Application of single-ended flyback circuit in inverter power supply

　　At present, battery-powered inverter power supplies generally consist of two stages. The front-stage DC/DC circuit converts the battery voltage into a DC voltage of about 350V, and the rear-stage DC/AC circuit converts the DC 350V voltage into an AC voltage of 220V. In this type of inverter power supply, the front-end DC/DC circuit generally has a low supply voltage (12V, 24V or 48V), a large input current, a high conduction voltage drop of the power tube, and a large loss, so it is difficult to improve the power supply efficiency. Its circuit forms include: single-ended flyback, single-ended forward, dual-tube forward, half-bridge and full-bridge, etc. For small and medium power (about 0.5~1kW), the single-ended flyback circuit has certain advantages, such as: circuit Simple, convenient to control, high efficiency, etc. This article takes 24V battery power supply and output 350V/1kW as an example to discuss the application of single-ended flyback circuit in the front-end DC/DC circuit of the inverter power supply.

　　1Conventional single-ended flyback circuit structure

　　The structure of the conventional single-ended flyback circuit is shown in Figure 1. The disadvantage of this circuit is that when the power tube VT is turned off, the reverse peak energy of the transformer primary is consumed by the absorption circuit composed of VD1, C1 and R1; and when the output power is the same In this case, the power tube current (compared to multiple tubes connected in parallel) is large, the conduction voltage drop is high, and the loss is large, so the efficiency and reliability are low.

　　Figure 1 Conventional single-ended flyback circuit structure

　　2 Multi-tube parallel single-ended flyback circuit structure

　　As shown in Figure 2, the characteristic of this circuit is that the main power circuit uses four power tubes connected in parallel. The current passing through each power tube is 1/4 of that of a single tube application (assuming that the parameters of the four power tubes are consistent), then the power The conduction voltage drop of the tube should also be 1/4 of that of a single tube. According to calculations, when the output is 550W, theoretically, four tubes in parallel can reduce the on-state loss by about 20W compared with a single tube, and improve the efficiency by nearly 3 percentage points. .

　　Figure 24: Power tubes connected in parallel to the main power circuit

　　3Single-ended flyback circuit structure using energy feedback technology

　　The structure of a single-ended flyback circuit using energy feedback technology is shown in Figure 3, and its main waveforms are shown in Figure 4. In this circuit, capacitor C2, inductor L1, diodes VD1 and VD2 are used to form the primary reverse peak absorption circuit of the transformer, which can make most of the reverse peak energy fed back to the input capacitor C1, reducing energy loss and improving circuit efficiency.

　　Figure 3 Primary reverse peak absorption circuit

　　Figure 4 Main waveforms of the primary reverse peak absorption circuit

　　Here's how it works:

　　(1) t0~t1 stage.

　　At time t0, the power tube is cut off, and the transformer primary inductor L, leakage inductance LK, capacitor C2 and power tube output capacitor C0 begin to resonate, and soon the C2 voltage reaches U0 (N1/N2). Then the secondary diode conducts, and the primary voltage is Clamped to U0 (N1/N2), the primary inductor L exits resonance, and IK is 0 at time t1. At the same time, the voltage on C2 and C0 reaches the maximum value, that is, the switching tube voltage US reaches the maximum value (UIN+UC2MXA).

　　(2) t1~t2 stage.

　　When LK, C2, and C0 continue to resonate, at the same time, the inductor L1 participates in the resonance. C2 and C0 feed back energy to the input capacitor C1 and replenish energy to L1. At t2, the resonance stops and the C2 voltage drops to U0 (N1/N2).

　　(3) t2~t3 stage.

　　Starting from time t2, the inductor L1 feeds back energy to the input capacitor C1.

　　The voltage of C2 is clamped at (N1/N2) U0, and the voltage on C0, that is, the voltage on the switch tube is UIN+ (N1/N2) U0, remains unchanged. By time t3, the energy in L1 has been released.

　　(4) t3~t4 stage.

　　The switch tube is completely cut off, and the C2 voltage and C0 voltage (that is, the switch tube voltage) continue to remain unchanged.

　　(5) t4~t5 stage.

　　At time t4, the power tube is turned on, its voltage US begins to decrease, C0 begins to discharge through the switching tube, and is quickly completed (all losses are on the power tube); C2 and L1 begin to resonate, that is, the energy in C2 is transferred to L1 , the current in L1 reaches the maximum value at time t5, and the power tube is fully turned on.

　　(6) t5~t6 stage.

　　At time t5, L1 feeds back energy to the input capacitor C1 through VD1 and VD2, and charges C2 to -UIN. By time t6, the energy in L1 is completely released.

　　(7) t6~t7 stage.

　　At this stage, the power tube continues to be fully conductive.

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