Power Electronics

SiC and GaN Vie for Slice of the Electric Vehicle Pie

MOSFETs using silicon carbide and gallium nitride technology are emerging to fill the power-controller need in electric and hybrid electric vehicles, and, while they are not yet in volume production, they show promise.

Electric vehicles (EVs) require compatible power electronic devices capable of efficient and effective operation at elevated temperatures. To meet this need, power modules are being developed with power MOSFETs that use silicon carbide (SiC) and gallium nitride (GaN) technologies.

Though these devices may not yet be ready for prime time, recent product introductions indicate an emerging generation of power-semiconductor controllers for EV/hybrid EV (HEV) applications. These devices offer lower loss during power conversion and operational characteristics that surpass traditional silicon counterparts.

ROHM'S SiC MODULE

One new solution is an SiC module combining ROHM's device technology with Honda R&D's high-power module that is claimed to be the world's first high-power module driven entirely by SiC devices. The device incorporates a one-phase converter and a three-phase inverter in a single package.

The Honda-ROHM module is rated at 1,200 V, 230 A (289-kVA equivalent). Specifically intended for EVs and HEVs, this high-power inverter module features SiC Schottky barrier diodes (SBDs) and SiC MOSFETs.

Useful for reducing total loss, SiC inverters meet the need for smaller cooling solutions and greater thermal-management design flexibility due to significantly less heat generation. As a result, SiC inverters enable a four-fold increase in drive frequency over conventional technology in step-up converter applications, promising significant improvements in output volume and density by making peripheral components smaller and lighter.

Also, the superior high temperature stability of SiC facilitates failsafe design in on-board applications. ROHM offers SiC SBDs with a footprint of 5.14 mm2, and MOSFETs with a footprint of 4.8 × 2.4 mm. SiC devices have been shown to dramatically reduce switching loss, to approximately one-seventh that of Si-IGBTs.

Switching loss even in power modules has been reduced to approximately one-quarter or less than that of conventional Si devices (Fig. 1). A 46% reduction in total loss is possible. SiC reduces total loss during power conversion, including R DS(ON). In addition, reducing switching loss enables a proportional increase in PWM drive frequency, from 20 kHz with conventional Si-IGBT-based modules to 80 kHz.

By applying these newly developed SiC high-power modules to HEVs and EVs, ROHM expects significant improvements in efficiency as well as considerable reductions in both size and weight. As the technology progresses, ROHM believes that even greater advancements in performance will be possible through refinements made to the structure of power modules, enabling them to take full advantage of the standalone performance of SiC MOSFETs, as well as through greater miniaturization.

POWEREX SiC MOSFET MODULES

Based on advanced work with Cree, Inc. and the United States Air Force Research Laboratory, Powerex offers two SiC MOSFET modules (QJD1210006 and QJD1210007) that operate at temperatures well beyond those possible with silicon IGBT-based modules. Compared to an equally rated silicon IGBT (junction temperature = 150°C), the SiC MOSFET module has 38% lower conduction losses and 60% lower switching losses — a total power-loss reduction of 54% at 20 kHz. Designed for power systems requiring low conduction and low switching losses, these devices enhance applications requiring high efficiency, high frequency, and/or high temperature.

With an innovative package design (Fig. 2), these modules are constructed in half-bridge configurations and are rated at 100 A per switch, with two switches per model. Both versions feature all-SiC Schottky diodes for reverse recovery. All components and interconnects are isolated from the heat-sinking baseplate to simplify assembly and thermal management.

Rated at 100 A, 1,200 V, the QJD1210006 and QJD1210007 feature increased junction temperature range (-40° to 200°C), industry-leading RDS(ON), high-speed switching, low switching losses, low capacitance, low drive requirements, and high power density.

SANKEN'S GaN MOSFETS

In 2008, Sanken Electric Co., Ltd. announced the development of a normally-off GaN FET using the company's proprietary epitaxial technology for high-quality GaN crystals on a silicon substrate, and adopting a new device structure. Sanken has proven its operation in an actual power-supply circuit and applications for 41 patents have been filed.

The device's normally-off design makes it easy to use because it operates in the same way as traditional silicon MOSFETs. Previously, that was difficult to realize with a conventional structure of GaN FET.

GaN has a breakdown electric field that is about 10 times higher than silicon's to reduce element loss and enable thinner layers with lower resistance. Sanken expects GaN to take over as the main material used in next-generation high-breakdown-voltage power devices. Usually, sapphire or SiC substrates are used for GaN devices. However, these substrates are extremely expensive and significantly raise device costs.

Sanken conducted lengthy research into the technology to enable GaN to be formed on a silicon substrate, which is low-cost and can be fabricated easily on large-diameter wafers. Despite the difficulty of forming a thick layer of GaN crystals on a silicon substrate, Sanken has succeeded with its proprietary epitaxial technology, resulting in a GaN FET offering ultra-low loss and high breakdown voltage (800 V).

Along with the GaN FET, Sanken has also developed a high-breakdown-voltage (800 V) GaN SBD that has been proven to operate in an actual PFC circuit. Forward voltage is 0.5 V lower than previous devices, achieved with the use of a new device structure (a barrier metal and a two-dimensional electron gas are brought into direct contact in excavations made in the substrate).

INTERNATIONAL RECTIFIER'S GaN-BASED POWER DEVICES

In 2008, International Rectifier announced the successful development of a GaN-based power device platform that improves key application-specific figures of merit (FOM) of up to a factor of 10 compared to state-of-the-art silicon-based technologies. The GaN-based power device technology platform is the result of five years of research and development by IR based on the company's proprietary GaN-on-silicon epitaxial technology.

IR's GaN-based power device technology platform enables revolutionary advancements in power-conversion solutions. The high-throughput, 150-mm GaN-on-Si epitaxy, together with subsequent device fabrication processes, is fully compatible with IR's silicon manufacturing facilities.

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