A method and apparatus for controlling a power supply to prevent instabilities due to dynamic loads in RF plasma processing systems, operating at frequencies of from 1 MHz and up, uses a feedforward type of control loop to tightly regulate the power supplied to the dynamic electrical load, such as loads caused by variable and inconsistent plasma impedance. A feedback control loop can also be used in combination with the feedforward loop, but at a slower rate, to help regulate the amount of power provided to the load.