A capacitor charging circuit with primary side output voltage sensing for generating a control signal indicative of whether the output voltage has reached a desired level. The control signal is unaffected by voltage spikes occurring when the main switch is turned off. In one embodiment, the primary side voltage is filtered for comparison to a reference voltage to provide the control signal. In another embodiment, the control signal transitions in response to the primary side voltage being greater than a reference voltage and the secondary current being discontinuous. In a further embodiment, the control signal transitions only in response to a predetermined delay occurring after the primary side voltage becomes greater than a reference voltage and the secondary current becomes discontinuous. According to a further aspect of the invention, an auxiliary switch reduces the reverse recovery time of the output diode to speed up the capacitor charging time.