An Improved Isolated Double Step Down DC-DC Converter With ZVZCS Operation

The conventional isolated double step-down DC-DC converter has attractive characteristics for high input voltage and low output voltage applications including a high step-down voltage ratio, low voltage stress for three primary switches, low primary transformer voltage, wide soft-switching range, and no saturation problem. However, this converter still has issues that limit practical application including losing zero-voltage switching in very light load conditions, high circulating current, hard-switching turn-off of primary switches, high voltage ringing, and hard-switching operation of rectifier diodes. This paper proposes a soft-switching isolated double step-down DC-DC converter. By using an active snubber on the secondary side, the proposed converter solves effectively all issues of the conventional double step-down DC-DC converter. The proposed converter can achieve zero-voltage switching turn-on, and zero-current switching turn-off for both primary and secondary switches in the entire load range, while the circulating current and output voltage ringing are eliminated. Moreover, the duty cycle is extended by the control signal of the snubber switch. In this paper, the theoretical analysis, a simulation, and a laboratory experiment are completed with 800 V input, 48 V-2 kW prototype.