Gallium nitride (GaN) switch technology has enabled a major advance in the miniaturization of chargers and adapters.

GaN transistors switch very efficiently. This allows the development of converters that can either operate at much higher switching frequency than a circuit using equivalent silicon devices, potentially reducing transformer size, or provide solutions that deliver significant system efficiency improvements, reducing or eliminating the need for heat sinks.

By using GaN-based transistors and ICs, designers have been able to deliver small chargers — often also incorporating USB PD interfaces and fast-charge protocols — that significantly increase the amount of power that can be delivered for a given size, and which can be used to drive a wide variety of personal portable devices in all corners of the world.

Power Integrations has been at the forefront of the GaN revolution, delivering complete power supply solutions in volume to major customers. This article explores the capabilities of GaN devices and discusses strategies for addressing the challenges raised by the technology.

Growing demand for power semiconductor devices is driving the wide bandgap semiconductor market. Key players have been investing in the development and mass production of materials and wafers for SiC and GaN.

Where is the WBG market headed? Who are the dominant players? How are they addressing the decades-old issues of high cost, limited volumes and constrained supply chains? This EE Times Special Project will unpack the technology, applications and dynamics of the WBG semiconductor market.

How good is GaN? Changes to power architectures The 1 cubic inch charger became an iconic footprint for low power flyback chargers a little over a decade ago. The technology pushed the size envelope as far as efficiency limits would allow and was the best that could be achieved with the technology available at the time. The power switch in any flyback design is the largest contributor to power loss, dissipating power during each switch transition and during conduction. Switching losses and conduction losses are inversely proportional to each other. As switch die area is increased to reduce RDS(ON) (conduction loss), switching loss increases.

Different silicon transistor technologies — super-junction, vertical, and lateral — all compete to reduce the combined losses in the device. GaN, however, dramatically improves switching efficiency in chargers and adapters by fundamentally reducing both switching and conduction losses. A comparison between technologies that illustrates this performance shift is shown in Figure 1. GaN devices are intrinsically rugged, the avalanche breakdown seen in conventional MOSFETs does not occur in GaN switches, making them ideal for use in offline power conversion in regions where mains voltage is subject to wide variation.

The change in switching efficiency created by the introduction of the GaN switch also dramatically reduces the thermal challenges, leading to a further miniaturization of chargers. A summary of those changes is shown in Figure 2 which compares the characteristic performance of legacy and previous high efficiency adapters with those powered by Power Integrations’ InnoSwitch AC-DC converter ICs, including the latest family members, which use GaN power switches.

The step change in efficiency from GaN switches first began appearing in chargers and adapters in 2018 and has led to a dramatic reduction in charger/adapter footprint and a volume ratio that closely matches the one described in Figure 2. Figure 3 shows the latest GaN charger which uses Power Integrations’ PowiGaN GaN transistor technology compared to both the groundbreaking 2008 design and a high performance design using the best available silicon switch technology.

Addressing the challenges of GaN GaN devices have reshaped power density thinking. The most successful power designs harness the increased switching efficiency to reduce converter size. Driving GaN devices presents challenges for the designer which must be overcome in a practical design. GaN devices switch very quickly. Parasitic capacitances between the gate and source connection, and the gate-drain capacitance normally seen between the Gate well and the Drain substrate (Miller capacitance), are very small (in the order of a few nC) which ensures very fast switch transition leading to low switching losses.

In order to provide a shutdown of the GaN device while avoiding false triggering, discrete current-sense circuits insert a series impedance that approaches (and in some cases exceeds!) the on-resistance of the GaN switch. The large resistance is necessary to ensure accurate short circuit detection and fast loop response for the protection circuit. In designs striving for maximum efficiency this is clearly a disadvantage; engineers are therefore turning to integrated lossless current sensing circuits that build a SenseFET into the structure of the GaN device.

Left unregulated, the fast switch transition will generate significant noise issues in the circuit. The combination of trace inductance and switch capacitance causes high frequency ringing during switching events which cause noise problems with circuit operation. For GaN switches it is important to reduce parasitic inductance by reducing the size of the switching loop (and secondary rectifier loop which appears as ‘extra’ leakage inductance in the transformer) by a combination of good layout and GaN integration. Figure 5 shows the circuit elements that contribute to ringing in a GaN switching circuit.

In addition to controlling loop inductance, consideration must be made in sizing the gate drive circuit appropriately for the size of power switch and gate charge characteristics. Fast gate transition is desirable to reduce crossover losses (gate voltage and current transitioning at the same time), but to reduce EMI it is important that this rate of change is limited by a combination of gate resistance and drive source/sink current which should be matched to the GaN device being used. Figure 6 compares the transition rates for GaN and Si switches driven from an appropriately-sized gate driver.

There are several other aspects to consider when driving power FETs, such as how to control the normally-on GaN structure during start-up; the comparison of the breakdown and avalanche associated with excess drain voltages in silicon switches to the more robust parametric-shift phenomenon seen in GaN devices; optimization of switching frequency and the trade-offs in transformer size versus smaller thermally limited volume; the limits in circuit efficiency imposed by programmable power conversion and USB PD and PPS. Each one is a separate article in its own right.

GaN devices provide an opportunity to provide dramatic improvements in the size, appearance, and even the appeal of power conversion devices in modern electronics equipment. The benefits are not limited to adapters. Appliance applications benefit from the removal of heat sinks which lessons mechanical issues reducing vibration and transport induced failure, while metering and industrial applications are beginning to take advantage of the ruggedness of GaN switches when there are exposed line voltage fluctuations. GaN is no longer nascent; market adoption is already well advanced with Power Integrations alone shipping millions of power supply ICs that include GaN switches. Engineers will continue to innovate and provide better switching solutions based on the benefits the technology provides, the future is bright — the future is GaN, and the future is now.

About the author — Chris Lee is director of product marketing at Power Integrations

Articles in this Special Project

GaN, SiC Offer a Power Electronics Alternative

By Maurizio Di Paolo Emilio

Wide bandgap materials may be posed to replace silicon for some low-power, high-frequency applications.

Where Is the Wide-Bandgap Market Going?

By Maurizio Di Paolo Emilio

While silicon still dominates the market, the emergence of GaN and SiC devices will soon direct technology toward new, more efficient solutions.

Designing with WBG semiconductors takes a little extra know-how

Electromagnetic interference, paralleling, and layout are all things that engineers understand, but when transitioning to silicon carbide or wide-bandgap devices, it requires a little more attention to these issues.

Wide-Bandgap Materials in Hybrid and Electric Vehicles

Electric vehicles are looking to wide-bandgap semiconductors, which offer greater power efficiency, smaller size, lighter weight, and lower overall cost.

Q&A: Wide Bandgap Semiconductors Poised to Make a Splash

By Maurizio Di Paolo Emilio

GaN Systems says wide bandgap chips will become ubiquitous across a range of industries for several applications.

How SiC Devices Have Changed the Face of Semiconductor Sector

SiC FETs are already opening up new applications at higher power and higher switching frequencies.

Wide Bandgap Technologies: New Norm for 21st century Power Electronic Applications

By Thomas Neyer and Mehrdad Baghaie

Researchers and universities have experimented with several wide bandgap materials, which showed high potential to replace incumbent silicon technologies.

GaN Enabling a Revolution in Charger Design

GaN switch technology is no longer nascent: engineers are leveraging it to innovate and provide better switching solutions.

Silicon Carbide Adoption Enters Next Phase

Demand continues to grow for silicon carbide (SiC) technology that maximizes the efficiency of today’s power systems while simultaneously reducing their size and cost.

Gallium Nitride: The Future of Grid Has Already Arrived

For years, designers have described a future where gallium nitride (GaN) can help realize unprecedented levels of power density, system reliability, and cost in grid applications.

Silicon Is Dead…and Discrete Power Devices Are Dying

Beyond performance and cost improvement, the most significant opportunity for GaN semiconductor technology to impact the power conversion market comes from its ability to integrate multiple devices a single substrate.

Solving the Challenges of Driving SiC MOSFETs

By Ming Su & Mitch Van Ochten

Silicon carbide (SiC) provides a number of advantages over silicon for making these power switching MOSFETs.

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