Design of bidirectional asymmetric CLLC resonant DC-DC converters for V2G and V2H application

Abstract

This thesis presents a detailed design and control methodology of asymmetric CLLC resonant converters suitable for high voltage gain bidirectional EV charging applications. A broad shift to 800 V EV systems is expected by 2025, thereby accelerating the need for 400 to 800V DC-DC converters for charging and discharging of the EVs at residential outlets. However, most of the reported CLLC converters are of symmetric or nearly symmetric structure with unity capacitance and transformer turns ratio, and hence are suitable for voltage gain close to unity. Unlike the existing CLLC design methods, the proposed methodology tunes the transformer turns ratio besides the resonant tank parameters to attain the desired voltage gain. For a design with 1: n (non-unity) turns ratio, the design becomes highly asymmetric, the forward and reverse gain characteristics become dissimilar, and hence, achieving near resonant point operation for high efficiency in both directions becomes extremely challenging. Furthermore, to reduce the converter size, in this thesis a 4-element resonant tank is considered compared to the commonly employed 5-element CLLC designs which further increases the asymmetry of the converter and makes the design even more challenging. A tool is proposed in this regard to appropriately determine the design parameters which fulfill all the design criteria, e.g., high voltage gain (400 to 800 V), soft switching, high efficiency and narrow and similar operating frequency range around the resonant frequency in forward and reverse power flow modes. The effect of the design parameters on the performance of the proposed CLLC converter is investigated and it is found that higher efficiency needs moderate turns ratio, moderate quality factor, and higher capacitance ratio. It is also found that symmetric CLLC structure with unity capacitance and transformer turns ratio fails to attain the required high voltage gain. In contrast to the typical control method for resonant converters that incorporates only pulse frequency modulation (PFM), in this thesis a novel hybrid control scheme is developed which combines pulse width modulation (PWM) technique along with PFM to efficiently regulate the output. The resulting design gives better performance in terms of output voltage regulation, dynamic response and efficiency which reaches up to 97.4%. The designed asymmetric CLLC converter outperforms its LLC counterpart and the existing CLLC designs reported in the literature. The design proposed in this thesis can be useful for bidirectional fast charging applications of new generation EVs.