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From this article: Power Tips: Power converter topology... they say that:

Moreover, buck-converter duty cycles become extremely small in high-voltage AC/DC conversion. For a 440VAC to 15VDC conversion, the duty cycle could be smaller than 2% in a DCM buck converter. When operating the converter at a high switching frequency, it is possible to hit the controller minimum on-time limitation; for example, the UCC28911 requires a 420nS minimum on time. In other words, if you use the UCC28911 in a 440VAC to 15VDC conversion as a buck-converter controller, the converter must operate at a switching frequency lower than 45kHz.

What I understand is this. Let's say the duty cycle is 2%. If the switching frequency is 60 kHz, then the minimum time would be 0.02*(1/60k) = 333 ns. And the UCC28911 wouldn't start because their minimum time is 420 ns according to the datasheet. Then the article says that the converter must operate at a switching frequency lower than 45kHz. However, the datasheet says the minimum switching frequency is 420 Hz. Would the controller adjust the frequency itself if this situation with the minimum time happen?

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  • \$\begingroup\$ I believe so, its a valley switching controller so it will adjust the frequency depending on the load. \$\endgroup\$
    – Delphesk
    Jun 11, 2020 at 14:50
  • \$\begingroup\$ The UCC28911 is a flyback converter and not a buck converter. It's not applicable to your title or the quoted text. \$\endgroup\$
    – Andy aka
    Jun 11, 2020 at 15:33

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As per the datasheet:
https://www.ti.com/lit/ds/symlink/ucc28911.pdf?ts=1591886894954&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FUCC28911

During voltage regulation, the controller operates in frequency modulation mode and amplitude modulation mode. The internal operating frequency limits of the controller are 115 kHz maximum and 420 Hz minimum. The transformer primary inductance and turns ratio sets the maximum operating frequency of the converter. The output preload resistor and efficiency at low power determines the converter minimum operating frequency. There is no external compensation required for the UCC2891x devices.

as long as the loading isn't too light and the voltage conversion range is too large that the lower frequency limit is hit, the controller shall regulate the switching frequency according to the operating case e

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