Design, analysis and implementation of discontinuous mode Dyna-C AC/AC converter for solid state transformer applications

The paper presents an improved control strategy for Dynamic Current (Dyna-C) Converter. Dyna-C is a bidirectional current-source based AC/AC converter for Solid State Transformer (SST) application. The proposed controller facilitates discontinuous mode operation which leads to zero current turn on and turn off. A non-linear average phase charge based controller is presented which ensures zero harmonic current in the input phase currents. The switching scheme has been so arranged that the diodes always turn off at zero voltage thereby reducing the reverse recovery stress and losses. These modifications lead to a considerable reduction of switching loss and device stress as compared to the conventional control scheme of Dyna-C. Owing to soft switching states, the frequency of operation can therefore be pushed further higher thereby reducing the size of the passive components. An alternate switching pattern has also been discussed which enables soft switched turn off of active switches. Design Considerations of packaging such a switch structure has also been discussed. It has been shown that direct connection of Emitter of IGBT to the Anode of the Diode leads to considerably lower parasitic inductance. Design guidelines to construct an optimized inductor has also been discussed. Simulation and experimental results are presented to validate operation of the control scheme. A comparison study has been carried out to choose the appropriate set of devices for this application. It has been shown that SiC-MOS with low cost Si-PiN Diode shows impressive reduction in overall losses.