Power gallium-nitride (GaN) FETs have been available for several years now, but some engineers have been slow to adopt them in power equipment despite their superior characteristics. Getting comfortable with those characteristics is part of the problem. Understanding their benefits and limitations is critical when designing power equipment, which must be more efficient and offer greater power densities while improving reliability.
The latest generations of GaN power devices are better than ever. The following discussion is a reminder that these devices can help improve your designs.
Power Design Objectives Influence Transistor Choice
Regardless of what power circuit you’re designing, the overall goals are to maximize efficiency and improve power density. These objectives are influenced by power switching devices, usually a MOSFET. In any switch-mode power circuit, you want low device “on” resistance and faster switching, which is usually the function of the input and output capacitances. Slew rate is also a contributing factor. Other desirable features are small junction capacitance, no body diode, and no reverse-recovery loss.
Your design goals may be met by traditional silicon MOSFETs, but significant performance and efficiency improvements can be had with GaN high-electron-mobility transistor (HEMT) power FETs. These devices switch faster at higher frequencies and can withstand voltages to 600 V.
Furthermore, one of the previously voiced downsides of using GaN FETs has been eliminated—namely, special gate drivers. The newer GaN devices have the critical gate drivers built into the device. Integrating GaN FETs with their own gate drivers improves switching performance and simplifies GaN-based power-stage design. Overall, these devices produce 50% lower losses in applications in the 65-W to 10-kW range. With the latest GaN FETs, you can obtain twice the speed with half the losses.
What kind of switch-mode power equipment are you designing? This includes:
- Power-factor-correction circuits
- Chargers for electric vehicles
- Welding equipment
- Energy-storage systems
- Solar inverters
- Industrial motor drives
- Uninterruptable power supplies (UPS)
- High-density consumer or industrial power supplies
A Basic Design
Most power circuits use a half-bridge or full H-bridge configuration. The figure shows a representative power-factor-correction (PFC) circuit. A good choice for implementing this circuit is Texas Instruments’ LMG342xRO70. The IC is a half-bridge made with GaN FETs and their integrated gate drivers. These devices can withstand up to 600 V on the bus.
However, a separate 12-V supply is needed to operate the gate drivers. This circuit, called a totem-pole PFC converter, is part of a TI reference design that demonstrates a two-phase interleaved topology. The driving signals designated by the red arrowheads are pulse-width-modulated from an external control micro. Pulse-width-modulation (PWM) rates up to 1.2 MHz are possible with critical-conduction-mode (CCM) operation.
The Body Diode Issue
A body diode is a diode that’s present between the source and drain in all silicon (Si) and silicon-carbide (SiC) MOSFETs. The parasitic device is inherent to the internal structure of the transistor. It’s just there—you can’t do anything about it.
Depending on the application, the body diode may or may not be a plus. Usually, it’s not a desirable trait. A typical problem is when a Si or SiC MOSFET in a half-bridge is driving an inductive load. When the high-side MOSFET turns off, the magnetic field in the inductor collapses, producing an opposite polarity voltage that forward-biases the body diode in the low-side MOSFET. The long recovery time from this condition limits switching speed.
GaN FETs don’t have a body diode. However, the GaN FETs do exhibit what’s called third quadrant operation, in which the device receives a voltage that causes the device to conduct from drain to source. A useful technique can be applied to minimize the third quadrant loss of GaN FETs used as rectifiers by leveraging a short dead time or adaptive dead-time control.
GaN FETs provide a viable alternative to Si MOSFETs in high power density applications due to their significant advantages over Si, including small junction capacitance, lack of body diode, and no reverse-recovery loss. These advantages enable more efficient and compact power-converter designs, driven by a higher switching frequency.
A Related Concern
“A New Approach to Validate GaN FET Reliability to Power-line Surges Under Use-conditions”: That’s the title of an article discussing in detail the effect of power-line surges, lightning strikes, and other transients that occur due to load switching and equipment failures in GaN-based power equipment. GaN FETs have better transient overvoltage tolerance than Si FETs, thereby providing the ability to operate through surges and transients without avalanching.
The article presents a comprehensive methodology for designing surge-resistant power supplies using GaN FETs. Surge ratings of GaN FETs provide superior transient overvoltage capability.
GaN FETs are a viable alternative to Si MOSFETs in high-power, high-density applications thanks to their significant advantages over Si, including small junction capacitance, lack of body diode, and no reverse-recovery loss. These advantages enable more efficient, compact power converter designs, driven by a higher switching frequency.