Variable Turns-Ratio Matrix Transformer based LLC Converter for Two-Stage Electric Vehicle Auxiliary Power Module Applications

An auxiliary power module (APM) is needed in electric vehicles, which converts traction battery voltage (270430V) to the low voltage auxiliary system (12.5∼15.5V). The single-stage design may achieve high power density but will suffer from poor efficiency and wide input voltage regulation capability. The design with two-stage can achieve wide-range regulation and good efficiency but will suffer from low power density. In order to achieve high efficiency while maintaining a good power density, a variable ratio matrix transformer based LLC is proposed to replace the fixed ratio transformer based LLC-DCX stage in two-stage AMP. The variable turns ratio matrix transformer reduces the low-side copper losses via the adaptive number of element transformers. Additionally, the buck stage achieves better efficiency with the variable intermediate bus voltage. As a result, it is potential for the system to reduce the cooling requirements with high-efficiency power conversion. A 1.6 kW prototype achieves 97.21% peak efficiency with 1.8 kW/L power density.

I. Introduction

The electrical vehicles (EVs) market is growing rapidly. However, EVs’ range anxiety is one factor that imperils market growth. Thus, efficient and compact power modules in EVs are highly demanded in the future. The auxiliary power module (AMP) is one of the power modules in EVs which is rated at kilowatts and converts traction battery high voltage (270V430V) to 12.5-15.5V for vehicle auxiliary systems as shown in Fig. 1. LLC resonant converter and phase-shift full-bridge converter are the most popular single-stage topologies for AMP due to their inherent soft switching ability. Unfortunately, the LLC resonant converter requires bulky external resonant inductors to achieve the required gain range [1]-[3]. It not only degrades the power density but also the efficiency. Phase-shift full bridge converter faces the dilemma of the trade-off between efficiency and zero-voltage switching (ZVS) range. In the past, two-stage solutions were adopted to achieve wide regulation ability and efficiency at the same time [4]-[6]. One stage is the LLC-DCX providing galvanic isolation and a high step-down ratio while the other stage, the buck converter, regulates the output. However, the efficiency drops at high input voltages as the intermediate bus voltage between the two stages is set low to accommodate the lowest input voltage. In addition, the volume is inevitably increased in the two-stage architecture.