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

• Published in: IEEE transactions on power electronics 2023-04, Vol.38 (4), p.1-15
• Main Authors: Forouzesh, Mojtaba, Liu, Yan-Fei, Sen, Paresh C.
• Format: Article
• Language: English
• Subjects: Complexity; Conduction losses; Efficiency Improvement; Electric potential; Energy gap; GaN HEMT; Integrated circuits; LLC Resonant Converter; Logic gates; Microcontrollers; Power flow; Resonant converters; Sensors; SiC MOSFET; Silicon carbide; Single-Stage AC-DC; Switching frequency; Synchronous Rectification; Time domain analysis; Transformers; Voltage; Wide Bandgap Devices
• ISSN: 0885-8993; 1941-0107
• DOI: 10.1109/TPEL.2023.3235968

Synchronous rectification that is being widely used in high power and current DC-DC LLC converters to reduce conduction losses can be challenging in single-stage AC-DC LLC converters with high output voltage levels (i.e., >200V) 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 microcontrollers. First, the operation of the single-stage LLC converter is briefly described for power factor correction (PFC) operation and then the time-domain analysis for AC operation is provided. 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 V to 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 for the 250 V output voltage condition which was improved by around 0.5% over the conventional fixed conduction time method. Moreover, it is shown that the proposed SR driving method obtains the same efficiency levels as more complex adaptive SR driving approaches.