Adaptive Dead Time Synchronous Rectification Control for High Efficiency LLC Resonant Converter

In low output voltage and high output current LLC resonant converter applications, a synchronous rectification (SR) is essential component for high efficiency and high power density. In the conventional SR control method, instantaneous drain voltage sensing method is widely used because it shows very simple control algorithm. However, the method has disadvantage against stray inductances in MOSFET package and PCB pattern. The stray inductances induce premature turn-off of SR gate and large SR dead time. Since the body diode conduction time is increased, the system efficiency is decreased in the conventional method. To compensate the stray inductance effect and increase system efficiency, an adaptive dead time control method is proposed. In the proposed control method, SR dead time is regulated to predetermined dead time target regardless of the stray inductances by using present drain voltage information and previous cycle dead time information. To verify the validity of the proposed control method, 240W prototype is built and experimented with the proposed controller which is implemented with 0.35-mu m BCDMOS technology.