Research of the soft start circuit for the high voltage power supply based on PSM technology

The power supply based on pulse step modulation (PSM) technology has been widely used as the high voltage power supply for additional heating systems, such as RF H&CD and NBI in tokamaks. The topology contains numbers of converters and multi-secondary transformers. For the magnetic inrush of the transformer and the charge current of the converters connecting with the secondary windings of transformer, the power up of the power supply system is quite complex. PSM power supply starts up by two steps: the soft start of transformer and the pre-charge of the converters. Resistor branches series with the transformer and each converter are used to limit the inrush current. A lot of resister branches and parallel contactors in the converter modules are required, as a result the dissipation and electromagnetic interference (EMI) are serious. In this paper, the soft start circuit is simplified to only one step for decreasing the dissipation and EMI. The pre-charge circuits of the converters which are connected to the secondary at first are diverted to the primary winding of transformer can also limit the magnetic inrush current. Since the multi-secondary transformer powers up as the same time as converters, the capacitors in all converters will limit the ramp up rate of the transformer voltage, and the magnetic inrush current can be ignored. To analyze the characters of the precharge branches, all converters can be equaled as parallel during the soft start process. The equivalent circuit, a series RLC circuit, is proposed in this paper. A 1.2kV power supply, contains 12 converters, with soft-start has been built up in laboratory. Simulation and experiment results are shown to verify the theoretical analysis. A new soft start circuit of 100kV high voltage power supply system was designed.