‘To Boldly go’ – Switching to the Future

  • Jun 18, 2020
  • UnitedSiC

Abstract

Mechanical switches and actuators are becoming a thing of the past in modern vehicle cockpits. Under the hood, mechanical switching and isolation of high current and voltage connections can now also be replaced by latest SiC semiconductors.

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It’s probably the most famous split infinitive, but ‘to boldly go’ or’ to go boldly’, either way, you get the idea.

Engineers are not particularly known for going boldly, it’s their job to be careful and question their own work, and no more so than in the automotive industry. We all have reason to be grateful for this, trusting that our car will be safe and reliable, with proven designs that change in an evolutionary, rather than revolutionary way. EVs, however, have forced a change in approach to design with modern cars more like mobile computing platforms than A to B transport.

Despite the high-tech innovations in EVs, there are some areas that still rely on simple mechanics; while wheel bearings aren’t going away anytime soon, electromechanical devices such as contactors and circuit breakers are still common, to break the high voltage/high current battery connections for safety isolation and in the event of faults. Prospective current can be in the thousands of amps, battery voltages are increasing to 800V, and with inherent shock and vibration, the devices have to be very rugged. Rupture of the casing is a concern on overload, along with wear that occurs when breaking DC, as the arc drawn burns off contact material and generates heat. An arc can be quenched with expensive gas-filled housings or even with exotic techniques such as blowing it away with compressed air or diverting it to a longer path with a magnetic field. All this along with the problem of slow response time means the application is crying out for a better solution.

SiC is now a viable alternative

Solid-state circuit breakers have been around for a long time but implementations with GTOs, IGBTs and more recently MOSFETs have not been ideal, with significant insertion losses which are unacceptable in EVs where every watt not used to achieve driving range is a problem. Now however, new generations of SiC FETs from UnitedSiC are changing the game; with latest 650V devices showing on-resistance of less than 7 milliohms and 1200V types less than 9 milliohms, conduction losses are much less of a problem and the inherent high junction temperature rating of SiC also helps. Available in TO-247 packages and with easy gate drives, the SiC FETs can often be a drop-in replacement for IGBTs and MOSFETs in existing breaker designs for lower losses with operation times 1000x quicker than mechanical types and, of course, with no arcing. Devices are rugged with good avalanche energy ratings for over-voltages.

Once breakers and contactors become solid-state, other control possibilities open up; conduction can be actively controlled for inrush current limiting or ‘pre-charging’ and short-circuits can be handled more intelligently. The JFET in SiC FETs for example, has an inherent ‘pinch-off’ effect which sets saturation current, which is quite constant with applied voltage and reduces with temperature for a benign power-limiting action (Figure 1).

Figure 1: SiC JFET saturation current is nearly constant with voltage and reduces with temperature

This characteristic can mean that an extremely simple current limiter can be implemented with just two SiC JFETs, chosen for the appropriate saturation current (Figure 2).

Figure 2: Two series SiC JFETs make an effective current limiter

Configured as a circuit breaker, normally-on SiC JFETs from UnitedSiC can also be configured as a true two-terminal circuit breaker module, without needing an external auxiliary power supply or internal DC-DC converters.

Fortune favors the bold

Engineers have always known that “if it moves, it will break” and latest EV cockpits with touch screen controls and drive-by-wire are a response to this. The latest SiC FET and JFET devices should encourage designers to boldly go, splitting infinitives and beyond, consigning mechanical contactors and circuit breakers to history.