A Zero-Voltage and Zero-Current Switching Three-Level DC–DC Converter With Reduced Rectifier Voltage Stress and Soft-Switching-Oriented Optimized Design

A novel zero-voltage zero-current switching (ZVZCS) three-level converter with pulsewidth modulation (PWM) phase-shift control is proposed. The ZCS of the lagging switch is obtained by using a regenerative passive snubber in the secondary. In order to reduce the voltage stress on the rectifier's diodes, a few passive elements are inserted into the primary: a small inductance, two diodes, and a small additional winding of the main transformer. In each half-cycle, one of these diodes will conduct for a short time in order to clamp the voltage of the snubber's capacitor, and thus, the rectifier stress, at (n₂ /n₁)(V_in/2), n₁, and n₂ being the transformer's primary and, respectively, secondary turns number. The three-level configuration allows for the reduction of the voltage stress across the power switches to half of the input voltage V_in. The conditions for assuring ZVS of the leading switch and ZCS of the lagging switch are found. Design constraints on the parallel capacitances of the switches of the leading switch, on the snubber's holding capacitor, and on the additional inductance and winding are hence established, allowing for an optimized design of the converter parameters. A dc analysis allows for the calculation of the effective duty cycle, which enjoys a boost effect due to the proposed snubber. Thus, a further reduction of the primary current stress and rectifier voltage stress is obtained. All the improvements conclude in a high efficiency. The influence of the choice of the parameters' values on the regulation capability is pointed out. Experiments on a prototype of 4.5kW confirm the results