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Can a X2 polypropylene film capacitor be used for dc smoothing at the output of a bridge rectifier? I want to use this capacitor at the input of a power factor correction circuit with a switching frequency of 130 kHz. This is the capacitor I want to use https://datasheet.lcsc.com/lcsc/1806081834_KEMET-PHE840MB6470MB16R17_C183568.pdf
The mains supply is 240Vrms. The capacitor I am asking about is the one labelled C105 and C106 enter image description here

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  • \$\begingroup\$ Do you have one in mind in particular? I mean, there may be some that aren't that good and nobody likes giving fully generic answers in case someone brings out an X2 capacitor that is good for AC and surge removal but is (somehow) awful as a smoother. So, pick a capacitor and link its data sheet. \$\endgroup\$
    – Andy aka
    Dec 31, 2021 at 15:34
  • \$\begingroup\$ Well it will smooth as well (or badly) as its value suggests, and probably be larger and more expensive than the alternatives. \$\endgroup\$ Dec 31, 2021 at 16:00
  • \$\begingroup\$ This is the capacitor i want to use datasheet.lcsc.com/lcsc/… \$\endgroup\$
    – p_karis
    Dec 31, 2021 at 17:03
  • \$\begingroup\$ Please provide a schematic or block diagram of your circuit. Typically when people say "smoothing at the output of a bridge rectifier" they are talking about using very large capacitors to minimize 100 Hz or 120 Hz ripple. Large capacitors are needed for this, typically. But you also mention power factor correction running at 130 kHz. So I really am not sure what you are trying to do. \$\endgroup\$
    – mkeith
    Dec 31, 2021 at 18:34
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    \$\begingroup\$ I have shown the circuit above \$\endgroup\$
    – p_karis
    Dec 31, 2021 at 18:56

3 Answers 3

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Yes they can and you have the benefit that if they fail they should fail short causing an over-current trip upstream (breaker/fuse/monitor).

You however might not be able to realise the same capacitance per unit volume as such capacitors are designed for high transients and as such have high spacing internally, which reduces the overall capacitance as capacitance is proportional to plate separation.

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  • \$\begingroup\$ Are they as good at filtering as electrolytic and ceramic capacitors? \$\endgroup\$
    – p_karis
    Dec 31, 2021 at 18:22
  • \$\begingroup\$ in their own way yes. I only use film ( eg MKP series vishay.com/docs/26015/mkp1848cdclink.pdf) but that is because I can't use electrolytics. You obviously need larger volume (and count) to realise the same capacitance then if you were to use Electrolytics so it is a tradeoff as to whether you want to use fewer electrolytics or high number of film. Sometimes you do not have a choice, say temperature extremes \$\endgroup\$
    – user16222
    Dec 31, 2021 at 18:25
  • \$\begingroup\$ does the film capacitors affect the power factor correction when used at the input of a boost PFC circuit? \$\endgroup\$
    – p_karis
    Dec 31, 2021 at 22:12
  • \$\begingroup\$ Film/ceramic/electrolytic ... The dielectric technology needs to be picked based upon their pros and cons. You need capacitance, that is 100%. How much and where (pre Vs post) depends what you are trying todo as each serves a purpose. google.com/url?sa=t&source=web&rct=j&url=https://… \$\endgroup\$
    – user16222
    Dec 31, 2021 at 22:49
  • \$\begingroup\$ pre PFC circuit. immediately after the bridge rectifier. \$\endgroup\$
    – p_karis
    Dec 31, 2021 at 23:41
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For a power factor correction circuit to work effectively the input voltage must not be smoothed. As much as possible it should be the same as the input line voltage but rectified as the following circuit requires unipolar voltage.

The purpose of those capacitors and the inductor L100 is to minimize the amount of the switching frequency that is conducted out of the power cord. There are FCC regulations limiting how much is acceptable.

The boost converter will be controlled so that at all times its current input is proportional to the input voltage. This is so that it acts like a resistor rather than taking current in short pulses as would normally occur with a rectifier and shooting capacitor input.

Power Factor Correction Explained

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  • \$\begingroup\$ what about the power tapped at the input (across the bridge rectifier)which will provide vcc bias to the PFC controller? Let's say I use a flyback converter which draws about 5W across the bridge rectifier, don't I need a smoothing capacitor here? \$\endgroup\$
    – p_karis
    Jan 1 at 16:46
  • \$\begingroup\$ @p_karis - Yes, if you need to supply a separate load that needs relatively smooth DC that would need a reservoir capacitor. However you would want to separate that from the supply feeding the PFC by a diode, as the PFC does not want smoothed DC or you will be destroying the benefits of the PFC. \$\endgroup\$ Jan 2 at 0:21
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to reduce 100Hz ripple from a rectifier at the low voltage output? No, this will not work. Or at least - it depends on the requested current.

Somewhere in the middle of a smps. Probably yes, but in any case a capacitor alone will make your powerfactor worse, because reactive components affect the powerfactor allways in either way.

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  • \$\begingroup\$ the output of a bridge rectifier is typically followed by a capacitor (or an LC if continuous current for PQ is required). This is done everywhere and while the type of capacitance (ceramic, electrolytic, film) might be different based upon other constraints, a capacitor is always included. \$\endgroup\$
    – user16222
    Dec 31, 2021 at 18:34
  • \$\begingroup\$ How do you calculate the inductor value of the LC filter in such a pfc circuit? \$\endgroup\$
    – p_karis
    Dec 31, 2021 at 18:58
  • \$\begingroup\$ I am designing for an output of 400V,1.045A \$\endgroup\$
    – p_karis
    Dec 31, 2021 at 19:16
  • \$\begingroup\$ @p_karis TIs Application Report SLUA704 is not very informative in this regard. I therefore recommend IR's Application Note AN-1150. In my opinion, this gives better information on how to calculate the values. This is the same kind of active pfc. \$\endgroup\$
    – arnisz
    Dec 31, 2021 at 20:06
  • \$\begingroup\$ @arnisz Thanks for the document. But I am having trouble choosing the right input filter from the bridge rectifier \$\endgroup\$
    – p_karis
    Dec 31, 2021 at 20:29

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