Basic auxiliary resonant pole (ARP) applied to three-level nested cells switching leg

A variant of the auxiliary resonant pole (ARP) concept is described, aiming at assisting the generation of PWM waveforms with soft transitions by multilevel nested-cells switching legs. The technique presented, requires modest passive components and only two additional switches to perform soft commutations, being especially suited for three-level switching legs. The merit of the presented concept is that the boost current, high RMS pole current, additional losses in the main switches, dc link center potential stability problems and the need for additional commutation transformers are avoided. That because the resonant current is interrupted during each soft-commutation cycle by an auxiliary hard turn-off to avoid freewheeling. Therefore the auxiliary pulse commutation can be substantially accelerated thereby reducing the RMS pole current and the volt-seconds loss of the main switching leg. The demerit of the approach is that additional turn-off losses in the pole need to be weighted against the overall losses of the switching leg. The results are that the overall loss reduction is not substantially (16%) but that the thermal profile of the converter can be much more improved, particularly when the main IGBTs are operating with high continuous load currents near maximum device temperature. To emphasize the simplicity of the approach, the paper treats a step by step commutation example and the trade-off between hard- and soft switching. The ideas are experimentally verified with of-the-shelf dual-packed 1200 V IGBTs and illustrated by means of temperature measurements.