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Constant off time Offline Flyback SMPS control chips?

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treez

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Hello,
The attached is the mains input voltage and current waveforms of a 67W, Offline Flyback converter that has no big smoothing capacitor after its mains rectifier. The flyback operates in constant off time mode. As you can see the power factor is overall better than an offline SMPS using a big smoothing capacitor after the mains rectifier.
I cannot understand why there are no off-the-shelf offline flyback control chips running in constant off time mode? For this one, I had to use a UCC28C43 chip with a signal FET connected common emitter to its clock pin and switched on and off by the gate drive output, to make it work in constant off time.
So why are there no ‘constant off time’ control offline flyback chips out there?

(All the flyabck does is repeatedly charge up a 1.5mF capacitor which is used to flash a xenon beacon every second)
 

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  • Mains voltage and current input.jpg
    Mains voltage and current input.jpg
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