An asymmetric half-bridge flyback conversion circuit

Abstract

The asymmetric half-bridge flyback conversion circuit comprises a first switch, a second switch connected in series with the first switch and in parallel with an input end, a third capacitor connected in series with a primary winding of a transformer and in parallel with the second switch, and a third switch connected in series with the third capacitor and in parallel with the input end. By adding the third switch and charging the voltage of the third capacitor to a certain value before the first switch and the second switch are turned on, the energy stored in the primary winding of the transformer can be effectively transmitted to the secondary winding in the first switching cycle.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of DC-DC conversion, and particularly relates to an asymmetric half-bridge flyback conversion circuit. BACKGROUND

[0002] The PD3.1 protocol specification increases the maximum output voltage of a PD (Power Delivery) from 20V to 48V, so that the maximum output power of the PD can be increased from 100W to 240W by using a standard 5A output line, and the increase of the voltage and the power expands the application range of the PD. High-power game notebook computers, electric bicycle chargers, and electric tools all have the opportunity to use the PD standard Type-C interface or PD charger or adapter. However, the expansion of the output voltage range brings great challenges to circuit design. The traditional flyback conversion circuit (Flyback circuit) cannot be applied to power levels above 150W due to its complete dependence on transformer energy storage. The LLC circuit cannot be applied to a 5V-48V wide output range due to its very limited output voltage regulation range. In contrast, the asymmetric half-bridge flyback conversion circuit (AHB Flyback circuit) is very suitable due to its characteristics of storing only part of the energy in the transformer and wide output voltage regulation range.

[0003] The asymmetric half-bridge flyback conversion circuit can achieve ZVS zero-voltage turn-on of the primary side switch tube and ZCS zero-current turn-off of the secondary side switch tube under heavy load, so it has very high efficiency. In order to improve the light load efficiency and standby power consumption, the asymmetric half-bridge flyback conversion circuit works in the hiccup mode, i.e. the intermittent operation mode, when it works under light load or no load, which is quite different from the working state under heavy load, and can cause some abnormal conditions, which can cause certain risks in performance and reliability of the circuit. SUMMARY

[0004] In view of the above problems, the asymmetric half-bridge flyback conversion circuit can effectively solve the above technical problems.

[0005] In order to achieve the above purpose, the application mainly adopts the following technical scheme:

[0006] An asymmetric half-bridge flyback conversion circuit, comprising,

[0007] a first switch,

[0008] a second switch connected in series with the first switch and connected in parallel with an input terminal,

[0009] a third capacitor connected in series with a primary winding of a transformer and connected in parallel with the second switch,

[0010] The third switch is connected in series with the third capacitor and in parallel with the input terminal.

[0011] The third switch is connected in series with the third capacitor through the first resistor.

[0012] The third switch is connected in series with the third capacitor through the first resistor and the primary winding of the transformer.

[0013] The secondary winding of the transformer is connected in parallel with the output terminal through the fourth switch.

[0014] The input terminal is connected in parallel with the first capacitor.

[0015] The output terminal is connected in parallel with the second capacitor.

[0016] The second switch and the first switch are complementary on and off when the third switch is on.

[0017] The present application increases the pre-charge module of the third capacitor and reasonably controls, so that the voltage of the resonance capacitor is charged to a certain value before the first and second switches are on, ensuring that the energy stored in the primary excitation inductance and the resonance capacitor can be effectively transmitted to the secondary side in the first switching cycle of each group of driving, eliminating the invalid switching action of the first and second switches, reducing the number of switches and the corresponding switching loss, improving the efficiency of the circuit under light load and no load, and reducing the standby loss of the circuit. BRIEF DESCRIPTION OF DRAWINGS

[0018] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings needed in the embodiments of the present application will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application, and other drawings can also be obtained by those skilled in the art based on these drawings.

[0019] Figure 1 The first embodiment of the present application is shown.

[0020] Figure 2 The second embodiment of the present application is shown.

[0021] Figure 3 The third embodiment of the present application is shown.

[0022] Figure 4 The fourth embodiment of the present application is shown.

[0023] Figure 5 The Figure 1 The waveform diagram of the key signal is shown.

[0024] Figure 6 The Figure 1The waveform diagram of the key signals when the middle switch Q3 is not working.

[0025] Figure 7 For Figure 1 The waveform diagram of the key signals when the middle switch Q3 is working. DETAILED DESCRIPTION

[0026] The technical solutions of the present application will be described clearly and completely below in combination with the drawings of the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. Based on the described embodiments of the present application, all the other embodiments obtained by those skilled in the art without any creative effort belong to the scope of protection of the present application.

[0027] As Figure 1 shown in the schematic diagram of the first embodiment of the present application, the asymmetric half-bridge flyback conversion circuit includes a switch Q1 and a switch Q2, the switch Q1 and the switch Q2 are connected in series and then connected in parallel with a capacitor C1, a capacitor Cr and a primary winding N1 of a transformer T1 are connected in series and then connected in parallel with the switch Q2, a secondary winding N2 of the transformer T1 is connected in series with a switch D1 and then connected in parallel with a capacitor C2. The switch Q3 is connected in series with the capacitor Cr through a resistor R1 and then connected in parallel with the capacitor C1.

[0028] The switch Q1 is a main switch, the switch Q2 is an auxiliary switch, when the switch Q1 is turned on and the switch Q2 is turned off, the input energy of the capacitor C1 is stored in the magnetizing inductance Lm of the transformer T1 and the capacitor Cr, when the switch Q1 is turned off and the switch Q2 is turned on, the stored energy in the magnetizing inductance Lm and the capacitor Cr is transmitted to the capacitor C2 through the switch D1. The switch D1 can be a diode or a MOS tube.

[0029] When the switch Q1 is turned on and the switch Q2 is turned off, the current of the magnetizing inductance Lm rises and the voltage of the capacitor Cr rises, the input energy is stored in the magnetizing inductance Lm and the capacitor Cr. When the switch Q1 is turned off and the switch Q2 is turned on, the energy stored in the magnetizing inductance Lm and the capacitor Cr is released to the secondary side. But there is a prerequisite for the energy to be released from the magnetizing inductance Lm and the capacitor Cr to the secondary side, that is, the voltage Vcr of the capacitor Cr should be higher than or equal to the reflected voltage Vp of the output voltage Vo to the primary side, Vp=Nps*Vo, Nps is the turns ratio of the primary and secondary sides of the transformer T1. If the voltage Vcr is less than the reflected voltage Vp, the rectifier D1 of the secondary side cannot be turned on, and the energy cannot be transmitted to the secondary side. Therefore, the voltage Vcr not less than the reflected voltage Vp is the prerequisite for the energy of the primary side of the transformer T to be transmitted to the secondary side of the transformer T.

[0030] When the asymmetric half-bridge flyback conversion circuit works at light load or no load, the asymmetric half-bridge flyback conversion circuit works in the hiccup mode. As Figure 5As shown, when the feedback signal COMP is higher than the first set value Vsetl, at time t1, the driving signal DRV_1 is high to drive the switch Q1 to turn on, the asymmetric half-bridge flyback conversion circuit works, the output voltage Vo rises, and the voltage Vcr on the capacitor Cr also rises. When the feedback signal COMP is lower than the second set value Vset2, at time t2, the driving signal DRV_1 is low to drive the switch Q1 to turn off, the asymmetric half-bridge flyback conversion circuit stops working, the output voltage Vo starts to discharge slowly due to the existence of the secondary side load, and the voltage Vcr on the capacitor Cr also drops slowly due to the leakage current.

[0031] Please refer to Figure 6 When the load is very light, the hiccup period is very long, and the output voltage Vo drops very slowly, and the voltage Vcr on the capacitor Cr drops faster, the voltage of the feedback signal COMP is higher than the first set value Vsetl, and the driving signal DRV_1 starts to be continuously given. When the driving signal DRV_1 of the first switch Q1 becomes high, the switch Q1 turns on, at time t1, the input energy starts to be stored in the excitation inductor Lm and the capacitor Cr, and the peak value of the current Ip in the primary winding N1 is Ipk1. Since the peak value of the current Ip is Ipk1 at the beginning, the voltage on the capacitor Cr cannot be charged high enough when the first switch Q1 is turned on, so that the voltage Vcr cannot be greater than the reflected voltage Vp when the switch Q2 is turned on for the first time, at time t2, which causes the energy stored in the primary side of the transformer T1 to be unable to be transmitted to the secondary side.

[0032] Only when the switch Q1 is turned on for the second time, at t3-t4, the peak value of the current Ip in the primary winding Np reaches Ipk2 (Ipk2>Ipk1), the input energy is stored on the capacitor Cr again, and when the auxiliary switch Q2 is turned on, at time t4, the voltage on the capacitor Cr is greater than the reflected voltage Vp, the energy stored in the primary side of the transformer T1 can be transmitted to the secondary side. In the subsequent several switching periods, the voltage Vcr can be greater than the reflected voltage Vp, so that the energy can be continuously transmitted to the secondary side. Until the output voltage Vo exceeds the third set value Vset3, the switch Q1 and the switch Q2 stop working, the circuit enters the rest mode, the output voltage Vo is discharged by the load, and the voltage on the capacitor Cr is discharged by the leakage current of the primary MOS tube. Until the output voltage Vo is lower than the fourth set value Vset4 again, the voltage of the feedback signal COMP is higher than the first set value Vsetl again, and the next group of pulses starts again.

[0033] In the first switching cycle of the driving signal DRV_1, the voltage Vcr can not be greater than the reflected voltage Vp, so the first switching cycle of the driving signal DRV_1 can not transmit energy to the secondary side, although the switches Q1 and Q2 have switching actions. This condition increases the switching loss of the circuit, especially the efficiency and standby power consumption of light load and no load, and if the secondary side uses synchronous rectification, it can also cause reliability problems of the synchronous rectification.

[0034] The switch Q3 of the present application precharges the voltage on the capacitor Cr to a certain value before the switch Q1 is turned on, ensuring that the voltage on the capacitor Cr is greater than the reflected voltage Vp when the switch Q1 is turned on, so that the energy stored in the primary side Lm and Cr can be transmitted to the secondary side when the switch Q2 is turned on. This eliminates the first invalid switching action of the primary side switch, reduces the number of switches, improves the efficiency of the circuit under light load and no load, and reduces the standby power consumption.

[0035] The switching Q3 control logic is as shown in Figure 7 In the belching mode, the driving signal of the switch Q3 is given at t1 before the start of a group of drives, and the switch Q3 is turned on for a period of Tp until t2. During the period of t1-t2, the input voltage Vin charges the capacitor Cr through the switch Q3 and the resistor R1, and the voltage of the capacitor Cr rises. The voltage rise ΔVcr can be approximately expressed as:

[0036]

[0037] The voltage V Cr is the average voltage on the capacitor Cr, V in is the input voltage, R1 is the resistance of the resistor R1, and Tp is the length of time when the switch Q3 is turned on. By setting the resistance of the resistor R1 and the length of time Tp, the voltage rise ΔVcr of the resonant capacitor in the precharge stage can be adjusted. Through reasonable control, it can be ensured that the voltage Vcr is not less than the reflected voltage Vp after the switch Q3 is turned off, so that the energy stored in the excitation inductor Lm and the capacitor Cr can be transmitted to the secondary side when the switch Q2 is turned on.

[0038] Figure 2 Another embodiment of the present application is shown in the figure, which is an asymmetric half-bridge flyback conversion circuit. The asymmetric half-bridge flyback conversion circuit includes switches Q1 and Q2, which are connected in series and connected in parallel with capacitor C1. The capacitor Cr and the primary winding Np of the transformer T1 are connected in series and connected in parallel with the switch Q2. The secondary winding Ns of the transformer T1 is connected in series with the switch D1 and connected in parallel with the capacitor C2. The switch Q3 is connected in series with the capacitor Cr through the resistor R1 and the primary winding N1, and connected in parallel with the capacitor C1.

[0039] Switch Q1 is a main switch, and switch Q2 is an auxiliary switch. When switch Q1 is turned on, the energy input by capacitor C1 is stored in magnetizing inductance Lm of transformer T1 and capacitor Cr. When switch Q2 is turned on, the energy stored in magnetizing inductance Lm and capacitor Cr is transmitted to capacitor C2 through switch D1. Switch D1 can be a diode or a MOS tube.

[0040] Figure 3 For another embodiment of the present application, in which the asymmetric half-bridge flyback conversion circuit includes switch Q1 and switch Q2, which are connected in series and then connected in parallel with capacitor C1. Switch Q2 is an upper tube, and switch Q1 is a lower tube. Capacitor Cr and primary winding Np of transformer T1 are connected in series and then connected in parallel with switch Q2. Secondary winding Ns of transformer T1 and switch D1 are connected in series and then connected in parallel with capacitor C2. Switch Q3 is connected in series with capacitor Cr and primary winding N1 through resistor R1 and then connected in parallel with capacitor C1.

[0041] Switch Q1 is a main switch, and switch Q2 is an auxiliary switch. When switch Q1 is turned on, the energy input by capacitor C1 is stored in magnetizing inductance Lm of transformer T1 and capacitor Cr. When switch Q2 is turned on, the energy stored in magnetizing inductance Lm and capacitor Cr is transmitted to capacitor C2 through switch D1. Switch D1 can be a diode or a MOS tube.

[0042] Figure 4 For another embodiment of the present application, in which the asymmetric half-bridge flyback conversion circuit includes switch Q1 and switch Q2, which are connected in series and then connected in parallel with capacitor C1. Switch Q2 is an upper tube, and switch Q1 is a lower tube. Capacitor Cr and primary winding Np of transformer T1 are connected in series and then connected in parallel with switch Q2. Secondary winding Ns of transformer T1 and switch D1 are connected in series and then connected in parallel with capacitor C2. Switch Q3 is connected in series with capacitor Cr and primary winding N1 through resistor R1 and then connected in parallel with capacitor C1.

[0043] Switch Q1 is a main switch, and switch Q2 is an auxiliary switch. When switch Q1 is turned on, the energy input by capacitor C1 is stored in magnetizing inductance Lm of transformer T1 and capacitor Cr. When switch Q2 is turned on, the energy stored in magnetizing inductance Lm and capacitor Cr is transmitted to capacitor C2 through switch D1. Switch D1 can be a diode or a MOS tube.

[0044] By increasing the switch Q3 and resistor R1 and reasonable control, so that the switch Q1 and switch Q2 open before, the voltage Vcr of capacitor Cr is charged to a certain value, ensure that in the first switch cycle of each group of drive signal can effectively transfer the energy stored in the excitation inductance Lm and capacitor Cr to the secondary side, avoid the switch Q1 and Q2 invalid switch action, reduce the number of switching and the corresponding switching loss, improve the circuit light load and no load efficiency, reduce the standby loss of circuit. In the secondary side switch D1 using synchronous rectification technology, also can improve the reliability of the circuit work, has very important practical value.

[0045] It can be understood that the above specific description of the present application is only for illustrating the present application and is not limited to the technical solutions described in the embodiments of the present application. Those skilled in the art should understand that the present application can still be modified or replaced equivalently to achieve the same technical effect; as long as the use needs are met, it is within the protection scope of the present application.

Claims

1. An asymmetrical half-bridge flyback conversion circuit, characterized by, comprising, a first switch, a second switch, in series with the first switch, and in parallel with the input of the asymmetric half-bridge flyback conversion circuit, a third capacitor, in series with the primary winding of the transformer, and in parallel with the second switch, a third switch, in series with the third capacitor, and in parallel with the input of the asymmetric half-bridge flyback conversion circuit; the third switch charges the third capacitor, so that the voltage on the third capacitor is greater than or equal to the reflected voltage of the output voltage to the primary winding before the first switch and the second switch are turned on; the third switch is turned on, and after a first time, the third switch is turned off, and then the second switch and the first switch are complementary on and off; the control logic of the third switch includes: in the hiccup mode, at a first time before the start of a group of drives, the drive signal of the third switch is given, the third switch is turned on for a first time, until the end of the second time, during the time from the first time to the second time, the input voltage charges the third capacitor through the third switch and the first resistor, and the voltage of the third capacitor rises; the voltage rise of the third capacitor is: ， The voltage Vcr is the average voltage on the third capacitor, Vin is the input voltage, R1 is the resistance value of the first resistor, T P is the first time length.

2. An asymmetrical half bridge flyback converter circuit as claimed in claim 1, wherein, the third switch is in series with the first resistor and the third capacitor.

3. An asymmetrical half bridge flyback converter circuit as defined in claim 2 wherein, the third switch is in series with the first resistor and the primary winding of the transformer.

4. An asymmetrical half bridge flyback converter circuit as claimed in claim 3, wherein, the secondary winding of the transformer is in parallel with the output through the fourth switch.

5. An asymmetrical half bridge flyback converter circuit as claimed in claim 4, wherein, the input is in parallel with the first capacitor.

6. An asymmetrical half bridge flyback converter circuit as claimed in claim 5, wherein, the output is in parallel with the second capacitor.

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