LLC resonant converter controller with novel light load control

LLC resonant converter is an attractive topology to achieve high power density with high efficiency. However, the high switching frequency and circulating current result the poor efficiency of a LLC resonant converter under light load condition. To improve efficiency further, a modified burst-mode control is implemented in the proposed controller. The switching frequency decreases gradually during burst-on time to reduce the number of switching cycles under the same output ripple. Moreover, in burst-on time, the first pulse drives the MOSFET at high side with limited pulse width to suppress the initial current in resonant tank. The last switching cycle in burst-on time must be complete to ensure that initial conditions of each burst cycle are the same. Consequently, the switching losses are reduced, and light-load efficiency is improved.