Power Electronics Europe Issue 4 - November 2022

www.epc-co.com AUTOMOTIVE POWER 13 www.power-mag.com Issue 4 2022 Power Electronics Europe GaN Power Behind Mild Hybrid Vehicle Electrification The hybrid vehicle market has more than doubled from 2017 from 2.0 to 5.1 percent [1] and by 2025, one of every 10 vehicles sold worldwide is projected to be a 48 V mild hybrid. 48 V systems boost fuel efficiency, deliver four times the power without increasing engine size, and reduce carbon-dioxide emissions without increasing system costs. A 48 V mild hybrid is estimated to provide 70 percent of the benefit of a high- voltage hybrid at 30 percent of the cost while boosting electrical power available in the vehicle from 2.5 kilowatts (kW) to 10 kW [2]. These systems will require a 48 V – 12 V bidirectional converter, with power range between 1.5 kW and 6 kW. The design priorities for these systems are size, cost, and high reliability. Michael de Rooij and Yuanzhe Zhang, Efficient Power Conversion Corporation (EPC), USA This article discusses the design of a 2 kW, two-phase 48 V/12 V bi-directional converter using GaN FETs in QFN packages, achieving 96% efficiency that is targeted for the 48 V mild hybrid system. The solution is scalable; two converters can be paralleled for 4 kW, three converters for 6 kW or only one phase can be used for 1 kW. Furthermore, the heatsinking capability can be considered infinite given the ultimately function will be inside a vehicle with the unit mounted to the chassis having a significantly larger heat flux capability compared to the losses generated. Design of the 48V/12V Bi-directional DC/DC Converter A simplified block diagram schematic of the bi-directional DC-DC converter is shown in Figure 1. The synchronous buck/boost converter is the simplest bi-directional converter, wss selected as the base topology. Other supporting circuitry includes current sensors, temperature sensor, digital controller, and housekeeping power supply. GaN FETs suitable for 48 V applications typically have 4 times better figure of merit (die area · times R DS(on) ) compared to equivalent MOSFETs [3]. For the same gate voltage of 5 V, GaN FETs have at least 5 times lower gate charge than MOSFETs. Other important advantages of GaN FETs include lower C OSS , faster voltage transition, zero reverse recovery and they are physically smaller. The GaN FET chosen for this design is the EPC2302 [4]. It has a low inductance 3 x 5 mm QFN package with exposed top for excellent thermal management. With 1.8 m R DS(on) , the rated peak DC current is 101 A. Therefore, the two-phase approach is selected so that the FET current requirement is reduced, i.e., at 14 V 2 kW output, the DC current in each phase is 70 A. This also reduces the current rating requirement for the inductors. The MPQ1918-AEC1 [5] gate drivers in this design are AEC-Q100 qualified and use bootstrap technique with voltage clamping for driving the high side FET. These drivers also have fast propagation times and excellent propagation delay matching of less than 1.5 ns typical. Vishay IHTH-1125KZ-5A series inductors [6] offer high current ratings for the inductance. In this design, the 1.0 µH inductor and 500 kHz switching frequency was selected, resulting in 80 A peak inductor current. To ensure accurate phase current balancing, current sensing using precision shunt resistor is preferred over inductor DCR current sensing. However, shunt resistors that are rated for above 70 A usually have large footprints, and therefore high parasitic inductance. This inductance can result in high noise that saturates the current sense amplifier and voids the measurement. A simple solution is to add an RC filter network with a matched time constant. MCP6C02 current sense amplifier is used in this design, with a maximum bandwidth of 500 kHz and 50 V/V gain. This results in 10 mV/A total current sensing gain for 0.2 m shunt. Symmetrical layout between the two phases is also critical in phase current Figure 1: Simplified schematic diagram of the multi-phase bi-directional converter