Soft switching in traction auxiliary power supplies

There is an increasing demand for efficient high power/weight traction auxiliary power supplies for use on trolley buses, light rail, metros, locos and carriages worldwide. Typical power ratings vary from 1 kW to 100 kW with a wide variety of input and output voltage demands. These requirements necessitate that a flexible front end high frequency power processing technique is adopted. Generally, when the conversion frequency of conventional power circuits nears high frequencies, their switching losses become excessive due to the simultaneous presence of a high current and a high voltage at the switch during turn-on and turn-off. Furthermore, energy stored in stray inductances would induce very high voltage spikes across the switching devices thereby causing device failure. Soft switching (resonant) circuits are gaining more acceptance in high frequency power applications at present. Their switching losses are low since the switch current is zero during the switching transition. They are less sensitive to parasitic inductances which can make up part of the resonant tank circuit. In addition, sinusoidal operation results in easier EMI/RFI filtering (due to lower dv/dt and di/dt). Here, the authors outline the possible topologies that could be used for achieving soft switching. Two examples of circuits currently being used in industry are given and their advantages/disadvantages highlighted