A power supply system includes a high-speed power supply providing a first output, operating in conjunction with an externally supplied DC source or low frequency power supply which provides a second output. A frequency blocking power combiner circuit combines the first and second outputs to generate a third output in order to drive a load, while providing frequency-selective isolation between the first and second outputs. A feedback circuit coupled to the combined, third output compares this combined, third output with a predetermined control signal and generates a control signal for controlling the high-speed power supply, based on a difference between the third output and the predetermined control signal. The feedback circuit does not control the DC source or the low frequency power supply, but controls only the high-speed power supply.