Why are film capacitors (post boost PFC) 800V rated?

[treez]

Hello,

This streetlight app note by TDK uses 800V rated film caps after the boost PFC stage…however, the Boost FET is only rated to 650V (STF21NM60N).
Why has it used film caps of such a high voltage rating?

Streetlight design app note:
https://www.digikey.hk/Web Export/Supplier Content/EPCOS_495/PDF/epcos-rd-lamp-ballast.pdf


Boost FET (STF21NM60N)
https://www.st.com/resource/en/datasheet/CD00066720.pdf

 

[betwixt]

I suspect it is simply because they are standard parts. It does mention that components are conservatively rated but if they really did have to withstand 800V the rest of the circuit would fail first. My neighbor has a similar circuit running to power their water sterilizer UV tubes, it uses the same ICs and PFC circuit but has electrolytic capacitors instead of film types.

Brian.

 

[KlausST]

Hi,

Voltage rating... it depends on application. Maybe it's just some headroom.

If not film capacitors.... what else?
* ceramics for that high voltage often are not suitable for DC..
* electrolytics maybe are too slow.

Klaus

 

[treez]

* ceramics for that high voltage often are not suitable for DC..

Thanks, do you mean that eg 1kV ceramic 1812 size caps should only be used on AC?

 

[FvM]

You won't ask the question if you had reviewed the TDK B32774 datasheet before. The simple point is that there no small DC link capacitors between 450 and 800V. 450V is too low, so choose 800V. Or a different manufacturer, but surely not in a TDK app note...

 

[KlausST]

Hi,

Thanks, do you mean that eg 1kV ceramic 1812 size caps should only be used on AC?

If you tell exactly what manufacturer snd what capacitor type you talk about..we could open the datasheet and see whether they are suitable for DC or not.

Klaus

 

[treez]

Thanks,
The below 100n, 1kV cap says its OK for DC.
I must admit I’ve never seen a datasheet of a ceramic capacitor that said it was “only for AC”.
Though I am certainly worried that there may be such capacitors out there.
We just made a filter for our offline LED lights which uses HV ceramics because we couldnt fit anything else under the visor.
The filter is in DC because its after the mains rectifier.

I remember some months back having a big discussion on edaboard about using HV ceramics in an AC mains filter. (because you cant fit films in there and are in that case we were only allowed surface mount)

Kemet 100n, 2220, 1kV , X7R capacitor
https://content.kemet.com/datasheets/KEM_C1010_X7R_HV_SMD.pdf

- - - Updated - - -

You won't ask the question if you had reviewed the TDK B32774 datasheet before. The simple point is that there no small DC link capacitors between 450 and 800V. 450V is too low, so choose 800V.

Thanks, of course you are right…I suspect many will simply reject the 800V films of TDK and use a different brand 630V rated film capacitor instead.
……..Its quite surprising that they didn’t just use their own TDK 450V rated films though…after all, it would have to be an enormous mains transient to take a 15uF film capacitor bank up above 450V….and even if it did….film capacitors can heal after short term overvoltage transients. So I am still a little puzzled as to the use of 800v Films.

 

[KlausST]

Hi,

I must admit I’ve never seen a datasheet of a ceramic capacitor that said it was “only for AC”.

Correct. My fault. I was confused.
HV ceramics capacitors may not be suitable for AC....but they should work with DC.

Klaus