Overcoming Challenges to Characterize 100V GaN Power FETs

Overcoming Challenges to Characterize 100V GaN Power FETs

By Ryo Takeda – Keysight Solution Architect, Takamasa Arai – Keysight Application Engineer, Ron Simpson – proprietor of GRAD Engineering LLC, and Mike Hawes – Keysight Power Solution Consultant

The applications for 100V (and less) GaN FETs are numerous - from reducing distortion in Class-D audio amplifiers to improving efficiency in synchronous rectifiers and motor drives. 100V GaN FETs are also popular in 48V automotive and server applications, as well as USB-C, lidar and LED lighting. However, the small size and minimal packaging parasitics creates multiple challenges to dynamically characterize these power devices. This article reviews the challenges GaN semiconductors manufacturers face to characterize these devices, as well as some new technologies that help address these challenges.

This article was originally published in Bodo’s Power Systems Magazine, in September, 2022. It is reprinted with permission.

In recent years, wide bandgap (WBG) devices are making significant progress in replacing Si based power MOSFETs and IGBTs in many power related applications. Their fundamental characteristics enable significant improvements in key areas for power applications. When comparing GaN to Si, it is well known that GaN’s higher bandgap, higher electron mobility and larger electric field potential enable important attributes, such as lower losses (i.e., higher efficiency), faster switching (e.g., > MHz) and a significantly reduced size (i.e., higher power density). However, WBG devices have a much shorter history of use in a variety of power applications compared to Si, especially ‘high uptime’ applications like automotive.

JEDEC® formed the JC-70 Committee in 2017 to develop needed new reliability, characterization, test methods, and datasheet enhancements to appropriately characterize GaN and SiC WBG power devices. The existing Si-based standards were not sufficient to enable designers to determine the most appropriate WBG devices for their application. For example Rds(on), the main parameter characterizing conduction losses, is a dynamic phenomenon in GaN, based on the charge being trapped in the transistor structure (current collapse). JEP-173 was JC-70’s first publication (issued in January 2019) to provide a standard for ‘Dynamic ON-Resistance Test Method Guidelines for GaN HEMT based Power Conversion Devices’.

Examples of 100V (or less) GaN FET Applications

One application of the initial Class D audio amplifiers was sound systems for automobiles. The amplifier’s lower power dissipation and superior efficiency( > 90%) compared to Class A amplifiers, enabled ‘limited power’ automobiles the ability to have multiple speakers and more sound (> 100W). However, the tradeoff for less power consumption was higher total harmonic distortion (THD) created by slower switching power Si MOSFETs. GaN FETs with significantly faster switching speeds (up to 10x) and no reverse recovery charge provide superior linear response and significantly reduced THD. In addition to automotive applications, recently you’ve probably noticed the boom in portable speakers. In addition to advances in battery technology, this application is enabled by efficient, compact Class D audio amplifiers designed with GaN FETs. Good audio quality is provided because of the lower distortion attributes of GaN, while the ability to run for extended times on batteries is possible because of GaN’s high efficiency. There are many other portable consumer devices that can leverage the same attributes as portable speakers

Automotive systems are moving toward higher voltage operation (e.g., 48V) as more electrical power needs develop for autonomous driving, including radar, cameras, ultrasonic sensors, and lidar. These functions require uninterrupted, highly reliable power. As the 48V bus emerges as one of the new higher voltage power systems, efficiency is again the key with a limited power source (i.e. car battery). GaN technology enables better power density than Si, minimizing additional weight, size, and thermal management. GaN’s higher frequency switching, and increased efficiency also reduces necessary passive component size (e.g., inductors) further minimizing the size of the power converter design. DC-DC converters (12V – 48V) made from these GaN FETs, enable the standard 12V power bus to supply power for these emerging automotive system requirements.

Motor drives (e.g. stepper motors, drones, etc.) is yet another large application for 100V and less GaN devices. Low losses often remove the need for heat sinks. GaN enables higher frequency PWM signals and significantly reduces switching losses. Higher frequency switching reduces/eliminates switch node oscillations, which often require snubber circuits in Si-based designs.

The are many evolving applications primed to take advantage of GaN’s superior performance compared to Silicon. But the challenges to characterize these devices follow the themes described above: small size (power density) and higher efficiency.