E-Mobility Engineering | November/December 2023 61 also has an engineering programme for 200 mm SiC wafer production. The trench design and the larger wafers will produce more devices on a wafer and thus bring down the cost. The planar SiC devices are also used in half-bridge power integrated modules with a low on-resistance in a standard package for DC fast-charging designs. The automotive-qualified 1200 V MOSFETs are tailored for high-power onboard chargers of up to 22 kW and high voltage to low voltage DC-DC converters. GaN GaN Systems has introduced its fourth generation of GaN architecture. The basic technology uses an ‘island’ structure that takes out the current from the planar HEMT transistor vertically, resulting in lower losses and frequencies as high as 20 MHz. “We continue to make improvements in performance,” says Jim Witham, CEO of GaN Systems. “There are two knobs to turn – the fab process and design. We work on the process and the structure, which gives a 20% improvement in the figure of merit, which is the product of the charge carrier saturation velocity in the material and the electric breakdown field and used to determine the overall performance of a transistor, whether that is a HEMT GaN, IGBT or SiC MOSFET. “The two major things we have done is electric field management and getting rid of white spaces – these are nonactive areas of the chip. Each customer application has a target on-resistance so we make a family of parts that then make the switching losses as low as possible. “If the die is too small you can struggle to get the heat out, but a smaller die is less expensive so we tend to try to push the limits of the die and use the packaging to get as much heat out as possible. It’s still the case that packaging is important – for an extra cent of packaging you can save more than a cent in the die. “Our fourth-generation technology is the same 100 V and 650 V products. We played a lot with 1200 V two-level versus 650 V multi-level, and we found a lot of value in the multi-level designs. “We like 650 V parts better in these applications. At 1200 V the dv/dt is much bigger than at 650 V, and that reduction in the dv/dt gives you lower EMI and noise and lower losses. The EMI and the noise are more important for the filtering you need, and interference issues are becoming bigger,” he says. This is even more important than the switching frequency increases. “The DC-DC converter and OBC combo designs are the first place everyone goes, as you can push the frequencies up, but the same thing is happening with the traction inverter,” Witham says, “We have lots of activity in the inverters with higher switching frequencies.” The OBC is typically at 10 to 200 kHz while the inverters are 10 to 20 kHz, driven mainly by the IGBT and SiC MOSFET requirements. High-speed switching Achieving higher switching speeds with GaN devices is a real challenge. “We have worked out how to switch GaN transistors incredibly fast – it’s a microwave black art,” says Rob Gwynne, founder and CEO of QPT. “We focused on GaN technology, as that’s the future, and quickly realised that people were having challenges,” he says. “The solution is a GaN transistor from GaN Systems with an isolated GaN is used across a range of onboard power electronics, starting with onboard chargers (Courtesy of GaN Systems)
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