The present invention provides a prestart control circuit that measures a reflected voltage on the primary side of a switching power converter. The prestart control circuit pulses the synchronous rectifier switch OPEN and CLOSED, which allows a measurement of the reflected voltage on the primary side of the switching power converter. The reflected voltage is proportionate to the output voltage of the switching power converter. The prestart control circuit uses the reflected voltage to establish the initial duty cycle of the switching power converter. The switching power converter may be any converter that includes a synchronous rectifier, such as a flyback converter or a forward converter, in a single-ended, double-ended and/or multi-phased configuration.