Control Method For Quasi-Resonant Flyback Converters Achieves Unity Power Factor With Zero THD
by Claudio Adragna and Giovanni Gritti, STMicroelectronics, Agrate Brianza, Italy, How2Power Today, Sep 15 2017
Focus:
The high-power-factor quasi-resonant (QR) flyback converter is popular in cost-sensitive
applications like solid-state lighting because it achieves two objectives using a single
conversion stage: compliance with harmonic current emission regulations such as IEC61000-3-2
and the generation of an isolated bus voltage. But with the conventional control scheme, the
primary current flows only during the on-time of the power switch, making it difficult to meet
the THD spec required in some areas. This distortion can be prevented if the flyback is
operated with a fixed switching frequency in DCM, but then some benefits of QR operation are
lost. This 19-page article describes a novel control method that retains the advantages of QR
operation yet draws a sinusoidal input current. The operating principles of this variable-
frequency control method are explained, models of two possible implementations are given and a
practical circuit implementation using ST’s L6561 PFC controller is shown. The article presents
large signal and small signal analysis, analysis of non-idealities, and simulated and
experimental results that validate the methodology.
What you’ll learn:
- How to achieve high power factor in a single PFC stage with benefits of QR operation and sinusoidal input current
- How to achieve low THD in a single-stage high power factor quasi-resonant flyback converter
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