Research on high input voltage DC-DC converter with low voltage stress on switches

Abstract

This thesis presents research results on high input voltage DC-DC converter with low switch stress. The voltage stress on the primary switches is only one-third of the input voltage, so switches of low voltage rating and thus of low on-resistance can be used. This leads to reduced conduction loss. By applying the asymmetrical duty cycle control of threephase DC-DC converter, the converter achieves zero-voltage-switching (ZVS) for all the switches, the switching loss can be reduced; And by using a novel phase shift pulse width modulation (PWM) control, the converter achieves zero-voltage and zero-current switching (ZVZCS), reduced the circulate energy loss during the freewheeling stage and increased the soft switching range. The contents of this thesis are as follows: In Chapter 1, the need of using high frequency solutions in Medium Voltage (MV) power conversion will be discussed. The existing topologies for high input voltage DC-DC converter will be compared. The suitable application range for each topology will be reviewed. Then, the need of a converter with six primary switches and each switch has the voltage stress of one-third of input voltage will be discussed, followed by the proposed high voltage three-phase DC-DC converter topology. In Chapter 2, the operating principles of the proposed converter using asymmetrical duty cycle control will be described. Different operating topologies will be discussed. The steady-state voltages across capacitors will be analyzed. Also the input-to-output voltage ratio and effective duty cycle will be studied and soft switching range will be evaluated. Moreover the small-signal model will be given. The theoretical predictions will be verified experimentally by a 5.1kWprototype with simplified design procedures In Chapter 3, Control strategies for the three-phase DC-DC converter will be discussed and a novel symmetrical phase shift PWM control strategy will be proposed for ZVZCS of the three-phase converter. The operating principles will be discussed. The steady-state output current distribution and effective duty cycle will be analyzed, and the soft-switching range for the three switching pairs will be given. The theoretical predictions will be verified experimentally by a 2.2kW prototype. In Chapter 4, a novel energy-state plane will be proposed for the control of boostderived converters which appear as the input stage of the DC-DC converter to improve the input power factor. The operating principles will be derived; the large-signal and steady state characteristics will be analysis. A boost prototype will verify the features of the new control strategy. This method can be extended to the control of the proposed medium voltage DC-DC converter. In Chapter 5, conclusions of the research and some suggestions for further studies will be given.