Investigation of HV-LV Isolated DC-DC Converter Topologies for Automotive Applications

Abstract

Abstract In this work the design of isolated DC/DC converter such as phase shifted full bridge with three different secondary topologies full bridge (FB), center tapped (CT) and current doubler(CD) are discussed which are to be used in auxiliary power module (APM) to power the LV battery in an electric vehicle. The converters are designed for a full load power of 7500 W and an input voltage range of 370 V - 920 V to achieve regulated output of 15 V at an operating frequency of 100 kHz. The main work involves the design of transformer, inductor, component level loss analysis and estimation of efficiency of the converters at various loads and input voltages. Phase shift is a modulation technique used to control the operation of the switches in the primary where all the switches in the primary operate for a fixed duty cycle of 50% but with a phase shift. The phase shift depends on the input voltage and turns ratio of the transformer. In this topology zero voltage switching (ZVS) is achieved through the leakage inductance of the transformer so turn on losses are zero but turn off losses are unavoidable. Silicon carbide (SiC) MOSFET’s are used in the primary and Silicon (Si) MOSFET’s are used in the secondary. Several design considerations were taken into account such as paralleling of switches on the secondary, variation of on state resistance ,Rds,on , junction temperature rise of the switches. All the converters were simulated in MATLAB/Simulink to find the currents in the components to estimate the conduction , switching losses in MOSFET’s, copper losses in transformer and inductor.The waveforms of the converters are verified theoretically and through simulations. Comparison of the converters based on the total losses and efficiency is done at the end of this work which made it evident that the PSFB-CT has a better performance at low load PSFB-CD has a better performance at high load. Further investigationss and comparison of topologies such as traditional half bridge and LLC can be considered. Different topology architectures involving interleaving/paralleling of the power stages or components can be considered to improve the performance.