This paper reports the occurrence of symmetry-breaking bifurcation in the series-parallel load-resonant dc-dc converter with L-C output filter. The constant frequency phase shift modulation technique is employed with proportional- integral controller in the output voltage regulation. It is shown that the dynamics are not only related to the resonant components but are also heavily dependent on the output filter design. The first instability can be caused by smooth and nonsmooth symmetry breaking bifurcations. Stable as well as unstable periodic orbits are identified by a newly developed algorithm based on fundamental solution matrix. As a significant result of this paper, it is shown that the symmetry-breaking can be caused by a grazing event.