Abstract: Based on generalized state space averaging, this paper presents a control strategy which could realize the active soft switch of switching devices and regulate output voltage simultaneously by means of pulse frequency modulation-pulse width modulation (PFM+PWM) with two degrees of control freedom. The proposed strategy could "actively" change the soft switch mode according to the loss profiles of the devices, taking the overall improvement of efficiency and the realization of wide load range into account. Three operation modes of full bridge series resonant DC converter when switch frequency is above resonant frequency are analyzed, in which the abnormal discontinuous current mode could be escaped by designing resonant tank parameters. The generalized state space averaging is used to model the frequency response dynamics of the system including the modulation section whose numerical sweep results coincide with the physical model well. The sensitivity matrix reflecting the controllability of MIMO system is defined, by which the linear double-link control strategy to realize output voltage control and active soft switch control is designed. MATLAB/ Simulink simulation and experiment prototype verify that the presented strategy could realize higher efficiency and wider load range of pulse frequency modulation.