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Does anyone know if choose the Active clamp forward as the topology in PoE system, according to this figure which from TI Design document, What's the function of C39. I do not understand why need to put the capacitor in there, is this DC block capacitor? can someone explain it to me? enter image description here

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Because you drive a P-channel, you need a negative bias to turn it on when the source is ground-referenced. The diode clamps on the positive excursion and, via the capacitor, shifts the 12-V drive voltage below ground, effectively turning the P-channel on. See the below waveform:

enter image description here

The best is to run small sims like in the above to test particular sections of this converter. You can freely use one of the 60+ SIMPLIS templates that I posted here as they work with the demo version. If you want to read more on the ACF, I published this article in How2Power in 2019 and it covers loop compensation.

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  • \$\begingroup\$ Thanks for your answer Do you know how to calculate this capacitor value? because the design guide doesn't teach how to calculate. Your article is great. \$\endgroup\$
    – EEC
    Commented Jul 15, 2021 at 0:07

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