It’s now common to see circuit boards that carry hundreds of amps, especially in intensive data-processing applications, electric vehicles, and ac distribution. Despite their designed current-handling capacity, such circuits still need current limiters and circuit breakers to protect them from fault conditions. Appropriate current limiters and circuit breakers need to drop as little voltage as possible under normal operating conditions, be robust over the long term, and yet operate immediately and effectively in response to undefined fault conditions.
Limiting Current
Traditional current limiters include resistors, low-current positive temperature coefficient thermistors, and active circuits that give minimum losses and maximum control at higher currents. Active circuits are often built with bipolar junction transistors (BJTs) or MOSFETs. At lower currents, MOSFETs exhibit a low drain-source resistance (RDS(on)) and thus offer near zero loss—at a cost. At higher currents, the constant saturation voltage between a BJT’s collector and emitter (VCE(SAT)) may mean lower losses than incurred through the resistive drop of a MOSFET.