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Auto Tech Outlook | Friday, June 18, 2021
Efficient drivetrains using semiconductor technologies such as Silicon Carbide (SiC) enable engineers to meet high voltage and power demands cost-effectively.
FREMONT, CA: At a more accessible price point, consumer demand for electric vehicles (EVs) with a range comparable to internal combustion engines currently outstrips technology itself. More efficient drivetrains, using semiconductor technologies like Silicon Carbide (SiC), enable engineers to achieve cost-effectively high voltage and power demands. As a result, some EV applications use SiC technology. These are primarily for low-power applications like battery chargers, auxiliary DC-DC converters, and solid-state circuit breakers. However, drivetrain power designers are reluctant to use this technology, preferring to wait for an acceptably low ON-resistance, improved robustness, and more accessibility. Now, a performance breakthrough addresses all those concerns—the latest SiC-FET generation of the US, or 'stacked cascode.'
A stacked cascode consists of two transistors mounted on top of each other: a high-voltage SiC JFET is connected to an optimised low-voltage Si-MOSFET. MOSFET shorts JFET gate-source up, turning it ON. MOSFET drain voltage rises at the low entrance, but only when the JFET closes at 10V. The result is an OFF-device easy drive. It also has all the advantages of a low-resistance, high-voltage, high-temperature operation SiC device and an integral body diode effect with excellent reverse recovery features. Cascode idea has been around for a while now, but JFET versions now achieve ON-resistance at high voltage ratings, labeling them close to the "ideal" switch.
SiC-FET also has short-circuited robustness. High channel resistance current produces a negative JFET gate bias that tends to disconnect the device. In addition, the channel's positive temperature coefficient further reduces short-circuit current through self-heating. This effect accessible to parallel SiC-FETs with automatic current balancing, further assisted by the relatively insensitive characteristics of stacked MOSFET threshold voltage and reverse temperature recovery.
In fast-charger applications, SiC-FETs are ideal. They offer peak efficiency in PFC's front ends and the main stage of DC-DC conversion—both typically using a phase-shifted full-bridge or LLC topology. Due to their low drop and lack of reverse recovery losses, SiC diodes are already used in high-voltage chargers to implement output rectification. This is because Si-MOSFET synchronous rectification (SR) is complex at high voltage and can save no diode losses. However, using SiC-FETS with low RDS(ON) may be better.
As UnitedSiC SiC-FETs are available in three- and four-lead TO-247 packages, they can replace many IGBTs and Si-MOSFETs in motor drives. This gives a significant efficiency boost with little circuit change, apart from perhaps gate drive resistors and small snubbers to tailor the switching edges. Moreover, with UnitedSiC's latest generation of low-RDS(ON) devices, SiC-FETs pave the way for the EV drivetrain revolution.
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