Efficient Power Conversion (EPC) demonstrates the design of a bi-directional DC-DC converter for automotive 48 V power distribution and showing how GaN technology is a powerful enabler for efficient electrification.
Compared to the traditional 12 V automotive power standard, 48 V distribution can deliver 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 48 V mild hybrid.
The trend towards increasing electrification in the automotive industry enables car makers both to deliver new innovations to market cost-effectively and to meet increasingly stringent emissions legislation. Raising the vehicle’s main bus voltage to 48 V helps meet the demands of power-hungry systems such as the start-stop motor/generator of a mild hybrid vehicle, as well as loads such as electric power steering, electric supercharging, and vacuum and water pumps.
However, dropping established 12 V electrical systems immediately is not an economical option. In practice, 48 V and 12 V infrastructures will coexist in vehicles for several generations to come. To make such a dual-voltage setup work satisfactorily, each being powered from 48 V and 12 V batteries respectively, a bidirectional DC-DC converter is needed to transfer power seamlessly between the two battery voltages. Depending on the vehicle, the required power rating of the converter can range from about 1.5 kW to 6 kW.
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. Maximizing energy efficiency is also paramount and, here, designers can take advantage of gallium nitride (GaN) technology to achieve significantly greater efficiency than is possible using traditional silicon power transistors. Gallium nitride benefits from exceptionally high electron mobility as well as low temperature coefficient, which allows power transistors to have very low on-resistance (RON) thereby minimizing 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 48 V applications have about four times better figure of merit (die area x RON) compared to similar MOSFETs. For the same gate voltage of 5 V, 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 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 presented by today’s vehicles.
The article written by Yuanzhe Zhang, Director, Applications Engineering, EPC Corporation, was published in the December 2021 edition of Bodo’s Power Systems.
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