A new ZCS PWM full-bridge converter of buck-type for applications with very high input voltage

For applications requiring power conversion from an input voltage of several kilovolts, like power supplies in railway systems, a new soft-switching PWM full-bridge converter of buck type is proposed. The active snubber used in this purpose is formed by two transistors operated alternatively in each half-cycle and one resonant capacitor, and is inserted in the secondary-side. All the main primary-side switches are turned-on and -off with ZCS, making them suitable for an IGBT implementation. The snubber's transistors are also turned-on and -off with ZCS. The resonant energy used to get soft-switching is recycled in each cycle to the load, by enhancing thus the effective duty-ratio. No additional resonant inductor is used, the transformer leakage inductance forms the resonant circuit with the snubber's capacitor. The soft-switching realization is independent of the load. The resonant capacitor is designed such that to assure ZCS starting from the required minimal input voltage. A design-oriented steady-state analysis leads to the expressions of the dc voltage conversion ratio and ZCS analytical conditions, allowing for a trade-off design of the resonant capacitance. The simulation and experimental results confirm the detailed theoretical analysis.