Driving down the on resistance of silicon carbide transistors: Page 2 of 3

December 13, 2019 //By Nick Flaherty
UnitedSiC has developed silicon carbide SiC transistors in standard packages with the world's lowest on resistance. Chris Dries, CEO, talks to Nick Flaherty about the significance of the move.
UnitedSiC has developed silicon carbide transistors in standard packages with the world's lowest on resistance. Chris Dries, CEO, talks to Nick Flaherty about the significance of the move.

“The problem with SiC is that it does not like to form a gate oxide," said Dries. "What we have done is use the perfect oxide of silicon as the gate interface and then we have the SiC JFET with the lowest  specific rds on, so we optimise the performance of both FETs. Its really hard to do it successfully but once you can it’s the best way," he added.

While the 4pin versions are aimed at high frequency switching, the three pin versions are drop in replacements for silicon IGBTs.

“The 3pin version is to directly replace CoolMOS [devices from Infineon] in low RDS on designs. Designers can move from 2.2, 2.4kW to 3kW by replacing CoolMOS silicon with drop in SiC. Most SiC MOSFETs need a negative gate drive from  -4 or -5V to +15 or 20V, but we design the gates so they are clamped at -20V so we can be pin for pin compatible with silicon IGBTs.”

Thermal management is a key part of the new devices. “What we do is silver sinter the die to the lead frame using die attach technology that allows us to get the heat out of the package. We can do a three lead (3L) package but the di/dt necessitates a four pin kelvin connected package," he said. “In the SiC industry we all have this four lead package – Infineon, Cree, ST all have it – the switching performance is far superior and we try to steer customers to it. Those parts can run at over 100A in a TO247 package.”

The low conduction losses mean the devices can also be used as solid-state circuit breakers and battery disconnect switches in electric vehicles. The devices can turn off very high currents very quickly, and when used as a circuit breaker have a self-limiting characteristic that controls the peak current that flows. This characteristic can also be used to limit inrush currents flowing into inverters and motors.

“Solid state circuit breakers s a new and emerging market for us, enabled by the low RDS on devices. Initial customers tend to be in the aerospace industry to route power in an aircraft,” said Dries. “When paralleled can surge rating over 100A and turn off safely. With six die in parallel you get a 2 mΩ switch in a SOT227 1200V module.”

The UF3SC065007K4S has a maximum operating voltage of 650V, a drain current of up to 120A, and an RDS(ON) of 6.7mΩ. The UF3SC120009K4S has a maximum operating voltage of 1200V, drain current of up to 120A, and an RDS(on) of 8.6mΩ. Both come in a four-lead Kelvin package, enabling cleaner drive characteristics.

For lower-power designs, UnitedSiC offers two parts with maximum operating voltages of 1200V, drain currents of up to 77A, and an RDS(on) of 16mΩ. The UF3SC120016K3S has a three-lead package, while the UF3SC120016K4S has a four-lead package.

Next: SiC production 


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