Soft-switched DC-DC converters for PHEV charger

An efficient power conditioning system plays a significant role in the design of battery charging systems for plug-in hybrid electric vehicles (PHEVs). A typical PHEV battery charging system consists of two stages. The first stage is an ac-dc conversion stage which regulates the input power factor, input current total harmonic distortion and intermediate dc bus voltage. The second stage is a dc-dc conversion stage which regulates the output voltage and provides galvanic isolation between utility grid and PHEV battery pack. The research works presented in this thesis focus on the dc-dc conversion stage of a PHEV charger. The phase-shift modulated (PSM) isolated full-bridge (FB) dc-dc converter topology is commonly preferred for the dc-dc stage. High efficiency, high power density, high reliability, and galvanic isolation are the main features of this topology. However, zero-voltage switching (ZVS) is not ensured for all switches at light-load conditions resulting in poor efficiency of the converter. Furthermore, high voltage spikes are present across output rectifier diodes due to voltage ringing on the secondary side. These voltage spikes are further intensified with the increase in series inductance or leakage inductance of the transformer, output filter inductance and switching frequency. In addition, the voltage spikes increase the electromagnetic interference (EMI) of the converter. Therefore, the motivation for the research work presented in this thesis is to address these drawbacks in order to obtain an improved performance for the dc-dc converter over the entire operating range. Auxiliary circuits can be added to the FB converter for improving the ZVS range. Additionally, snubber circuits can minimize the voltage spikes. In this research work, two new gating techniques, namely asymmetrical pulse-width modulation (APWM) and trailing-edge pulse-width modulation (TEPWM) gating techniques and a passive auxiliary circuit assisted topology are proposed to improve the performance of the dc-dc conversion stage in a PHEV charger. APWM/TEPWM gating techniques minimize the auxiliary inductance value by a factor close to 2 compared to PSM for zero-voltage and zero-current switching (ZVZCS) FB dc-dc converter with auxiliary inductor at both legs. An adaptive frequency control, namely asymmetrical duty cycle frequency control (ADFC) method is also proposed to further reduce the auxiliary inductance and improve the efficiency for the same converter. Another research work presented in this thesis comprising a FB dc-dc converter with inductive and capacitive output filter topologies assisted by an auxiliary transformer and auxiliary inductor is proposed. The proposed converter topologies achieve ZVS for all switches over the entire battery charging range. The proposed converter topology with inductive output filter is employed with a diode clamping network on the primary side to minimize voltage spikes across output rectifier diodes. The proposed converter topology with capacitive output filter is operated as a current-driven rectifier to eliminate voltage spikes. These converter topologies can achieve higher efficiency with the proposed APWM technique compared to the conventional PSM technique. In this thesis, the operation and steady-state analysis of each proposed converter topology are explained in detail. The design considerations for the circuit parameters are also given. The improved performance of the proposed gating techniques and converter topologies is validated with experimental results obtained from 100 kHz, 1.2 kW converter hardware prototypes.Doctor of Philosophy (IGS