By: Ester Spitale, Technical Marketing Manager, STMicroelectronics and Albert Boscarato, Application Lab Manager, STMicroelectronics
GaN benefits in different applications
The major challenge of power electronics today is dealing with the growing need for improved efficiency and power performance, and at the same time, the constant pursuit of cost and size reductions.
The introduction of Gallium Nitride (GaN) technology, a relatively new wide bandgap compound, moves in this direction, as it becomes increasingly available commercially, its use is growing tremendously.
With a better figure-of-merit (FOM), on-resistance RDS(on), and total gate charge (QG) than silicon counterparts, High-electron-mobility transistor (HEMT) devices based on gallium nitride (GaN) also offer a high drain to source voltage capability, zero reverse recovery charge and very low intrinsic capacitances.
The first application where GaN technology has spread is power conversion: GaN represents the leading solution for improving efficiency, making it possible to meet the most stringent energy requirements. The capability to work at higher switching frequencies enables higher power densities, and therefore reduction of the system dimensions, weight and cost.
Size and energy efficiency are also crucial in electronic motor designs: minimising conduction and switching losses in the drive is key for reducing energy waste.
Performance improvement in motor drivers relying on classic silicon MOSFETs and IGBTs is becoming more difficult as silicon technology approaches theoretical limits for power density, breakdown voltage, and switching frequency. Due to their superior electrical characteristics, GaN transistors are a valid alternative to MOSFETs and IGBTs in high-voltage motor control applications.
Fueling the next generation of motor inverters
GaN is promising important benefits even in applications operating at low frequencies (up to 20kHz). In the realm of home appliances, motor-driven systems such as washing machines, refrigerators, air conditioners, and vacuum cleaners rely heavily on motor inverters to control speed, torque, and efficiency. Unlike industrial servo or precision motors, the physical size of these motors is largely fixed due to mechanical and functional constraints. This means that the traditional approach of reducing overall system size by shrinking the motor itself is not feasible. Instead, improvements must be sought in the inverter and power electronics that drive these motors.
In this sense, it is important to point out that the benefit of GaN over traditional silicon transistors does not come from a single parameter that stands out. It is rather the sum of different aspects concatenating together.
GaN has a de facto negligible reverse recovery charge (Qrr) and low parasitic capacitances, which in turn enable working with slightly higher dV/dt. While the motor winding and insulation limit the maximum allowable dV/dt, GaN’s capability to operate at higher switching speeds allows designers to optimise switching edges carefully.
Moreover, a safe and drastic reduction of dead-time is also achievable without risking shoot-through faults. Time between high-side and low-side switching can be easily lowered by a factor of 10. This can improve inverter efficiency and reduce switching losses without compromising motor reliability.
As remarkable as it gets, the performance is not over yet. In fact, all these “little” improvements combined lead to what may be considered the most relevant of them all: the removal of the heatsink.
Kiss your heatsink goodbye
The considerable reduction in power dissipation allows designers to reduce or even remove bulky heatsinks in the inverter power stage. The assembly line may now require fewer steps in the manufacturing process. No heatsink also means no screws or mounting joints, thus avoiding mechanical failures that can appear when the appliance is already long in the field. An interesting potential saving of service and warranty costs.
The overall result is a more compact, lightweight, and cost-effective inverter design that fits better within the demanding and highly competitive space of the home appliances market.
The waveforms show how smooth and cold a GaN can be. In the example above, the device under test has a typical RDS(on) of 80mΩ. The motor inverter runs at a switching frequency of 16 kHz, with a maximum dV/dt slightly under 10V/ns.
A power level of about 800 W can be safely achieved without incurring thermal runaway. The increase in temperature Δt is less than 70 °C, which leaves a good margin before reaching the maximum operating junction temperature (TJmax) of 150 °C.
This remarkable result is achieved without a heatsink, with GaNs mounted on and cooled down through a common 2-layer PCB.
STPOWER GaN Transistors
STPOWER GaN Transistors are intrinsically normally off, p-GaN gate e-mode transistors that offer a zero reverse recovery charge. ST offers today seven part numbers rated 700 V breakdown voltage (VDS), with typical on-resistance RDS(on) ranging from 270 mΩ down to 53 mΩ in DPAK, PowerFLAT 8×8, and TO-LL packages.
The portfolio is rapidly growing, adding on different packages, RDS(on) and breakdown voltage levels.
