[Introduction]In power electronics applications, in order to meet the requirements of higher energy efficiency and higher switching frequency, power density is becoming one of the key indicators. Silicon (Si)-based technology is approaching the development limit, high-frequency performance and energy density continue to decline, and power semiconductor materials are also developing from the first-generation silicon-based materials to the second-generation gallium arsenide, officially opening the third The gateway to the application of generation wide bandgap technologies such as Silicon Carbide (SiC) and Gallium Nitride (GaN).

The high pressure resistance of SiC is 10 times that of silicon, the high temperature resistance is 2 times that of silicon, and the high frequency capability is 2 times that of silicon. For products with the same electrical parameters, the use of SiC material can reduce the volume by 50% and reduce energy loss by 80%. Likewise, GaN has many excellent properties, with a bandgap of 3.2eV, which is almost three times higher than silicon’s 1.1eV bandgap, giving it the potential to conduct electrons 1,000 times more efficiently than silicon. What’s more, GaN can operate at frequencies up to 1MHz with no loss of efficiency, while silicon can struggle to reach frequencies above 100kHz. Therefore, with its extremely high power conversion efficiency, GaN can almost become a substitute for silicon in the production of high-efficiency voltage converters, power MOSFETs and Schottky diodes.

In terms of application, SiC is mainly used in high-voltage environments, while GaN is concentrated in the field of medium and low voltages. Among them, SiC devices can provide voltage levels up to 1,200V and have high current-carrying capabilities, making them ideal for applications such as automotive and locomotive traction inverters, high-power solar farms, and large-scale three-phase grid converters.

GaN FETs are typically 600V and can be used as high power density converters in the 10kW and beyond range. The main applications that initially adopted GaN technology and grew are low power fast charging USB PD power adapters and high power adapters for gaming laptops. With the advancement of technology and cost optimization, the application range of GaN has been greatly expanded, and the application in electric vehicles (EV) is no longer limited to on-board chargers. In DC/DC converters, traction inverters, GaN-based solutions are being rolled out at scale.

Can electric vehicles be the future of GaN applications?

Previously, the key applications of GaN were mainly in 5G base stations and smartphones, where GaN devices were used to provide the last stage of power amplification between the RF signal and the antenna. Due to its high efficiency, high power density and small footprint, GaN devices can effectively increase data throughput and reduce congestion in base stations. In the past 2-3 years, GaN technology has developed rapidly. If GaN FETs are used in automobiles, electric vehicle (EV) vehicles can be made lighter and more energy efficient.

Why use GaN FETs in EVs? There are two aspects to this question. First, a GaN FET is a high electron mobility transistor (HEMT) whose excellent material and device properties make it ideal for more advanced applications in automotive power systems and RF devices. Second, EV power systems often operate at high switching frequencies, high output currents, and high voltages, where the excellent high-frequency characteristics of GaN come in handy.

Specifically, the advantages of GaN FETs in RF and automotive power electronics are mainly due to the following material properties:

breakdown electric field

GaN has a higher breakdown electric field than Si (about 15 times that of Si), so GaN devices can operate at higher voltages than Si MOSFETs of the same size.

Electron mobility

GaN has higher electron mobility than Si, so GaN transistors can be physically smaller than Si transistors with the same on-resistance.

Thermal conductivity

GaN has about twice the thermal conductivity of Si, so it can dissipate heat more efficiently into the substrate or heat sink.

capacitance

When the physical dimensions of the two devices are approximately the same, the capacitance between the inputs on a GaN FET is smaller than that in a Si MOSFET.

Today, GaN power devices are already present in small-capacity, high-end photovoltaic inverters and are increasingly used in fast chargers for a range of mobile devices, including smartphones. According to Yole’s analysis data, GaN power device revenue is expected to reach about $100 million in 2021. But that figure will grow to $1 billion by 2026 as GaN device suppliers seek to enter other markets. Among them, the EV/HEV market is a market that has received considerable attention.

Figure 1: Power GaN Device Market Analysis and Forecast

(Image source: Yole)

Yole’s technology and market analysts believe that GaN can operate at higher frequencies with higher efficiency, a feature that significantly outperforms Si MOSFET devices, effectively reducing the number of passive components in the system and increasing power density. From 2022, GaN is expected to penetrate the new energy vehicle market on a small scale, with the main penetration target being 48V to 12V DC/DC converters, i.e. 48V systems in standardized mild hybrid electric vehicles (MHEVs), to Increase power delivery and reduce resistive losses.

Separation of GaN and SiC in Automotive Applications

Both GaN and SiC are wide bandgap materials. While these materials have excellent properties, their properties, applications, and gate drive requirements vary. At present, in higher power and higher voltage applications (above 1,200V), especially in the inverter stage, SiC is more favored. SiC can compete with IGBT transistors in high power and ultra-high voltage (above 650V) applications.

Likewise, GaN’s original niche came from on-board chargers (OBCs), where GaN is very well-positioned. In power applications with voltages up to 650V, GaN can compete with current MOSFETs and superjunction (SJ) MOSFETs. With the on-board charger and DC/DC segments gaining momentum, this could be a multi-billion dollar market for GaN. The key question, though, is whether this technology can be applied to the main inverter of an electric vehicle powertrain to achieve the astonishingly high capacity comparable to SiC technology. Early industry developments have shown that this solution is feasible, and perhaps, we can now say that after 5G applications, new energy vehicles will become another killer application of GaN.

Selection of GaN FETs for Automotive Applications

There are two common types of GaN FETs on the market, those grown on Si and those grown on SiC. The thermal conductivity of SiC is approximately 170% of that of GaN, so GaN FETs formed by heteroepitaxial growth of GaN on SiC are preferred for high power applications. For switching applications, such as high-power switching regulators, lower capacitance and smaller R_ON values ​​enable very fast power transfer with rise times on the order of nanoseconds. These properties mean that GaN FETs can operate at both high frequencies and higher powers, both of which are required for power electronics in RF and automotive applications.

There are many types of GaN FETs on the market, and we must fully consider the characteristics and application scenarios of electric vehicles when selecting GaN FETs for electric vehicles.

Currently, the promotion of electric vehicles faces two major challenges, namely cost and mileage. One of the more effective ways to reduce costs and improve system efficiency is to integrate the powertrain. Integration includes careful design and a thorough understanding of security concepts and potential interactions. The integration also reduces the need for excess packaging material, eliminates redundant hardware, and significantly reduces the weight and bulk of the system. Integrating the drivetrain into a compact mechanical enclosure could result in a more affordable and efficient electric vehicle. With its low switching power losses, GaN FETs can do the job.

For EV powertrains, GaN FET solutions will double power density while reducing size by about 60% with integrated gate drivers and switching speeds up to 2.2MHz. Therefore, GaN FETs are well suited for AC/DC on-board chargers (OBCs) and high-voltage-to-low-voltage (HV-to-LV) DC/DC converters in electric vehicles. In on-board charger solutions, GaN FETs help improve space efficiency, freeing up space for other on-board components to be integrated with the OBC. In DC/DC converters, which require power conversion from the vehicle battery (eg 400V to 12V or 48V to 12V), GaN FETs have strong size and efficiency advantages. In traction inverters, GaN power semiconductors are the key to the development of traction inverters, which can provide more than 70% power increase compared to inverters using traditional IGBTs.

Nexperia’s GAN063-650WSA 650V, 50mΩ Gallium Nitride FET is a normally-off device that combines Nexperia’s latest high-voltage GaN HEMT and low-voltage Si MOSFET technologies. The device features a gate voltage of up to 20V, a fast turn-on time of 57ns (10ns output rise time), and a peak DC drain-to-source voltage of 650V. At only 10V gate voltage, this FET provides 34.5A DC with a peak transient current of 150A and fast pulses less than 10μs. On-resistance is only 50mΩ at room temperature, rising to only 120mΩ at 175°C. The GAN063-650WSA 650V, 50mΩ Gallium Nitride FET has excellent reliability and performance, and is AEC-Q101 qualified, ideal for bridgeless push-pull output circuits PFC, servo motor drives and UPS inverters.

Figure 2: GAN063-650WSA 650V, 50mΩ Gallium Nitride FET

(Image source: Nexperia)

EPC’s EPC9163 is a 2kW, two-phase, 48V/12V bidirectional converter demo board that uses eight 100V EPC2218 eGaN FETs to achieve 96.5% efficiency in a very small footprint. The fast switching and low loss characteristics of eGaN FETs enable converters to operate at 500kHz, significantly reducing the size of the solution. The high switching frequency capability allows the use of miniature inductors in the design, which saves system space and reduces cost. Compared to Si MOSFET solutions, eGaN FET-based DC-DC converters are three times faster, more than 35% smaller and lighter, more than 1.5% more efficient, and have a lower overall system cost.

Figure 3: Eight 100V EPC2218 eGaN FETs are used in the EPC9163 48V/12V bidirectional converter to achieve high efficiency of 96.5% in a very small footprint (Credit: EPC)

Epilogue

Technological advancements in electric vehicles are steadily reducing the material cost of vehicles, and the combination of higher-power-density batteries and more efficient motors, inverters and on-board chargers helps reduce vehicle mass for greater range. GaN FETs offer higher efficiency and power density in EV traction inverters, on-board chargers (OBCs) and DC/DC converters. For this reason, system developers of electric vehicles are bringing GaN into a larger focus. Next, GaN has the potential to replace silicon as the core of automotive Electronic chips to meet the growing demand for faster, more efficient circuits in high-power environments.

GaN is a rapidly maturing technology. It allows for new design approaches that benefit the many power electronic systems on the vehicle. Conventional IGBTs (insulated gate bipolar transistors) are currently the workhorse for power control in electric and hybrid vehicles, and GaN-based inverters can improve efficiency by more than 70% compared to current inverters using IGBTs.

It may be premature to assert that EVs are the future of GaN. However, what we can see is the fact that many power GaN companies have developed and certified 650V GaN devices for vehicles, mainly for on-board chargers and DC/DC conversion in EVs/HEVs, and with some automotive Businesses have established partnerships. There are some examples of vehicle-related applications that could benefit automobiles from GaN.

However, under no circumstances should GaN devices be considered a “replacement” for silicon devices in existing designs, as this is a new design approach to achieve high performance and efficiency in a small space, GaN devices The benefits for new energy vehicles are obvious.