Control strategies design and comparison of DC-DC LLC converter in V2X mode for electric vehicle charger application

Abstract

Vehicle to Grid (V2G) and Vehicle to Home or Load (V2H/L) modes, in one word V2X mode, for Electric Vehicle (EV) chargers gets more attention in these days. AC-DC and DC-DC converters are usually used in an EV charger. In this paper, motivated by an industrial collaboration, the isolated bidirectional DC-DC LLC resonant converter is considered for the control purpose to improve the charger efficiency within both battery power and voltage ranges in V2X mode. For our best of knowledge, existing control solutions for this type of converter are mostly related to battery voltage or DC current regulations. The new challenge of this paper to achieve the improvements consists of input DC bus voltage control for such DC-DC converter. The main contribution leads to design the control laws based on the DC-DC LLC transfer function gain inversion. Three conventional modulation strategies based on an averaged small signal DC-DC LLC dynamic model converter with first harmonic approximation are presented. Pulse Frequency Modulation (PFM) control is firstly studied. Then, Pulse Width Modulation (PWM) and Phase Shift Modulation (PSM) based control are designed to avoid PFM drawbacks. The benefits and the limitations of each strategy are highlighted. A comparative study between the different strategies is generated with respect to control performances and converter efficiency.