AUTOMOTIVE
Electrifying power- hungry loads
Yuanzhe Zhang, Director of applications engineering at Efficient Power Conversion Corporation presents an automotive buck/reverse-boost converter with GaN, for efficient 48V power distribution
T
he trend towards increasing electrification in the automotive industry enables car makers to both deliver new market
innovations cost-effectively and meet increasingly stringent emissions legislation. Raising the vehicle’s main bus voltage to 48V helps meet the
demands of power-hungry systems such as the start-stop motor/generator of a mild hybrid vehicle, as well as electric power steering, electric supercharging, and vacuum and water pumps. Compared to the traditional 12V automotive power standard, 48V distribution delivers four times the electrical power without increasing cable thickness, weight, and cost. By 2025, one in every 10 vehicles sold worldwide, is projected to be a 48V mild hybrid. However, dropping established 12V electrical systems immediately
is not an economical option. In practice, 48V and 12V infrastructures will coexist in vehicles for several generations to come. To make such a dual-voltage setup work satisfactorily, each being powered from 48V and 12V batteries respectively, a bidirectional DC-DC converter is needed to transfer power seamlessly between the two battery voltages. The required power rating of the converter can range from about 1.5kW to 6kW.
GaN and power conversion When designing an automotive converter, size, cost, and reliability are critical factors. To meet these criteria, the simplest bi-directional topology; the synchronous buck/reverse-boost converter, is chosen. Maximising energy efficiency is also paramount and, designers can take advantage of gallium nitride (GaN) technology to achieve significantly greater efficiency than is possible using traditional silicon power transistors. GaN benefits from exceptionally high electron mobility as well as
low temperature coefficient, which allows power transistors to have very low on-resistance (RON) thereby minimising on-state conduction losses. The lateral transistor structure also results in exceptionally low gate charge (QG) with zero reverse-recovery charge (QRR). In addition, GaN FETs also have much lower output capacitance (COSS) than comparable MOSFETs. GaN FETs suitable for 48V applications have about four times better
figure of merit (die area x RON) compared to similar MOSFETs. For the same gate voltage of 5V, GaN FETs have at least five times lower gate charge than silicon MOSFETs. As a result, GaN FETs can operate more efficiently and at high
switching frequencies than silicon MOSFETs, allowing designers to specify smaller capacitors and inductors in their designs. With lower losses across the switching and on states, the heatsink size can also
8 DECEMBER/JANUARY 2022 | ELECTRONICS TODAY
Figure 1: Simplified schematic of two-phase bi-directional converter with eGaN FETs
be reduced, ultimately enabling smaller, slimmer modules or permitting higher power ratings within the same footprint. Ultimately, this gives vehicle designers extra freedom to pack more new features within the tight space constraints of today’s vehicles.
Designing the converter Figure 1 shows a simplified schematic block diagram for a 1.5kW bi- directional 48V/ 12V converter, which can be scaled to 3kW relatively easily by paralleling two converters to make it 4 phases. The two- phase design shown in the diagram can operate up to 1.5kW with a maximum current of 62.5A per phase on the 12V port. This is possible by using the EPC2206 eGaN (enhancement-mode GaN) AEC-Q101 qualified FET, which has 2.2mΩ RON and rated peak DC current of 90A. The two-phase design also reduces the required current rating of the inductors. In this design, the inductor values and switching frequency are
determined using an analytical loss model so that the efficiency at 50% of full rated power is maximised. With the selected 2.2µH inductor, as shown, and 250kHz switching frequency, the peak inductor current is 70A. To ensure accurate phase-current balancing, current sensing using a precision shunt resistor is preferred over inductor DCR current
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