first switch 46 is opened when DRV is not higher than the internal power supply VCCI, and the first switch 46 is opened when one of two conditions for forcibly opening the first switch 46 when the drain voltage is greater than the second threshold voltage, i.e., the first switch 46 is closed, is not satisfied.

In one example, the first switch 46 may be an NMOS switch tube or a transmission gate circuit.

The second control circuit is described in detail below.

When DRV is not reduced to the desired target value V _REG And is greater than the third threshold Vth3, the second control circuit controls the controlled circuit 413 to be turned on.

When DRV is equal to or less than the third threshold Vth3, the second control circuit controls the controlled circuit 413 to be turned off.

Still referring to fig. 1, the second control circuit illustratively includes at least N discharge circuits and a fourth comparator 49, a second switch 410, and an amplifier 411. The N discharge circuits include an always-on discharge circuit 412 and a controlled circuit 413.

The second control circuit may be implemented using a variety of circuit configurations. In one example, the second control circuit includes at least N discharge circuits and a second controller.

The second controller is specifically configured to:

the magnitude of DRV and the third threshold voltage Vth3, and the magnitude of the drain voltage and the second threshold voltage Vth2 are compared.

When DRV is equal to or greater than the third threshold voltage and the drain voltage is equal to or greater than the second threshold voltage (Vth 2), the second controller controls the controlled circuit 413 to be turned on. When DRV is less than the third threshold voltage and the drain voltage is equal to or greater than the second threshold voltage, the second controller controls the controlled circuit 413 to be turned off. The N discharge circuits are connected with the MOSFET; both the normally-on discharge circuit 412 and the controlled circuit 413 can discharge to reduce the voltage value of DRV when turned on.

In one example, the always-on discharge circuit 412 and the controlled circuit 413 discharge faster at the same time than only the always-on discharge circuit 412.

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

In one example, the always-on discharge circuit 412 may include a second NMOS transistor. The controlled circuit 413 may include one or more first NMOS transistors, and those skilled in the art may flexibly design the number of the first NMOS transistors, such as 1, 2, 3, etc., which is not described herein.

The first NMOS transistor and the second NMOS transistor have different parameters, and the parameters may be different sizes.

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 turned off, the first NMOS transistor does not adjust DRV any more, and there is no discharge current, and only the second NMOS transistor adjusts DRV, so the discharge speed is slowed down. The first NMOS tube is used as a rapid adjusting tube of the DRV; the second NMOS tube is used as a regulating tube of the DRV.

The second controller may specifically be a chip.

Of course, the circuit structure may also be adopted to realize the function of the second controller. Illustratively, still referring to fig. 1, the second controller includes at least: a fourth comparator 49, a second switch 410 and an amplifier 411. Wherein:

the positive input terminal of the fourth comparator 49 is used for inputting DRV, and the negative input terminal of the fourth comparator 49 is used for inputting the third threshold voltage Vth3.

The second switch 410 is connected to the output of the fourth comparator 49, and the second switch 410 is used to open or close the controlled circuit 413.

A forward input terminal of the amplifier 411 is used to input the drain voltage, and an inverting input terminal of the amplifier 411 is used to input the second threshold voltage Vth2. The output end of the amplifier 411 is connected to the gate of the first NMOS transistor (MN 1) and the gate of the second NMOS transistor (MN 2), respectively. The drain electrode of the first NMOS tube is connected with the MOSFET through a second switch 410; the drain electrode of the second NMOS tube is connected with the DRV; the source electrodes and the substrate of the first NMOS tube and the second NMOS tube are grounded.

In one example, the drain voltage is greater than the second threshold voltage Vth2, and the amplifier 411 outputs a high level. The output end of the amplifier 411 is connected to the drain of the first NMOS transistor and the drain of the second NMOS transistor, respectively, the drain of the first NMOS transistor is connected to the MOSFET through the second switch 410, and when the amplifier 411 outputs a high level and the second switch 410 is turned on, 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, and after the second switch 410 is closed, the first NMOS transistor MN1 and the second NMOS transistor MN2 discharge together, and enter a fast discharge stage. The other end of the second switch 410 is connected to the DRV. If the DRV voltage is equal to the third threshold voltage Vth3, the second switch 410 is turned on, the first NMOS transistor MN1 stops discharging, and only the second NMOS transistor MN2 discharges the DRV more slowly, so that the DRV is prevented from being over-discharged, and the DRV adjustment circuit is more easily brought into a stable adjustment state.

The functions of the above devices are as follows:

the fourth comparator 49 is used for comparing DRV with the third threshold voltage Vth3, and when DRV is greater than the third threshold voltage Vth3, the fourth comparator 49 outputs a high level. The fourth comparator 49 outputs a high level as a condition that the second switch 410 is closed.

In one example, the third threshold voltage Vth3 may be specifically negative 10 millivolts or negative 5 millivolts.

The amplifier 411 is used for comparing the drain voltage with the second threshold voltage Vth2, and when the drain voltage is greater than the second threshold voltage Vth2, the amplifier 411 outputs a high level.

Typical applications of the synchronous rectification control circuit may include two types:

fig. 2 is a partial node voltage curve diagram of the synchronous rectification control circuit according to the embodiment of the present invention. As shown in fig. 2, when the secondary side freewheeling starts, the MOSFET is in a closed state, the secondary side current flows through the parasitic diode D0 of the MOSFET to freewheel, and a negative source-drain Voltage (VDS) is formed across the parasitic diode D0, i.e., the VDET is changed from a positive voltage drop to a negative voltage. When VDET drops to the turn-on threshold (Vth _ on), the MOSFET will turn on after a delay (td 1) and DRV goes from low to high. In 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 freewheel current decreases, VDET gradually rises (changes from negative to positive), and when VDET rises to the second threshold voltage Vth2, the second switch 410 closes, rapidly discharging DRV during tc1, causing the DRV regulation circuit to enter a stable regulation state more quickly. When the DRV rapidly drops to the third threshold voltage Vth3, the second switch 410 is turned on, and the discharging speed of the DRV is slowed down, so that the DRV can be prevented from being over-discharged, and the DRV adjusting circuit can more easily enter a stable adjusting state.

After tc2 time, DRV dropped to V _REG And enters a stable regulation state. DRV slave V subsequently _REG Continuing to ramp down, by this mechanism VDS is adjusted around the second threshold voltage Vth2 when the current through the MOSFET is relatively low. As the freewheel current continues to decrease, VDET rises to the turn-off threshold voltage (Vth _ off), the MOSFET turns off after a time td2, where DRV is already very low, which speeds the MOSFET turn-off.

Second application, fig. 3 is a partial node voltage curve diagram of the synchronous rectification control circuit in the second application according to the embodiment of the present invention. As shown in fig. 3, when the secondary side freewheeling starts, the MOSFET is in an off state, the secondary side current flows through the parasitic diode D0 of the MOSFET to realize freewheeling, and a negative VDS voltage is formed across the parasitic diode, i.e., VDET decreases from a positive voltage to a negative voltage. When VDET drops to the turn-on threshold Vth _ on, the MOSFET will turn on after a time delay td1, and DRV changes from low to high. In 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 negative to positive), and when VDET rises to the first threshold voltage Vth1, the first switch 46 closes, discharging DRV to the VCCI voltage, and then closing the first switch 46. The subsequent DRV adjustment process is the same as the application scenario one in fig. 2, and is not described herein.

By comparison, in the second application, the pull-down current source circuit 48 is added to discharge DRV from VCC to VCCI, so that DRV is reduced from VCCI to VCCI _REG The procedure of (2) is independent of VCC, so that the adjustment time required in the second occasion is consistent with the adjustment time required in the first occasion.

In summary, the synchronous rectification control circuit reduces DRV from high level to V _REG And preventing excessive drop of DRV, thereby increasing both the turn-off speed of the MOSFET and the stability.

Referring to fig. 4, a switching power supply using a synchronous rectification control circuit is shown. Illustratively, the switching power supply using the synchronous rectification control circuit at least comprises: synchronous rectification control circuit, electric capacity 5, transformer 6, anodal interface 7 and negative pole interface 8.

In one example, the capacitor 5 is connected to the positive interface 7 and the negative interface 8, respectively, and the capacitor 5 is used for storing electric energy.

The negative interface 8 is connected to ground.

The positive electrode interface 7 is connected with one end of the transformer 6, and the other end of the transformer 6 is connected with the drain terminal of the MOSFET; the source terminal and the substrate of the MOSFET are grounded. The transformer 6 includes a primary side inductance and a secondary side inductance. Illustratively, the left side is the primary side inductance and the right side is the secondary side inductance. The secondary freewheeling means that current flows from the "Ground (GND) -MOSFET-secondary inductor-positive interface 7 (VOUT +) on the secondary side".

The synchronous rectification control circuit is connected with the MOSFET.

Fig. 4 is a switching power supply employing a synchronous rectification control circuit. As shown in fig. 4, the VDET terminal of the synchronous rectification control circuit detects the VDS voltage of the power transistor MOSFET (MN 0), and outputs DRV to control the power transistor MN0 to turn on and off. The gate of MN0 is connected with DRV, the source end and the substrate are grounded, and the drain end is connected with one end of the transformer 6. During the period when the secondary side starts freewheeling and the MOSFET is not turned on, freewheeling occurs through parasitic diode D0. The other end of the transformer 6 is a positive electrode interface 7 (VOUT +), a negative electrode interface 8 (VOUT-) is a grounding terminal, and the capacitor 5 is connected between the positive electrode interface 7 and the negative electrode interface 8.

The embodiments of the present invention also claim the DRV adjustment circuit in all the embodiments.

The embodiments in the present description are described in a progressive manner, each embodiment focuses on differences from other embodiments, and the same and similar parts among the embodiments are referred to each other.

The principle and implementation of the embodiments of the present invention are explained herein by applying specific examples, and the above descriptions of the embodiments are only used to help understand the core ideas of the embodiments of the present invention; meanwhile, for a person skilled in the art, according to the idea of the embodiment of the present invention, the specific implementation manner and the application range may be changed. In view of the foregoing, the description is not intended to limit embodiments of the invention.

Claims (10)

1. A synchronous rectification control circuit, comprising:

the conduction control circuit is used for sending out a conduction signal;

the turn-off control circuit is used for sending a turn-off signal;

the driving circuit is respectively connected with the turn-on control circuit, the turn-off control circuit and the MOSFET and is used for turning on the MOSFET according to the turn-on signal or turning off the MOSFET according to the turn-off signal;

the grid driving voltage DRV adjusting circuit is connected with the MOSFET and is used for reducing the grid driving voltage before the MOSFET is turned off; the DRV adjusting circuit comprises a first control circuit, a second control circuit and N discharging circuits, wherein N is not less than 2;

when the DRV is higher than the reference voltage, the first control circuit is in a conducting state to reduce the voltage value of the 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 a second threshold voltage, the first control circuit is in an off state;

the N discharge circuits are connected with the MOSFET; when any discharge circuit is conducted, the discharge circuit can discharge to reduce the voltage value of the DRV; any discharge circuit is a normally-on discharge circuit or a controlled circuit controlled by the second control circuit;

the second control circuit is used for controlling the second control circuit to control the second control circuit when the DRV is not reduced to the required target value V _REG And when the DRV is less than or equal to the third threshold value, the controlled circuit is controlled to be switched on, and when the DRV is less than or equal to the third threshold value, the controlled circuit is controlled to be switched off.

2. The synchronous rectification control circuit of claim 1, wherein the first control circuit comprises:

a pull-down current source circuit;

the first controller is used for controlling the pull-down current source circuit to be conducted when the DRV is higher than the reference voltage so as to reduce the voltage value of the DRV to the reference voltage; and controlling the pull-down current source circuit to be switched off when the voltage value of the DRV is equal to the reference voltage.

3. The synchronous rectification control circuit of claim 2,

the first controller is specifically configured to:

comparing the DRV with the reference voltage, the drain voltage with a first threshold voltage, and the drain voltage with a second threshold voltage; the first threshold voltage is less than the second threshold voltage;

if the DRV is larger than the reference voltage, and the drain voltage is larger than a first threshold voltage and smaller than a second threshold voltage, controlling the pull-down current source circuit to be closed;

and if the 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 be disconnected.

4. The synchronous rectification control circuit of claim 3, wherein the first controller comprises:

a first comparator, wherein the inverting input terminal of the first comparator is used for inputting the DRV, and the forward input terminal of the first comparator is used for inputting the reference voltage;

a second comparator, wherein a positive input terminal of the second comparator is used for inputting the first threshold voltage, and a negative input terminal of the second comparator is used for inputting the drain voltage;

a first input end of the NOR gate is connected with the output end of the first comparator, and a second input end of the NOR gate is connected with the output end of the second comparator;

a positive input end of the third comparator is used for inputting the drain voltage, and a negative input end of the third comparator is used for inputting the second threshold voltage;

the input end of the inverter is connected with the output end of the third comparator;

a first switch;

the first input end of the NAND gate is connected with the output end of the NOR gate, and the second input end of the NAND gate is connected with the output end of the inverter; and the output end of the NAND gate is used for outputting a control signal for controlling the first switch.

5. The synchronous rectification control circuit of claim 1,

the second control circuit is specifically configured to:

if DRV is larger than or equal to the third threshold voltage and the drain voltage is larger than or equal to the second threshold voltage, controlling the controlled circuit to be conducted;

and if the DRV is less than the third threshold voltage and the drain voltage is greater than or equal to the second threshold voltage, controlling the controlled circuit to be switched off.

6. The synchronous rectification control circuit of claim 5, wherein the second control circuit comprises:

a fourth comparator, a positive input end of which is used for inputting the DRV, and a negative input end of which is used for inputting the third threshold voltage;

a control end of the second switch is connected with an output end of the fourth comparator, and the second switch is used for opening or closing the controlled circuit;

and the positive input end of the amplifier is used for inputting the drain voltage, the negative input end of the amplifier is used for inputting the second threshold voltage, and the output end of the amplifier is connected with the discharge circuit.

7. A DRV adjustment circuit, for use in a synchronous rectification control circuit as claimed in any one of claims 1 to 6,

the DRV adjusting circuit is connected with the MOSFET and used for reducing the grid driving voltage before the MOSFET is turned off;

the DRV adjustment circuit includes: the device comprises a first control circuit, a second control circuit and N discharge circuits, wherein N is not less than 2; wherein:

when the DRV is higher than the reference voltage, the first control circuit is in a conducting state to reduce the voltage value of the 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 a second threshold voltage, the first control circuit is in an off state;

the N discharge circuits are connected with the MOSFET; any discharge circuit can discharge when being conducted so as to reduce the voltage value of the DRV; any discharge circuit is a normally-on discharge circuit or a controlled circuit controlled by the second control circuit;

the second control circuit is configured to: and controlling the controlled circuit to be switched on when the DRV does not fall to the required target value VREG and is greater than a third threshold value, and controlling the controlled circuit to be switched off when the DRV is less than or equal to the third threshold value.

8. The synchronous rectification control circuit of claim 6, wherein the controlled circuit comprises a first NMOS transistor; the normally-on discharge circuit comprises a second NMOS tube;

the output end of the amplifier is respectively connected with the grid electrode of the first NMOS tube and the grid electrode of the second NMOS tube; the drain electrode of the first NMOS tube is connected with the grid electrode of the MOSFET through the second switch;

the drain electrode of the second NMOS tube is connected with the grid electrode of the MOSFET;

the source electrodes and the substrate of the first NMOS tube and the second NMOS tube are grounded.

9. The synchronous rectification control circuit of claim 8, wherein the first NMOS transistor and the second NMOS transistor have different parameters.

10. A switching power supply using a synchronous rectification control circuit, comprising: a synchronous rectification control circuit as claimed in any one of claims 1 to 9, and, a capacitor, a transformer, a positive interface and a negative interface;

the capacitor is respectively connected with the positive electrode interface and the negative electrode interface and is used for storing electric energy;

the negative electrode interface is grounded;

the positive electrode interface is connected with one end of the transformer; the other end of the transformer is connected with the drain end of the MOSFET; the source end and the substrate of the MOSFET are grounded;

the synchronous rectification control circuit is connected with the MOSFET.

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