A multi-variable control technique for ZVS phase-shift full-bridge DC/DC converter

In this paper a multivariable control system is proposed for an efficient ZVS full-bridge dc-dc converter used in a Plug-in Hybrid Electric Vehicle (PHEV). This converter processes the power between the high voltage traction battery and low voltage (12V) battery. Generally, Phase-shift between the two legs of the full-bridge converter is the main control parameter to regulate the output power. However, the zero voltage switching cannot be guaranteed by merely controlling the phase-shift particularly for light load conditions. In order to extend the soft switching operation of the converter for light loads, asymmetrical passive auxiliary circuits are used to provide reactive current. However, the auxiliary circuits increase extra current burden on the power MOSFETs, leading to lower efficiency. In this paper, the duty cycle of bridge legs (as another control parameter) is also controlled to minimize the conduction losses of the converter. Basically, the multivariable controller has to adjust the control parameters in such a way that the circulating currents are kept at their minimum level for soft switching while the output power is regulated. The system operating principle, soft switching and mathematical model are discussed. Experimental results are also presented that validate the effectiveness of the control method for a 2KW prototype.