A buck regulator or impedance converter is disclosed for operating a current limited power supply such as a photovoltaic array at or near its maximum power point. The maximum power point for such a supply is achieved at the compliance voltage which is an almost constant voltage. Means are provided for sensing the voltage at the input of the regulator and for using this sensed voltage to control a logic circuit which, in turn, controls the duty cycle of a switching transistor. The output from the switching transistor is coupled to an LC filter and from there to the load. The logic circuitry, responsive to the sensed voltage, controls the duty cycle of the switching transistor in such a manner as to maintain the input voltage approximately constant and thus to maintain the power supply at or near its peak power operating point.