In the field of high voltage power conversion, low or medium voltage inverters are generally used with high voltage transformers. This article presents a high voltage chopper supplied directly by a high voltage source. At the same power level, higher input voltage means lower current stress in the switches, making realisation on printed board easier. Such a solution requires serial connection to realise the high voltage switch. To reach a switching frequency of several tens of kHz, MOS or IGBT are used. Serial connection of power components encounters two major problems in hard switching: voltage unbalance and overcurrents due to parasitic capacitances to ground during turn on. As the number of stages connected in series increases, those problems become more crucial.
Voltage unbalance is mainly due to differential delay time of drive circuit (switching on and off phases) and speed scattering (switching off phases). Synchronous orders and equal switching speeds are necessary to obtain a good voltage sharing. Otherwise, active clamping can be used for grid components like IGBTS. During the off-state, scattering of leakage current between semiconductors gives rise to significant voltage unbalance. Sharing resistor can be added in parallel of each switch for current unbalancing. At high frequency, they are no more needed. Parasitic overcurrents in hard commutation, for high voltage low current switches using serial connection, are one of the main problems. Soft switching can be used to reduce switching losses, voltage unbalance and parasitic overcurrents. This article presents a converter combining serial connection with soft switching.
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