Abstract: There is provided by this invention an apparatus and method for controlling a dc magnetron plasma processing system that automatically adjusts the control signal to the power supply based upon the dynamic impedance of the load to control the output power to the plasma. The output voltage and the output current of the power supply that supplies power to the plasma is sampled over at a sampling frequency at least four to five times higher than the switching frequency and the dynamic impedance of the plasma is calculated based upon the sampled voltage and current from the algorithm R plasma = ? ? ? ? V n ? ? ? ? I n wherein ?Vn and ?In is the maximum difference among samples on one switching cycle. If the dynamic impedance seen is negative in nature then the control signal is compensated accordingly.