Hybrids, EVs offer sweet spot for GaN power devices

Article By : EEWeb

The use of GaN in applications such as power converters enables significant improvements compared with traditional solutions based on Si: greater power efficiency, smaller size, lighter weight, and lower overall cost.

Due to its chemical-physical properties and to highly reliable manufacturing processes, silicon (Si) has been for many years the most used semiconductor for manufacturing passive and active electronic components. The introduction of MOSFET and IGBT technologies has also enabled the use of Si in power applications, characterized by particularly high currents and voltages. Today, however, the performance of this semiconductor has almost completely reached its theoretical limits, highlighting some disadvantages offered by Si-based technology, in particular: limited heat dissipation, limited efficiency, and non-negligible conduction losses. Research activities carried out in recent years have made it possible to identify some materials, known as wide-bandgap (WBG) semiconductors, whose properties are able to overcome the limits offered by Si.

Gallium nitride (GaN) belongs to this category of semiconductors, particularly suitable for power applications due to its superior characteristics compared with Si — specifically, its ability to switch faster internally when operated at the same operating frequency as Si or silicon carbide (SiC); lower internal switching losses from its superior electron mobility, which is 2× higher; higher operating frequencies from its lower parasitic inductances, especially in soft-switching topologies; and higher working voltage for a given size of die based on its higher critical electric field strength (3.3 MV/cm) versus 0.3 MV/cm with Si, all resulting in higher efficiency. The use of GaN in applications such as power converters enables significant improvements compared with traditional solutions based on Si: greater power efficiency, smaller size, lighter weight, and lower overall cost. Though switching loss increases, as frequency is propositional to power loss, increasing the frequency of operation can result in smaller form factors and an overall lower system cost. Along with GaN’s superior electron mobility, reducing crossover losses, thermal performance can be achieved by reducing the need for bulky heat sinks and cooling systems, reducing the weight and size of the power supply.

GaN properties

The WBG semiconductor family, which in addition to GaN includes the equally well-known SiC, comprises materials characterized by a relatively large energy bandgap, especially when compared with that of Si. Also known as forbidden band, this band represents the energy gap existing between the upper limit of the valence band and the lower limit of the conduction band. It is precisely the presence of this bandgap that allows semiconductors to switch between on and off states through some externally controllable electrical parameters. GaN’s bandgap is equal to 3.4 eV, significantly higher than that of Si (1.2 eV). The greater mobility of the GaN electrons leads to a higher switching speed, as the electrical charges that would normally accumulate on the junctions can be dispersed more quickly. A wider bandgap also allows higher-temperature operation. With increasing temperature, the electrons’ thermal energy in the valence band increases until, once a certain temperature threshold is exceeded, they enter the conduction zone. For Si, this temperature threshold is approximately 150°C, while for GaN, it is even higher than 400°C. A broad bandgap also implies a greater breakdown voltage. At the same breakdown voltage, it is therefore possible to create thinner layers, increasing the doping levels of the semiconductor and obtaining much lower on-resistance values, as shown in Figure 1.

Figure 1: On-resistance graph in relation to breakdown voltage

Compared with traditional Si technology, the main advantages offered by GaN can be summarized as follows:

• Great efficiency, small footprint, and low weight
• High breakdown voltage (above 600 V)
• High power density
• Excellent thermal conductivity
• High operating and switching frequencies
• Low on-resistance
• High temperature operation (up and above 300°C)
• High reliability
• Near-zero reverse-recovery time

Leading the GaN revolution

Thanks to its superior properties compared with Si, GaN is rapidly spreading in sectors demanding for power devices that are efficient, reliable, and able to reduce the application size, weight, and cost. The automotive industry, increasingly oriented toward hybrid and electric vehicles, can benefit significantly from the use of GaN in devices such as DC/AC inverters, DC/DC converters, AC/DC on-board chargers, EV powertrains, and more (see Figure 2).

GaN is now a popular choice for power conversion. High-voltage GaN HEMTs (GaN FETs) in the range of 650 to 900 V are emerging as the next standard for power conversion. With its proven ability to reduce size (form factor) and save energy (high efficiency), 650-V GaN FETs have now been adopted in the mass market.

In systems, GaN offers high value within the AC-to-DC bridgeless totem-pole PFC, which, unlike the well-established analog-based classic boost PFC, uses digital programming. GaN provides cost-competitive, easy-to-embed solutions that reduce energy loss by more than 50%, shrink system sizes by more than 40%, and simplify converter/inverter design and manufacturing, contributing to system cost reduction.

Figure 2: EV/HEV GaN applications

Transphorm’s vertically integrated business approach leverages the industry’s most experienced GaN engineering team at every development stage: design, fabrication, device, and application support. This approach, backed by one of the industry’s largest IP portfolios with over 1,000 patents, has yielded the industry’s only JEDEC- and AEC-Q101–qualified GaN FETs. Transphorm’s innovations are moving power electronics beyond the limitations of Si to achieve over 99% efficiency, 40% more power density, and 20% lower system cost.

High-voltage GaN technology benefits numerous markets that require reliable higher-efficiency and higher-performance power conversion. The main application areas are the following:

• Data center, infrastructure, and IT power supplies: GaN increases clean power output in standardized server and telecom form factors.
• Industrial UPS and battery chargers: GaN technology reduces size and weight of systems that run industrial factories, charge battery-powered forklifts and electric vehicles, and keep critical data accessible.
• Automotive and EV charging: GaN generates longer distance per charge with a lower overall system cost.
• Consumer and computing: GaN technology improves efficiency in adapters, gaming, and power supplies, resulting in better thermal management, improved power density, and lower system cost.

High-voltage GaN 650- to 950-V FETs are becoming the next standard for power conversion. They provide cost-competitive, easy-to-embed solutions that reduce energy loss by more than 50%, shrink system size by more than 40%, and simplify power converter/inverter design and manufacturing.

“Transphorm’s GaN FETs switch up to 4× faster than silicon solutions,” said a Transphorm spokesperson. “Furthermore, unlike Si MOSFETs, the GaN transistors are inherently bidirectional and optimized in a bridgeless totem-pole power-factor–correction design.”

Today’s EV on-board chargers require low weight and small size. “High-efficiency and high-frequency operation help to achieve this, the benefits offered by Transphorm GaN FETs, and the LLC topology,” said the spokesperson.

Transphorm GaN FETs are particularly well-suited for LLC and other high-frequency resonant applications for the following reasons: fast switching, low drain charge (Q_OSS = C_OSS(tr) × V_DS), very fast body diode (low Q_RR), and low gate drive current requirement.

Figure 3: LLC topology and simulation schematic with full-bridge primary and secondary

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