A zero-voltage and zero-current switching three-level DC-DC converter with reduced rectifier voltage stress

A novel ZVZCS three-level converter with PWM phase-shift control is proposed. The ZCS of the inner leg 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 conduct for a short moment in order to clamp the voltage of the snubber's capacitor at (n₂/n₁)(V_in/2), n₁ and n₂ being the primary, and respectively the secondary's turns number. Thus, the voltage stress on the rectifier is clamped. 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 zero-voltage-switching of the outer leg is realized in a large load range by using the energy stored in the output filter's inductor. The regenerative snubber serves to resetting the primary current for achieving ZCS, and thus eliminates high circulating energy in the freewheeling stage. As a result of the above improvements, a high efficiency is obtained. The theoretical analysis and the performance expectations are confirmed by experiments carried on a 4.5 kW prototype.