An LLC Converter Family with Auxiliary Switch for Hold-Up Mode Operation

Hongliang Wang, Yang Chen, Peng Fang, Yan Fei Liu, Jahangir Afsharian, Zhihua Yang

Research output: Contribution to journalArticlepeer-review

38 Scopus citations


This paper proposed a new half-bridge (HB) LLC resonant converter family with pulse-width modulated (PWM) auxiliary switch (sLLC converter) for hold-up mode operation. The proposed sLLC converters could operate with synchronous rectifier, which is suitable for low-output voltage applications. In the proposed converters, an auxiliary switch is added at the primary side to provide charging path for the series resonant inductor. For nominal 400-V input, sLLC achieves same performance as conventional LLC converter, and all the good features such as soft switching are naturally retained. For slight input voltage fluctuation, frequency modulation is used to regulate the output voltage. When the input voltage reduces further, HB switches will operate at constant minimum frequency, and the auxiliary switch will operate in the PWM mode to energize the resonant inductor with the bus voltage directly during hold-up period. Thus, the converter achieves higher voltage gain, and the output voltage can be maintained at desired level. To verify the effectiveness of the proposed sLLC converter, operational principle and equivalent circuits will be carefully explained and analyzed in this paper. A 300-W prototype is built for 250-400-V input 12-V output application.

Original languageEnglish (US)
Pages (from-to)4291-4306
Number of pages16
JournalIEEE Transactions on Power Electronics
Issue number6
StatePublished - Jun 2017
Externally publishedYes

Bibliographical note

Publisher Copyright:
© 1986-2012 IEEE.


  • Auxiliary switch
  • LLC
  • high-voltage gain
  • hold up
  • pulse-width modulation (PWM)
  • server power supply


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