Isolated Three-Level Converter

The conversion method and steps of the non-isolated three-level converter introduced above can also be basically applied to the conversion of the isolated three-level converter. Commonly used isolated basic converters include forward, flyback, push-pull, half-bridge, full-bridge, etc. The following takes the isolated forward converter as an example to illustrate the conversion of the isolated three-level converter.

1 Three-level circuit of isolated forward converter

The process of converting an isolated forward converter (with magnetic reset winding) into a three-level circuit is shown in Figure 1. Using the conversion method and steps given above, an isolated forward three-level converter as shown in Figure 1 (a) can be obtained. If WP = Wr of a general forward converter is considered, the voltage stress of the two switches is 2Ui, and the electrical stress of one switch is Ui. For this reason, the power supply voltage should be taken as the clamping power supply. In this way, the cathode of the clamping diode D4 should be toward the midpoint of the clamping voltage, as shown in Figure 1 (a).

Figure 1 Schematic diagram of the evolution of the three-level circuit of the isolated forward converter

In order to simplify the circuit, first move the upper switch tube in the figure to the primary winding Wp, and keep the others unchanged. At this time, the two switch tubes are turned on and off at the same time. In the figure, Wp is the primary working winding and Wr is the magnetic reset winding. However, if the lower switch tube is turned off first and the upper switch tube is turned off after a delay, the current ip becomes a circulating current due to the guidance of the clamping diode D4. This circulating current flows from the same end, but the voltage is 0, which is similar to the role of the diode D3 in Figure 1 (b) connecting the same end of the magnetic reset winding Wr to 0 level to play the role of magnetic reset. Therefore, only the diode D3 can be left, and the magnetic reset winding Wr can be removed, so that the working winding Wp can also play the role of magnetic reset wr. In this way, the simplified circuit shown in Figure 1 (c) is formed, and the voltage stress on each switch tube is only Ui. This circuit is essentially the dual-tube forward PWM DC/DC converter shown in Figure 1. In recent years, it has been a hot project for development. The features of the development are: using power MOSFET tubes with high frequency and low voltage resistance as switch tubes; using four switch tubes (two series and two parallel) dual forward converter three-level circuit, so that the stress of the switch tube is only Ui/22; in order to reduce the size and improve work efficiency, soft switching (ZVZCS) technology should be used, which will not cause the bridge arm to be directly connected, and ZVS can be achieved in the secondary series resonant inductor.