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Power factor corrected forward converter with big input capacitor

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Advanced Member level 5
Jun 13, 2021
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A customer has sent us an 150W isolated SMPS, which is "supposed to be" Power Factor corrected. Its a One Tran Forward, with large L leakage in the transformer, (instead of an output inductor). It has a high voltage reset cap (for resetting the transformer) which is kept below 500V by a regenerative SMPS snubber, which bleeds the reset cap down so it stays below 500V. (just shovels the energy into the primary DC Bus)
It has a 5u6 Film cap after the mains rectifier bridge.........we dont have a harmonics tester, but do you confirm with us that there is no way that this thing can pass mains harmonics to EN61000-3-2 , class D with that size of capcaitor after the bridge?

Just so we know what i am talking about...the attached is a Flyback PFC in LTspice and jpeg....changing C7 to 5u6 (it should be around 330n) shows how poor the mains harmonics become with this size of cap ...i know its a different topology, but the principle is the same,.


  • PFC with big input cap.jpg
    PFC with big input cap.jpg
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  • PFC Flybk BCM LT3799 150w 240vac_big
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Looks like you did not even measure harmonic currents in your simulation setup. Does it exceed class D limits?

Also as mentioned in your other recent PFC thread, your TBS1052 FFT is good enough for a pre-compliance estimation.
Thanks, i will get round to measuring it...but i dont know if i trust the sim to do it do it like the actual test instrument on the real smps.
Also, as you know, there is a general rule of thumb in PFC stages, that your inut capacitance must be limited to some 220nF/ using 5.6uF for 150W is way , way too much capacitance.
...we are talking Mains Harmonics fail, surely?...though i cant confirm without the actual test box......(You know the one i quickly gives you all 39 Mains Harmonics when you power your SMPS through it.)

How about your initial question? Post #3 and #4 don't contribute to an answer.

I see harmonics clearly below class A and D limits with 5.6 uF cap at 150 W.
Thanks, Harmonics from 3 to 39? checked all of them?

To provide further proof, adding a decent size input capacitor is the easiest way to solve input filter oscillations in SMPS, and also to get a conducted EMC if adding a relatively big input cap was OK, then everyone would do it...but they dont....This is the proof-without-measurement, that PFC with > say 2uF per 100W is going to fail on Mains Harmonics......or be harder to pass
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