This circuit presents an easy way to control the current for a 115-Vac input using the floating capabilities of the UCC3917 hot-swap power manager. The full bridge topology (of diodes D1-D4) ensures that the UCC3917 and the external N-channel MOSFET, Q1, see only positive voltages. Power for the UCC3917 is derived from transformer T1. R1 is used to set the current into Vdd, which operates as a shunt regulator to provide 10 V to the IC as well as to protect against excess voltage.
In addition, the UCC3917 provides overcurrent fault protection. During an output overcurrent (a short-circuit, for example), the maximum current (IMAX) sourced to the load is controlled by the UCC3917 and the N-channel MOSFET. The maximum current source capability is programmed by R2, R3 and RS. If the load current rises above a user-programmable threshold, capacitor C3 begins to charge at a constant rate. Once it reaches 2.5 V, Q1 will be disabled. The UCC3917 will re-enable Q1 at a 3 percent duty cycle for the on-time determined by C3. R5 provides additional power limiting for Q1 by further decreasing the duty cycle.
U2 provides remote shutdown to remove power to the load. For the circuit shown, the fault current threshold is 0.2 A, IMAX is 0.5 A and fault time is 6 microseconds. On-time for the current fault should be programmed to ensure the external N-channel MOSFET remains in its safe operating area during fault conditions.