10 kW Applications Gain High Efficiency Power Conversion from Highly Scaled GaN HEMTs
(25% lower loss than SiC in the standard TO-247 package)
Carl Neufeld, Senior Member of Technical Staff & Manager, Transphorm
Yifeng Wu, Senior Vice President of Engineering, Transphorm
Numerous application markets from broad industrial and consumer electronics to the rapidly growing electric vehicle industry are driving considerable demand for high efficiency power conversion solutions. As a result, significant advancement in power conversion technologies have been and continue to be achieved in an effort to meet the related power systems’ high quality, high performance requirements.
A Gallium Nitride on Silicon (GaN-on-Si; hereafter referred to as GaN) platform offers several advantages that make it an appealing choice to lead this new wave of power conversion solutions.
Intrinsic GaN Performance Advantages vs. Silicon (Si) and Silicon Carbide (SiC):
- GaN has superior material properties.
- GaN offers higher efficiencies with lowest losses in all power conversions.
- GaN can operate at much higher frequencies.
Relative Cost Advantages of GaN:
- GaN is less expensive than SiC.
- GaN offers lower overall system cost than Si.
- Roadmap for GaN to approach cost parity with Si at device-level exists.
Since the first 600 V GaN power devices launched in 2013, GaN power electronics have been increasingly accepted into the market with strong projected growth. Today, the most commonly available GaN transistors are qualified at 600 V or 650 V; these devices, however, are capable of only 2 kW to 5 kW of output power per device.
Transphorm’s GaN Revolution
There is demand for individual discrete devices that can deliver 5 kW+ power levels in many different markets and applications such as datacenter rack-mounted power supplies, renewable string inverters, and electric vehicle on-board charges to name a few. Transphorm is meeting that demand by effectively scaling its high voltage D-mode GaN HEMT and integrating it with a low-voltage e-mode Si MOSFET in a TO-247 package to form a high-power cascode switch. The integration combines the high performance of Transphorm’s GaN with the ease of driving Si MOSFETs—giving the best of both worlds in an industry-standard thru-hole package capable of handling high power applications.
The resulting device is a 650 V rated power FET with a high threshold of 4 V, an on-resistance of less than 15 mΩ, and single-chip-like package simplicity. Using a simple gate drive (as all of Transphorm’s FETs do), the GaN device exhibits faster switching transitions and 2x lower switching losses than a competing SiC MOSFET. When implemented in a 240 V:400 V half-bridge synchronous boost converter, 12 kW output power with greater than 99 percent peak efficiency is achieved.
Device Design and Performance
The integrated GaN HEMT and Si MOSFET is shown in Fig.1. The simple single-chip-like wire bonding configurations are shown in Fig.1(b). Currently available e-mode GaN devices have relatively poor gate drive margin and are therefore limited to using surface mount packages. This makes such e-mode devices generally only suitable for sub-2 kW applications. This is in contrast to the robust gate of the cascode switch, which allows for the use of the TO-247 thru-hole package for several times higher heatsinking capability.
Transphorm’s final GaN device in the TO-247 package is shown in Fig.1(c) with G-S-D pin out and the metal pad as the common source for a natural signal flow to promote good input-output isolation. It has a reliable gate voltage rating of +/- 20 V and a high threshold (VTH) of +4 V. It also has the combination of a high VTH and low QG makes the GaN FET the easiest to drive among competing devices with no need for spike clamping or negative VG.
Fig. 1. The 2-chip normally off cascode GaN FET (a) Circuit Representation. (b) Internal package configuration. (c) Finished device in TO-247 with G-S-D pin allocation and the metal pad as another S terminal.
Off-state leakage measurements show a large voltage margin above the 650 V rating with soft breakdown of 1200 V at 25oC and 1000 V at 175oC. Additionally, the GaN FET exhibits excellent reverse conduction capability in off-state with a low voltage drop of 1.5 V at the 100oC rated current of 60A due to the Si MOSFET body diode and the open GaN HEMT channel.
Switching Operation and Converter Performance
To evaluate ultimate switching performance, a half-bridge synchronous boost converter circuit was built with a low- side and a high-side device (Q1 & Q2 as in Fig. 2(a)). The 240 V:400 V boost converter was tested at 70 kHz with performance as a function of output power plotted in Fig. 2(b) along with that of a competing SiC MOSFET, resulting in 25 percent lower loss (11 kW) than SiC in a standard TO-247 package shown in Fig. 2(c).
The GaN converter outperforms that of the SiC, delivering 12 kW output power and a peak efficiency well above 99%. The lower loss in the GaN converter is credited to the reduced switching loss as indicated in the loss breakdown in Fig. 2(c), which is enabled by faster switching speed due to superior electron transport in GaN vs SiC. The GaN device efficiency is estimated to be close to 99 percent even at 12 kW when considering all other losses in the system, including the significant ESR of the inductor. Although the GaN devices have higher junction-to-case thermal resistance than that of the SiC device (0.4oC/W and 0.35oC/W respectively), the junction temperature of the Q1 GaN FET was estimated to be 139oC at 12 kW output power, while that of the SiC MOSFET was 166oC at only 11 kW and would exceed the 175oC rating at 12 kW. This exemplifies the importance of the electrical performance enabled by GaN.
Fig. 2.(a) Simplified schematic of the bridge boost converter circuit. (b) Efficiency performance comparison of the 240 V:400 V half-bridge converter using the GaN FETs and the SiC MOSFETs at 70 kHz. (c) Conduction, switching, passive and total power loss at 11 kW.
Transphorm’s GaN platform evolution continues. The company’s fifth generation SuperGaN™ devices are currently sampling in TO-247 packages and are capable of driving more than 10 kW of power (application dependent) complete with the best-in-class reliability and matching performance that has become synonymous with the Transphorm brand.
The authors gratefully acknowledge the contributions of team members from Transphorm and AFSW in various aspects of the project including design, epi, wafer fab, package, test, and quality in a strong manufacturing environment.
 Wu et al., “Performance and Robustness of First Generation 600-V GaN- on-Si Power Transistors,” Proceeding, 1st IEEE Workshop on Wide Bandgap Devices and Applications, S1-002, Ohio, Oct 27-29, 2013
 Chen et al., “GaN-on-Si power technology: Devices and applications.” IEEE Transactions on Electron Devices 64.3 (2017): 779-795.
 “Power GaN Device Market Grows 16% in Q4/2019.” Semiconductor Today, March 2020. http://www.semiconductor-today.com/news_items/2020/mar/yole,-230320.shtml
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