It’s automotive, but not as we know it

  • Dec 03, 2019
  • UnitedSiC

As the speed of automotive invention accelerates beyond what Henry Ford could ever have imagined, automotive engineers are finding themselves faced with a fundamental shift away from electro-mechanical engineering.

With consumer demand and industry focus moving emphatically towards hybrid and fully electric vehicles, this change is even more keenly felt, meaning that engineers must look beyond ‘what we’ve always done’ and seek out innovative solutions to problems such as breaking high-current, high-voltage power lines.

Fun fact – in the Star Trek series, Spock never actually said “It’s life, Jim, but not as we know it”– this comes from a parody song by British band ‘The Firm’ in 1987. It had chart success in several countries and the phrase somehow became part of ‘Trekkie’ folklore. In those days, the technology of viable electric cars would have been just as futuristic as warp drive and tractor beams but today it is mature to the point of mass production.

Some areas of vehicle design, even in EVs, have been stuck in in the 1980s though with electromechanical switches and contactors still used to break high currents to disconnect power rails under fault conditions. With prospective fault currents in the thousands of amps and battery voltages heading towards 800V and beyond, the mechanical option becomes complex and difficult. Wear is the main worry – breaking DC causes an arc which has a length proportional to voltage and an intensity proportional to current. Material is burnt off contacts and the heat from the arc degrades the whole component. While measures can be taken to quench the arc with oil filling or more exotic techniques such as blowing the arc away with compressed air, the problem just gets worse as current and voltage levels increase. Being a mechanical device, a breaker or contactor is also susceptible to shock and vibration, a particular problem in automotive environments, and operates relatively slowly, with typical values measured in tens of milliseconds.

Solid-state breakers have been an option for some time using GTOs, IGBTs and more recently MOSFETs, but insertion loss is significant, with tens of watts potentially dissipated in the devices under normal running conditions, stressing the components with heat and losing system efficiency.

Latest generation silicon carbide (SiC) switches are opening up new possibilities, however. Configured as breakers, they operate within 100ns to a few microseconds and arcs are avoided. New parts from UnitedSiC such as the UF3SC065007K4S (650V, 6.7 milliohm) and UF3SC120009K4S (1200V, 8.6 milliohm) have minimal conduction losses and can work reliably with continuous junction temperatures of 175°C with even higher peak ratings. In TO-247 packages, the SiC switches are cascodes of a SiC JFET and Si-MOSFET and with non-critical gate drives are easily implemented in new designs or can often drop in as replacements in existing discrete solid state breaker designs using MOSFETs or IGBTs.

Compared with mechanical breakers, solid state switches using SiC cascodes allow other functionality. For example, conduction can be controlled to limit inrush current or to pre-charge capacitors. Short circuit current can also be limited by a ‘pinch-off’ effect in the integral JFET providing a well-controlled saturation current which usefully reduces as temperature increases due to electron mobility increase (Figure 1).

Figure 1: SiC JFET saturation current is nearly constant with drain-source voltage

In the off-state, the SiC devices also withstand voltage transients well with an effective high-energy avalanche rating.

SiC cascodes are intended to be compatible with MOSFET and IGBT gate drives so require a positive Vg for saturation. SiC JFETs are also available from UnitedSiC which are normally on – that is, conducting at Vg = 0V. This opens up the possibility of a true two-terminal circuit breaker module which does not require external auxiliary power rails or internal DC-DC converters, Figure 2.

Figure 2: Two-terminal self-biasing circuit breaker concept

Modern automotive design increasingly does away with moving parts, with touch-screen displays and electronic switching becoming standard. There is now a high-efficiency and robust solution for switching the high voltages and currents in the drive and battery system using SiC cascodes from UnitedSiC.

Automotive technology is increasingly ‘not life as we have known it’ – use of solid-state circuit breakers, switches and current limiters using SiC devices is another opportunity for engineers to split their infinitives and ‘boldly go’ towards the next generation of EV designs.