Active Clamp Flyback Converter-Forward Converter-EEWORLD

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Fly-Forward Converter The forward converter (Fly-Forward CONVERTER) was first proposed by IR Corporation of the United States. As we all know, both the forward converter and the flyback converter can be applied to small and medium power high frequency switching power supplies . The main disadvantage is that the voltage stress of the switch tube is high, the forward converter needs to adopt special magnetic reset measures, and the output ripple of the flyback converter is large. Combining the forward converter and the flyback converter can combine the advantages of the two converters and overcome the disadvantages of both to a certain extent. And realize ZVS, automatic and reliable magnetic reset, lower voltage stress, etc.

The active clamp flyback-forward converter circuit proposed by Yayoi of Japan in 1994 is shown in Figure 5-13. The primary windings of the transformers Tr1 and Tr2 of the forward converter and the flyback converter are connected in series, sharing a main switch tube V1 and a clamping circuit, and the clamping circuit is connected in parallel to the primary series windings of Tr1 and Tr2. Some flyback-forward converters derived later use only one transformer, and its secondary uses a midpoint tap rectifier or a current doubler rectifier output circuit. Studying the circuit of Figure 5-13, it can be found that there is no filter inductor at the output end. The combination of the circuit of Figure 5-13 enables the two converters to supply power to the load separately in one cycle, and the secondary of the transformer is connected in parallel and staggered for output. Therefore, there is no need to connect a freewheeling diode separately, and because the output ripple is small, there is no need to add a filter inductor. For the forward converter, because the primary is connected in series with an inductor (i.e., a transformer) of a flyback converter, it is equivalent to moving the output filter inductor from the secondary to the primary.

The flyback-forward converter shown in Figure 5-13

The flyback-forward converter shown in Figure 5-13 has 8 switching modes in one switching cycle, see Table 5-3. In the table, uDS1 is the voltage at the terminal of the main switch tube V1.

Table 5-3

The main feature of Figure 5-13 is that both the main switch and the clamp switch achieve ZVS, which reduces switching losses and improves circuit conversion efficiency. Like the active clamp forward converter, the voltage on the clamp capacitor is