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A three level natural clamped inverter (NCI)

This paper describes a zero voltage switched resonant inverter where focus is put on obtaining low power losses. Using zero voltage switching reduces the power losses of the inverter switching devices, however it is realized that the resonant circuit, introduced to obtain the zero voltage switching, is a major source of power loss that must be limited. This paper presents an inverter topology where low resonant circuit losses are obtained by the following strategy: keep IGBT switches out of the resonant circuit. The result is a three level converter with a voltage stress quite similar to that of the auxiliary resonant commutated pole (ARCP) inverter, but with a significant different control of the resonant circuit. Next, it is discussed how to obtain a low power loss design of the resonant circuit. It is concluded with help from experimental results that the use of an air core inductor instead of a magnetic core will give lower losses, particular at no-load or light load situations, and a quality factor of the resonant circuit of 200 is obtained. Finally measurement on a 55 kW resonant inverter is given.