CN111865096A - Synchronous rectification control circuit and flyback switching power supply - Google Patents
Synchronous rectification control circuit and flyback switching power supply Download PDFInfo
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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
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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
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Abstract
The invention discloses a synchronous rectification control circuit and a flyback switching power supply. The voltage-second product comparison circuit is used for generating a magnitude-second product threshold value positively correlated with the output voltage of the switching power supply, comparing the voltage-second product of the voltages at two ends of the secondary winding of the switching power supply with the voltage-second product threshold value, and generating a switching-on allowing signal when the voltage-second product is greater than the voltage-second product threshold value; the rectifier tube control circuit is used for controlling the conduction of the rectifier tube after receiving the opening permission signal. Therefore, the voltage-second product of the voltages at the two ends of the secondary winding can be used as a judgment standard to distinguish the voltages at the two ends of the secondary winding and the parasitic ringing excited by normal primary side switching action, so that the phenomenon that the secondary rectifying tube is switched on by mistake during the parasitic ringing is avoided; moreover, the volt-second product threshold value can be set in a self-adaptive mode according to the output voltage of the switching power supply, and therefore the method is suitable for the switching power supply system with multiple output voltages.
Description
Technical Field
The invention relates to the field of control of switching power supplies, in particular to a synchronous rectification control circuit and a flyback switching power supply.
Background
The primary side controlled flyback switching power supply is gradually an important electronic component power supply device due to small volume and high efficiency, and the output end of the primary side controlled flyback switching power supply is generally connected with a rectifier diode in series to provide direct current output voltage. With the development of electronic technology, the output voltage and the output power required by the load electronic component are lower and higher, so that the forward conduction voltage drop of the rectifier diode becomes a main factor for limiting the improvement of the efficiency of the switching power supply.
The conventional solution is to use a rectifier tube to simulate a diode for rectification, i.e. so-called synchronous rectification technology, and generally a Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) is used as the rectifier tube. Synchronous rectification utilizes low resistance when the MOSFET is conducted to reduce loss on a rectifier tube, and a grid control signal needs to be synchronous with the phase of rectified current.
In the prior art, the implementation manner generally adopted by the synchronous rectification control is as follows: referring to fig. 1, a typical synchronous rectification control circuit applied to the secondary side of a primary-side controlled flyback switching power supply is shown. In the primary side controlled flyback switching power supply shown in fig. 1, the switching action of the primary side switch M1 is converted by the transformer, and the voltages at the two ends of the secondary side winding have corresponding responses, so that the switching state of the primary side switch M1 can be known by detecting the change of the voltages at the two ends of the secondary side winding, and further, the synchronous control of the secondary side rectifier tube M2 is realized.
However, when the primary-side-controlled flyback switching power supply operates in DCM (Discontinuous Current Mode), the undesired parasitic element causes damped harmonic oscillation of the voltage across the secondary winding, as shown in fig. 2 a. In fig. 2a, R denotes an equivalent source-drain on-resistance of the rectifying tube M2 when it is turned on, corresponding to the linear rising segment in fig. 2 a; diode indicates that the parasitic body diode of the rectifier M2 is turned on, corresponding to the exponential section at both ends of the linear section in fig. 2 a. Because the synchronous rectification has turn-on delay and turn-off delay, namely front and back exponential sections, the synchronous rectification is conducted by a body diode.
As can be seen from fig. 2a, simply determining the polarity of the voltage across the secondary winding 103 cannot avoid the erroneous control of the rectifier M2, which may cause the reverse current in the secondary loop, resulting in unnecessary energy loss. Therefore, it is necessary to accurately distinguish between the voltage change in the secondary winding and the parasitic ringing that is excited by the normal operation of the primary switch M1.
Voltage oscillation on the secondary winding due to parasitic capacitance and leakage inductance is inevitable, and its period and amplitude also vary depending on the application environment. In the case of a low input voltage on the primary side and a high output voltage on the secondary side, the amplitude of the parasitic ringing may reach the value of the voltage across the secondary winding that was excited when the primary switch M1 was turned off, as shown in fig. 2 b. Therefore, it is difficult to avoid the malfunction due to the parasitic ringing by controlling the rectifying tube M2 according to the amplitude of the voltage across the secondary winding.
Therefore, how to provide a solution to the above technical problem is a problem that needs to be solved by those skilled in the art.
Disclosure of Invention
The invention aims to provide a synchronous rectification control circuit and a flyback switching power supply, which can distinguish the voltage at two ends of a secondary winding and parasitic damped oscillation excited by normal primary side switching action by using the voltage-second product of the voltage at two ends of the secondary winding as a judgment standard, thereby avoiding the phenomenon that a secondary rectifier tube is switched on by mistake during the parasitic damped oscillation; moreover, the volt-second product threshold value can be set in a self-adaptive mode according to the output voltage of the switching power supply, and therefore the method is suitable for the switching power supply system with multiple output voltages.
In order to solve the above technical problem, the present invention provides a synchronous rectification control circuit, including:
the voltage-second product comparison circuit is used for generating a magnitude-second product threshold value positively correlated with the output voltage of the switching power supply, comparing the voltage-second product of the voltages at two ends of a secondary winding of the switching power supply with the voltage-second product threshold value, and generating a switching-on allowing signal when the voltage-second product is larger than the voltage-second product threshold value;
and the rectifier tube control circuit is used for controlling the conduction of the rectifier tube after receiving the opening permission signal.
Preferably, the volt-second product comparison circuit comprises a volt-second integration circuit for acquiring a volt-second product of the voltage at two ends of the secondary winding, a volt-second product threshold setting circuit for setting a volt-second product threshold according to the output voltage of the switching power supply, and a comparator; wherein:
the first detection end of the volt-second integral circuit is connected with one end of a secondary winding of the switching power supply, the second detection end of the volt-second integral circuit is connected with the other end of the secondary winding, the output end of the volt-second integral circuit is connected with the positive input end of the comparator, the input end of the volt-second integral threshold setting circuit is connected with the output voltage of the switching power supply, the output end of the volt-second integral threshold setting circuit is connected with the negative input end of the comparator, and the output end of the comparator is connected with the rectifier tube control circuit;
and the comparator is used for generating a high-level signal as an opening allowing signal when the volt-second product is larger than the volt-second product threshold value.
Preferably, the volt-second product threshold setting circuit comprises a voltage division circuit, an operational amplifier, a first resistor, a transistor, a current mirror and a second resistor; wherein:
the input end of the voltage division circuit is connected with the output voltage of the switching power supply, the output end of the voltage division circuit is connected with the input positive end of the operational amplifier, the input negative end of the operational amplifier is respectively connected with the first end of the first resistor and the first end of the transistor, the output end of the operational amplifier is connected with the control end of the transistor, the second end of the transistor is connected with the first end of the current mirror, the second end of the current mirror is connected with the first end of the second resistor, the common end of the current mirror is connected with the input negative end of the comparator, and the second end of the first resistor and the second end of the second resistor are both grounded; the transistor is switched on at a high level and switched off at a low level; the current of the second resistor/the current of the first resistor is equal to the current amplification factor corresponding to the current mirror.
Preferably, the voltage-second product threshold setting circuit further includes:
the voltage maximum circuit is used for comparing the preset reference voltage with the output voltage of the voltage division circuit and outputting the maximum value of the preset reference voltage and the output voltage to the input positive end of the operational amplifier.
Preferably, the voltage divider circuit, the voltage maximum value circuit, the operational amplifier, the first resistor, the transistor, and the current mirror of the volt-second product threshold setting circuit are integrated in the same chip, and the second resistor of the volt-second product threshold setting circuit is set independently of the same chip.
Preferably, the voltage dividing circuit includes a third resistor and a fourth resistor; wherein:
the first end of the third resistor is used as the input end of the voltage division circuit, the second end of the third resistor is connected with the first end of the fourth resistor, the common end of the third resistor is used as the output end of the voltage division circuit, and the second end of the fourth resistor is grounded.
Preferably, the volt-second integration circuit comprises a reset switch, a capacitor and a voltage-to-current circuit; wherein:
the first end of the reset switch is respectively connected with the first end of the capacitor and the current output end of the voltage-to-current circuit, the common end of the reset switch is connected with the input positive end of the comparator, the voltage input end of the voltage-to-current circuit is connected with the voltages at the two ends of the secondary winding, and the second end of the reset switch and the second end of the capacitor are both grounded; wherein the capacitor discharges to ground when the reset switch is closed;
and the voltage-to-current circuit is used for converting the voltage at two ends of the secondary winding into current flowing into the capacitor according to a certain proportion.
In order to solve the technical problem, the invention also provides a flyback switching power supply which comprises a transformer comprising a primary winding and a secondary winding, a secondary rectifier tube and any one of the synchronous rectification control circuits.
The invention provides a synchronous rectification control circuit which comprises a volt-second product comparison circuit and a rectifier tube control circuit. The voltage-second product comparison circuit is used for generating a magnitude-second product threshold value positively correlated with the output voltage of the switching power supply, comparing the voltage-second product of the voltages at two ends of the secondary winding of the switching power supply with the voltage-second product threshold value, and generating a switching-on allowing signal when the voltage-second product is greater than the voltage-second product threshold value; the rectifier tube control circuit is used for controlling the conduction of the rectifier tube after receiving the opening permission signal. Therefore, the voltage-second product of the voltages at the two ends of the secondary winding can be used as a judgment standard to distinguish the voltages at the two ends of the secondary winding and the parasitic ringing excited by normal primary side switching action, so that the phenomenon that the secondary rectifying tube is switched on by mistake during the parasitic ringing is avoided; moreover, the volt-second product threshold value can be set in a self-adaptive mode according to the output voltage of the switching power supply, and therefore the method is suitable for the switching power supply system with multiple output voltages.
The invention also provides a flyback switching power supply which has the same beneficial effect as the synchronous rectification control circuit.
Drawings
In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings needed in the prior art and the embodiments will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings without creative efforts.
Fig. 1 is a circuit diagram of a synchronous rectification control circuit applied to a secondary side of a flyback switching power supply controlled by a primary side in the prior art;
FIG. 2a is a waveform diagram illustrating a first condition of the drain terminal voltage of the secondary rectifier of FIG. 1;
FIG. 2b is a waveform diagram illustrating a second condition of the drain terminal voltage of the secondary rectifier of FIG. 1;
fig. 3 is a schematic structural diagram of a synchronous rectification control circuit according to an embodiment of the present invention;
fig. 4 is a characteristic waveform diagram of a single voltage system chip VDET pin with respect to GND according to an embodiment of the present invention;
fig. 5 is a schematic structural diagram of a synchronous rectification control circuit according to an embodiment of the present invention;
fig. 6 is a schematic structural diagram of a voltage-second product comparison circuit according to an embodiment of the present invention.
Detailed Description
The core of the invention is to provide a synchronous rectification control circuit and a flyback switching power supply, which can distinguish the voltage at two ends of a secondary winding and parasitic damped oscillation excited by normal primary side switching action by using the voltage-second product of the voltage at two ends of the secondary winding as a judgment standard, thereby avoiding the phenomenon that a secondary rectification tube is switched on by mistake during the parasitic damped oscillation; moreover, the volt-second product threshold value can be set in a self-adaptive mode according to the output voltage of the switching power supply, and therefore the method is suitable for the switching power supply system with multiple output voltages.
In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all, embodiments of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
Referring to fig. 3, fig. 3 is a schematic structural diagram of a synchronous rectification control circuit according to an embodiment of the present invention.
The synchronous rectification control circuit includes:
the voltage-second product comparison circuit 1 is used for generating an amplitude-second product threshold value positively correlated with the output voltage of the switching power supply, comparing the voltage-second product of the voltages at two ends of a secondary winding of the switching power supply with the voltage-second product threshold value, and generating a switching-on allowing signal when the voltage-second product is greater than the voltage-second product threshold value;
and the rectifier tube control circuit 2 is used for controlling the conduction of the rectifier tube M2 after receiving the opening permission signal.
Specifically, the synchronous rectification control circuit of the present application includes a volt-second product comparison circuit 1 and a rectifier tube control circuit 2, and its operating principle is:
referring to fig. 4, Tonp represents the primary side on-time, and Area _ Tonp represents the volt-second product of the voltage across the secondary winding that is excited when the normal primary side switch M1 is operated; tons represents the secondary side conduction time; toff represents the time when both the primary and secondary are off, and Area _ peak1, Area _ peak2 are volt-second products of the voltage across the secondary winding during parasitic ringing. As can be seen from the analysis of fig. 4, the voltage-second product of the voltage across the secondary winding excited when the normal primary switch M1 operates is greater than the voltage-second product of the voltage across the secondary winding during the parasitic ringing, and it can be understood that, in order to distinguish the voltage across the secondary winding excited when the normal primary switch M1 operates from the parasitic ringing, a one-volt-second threshold is set in the present application, and the setting of the one-volt-second threshold needs to be satisfied: the voltage product at two ends of the secondary winding excited by the normal action of the primary side switch M1 is larger than the voltage product threshold value and larger than the voltage product at two ends of the secondary winding in the parasitic damped oscillation, so that the purpose is realized: and comparing the voltage-second product of the voltages at the two ends of the secondary winding with a set voltage-second product threshold, and allowing the secondary rectifying tube M2 to be switched on when the voltage-second product of the voltages at the two ends of the secondary winding is greater than the set voltage-second product threshold. Therefore, the voltage-second product of the voltages at the two ends of the secondary winding is used as a judgment standard, the voltages at the two ends of the secondary winding and the parasitic ringing excited by normal primary side switching action are distinguished, and therefore the phenomenon that the secondary rectifier tube is switched on by mistake during the parasitic ringing is avoided.
Meanwhile, considering that for a switching power supply system with multiple output voltages (such as a rapid charging power supply system), the voltage-second product of the voltage at two ends of the secondary winding changes along with the change of the output voltage or the load of the system, if the set voltage-second product threshold is a fixed value, the phenomenon that the secondary rectifier tube is turned on by mistake when spurious damped oscillation still occurs under certain working conditions can be caused, so in order to more accurately distinguish the voltage at two ends of the secondary winding excited when the normal primary side switch M1 acts and the spurious damped oscillation under different output voltages, the voltage-second product threshold is set to be an adjustable value and is particularly in positive correlation with the output voltage of the switching power supply, namely the voltage-second product threshold can be set in a self-adaptive manner according to the output voltage of the switching power supply, and the switching power supply system with multiple output voltages is suitable for the switching power supply system with multiple.
Based on this, the switching principle of the secondary rectifier M2 of the switching power supply is as follows: the voltage-second product comparison circuit 1 obtains a voltage-second product of voltages at two ends of a secondary winding of the switching power supply on one hand, and generates a magnitude-second product threshold value positively correlated with the output voltage of the switching power supply on the other hand, then compares the voltage-second product of the voltages at two ends of the secondary winding of the switching power supply with the currently generated voltage-second product threshold value, and if the voltage-second product is larger than the currently generated voltage-second product threshold value, generates a turn-on permission signal to the rectifier tube control circuit 2 to inform the rectifier tube control circuit 2 that the secondary rectifier tube M2 meets a voltage-second product turn-on condition. After receiving the turn-on permission signal, the rectifier control circuit 2 determines that the secondary rectifier M2 satisfies the volt-second product turn-on condition, and controls the rectifier M2 to be turned on (if the rectifier has other turn-on conditions, the rectifier M2 is controlled to be turned on when all the turn-on conditions are satisfied at the same time), so that the turn-on accuracy of the rectifier M2 is improved.
The invention provides a synchronous rectification control circuit which comprises a volt-second product comparison circuit and a rectifier tube control circuit. The voltage-second product comparison circuit is used for generating a magnitude-second product threshold value positively correlated with the output voltage of the switching power supply, comparing the voltage-second product of the voltages at two ends of the secondary winding of the switching power supply with the voltage-second product threshold value, and generating a switching-on allowing signal when the voltage-second product is greater than the voltage-second product threshold value; the rectifier tube control circuit is used for controlling the conduction of the rectifier tube after receiving the opening permission signal. Therefore, the voltage-second product of the voltages at the two ends of the secondary winding can be used as a judgment standard to distinguish the voltages at the two ends of the secondary winding and the parasitic ringing excited by normal primary side switching action, so that the phenomenon that the secondary rectifying tube is switched on by mistake during the parasitic ringing is avoided; moreover, the volt-second product threshold value can be set in a self-adaptive mode according to the output voltage of the switching power supply, and therefore the method is suitable for the switching power supply system with multiple output voltages.
On the basis of the above-described embodiment:
referring to fig. 5, fig. 5 is a schematic diagram of a specific structure of a synchronous rectification control circuit according to an embodiment of the present invention.
As an alternative embodiment, the volt-second product comparison circuit 1 includes a volt-second integration circuit 11 for obtaining a volt-second product of the voltage across the secondary winding, a volt-second product threshold setting circuit 12 for setting a volt-second product threshold according to the output voltage of the switching power supply, and a comparator 13; wherein:
a first detection end of the volt-second integration circuit 11 is connected with one end of a secondary winding of the switching power supply, a second detection end of the volt-second integration circuit 11 is connected with the other end of the secondary winding, an output end of the volt-second integration circuit 11 is connected with an input positive end of a comparator 13, an input end of a volt-second product threshold value setting circuit 12 is connected with an output voltage of the switching power supply, an output end of the volt-second product threshold value setting circuit 12 is connected with an input negative end of the comparator 13, and an output end of the comparator 13 is connected with the rectifier control circuit 2;
the comparator 13 is configured to generate a high level signal as an on-enable signal when the voltage-second product is greater than a voltage-second product threshold.
Specifically, the voltage-second product comparison circuit 1 of the present application includes a voltage-second integration circuit 11, a voltage-second product threshold setting circuit 12, and a comparator 13, and its working principle is:
the volt-second integration circuit 11 detects a potential VDET at one end of the secondary winding of the switching power supply on the one hand, and a potential VOUT at the other end of the secondary winding of the switching power supply on the other hand, to obtain a voltage (VDET-VOUT) at both ends of the secondary winding, then performs time integration on the voltage (VDET-VOUT) at both ends of the secondary winding to obtain a volt-second product of the voltages at both ends of the secondary winding, and sends the volt-second product of the voltages at both ends of the secondary winding to the positive input end of the comparator 13.
The volt-second product threshold setting circuit 12 adaptively sets a volt-second product threshold based on the output voltage of the switching power supply, and transmits the currently generated volt-second product threshold to the input negative terminal of the comparator 13. When the voltage-second product of the voltage across the secondary winding is greater than the current generated voltage-second product threshold, the comparator 13 generates a high level signal (i.e., the turn-on permission signal of the above embodiment) to the rectifier control circuit 2, so as to inform the rectifier control circuit 2 that the secondary rectifier M2 satisfies the voltage-second product turn-on condition.
It should be noted that the volt-second integration circuit 11 needs to integrate the voltages at the two ends of the secondary winding again each time the potential VDET of the secondary winding has a positive voltage, so as to avoid the influence of the last integration value on the subsequent volt-second product of obtaining the voltages at the two ends of the secondary winding.
Referring to fig. 6, fig. 6 is a schematic structural diagram of a voltage-second product comparison circuit according to an embodiment of the present invention.
As an alternative embodiment, the volt-second product threshold setting circuit 12 includes a voltage dividing circuit, an operational amplifier 120, a first resistor R1, a transistor M21, a current mirror 121, and a second resistor R2; wherein:
the input end of the voltage dividing circuit is connected with the output voltage of the switching power supply, the output end of the voltage dividing circuit is connected with the input positive end of the operational amplifier 120, the input negative end of the operational amplifier 120 is respectively connected with the first end of the first resistor R1 and the first end of the transistor M21, the output end of the operational amplifier 120 is connected with the control end of the transistor M21, the second end of the transistor M21 is connected with the first end of the current mirror 121, the second end of the current mirror 121 is connected with the first end of the second resistor R2, the common end of the second resistor R2 is connected with the input negative end of the comparator 13, and the second end of the first resistor R1 and the second end of the second resistor R2 are both grounded; the transistor M21 is a transistor that is turned on at a high level and turned off at a low level; the current of the second resistor R2/the current of the first resistor R1 is the current amplification factor corresponding to the current mirror 121.
Specifically, the volt-second product threshold setting circuit 12 of the present application includes a voltage dividing circuit, an operational amplifier 120, a first resistor R1, a transistor M21, a current mirror 121, and a second resistor R2, and its operating principle is:
the voltage divider circuit divides the output voltage VOUT of the switching power supply to obtain a divided voltage signal, and sends the divided voltage signal to the operational amplifier 120. The operational amplifier 120 amplifies the divided voltage signal to control the transistor M21 to be in a conducting state. When the output voltage of the voltage divider circuit is VOUT × K1(K1 is a positive number smaller than 1), the current of the first resistor R1 is VOUT × K1/R1. Under the operation of the current mirror 121, the current of the second resistor R2 is equal to the current of the first resistor R1, which is equal to the current amplification factor corresponding to the current mirror 121. If the current amplification factor corresponding to the current mirror 121 is K2, the current of the second resistor R2 is equal to K
Based on VOUT × K1 × K2/R1, the voltage input to the negative input terminal of the comparator 13 is VOUT × K1 × K2 × R2/R1 across the second resistor R2.
As an alternative embodiment, the volt-second product threshold setting circuit 12 further includes:
the voltage maximum value circuit 122, of which the input end is connected to the output end of the voltage divider circuit and the output end is connected to the input positive end of the operational amplifier 120, is configured to compare the preset reference voltage with the output voltage of the voltage divider circuit, and output the maximum value of the preset reference voltage and the output voltage of the voltage divider circuit to the input positive end of the operational amplifier 120.
Further, the volt-second product threshold setting circuit 12 of the present application further includes a voltage maximum value circuit 122, and the operating principle thereof is as follows:
considering that the output voltage VOUT of the switching power supply is small at the initial stage of the secondary side conduction of the switching power supply, the voltage-second product threshold setting circuit 12 of the present application adds the voltage-taking circuit 122 between the voltage dividing circuit and the operational amplifier 120, and the voltage-taking circuit 122 selects the maximum value of the voltage-second product threshold between a fixed reference voltage Vref1 set in advance and the output voltage of the voltage dividing circuit and outputs the maximum value to the operational amplifier 120, thereby more reasonably setting the voltage-second product threshold.
As an alternative embodiment, the voltage divider circuit, the voltage maximum value circuit 122, the operational amplifier 120, the first resistor R1, the transistor M21, and the current mirror 121 of the volt-second product threshold setting circuit 12 are integrated in the same chip, and the second resistor R2 of the volt-second product threshold setting circuit 12 is set independently of the same chip.
Specifically, the voltage divider circuit, the voltage maximum value circuit 122, the operational amplifier 120, the first resistor R1, the transistor M21, and the current mirror 121 of the volt-second product threshold setting circuit 12 of the present application may be integrated in the same chip, and two external pins are provided on the chip: the VOUT pin is used for connecting an output voltage of the switching power supply, and the Rx pin is used for connecting a second resistor R2 of the volt-second product threshold setting circuit 12. The reason why the second resistor R2 is provided separately from the chip is that it is convenient to replace the second resistor R2, so that the resistance value of the second resistor R2 can be adjusted, and thus the proportional relationship between the volt-second product threshold and the output voltage of the switching power supply can be adjusted.
As an alternative embodiment, the voltage dividing circuit includes a third resistor R3 and a fourth resistor R4; wherein:
the first end of the third resistor R3 is used as the input end of the voltage dividing circuit, the second end of the third resistor R3 is connected with the first end of the fourth resistor R4, the common end is used as the output end of the voltage dividing circuit, and the second end of the fourth resistor R4 is grounded.
Specifically, the voltage divider circuit of the present application includes a third resistor R3 and a fourth resistor R4, and its operating principle is:
the output voltage VOUT of the switching power supply is divided by the third resistor R3 and the fourth resistor R4, and the voltage across the fourth resistor R4 is used as the output voltage of the voltage divider circuit, i.e., the output voltage of the voltage divider circuit
＝VOUT*R4/(R3+R4)。
As an alternative embodiment, the volt-second integration circuit 11 includes a reset switch K, a capacitor C, and a voltage-to-current circuit 110; wherein:
the first end of the reset switch K is respectively connected with the first end of the capacitor C and the current output end of the voltage-to-current circuit 110, the common end of the reset switch K is connected with the input positive end of the comparator 13, the voltage input end of the voltage-to-current circuit 110 is connected with the voltages at the two ends of the secondary winding, and the second end of the reset switch K and the second end of the capacitor C are both grounded; when the reset switch K is closed, the capacitor C discharges to the ground;
the voltage-to-current circuit 110 is used for converting the voltage across the secondary winding into a current flowing into the capacitor C according to a certain proportion.
Specifically, the volt-second integration circuit 11 of the present application includes a reset switch K, a capacitor C, and a voltage-to-current circuit 110, and its working principle is:
the voltage-to-current circuit 110 converts the voltage at the two ends of the secondary winding of the switching power supply into a current according to a certain proportion, and the current flows into the capacitor C to charge the capacitor C, and the charging voltage of the capacitor C is input to the input positive terminal of the comparator 13. Let the charging current of the capacitor C be (VDET-VOUT)/R, the charging voltage of the capacitor C be ═ [ (VDET-VOUT) × C/R ] dt, i.e., the volt-second product of the voltages across the secondary winding.
It should be noted that, when the secondary winding potential VDET of the switching power supply has a positive voltage each time, the reset switch K is turned on for a short time (much shorter than the duration of the positive voltage of the secondary winding potential VDET) and then turned off (which may be controlled by the rectifier control circuit 2), so that the volt-second integrator circuit 11 integrates the voltages at the two ends of the secondary winding again, so as to avoid the influence of the previous integral value on the subsequent volt-second product for obtaining the voltages at the two ends of the secondary winding.
The application also provides a flyback switching power supply which comprises a transformer comprising a primary winding and a secondary winding, a secondary rectifier tube and any one of the synchronous rectification control circuits.
For the introduction of the flyback switching power supply provided in the present application, reference is made to the above-mentioned embodiment of the synchronous rectification control circuit, and details are not repeated herein.
It is further noted that, in the present 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. Also, 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 an … …" does not exclude the presence of other identical 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 (8)
1. A synchronous rectification control circuit, comprising:
the voltage-second product comparison circuit is used for generating a magnitude-second product threshold value positively correlated with the output voltage of the switching power supply, comparing the voltage-second product of the voltages at two ends of a secondary winding of the switching power supply with the voltage-second product threshold value, and generating a switching-on allowing signal when the voltage-second product is larger than the voltage-second product threshold value;
and the rectifier tube control circuit is used for controlling the conduction of the rectifier tube after receiving the opening permission signal.
2. The synchronous rectification control circuit of claim 1, wherein the volt-second product comparison circuit comprises a volt-second integration circuit for obtaining a volt-second product of the voltage across the secondary winding, a volt-second product threshold setting circuit for setting a volt-second product threshold according to the output voltage of the switching power supply, and a comparator; wherein:
the first detection end of the volt-second integral circuit is connected with one end of a secondary winding of the switching power supply, the second detection end of the volt-second integral circuit is connected with the other end of the secondary winding, the output end of the volt-second integral circuit is connected with the positive input end of the comparator, the input end of the volt-second integral threshold setting circuit is connected with the output voltage of the switching power supply, the output end of the volt-second integral threshold setting circuit is connected with the negative input end of the comparator, and the output end of the comparator is connected with the rectifier tube control circuit;
and the comparator is used for generating a high-level signal as an opening allowing signal when the volt-second product is larger than the volt-second product threshold value.
3. The synchronous rectification control circuit of claim 2, wherein the volt-second product threshold setting circuit comprises a voltage divider circuit, an operational amplifier, a first resistor, a transistor, a current mirror, and a second resistor; wherein:
the input end of the voltage division circuit is connected with the output voltage of the switching power supply, the output end of the voltage division circuit is connected with the input positive end of the operational amplifier, the input negative end of the operational amplifier is respectively connected with the first end of the first resistor and the first end of the transistor, the output end of the operational amplifier is connected with the control end of the transistor, the second end of the transistor is connected with the first end of the current mirror, the second end of the current mirror is connected with the first end of the second resistor, the common end of the current mirror is connected with the input negative end of the comparator, and the second end of the first resistor and the second end of the second resistor are both grounded; the transistor is switched on at a high level and switched off at a low level; the current of the second resistor/the current of the first resistor is equal to the current amplification factor corresponding to the current mirror.
4. The synchronous rectification control circuit of claim 3, wherein the volt-second product threshold setting circuit further comprises:
the voltage maximum circuit is used for comparing the preset reference voltage with the output voltage of the voltage division circuit and outputting the maximum value of the preset reference voltage and the output voltage to the input positive end of the operational amplifier.
5. The synchronous rectification control circuit of claim 4, wherein the voltage divider circuit, the voltage maximum circuit, the operational amplifier, the first resistor, the transistor, and the current mirror of the volt-second product threshold setting circuit are integrated in a same chip, and the second resistor of the volt-second product threshold setting circuit is set independently of the same chip.
6. The synchronous rectification control circuit of claim 4, wherein the voltage divider circuit includes a third resistor and a fourth resistor; wherein:
the first end of the third resistor is used as the input end of the voltage division circuit, the second end of the third resistor is connected with the first end of the fourth resistor, the common end of the third resistor is used as the output end of the voltage division circuit, and the second end of the fourth resistor is grounded.
7. The synchronous rectification control circuit of claim 2 wherein the volt-second integration circuit comprises a reset switch, a capacitor and a voltage-to-current circuit; wherein:
the first end of the reset switch is respectively connected with the first end of the capacitor and the current output end of the voltage-to-current circuit, the common end of the reset switch is connected with the input positive end of the comparator, the voltage input end of the voltage-to-current circuit is connected with the voltages at the two ends of the secondary winding, and the second end of the reset switch and the second end of the capacitor are both grounded; wherein the capacitor discharges to ground when the reset switch is closed;
and the voltage-to-current circuit is used for converting the voltage at two ends of the secondary winding into current flowing into the capacitor according to a certain proportion.
8. A flyback switching power supply comprising a transformer including a primary winding and a secondary winding, a secondary rectifier, and a synchronous rectification control circuit as claimed in any one of claims 1 to 7.
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| CN202010880101.8A CN111865096B (en) | 2020-08-27 | 2020-08-27 | Synchronous rectification control circuit and flyback switching power supply |
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| CN202010880101.8A CN111865096B (en) | 2020-08-27 | 2020-08-27 | Synchronous rectification control circuit and flyback switching power supply |
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Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN102231605A (en) * | 2011-06-30 | 2011-11-02 | 上海新进半导体制造有限公司 | Synchronous rectification control circuit of switch power supply secondary and flyback switch power supply |
| CN108539986A (en) * | 2017-12-15 | 2018-09-14 | 杰华特微电子（杭州）有限公司 | A kind of inverse-excitation type switch power-supply |
| CN213094077U (en) * | 2020-08-27 | 2021-04-30 | 上海新进芯微电子有限公司 | Synchronous rectification control circuit and flyback switching power supply |
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2020
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Patent Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN102231605A (en) * | 2011-06-30 | 2011-11-02 | 上海新进半导体制造有限公司 | Synchronous rectification control circuit of switch power supply secondary and flyback switch power supply |
| CN108539986A (en) * | 2017-12-15 | 2018-09-14 | 杰华特微电子（杭州）有限公司 | A kind of inverse-excitation type switch power-supply |
| CN213094077U (en) * | 2020-08-27 | 2021-04-30 | 上海新进芯微电子有限公司 | Synchronous rectification control circuit and flyback switching power supply |
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