Multi-Variable Hybrid Switching Frequency- Duty Cycle Based Phase-Shift Control for DC-DC Resonant Converters

This paper presents a multi-variable hybrid control strategy that can simultaneously adjust both operating switching frequency, f
s
, and duty cycle of a dc-dc resonant converter in response to variation of either input voltage or load conditions. Conventional resonant converter control strategies are mono--variable i.e., either f
s
or duty cycle is changed at a time in response to varying operating conditions. This strategy might eliminate MOSFET turn-ON losses due to zero voltage switching (ZVS). However, such control strategy may inadvertently cause large circulating reactive current flow which can nullify ZVS power savings. Hence, it is necessary to operate the converter such that ZVS operation with minimum primary circulating current is guaranteed while ensuring output power regulation for a wide range of operating conditions. A multi-variable ZVS control strategy that achieves simultaneous variation of f
s
and duty cycle to changing operating conditions is implemented using a F-D control law relationship. This control law has been developed for a hybrid Frequency modulated (FM)-Phase-shift modulated (PSM) LLC resonant converter based on ZVS boundary conditions established using FHA. Efficiency improvements of at most 1.5% are observed when compared with mono-variable FM-PSM control.