October / November 2021

www.unitedsic.com POWER SEMICONDUCTORS 23 www.power-mag.com Issue 4 2021 Power Electronics Europe A New Approach to Circuit Breaker Design Using Silicon Carbide Switches Mechanical circuit breakers can be low cost with minimal losses, but they operate slowly and wear out. Solid state versions overcome the problems and are becoming increasing viable as replacements at ever-high currents. This article discusses the issues and introduces a new Silicon Carbide semiconductor, the DG-FET as an enabler for better performance. Anup Bhalla, VP Engineering, UnitedSiC, Princeton, USA Mechanical circuit breakers have been around for a long time and do the basic job very well – they are low loss when on and provide excellent isolation when off. They do have their downsides though, with slow make/break times and arcing across the contacts, degrading their operating lifetime, particularly with DC power lines. Complex arrangements have been devised to disconnect quickly but it might still take around 10 milliseconds, allowing significant and possibly damaging energy to pass after a short circuit is detected, for example. Similarly, DC arcs can be suppressed to an extent with labyrinth clearances, a magnetic bias field or even blasts of compressed air. However, there are new applications for high current DC circuit breakers where this complexity cannot be afforded such as in EVs, where cost, size and weight are major issues. Solid-state breakers have been a partial solution Solid-state circuit breakers (SSCBs), typically using IGBTs, can break power lines perhaps a thousand times faster than mechanical types with no arcing or aging problems, but they are imperfect switches, dropping around 1.5 V, producing tens or hundreds of watts of dissipation at the current levels seen in EV traction drives, for example. The unit and heatsinking costs are substantial and the power loss effectively reduces battery energy available for driving range, so in EVs, IGBT-based SSCBs are not seen as viable. Silicon MOSFET-based SSCBs have been an alternative, with their on-resistance potentially dissipating less power than IGBTs which have a fixed saturation voltage. However, at high current levels with high voltage devices, sufficiently low on-resistances are not yet available. For example, at 500 A, an IGBT could drop Table 1: Mechanical and solid-state circuit breakers compared Figure 1: A SiC JFET (left), SiC FET (middle) and a DG-FET (right)