Series-Connected JFETs in Active Current Limiter

A simple circuit is proposed that simultaneously balances voltage across and controls current through series-connected FETs for both DC and AC applications, all with a single current control input. This circuit enables active mode (partially-on) operation at high voltage by series-connecting FETs. It is also a low-cost option for higher power active mode operation because one controller regulates the current through multiple FETs, as opposed to one controller per FET when paralleling. A fast turn-off feature blocks current for both emergency and normal turn-off. A way to improve voltage balance is presented, which is especially useful at low operating voltage. Such a circuit is useful as an active current limiter, soft starter, and a solid-state circuit breaker. Experimental results are provided.