Switch transitions in the soft-switching full-bridge PWM phase-shift DC/DC converter: analysis and improvements

An analysis is presented for determining the commutating energy available for the clocked or passive-to-active leg of a full-bridge soft-switching power converter with an external commutating aid, as a function of the load current. It is shown that by properly choosing the values of the circuit inductances, soft switching can be achieved over a wide range of load current without a significant penalty in conduction loss. The currents in the clamp diodes of the commutating aid depend on the location of the commutating inductor. If the inductor is connected between the passive-to-active leg and the transformer, excess current circulates in the clamp diodes, leading to switching losses and potential failure of the diodes. The excess current is not present if the commutating inductor is connected between the active-to-passive leg and the transformer. The predictions of analysis are verified by computer simulation and by experimental results taken from a 3 kW converter with 200 kHz clock frequency