CN109286321A - switching power supply circuit - Google Patents

Abstract

Translated fromChinese

公开了一种开关电源电路，包括第一变压器和第二变压器、位于第一变压器的副边侧的第一脉宽调制控制芯片、位于第一变压器的原边侧的第二脉宽调制控制芯片、以及连接在第一变压器的原边绕组与参考地之间的第一功率开关，其中：交流电源提供的电能经由第一变压器从开关电源电路的电路输入端传送到电路输出端；第一脉宽调制控制芯片经由第二变压器向第二脉宽调制控制芯片传送用于控制第一功率开关的导通与关断的脉宽调制控制信号；第二脉宽调制控制芯片基于来自第一脉宽调制控制芯片的脉宽调制控制信号来控制第一功率开关的导通与关断。

A switching power supply circuit is disclosed, comprising a first transformer and a second transformer, a first pulse width modulation control chip located on the secondary side of the first transformer, and a second pulse width modulation control chip located on the primary side of the first transformer , and a first power switch connected between the primary winding of the first transformer and the reference ground, wherein: the electrical energy provided by the AC power supply is transmitted from the circuit input end of the switching power supply circuit to the circuit output end through the first transformer; the first pulse The width modulation control chip transmits the pulse width modulation control signal for controlling the turn-on and turn-off of the first power switch to the second pulse width modulation control chip via the second transformer; The pulse width modulation control signal of the modulation control chip is used to control the turn-on and turn-off of the first power switch.

Description

Switching power circuit

Technical field

The present invention relates to circuit fields, relate more specifically to a kind of switching power circuit.

Background technique

In recent years, with such as smart phone, tablet computer and laptop etc portable equipment screenBecome larger, processor becomes faster, need to increase the battery capacity of portable equipment maintain or extend portable equipment use the time.However, battery charge time increases significantly with the increase of battery capacity, this is that the most users of portable equipment are not intended toThe case where seeing.

In order to maintain battery charge time constant or shorten battery charge time, need to increase charger and adapterOutput power.By propositions such as QualCom company, Huawei Technologies company and companies of TaiWan, China MediaTek Inc.The electric power output voltage of fast charge agreement and PD2.0, PD3.0 agreement etc. adjust agreement with the appearance of this demand andOccurring, these agreements also require higher average efficiency and smaller outer dimension other than requiring output voltage variable,Therefore it needs to use synchronous rectification controlling mechanism in switching power circuit.

Summary of the invention

In view of above-described one or more problems, the present invention provides a kind of switching power circuits.

Switching power circuit according to an embodiment of the present invention, including the first transformer and the second transformer, be located at first and becomeFirst PWM controller chip of the secondary side of depressor, the second pulse width modulation controlled positioned at the primary side side of the first transformerChip and the first power switch being connected between the primary side winding of the first transformer and reference ground, in which: AC power sourceThe electric energy of offer is transmitted to circuit output end from the circuit input end of switching power circuit via the first transformer；First pulsewidthModulation control chip is via the second transformer to the transmission of the second PWM controller chip for controlling leading for the first power switchThe logical pulse-width modulation control signal with shutdown；Second PWM controller chip is based on coming from the first PWM controller chipPulse-width modulation control signal control the conducting and shutdown of the first power switch.

Detailed description of the invention

The present invention may be better understood from the description with reference to the accompanying drawing to a specific embodiment of the invention,In:

Fig. 1 shows the circuit diagram of traditional switching power circuit with synchronous rectification controlling mechanism；

Fig. 2 shows the internal frame diagrams of PWM controller chip shown in FIG. 1；

Fig. 3 A-3B, which is respectively illustrated, to be led in PWM controller chip work in discontinuous conduction mode (DCM) and continuouslyThe waveform diagram of the feedback partial pressure and current detection voltage Vcs of output voltage VO when logical mode (CCM)；

Fig. 4 shows the circuit diagram of switching power circuit according to an embodiment of the present invention；

Fig. 5 shows the internal frame diagram of the PWM controller chip of the secondary side positioned at transformer T1 shown in Fig. 4；

Fig. 6 shows the internal frame diagram of the PWM controller chip positioned at the primary side side of transformer T1 shown in Fig. 4；

Fig. 7 shows the circuit diagram of ramp generator shown in fig. 5；

Fig. 8 shows the circuit of the switching power circuit according to an embodiment of the present invention with synchronous rectification controlling mechanismFigure；

Fig. 9 shows the internal frame diagram of the PWM controller chip of the secondary side positioned at transformer T1 shown in Fig. 8.

Specific embodiment

The feature and exemplary embodiment of various aspects of the invention is described more fully below.In following detailed descriptionIn, many details are proposed, in order to provide complete understanding of the present invention.But to those skilled in the artIt will be apparent that the present invention can be implemented in the case where not needing some details in these details.Below to implementationThe description of example is used for the purpose of providing by showing example of the invention and better understanding of the invention.The present invention is neverIt is limited to any concrete configuration and algorithm set forth below, but covers member under the premise of without departing from the spirit of the present inventionAny modification, replacement and the improvement of element, component and algorithm.In the the accompanying drawings and the following description, well known structure is not shownAnd technology, it is unnecessary fuzzy to avoid causing the present invention.

Fig. 1 shows the circuit diagram of traditional switching power circuit with synchronous rectification controlling mechanism.It is shown in Fig. 1Switching power circuit in, synchronous rectification control chip (SR IC) is located at the secondary side of transformer T1, by detecting transformationWhether device T1 is in freewheeling state (that is, state that the energy stored in transformer is discharged into circuit output end) to control connectionThe conducting and shutdown of power switch M2 between the vice-side winding and circuit output end of transformer T1；Pulse width modulation controlled corePiece (PWM IC) is located at the primary side side of transformer T1, passes through the variation of the output voltage of detection circuit output end and flows through changeThe variation of the primary current of the primary side winding of depressor T1 is connected to the primary side winding of transformer T1 and with reference between ground to controlThe conducting and shutdown of power switch M1.

Specifically, whether synchronous rectification control chip is in freewheeling state by VD terminal test transformer T1, is detectingIt is in the conductive state that power switch M2 is controlled when being in freewheeling state to transformer T1, and is detecting transformer T1 afterflow knotPower switch M2 is controlled when beam or power switch M1 become on state from off state to be become turning off shape from stateState.

Due to PWM controller chip and synchronous rectification control chip independently controls power switch M1 and power is openedThe conducting and shutdown of M2 are closed, therefore there are power under certain conditions (for example, under the conditions of dynamic load switching or short circuit etc.)The case where switch M1 and power switch M2 are simultaneously turned in a short time, this, which will lead to, flows through power switch M1 and power switchThe instantaneous peak current of M2 very so as to cause the damage of these power switch or causes aircraft bombing greatly.

Fig. 2 shows the internal frame diagrams of PWM controller chip shown in FIG. 1.Pulse width modulation controlled shown in Fig. 2In chip, divider resistance Ru and Rd is anti-to the secondary side from transformer T1, characterization circuit output end output voltage VOFeedthrough voltage VFB is divided, and the feedback partial pressure of output voltage VO is generated；Lead-edge-blanking (LEB) circuit is to flowing through transformer T1'sThe current detection voltage Vcs that the primary current of primary side winding generates on current sense resistor Rs carries out lead-edge-blanking processing,Generate the blanking voltage of current detection voltage Vcs；Feedback partial pressure and current detecting electricity of the PWM comparator based on output voltage VOThe blanking voltage of Vcs is pressed to generate PWM modulation signal；Oscillator generates pulsewidth based on the feedback voltage V FB of characterization output voltage VOFixed oscillator signal；Rest-set flip-flop and driver based on the PWM modulation signal from PWM comparator and carry out self-oscillatorOscillator signal generates conducting and shutdown of the pwm control signal to control power switch M1.

Here, the voltage at the CS terminal of PWM controller chip, that is, flow through the primary side of the primary side winding of transformer T1The current detection voltage Vcs that electric current generates on current sense resistor Rs is indicated by following equation 1, and current detection voltageThe rate of rise Kr_cs of Vcs is indicated by following equation 2:

Wherein, VIN is that switching power circuit shown in FIG. 1 carries out electromagnetic interference (EMI) filter to exchange (AC) input voltageThe line voltage obtained after wave and rectification, Lm is the sensibility reciprocal of the primary side winding of transformer T1, when ton is the conducting of power switch M1Between, Rs is the resistance value of current sense resistor Rs.

After switching power circuit shown in Fig. 1 enters Closed loop operation, two input terminals of PWM comparator are received defeatedIt is equal to enter voltage, that is, the feedback partial pressure of output voltage VO is equal with the blanking voltage of current detection voltage Vcs.

Fig. 3 A-3B, which is respectively illustrated, to be led in PWM controller chip work in discontinuous conduction mode (DCM) and continuouslyThe waveform diagram of the feedback partial pressure and current detection voltage Vcs of output voltage VO when logical mode (CCM).

As shown in Figure 3A, in dcm mode, the voltage at the FB terminal of PWM controller chip, i.e. characterization output electricityPress the feedback voltage V FB of VO are as follows:

As shown in Figure 3B, in ccm mode, the voltage at the FB terminal of PWM controller chip, i.e. characterization output electricityPress the feedback voltage V FB of VO are as follows::

Wherein, Ru and Rd be respectively be connected in PWM controller chip FB terminal and systematically between partial pressure electricityThe resistance value of Ru and Rd is hindered, Vcs_peak is the maximum value of current detection voltage Vcs, and Vcs0 is the minimum of current detection voltage VcsValue.

Here, the voltage at the CS terminal of PWM controller chip, that is, flow through the primary side of the primary side winding of transformer T1The descending slope Kf_cs for the current detection voltage Vcs that electric current generates on current sense resistor Rs are as follows:

Kf_cs=Vcs/tdem=Np/Ns (Vo+Vd) Rs/Lm (equation 5)

Wherein, Np and Ns is the primary side winding of transformer T1 and the number of turns of vice-side winding respectively, and Vd is to be connected to transformerThe conducting voltage of diode between the vice-side winding and circuit output end of T1.

In the switching power circuit for combining Fig. 1, Fig. 2 and Fig. 3 A-3B description, when load transients load or load sheddingWhen, it is located at the primary side side of transformer T1 and the power switch M1 of secondary side and power switch M2 is simultaneously turned on, this will increaseThe risk of switching power circuit damage.

Fig. 4 shows the circuit diagram of switching power circuit according to an embodiment of the present invention.Fig. 5 is shown positioned at shown in Fig. 4Transformer T1 secondary side PWM controller chip internal frame diagram.Fig. 6 is shown positioned at transformer shown in Fig. 4The internal frame diagram of the PWM controller chip of the primary side side of T1.Below with reference to fig. 4 to fig. 6, detailed description is real according to the present inventionApply the switching power circuit of example.

As shown in figure 4, switching power circuit according to an embodiment of the present invention includes transformer T1 and T2, is located at transformerThe PWM controller chip 302 of the secondary side of T1, positioned at the primary side side of transformer T1 PWM controller chip 304, withAnd it is connected to the power switch M1 between the primary side winding of transformer T1 and reference ground.Here, the electric energy warp that AC power source providesCircuit output end is transmitted to from the circuit input end of switching power circuit by transformer T1；PWM controller chip 302 viaThe PWM control that transformer T2 transmits conducting and shutdown for controlling power switch M1 to PWM controller chip 304 is believedNumber (here, transformer T2 is by the pwm control signal and pulse width modulation controlled at the PWM terminal of PWM controller chip 304Pwm control signal at the PWM terminal of chip 302 is kept apart)；PWM controller chip 304 is based on coming from pulsewidth modulation controlThe pwm control signal of coremaking piece 302 controls the conducting and shutdown of power switch M1.

As shown in figure 5, error amplifier (EA) is based on the voltage at FB terminal in PWM controller chip 302(that is, characterization voltage of output voltage VO) and reference voltage Vref generate error amplification signal；Ramp generator is based on to switchThe AC-input voltage of power circuit carries out the line voltage that EMI filtering and rectification obtain, the voltage at PWM terminal (that is, pulsewidthModulation control chip 302 export pwm control signal) and VDD terminal at voltage (that is, output voltage VO) generate slopeVoltage signal；PWM comparator is based on the error amplification signal from error amplifier and from the ramp voltage of ramp generatorSignal generates PWM modulation signal；Oscillator generates the fixed oscillation of pulsewidth based on the ramp voltage signal from ramp generatorSignal；Rest-set flip-flop and driver are given birth to based on the PWM modulation signal from PWM comparator with the oscillator signal for carrying out self-oscillatorAt pwm control signal.Here, the characterization voltage of output voltage VO divides output voltage VO by divider resistance R1 and R2It generates.

As shown in fig. 6, the reduction of PWM detection unit comes from pulse width modulation controlled core in PWM controller chip 304The pwm control signal of piece 302 simultaneously carries out shaping to it；Driver is based on by the PWM detection unit reduction PWM that simultaneously shaping obtainsSignal is controlled to control the conducting and shutdown of power switch M1.

Here, when the pwm control signal that PWM controller chip 302 exports is " 1 " (that is, high level), pulsewidth tuneThe driving signal of the GATE terminal output high level of system control chip 304, it is in the conductive state to control power switch M1；WhenWhen the PWM control signal that PWM controller chip 302 exports is " 0 " (that is, low level), PWM controller chip 304GATE terminal exports low level driving signal, in an off state to control power switch M1.

In order to reach with the switching power circuit in conjunction with Fig. 1, Fig. 2 and Fig. 3 A-3B description in DCM mode and CCM mouldThe identical effect of PWM control mode under formula, the rise/fall for the ramp voltage signal that ramp generator shown in fig. 5 generatesThe current detection voltage Vcs's in switching power circuit that slope needs and combination Fig. 1, Fig. 2 and Fig. 3 A-3B is described is upperLiter/descending slope is identical or proportional relationship.

By the upper of the current detection voltage Vcs in the switching power circuit of combination Fig. 1, Fig. 2 and Fig. 3 A-3B descriptionThe equation of liter/descending slope Kr_cs/Kf_cs it is found that the rate of rise Kr_cs and VIN of current detection voltage Vcs are directly proportional,The descending slope Kf_cs and VO+Vd of current detection voltage Vcs is directly proportional.Therefore, as long as ramp generator shown in fig. 5 generatesThe rate of rise and the ramp voltage signal that VIN is directly proportional, descending slope and VO+Vd are directly proportional, it is according to an embodiment of the present inventionThe PWM control mode of switching power circuit can be with the switching power circuit for combining Fig. 1, Fig. 2 and Fig. 3 A-3B descriptionPWM control mode is fully equivalent.

In switching power circuit shown in Fig. 4, when power switch M1 is in the conductive state, the secondary side of transformer T1Voltage on winding is VIN*Ns/Np；In the case where transformer T1 is flyback transformer, on the vice-side winding of transformer T1Voltage be relative to reference negative pressure, i.e.-VIN*Ns/Np.With the switch electricity for combining Fig. 1, Fig. 2 and Fig. 3 A-3B descriptionSource circuit is similar, and Np and Ns are the primary side winding of transformer T1 and the number of turns of vice-side winding respectively, and VIN is shown in Fig. 4 opensPowered-down source circuit carries out the line voltage obtained after EMI filtering and rectification to ac input voltage.

Fig. 7 shows the circuit diagram of ramp generator shown in fig. 5.In ramp generator shown in Fig. 7, operation is putBig device (OPA) is by the voltage clamping at VD terminal in " 0 ", therefore the electric current for flowing through operational amplifier is (VIN*Ns/Np)/R0,Wherein R0 is the resistance value for the resistance R0 being connected between circuit output end and VD terminal；Flow through the 1/M mirror image electricity of operational amplifierIc is flowed to charge to capacitor Cramp when pwm control signal is " 1 "；Voltage (that is, output voltage VO) superposition one at VDD terminalA fixed voltage Va generates discharge current Id after overvoltage turns current module；Discharge current Id is " 0 " in pwm control signalWhen to capacitor Cramp discharge；Ic/Id electric current forms ramp voltage signal Vramp to the charging and discharging of capacitor Cramp.

Here, 1/M image current Ic and discharge current Id are indicated by following equation 6 and equation 7 respectively:

Wherein, R0 is the resistance value of the resistance R0 in Fig. 4, and M is the mirror image coefficient of current mirror shown in fig. 7, Rv Fig. 7Shown in voltage turn the resistance value of the resistance in current module.

Therefore, the rate of rise Kr_ramp and descending slope Kf_ramp of ramp voltage signal Vramp are as follows:

Enable Kr_cs=Kr_ramp it can be concluded that

After system determines, Ns, Np, Lm, Rs are fixed values.Therefore, can by choose meet equation 10 M,R0, Cramp, so that current detection voltage Vcs's in the rate of rise and Fig. 1 of the ramp voltage signal Vramp in Fig. 5 is upperIt is equal to rise slope.

Equally, enable Kf_cs=Kf_ramp it can be concluded that

VDD=VO in equation 11, takes Va=Vd, can be obtained:

After system determines, Ns, Np, Lm, Rs are fixed values.Therefore, can by choose meet equation 12 Rv,Cramp, so that the lower drop angle of the current detection voltage Vcs in the descending slope and Fig. 1 of the ramp voltage signal Vramp in Fig. 5Rate is equal.

In conclusion choosing suitable M, Rv, Cramp inside PWM controller chip 302 can make in Fig. 5The rate of rise of current detection voltage Vcs in the rate of rise and descending slope and Fig. 1 of ramp voltage signal Vramp is underDrop angle rate is the same, so that PWM control mode according to an embodiment of the present invention and traditional PWM control mode are fully equivalent.

Fig. 8 shows the circuit of the switching power circuit according to an embodiment of the present invention with synchronous rectification controlling mechanismFigure.Fig. 9 shows the internal frame diagram of the PWM controller chip of the secondary side positioned at transformer T1 shown in Fig. 8.In Fig. 8In switching power circuit shown in Fig. 9, when pwm control signal is " 1 ", it is connected to the primary side winding and ginseng of transformer T1The power switch M1 examined between ground is in the conductive state, is connected between the vice-side winding and circuit output end of transformer T1Power switch M2 is in an off state, therefore the secondary common problem in side of original of short time can solve；Meanwhile it controlling and believing in PWMNumber be " 0 " when, synchronous rectification (SR) detection module detection transformer T1 whether be in freewheeling state, detecting transformer T1Control power switch M2 is in the conductive state when in freewheeling state (that is, the voltage at VD terminal being negative pressure), continuous detectingIt is in an off state that stream terminates control power switch M2 when (that is, the voltage at VD terminal is positive pressure).

Those skilled in the art will be understood that above-described embodiment is illustrative and not restrictive.In different embodimentsThe different technologies feature of middle appearance can be combined, to obtain beneficial effect.Those skilled in the art are studying attached drawing, are sayingOn the basis of bright book and claims, the embodiment of other variations of revealed embodiment is will be understood that and realized.RightAny appended drawing reference in it is required that is understood not to limiting of its scope.The multiple portions occurred in claimFunction can be realized by an individual hardware or software module.Certain technical characteristics appear in different appurtenancesIt is not meant to these technical characteristics cannot be combined in it is required that obtain beneficial effect.

Claims (6)

Translated fromChinese

1.一种开关电源电路，包括第一变压器和第二变压器、位于所述第一变压器的副边侧的第一脉宽调制控制芯片、位于所述第一变压器的原边侧的第二脉宽调制控制芯片、以及连接在所述第一变压器的原边绕组与参考地之间的第一功率开关，其中：1. A switching power supply circuit, comprising a first transformer and a second transformer, a first pulse width modulation control chip located on the secondary side of the first transformer, and a second pulse width modulation control chip located on the primary side of the first transformer. A wide modulation control chip, and a first power switch connected between the primary winding of the first transformer and the reference ground, wherein:交流电源提供的电能经由所述第一变压器从所述开关电源电路的电路输入端传送到电路输出端；The electrical energy provided by the AC power source is transmitted from the circuit input end of the switching power supply circuit to the circuit output end via the first transformer;所述第一脉宽调制控制芯片经由所述第二变压器向所述第二脉宽调制控制芯片传送用于控制所述第一功率开关的导通与关断的脉宽调制控制信号；The first PWM control chip transmits a PWM control signal for controlling the turn-on and turn-off of the first power switch to the second PWM control chip via the second transformer;所述第二脉宽调制控制芯片基于来自所述第一脉宽调制控制芯片的所述脉宽调制控制信号来控制所述第一功率开关的导通与关断。The second PWM control chip controls on and off of the first power switch based on the PWM control signal from the first PWM control chip.2.如权利要求1所述的开关电源电路，其中，所述第一脉宽调制控制芯片包括误差放大器、斜坡生成器、脉宽调制比较器、振荡器、RS触发器、以及第一驱动器，其中：2. The switching power supply circuit of claim 1, wherein the first PWM control chip comprises an error amplifier, a ramp generator, a PWM comparator, an oscillator, an RS flip-flop, and a first driver, in:所述误差放大器基于所述开关电源电路的输出电压的表征电压和基准电压生成误差放大信号；The error amplifier generates an error amplification signal based on the characteristic voltage and the reference voltage of the output voltage of the switching power supply circuit;所述斜坡生成器基于对所述开关电源电路的交流输入电压进行电磁干扰滤波和整流得到的线电压、所述脉宽调制控制信号、以及所述开关电源电路的输出电压生成斜坡电压信号；The ramp generator generates a ramp voltage signal based on a line voltage obtained by performing electromagnetic interference filtering and rectification on an AC input voltage of the switching power supply circuit, the pulse width modulation control signal, and an output voltage of the switching power supply circuit;所述脉宽调制比较器基于所述误差放大信号和所述斜坡电压信号生成脉宽调制信号；the pulse width modulation comparator generates a pulse width modulation signal based on the error amplified signal and the ramp voltage signal;所述振荡器基于所述斜坡电压信号生成脉宽固定的振荡信号；The oscillator generates an oscillation signal with a fixed pulse width based on the ramp voltage signal;所述RS触发器和所述驱动器基于所述脉宽调制信号和所述振荡信号生成所述脉宽调制控制信号。The RS flip-flop and the driver generate the pulse width modulation control signal based on the pulse width modulation signal and the oscillation signal.3.如权利要求2所述的开关电源电路，其中，所述第二脉宽调制控制芯片包括脉宽调制检测单元和第二驱动器，其中：3. The switching power supply circuit according to claim 2, wherein the second pulse width modulation control chip comprises a pulse width modulation detection unit and a second driver, wherein:所述脉宽调制检测单元对来自所述第一脉宽调制控制芯片的所述脉宽调制控制信号进行还原和整形；The PWM detection unit restores and shapes the PWM control signal from the first PWM control chip;所述第二驱动器基于由所述脉宽调制检测单元还原并整形得出的所述脉宽调制控制信号来控制所述第一功率开关的导通与关断。The second driver controls the turn-on and turn-off of the first power switch based on the pulse width modulation control signal restored and shaped by the pulse width modulation detection unit.4.如权利要求2所述的开关电源电路，其中，所述斜坡生成器通过电容的充电和放电来生成所述斜坡电压信号，用于所述电容的充电电流取决于基于对所述开关电源电路的交流输入电压进行电磁干扰滤波和整流得到的线电压、所述第一变压器的副边绕组与原边绕组的匝数比、以及连接在所述运算放大器的负极输入端与所述开关电源电路的电路输出端之间的电阻的阻值，用于所述电容的放电电流取决于所述开关电源电路的输出电压和预定电压。4. The switching power supply circuit of claim 2, wherein the ramp generator generates the ramp voltage signal by charging and discharging a capacitor, and the charging current for the capacitor depends on the switching power supply based on the charging and discharging of the capacitor. The AC input voltage of the circuit is subjected to electromagnetic interference filtering and rectification to obtain the line voltage, the turns ratio of the secondary winding and the primary winding of the first transformer, and the negative input terminal connected to the operational amplifier and the switching power supply. The resistance value of the resistor between the circuit output terminals of the circuit, and the discharge current for the capacitor depends on the output voltage of the switching power supply circuit and a predetermined voltage.5.如权利要求1所述的开关电源电路，还包括连接在所述第一变压器的副边绕组与所述开关电源电路的电路输出端之间的第二功率开关，其中5. The switching power supply circuit of claim 1, further comprising a second power switch connected between the secondary winding of the first transformer and the circuit output of the switching power supply circuit, wherein在所述脉宽调制控制信号为高电平时，所述第一脉宽调制控制芯片控制所述第一功率开关处于导通状态并控制所述第二功率开关处于关断状态，When the PWM control signal is at a high level, the first PWM control chip controls the first power switch to be in an on state and controls the second power switch to be in an off state,在所述脉宽调制控制信号为低电平时，所述第一脉宽调制控制芯片控制所述第一功率开关处于关断状态，检测所述第一变压器是否处于续流状态，在检测到所述第一变压器处于续流状态时控制所述第二功率开关处于导通状态，并在检测到所述第一变压器续流结束时控制所述第二功率开关从导通状态变为关断状态。When the PWM control signal is at a low level, the first PWM control chip controls the first power switch to be in an off state, detects whether the first transformer is in a freewheeling state, and detects whether the first transformer is in a freewheeling state. When the first transformer is in a freewheeling state, the second power switch is controlled to be in an on state, and when the end of the freewheeling of the first transformer is detected, the second power switch is controlled from an on state to an off state .6.如权利要求5所述的开关电源电路，其中，所述第一脉宽调制控制芯片还包括同步整流检测单元，所述同步整流检测单元检测所述第一变压器是否处于续流状态，在检测到所述第一变压器处于续流状态时控制所述第二功率开关处于导通状态，并在检测到所述第一变压器续流结束时控制所述第二功率开关从导通状态变为关断状态。6 . The switching power supply circuit according to claim 5 , wherein the first pulse width modulation control chip further comprises a synchronous rectification detection unit, the synchronous rectification detection unit detects whether the first transformer is in a freewheeling state, and when 6 . When it is detected that the first transformer is in a freewheeling state, the second power switch is controlled to be in a conducting state, and when it is detected that the freewheeling state of the first transformer is over, the second power switch is controlled to change from a conducting state to off state.