CN111628654B - Switching power supply circuit - Google Patents

Abstract

The application relates to a switching power supply circuit comprising: the primary circuit comprises a transformer, a primary winding of the transformer is connected with the first switching tube in series, a secondary winding of the transformer is connected with the second switching tube in series, and a resonance capacitor is connected between the second switching tube and the secondary winding and connected with the secondary winding in parallel; the control circuit is connected with the control end of the first switching tube on one hand and is used for controlling the first switching tube to be turned on and off, and is connected with the control end of the second switching tube on the other hand and is used for controlling the second switching tube to be turned off when the secondary side current is reduced to zero during the conduction period of the first switching tube and controlling the second switching tube to be turned on when the resonance capacitor is charged during the turn-off period of the second switching tube. According to the switching power supply, the switching tube is used for rectification, and compared with the traditional diode rectification, the rectifying loss of the circuit can be reduced.

Description

A switching power supply circuit

Technical field

The present invention relates to the field of switching power supplies, and in particular, to a switching power supply circuit.

Background technique

The switching power supply circuit usually includes a power conversion circuit at the input end and a rectification and filtering circuit at the output end. The power conversion circuit includes a transformer and a switching tube connected to the transformer. Power conversion is performed by controlling the opening and closing of the switching tube; the rectification and filtering circuit includes a rectifier. Diodes use the unidirectional conduction characteristics of diodes to output direct current. Figure 1 shows a switching power supply circuit in traditional technology, in which the switching tube Q is connected to the primary winding of the transformer, and the rectifier diode D is connected to the secondary winding of the transformer to control the turning on and off of the switching tube Q. During the turn-on and turn-off process of the switch Q, when the secondary winding current is positive, the rectifier diode is turned on, and when the secondary winding current is negative, the rectifier diode is turned off, thereby utilizing the one-way conduction of the diode for rectification. Output DC power. However, since the rectifier diode has a certain on-resistance, the rectifier diode will also produce a certain loss during the operation of the switching power supply circuit, which increases the overall power consumption of the switching power supply circuit and is not conducive to improving the power density of the switching power supply circuit.

Contents of the invention

Based on this, it is necessary to propose a new switching power supply circuit to address the problem of large rectifier diode losses in switching power supply circuits.

A switching power supply circuit, including:

Main circuit, the main circuit includes a transformer, a resonant capacitor, a first switching tube and a second switching tube, wherein the transformer includes a primary winding and a secondary winding, and the connection between the primary winding and the first switching tube is The input end and the output end are connected in series to form a primary side branch. The primary side branch is used to connect to the input power supply. The secondary side winding is connected in series with the input end and output end of the second switch tube to form a secondary side branch. , the resonant capacitor is connected between the second switch tube and the secondary winding and in parallel with the secondary winding, and the secondary branch is used to connect a load;

A control circuit that controls the turn-on and turn-off of the first switch tube and the second switch tube, wherein when the first switch tube is turned on and when the input end of the second switch tube flows from the input end of the second switch tube to the When the current at the output end of the second switch tube drops to zero, the second switch tube is controlled to turn off; when the first switch tube is turned off and the resonant capacitor is charging, the second switch tube is controlled to conduct Pass.

In the above switching power supply circuit, the secondary branch uses a second switch tube for rectification. Since the second switch tube itself does not have one-way conduction characteristics, in order to use the second switch tube for rectification, this application uses a control circuit to timely control the second switch tube. on and off. Since during the switching and turn-off process of the first switch tube, the current of the secondary branch is controlled by the current of the primary branch and fluctuates. It is defined that the secondary branch flows from the input end of the second switch to the second switch. The current at the output end of the tube is a forward current, and the direction opposite to the forward current is a reverse current. After the first switch tube is turned on, the forward current in the secondary branch gradually decreases, and when the secondary branch When the forward current drops to zero, the second switch is controlled to turn off, which is equivalent to the traditional technology that when the secondary branch generates a reverse current, the diode is turned off; after the first switch is turned off, the secondary resonant capacitor and The inductance in the circuit, such as the leakage inductance of the transformer, generates resonance. The resonant capacitor discharges in the reverse direction and then charges forward during the resonance process. When the resonant capacitor is charged, the charging current is a forward current, and the control circuit controls the second switch to turn on. , which is equivalent to the diode conducting when the secondary branch generates forward current in traditional technology. The control circuit controls the turn-on and turn-off of the second switch tube in a timely manner, so that the second switch tube has rectification characteristics and can replace the traditional rectifier diode. At the same time, since the second switch tube has a lower on-resistance than the rectifier diode, accordingly, during the operation of the switching power supply circuit, the forward voltage drop of the second switch tube is smaller and the rectification loss is lower, which is beneficial to reducing The overall power consumption of the switching power supply circuit.

In one embodiment, the main circuit further includes a resonant inductor, and the resonant inductor is connected in series to the primary branch.

In one embodiment, in the primary branch, the input end of the resonant inductor is used to connect to the input power supply, and the output end of the resonant inductor is connected to the first end of the primary winding, so The second end of the primary winding is connected to the input end of the first switching tube, and the output end of the first switching tube is grounded;

In the secondary branch, the third end of the secondary winding is connected to the input end of the second switching tube, and the fourth end of the secondary winding and the output end of the second switching tube are connected by When connecting to a load, the resonant capacitor is connected in parallel with the secondary winding and one end of the resonant capacitor is connected to the input end of the second switch tube, and the second end and the third end are the same end.

In one embodiment, the main circuit further includes an input filter capacitor and an output filter capacitor, the input filter capacitor is connected between the input end of the resonant inductor and ground, and the output filter capacitor is connected to the first between the output terminal of the second switching tube and the fourth terminal of the secondary winding.

In one embodiment, the control circuit is used to when the potential between the input terminal of the second switch tube and the output terminal of the second switch tube drops to zero during the charging process of the resonant capacitor. Control the second switch tube to conduct.

In one embodiment, the control circuit is used to control the first switching tube to conduct when the potential between the input terminal and the output terminal of the first switching tube is at the bottom, and when the electric potential flowing through the first switching tube is at the bottom, When the current of a switch tube is zero, the first switch tube is controlled to be turned off.

In one embodiment, the transformer further includes a primary auxiliary winding, and the main circuit further includes a first sampling resistor, a second sampling resistor, and a delay resistor. The first sampling resistor and the second sampling resistor It is connected in series between the fifth end and the sixth end of the primary auxiliary winding, and the sixth end of the primary auxiliary winding is grounded. One end of the first sampling resistor connected to the second sampling resistor serves as the third A sampling point, one end of the delay resistor is connected to the first sampling point, and the other end serves as the second sampling point. The fifth end of the primary auxiliary winding and the third end of the secondary winding have the same name. end;

The control circuit includes a sampling module, a calculation module and a driving module. The sampling module is connected to the first sampling point and the second sampling point respectively to sample the first sampling voltage of the first sampling point and the first sampling point. The second sampling voltage of the second sampling point, the calculation module includes a comparator, the positive input terminal of the comparator is connected to the first sampling voltage, and the negative input terminal is connected to the second sampling voltage, and the driver The module is connected to the calculation module and is used to control the conduction of the second switch when the output end of the comparator jumps from high level to low level; the calculation module is also used to calculate the The first inflection point of the first sampling voltage during the conduction period of the first switch tube, the driving module is used to control the second switch tube to turn off at the inflection point.

In one embodiment, the calculation module further includes a delay circuit and an XOR gate. On the one hand, the output end of the comparator is connected to the first input end of the XOR gate, and on the other hand, it is connected to the delay circuit. The input terminal of the time delay circuit is connected, the output terminal of the delay circuit is connected with the second input terminal of the XOR gate, and the driving module is used to control the XOR gate when the output terminal outputs a high level. The second switch tube is turned on, and the state of the second switch tube remains unchanged when the output terminal of the XOR gate outputs a low level.

In one embodiment, the second sampling voltage is delayed from the first sampling voltage by 1 ns to 2 ns.

In one embodiment, the main circuit further includes a third sampling resistor. The primary winding is connected in series with the first switching tube and the third sampling resistor and then connected to ground. The third sampling resistor is connected to the third sampling resistor. One end connected to the first switch tube is used as a third sampling point, the control circuit is used to sample the third sampling voltage of the third sampling point, and is used to control all the sampling voltages when the third sampling voltage reaches an even number of zero points. The first switch tube is turned on, and during the period when the first switch tube is turned on, when the third sampling voltage reaches an even number of zero points, the first switch tube is controlled to be turned off.

Description of drawings

Figure 1 is a circuit diagram of a switching power supply in traditional technology;

Figure 2 is a circuit diagram of a switching power supply in an embodiment of the present invention;

Figure 3 is a circuit diagram of a switching power supply in another embodiment of the present invention;

Figure 4 is a waveform diagram of relevant current and voltage in a switching power supply circuit within a control cycle according to an embodiment of the present invention;

Figure 5 is a structural framework diagram of a computing module in an embodiment of the present invention;

Figure 6 is a schematic diagram of waveform processing in the computing module in one embodiment of the present invention;

FIG. 7 is a schematic diagram of the secondary branch resonant capacitor and parasitic inductance resonant circuit in an embodiment of the present invention.

Detailed ways

In order to facilitate understanding of the present invention, the present invention will be described more fully below with reference to the relevant drawings. There is shown in the drawing a preferred embodiment of the invention. However, the invention may be embodied in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the technical field to which the invention belongs. The terminology used herein in the description of the invention is for the purpose of describing specific embodiments only and is not intended to limit the invention. As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items.

Figure 2 shows a switching power supply circuit in an embodiment of the present application, including a main circuit and a control circuit. Among them, the main circuit includes a transformer Lp, a resonant capacitor Cr, a first switching tube Q1 and a second switching tube Q2. The transformer Lp includes a primary winding Np and a secondary winding Ns. The primary winding Np and the input end of the first switching tube Q1 It is connected in series with the output terminal to form a primary side branch. This primary side branch is used to connect to the input power supply. The current of the input power supply flows through the input terminal and output terminal of the first switching tube Q1 in sequence and then flows back to the input power supply. The secondary winding Ns It is connected in series with the input end and output end of the second switching tube Q2 to form a secondary side branch. In the secondary side branch, the resonant capacitor Cr is connected between the second switching tube Q2 and the secondary winding Ns and is connected in parallel with the secondary winding Ns. , the secondary branch is used to connect the external load and provide the output voltage V _O for the external load. The induced current generated by the secondary winding Ns flows through the input terminal and the output terminal of the second switch Q2 in turn and then flows into the external load. The control circuit, on the one hand, is connected to the control terminal of the first switching tube Q1 and outputs the first control signal duty, which is used to control the on and off of the first switching tube Q1. On the other hand, it is connected to the control terminal of the second switching tube Q2. connected to output the second control signal dutySR for controlling the turn-on and turn-off of the second switch Q2. Among them, the control of the second switch Q2 specifically includes controlling the second switch Q2 to turn off when the current flowing from the input end to the output end of the second switch Q2 drops to zero during the conduction period of the first switch Q1. , and during the turn-off period of the first switch Q1, the resonant capacitor Cr resonates with the inductor in the circuit. The resonant capacitor Cr is discharged in the reverse direction and then charged forward. When the resonant capacitor Cr is charged forward, the second switch Q2 is controlled. conduction.

In this application, the control circuit controls the turning on and off of the first switching tube Q1 through the first control signal duty. During the turning on and off periods of the first switching tube Q1, the primary side current Ip will fluctuate, and the secondary side branch In the controlled and primary side branches, the secondary side current I _D will also fluctuate accordingly. Since the second switching transistor Q2 used for rectification in the secondary side branch itself does not have the one-way conduction characteristics of the traditional rectifier diode, it will not pass according to the It is automatically turned off and on due to the fluctuation of current. Therefore, a control circuit is required to control the turn-on and turn-off of the second switch Q2 in a timely manner. The secondary current I _D in the secondary branch is defined from the input end of the second switch Q2. The direction of flow to the output terminal is forward, and vice versa. When the secondary current I _D is reversed, the second switch Q2 is turned off. When the secondary current I _D is forward, the second switch Q2 is turned on. The control circuit is used to timely control the turning on and off of the second switching tube Q2. The specific details are as follows: after the first switching tube Q1 is turned on, the primary side current Ip will increase, and the forward secondary side current I _D will decrease. When the current I _D drops to zero, that is, when the secondary side current I _D is about to reverse direction, the control circuit is used to determine whether the secondary side current I _D drops to zero. If it drops to zero, it controls the second switch Q2 to turn off; After the first switch Q1 is turned off, the resonant capacitor Cr resonates with the inductor in the circuit such as the leakage inductance of the transformer Lp. The resonant capacitor Cr is discharged in the reverse direction and then charged forward. The current generated by the forward charge is a forward current. The control circuit is used to determine whether the resonant capacitor is charging forward. If it is charging forward, it controls the second switch Q2 to turn on. During the above control process, when the secondary current I _D is reversed, the second switch Q2 is turned off. When the secondary current I _D is forward, the second switch Q2 is turned on, which is equivalent to the rectifier diode in traditional technology. It is cut off when the reverse current is connected and turned on when the forward current is connected, thus realizing the rectification function of the secondary branch. In this application, the second switching tube Q2 is sampled for rectification. Since the second switching tube Q2 has a lower conduction resistance than a traditional diode, it has a lower conduction voltage drop and smaller rectification loss.

In one embodiment, as shown in FIG. 3 , the main circuit further includes a resonant inductor Lres, and the resonant inductor Lres is connected in series in the primary branch. In the switching power supply circuit, the transformer Lp has leakage inductance, and the leakage inductance can participate in resonance. When the current leakage inductance of the transformer Lp meets the resonant frequency requirements, no additional resonant inductor Lres is needed. When the resonant frequency is required to be at the megahertz level, the required resonant inductance Lres is at the micro-Henry level, and the leakage inductance of the transformer Lp is generally at the nano-Henry level, which is smaller than the required resonant inductance Lres. Therefore, additional resonant inductance Lres needs to be added to achieve resonance. The parameters meet the requirements.

In a specific embodiment, as shown in Figure 3, the primary branch of the main circuit is: the input end of the resonant inductor Lres is used as the input end of the switching power supply circuit for connecting the input power supply, and the output end of the resonant inductor Lres is connected to The first end of the primary winding Np is connected, the second end of the primary winding Np is connected to the input end of the first switching tube Q1, and the output end of the first switching tube Q1 is grounded; the secondary side branch of the main circuit is: secondary side The third end of the winding Ns is connected to the input end of the second switching tube Q2, and the fourth end of the secondary winding Ns and the output end of the second switching tube Q2 serve as the output end of the switching power supply circuit for connecting the load and the resonant capacitor. One end of Cr is connected to the input end of the second switching tube Q2, and the resonant capacitor Cr is connected in parallel with the secondary winding Ns, and the second end of the primary winding Np and the third end of the secondary winding Ns are the same ends. In this embodiment, in the primary branch, the direction in which the primary current Ip flows from the input end of the resonant inductor Lres to the output end of the resonant inductor Lres is defined as forward, and vice versa is defined as reverse. When the control circuit controls the first switch Q1 to turn on, the primary current Ip is a forward current and gradually increases, and the secondary current I _D is a forward current and gradually decreases. When the secondary current I _D decreases to zero When, the control circuit controls the second switch Q2 to turn off, which is equivalent to when the secondary current I _D reverses, the diode turns off. Then the first switch Q1 is controlled to turn off. During the turn-off period of the first switch Q1, the resonant capacitor Cr forms a resonant circuit with the leakage inductance of the transformer Lp, the resonant inductor Lres and the output capacitor Coss of the first switch Q1, and the resonant capacitor Cr reacts After being discharged and then charged forward, a forward charging current is formed during the charging process of the resonant capacitor Cr. At this time, the control circuit turns on the second switch Q2, which is equivalent to the diode conducting when the secondary branch induces a forward current. Pass. The rectification characteristics of the secondary branch are realized by controlling the turn-on and turn-off of the second switch Q2 in a timely manner by the control circuit.

In one embodiment, as shown in Figure 3, the main circuit also includes an input filter capacitor C1 and an output filter capacitor C _L. The input filter capacitor C1 is located in the primary branch and is connected between the input end of the resonant inductor Lres and ground. , the output filter capacitor C _L is located in the secondary branch and is connected between the output terminal of the second switching tube Q2 and the fourth terminal of the secondary winding Ns. By setting the input filter capacitor C1 and the output filter capacitor C _L , the input electrical signal and the output electrical signal can be filtered to generate a stable direct current. In one embodiment, as shown in Figure 3, the primary branch also includes a rectifier bridge. The output end of the rectifier bridge is connected to the input end of the resonant inductor Lres, and the input end of the rectifier bridge is used as the input end of the switching power supply circuit. , can be connected to AC power. The external AC power is first rectified by the rectifier bridge and then converted into DC power, and then pulse width modulated to obtain the required output voltage.

In one embodiment, the control circuit is used to control the second switch Q2 to turn on, specifically when the potential between the input terminal and the output terminal of the second switch Q2 drops to zero during the charging process of the resonant capacitor Cr. The second switch Q2 is controlled to be turned on. During the forward charging process of the resonant capacitor Cr, the voltage V _Cr across the resonant capacitor Cr gradually increases, and the potential between the input terminal and the output terminal of the second switching tube Q2 gradually decreases. When the voltage between the output terminal and the output terminal of the second switching tube Q2 When the potential between the two switches drops to zero, the second switch transistor Q2 is controlled to be turned on. Since the switch tube has switching losses during the switching process, in order to reduce the switching losses, soft switching technology is generally used. Soft switching technology refers to zero voltage switching (Zero Voltage Switching, ZVS) or zero current switching (Zero Current Switching, ZCS). , when the voltage is zero, the device is turned on, and when the current is zero, the device is turned off, thereby achieving zero switching loss and reducing circuit power consumption. In this embodiment, the second switch Q2 in the switching power supply circuit is turned off when the current drops to zero, and is turned on when the voltage drops to zero, realizing zero voltage switching (ZVS) or zero current switching (ZCS). The switching loss of the second switch transistor Q2 is reduced, and the power consumption of the circuit is further reduced. In one embodiment, when the resonant capacitor Cr is directly connected to the external load through the second switch Q2, and when the voltage of the resonant capacitor Cr rises to the output voltage V _O of the switching power supply circuit, the input terminal of the second switch Q2 and The potential between the output terminals is zero.

In one embodiment, the control circuit is used to control the turn-on and turn-off of the first switch Q1. Specifically, it is used to control the first switch Q1 when the potential between the input terminal and the output terminal of the first switch Q1 is at the bottom. Q1 is turned on, and when the current flowing through the first switching tube Q1 is zero, the first switching tube Q1 is controlled to be turned off. The switching of the first switch tube is designed as a soft switching form to realize zero voltage switching (ZVS) or zero current switching (ZCS), thereby reducing the switching loss of the first switch tube Q1 and further reducing the power consumption of the circuit.

In the above embodiment, the control circuit controls the turn-on and turn-off of the second switch Q2 according to each current and voltage information in the main circuit. Specifically, the required voltage and current information can be collected directly, or the required voltage and current information can be obtained indirectly. voltage or current information. In a specific embodiment, as shown in Figure 3, in the main circuit, the transformer Lp also includes a primary auxiliary winding Naux, and the main circuit also includes a first sampling resistor R1, a second sampling resistor R2 and a delay resistor Rs. The first sampling resistor R1 and the second sampling resistor R2 are connected in series between the fifth terminal and the sixth terminal of the primary auxiliary winding Naux, and the sixth terminal of the primary auxiliary winding Naux is grounded. The first sampling resistor R1 and the second sampling resistor R1 One end connected to the resistor R2 is used as the first sampling point, one end of the delay resistor Rs is connected to the first sampling point, the other end is used as the second sampling point, the fifth end of the primary auxiliary winding Naux and the third end of the secondary winding Ns have the same name as each other; in the control circuit, the control circuit includes a sampling module, a calculation module and a driving circuit module, in which the sampling module is connected to the first sampling point to sample the first sampling voltage Vsense of the first sampling point, and the sampling module is also connected to The second sampling point is connected to sample the second sampling voltage V'sense of the second sampling point. The calculation module includes a comparator. The positive input terminal of the comparator is connected to the first sampling voltage Vsense, and the negative input terminal is connected to the second sampling voltage V 'sense, the drive module is connected to the calculation module, and is used to control the second switch Q2 to turn on when the output terminal of the comparator jumps from high level to low level, and the calculation module is also used to calculate the signal in the first switch tube. The driver module is used to control the second switching transistor Q2 to turn off at the first inflection point of the first sampling voltage Vsense during the conduction period of Q1. In this embodiment, the primary side modulation mode is used, that is, the primary side auxiliary winding Naux is used to obtain the current and voltage information in the primary side branch and the secondary side branch. Compared with using an optocoupler element to obtain the secondary side electrical signal, the original side The edge modulation mode does not require the use of optocoupler components, has lower cost, reduces the design difficulty of the peripheral feedback loop, and has higher reliability.

In the above embodiment, as shown in Figure 4, after the first switch Q1 is turned on at time t0, when the secondary current I _D gradually decreases to zero, the waveform of the first sampling voltage Vsense appears an inflection point and drops rapidly. This inflection point is the first inflection point of the first sampling voltage Vsense during the conduction period of the first switch Q1, which is defined as the turn-off inflection point B. The calculation module is used to detect the turn-off inflection point B. The driving module detects the turn-off inflection point B when the turn-off inflection point occurs. At time t1, the second switching transistor Q2 is turned off. In this embodiment, after the second switch Q2 is turned off, the resonant capacitor Cr is discharged to zero in the reverse direction and then charged forward. The first sampling voltage Vsense drops rapidly to a negative value and then gradually rises. When the voltage of the resonant capacitor Cr When rising to the output voltage, the first sampling voltage Vsense has an inflection point, which is defined as the conduction inflection point C. Therefore, the calculation module is used to detect the conduction inflection point C, and the driver module can turn on the second switch transistor Q2. In this embodiment, the calculation module detects the conduction inflection point C through a comparator. As shown in Figure 6, the second sampling voltage V'sense is the first sampling voltage Vsense delayed by the delay resistor Rs. The delay range of the delay resistor Rs is 1ns~2ns. During the turn-off period of the second switch Q2 , input the first sampling voltage Vsense and the second sampling voltage V'sense into the comparator. When the comparator output voltage Vcomp jumps from high level to low level for the first time, it is the first intersection point A of the two voltage waveforms during this period. , that is, before intersection A, the first sampling voltage Vsense is greater than the second sampling voltage V'sense, and the comparator output is high level. After intersection A, the first sampling voltage Vsense is less than the second sampling voltage V'sense, comparison The output of the device is low level, and the intersection point A slightly lags behind the above-mentioned conduction inflection point C. The calculation module detects that the output voltage jumps from high level to low level for the first time during the turn-off period of the second switch, which corresponds to the Intersection A, when intersection A appears, the driver module turns on the second switch Q2. It should be noted that the second switching tube Q2 is controlled to be turned on at the intersection point A, and the intersection point A actually lags behind the conduction inflection point. Since the conduction time of the second switching tube Q2 is greater than the ideal value, the current will flow back to the original side. branch, the first switching tube Q1 is damaged, and after reaching the conduction inflection point C, the second switching tube Q2 is delayed for a period of time, which can protect the circuit.

In one embodiment, as shown in Figure 5, the above calculation module also includes a delay circuit and an XOR gate. On the one hand, the output end of the comparator is connected to the first input end of the XOR gate, and on the other hand, it is connected to the delay circuit. The input terminal is connected, the output terminal of the delay circuit is connected to the second input terminal of the XOR gate, the driver module is used to control the second switch Q2 to turn on when the output terminal of the XOR gate outputs a high level, and when the XOR gate outputs a high level, When the output terminal of the gate outputs a low level, the state of the second switch transistor Q2 remains unchanged. In this embodiment, as shown in Figure 6, the comparator output voltage Vcomp is a rectangular pulse. At the intersection point A, the comparator output voltage Vcomp jumps from high level to low level. The comparator output voltage Vcomp is delayed by The delay pulse Vcomp_de is generated after the time circuit is delayed. The delay pulse Vcomp_de and the comparator output voltage Vcomp undergo an XOR logic operation to generate the voltage dutySR'. When the delay pulse Vcomp_de and the comparator output voltage Vcomp have opposite levels, the XOR gate The output voltage dutySR' is high level, and the other outputs are low level. Therefore, at the intersection point A, the XOR gate output voltage dutySR' is high level. When the XOR gate output voltage dutySR' is high level, the second output voltage can be controlled. The switch Q2 is turned on, and when the XOR gate output is low level, the state of the second switch Q2 remains unchanged.

The control circuit controls the turning on and off of the first switching tube Q1 according to the current and voltage information of the primary branch. In one embodiment, the main circuit also includes a third sampling resistor Rsense, and the primary winding Np is connected to the first switching tube Q1 in turn. Q1 and the third sampling resistor Rsense are connected in series and then grounded. One end of the third sampling resistor Rsense connected to the first switching tube Q1 is used as the third sampling point. The control circuit is used to sample the third sampling voltage Vp of the third sampling point. When the When the third sampling voltage Vp reaches an even number of zero points, the first switching tube Q1 is controlled to be turned on. During the conduction period of the first switching tube Q1, when the third sampling voltage Vp reaches an even number of zero points, the first switching tube Q1 is controlled to be turned off. The third sampling voltage Vp is positively correlated with the primary current Ip, V _P =R _sense * I _P . The zero point of the third sampling voltage Vp corresponds to the zero point of the primary current Ip. When the third sampling voltage Vp is at the even-numbered zero point, That is, when the primary current Ip is at the even-numbered zero point, the potential between the input terminal and the output terminal of the first switching tube Q1 is at the bottom. At this time, the first switching tube Q1 is turned on to achieve zero-voltage conduction; at the When the first switch Q1 is on, when the third sampling voltage Vp is at an even zero point, that is, when the primary current Ip is at an even zero point, the first switch Q1 is controlled to be turned off to achieve zero current turn-off, thus reducing the first Switching loss of switch Q1.

In one embodiment, the first switching transistor Q1 and the second switching transistor Q2 are both Metal-Oxide-Semiconductor Field-Effect Transistor (hereinafter referred to as MOS transistor), and the first switching transistor Q1 and the second switching transistor Q2 The second switching tubes Q2 are both NMOS tubes. The following takes the first switching tube Q1 as the first NMOS tube and the second switching tube Q2 as the second NMOS tube as an example to illustrate the working process of the switching power supply circuit in this application, where the input terminal and the output terminal of the first switching tube Q1 The potential between is the source-drain voltage V _ds of the first NMOS transistor. As shown in Figure 4, within a control cycle, the work process is divided into the following four time stages:

Time period t0~t1: At time t0, the primary current Ip passes through the even-numbered zero point, and the source-drain voltage V _ds of the first NMOS tube is at the bottom. At this time, the first control signal duty is high level, controlling the first switch. When the tube Q1 is turned on, the primary current Ip increases linearly, and the secondary current I _D decreases linearly. At time t1, the secondary current I _D drops to zero, and the first sampling voltage Vsense appears at the turn-off inflection point B. At this time, the second The control signal dutySR jumps to high level, controlling the second switch Q2 to turn off, achieving zero current turn-off;

Time period t1 ~ t2: At time t1, the second switch Q2 is turned off, the resonant capacitor Cr, the resonant inductor Lres and the leakage inductance of the transformer Lp resonate, the energy of the primary winding Np is transferred to the secondary winding Ns, and the resonant capacitor Cr discharges in the reverse direction. Then it is charged forward, causing the third sampling voltage Vp to rapidly drop to a negative value and then gradually increase. The primary current Ip gradually increases and then resonates to a negative value. When the primary current Ip reaches an even number of zero points, the first control signal duty jumps to a low level, controlling the first switching tube Q1 to turn off. In this embodiment, when the first switching tube Q1 turns on the device, when the primary current Ip reaches the second zero point, the first switching tube Q1 is controlled to turn off, achieving zero current turn-off. Turn off the first switching transistor Q1 at the second even number of zero points of the primary current Ip, so that the primary branch will perform switching action after it has experienced an integer number of resonance cycles;

Time period t2 ~ t3: At time t2, the first switching tube Q1 is turned off, the resonant capacitor Cr, the resonant inductor Lres, the leakage inductance of the transformer Lp and the source-drain output capacitance of the first switching tube Q1 resonate, and the resonant capacitor Cr is charged forward, and The output voltage V _O is reached at time t3. At this time, the source-drain voltage of the second switching tube Q2 is zero, the first sampling voltage Vsense has a conduction inflection point C, and the second control signal dutySR jumps to high level, controlling the The second switch Q2 is turned on to achieve zero-voltage conduction;

Time period t3~t4: At time t3, the second switching tube Q2 is turned on, and resonance occurs in the primary branch, that is, the resonant inductor Lres, the leakage inductance of the transformer Lp and the source-drain output capacitance of the first switching tube Q1 form a resonant circuit. The current Ip is the resonant current. During the conduction period of the first switch Q1, when the primary current Ip is at the even-numbered zero point, the source-drain voltage of the first switch Q1 is just at the bottom. At this time, the first control signal duty becomes is a high level, controlling the first switch Q1 to conduct, achieving zero-voltage conduction. This completes a cycle of control, in which the on-time and off-time of the first switch Q1 are determined according to specific conditions.

It should be noted that during the time period t3 to t4, when the second switch Q2 is on, due to the existence of parasitic inductance in the circuit, the parasitic circuit includes the leakage inductance of the transformer Lp and the parasitic inductance of the wire. In the secondary branch, the parasitic inductance and the resonant capacitor Cr will form a resonant circuit as shown in Figure 7. The voltage of the resonant capacitor Cr That is, the voltage of the resonant capacitor Cr fluctuates due to resonance at this stage, causing the first sampling voltage Vsense to fluctuate accordingly during this time period.

The above embodiments only express several embodiments of the present invention, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the invention. It should be noted that, for those of ordinary skill in the art, several modifications and improvements can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention. Therefore, the scope of protection of the patent of the present invention should be determined by the appended claims.

Claims (9)

1. A switching power supply circuit, characterized in that it includes: Main circuit, the main circuit includes a transformer, a resonant capacitor, a first switching tube and a second switching tube, wherein the transformer includes a primary winding and a secondary winding, and the connection between the primary winding and the first switching tube is The input end and the output end are connected in series to form a primary side branch. The primary side branch is used to connect to the input power supply. The secondary side winding is connected in series with the input end and output end of the second switch tube to form a secondary side branch. , the resonant capacitor is connected between the second switching tube and the secondary winding and in parallel with the secondary winding, and the secondary branch is used to connect a load; and A control circuit that controls the turn-on and turn-off of the first switch tube and the second switch tube, wherein when the first switch tube is turned on and when the input end of the second switch tube flows from the input end of the second switch tube to the When the current at the output end of the second switch tube drops to zero, the second switch tube is controlled to turn off; while the first switch tube is turned off and the resonant capacitor is charging, when the second switch tube When the potential between the input terminal and the output terminal of the second switch tube drops to zero, the second switch tube is controlled to be turned on. 2. The switching power supply circuit of claim 1, wherein the main circuit further includes a resonant inductor, and the resonant inductor is connected in series to the primary branch. 3. The switching power supply circuit according to claim 2, characterized in that, In the primary branch, the input end of the resonant inductor is used to connect to the input power supply, the output end of the resonant inductor is connected to the first end of the primary winding, and the second end of the primary winding The terminal is connected to the input terminal of the first switch tube, and the output terminal of the first switch tube is grounded; In the secondary branch, the third end of the secondary winding is connected to the input end of the second switching tube, and the fourth end of the secondary winding and the output end of the second switching tube are connected by When connecting to a load, the resonant capacitor is connected in parallel with the secondary winding and one end of the resonant capacitor is connected to the input end of the second switch tube, and the second end and the third end are the same end. 4. The switching power supply circuit of claim 3, wherein the main circuit further includes an input filter capacitor and an output filter capacitor, the input filter capacitor is connected between the input end of the resonant inductor and ground, The output filter capacitor is connected between the output end of the second switch tube and the fourth end of the secondary winding. 5. The switching power supply circuit of claim 1, wherein the control circuit is configured to control the first switching tube when the potential between the input terminal and the output terminal of the first switching tube is at the bottom. Turn on, and control the first switch to turn off when the current flowing through the first switch is zero. 6. The switching power supply circuit as claimed in claim 5, characterized in that, The transformer also includes a primary auxiliary winding. The main circuit also includes a first sampling resistor, a second sampling resistor and a delay resistor. The first sampling resistor and the second sampling resistor are connected in series to the primary auxiliary winding. Between the fifth end and the sixth end of the winding, and the sixth end of the primary auxiliary winding is grounded, one end of the first sampling resistor connected to the second sampling resistor serves as the first sampling point, and the extension One end of the resistor is connected to the first sampling point, the other end serves as the second sampling point, and the fifth end of the primary auxiliary winding and the third end of the secondary winding are the same ends; The control circuit includes a sampling module, a calculation module and a driving module. The sampling module is connected to the first sampling point and the second sampling point respectively to sample the first sampling voltage of the first sampling point and the first sampling point. The second sampling voltage of the second sampling point, the calculation module includes a comparator, the positive input terminal of the comparator is connected to the first sampling voltage, and the negative input terminal is connected to the second sampling voltage, and the driver The module is connected to the calculation module and is used to control the conduction of the second switch when the output end of the comparator jumps from high level to low level; the calculation module is also used to calculate the The first inflection point of the first sampling voltage during the conduction period of the first switch tube, the driving module is used to control the second switch tube to turn off at the inflection point. 7. The switching power supply circuit of claim 6, wherein the calculation module further includes a delay circuit and an XOR gate, and the output end of the comparator is connected to the first input of the XOR gate on the one hand. terminal is connected, on the other hand, it is connected to the input terminal of the delay circuit, the output terminal of the delay circuit is connected to the second input terminal of the XOR gate, and the driving module is used to When the output terminal of the XOR gate outputs a high level, the second switch tube is controlled to be turned on, and when the output terminal of the XOR gate outputs a low level, the state of the second switch tube is kept unchanged. 8. The switching power supply circuit of claim 6, wherein the second sampling voltage is delayed by 1 ns to 2 ns compared with the first sampling voltage. 9. The switching power supply circuit of claim 5, wherein the main circuit further includes a third sampling resistor, and the primary winding is connected in series with the first switching tube and the third sampling resistor in sequence. Grounded, one end of the third sampling resistor connected to the first switching tube is used as a third sampling point, the control circuit is used to sample the third sampling voltage of the third sampling point, and is used to when the third sampling point When the third sampling voltage reaches an even number of zero points, the first switching tube is controlled to be turned on. During the conduction period of the first switching tube, when the third sampling voltage reaches an even number of zero points, the first switching tube is controlled to be turned off. .