Wide Bandgap – The material of the future?
Managing and using power efficiently is one of the challenges of modern times. Power is a common theme in almost every electronic design and is becoming increasingly common in vehicles, to store the incredible amount of data we produce, harvesting energy from nature and also in every small portable device we use to help us manage our busy daily lives.
At the heart of any power solution is a semiconductor switching device. Conventionally, these have been made from silicon, yet these have reached a point where further efficiency gains are unlikely and other materials are being explored. So-called wide bandgap (WBG) materials are emerging that can increase the efficiency of DC/DC converters from 85% to around 95%, or DC/AC inverters can go from 96% to 99% – clearly a significant improvement.
Gallium nitride (GaN) is one of the first WBG materials to become commercially available and is commonly used in high electron mobility transistors (HEMT). When compared with silicon-based superjunction transistors, GaN-based HEMT offer much lower switching losses due to their lower input and output capacitances (Ciss and Coss). Switching speeds are also faster, mainly due to the lower Miller capacitance which means that higher frequency topologies can be used, thereby reducing the size, weight and cost of components, especially magnetic devices. GaN also exhibits a lower on-resistance per die area than silicon, reducing static losses and lowering heat generation from the device. As a result, devices can be smaller and the cost and size of thermal management (such as heatsinks or fans) can be reduced, further reducing the system size and cost.
Commercial availability of GaN
Until now, the availability and adoption of GaN has been relatively limited. In part, this is due to the silicon-based superjunction transistor that has extended the figure-of-merit to a point where acceptable performance is achievable. The other factor, and possibly the most important, has been the high cost of these devices, linked to the low sales volumes.
The situation is changing. As users demand ever-higher performance and efficiency from power supplies, designers have little alternative but to embrace GaN technology. As a result of the greater usage, and some further development of the technology, the economies of scale are beginning to make these devices more economically viable – which will lead to greater adoption and further cost reductions.
There are two primary types of GaN power transistor; the normally-on depletion-mode devices that require a negative gate voltage (relative to the drain and source potential) to turn off and the normally-off enhancement-mode (e-mode) devices that require a positive gate voltage to turn on.
One area for careful consideration, especially with depletion-mode GaN FETs, is the start-up phase. In a half-bridge topology both the upper and lower switches would normally be on creating a short circuit, so the gate control circuits must be started first to apply a negative bias to the GaN FETs to avoid powering up into a short circuit.
Alternatively, depletion-mode GaN transistors can be used with a low-voltage silicon MOSFET in cascode configuration. In this arrangement, the GaN transistor source is connected to the silicon MOSFET drain, and the silicon MOSFET source is connected to the GaN transistor gate as shown in Figure 1. 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. GaN FETs are available co-packaged in cascode configuration – one example being the ON Semiconductor NTP8G202NG.
Another way to eliminate start-up short circuits is to use an enhancement-mode GaN HEMT that is normally off. The GaN Systems GS66516B is a 650V device that operates from a low (0-6V) gate voltage, thereby simplifying the design challenge. Despite the low gate voltage, the device tolerates gate transient voltages from -20V to +10V. The device is very suitable for modern power applications, allowing operation at speeds up to 10MHz and providing six contacts in its bottom-side cooled package. The GS66516B can handle drain-source currents up to 10A, while operating efficiently due to it 25mΩ on-resistance.
Design of switching circuits
Designers must ensure that the devices are turned fully on and fully off quickly to ensure proper operation. To achieve this, a well matched and configured gate driver is required, as is common with all power devices. One essential consideration is ensuring that, when turning on, the driver charges the transistor’s gate capacitance quickly with no ringing or overshoot. The same applies to the turn off, where discharging the gate capacitance but also be accomplished cleanly. In bridge configurations, “Shoot through” short circuits are another potential issue – these can be avoided by ensuring the skew time is well controlled and that the performance of the driver is consistent.