A power controller for an LLC resonant converter controls a high-side switch and a low-side switch. An ON-time generator in the power controller determines a high-side ON time of the high-side switch and a low-side ON time of the low-side switch in response to the bigger one between a feedback voltage and a burst voltage, where the feedback voltage is generated in response to an output voltage of the LLC resonant converter. A burst-mode controller in the power controller has a triangular-wave generator providing a triangular-wave signal with an amplitude in association with the burst voltage. A comparator comparing the triangular-wave signal and the feedback voltage to determine a break time when both the high-side and low-side switches are turned OFF. The LLC resonant converter operates in a burst mode when the break time is introduced.