Choosing and Using GaN Technology for High-Efficiency Power Conversion

April 30, 2018 // By Mark Patrick
Choosing and Using GaN Technology for High-Efficiency Power Conversion

Wide bandgap (WBG) power switches have emerged to tackle these challenges. Replacing conventional silicon power semiconductors with WBG devices in could increase typical DC/DC converter efficiency from about 85% to nearer 95% or boost typical DC/AC inverter efficiency from 96% to 99%.

Among WBG devices in the market today, gallium nitride (GaN) high electron mobility transistors (HEMT) offer significant advantages over existing silicon-based alternatives such as superjunction transistors, up to voltage ratings of about 600V. Advantages include significantly lower input and output capacitances (Ciss and Coss), which results in lower switching losses. Also, the Miller capacitance of a GaN transistor is much lower than for a MOSFET of comparable RDS(ON). Hence the GaN device can be turned on and off much faster, which, in turn, permits the use of smaller transformers and passive components. Also, lower on-resistance per die area leads to reduced conduction losses and frees designers to achieve a favorable trade-off between energy losses, device size, and the cost and size of thermal management such as heatsinks.

GaN in the Market

Until recently, GaN technology has been prohibitively expensive compared to more established silicon-based alternatives. The development of the superjunction transistor, unlocking further improvements in the figure of merit for silicon technology, has been one factor that has held back widespread adoption of GaN devices. Now, however, as further development and economies of scale make GaN more economically viable, and the pressing demand to improve power-conversion performance and efficiency further still, GaN devices are ready to gain more and more design wins.

Fundamentally, GaN power transistors are either depletion-mode devices, which are normally-on and require a negative gate voltage relative to the drain and source electrodes to turn off, or enhancement-mode, or e-mode) devices. These usually are off and are turned on by a positive gate voltage.

Depletion-mode devices can deliver higher performance and robustness, although careful management of system start-up to avoid potentially dangerous short circuits. In a half-bridge topology containing depletion-mode GaN FETs as both the upper and lower switches, for example, the gate control circuits must be started first to supply negative bias and keep the transistors off, to prevent powering-up the DC bus into a short circuit. An alternative is to use a depletion-mode GaN transistor in cascode configuration with a low-voltage silicon MOSFET. As figure 1 shows, the GaN transistor source is connected to the silicon MOSFET drain, and the silicon MOSFET source is connected to the GaN transistor gate. When no bias is applied to the silicon MOSFET gate, its drain-source voltage (Vds) negatively biases the GaN transistor gate, to keep the device turned off. Co-packaged cascode GaN power transistors such as the ON Semiconductor NTP8G202NG are already in the market.

Figure 1: Cascode configuration delivers GaN performance advantages with normally-off convenience.

An enhancement-mode GaN HEMT, being normally off, eliminates short-circuit concerns on start-up. A 650V GaN device further simplifies design-in by operating from a low gate voltage of only 0-6V and tolerating transient voltages at the gate as large as-20 to +10V. With six contacts and a package designed for bottom-side-cooling, as well

The need to increase energy efficiency and reliability, while reducing overall solution size, is prevalent in modern power-electronics applications. Such applications include electric-vehicle traction inverters, data center converters and UPS, solar/wind-energy harvesting, and the hundreds of millions of small converters powering high-tech devices used every day in homes and offices all over the world.

GaN, transistor, gate driver

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