High Accuracy Knee Voltage Detection for Primary-Side Control in Flyback Battery Charger

Shang Hsien Yang, Tsung Hsun Tsai, Hsin Chen, Chao Chang Chiu, Ke-Horng Chen*, Ying Hsi Lin, Jian Ru Lin, Tsung Yen Tsai

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

18 Scopus citations


In a primary-side control flyback charger, the accuracy of a conventional knee voltage detection (KVD) approach to obtain the output voltage is influenced by the inclusion of a snubber circuit. Although the snubber circuit dampens the ringing voltages due to switching, it also affect the resonance frequency which would reduce the timing of the sampling circuit, resulting in the inaccurate sampling of output voltage. By analyzing the snubber circuit and its resonance on the primary side, this paper implements a proposed accuracy knee voltage detection (AKVD) technique to compensate for the induced resonance frequency. Furthermore, the proposed constant current (CC) regulator ensures the accuracy of CC mode by compensating the sensing current error caused by the switching delay of the power transistor. A prototype consisting of a test chip fabricated with a 0.5μm BCD process demonstrates a voltage accuracy exceeding 99.63%. This is achieved by mitigating abnormal detections and thus ensuring correct control of charging current throughout the charging period.

Original languageEnglish
Article number7786866
Pages (from-to)1003-1012
Number of pages10
JournalIEEE Transactions on Circuits and Systems I: Regular Papers
Issue number4
StatePublished - 1 Apr 2017


  • Band-pass filter
  • knee voltage detection (KVD) technique
  • primary side regulator
  • snubber circuit

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