A Line Cycle Synchronous Rectification Strategy Based on Time-Domain Analysis for Single- Stage AC–DC LLC Converters

Synchronous rectification that is being widely used in high power and current DC-DC
LLC
resonant converters to reduce conduction losses can be challenging in single-stage AC-DC
LLC
converters with high output voltage levels (i.e., >200 V) where synchronous rectifier (SR) driving ICs cannot be used. In this paper, a simple AC line cycle synchronous rectification strategy with direct control by a cost-effective microcontroller unit (MCU) is proposed for single-stage AC-DC
LLC
converters with high switching frequencies using wide bandgap devices (i.e., GaN or SiC). The SR gate pulse is generated based on the time-domain calculated conduction time, which is then switched
on
and
off
over the AC line cycle to avoid reverse power flow in light load conditions. The proposed strategy reduces the complexity of implementation over any adaptive online calculation or model-based methods that require powerful and expensive MCUs. First, the operation is briefly described followed by the time-domain analysis for AC operation. Next, the calculation and methodology behind the proposed AC line cycle SR driving strategy are discussed in detail. A scaled-down wide bandgap-based AC-DC
LLC
converter prototype with a 250--400 V output voltage range is used with digital control implementation to validate the performance of the proposed synchronous rectification strategy. It is found that maximum efficiency of 98.1% can be achieved which is improved by around 0.5% over the conventional fixed conduction time method. Moreover, it is shown that the proposed method obtains the same efficiency levels as more complex adaptive SR driving approaches.