Modelling and design of a robust controller for fixed-frequency PWM DC-to-DC power converters are well-known problems and have been intensively investigated in the literature. However, none of the existing studies considers the variable-frequency applications recently employed to improve efficiency. This paper focuses on synthesizing a robust compensator for a variable-frequency, wide operating range, flyback converter using the H_∞ framework. The simulation results show that it is possible to design a single controller that can preserve stability over the whole working range of the converter, but robust performance may be compromised.