CN113162440A - 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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- CN113162440A CN113162440A CN202110447210.5A CN202110447210A CN113162440A CN 113162440 A CN113162440 A CN 113162440A CN 202110447210 A CN202110447210 A CN 202110447210A CN 113162440 A CN113162440 A CN 113162440A
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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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- 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
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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 output terminal of the flyback switching power supply is rectified by a diode D1, and during the conduction time of the secondary winding, the ratio of the energy loss generated by the diode D1 to the total energy of the secondary winding is relatively large, 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 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, and 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 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.
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, the conduction voltage drop of the synchronous rectifier Q2 can be reduced to below 0.1V, and the generated energy loss is very small.
The MOS transistor Q2 is a gate control device and is cooperatively provided with a synchronous rectification control circuit, 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 transistor Q2 and controlling the switching of the synchronous rectification transistor 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 both a flyback switching power supply and 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 switching control signal by monitoring the VS voltage of the synchronous rectifier tube so as to control the turn-on and turn-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 rectifier tube according to the switching control signal of the control module, so as to control the on and off of the synchronous rectifier 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 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 start control circuit outputs a high-level control signal and the turn-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 is 1, namely the start control signal. The driving module outputs a driving signal to control the synchronous rectifier tube Q2 to be turned on according to the received turn-on 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 is 0, that is, the off control signal. The driving module outputs a driving signal to control the synchronous rectifier 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 functions of turning on the control signal and turning off the control signal, and this embodiment of the present 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, the synchronous rectification control circuit of the secondary winding detects that the drain-source voltage VS of the synchronous rectifier Q2 decreases or that a current flows from the source to the drain, and detects that the slope of the decrease in the voltage VS satisfies the requirement, the GATE driving signal for turning on the synchronous rectifier 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 both approach 0, a GATE driving signal for turning off 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 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 Q2 is very easy to occur.
The synchronous rectifier tube Q2 is turned off in advance, which easily causes the reduction of the conversion efficiency of the flyback switching power supply, and the delayed turn-off easily causes the 'common' phenomenon of the primary coil and the secondary coil of the flyback switching power supply, which brings the damage of the circuit and reduces the safety of the circuit.
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, an output SW2 of the second delay circuit Tdly2 at the beginning is 0, and at this time, an output S2 of the dynamic monitoring circuit is 0. When the switch control signal SW changes from low level to high level, that is, when SW is equal to 1, the second delay circuit delays SW by a second set time Tdly2, and then outputs SW2 equal to 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, and the embodiment of the present application is not limited herein.
Fig. 7 shows a waveform diagram of the switching power supply in the heavy load operation 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 heavily loaded, the MOS transistor Q1 of the primary winding is turned on, which causes the VS voltage to rapidly rise when the VS voltage is relatively low.
At this time, if whether the synchronous rectifier is turned off is determined 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 comparator, CMP2, where the non-inverting input of the second voltage comparator CMP2 is connected to the drain-source voltage signal, i.e., the VS voltage.
And the VS voltage is connected to the inverting input end of the second voltage comparator CMP2 through an RC delay branch and a constant voltage difference generation branch which are connected in sequence.
The RC delay branch of the present embodiment includes a parallel circuit formed by 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.
On the contrary, the voltage VA at the positive terminal of the capacitor C1 slowly rises due to the presence of the RC delay branch, and the voltage at 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 decision 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 an output of the second AND circuit AND2 is connected to a reset terminal of the RS flip-flop.
The dynamic detection circuit of the embodiment of the application delays the turn-off control signal by the second set time Tdly2 by setting the second delay circuit Tdly 2. Because the turn-off control signal is delayed and not immediately output during the unstable 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 the present embodiment can detect the change of the VS voltage in time when the VS voltage rapidly rises, and output the turn-off control signal after the second delay time Tdly2 is reached, thereby ensuring 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 determining circuit of this embodiment includes: the circuit comprises a first delay circuit, a first voltage comparison circuit and a first judgment circuit. Wherein,
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 shutdown threshold determination 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, an output SW1 of the first delay circuit Tdly1 at the beginning is 0, and at this time, an output S1 of the shutdown threshold detection circuit is 0. When the switch control signal SW changes from low level to high level, that is, when SW is equal to 1, the first delay circuit delays the SW by a first set time Tdly1, and then outputs SW1 equal to 1. Wherein Tdly2> Tdly 1. 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, and the embodiment of the present application 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 comparator 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. The first comparator CMP1 outputs a high signal when the VS voltage is greater than the reference voltage, and otherwise outputs a low 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, AND the control signal S1 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 is equal to 1 at this time, the first AND circuit AND1 outputs a high-level signal, the output S1 of the turn-off threshold detection circuit is equal to 1, the reset terminal of the RS flip-flop is turned on to a high level, AND the RS flip-flop is reset, that is, outputs a turn-off control signal.
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 unstable 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 application, by using the turn-off threshold judgment circuit in the above embodiment, when the VS voltage is detected to rise to the reference voltage, the turn-off control signal is immediately output to immediately turn off the synchronous rectifier tube, so that the problem that the turn-off of the synchronous rectifier tube is too slow due to the start of resonance of the VS voltage is avoided.
Therefore, the turn-off threshold 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 period when the switching power supply is in light load, but also avoids the delayed turn-off when the switching power supply is in light load, and greatly improves the turn-off control accuracy of the synchronous rectifier control circuit.
The turn-off control circuit of the embodiment of the present application may set the turn-off threshold determination circuit and the dynamic detection circuit at the same time, referring to fig. 4, the turn-off threshold determination circuit and the dynamic detection circuit are used as inputs of the or gate circuit in the embodiment of the present application, and as long as any one of the outputs of the turn-off threshold determination circuit and the dynamic detection circuit is 1, the output of the turn-off 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 primary winding MOS transistor Q1 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 within an 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 switching-off threshold judging circuit and the dynamic detection circuit, the synchronous rectifying tubes are controlled to be switched off 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 (10)
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:
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 a turn-off control signal to control the turn-off of the synchronous rectifier tube.
2. The synchronous rectification control circuit of the switching power supply according to claim 1, wherein the dynamic detection circuit comprises:
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.
3. The synchronous rectification control circuit of the switching power supply according to claim 2, wherein the drain-source voltage monitoring circuit comprises a second voltage comparison circuit, and a non-inverting input terminal of the second voltage comparison circuit is connected with a 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.
4. 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 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.
5. The synchronous rectification control circuit of the switching power supply according to claim 4, 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 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.
6. The synchronous rectification control circuit of the switching power supply according to claim 1, further comprising a reference voltage generation circuit configured to generate a reference voltage required for the shutdown control circuit.
7. The synchronous rectification control circuit of the switching power supply as claimed in claim 1, further comprising a turn-on control circuit 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.
8. The synchronous rectification control circuit of claim 7, further comprising a driving module 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 transistor.
9. The synchronous rectification control circuit of the switching power supply according to claim 8, further comprising a switching 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.
10. 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 9.
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