CN112234837B - Synchronous rectification circuit and flyback switching power supply - Google Patents

Synchronous rectification circuit and flyback switching power supply Download PDF

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Publication number
CN112234837B
CN112234837B CN202011189612.1A CN202011189612A CN112234837B CN 112234837 B CN112234837 B CN 112234837B CN 202011189612 A CN202011189612 A CN 202011189612A CN 112234837 B CN112234837 B CN 112234837B
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circuit
signal
timing
turn
turned
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CN112234837A (en
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彭韶华
张美玲
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BCD Shanghai Micro Electronics Ltd
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BCD Shanghai Micro Electronics Ltd
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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
    • H02M3/00—Conversion of dc power input into dc power output
    • H02M3/22—Conversion of dc power input into dc power output with intermediate conversion into ac
    • H02M3/24—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters
    • H02M3/28—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac
    • H02M3/325—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal
    • H02M3/335—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only
    • H02M3/33569—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only having several active switching elements
    • H02M3/33576—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only having several active switching elements having at least one active switching element at the secondary side of an isolation transformer
    • H02M3/33592—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only having several active switching elements having at least one active switching element at the secondary side of an isolation transformer having a synchronous rectifier circuit or a synchronous freewheeling circuit at the secondary side of an isolation transformer
    • 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
    • H02M1/32—Means for protecting converters other than automatic disconnection
    • 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

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Rectifiers (AREA)
  • Dc-Dc Converters (AREA)

Abstract

The application discloses a synchronous rectification circuit and a flyback switching power supply. The timing circuit starts timing when the secondary rectifying tube of the switching power supply is turned on, and if the timing time reaches a preset time threshold value, a turn-off permission signal is generated; otherwise, generating a turn-off prohibition signal; the releasing circuit judges whether a primary side switch of the switching power supply is turned on during the timing period of the timing circuit according to the drain voltage of the secondary side rectifying tube; if not, directly outputting the signal generated by the timing circuit; if yes, all signals generated by the timing circuit are converted into turn-off permission signals to be output; the control circuit controls the secondary rectifying tube to be turned on when receiving the turn-on signal; and when receiving the turn-off permission signal output by the release circuit, controlling the turn-off of the secondary rectifying tube. Therefore, when the secondary rectifying tube is turned on, if the primary switch is turned on by mistake, the secondary rectifying tube is directly turned off, the allowable turn-off time of the secondary rectifying tube is not limited, the common phenomenon of the primary and secondary sides is avoided, and the reliability of the system is improved.

Description

Synchronous rectification circuit and flyback switching power supply
Technical Field
The invention relates to the field of switching power supplies, in particular to a synchronous rectification circuit and a flyback switching power supply.
Background
The flyback switching power supply controlled by the primary side gradually becomes an important electronic element power supply device due to small volume and high efficiency, and the output end of the flyback switching power supply is generally connected in series with a rectifier diode to provide direct-current output voltage. With the development of electronic technology, the output voltage required by the load electronic element is lower and the output power is higher, so that the forward turn-on voltage drop of the rectifier diode becomes a main factor for limiting the improvement of the efficiency of the switching power supply.
The current common solution is to use a rectifier tube analog diode for rectification, so-called synchronous rectification technology, and MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor, metal-Oxide-semiconductor-field effect transistor) can be generally used as the rectifier tube. Synchronous rectification utilizes the low resistance of MOSFET when it is on to reduce loss on rectifying tube, and its grid control signal needs to be phase-synchronous with rectified current.
Referring to fig. 1, fig. 1 is a schematic structural diagram of a flyback switching power supply in the prior art. When the synchronous rectification circuit of the secondary side of the flyback switching power supply works normally in a DCM (Discontinuous Current Mode, current interruption mode), the primary side switch M1 and the secondary side rectification tube M2 are not simultaneously turned on. However, when the secondary rectifying tube M2 is turned on, if the system has abnormal conditions such as interference, the primary rectifying tube M1 may be turned on by mistake, and the secondary rectifying tube M2 is allowed to be turned off after the on time of the secondary rectifying tube M2 is maintained for a certain time, so when the primary rectifying tube M1 is turned on by mistake, if the secondary rectifying tube M2 does not reach the time of allowing the secondary rectifying tube M2 to be turned off, the primary and secondary side sharing phenomenon is caused, and the reliability risk of the system is caused.
Therefore, how to provide a solution to the above technical problem is a problem that a person skilled in the art needs to solve at present.
Disclosure of Invention
The invention aims to provide a synchronous rectifying circuit and a flyback switching power supply, when a secondary rectifying tube is turned on, if a primary switch is turned on by mistake, the secondary rectifying tube is directly turned off, and the allowable turn-off time of the secondary rectifying tube is not limited any more, so that the common phenomenon of the primary and secondary sides is avoided, and the reliability of the system is improved.
In order to solve the above technical problems, the present invention provides a synchronous rectification circuit, comprising:
The timing circuit is used for starting timing from the moment when the secondary rectifying tube of the switching power supply is turned on, and generating a turn-off permission signal if the timing time reaches a preset time threshold; otherwise, generating a turn-off prohibition signal;
The releasing circuit is used for judging whether the primary side switch of the switching power supply is turned on during the timing period of the timing circuit according to the drain voltage of the secondary side rectifying tube; if not, directly outputting the signal generated by the timing circuit; if yes, all signals generated by the timing circuit are converted into the turn-off permission signals to be output;
the control circuit is used for controlling the secondary rectifying tube to be turned on when receiving the turn-on signal; and when receiving the turn-off permission signal output by the release circuit, controlling the turn-off of the secondary rectifying tube.
Preferably, the control circuit comprises a volt-second product comparison circuit, a first RS trigger, a second RS trigger, a NOT gate, a NAND gate, a first switching tube and a second switching tube; wherein:
The output end of the volt-second product comparison circuit is connected with the S end of the first RS trigger, the Q end of the first RS trigger is respectively connected with the input end of the NOT gate and the control end of the first switching tube, the output end of the NOT gate is connected with the first input end of the NOT gate, the second input end of the NOT gate is input with a high-level opening signal, the output end of the NOT gate is connected with the S end of the second RS trigger, the Q end of the second RS trigger is connected with the control end of the second switching tube, the R end of the first RS trigger and the R end of the second RS trigger are both connected with the output end of the release circuit, the first end of the second switching tube is connected with the first end of the first switching tube, the common end of the second switching tube is connected with the control end of the auxiliary rectifying tube, and the second end of the first switching tube is grounded; the first switching tube is a switching tube with high-level conduction and low-level cut-off; the second switching tube is a switching tube with low-level conduction and high-level cut-off; the turn-off permission signal is a low level signal; the turn-off prohibition signal is a high level signal;
The volt-second product comparison circuit is used for judging whether the volt-second product of the voltages at the two ends of the secondary winding of the switching power supply is larger than a preset volt-second product threshold value, and if so, a high-level signal is generated; if not, a low level signal is generated.
Preferably, the volt-second product comparison circuit comprises a volt-second integration circuit and a comparator, wherein the volt-second integration circuit is used for obtaining the volt-second product of the voltage across the secondary winding of the switching power supply; wherein:
The output end of the volt-second integrating circuit is connected with the input positive end of the comparator, the input negative end of the comparator inputs a preset volt-second integrating threshold value, and the output end of the comparator is connected with the S end of the first RS trigger.
Preferably, the timing circuit includes a third RS flip-flop and an LEB circuit; wherein:
The R end of the third RS trigger is connected with the output end of the NOT gate, the S end of the third RS trigger is connected with the opening signal, the Q end of the third RS trigger is connected with the input end of the LEB circuit, and the output end of the LEB circuit is connected with the release circuit;
The LEB circuit is used for starting timing when receiving a high-level signal output by the third RS trigger, and generating a low-level signal as the turn-off permission signal when the timing time reaches a preset time threshold; otherwise, a high level signal is generated as the off prohibition signal.
Preferably, the release circuit includes:
The voltage comparison circuit is used for judging whether the VDET voltage at the connection part of the secondary side rectifying tube and the secondary side winding of the switching power supply is larger than a preset voltage threshold value, and if so, a timing release signal is generated;
A timing release circuit for directly outputting a signal generated by the timing circuit when the timing release signal is not received; and when the timing release signal is received, converting signals generated by the timing circuit into the turn-off permission signal to be output.
Preferably, the voltage comparison circuit is specifically a fast comparator; wherein:
The input positive end of the fast comparator is connected with VDET voltage, the input negative end of the fast comparator is connected with the preset voltage threshold, and the output end of the fast comparator is connected with the timing release circuit;
the fast comparator is used for generating a high-level signal when the VDET voltage is larger than the preset voltage threshold value; otherwise, a low level signal is generated.
In order to solve the technical problems, the invention also provides a flyback switching power supply which comprises a transformer comprising a primary winding and a secondary winding, a secondary rectifying tube and any synchronous rectifying circuit.
The application provides a synchronous rectification circuit which comprises a timing circuit, a release circuit and a control circuit. The timing circuit is used for starting timing from the moment when the secondary rectifying tube of the switching power supply is turned on, and generating a turn-off permission signal if the timing time reaches a preset time threshold; otherwise, generating a turn-off prohibition signal; the releasing circuit is used for judging whether the primary side switch of the switching power supply is turned on during the timing period of the timing circuit according to the drain voltage of the secondary side rectifying tube; if not, directly outputting the signal generated by the timing circuit; if yes, all signals generated by the timing circuit are converted into turn-off permission signals to be output; the control circuit is used for controlling the secondary rectifying tube to be turned on when receiving the turn-on signal; and when receiving the turn-off permission signal output by the release circuit, controlling the turn-off of the secondary rectifying tube. Therefore, when the secondary rectifying tube is turned on, if the primary switch is turned on by mistake, the secondary rectifying tube is directly turned off, and the allowable turn-off time of the secondary rectifying tube is not limited, so that the phenomenon of common use of the primary and secondary sides is avoided, and the reliability of the system is improved.
The invention also provides a flyback switching power supply which has the same beneficial effects as the synchronous rectification circuit.
Drawings
In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required in the prior art and the embodiments will be briefly described below, and it is apparent that the drawings in the following description are only some embodiments of the present invention, and other drawings may be obtained according to these drawings without inventive effort for a person skilled in the art.
Fig. 1 is a schematic structural diagram of a flyback switching power supply in the prior art;
Fig. 2 is a schematic structural diagram of a synchronous rectification circuit according to an embodiment of the present invention;
fig. 3 is a schematic diagram of a specific structure of a synchronous rectification circuit according to an embodiment of the present invention;
Fig. 4 is a signal timing diagram of a synchronous rectification circuit under an abnormal condition according to an embodiment of the present invention.
Detailed Description
The core of the invention is to provide a synchronous rectification circuit and a flyback switching power supply, when a secondary rectifying tube is turned on, if a primary switch is turned on by mistake, the secondary rectifying tube is directly turned off, and the turn-off time of the secondary rectifying tube is not limited, so that the common phenomenon of the primary and secondary sides is avoided, and the reliability of the system is improved.
For the purpose of making the objects, technical solutions and advantages of the embodiments of the present invention more apparent, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention, and it is apparent that the described embodiments are some embodiments of the present invention, but not all embodiments of the present invention. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.
Referring to fig. 2, fig. 2 is a schematic structural diagram of a synchronous rectification circuit according to an embodiment of the invention.
The synchronous rectification circuit includes:
the timing circuit 1 is used for starting timing from the moment when the secondary rectifying tube M2 of the switching power supply is turned on, and generating a turn-off permission signal if the timing time reaches a preset time threshold; otherwise, generating a turn-off prohibition signal;
a release circuit 2 for judging whether the primary side switch M1 of the switching power supply is turned on during the timing period of the timing circuit 1 according to the drain voltage of the secondary side rectifier tube M2; if not, directly outputting the signal generated by the timing circuit 1; if yes, all signals generated by the timing circuit 1 are converted into turn-off permission signals to be output;
A control circuit 3, configured to control the secondary rectifying tube M2 to be turned on when receiving an on signal; when receiving the off permission signal outputted from the release circuit 2, the secondary rectifying tube M2 is controlled to be turned off.
It should be noted that, the preset of the present application is set in advance, and only needs to be set once, and no resetting is needed unless the modification is needed according to the actual situation.
Specifically, the synchronous rectification circuit of the application comprises a timing circuit 1, a release circuit 2 and a control circuit 3, and the working principle is as follows:
The control circuit 3 controls the secondary rectifying tube M2 of the switching power supply to be turned on, and particularly controls the secondary rectifying tube M2 to be turned on when receiving an on signal. When the secondary rectifying tube M2 is turned on, the timer circuit 1 starts timing, and generates a turn-off prohibition signal to the release circuit 2 when the timing time does not reach the preset time threshold, and generates a turn-off permission signal to the release circuit 2 when the timing time reaches the preset time threshold. That is, the timer circuit 1 is configured to allow the secondary rectifying tube M2 to be turned off after the on time of the secondary rectifying tube M2 maintains a preset time threshold.
The release circuit 2 receives the off prohibition signal or the off permission signal output from the timer circuit 1, and determines whether the primary switch M1 of the switching power supply is turned on during the timer period of the timer circuit 1 according to the magnitude of the drain voltage (VDET voltage in fig. 2) of the secondary rectifier M2. If the canceling circuit 2 receives the shutdown prohibition signal output by the timer circuit 1, it is considered that the timer circuit 1 does not allow the secondary rectifying tube M2 to be turned off at this time, and if it is determined that the primary switch M1 is not turned on at this time, the primary and secondary common phenomenon is not caused, the shutdown prohibition signal output by the timer circuit 1 does not need to be canceled, and the shutdown prohibition signal output by the timer circuit 1 is directly output to the control circuit 3; if it is determined that the primary switch M1 is turned on, the primary-secondary common phenomenon is caused, and the shutdown prohibition signal output by the timer circuit 1 needs to be released, specifically, the shutdown prohibition signal output by the timer circuit 1 is converted into a shutdown permission signal and output to the control circuit 3. When receiving the turn-off permission signal outputted from the release circuit 2, the control circuit 3 controls the secondary rectifying tube M2 to turn off, thereby avoiding the common phenomenon of primary and secondary sides.
When the release circuit 2 receives the off permission signal output from the timer circuit 1, it is considered that the timer circuit 1 permits the secondary rectifying tube M2 to be turned off at this time, in this case, the off permission signal output from the timer circuit 1 may be directly output to the control circuit 3 regardless of the state of the primary switch M1, and the control circuit 3 may control the secondary rectifying tube M2 to be turned off when receiving the off permission signal output from the release circuit 2.
It should be noted that the present application does not have any influence on the normal operation of the secondary rectifying tube M2.
The application provides a synchronous rectification circuit which comprises a timing circuit, a release circuit and a control circuit. The timing circuit is used for starting timing from the moment when the secondary rectifying tube of the switching power supply is turned on, and generating a turn-off permission signal if the timing time reaches a preset time threshold; otherwise, generating a turn-off prohibition signal; the releasing circuit is used for judging whether the primary side switch of the switching power supply is turned on during the timing period of the timing circuit according to the drain voltage of the secondary side rectifying tube; if not, directly outputting the signal generated by the timing circuit; if yes, all signals generated by the timing circuit are converted into turn-off permission signals to be output; the control circuit is used for controlling the secondary rectifying tube to be turned on when receiving the turn-on signal; and when receiving the turn-off permission signal output by the release circuit, controlling the turn-off of the secondary rectifying tube. Therefore, when the secondary rectifying tube is turned on, if the primary switch is turned on by mistake, the secondary rectifying tube is directly turned off, and the allowable turn-off time of the secondary rectifying tube is not limited, so that the phenomenon of common use of the primary and secondary sides is avoided, and the reliability of the system is improved.
Based on the above embodiments:
Referring to fig. 3, fig. 3 is a schematic diagram of a specific structure of a synchronous rectification circuit according to an embodiment of the invention.
As an alternative embodiment, the control circuit 3 includes a volt-second product comparison circuit, a first RS flip-flop U1, a second RS flip-flop U2, an NOT gate NOT, a NAND gate NAND, a first switching transistor M11, and a second switching transistor M12; wherein:
The output end of the volt-second product comparison circuit is connected with the S end of a first RS trigger U1, the Q end of the first RS trigger U1 is respectively connected with the input end of a NOT and the control end of a first switching tube M11, the output end of the NOT is connected with the first input end of the NOT, the second input end of the NOT is input with a high-level opening signal, the output end of the NOT is connected with the S end of a second RS trigger U2, the Q end of the second RS trigger U2 is connected with the control end of a second switching tube M12, the R end of the first RS trigger U1 and the R end of the second RS trigger U2 are both connected with the output end of a release circuit 2, the first end of the second switching tube M12 is connected with a direct-current power supply, the second end of the second switching tube M12 is connected with the first end of the first switching tube M11, the common end of the second switching tube M11 is connected with the control end of a secondary rectifying tube M2, and the second end of the first switching tube M11 is grounded; the first switching tube M11 is a switching tube with high-level conduction and low-level cut-off; the second switching tube M12 is a switching tube with low-level conduction and high-level cut-off; the off enable signal is a low level signal; the turn-off disable signal is a high level signal;
The volt-second product comparison circuit is used for judging whether the volt-second product of the voltages at the two ends of the secondary winding of the switching power supply is larger than a preset volt-second product threshold value, and if so, a high-level signal is generated; if not, a low level signal is generated.
Specifically, the control circuit 3 of the present application includes a volt-second product comparison circuit, a first RS flip-flop U1, a second RS flip-flop U2, a NOT, a NAND gate NAND, a first switching tube M11 and a second switching tube M12, and the working principle thereof is as follows:
The principle of the RS flip-flop is: when the state of the input signal of the S end of the RS trigger changes, the Q end of the RS trigger outputs a low level no matter how the state of the input signal of the R end of the RS trigger is kept; when the state of the input signal at the R terminal of the RS flip-flop changes, the Q terminal of the RS flip-flop outputs a high level regardless of the state of the input signal at the S terminal of the RS flip-flop.
Based on this, in the primary on time TONP of the switching power supply, the volt-second product comparison circuit compares the volt-second product of the voltages at the two ends of the secondary winding of the switching power supply with the preset volt-second product threshold v×s_th, if the current volt-second product of the voltages at the two ends of the secondary winding is greater than the preset volt-second product threshold v×s_th, the signal TONPDET output by the volt-second product comparison circuit is converted from low level to high level signal and is input to the S end of the first RS trigger U1, the first RS trigger U1 sets the signal Pulldown output by the Q end low, and the first switching tube M11 is turned off to prepare for the turn-on of the secondary rectifying tube M2. The Von signal is an on signal of the secondary rectifier M2, and is at a high level when the VDET voltage is smaller than the preset on threshold von_th. After the primary-side on time TONP is finished, the VDET voltage drops rapidly from high voltage to around-600 mV, the Von signal is changed from low level to high level, the inverted signal Pulldown _n of the signal Pulldown is at high level at this time, the output of the NAND gate NAND is changed from high level to low level, and is input to the S end of the second RS flip-flop U2, the second RS flip-flop U2 sets the signal Pullup output by the Q end low, the second switching tube M12 is turned on, the driving signal DRISR of the secondary rectifying tube M2 is changed from low level to high level, and the secondary rectifying tube M2 is turned on.
When the secondary rectifying tube M2 is turned on, the timing circuit 1 starts timing, if the timing time reaches the preset time threshold, the output signal of the timing circuit 1 is changed from high level to low level, and the release circuit 2 inputs the low level signal output by the timing circuit 1 to the R ends of the first RS trigger U1 and the second RS trigger U2, the first RS trigger U1 rapidly sets the signal Pulldown output by the Q end high, and simultaneously rapidly sets the Q end output signal Pullup high through the second RS trigger U2, and the secondary rectifying tube M2 is rapidly turned off. If the timing time does not reach the preset time threshold, but the releasing circuit 2 determines that the primary side switch M1 is turned on, the releasing circuit 2 actively converts the high level signal output by the timing circuit 1 into the low level signal and inputs the low level signal to the R ends of the first RS trigger U1 and the second RS trigger U2, and the secondary side rectifying tube M2 is rapidly turned off, so as to effectively avoid the common use of the primary side and the secondary side.
As an alternative embodiment, the volt-second product comparison circuit comprises a volt-second integration circuit for obtaining the volt-second product of the voltages across the secondary winding of the switching power supply and a comparator; wherein:
the output end of the volt-second integrating circuit is connected with the input positive end of the comparator, the input negative end of the comparator inputs a preset volt-second integrating threshold value, and the output end of the comparator is connected with the S end of the first RS trigger U1.
Specifically, the volt-second product comparison circuit comprises a volt-second integration circuit and a comparator, and the operating principle is as follows:
The voltage-second integrating circuit detects the potential VDET at one end of the secondary winding of the switching power supply on one hand, and detects the potential VOUT at the other end of the secondary winding of the switching power supply on the other hand so as to acquire the voltage (VDET-VOUT) at two ends of the secondary winding, then integrates the voltage (VDET-VOUT) at two ends of the secondary winding for time to acquire the voltage-second product of the voltage at two ends of the secondary winding, and sends the voltage-second product of the voltage at two ends of the secondary winding to the input positive end of the comparator.
The comparator compares the volt-second product of the voltages at the two ends of the secondary winding with a preset volt-second product threshold V.s_th, and if the current volt-second product of the voltages at the two ends of the secondary winding is larger than the preset volt-second product threshold V.s_th, the comparator outputs a high-level signal (TONPDET); otherwise, a low level signal is output.
As an alternative embodiment, the timing circuit 1 comprises a third RS flip-flop U3 and an LEB circuit 11; wherein:
The R end of the third RS trigger U3 is connected with the output end of the NOT, the S end of the third RS trigger U3 is connected with an opening signal, the Q end of the third RS trigger U3 is connected with the input end of the LEB circuit 11, and the output end of the LEB circuit 11 is connected with the release circuit 2;
The LEB circuit 11 is configured to start timing from receiving a high level signal output from the third RS flip-flop U3, and generate a low level signal as a turn-off permission signal when the timing time reaches a preset time threshold; otherwise, a high level signal is generated as the off prohibition signal.
Specifically, the timing circuit 1 of the present application includes a third RS flip-flop U3 and an LEB circuit 11, and its operation principle is:
When the secondary rectifying tube M2 is turned on, the signal Start output by the Q end of the third RS flip-flop U3 is set high to trigger the LEB circuit 11 to Start timing, and after a preset time threshold, the signal LEB output by the LEB circuit 11 is turned from high level to low level and is input to the release circuit 2.
As an alternative embodiment, the cancellation circuit 2 comprises:
the voltage comparison circuit 21 is configured to determine whether the VDET voltage at the connection between the secondary rectifier tube M2 and the secondary winding of the switching power supply is greater than a preset voltage threshold, and if yes, generate a timing release signal;
A timing release circuit 22 for directly outputting the signal generated by the timing circuit 1 when the timing release signal is not received; when the timer release signal is received, the signals generated by the timer circuit 1 are all converted into off-enable signal outputs.
Specifically, the canceling circuit 2 of the present application includes a voltage comparing circuit 21 and a timing canceling circuit 22, and its operation principle is:
Considering that when the primary side switch M1 of the switching power supply is turned on by mistake, the VDET voltage correspondingly rises rapidly, the application sets a voltage threshold Vth in advance (which can be set to be larger than the on threshold VON_th), and considers that: if the VDET voltage is greater than the preset voltage threshold Vth, the primary switch M1 of the switching power supply is turned on, so the voltage comparison circuit 21 compares the VDET voltage with the preset voltage threshold Vth, and if the VDET voltage is greater than the preset voltage threshold Vth, generates a timing release signal to the timing release circuit 22. The timer release circuit 22 directly outputs the signal generated by the timer circuit 1 to the control circuit 3 when the timer release signal is not received; when the timing release signal is received, the signals generated by the timing circuit 1 are converted into turn-off permission signals and output to the control circuit 3 so as to avoid the common use of primary and secondary sides.
As an alternative embodiment, the voltage comparison circuit 21 is embodied as a fast comparator; wherein:
The input positive end of the fast comparator is connected with VDET voltage, the input negative end of the fast comparator is connected with a preset voltage threshold value, and the output end of the fast comparator is connected with the timing release circuit 22;
The fast comparator is used for generating a high-level signal when the VDET voltage is larger than a preset voltage threshold value; otherwise, a low level signal is generated.
Specifically, the voltage comparison circuit 21 of the present application selects a fast comparator, the fast comparator compares the VDET voltage with a preset voltage threshold Vth, and if the VDET voltage is greater than the preset voltage threshold, a high level signal (i.e. the timing release signal) is generated to the timing release circuit 22; if the VDET voltage is not greater than the predetermined voltage threshold, a low signal is generated to the timing release circuit 22.
The voltage comparison circuit 21 is a fast comparator because there is little delay from the start of the disturbance to the secondary side rectifier M2 being turned off, thus ensuring that the primary and secondary sides are hardly shared.
In summary, the signal timing sequence of the synchronous rectification circuit under the abnormal condition is shown in fig. 4, and as can be seen from fig. 4, the synchronous rectification circuit of the application can effectively prevent the common primary side and secondary side under the abnormal condition, and the system reliability is greatly enhanced.
The application also provides a flyback switching power supply, which comprises a transformer comprising a primary winding and a secondary winding, a secondary rectifying tube and any synchronous rectifying circuit.
The description of the flyback switching power supply provided by the application refers to the embodiment of the synchronous rectification circuit, and the application is not repeated here.
It should also be noted that in this specification, relational terms such as first and second, and the like are used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. Moreover, the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one … …" does not exclude the presence of other like elements in a process, method, article, or apparatus that comprises the element.
The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims (7)

1. A synchronous rectification circuit, comprising:
The timing circuit is used for starting timing from the moment when the secondary rectifying tube of the switching power supply is turned on, and generating a turn-off permission signal if the timing time reaches a preset time threshold; otherwise, generating a turn-off prohibition signal;
The releasing circuit is used for judging whether the primary side switch of the switching power supply is turned on during the timing period of the timing circuit according to the drain voltage of the secondary side rectifying tube; if not, directly outputting the signal generated by the timing circuit; if yes, all signals generated by the timing circuit are converted into the turn-off permission signals to be output;
the control circuit is used for controlling the secondary rectifying tube to be turned on when receiving the turn-on signal; and when receiving the turn-off permission signal output by the release circuit, controlling the turn-off of the secondary rectifying tube.
2. The synchronous rectification circuit of claim 1, wherein said control circuit comprises a volt-second product comparison circuit, a first RS flip-flop, a second RS flip-flop, a not gate, a nand gate, a first switching tube and a second switching tube; wherein:
The output end of the volt-second product comparison circuit is connected with the S end of the first RS trigger, the Q end of the first RS trigger is respectively connected with the input end of the NOT gate and the control end of the first switching tube, the output end of the NOT gate is connected with the first input end of the NOT gate, the second input end of the NOT gate is input with a high-level opening signal, the output end of the NOT gate is connected with the S end of the second RS trigger, the Q end of the second RS trigger is connected with the control end of the second switching tube, the R end of the first RS trigger and the R end of the second RS trigger are both connected with the output end of the release circuit, the first end of the second switching tube is connected with the first end of the first switching tube, the common end of the second switching tube is connected with the control end of the auxiliary rectifying tube, and the second end of the first switching tube is grounded; the first switching tube is a switching tube with high-level conduction and low-level cut-off; the second switching tube is a switching tube with low-level conduction and high-level cut-off; the turn-off permission signal is a low level signal; the turn-off prohibition signal is a high level signal;
The volt-second product comparison circuit is used for judging whether the volt-second product of the voltages at the two ends of the secondary winding of the switching power supply is larger than a preset volt-second product threshold value, and if so, a high-level signal is generated; if not, a low level signal is generated.
3. The synchronous rectification circuit of claim 2, wherein said volt-second product comparison circuit comprises a volt-second integration circuit and a comparator for obtaining a volt-second product of voltages across a secondary winding of said switching power supply; wherein:
The output end of the volt-second integrating circuit is connected with the input positive end of the comparator, the input negative end of the comparator inputs a preset volt-second integrating threshold value, and the output end of the comparator is connected with the S end of the first RS trigger.
4. The synchronous rectification circuit of claim 2, wherein said timing circuit includes a third RS flip-flop and LEB circuit; wherein:
The R end of the third RS trigger is connected with the output end of the NOT gate, the S end of the third RS trigger is connected with the opening signal, the Q end of the third RS trigger is connected with the input end of the LEB circuit, and the output end of the LEB circuit is connected with the release circuit;
The LEB circuit is used for starting timing when receiving a high-level signal output by the third RS trigger, and generating a low-level signal as the turn-off permission signal when the timing time reaches a preset time threshold; otherwise, a high level signal is generated as the off prohibition signal.
5. The synchronous rectification circuit of claim 1, wherein said cancellation circuit comprises:
The voltage comparison circuit is used for judging whether the VDET voltage at the connection part of the secondary side rectifying tube and the secondary side winding of the switching power supply is larger than a preset voltage threshold value, and if so, a timing release signal is generated;
A timing release circuit for directly outputting a signal generated by the timing circuit when the timing release signal is not received; and when the timing release signal is received, converting signals generated by the timing circuit into the turn-off permission signal to be output.
6. The synchronous rectification circuit of claim 5, wherein said voltage comparison circuit is embodied as a fast comparator; wherein:
The input positive end of the fast comparator is connected with VDET voltage, the input negative end of the fast comparator is connected with the preset voltage threshold, and the output end of the fast comparator is connected with the timing release circuit;
the fast comparator is used for generating a high-level signal when the VDET voltage is larger than the preset voltage threshold value; otherwise, a low level signal is generated.
7. A flyback switching power supply comprising a transformer comprising a primary winding and a secondary winding, a secondary rectifier and a synchronous rectifier circuit according to any one of claims 1 to 6.
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