Synchronous rectifier reduces conduction loss in LLC resonant power supplies

With increasing drive to shrink electronic solutions, the merits of resonant power converters are now attracting increased attention among power supply engineers. This will especially affect the LCD TV 80PLUS Silver or the more recent 85PLUS ATX power supplies segment, where there is more and more pressure to shift towards higher switching frequencies in an attempt to reduce the physical size and cost of key system elements, such as the power transformer, filtering capacitors and heat sinking component. Resonant LLC converters can enable higher frequency through minimization of the primary switches’ switching loss. Critically, the rectifier stage can constitute a significant source of conduction loss, since even the Schottky will have a relatively large voltage drop at high current ratings. This article discusses the limitations of using a diode and shows that the ultimate efficiency could be realized by the more elaborate scheme of using synchronous controller in conjunction with low resistance MOSFET as the output rectifier.