A robust controller is designed for the series parallel resonant DC/DC converter based on the linear quadratic Gaussian/loop transfer recovery (LQG/LTR) methodology. The controller structure, comprising a servo-compensator with an internal model, reference state observer, plant and disturbance state observer, ensures tracking of the reference voltage and rejection of system disturbances. The controller performance is insensitive to converter parametric and operational variations. The controller design is based on a converter small-signal model derived using the principles of the describing function and harmonic balance on the nonlinear time-varying converter equations. The controlled converter is shown by computer simulation to perform excellently well, with very good tracking and disturbance capabilities in the presence of changes of load and input voltage.