(Invited) Low Loss Power Conversion with Gallium Nitride Based Devices

Power converters are an integral part of energy conversion and energy storage systems. High efficiency and high-speed compact power converters are an ever-increasing need to offer more versatility, value and create new platforms. All power converters rely on a switch that is typically a solid-state device. Si-based devices have enabled solid-state power conversion leading to several advances so far, but any further improvement in performance using Si-based transistors is now incremental. New functionality at the system level requires switches that can switch faster and at higher temperature without adding to the system losses.  It is now well agreed that the overall performance and cost of the system drives the technology over its component’s cost and hence it is crucial to evaluate the potential of any new technology at the system level. Current trends in GaN based power conversion system show that the impact of a high efficiency switch can result in superior system performance, reduced form-factor, and lower system’s cost by reducing number of active components (transistor and diodes), shrinking the size of the passive components (capacitors and inductors), and eliminating or reducing heat sinks.

GaN-on-Si technology has been developed for providing scalable and cost-effective power conversion switches in the form of HEMTs. 600V lateral HEMTs, where the gate to drain distance absorbs the off-state high voltage allowing very low leakage current, can switch 1.5 times higher operating current and at a frequency which is at least 10 times or higher compared to the state-of-the art Si-based devices without compromising the efficiency of the converter [1]. GaN switched converters can be designed to operate at higher frequencies (100s of kHz), with reduced size of the passive components leading to an overall reduction in the form-factor. In addition to performing at high frequencies, GaN-based devices can operate at higher temperature thereby requiring smaller or no heat sinks and hence further reducing the form-factor of the converter. A 40% reduction in volume of a Photovoltaic inverter over the state-of-the art Si based inverter was realized by Yaskawa Electric Corporation using Transphorm’s GaN switches, due to reduction in size of the output filter and the heat sink.

Electromechanical efficiency of a motor increases when unwanted harmonics are screened off from the input of the motor. A 100kHz switching frequency enabled a compact output filter that filtered out the harmonics demonstrating an overall increase of the electromechanical and hence the system efficiency by 5% at 50% load [2]. Very stable 1.2kV class AlGaN/GaN HEMTs clearly gives GaN a leading edge in medium power (up to 10kW) market.

Lateral GaN HEMTs although well suited for the medium power market, cannot address high power (10kW-1MW) application space since the chip area becomes large and hence uneconomical in terms of both cost and switching performance. Vertical GaN switches enabled by high quality bulk GaN substrates showing a promising roadmap towards a scalable bulk GaN technology is capable of delivering a sustainable solution for the high power market. Early results from CAVETs have established the fact that bulk GaN devices offer breakdown electric field almost 3 times higher compared to lateral HEMTs. CAVETs have also demonstrated dispersion less output current without the need of complex field plate structures (an integral part of the lateral HEMT design) [3]. Our recent research has identified vertical device designs that warranties single chip normally-off devices, which will further improve switching performance by eliminating bond wire and PCB trace related inductances. Simplicity of device design plays a key role in reducing the device cost and is favoring the vertical GaN-based device to penetrate the high power application space.

GaN substrates with defect densities lower that 10⁴cm^-2 and allowing electron mobility >1100cm²V^-1s^-1create a very encouraging picture for bulk GaN power devices. Next generation power conversion will rely on cost and reliability just like the present generation, but functionality and form-factor will play bigger roles than ever and GaN has the portfolio to address it all.

Reference:

[1] S.Chowdhury and U.K Mishra, Lateral and Vertical transistors using the AlGaN/GaN heterostructure, IEEE Transaction on Electron Devices, vol. 60 (2013) (7pp).

[2] J.Honea and J. Kang, "High-Speed GaN Switches for Motor Drives", Power Electronics Europe, 3 (2012) 38-41.

[3] S. Chowdhury, B.L Swenson, M.H. Wong and U.K. Mishra, "Current status and scope of gallium nitride-based vertical transistors for high-power electronics application", Semicond. Sci. Technol. vol. 28 (2013) (8pp).