13 Nov 2017

Gallium Nitride: its rise and rise

Mark Patrick, Mouser Electronics, looks at Gallium Nitride technology and finds out how many areas GaN can be used to good advantage.

Gallium nitride (GaN) technology has been around for many years. Initially gallium nitride was used in higher performance microwave radio systems to take care of the high frequency characteristics.

Then it formed the basis of the boom in LED production, in recent years it has started to show its value in power systems.

Wide GaN bandgap

Because of the wide band gap that characterises GaN it is suitable for high speed operation at 60/70GHz. The technology was originally taken up for microwave communications applications, with devices from companies such as Wolfspeed (part of Cree) hitting the market. Researchers at the University of Bristol are currently working high electron mobility transistors (HEMTs) made from GaN deposited onto a diamond substrate, for the next generation of 5G and 6G cellular networks.

Gallium Nitride from Mouser

The wide band gap has also made GaN popular for use in LED devices. Companies such as Broadcom/Avago, STMicroelectronics and LiteOn have used this technology to enable more efficient and longer lifespan solid state lighting systems.

All this has driven the costs of fabricating GaN downwards, as economies of scale start to kick in. It must be acknowledged, however, that production yields are still lower than for silicon, and this is still having some implications for the price tag of GaN devices (though over time yields will improve and the associated cost premiums will lessen). Now the push to electric cars is seeing more opportunities emerge for the technology in power related applications. Alongside this, different designs of GaN transistors are allowing demands for higher frequency operation and greater efficiency to be met, while also reducing the size of power supplies. This is being witnessed in all kinds of systems - from smartphones and laptops to data centre racks.

Wireless capabilities

The technology is even seeing the combination of respective wireless and power capabilities - with a new breed of wireless charger systems being introduced that eliminate the need for a power cable. All these emerging application areas are contributing to GaN’s dramatic growth in popularity. The enhanced mode can control slew rate by simply adjusting the gate resistance, reducing any electromagnetic interference (EMI) from the high speed switching and allowing motor control applications to be substantially smaller.

Hybrid and electric vehicles have substantial power conversion requirements. A typical drivetrain can be switching 100kW, and while a typical silicon converter can reach efficiencies of 95%, GaN converters can reach 98% or even 99% across a wider range of operating loads. This may not seem like much, but the power loss of a silicon design would dissipate close to 5kW, compared to as little as 1kW for an equivalent GaN design. This makes a tremendous difference with regard to thermal management, reducing the weight of the heatsinks or even eliminating them entirely. The 300% reduction in power dissipation that can be witnessed allows the motors to be air-cooled rather than relying on radiators. This is even more critical when silicon converter efficiency falls as low as 70% at lower loads, while GaN devices only drop to 90%.

There are a number of semiconductor companies developing GaN technology for power designs, from established global players such as ST Microelectronics and Panasonic to start-ups, such as GaN Systems. The key advantage of using GaN is the efficiency of conversion, but a number of steps have had to be overcome in order for this technology to become more widely adopted. These include demonstrating ongoing reliability, as well as providing the SPICE models and design guides needed to help developers use the new devices.

As GaN has a much wider band gap than silicon it also exhibits a higher breakdown voltage, with GaN transistors consequently capable of targeting designs where the voltages are as high as 650V and 700V. The smaller size format they occupy compared to silicon MOSFETS or IGBTs helps to reduce the parasitic capacitance involved. This in turn improves switching speeds.

Low parasitic capacitance

This low parasitic capacitance, along with high speed switching and higher voltages, means that smaller components, such as transformers and inductors, can be used. This then allows more compact, efficient power converters, so that mobile phone and laptop chargers are barely any larger than the plugs that go into the wall socket. While the devices are high performance, they also need specialised controllers, or gate drivers.

Gallium nitride GS6650B

Panasonic has started mass production of its 600V enhancement mode X-GaN transistors and the high speed gate driver controller. This drives the transistors at frequencies of up to 4MHz (which are high for power designs) and integrates the active miller clamp function that prevents problems during high speed switching processes. These can be used for power applications from 100W to 5kW - such as inverters for solar cells, racks in data centres and mobile base station power sources, as well as audio-visual equipment and medical devices.

GaN Systems has developed its own process technology, device designs and packaging to eliminate wire bonding - and, as a result, is able to provide ICs with elevated levels of reliability and smaller packages. The combination of its proprietary Island™ technology with GaNPX packaging allows currents of hundreds of amperes to be supported, while reducing the size of heatsinks or fans. The devices have a positive temperature coefficient, which limits the current as temperature increases. This assists in high current multi-die packaged IC products.

The Island™ structure has the dual advantage of a reduction in the size and cost of GaN devices, while transferring substantial current from the on-chip metal to a separate carrier. GaN Systems has developed a high efficiency bridge-less totem pole (BTP) power factor correction (PFC) 3kW reference design - using its GaN E-HEMT devices to take a 176V to 264V input and produce a 400V output. While the BTP-PFC architecture has been known about for several years as a more efficient converter topology, it is only the higher performance that is derived from the GaN transistors that allows a practical implementation.

A conventional PFC circuit consists of a full bridge rectifier and a boost pre-regulator, but a lot of the system losses are in the diode bridge and cannot be avoided even with zero voltage switching on the boost stage. This inherently limits the peak efficiency of the conventional PFC stage.

A well-designed PFC stage can achieve an efficiency of 97% or 98%, but efficiency higher than 98% becomes very challenging for standard PFC due to the fixed diode bridge loss. For example, the 80PLUS Titanium efficiency standard demands half load efficiency of 94% at low line and 96% at high line. As a typical DC/DC converter has an efficiency of 97.5%, the PFC stage efficiency needs to be greater than 98.5% to meet the standard. A bridge-less PFC avoids the need for diodes and the associated losses and, consequently, it can achieve efficiencies of 99% or higher. BTP-PFC has been proposed before, but its application has been very limited until recently. The major challenge is the poor reverse recovery performance of conventional silicon MOSFETs in the half bridge configuration. GaN devices do not need a body diode and the high frequency operation allows for hard switching in the half bridge power stage.

The related evaluation board consists of three major parts. They are a 3.3V PFC controller daughter board, a 5V GaN half-bridge daughter board and the mother board. The mother board comprises an EMI filter, start-up circuit, line frequency silicon MOSFETs and their gate drive circuits, plus voltage and current sensing circuits. The PFC controller daughter board includes current, input line voltage and output voltage sampling pins and has four pulse-width modulated (PWM) outputs, two leading to the GaN half bridge and the other two to the line frequency silicon MOSFETs.

The high frequency and high power of GaN technology is being combined for wireless charging. Standards, such as the AirFuel Alliance, require switching frequencies of 6.78MHz and 13.56MHz which are difficult to achieve with silicon MOSFETs. The GaN devices allows both the high frequency switching and the smaller size, so that the wireless chargers can fit into small 700W units for laptops. But this also allows wireless charging at higher power ratings of 3kW, 7kW and 11kW for electric vehicles.

Conclusion

Starting out in high frequency wireless applications, GaN has moved into LED and now into mainstream power designs. The unique characteristics of the technology combine to provide the 99% efficiency levels needed to meet today’s power standards across the whole system. With portable electronics and wearables both needing more and more power from smaller chargers, GaN is becoming an essential part of the designer’s portfolio. Likewise, as electric vehicles enter into the mainstream, the demand for efficient, high power charging systems is getting even stronger and GaN is a key technology for this. From plug-in hybrids and pure electric vehicles right through to the next generation of wirelessly charged cars, GaN transistors are well positioned to contribute to the future of the automobile industry, much as they will do in a variety of other industry sectors.

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About the author

Mark joined Mouser Electronics in July 2014, having previously held senior marketing roles at RS Components. Prior to RS, Mark spent 8 years at Texas Instruments in Applications Support and Technical Sales roles. He holds a first class Honours Degree in Electronic Engineering from Coventry University.

Mouser Electronics, a subsidiary of TTI, Inc., is part of Warren Buffett's Berkshire Hathaway family of companies. Mouser is an award-winning, authorized semiconductor and electronic component distributor, focused on the rapid introduction of new products and technologies to electronic design engineers and buyers. Mouser.com features more than 4 million products online from more than 500 manufacturers. Mouser publishes multiple catalogs per year providing designers with up-to-date data on the components now available for the next generation of electronic devices. Mouser ships globally to over 500,000 customers in 170 countries from its 492,000 sq. ft. state-of-the-art facility south of Dallas, Texas.

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