CN113162440B - Switching power supply and synchronous rectification control circuit thereof - Google Patents
Switching power supply and synchronous rectification control circuit thereof Download PDFInfo
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- CN113162440B CN113162440B CN202110447210.5A CN202110447210A CN113162440B CN 113162440 B CN113162440 B CN 113162440B CN 202110447210 A CN202110447210 A CN 202110447210A CN 113162440 B CN113162440 B CN 113162440B
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/02—Conversion of ac power input into dc power output without possibility of reversal
- H02M7/04—Conversion of ac power input into dc power output without possibility of reversal by static converters
- H02M7/12—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/21—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
- H02M7/217—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M1/00—Details of apparatus for conversion
- H02M1/08—Circuits specially adapted for the generation of control voltages for semiconductor devices incorporated in static converters
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02B—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO BUILDINGS, e.g. HOUSING, HOUSE APPLIANCES OR RELATED END-USER APPLICATIONS
- Y02B70/00—Technologies for an efficient end-user side electric power management and consumption
- Y02B70/10—Technologies improving the efficiency by using switched-mode power supplies [SMPS], i.e. efficient power electronics conversion e.g. power factor correction or reduction of losses in power supplies or efficient standby modes
Abstract
The application relates to a switching power supply and synchronous rectification control circuit thereof, synchronous rectification control circuit is including shutting down control circuit, it includes dynamic detection circuit to shut down control circuit, the configuration of dynamic detection circuit is: and after delaying the switching control signal of the synchronous rectifier tube for a second set time, combining a drain-source voltage change monitoring result of the synchronous rectifier tube and outputting the turn-off control signal so as to control the turn-off of the synchronous rectifier tube. The problem of circuit damage caused by the fact that a synchronous rectifier tube is turned off in advance or turned off after delay in the prior art is solved.
Description
Technical Field
The application relates to a switching power supply and a synchronous rectification control circuit thereof, belonging to the technical field of switching power supplies.
Background
A switching power supply, namely a switching stabilized voltage supply, is a novel stabilized voltage supply circuit relative to a linear stabilized voltage supply, and stabilizes output voltage by monitoring the output voltage in real time and dynamically controlling the on-off time ratio of a switching tube in a switching power supply circuit.
Due to its efficient control and accurate output, the switching power supply is widely used in the power supply system. As shown in fig. 1, the flyback switching power supply circuit is a flyback switching power supply circuit, the output terminal of the flyback switching power supply is rectified by using a diode D1, the ratio of the energy loss generated by the diode D1 to the total energy of the secondary winding is relatively large during the conduction time of the secondary winding, and the conversion efficiency of the flyback switching power supply is greatly reduced due to the relatively high voltage drop of the diode D1.
Disclosure of Invention
The application provides a switching power supply and synchronous rectification control circuit thereof, can solve among the prior art switching power supply output and pass through diode voltage drop big, and the energy loss of its production can reduce the problem of flyback switching power supply's conversion efficiency greatly.
The application provides the following technical scheme:
in a first aspect, a synchronous rectification control circuit of a switching power supply is provided, where the synchronous rectification control circuit is configured to control switching of a synchronous rectifier tube at an output end of the switching power supply, the synchronous rectification control circuit includes a turn-off control circuit, the turn-off control circuit includes a dynamic detection circuit, and the dynamic detection circuit is configured to:
and after delaying the switching control signal of the synchronous rectifier tube for a second set time, combining a drain-source voltage change monitoring result of the synchronous rectifier tube and outputting the turn-off control signal so as to control the turn-off of the synchronous rectifier tube.
Optionally, in an embodiment of the first aspect of the present application, the dynamic detection circuit includes:
the second delay circuit is configured to delay the switch control signal for a second set time and then output the switch control signal;
the drain-source voltage monitoring circuit is configured to monitor drain-source voltage change of the synchronous rectifier tube and output a monitoring result;
and the second judgment circuit is configured to output the turn-off control signal when both the output of the second delay circuit and the output of the drain-source voltage monitoring circuit meet a second preset condition.
Optionally, in an embodiment of the first aspect of the present application, the drain-source voltage monitoring circuit includes a second voltage comparison circuit, and a non-inverting input of the second voltage comparison circuit is connected to the drain-source voltage signal; and the drain-source voltage signal is connected to the inverting input end of the second voltage comparison circuit through an RC delay branch and a constant voltage difference generation branch which are sequentially connected.
Optionally, in an embodiment of the first aspect of the present application, the shutdown control circuit further includes a shutdown threshold determination circuit, where the shutdown threshold determination circuit is configured to:
after the switch control signal of the synchronous rectifier tube is delayed for the first set time, the turn-off control signal is output by combining the comparison result of the drain-source voltage and the reference voltage of the synchronous rectifier tube so as to control the turn-off of the synchronous rectifier tube, wherein the first set time is less than the second set time.
Optionally, in an embodiment of the first aspect of the present application, the shutdown threshold determining circuit includes:
the first delay circuit is configured to delay the switch control signal of the synchronous rectifier tube for a first set time and then output the delayed switch control signal;
the first voltage comparison circuit is configured to monitor the drain-source voltage of the synchronous rectifier tube, compare the drain-source voltage with a reference voltage and output a comparison result;
the first judgment circuit is configured to output the turn-off control signal when both the output of the first delay circuit and the output of the voltage comparison circuit meet a first preset condition.
Optionally, in an embodiment of the first aspect of the present application, the synchronous rectification control circuit further includes a reference voltage generating circuit configured to generate a reference voltage required by the shutdown control circuit.
Optionally, in an embodiment of the first aspect of the present application, the synchronous rectification control circuit further includes a turn-on control circuit, and the turn-on control circuit is configured to output a turn-on control signal by monitoring a drain-source voltage of the synchronous rectification tube, so as to control the turn-on of the synchronous rectification tube.
Optionally, in an embodiment of the first aspect of the present application, the synchronous rectification control circuit further includes a driving module, and the driving module is configured to output a driving signal according to the turn-on control signal and the turn-off control signal to control the turn-on or turn-off of the synchronous rectification tube.
Optionally, in an embodiment of the first aspect of the present application, the synchronous rectification control circuit further includes a switch control signal output circuit configured to:
and determining the signal output to the driving module to be an on control signal or an off control signal according to the output of the on control circuit and the output of the off control circuit.
In a second aspect, a switching power supply is provided, which includes a synchronous rectifier disposed at an output end of the switching power supply and the synchronous rectification control circuit according to any embodiment of the first aspect of the present application.
The beneficial effect of this application lies in: the turn-off control circuit of the embodiment of the application can timely detect the change of the VS voltage when the VS voltage rapidly rises after delaying the turn-off control signal for the second set time through the second delay circuit by arranging the dynamic detection circuit, and outputs the turn-off control signal, so that the rapid turn-off of the synchronous rectifier tube Q2 is ensured. The timeliness of the turn-off of the synchronous rectifier tube is guaranteed, the power supply efficiency is improved, and meanwhile the problem that the synchronous rectifier tube is not turned off timely when the switching power supply is heavily loaded is avoided.
The turn-off control circuit of the embodiment of the application, through setting the turn-off threshold judging circuit, when the switch power supply is lightly loaded, after the turn-off control signal is delayed for the first set time through the first delay circuit, when the VS voltage is detected to rise to the reference voltage, the turn-off control signal is immediately output, the synchronous rectifier tube is turned off, the timeliness of turn-off of the synchronous rectifier tube is guaranteed, the power efficiency is improved, and meanwhile, the problem that the synchronous rectifier tube is turned off too slowly due to resonance when the VS voltage is lightly loaded on the switch power supply is avoided.
The foregoing description is only an overview of the technical solutions of the present application, and in order to make the technical solutions of the present application more clear and clear, and to implement the technical solutions according to the content of the description, the following detailed description is made with reference to the preferred embodiments of the present application and the accompanying drawings.
Drawings
Fig. 1 is a circuit schematic diagram of a conventional switching power supply;
FIG. 2 is a schematic diagram of a switching power supply circuit provided by an embodiment of the present application;
fig. 3 is a schematic diagram of a switching power supply circuit according to another embodiment of the present application;
FIG. 4 is a circuit schematic of a shutdown control circuit provided by one embodiment of the present application;
FIG. 5 is a circuit schematic of a dynamic detection circuit provided by one embodiment of the present application;
FIG. 6 is a timing diagram illustrating the operation of the dynamic detection circuit according to one embodiment of the present application;
fig. 7 is a waveform diagram of a switching power supply in a heavy-load operation mode according to an embodiment of the present application;
fig. 8 is a waveform diagram of a switching power supply in a light-load operation mode according to an embodiment of the present application;
FIG. 9 is a circuit schematic of a turn-off threshold decision circuit provided by one embodiment of the present application;
fig. 10 is a timing diagram illustrating the operation of the shutdown threshold determination circuit according to an embodiment of the present application.
Detailed Description
The following detailed description of embodiments of the present application will be described in conjunction with the accompanying drawings and examples. The following examples are intended to illustrate the present application but are not intended to limit the scope of the present application.
Aiming at the problem that the conversion efficiency of the flyback switching power supply is greatly reduced due to the energy loss generated by the large conduction voltage drop of the diode because the diode is adopted for rectifying the output end of the switching power supply, a Metal-Oxide-Semiconductor Field effect transistor (MOSFET) with low conduction resistance can be adopted to replace a rectifying diode at present.
As shown in fig. 2, the synchronous rectifier (i.e., MOS transistor) Q2 is used instead of the diode D1, so that the conduction voltage drop of the synchronous rectifier Q2 can be reduced to 0.1V or less, and the generated energy loss is very small.
The MOS tube Q2 is a grid control device and is provided with a synchronous rectification control circuit in a matching way, and the synchronous rectification control circuit is used for outputting a driving signal according to the drain-source voltage or the drain current of the synchronous rectification tube Q2 and controlling the switch of the synchronous rectification tube Q2.
Fig. 3 is a synchronous rectification control circuit applied to a switching power supply according to an embodiment of the present disclosure, where the synchronous rectification control circuit according to the embodiment of the present disclosure may be used for a flyback switching power supply as well as a buck switching power supply. The synchronous rectification control circuit according to the embodiment of the present application will be described in detail below, taking application to a flyback switching power supply as an example.
As shown in fig. 3, the synchronous rectification control circuit of the present embodiment includes:
and the control module is configured to output a switch control signal by monitoring the VS voltage of the synchronous rectifier tube so as to control the on and off of the synchronous rectifier tube Q2.
And the driving module is configured to output a driving signal to the control end of the synchronous rectifying tube according to the switch control signal of the control module, and control the on and off of the synchronous rectifying tube Q2.
Optionally, the control module of the embodiment of the present application includes:
and the turn-on control circuit is configured to monitor the VS voltage and output a turn-on control signal of the synchronous rectifier tube Q2 according to the monitoring result.
And the turn-off control circuit is configured to monitor the VS voltage and output a turn-off control signal of the synchronous rectifier tube Q2 according to the monitoring result.
In the embodiment of the application, the starting control circuit and the turning-off control circuit are respectively controlled and are independent from each other and are not interfered.
Optionally, the embodiment of the present application is further provided that the synchronous rectification control circuit further includes a switch control signal output circuit.
Specifically, the switch control signal output circuit is configured to: and determining the signal output to the driving module to be an on control signal or an off control signal according to the output of the on control circuit and the output of the off control circuit.
As shown in fig. 3, the switch control signal output circuit according to the embodiment of the present application may be, for example, an RS flip-flop circuit.
The output end of the turn-on control circuit is connected with the set end of the RS trigger circuit, and the output end of the turn-off control circuit is connected with the reset end of the RS trigger circuit.
The output end of the RS trigger circuit is connected with the input end of the driving module, and the RS trigger is used for outputting a switch control signal SW.
When the on control circuit outputs a high level control signal and the off control circuit outputs a low level control signal, the RS trigger circuit is set, the output switch control signal SW is high level, and SW =1 is the on control signal. The driving module outputs a driving signal to control the synchronous rectifier tube Q2 to be opened according to the received opening control signal.
When the on control circuit outputs a low level control signal and the off control circuit outputs a high level control signal, the RS trigger circuit is reset, the output switch control signal SW is at a low level, and SW =0 is the off control signal. The driving module outputs a driving signal to control the synchronous rectifying tube Q2 to be switched off according to the received switching-off control signal.
The switch control signal output circuit of this embodiment may also be other circuits capable of implementing the output function of turning on the control signal and turning off the control signal, and this embodiment of this application is not limited herein.
The synchronous rectification control circuit of the embodiment of the application further comprises a power supply circuit, and the power supply circuit simultaneously generates the working voltage VDD and the reference voltage VREF required by the synchronous rectification control circuit.
When introducing the principle of the synchronous rectification control circuit, the embodiment of the application simply introduces the starting control circuit, the starting control circuit can adopt the existing circuit structure, and the embodiment of the application mainly aims at the technical problem of the synchronous rectification tube existing in the turn-off process and specifically describes the turn-off control circuit.
In the above embodiment, when the MOS transistor Q1 of the primary winding is turned off, and the synchronous rectification control circuit of the secondary winding detects that the drain-source voltage VS of the synchronous rectification transistor Q2 decreases or that a current flowing from the source to the drain occurs, and at the same time detects that the slope of the decrease in the voltage VS meets the requirement, a GATE driving signal for turning on the synchronous rectification transistor Q2 is output.
When the synchronous rectification control circuit detects that the drain-source voltage VS of the synchronous rectification tube Q2 or the current flowing from the source to the drain approaches to 0, a GATE driving signal for closing the synchronous rectification tube Q1 is output.
When the load of the switching power supply works in a continuous mode, a discontinuous mode or other modes, when the drain-source voltage VS of the synchronous rectifier tube Q2 rises, the condition of abrupt change or slow change of the waveform slope occurs, so that the reaction time and the detection result of the system are influenced, and the phenomenon of early turn-off or delayed turn-off of the synchronous rectifier tube Q2 is easily caused.
The synchronous rectifier tube Q2 is turned off in advance, so that the conversion efficiency of the flyback switching power supply is easily reduced, the phenomenon of 'common use' of a primary coil and a secondary coil of the flyback switching power supply is easily caused by delayed turn-off, the circuit damage is caused, and meanwhile, the safety of the circuit is reduced.
In view of the above technical problem, optionally, as shown in fig. 4, the shutdown control circuit of the embodiment of the present application includes a dynamic detection circuit.
Specifically, the dynamic detection circuit is configured to:
and after the switch control signal of the synchronous rectifier tube is delayed for a second set time, combining a drain-source voltage change monitoring result of the synchronous rectifier tube, and outputting the turn-off control signal to control the synchronous rectifier tube to be normally turned off, wherein the second set time is longer than the first set time.
Optionally, the dynamic detection circuit of the embodiment of the present application includes:
and the second delay circuit is configured to delay the switch control signal for a second set time and then output the delayed switch control signal.
Specifically, fig. 5 shows a schematic diagram of the dynamic monitoring circuit according to an embodiment of the present application, as shown in fig. 5, an input terminal of the second delay circuit Tdly2 is connected to the switch control signal SW, and an output SW2=0 of the second delay circuit Tdly2 at the beginning, at which time, the output S2=0 of the dynamic detection circuit. When the switch control signal SW changes from low level to high level, that is, SW =1, the second delay circuit delays SW by a second set time Tdly2, and then outputs SW2=1. See the timing diagram shown in fig. 6.
The second delay circuit Tdly2 may adopt an RC delay circuit or a timing delay circuit implemented by software, which is not limited herein.
Fig. 7 shows a waveform diagram of the switching power supply in the heavy load operating mode, as shown in fig. 7, when the load of the switching power supply operates in the continuous mode (CCM), that is, the switching power supply is under heavy load, the MOS transistor Q1 of the primary winding is turned on, which may cause the VS voltage to rapidly rise when the VS voltage is relatively low.
At this time, if the synchronous rectifier is turned off only by comparing the VS voltage with the reference voltage, the problem that the synchronous rectifier Q2 is not turned off in time is caused, so that the primary coil and the secondary coil are shared, and the circuit is damaged.
Aiming at the problem of untimely turn-off, the dynamic detection circuit of the embodiment of the application is also provided with a drain-source voltage monitoring circuit.
And the drain-source voltage monitoring circuit is configured to monitor the drain-source voltage change of the synchronous rectifier tube and output a monitoring result.
Specifically, referring to fig. 5, the drain-source voltage monitoring circuit in one embodiment of the present application includes a second voltage comparison circuit, i.e., a second voltage comparator CMP2, and a non-inverting input of the second voltage comparator CMP2 is connected to a drain-source voltage signal, i.e., a VS voltage.
And the VS voltage is connected to the inverting input terminal of the second voltage comparator CMP2 through the sequentially connected RC delay branch and the constant voltage difference generation branch.
The RC delay branch circuit of the present embodiment includes a parallel circuit composed of a resistor R1 and a capacitor C1, and the constant voltage difference generating circuit is a dc source with a constant voltage difference, and the dc source has a constant voltage difference Δ V.
When the VS voltage is in a steady state, the voltage at the non-inverting input terminal of the second comparator CMP2 is lower than the voltage at the inverting input terminal by Δ V, and thus the second comparator CMP2 outputs a low level signal.
When the VS voltage suddenly rises, the voltage at the non-inverting input terminal of the second comparator CMP2 immediately rises to the VS voltage.
And the voltage VA of the positive terminal of the capacitor C1 slowly rises due to the existence of the RC delay branch circuit, and the voltage of the inverting input terminal cannot immediately rise to the VS voltage. Therefore, the voltage at the inverting terminal of the second comparator CMP2 also rises only slowly.
In the case where the VS voltage is higher than VA + Δ V, the second comparator CMP2 outputs a high level signal.
And the second judgment circuit is configured to output the turn-off control signal when both the output of the second delay circuit and the output of the drain-source voltage monitoring circuit meet a second preset condition.
Specifically, the second preset condition means that the outputs of the second delay circuit and the drain-source voltage monitoring circuit are both at a high level. The second determining circuit may be a second AND circuit AND2, an output of the second voltage comparator CMP2 AND an output of the second delay circuit are connected to an input terminal of the second AND circuit AND2, AND a control signal S2 of the output of the second AND circuit AND2 is connected to a reset terminal of the RS flip-flop.
The dynamic detection circuit in the embodiment of the application delays the turn-off control signal by the second set time Tdly2 by setting the second delay circuit Tdly2. Because the turn-off control signal is delayed and not immediately output during the instable period of the VS voltage, the condition that the synchronous rectifier tube is turned off in advance due to misjudgment when the signal is unstable is avoided.
The dynamic detection circuit of this embodiment can detect the change of the VS voltage in time when the VS voltage rises rapidly, and output the turn-off control signal after reaching the second delay time Tdly2, so as to ensure the rapid turn-off of the synchronous rectifier Q2. Therefore, the problem that the turn-off of the synchronous rectifier tube is delayed when the switching power supply is heavily loaded is solved.
Therefore, the dynamic detection circuit of the embodiment of the application not only avoids the early turn-off of the synchronous rectifier tube during the unstable VS voltage period when the switching power supply is heavily loaded, but also avoids the delayed turn-off when the switching power supply is heavily loaded, and greatly improves the turn-off control accuracy of the synchronous rectifier control circuit.
Fig. 8 shows a waveform diagram of the switching power supply in the light-load operating mode, as shown in fig. 8, when the load of the flyback switching power supply operates in the discontinuous mode (DCM), that is, when the switching power supply is in light load, the VS voltage will slowly rise along with the decrease of the current of the secondary winding, and when the current in the secondary winding is exhausted, the VS voltage starts to resonate until the primary winding is turned on.
In this case, if the synchronous rectifier is turned off only by the dynamic detection circuit, the turn-off of the synchronous rectifier is too slow.
In view of the above technical problem, further optionally, as shown in fig. 4, the shutdown control circuit according to the embodiment of the present application further includes a shutdown threshold judgment circuit.
Specifically, the turn-off threshold determination circuit is configured to:
and after delaying the switching control signal of the synchronous rectifier tube for a first set time, combining a comparison result of drain-source voltage and reference voltage of the synchronous rectifier tube, and outputting the turn-off control signal to control the synchronous rectifier tube to be normally turned off.
Optionally, the shutdown threshold determination circuit of this embodiment includes: the circuit comprises a first delay circuit, a first voltage comparison circuit and a first judgment circuit. Wherein, the first and the second end of the pipe are connected with each other,
and the first delay circuit is configured to delay the switching control signal of the synchronous rectifier tube for a first set time and then output the delayed switching control signal.
Fig. 9 shows a schematic diagram of the turn-off threshold judging circuit according to an embodiment of the present application, as shown in fig. 9, an input terminal of the first delay circuit Tdly1 is connected to the switch control signal SW, and an output SW1=0 of the first delay circuit Tdly1 at the beginning, at this time, an output S1=0 of the turn-off threshold detecting circuit. When the switch control signal SW changes from low level to high level, that is, SW =1, the first delay circuit delays the SW by the first set time Tdly1 and then outputs SW1=1. Wherein Tdly2> Tdly1. See the timing diagram shown in fig. 10.
The first delay circuit Tdly1 may adopt an RC delay circuit or a timing delay circuit implemented by software, which is not limited herein.
And the first voltage comparison circuit is configured to monitor the drain-source voltage of the synchronous rectifier tube, compare the drain-source voltage with a reference voltage and output a comparison result.
Specifically, referring to fig. 9, the first voltage comparison circuit is a first voltage comparator CMP1, a non-inverting input terminal of the first comparator CMP1 is connected to the VS voltage, and an inverting input terminal of the first comparator CMP1 is connected to the reference voltage VREF. When the VS voltage is greater than the reference voltage, the first comparator CMP1 outputs a high level signal, and otherwise outputs a low level signal.
The first judging circuit is configured to output the turn-off control signal when the output of the first delay circuit and the output of the voltage comparing circuit both meet a first preset condition.
Specifically, the first preset condition means that the outputs of the first delay circuit and the voltage comparison circuit are both at a high level. Referring to fig. 8, the first determining circuit is a first AND circuit AND1, the output of the first comparator CMP1 AND the output of the first delay circuit Tdly1 are connected to the input terminal of the first AND circuit AND1 in this embodiment, AND the control signal S1 of the output of the first AND circuit AND1 is connected to the reset terminal of the RS flip-flop.
When the first comparator CMP1 outputs a high level signal, if SW1=1 at this time, the first AND circuit AND1 outputs a high level signal, the output S1=1 of the shutdown threshold detection circuit, the reset terminal of the RS flip-flop is switched to a high level, AND the RS flip-flop is reset, that is, the shutdown control signal is output.
In the embodiment of the application, the first delay circuit Tdly1 is arranged, the turn-off threshold judgment circuit delays the switch control signal for the first set time Tdly1, and outputs the turn-off control signal after detecting that the VS voltage is greater than the reference voltage. Because the turn-off control signal is delayed and not immediately output during the instable period of the VS voltage, the condition that the synchronous rectifier tube is turned off in advance due to misjudgment when the signal is unstable is avoided.
In addition, according to the embodiment of the present application, by using the turn-off threshold determining circuit in the above embodiment, when it is detected that the VS voltage rises to the reference voltage, the turn-off control signal is immediately output to immediately turn off the synchronous rectifying tube, so that the problem that the turn-off of the synchronous rectifying tube is too slow due to the start of resonance of the VS voltage is avoided.
Therefore, the turn-off threshold value judging circuit of the embodiment of the application not only avoids the early turn-off of the synchronous rectifier tube during the unstable VS voltage when the switch power supply is in light load, but also avoids the delayed turn-off when the switch power supply is in light load, and greatly improves the turn-off control accuracy of the synchronous rectifier control circuit.
In the shutdown control circuit of the embodiment of the present application, the shutdown threshold determination circuit and the dynamic detection circuit may be set at the same time, referring to fig. 4, in the embodiment of the present application, the shutdown threshold determination circuit and the dynamic detection circuit are used as inputs of the or gate circuit, and as long as an output of any one of the shutdown threshold determination circuit and the dynamic detection circuit is 1, an output of the shutdown control circuit is 1.
In addition, the shutdown control circuit in the embodiment of the present application may further include a shutdown circuit for turning off the synchronous rectifier tube, in addition to the shutdown threshold determination circuit and the dynamic detection circuit, which is not limited herein.
In summary, after the MOS transistor Q1 of the primary winding is turned off, the drain-source voltage VS of the synchronous rectifier Q2 rapidly drops and generates resonant oscillation, and in order to avoid controlling the turn-on and turn-off of the synchronous rectifier in the oscillation time interval, in the embodiment of the present application, the first delay circuit Tdly1 is provided in the turn-off threshold determination circuit, and the second delay circuit Tdly2 is provided in the dynamic detection circuit, so that the situation that the turn-off control circuit determines erroneously when the VS voltage is unstable can be avoided, and it is ensured that the synchronous rectifier is not turned off in advance due to erroneous determination.
The turn-off control circuit of the embodiment of the application, through setting the turn-off threshold judging circuit, when the switch power supply is lightly loaded, after the turn-off control signal is delayed for the first set time through the first delay circuit, when the VS voltage is detected to rise to the reference voltage, the turn-off control signal is immediately output, the synchronous rectifier tube is turned off, the timeliness of turn-off of the synchronous rectifier tube is guaranteed, the power efficiency is improved, and meanwhile, the problem that the synchronous rectifier tube is turned off too slowly due to resonance when the VS voltage is lightly loaded on the switch power supply is avoided.
The turn-off control circuit of the embodiment of the application can timely detect the change of the VS voltage when the VS voltage rapidly rises after delaying the turn-off control signal for the second set time through the second delay circuit by arranging the dynamic detection circuit, and outputs the turn-off control signal, so that the rapid turn-off of the synchronous rectifier tube Q2 is ensured. The timeliness of the turn-off of the synchronous rectifier tube is guaranteed, the power supply efficiency is improved, and meanwhile the problem that the synchronous rectifier tube is not turned off timely when the switching power supply is heavily loaded is avoided.
According to the embodiment of the application, the turn-off threshold judging circuit and the dynamic detection circuit are used for controlling the turn-off of the synchronous rectifier tubes in parallel, the corresponding switching power supply can be suitable for a heavy-load working mode and a light-load working mode, and the application range of the switching power supply is widened.
The embodiment of the present application further provides a switching power supply, which includes a turn-off control circuit, and the turn-off control circuit can refer to the related description of the above turn-off control circuit embodiment, which is not described herein again.
The technical features of the embodiments described above may be arbitrarily combined, and for the sake of brevity, all possible combinations of the technical features in the embodiments described above are not described, but should be considered as being within the scope of the present specification as long as there is no contradiction between the combinations of the technical features.
The above-mentioned embodiments only express several embodiments of the present application, and the description thereof is more specific and detailed, but not construed as limiting the scope of the invention. It should be noted that, for a person skilled in the art, several variations and modifications can be made without departing from the concept of the present application, which falls within the scope of protection of the present application. Therefore, the protection scope of the present patent shall be subject to the appended claims.
Claims (7)
1. A synchronous rectification control circuit of a switching power supply, characterized in that the synchronous rectification control circuit comprises a turn-off control circuit, the turn-off control circuit comprises a dynamic detection circuit, and the dynamic detection circuit is configured to:
after delaying the on-off control signal of the synchronous rectifier tube for a second set time, combining the drain-source voltage change monitoring result of the synchronous rectifier tube, and outputting a turn-off control signal to control the synchronous rectifier tube to be turned off;
the dynamic detection circuit includes:
the second delay circuit is configured to delay the switch control signal for a second set time and then output the switch control signal;
the drain-source voltage monitoring circuit is configured to monitor drain-source voltage change of the synchronous rectifier tube and output a monitoring result;
the second judgment circuit is configured to output the turn-off control signal when both the output of the second delay circuit and the output of the drain-source voltage monitoring circuit meet a second preset condition; the second preset condition means that the outputs of the second delay circuit and the drain-source voltage monitoring circuit are both high levels;
the drain-source voltage monitoring circuit comprises a second voltage comparison circuit, and the in-phase input end of the second voltage comparison circuit is connected with a drain-source voltage signal; the drain-source voltage signal is connected to the inverting input end of the second voltage comparison circuit through an RC delay branch and a constant voltage difference generation branch which are connected in sequence;
the output of the second voltage comparison circuit and the output of the second delay circuit are connected to the input end of the second judgment circuit.
2. The synchronous rectification control circuit of the switching power supply according to claim 1, wherein the shutdown control circuit further comprises a shutdown threshold determination circuit configured to:
after delaying a switching control signal of the synchronous rectifier tube for a first set time, combining a comparison result of drain-source voltage and reference voltage of the synchronous rectifier tube, and outputting a turn-off control signal to control the synchronous rectifier tube to turn off, wherein the first set time is less than the second set time;
the turn-off threshold judging circuit and the dynamic detection circuit are used as the input of an OR gate circuit, so that when the output of any one of the turn-off threshold judging circuit and the dynamic detection circuit is 1, the output of the turn-off control circuit is 1.
3. The synchronous rectification control circuit of the switching power supply according to claim 2, wherein the turn-off threshold judging circuit comprises:
the first delay circuit is configured to delay the switch control signal of the synchronous rectifier tube for a first set time and then output the delayed switch control signal;
the first voltage comparison circuit is configured to monitor the drain-source voltage of the synchronous rectifier tube, compare the drain-source voltage with a reference voltage and output a comparison result;
the first judgment circuit is configured to output the turn-off control signal when the output of the first delay circuit and the output of the first voltage comparison circuit both meet a first preset condition;
the first preset condition means that the outputs of the first delay circuit and the first voltage comparison circuit are both high levels;
the output of the first voltage comparison circuit and the output of the first delay circuit are connected to the input end of the first judgment circuit.
4. The synchronous rectification control circuit of the switching power supply according to claim 3, further comprising a power supply circuit configured to generate a reference voltage required for the shutdown control circuit; the reference voltage is input to an inverting input terminal of the first voltage comparison circuit.
5. The synchronous rectification control circuit of the switching power supply according to claim 1, further comprising a turn-on control circuit and a switching control signal output circuit, wherein the turn-on control circuit is configured to output a turn-on control signal to control the turn-on of the synchronous rectifier by monitoring a drain-source voltage of the synchronous rectifier;
the switch control signal output circuit is configured to determine a signal output to the driving module to be an on control signal or an off control signal according to the output of the on control circuit and the output of the off control circuit;
the output end of the starting control circuit is connected with the position end of the switch control signal output circuit; the output end of the turn-off control circuit is connected with the reset end of the switch control signal output circuit.
6. The synchronous rectification control circuit of the switching power supply according to claim 5, further comprising the driving module, wherein the driving module is configured to output a driving signal according to the turn-on control signal and the turn-off control signal to control the turn-on or turn-off of the synchronous rectification tube;
the input end of the driving module is connected with the output end of the switch control signal output circuit; the output end of the driving module is connected with the control end of the synchronous rectifier tube.
7. A switching power supply comprising a synchronous rectifier disposed at an output of the switching power supply and the synchronous rectification control circuit of any one of claims 1 to 6.
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