ISSUE 012 Winter 2021 Sigma Powertrain EMAX transmission dossier l In conversation: David Hudson l 48 V systems focus l 2021 Battery Show North America and Cenex-LCV reports l Everrati Porsche 911 digest l Switching insight l Motor laminations focus

Nick Flaherty examines how new technologies are boosting the performance of inverters and onboard chargers Power lifters S witching is at the heart of the electrical systems for e-mobility platforms. The performance of the switching devices – the transistors – determines the power levels, the efficiency and even the size of the systems. Switching at higher frequencies reduces the size of the magnetics and allows more integration, lower weight and longer range. New types of transistors are opening up the use of switching topologies such as totem poles and Vienna rectifiers, which offer higher efficiencies than present switching topologies using silicon MOSFETs and IGBTs. However, there are distinctly different choices of devices, topologies and control schemes that can be used for switching in the inverter and onboard charger. The 900 and 1200 V silicon IGBTs used in inverters to drive an EV’s motor typically switch at 10-20 kHz. MOSFET transistors built on silicon carbide (SiC) can switch at frequencies of 100- 200 kHz, with efficiencies of about 97%. This higher efficiency means smaller, lighter heat sinks can be used, saving weight and cost. IGBTs have better short-circuit withstand time, but higher switching losses. For SiC, the withstand time is shorter but there are lower losses that don’t increase with the breakdown voltage, giving designers more margin. However, the SIC devices can be three times more expensive than IGBTs, and currently extend to 1200 V, which is sufficient for 800 and 900 V battery packs in e-mobility designs but can struggle as packs move to 1000 V and above. IGBTs meanwhile can extend to 1700 V and even up to 3300 V for higher voltage switching systems in agricultural and construction platforms, where faster switching is not so important. There are also hybrid devices, called cascodes, that combine an SiC transistor with a silicon IGBT or MOSFET in the same package to provide a balance of switching performance and cost. The higher withstand voltage allows a single transistor to be used for each ‘leg’ of a switching architecture to handle a single phase, although lower voltage parts can be used in multi-stage designs. This becomes more complex though with multiple transistors for each stage and for each phase of the switching architecture. “For 1200 V devices with an 800 V battery, the IGBT is less competitive compared with SiC, but there are opportunities in charging applications,” says Luigi Di Turi, applications manager for HV products at STMicroelectronics. A newer generation of transistors using a layer of gallium nitride on a silicon substrate is being used in onboard chargers with new topologies such as a totem pole or a Vienna rectifier. This is also leading to new algorithms to control the switching devices. Traditional switching control schemes such as pulse width modulation (PWM) are being enhanced with machine learning and new techniques to make the switching even more efficient. Onboard chargers With onboard chargers (OBCs), the silicon superjunction MOSFETs can be replaced by GaN or SiC devices, with the smaller size allowing the OBC to be combined with DC-DC converters. “We start with the switching topologies rather than the technology,” says Di Turi. “The half-bridge arrangement is very versatile and can be implemented in an inverter with a leg of the bridge based on IGBT, SiC or a superjunction MOSFET. We also develop full-bridge and more complex switching topologies, implemented on A 22 kW onboard charger design using SiCs and IGBTs (Courtesy of 0nfineon ;ecOnoloNies) 56 Winter 2021 | E-Mobility Engineering