A new soft-switching PWM scheme for a three-phase, ac-dc, step-down, unity
power factor converter is described. The main advantage of the scheme is th
at it requires no additional hardware components to achieve a combination o
f zero current and voltage turn-off and zero and reduced voltage turn-on. T
he scheme relies on the repositioning of the PWM pulses within the carrier
period to obtain natural commutation from one switching device to the next.
The theoretical reduction in the total switching losses is mathematically
shown to be 42% compared with the standard center-aligned PWM scheme used w
ith the converter. A PSpice (TM) simulation of the new scheme reveals soft-
switching waveforms. Practical results are given to support the simulation
results and heatsink temperature rise results show a reduction in heatsink
temperature rise for the new scheme compared to the old.