Abstract
Electric Vehicle (EV) bidirectional charger technology is growing in importance. It defines the fact of returning the electricity stored in the batteries of EV to Grid (V2G), to Home (V2H), to Load (V2L), or in one word V2X mode. The EV onboard charger is divided into two parts: AC-DC and DC-DC converters. The isolated bidirectional DC-DC LLC resonant converter is used to improve the charger efficiency within both battery power and voltage ranges. It is controlled by varying the switching frequency based on a small signal modeling approach using the gain transfer function inversion method. The dimensions of the DC-DC LLC converter directly affect the charger cost. Moreover, they cause an important control frequency saturation zone, especially in V2X mode, where the switching frequency is out of the feasibility zone. The new challenge in this paper is to design an optimization strategy to minimize the LLC converter cost and improve the control frequency feasibility zone, for a wide variation of battery voltage and converter power, in the charging (G2V) and discharging (V2X) modes simultaneously. For our best knowledge, this optimization problem, in the case of a bidirectional (G2V and V2X) charger, is not yet considered in the literature. An optimal design that considers the control stability equations in the optimization algorithm is elaborated. The obtained results show a significant converter cost decrease and important expansion of control frequency feasibility zones. A comparative study between initial and optimized values, in G2V and V2X modes, is generated according to the converter efficiency.
Highlights
Energy transport is today a major component of the energy transition
It should be noted that Lm0, Lr0, and Cr0 are the initial values of the DC-DC LLC converter
Based on this study of the LLC parameters effect on the feedforward switching frequency in G2V and V2X modes, it is confirmed that f 0c and f 0d are inversely proportional to Lr, Cr, and Lm . f 0c and f 0d should respect Zero Voltage Switching (ZVS)
Summary
Energy transport is today a major component of the energy transition. By using EV as a transport vector, it becomes a significant technology allowing different uses. The onboard charger is often designed for lower kilowatts of power transfer and adds a significant weight to the vehicle It is responsible for the final stage of the battery pack charging inside the EV. An onboard battery charger is constrained by sizing, weight, and cost It should be implemented either with unidirectional or bidirectional power flow (Figure 1). The switching frequency feasible zone is defined between a minimum and maximum authorized value to guarantee Zero Voltage Switching (ZVS) condition This frequency feasibility condition provides a cost minimization challenge related to both software (FPGA operation) and hardware (charger sizing) implementations in the EV charger. When the PFM strategy is adopted for wide input/output range application in the onboard battery charger in V2X mode, a wide operating switching frequency range is required to meet the system voltage gain requirement. A wide switching frequency range causes soft switching operation loss, which results in low conversion efficiency and control performances
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