CN211429196U - Synchronous rectifier control circuit and flyback voltage conversion circuit thereof - Google Patents

Abstract

The utility model provides a control circuit and flyback voltage conversion circuit for controlling synchronous rectifier tube. A control circuit for controlling a synchronous rectifier tube comprises a controllable current source and a current control circuit, wherein the controllable current source is used for being coupled with a control end of the synchronous rectifier tube, the current control circuit is used for being coupled with the synchronous rectifier tube and the controllable current source, and when the synchronous rectifier tube is conducted, the current control circuit controls the controllable current source at least based on the voltage difference between two ends of the synchronous rectifier tube. The utility model discloses a control circuit and flyback voltage conversion circuit are used for accurately turning off synchronous rectifier, improve system efficiency and stability.

Description

Synchronous rectifier control circuit and flyback voltage conversion circuit thereof

Technical Field

The utility model relates to an electron field, concretely but not limited to relate to a control circuit and flyback voltage conversion circuit for controlling synchronous rectifier tube.

Background

The flyback voltage converter includes a primary circuit and a secondary circuit, and as shown in fig. 1, the primary current and the secondary circuit are isolated by a transformer T. The primary side switch Q of the primary side circuit transfers energy to the secondary side through switching action. The secondary side circuit comprises a rectifying device D, when the primary side switch Q is switched off, the rectifying device D is switched on and used for supplying power to the output capacitor Co and the load through follow current, and when the follow current is reduced to zero, the rectifying device D is switched off and supplies power to the load through the output capacitor Co. In order to improve the power efficiency, the secondary rectifier device usually employs a synchronous rectifier, and the high-efficiency rectification function is realized by timely controlling the on and off of the synchronous rectifier. However, this poses a challenge to the accuracy of the turn-off time point of the secondary-side synchronous rectifier. Because the turn-off delay will result in the common use of the secondary synchronous rectifier and the primary switch, the reliability and stability of the system will be raised. If the turn-off is advanced, the synchronous rectification function is turned off when the follow current is large, and the system efficiency is reduced.

In view of the above, there is a need to provide a new structure or control method to solve at least some of the above problems.

SUMMERY OF THE UTILITY MODEL

To one or more problems among the prior art, the utility model provides a control circuit and flyback voltage conversion circuit for controlling synchronous rectifier tube.

According to an aspect of the utility model, a control circuit for controlling synchronous rectifier is provided, include: the controllable current source is coupled with the control end of the synchronous rectifier tube; and the current control circuit is provided with an input end and an output end, the input end of the current control circuit is coupled with the synchronous rectifier tube, and the output end of the current control circuit is coupled with the control end of the controllable current source. When the synchronous rectifier tube is conducted, the current control circuit controls the controllable current source at least based on the voltage difference between two ends of the synchronous rectifier tube.

In one embodiment, the controllable current source comprises a current source and a switch connected in series, an output terminal of the current control circuit is coupled to a control terminal of the switch, and the current control circuit selectively controls the switch to be turned on based on at least a voltage difference between two terminals of the synchronous rectifier. In another embodiment, the controllable current source comprises a voltage controlled current source.

In one embodiment, when a preset condition is met, the switch is controlled to be conducted, and the current of the current source flows out from the control end node of the synchronous rectifier tube.

In one embodiment, the control circuit further includes a voltage detection circuit having an input terminal and an output terminal, the input terminal of the voltage detection circuit is coupled to the synchronous rectifier for obtaining a detection signal of a voltage difference across the synchronous rectifier.

In one embodiment, the current control circuit includes a comparison circuit having a first input terminal, a second input terminal, and an output terminal, the first input terminal of the comparison circuit is coupled to the output terminal of the voltage detection circuit, the second input terminal of the comparison circuit is coupled to the first threshold signal, the output terminal of the comparison circuit is coupled to the control terminal of the switch, and the comparison circuit controls the switch to be turned on based on the comparison of the detection signal and the first threshold signal. In one embodiment, the detection signal is the drain-source voltage of the MOSFET synchronous rectifier, and when the detection signal is greater than the first threshold signal, the control switch is turned on, so that current flows out from the control end node of the synchronous rectifier, and the gate voltage of the MOSFET is reduced.

In one embodiment, the comparison circuit comprises a hysteresis comparator.

In one embodiment, the current control circuit further comprises a signal generating circuit having an input terminal and an output terminal, the input terminal of the signal generating circuit is coupled to the output terminal of the comparing circuit, and the output terminal of the signal generating circuit is coupled to the control terminal of the switch. When the preset condition is met, the signal generating circuit controls the switch to be intermittently conducted at the set duty ratio.

In one embodiment, the control circuit further comprises: the first input end of the second comparison circuit is coupled with the second threshold signal, and the second input end of the second comparison circuit is coupled with the output end of the voltage detection circuit; the first input end of the third comparison circuit is coupled with the output end of the voltage detection circuit, and the second input end of the third comparison circuit is coupled with a third threshold signal; and the trigger circuit is provided with a first input end, a second input end and an output end, wherein the first input end of the trigger circuit is coupled with the output end of the second comparison circuit, the second input end of the trigger circuit is coupled with the output end of the third comparison circuit, the output end of the trigger circuit is coupled with the control end of the synchronous rectifier tube, and the third threshold signal is greater than the second threshold signal.

In one embodiment, the synchronous rectifier comprises a field effect transistor, and the detection signal is a drain-source voltage sampling signal representing the drain-source voltage of the synchronous rectifier.

According to another aspect of the present invention, a flyback voltage converter circuit includes a primary circuit and a secondary circuit, wherein the secondary circuit includes a synchronous rectifier and a control circuit as described in any of the above embodiments.

The utility model provides a control circuit and flyback voltage conversion circuit can reduce the control end voltage of synchronous rectifier tube through the current source for synchronous rectifier tube is turn-offed by the accuracy, has improved system efficiency and stability.

Drawings

Fig. 1 shows a flyback voltage converter circuit;

fig. 2 shows a control circuit 20 for controlling the synchronous rectifier tube 10 according to an embodiment of the present invention;

fig. 3 shows a circuit schematic of the control circuit 30 according to an embodiment of the present invention;

fig. 4 is a schematic diagram of a flyback voltage converter according to an embodiment of the present invention;

fig. 5 shows a schematic diagram of a waveform according to an embodiment of the invention;

fig. 6 shows a schematic diagram of a waveform according to another embodiment of the present invention;

fig. 7 shows a control circuit according to another embodiment of the present invention;

the same reference numbers in different drawings identify the same or similar elements or components.

Detailed Description

For further understanding of the present invention, preferred embodiments of the present invention will be described below with reference to examples, but it should be understood that these descriptions are only for the purpose of further illustrating the features and advantages of the present invention, and are not intended to limit the claims of the present invention.

The description in this section is for exemplary embodiments only, and the present invention is not limited to the scope of the embodiments described. Combinations of different embodiments, or technical features of different embodiments, or similar prior art means, or technical features of embodiments, may be substituted for each other within the scope of the present invention.

The term "coupled" or "connected" in this specification includes both direct and indirect connections. An indirect connection is a connection made through an intermediate medium, such as a connection through an electrically conductive medium, such as a conductor, where the electrically conductive medium may contain parasitic inductance or parasitic capacitance; indirect connections may also include connections through other active or passive devices that perform the same or similar function, such as connections through switches, driver circuits, signal amplification circuits, or follower circuits, among other circuits or components.

Fig. 2 shows a control circuit 20 for controlling the synchronous rectifier tube 10 according to an embodiment of the present invention. The control circuit 20 comprises a controllable current source 21 and a current control circuit 22. Wherein the controllable current source 21 is coupled to the control terminal of the synchronous rectifier 10. The current control circuit 22 is coupled to the synchronous rectifier 10 and the controllable current source 21. The current control circuit 22 has an input terminal and an output terminal, the input terminal of the current control circuit 22 is coupled to the synchronous rectifier 10, and the output terminal of the current control circuit 22 is coupled to the control terminal of the controllable current source 21. When the synchronous rectifier 10 is turned on, the current control circuit 22 controls the controllable current source 21 based on at least the detection signal Vds representing the voltage difference between the two ends of the synchronous rectifier 10. In one embodiment, synchronous rectifier tube 10 comprises a Field Effect Transistor (FET). In a preferred embodiment, synchronous rectifier tube 10 comprises a Metal Oxide Semiconductor Field Effect Transistor (MOSFET). In the illustrated embodiment, the synchronous rectification tube 10 is a PNP MOSFET, and the synchronous rectification tube 10 has a drain, a gate and a source, where the gate is a control terminal of the synchronous rectification tube 10, and a voltage difference between two terminals of the synchronous rectification tube 10 is a drain-source voltage of the MOSFET. In other embodiments, the synchronous rectifier may also be another type of switching tube, in which the synchronous rectifier is connected in parallel with a diode, and the diode may be a parasitic diode. In one embodiment, the synchronous rectifier comprises a junction field effect transistor. The current control circuit 22 is coupled to the drain and/or source of the synchronous rectifier 10, and is configured to obtain a detection signal Vds representing the drain-source voltage of the synchronous rectifier 10. The detection signal Vds may be a voltage proportional to the drain voltage of the synchronous rectifier 10, wherein the ratio may be 1 or less than 1. The detection signal Vds may be a signal proportional to the drain-source voltage (the difference between the drain voltage and the source voltage) of the synchronous rectifier 10, and the ratio may be 1 or smaller or larger than 1. In one embodiment, the detection signal Vds is a drain-source voltage sampling signal of the synchronous rectifier 10. The anode of the controllable current source 21 is coupled to the control end node of the synchronous rectifier tube 10, and the cathode of the controllable current source 21 is coupled to the low-order voltage node. In the embodiment shown in fig. 2, the control terminal node is coupled to the gate of the synchronous rectifier 10. In one embodiment, the cathode of the controllable current source 21 is coupled to the ground reference of the control circuit 20. When the voltage at the gate voltage of the synchronous rectifier 10 is pulled high by the driving circuit, the synchronous rectifier 10 is turned on. When the current control circuit 22 detects that the drain-source voltage Vds satisfies the preset condition, the current control circuit 22 outputs a control signal for controlling the current of the current source 21 to flow out from the control end node of the synchronous rectifier tube 10, so that the control end voltage of the synchronous rectifier tube 10 is reduced. By dynamically reducing the voltage at the control end of the synchronous rectifier tube, the synchronous rectifier tube can be turned off under lower continuous current, and the system efficiency is improved. Meanwhile, the turn-off delay can be avoided, and the condition that the system works abnormally due to the fact that the primary side switch and the secondary side synchronous rectifier tube are simultaneously conducted can be prevented in the application of the flyback voltage conversion circuit.

Fig. 3 shows a circuit schematic diagram of the control circuit 30 according to an embodiment of the present invention. The control circuit 30 includes a controllable current source and current control circuit 32. In the illustrated embodiment, the controllable current source includes a current source I1 and a switch S1 connected in series, and the output terminal of the current control circuit 32 is coupled to the control terminal of the switch S1. The anode terminal of the current source I1 is coupled to the control terminal of the synchronous rectifier 10, the cathode terminal of the current source I1 is coupled to the first terminal of the switch S1, and the other terminal of the switch S1 is grounded. In another embodiment, the positions of the current source I1 and the switch S1 may be reversed.

In one embodiment, the reference ground of the control circuit 30 is coupled to the source terminal of a synchronous rectifier, wherein the synchronous rectifier comprises a field effect transistor. In one embodiment, the synchronous rectifier is coupled between the output of the flyback voltage converter and the secondary winding, and the ground reference of the control circuit 30 on the secondary side is the source terminal of the transistor on the synchronous rectifier, which is not common to the ground reference of the secondary side circuit. In another embodiment, the synchronous rectifier tube is coupled as a low tube to the reference ground of the secondary circuit, and the reference ground of the control circuit 30 is common to the reference ground of the secondary circuit.

When the preset condition is met, the current control circuit 32 controls the switch S1 to be turned on, and the current of the current source I1 flows out from the control end node of the synchronous rectifier tube, so as to pull down the voltage at the control end of the synchronous rectifier tube.

In the illustrated embodiment, the current control circuit 32 includes a comparison circuit C1. The comparator C1 has a first input terminal (in the illustrated embodiment, the non-inverting input terminal) receiving the detection signal Vds, a second input terminal (in the illustrated embodiment, the inverting input terminal) coupled to the first threshold signal Vref1, and an output terminal coupled to the control terminal of the switch S1. The comparison circuit C1 is used to compare the detection signal Vds representing the voltage difference between two ends of the synchronous rectifier with the first threshold signal Vref1, and when the detection signal Vds is greater than the first threshold signal Vref1, the control switch S1 is turned on.

In one embodiment, the control circuit 30 further comprises a voltage detection circuit having an input terminal coupled to the synchronous rectifier 10 for obtaining a voltage difference across the synchronous rectifier 10 and an output terminal for providing a detection signal, such as a drain-source voltage Vds, wherein the first input terminal of the comparison circuit C1 is coupled to the output terminal of the voltage detection circuit.

In one embodiment, the preset condition is that the switch S1 is controlled to be turned on for a period of time when the synchronous rectifier 10 is detected to be turned on and the voltage difference across the synchronous rectifier rises to be greater than the threshold (Vref 1).

In one embodiment, when the predetermined condition is satisfied, the control switch S1 is turned on for a predetermined time.

In one embodiment, the comparison circuit includes a hysteresis comparator for controlling the turn-off of the switch S1.

In one embodiment, the control switch S1 is turned off when the gate signal is lower than a predetermined value.

In another embodiment, when the preset condition is met, the current control circuit controls the switch S1 to conduct intermittently at a set duty cycle for a period of time. The current control circuit may further include a signal generating circuit, an input terminal of the signal generating circuit is coupled to the output terminal of the comparing circuit, and an output terminal of the signal generating circuit is coupled to the control terminal of the switch for providing a signal of intermittently turning on the switch S1 with alternating high and low levels.

In the embodiment shown in fig. 3, the control circuit further includes a turn-on control circuit and a turn-off control circuit, wherein the turn-on control circuit is coupled to the detection signal Vds for turning on the synchronous rectifier 10 when the detection signal Vds is less than the second threshold signal Vref 2. The turn-off control circuit is coupled to the detection signal Vds, and turns off the synchronous rectifier 10 when the detection signal Vds is greater than a third threshold signal Vref3, wherein the third threshold signal Vref3 is greater than the second threshold signal Vref 2. Wherein the first threshold signal Vref1 is less than the third threshold signal Vref2 and greater than the second threshold signal Vref 2. In the illustrated embodiment, the conduction control circuit includes a second comparison circuit C2 having a non-inverting input receiving a second threshold signal Vref2 and an inverting input receiving the detection signal Vds; the shutdown control circuit comprises a third comparison circuit C3, the non-inverting input of which receives the detection signal Vds and the inverting input of which receives a third threshold signal Vref 3. The control circuit 30 further includes a flip-flop circuit 33 and a driving circuit 34, the flip-flop circuit providing a Pulse Width Modulation (PWM) signal SR for turning on the synchronous rectifier 10 when the PWM signal SR is in a first state such as a high level and for turning off the synchronous rectifier 10 when the PWM signal SR is in a second state such as a low level based on output signals of the comparison circuit C2 and the comparison circuit C3. The input terminal of the driving circuit 34 is coupled to the trigger circuit 33 for receiving the signal SR, the output terminal of the driving circuit 34 is coupled to the control terminal of the synchronous rectifier 10, and the driving circuit 34 amplifies the signal SR to provide a driving signal suitable for driving the synchronous rectifier 10, so as to control the on/off of the synchronous rectifier 10. The conduction control circuit may further include an and gate and a primary side switch conduction detection circuit, a first input terminal of the and gate is coupled to the output terminal of the comparison circuit C2, a second input terminal of the and gate is coupled to the output terminal of the primary side switch conduction detection circuit, and an output terminal of the and gate is coupled to a set input terminal of the trigger circuit, when the primary side switch conduction detection circuit detects that the primary side switch is conducted, and when a difference between voltages Vds at two ends of the synchronous rectifier tube is smaller than a second threshold Vref2, the conduction of the synchronous rectifier tube 10 is controlled.

Fig. 4 shows a schematic diagram of a flyback voltage converter circuit according to an embodiment of the present invention, where the flyback voltage converter circuit includes a primary circuit and a secondary circuit, the primary circuit is coupled to a primary winding of a transformer T, and the primary circuit includes a primary switch Q. The secondary circuit is coupled to the secondary winding of the transformer T, and includes the synchronous rectifier 10 and the control circuit 40. The first end of the control circuit 40 is coupled to the first end D of the synchronous rectifier 10, the other end of the control circuit 40 is coupled to the second end S of the synchronous rectifier 10, and the output end of the control circuit 40 is coupled to the control end G of the synchronous rectifier 10. The control circuit 40 may be the control circuit described in any of the embodiments of the present disclosure.

In one embodiment, the control circuit 40 is fabricated on the same semiconductor substrate as the semiconductor die.

In another embodiment, the control circuit comprises the synchronous rectifier itself. In one embodiment, control circuit 40 and synchronous rectifier tube 10 are packaged in the same electronic package to form a semiconductor electronic package.

In another embodiment, the controllable current source may comprise a voltage-controlled current source, and the voltage of the gate terminal of the synchronous rectifier is controlled by controlling the magnitude of the current source.

Fig. 5 shows a schematic waveform diagram according to an embodiment of the present invention. Referring to fig. 3, the signals from top to bottom are the voltage difference Vds between two ends of the synchronous rectifier (i.e. the drain-source voltage Vds in the embodiment shown in fig. 3), the control signal SR for controlling the synchronous rectifier, the control signal CT for controlling the switch S1, and the control terminal voltage Gate of the synchronous rectifier 10, respectively. At time t1, the voltage difference signal Vds between the two ends of the synchronous rectifier is smaller than the second threshold signal Vref2, and the control signal SR is converted into an effective value (shown as a high level) for turning on the synchronous rectifier. At time T2, the current control circuit detects that the signal Vds is greater than the first threshold signal Vref1, and the preset condition is satisfied, the current source control signal CT is converted to an effective value (high level) to turn on the switch S1 for a period of time T0. At this time, the current of the current source I1 flows out from the synchronous rectifier control end node, and the synchronous rectifier Gate voltage Gate decreases. In continuous current mode control, the reduced gate voltage is such that when Vds rises and reaches the third threshold Vref3, the gate voltage can drop below the MOSFET turn-on threshold more quickly, preventing the synchronous rectifier and primary switch from sharing. In the discontinuous current mode control, the reduced grid voltage enables the on-resistance to be increased, the synchronous rectifier tube is turned off under lower continuous current, and the efficiency of the synchronous rectifier tube is improved.

Fig. 6 shows a schematic waveform diagram according to another embodiment of the present invention. Referring to fig. 3, when the voltage difference signal Vds between the two terminals of the synchronous rectifier 10 is smaller than the second threshold signal Vref2, the control signal SR is converted to an effective value (shown as a high level) for turning on the synchronous rectifier. When the signal Vds is greater than the first threshold signal Vref1 and the preset condition is satisfied, the control signal CT is converted into a valid value (high level) to turn on the switch S1, and the current source control signal CT may be represented as a pulse width modulation signal having a duty ratio for intermittently turning on the switch S1. The duration T0 of the pwm signal may be a preset value, or may be adjusted according to the Gate voltage Gate or the voltage difference Vds between two ends of the synchronous rectifier. In one embodiment, the switch S1 is turned off when the Gate voltage Gate is less than a threshold signal.

Fig. 7 shows a control circuit according to another embodiment of the present invention, wherein the controllable current source comprises a current source I1 and a switch S1 connected in series and coupled to the control terminal of the synchronous rectifier 10. The current control circuit 72 includes two comparison circuits and a trigger circuit. When the voltage difference Vds between the two ends of the synchronous rectifier tube is greater than the first threshold Vref1, the first comparison circuit outputs a high level signal, and the trigger circuit outputs the high level signal for turning on the switch S1. When the voltage difference Vds between the two ends of the synchronous rectifier tube is smaller than the fourth threshold Vref1, the second comparison circuit outputs a high level signal, and the output signal of the trigger circuit is set to be low, so as to turn off the switch S1.

Those skilled in the art should understand that the logic controls such as "high" and "low", "set" and "reset", "and" in the above logic control can be exchanged or changed, and the subsequent logic control can be adjusted to achieve the same function or purpose as the above embodiments.

The description and applications of the present invention are illustrative and are not intended to limit the scope of the invention to the embodiments described above. The descriptions related to the effects or advantages in the specification may not be reflected in practical experimental examples due to uncertainty of specific condition parameters or influence of other factors, and the descriptions related to the effects or advantages are not used for limiting the scope of the invention. Variations and modifications of the embodiments disclosed herein are possible, and alternative and equivalent various components of the embodiments will be apparent to those skilled in the art. It will be clear to those skilled in the art that the present invention may be embodied in other forms, structures, arrangements, proportions, and with other components, materials, and parts, without departing from the spirit or essential characteristics thereof. Other variations and modifications of the embodiments disclosed herein may be made without departing from the scope and spirit of the present invention.

Claims (9)

1. A control circuit for controlling a synchronous rectifier, the control circuit comprising:

the controllable current source is coupled with the control end of the synchronous rectifier tube; and

the current control circuit is provided with an input end and an output end, the input end of the current control circuit is coupled with the synchronous rectifier tube, and the output end of the current control circuit is coupled with the control end of the controllable current source.

2. The control circuit of claim 1, wherein the controllable current source comprises a current source and a switch connected in series, and wherein the output terminal of the current control circuit is coupled to the control terminal of the switch.

3. The control circuit of claim 2, wherein when a predetermined condition is satisfied, the switch is turned on and the current of the current source flows from the control end node of the synchronous rectifier.

4. The control circuit of claim 2, wherein:

the control circuit further comprises a voltage detection circuit, the voltage detection circuit is provided with an input end and an output end, and the input end of the voltage detection circuit is coupled with the synchronous rectifier tube;

the current control circuit comprises a comparison circuit, wherein the comparison circuit is provided with a first input end, a second input end and an output end, the first input end of the comparison circuit is coupled with the output end of the voltage detection circuit, the second input end of the comparison circuit is coupled with the first threshold signal, and the output end of the comparison circuit is coupled with the control end of the switch.

5. The control circuit of claim 4, wherein the comparison circuit comprises a hysteresis comparator.

6. The control circuit of claim 4, wherein the current control circuit further comprises a signal generating circuit having an input and an output, the input of the signal generating circuit being coupled to the output of the comparing circuit, the output of the signal generating circuit being coupled to the control terminal of the switch.

7. The control circuit of claim 4, wherein the control circuit further comprises:

the first input end of the second comparison circuit is coupled with the second threshold signal, and the second input end of the second comparison circuit is coupled with the output end of the voltage detection circuit;

the first input end of the third comparison circuit is coupled with the output end of the voltage detection circuit, and the second input end of the third comparison circuit is coupled with a third threshold signal; and

the trigger circuit is provided with a first input end, a second input end and an output end, wherein the first input end of the trigger circuit is coupled with the output end of the second comparison circuit, the second input end of the trigger circuit is coupled with the output end of the third comparison circuit, the output end of the trigger circuit is coupled with the control end of the synchronous rectifier tube, and the third threshold signal is greater than the second threshold signal.

8. A control circuit as claimed in any one of claims 1 to 7 wherein the synchronous rectifier comprises a field effect transistor.

9. A flyback voltage converter circuit comprising a primary circuit and a secondary circuit, characterized in that the secondary circuit comprises a synchronous rectifier and a control circuit as claimed in any of claims 1 to 7.

Images