A DC-to-AC inverter provides an AC voltage to the primary winding of an output isolation transformer having at least one secondary winding. An AC output voltage from the secondary winding is rectified to generate a DC voltage, which is applied to a load. The current flowing through the load is sensed and compared to a reference magnitude to produce a feedback signal. The feedback signal controls a voltage superposition circuit via an output stage of an optocoupler. The superposition circuit generates a superposition voltage that is applied to a current control circuit, which responds to the superposition voltage to vary a control current to a switching controller, which varies the frequency of the AC voltage to thereby vary the load current. A fault detection circuit senses a short in the output stage of the optocoupler to disable the operation of switching controller to prevent an overvoltage and an overcurrent.