Three-phase soft-switching voltage source converters for motor drives. II. Fundamental limitations and critical assessment

Fundamental limits to soft-switching converter performance are established by examining the detailed physical behaviour of bipolar power semiconductors. It is found that the soft-switched resonant transition time must be much longer than the semiconductor high-level carrier lifetime for effective reduction of switching losses. Constraints on PWM range and PWM resolution as a function of switching frequency are established for a range of soft-switching converter topologies and the standard hardswitched inverter. For quasi-resonant DC-link topologies, the degradation in PWM resolution causes low-frequency harmonics to be introduced into the output spectrum. Topology-specific features are used to form a comparative assessment of the principal classes of softswitched converters. Resonant DC-link topologies are shown to produce the poorest output performance, although they offer the cheapest solution. Auxiliary resonant pole inverters can achieve levels of performance approaching those of the hard-switched topology, while retaining the benefits of soft-switching. It is concluded that the auxiliary commutated pole inverter offers the greatest potential for exploitation despite its relatively high capital cost. Modularisation of the active devices and optimisation of semiconductor design are identified as key objectives in the realisation of an economically viable system.