The road to the EV revolution is paved with SiC

  • Dec 04, 2019
  • UnitedSiC

Silicon carbide semiconductor switches have many attributes that make them serious contenders to replace IGBTs in EV inverter applications. Conduction losses though have been comparable at high currents and rated voltages. Latest generation of SiC cascodes, however, breakthrough this barrier with ultra-low on-resistance.

When Dorothy and Toto followed the yellow brick road it might have been paved with silicon carbide – an iridescent yellow crystal. (SiC can also be green or bluish-black but let’s not spoil the metaphor). It would have been pretty good for grip and durability – its other name is carborundum.

In a modern version of the story, the characters might travel in an electric vehicle towards the Emerald City but in the real world, mass-market adoption of EVs is hindered by price and limited range. The demand is there though for a zero-carbon emission future for transportation. Initiatives such as the EV30@30 to reach 30% market share for EVs by 2030 are tough but advances in power conversion components help to make the switch to electric propulsion more viable in terms of range and acquisition and running costs. While battery and motor technology is advancing, the conversion efficiency, size and cost of the motor drive electronics is still critical. Every percentage point of efficiency gained contributes to a virtuous circle of less load on the battery, less heat dissipated, smaller size and less weight, in turn helping get more miles per charge.

This is where SiC as a semiconductor switch comes in rather than as a road surface. The first generation of EVs exclusively used IGBTs as switches, a mature technology tried and tested over decades, proven to be robust and reliable. Many paralleled leaded packages have been used, such as in Tesla models, but IGBT modules are also considered. IGBTs in motor drives do not need to switch at high frequency but their switching edge rates are also slow, dissipating significant power on each transition. Si-MOSFETs are substantially better with nanosecond slew rates but are limited in power rating and with peak powers in EVs now measured in fractions of a megawatt, this is a problem. SiC, however, has the speed and low on-resistance to handle the hundreds of amps typically involved. Early generations with higher on-resistance, particularly for parts with high voltage ratings, still dissipated significant I2R power but latest generations achieve sub-10milliohms even at 1200V ratings, making them suitable for the next generation of EVs with 800V batteries.

Examples are the latest in the UF series of SiC switches from UnitedSiC with part UF3SC065007K4S rated at 650V and 6.7 milliohms and part UF3SC120009K4S rated at 8.8 milliohms at 1200V. Complementing the range are parts with 16 milliohms Rds(on) at 1200V rating for lower power applications. All the devices are available in TO-247 packages with 4-lead Kelvin connections for optimum gate drive. 

New ultra-low Rds(on) SiC cascodes from UnitedSiC in a 4-lead TO247 package 

These are cascodes – a combination of SiC JFET and Si-MOSFET which give the switching performance of SiC but with the ease of gate drive of a MOSFET. The gate drive voltages required are compatible with MOSFETs and IGBTs so the possibility opens up of dropping in the devices into existing applications that use TO-247 IGBTs, for an instant boost in efficiency performance without reinventing the entire power platform. More benefit can be obtained by adjusting components around the inverter, for example snubber sizes can be reduced and IGBT parallel didoes removed as the cascodes have an intrinsic high-performance body diode effect. Following this process, efficiencies of better than 99% are easily achieved. Power loss is less, so heatsinking can be smaller, cheaper and lighter leading on to better range.

Reliability of the SiC cascodes is excellent with proven high-energy avalanche ratings and short circuit withstand with self-limiting of peak current as the device heats. The inherent high-temperature operation of the wide band-gap material also gives peace of mind with parts rated at 175°C continuous junction temperature with higher peak values allowed.

When the land of Oz declares itself emissions-free, those yellow SiC crystals are surely going to be in Dorothy’s EV as well as paving the roads.