Why is offline SMPS DC bus capacitor a big X2 capacitor instead of small ceramic?

[treez]

Hello,
The following offline 11W switch mode LED driver circuit uses a primary side DC bus capacitor (C4) which is EPCOS BFC233822104. This capacitor is an X2 capacitor of size 6*12*12.5mm

Schematic of 11W LED driver from ti.com of LED driver:
**broken link removed**

DATASHEET: EPCOS BFC233822104 capacitor (630vdc)
**broken link removed**

Why have they used such a big capacitor? The following 630VDC ceramic capacitor is less than half the size and has a higher temperature rating….

DATASHEET: Murata 630VDC ceramic capacitor: (7.5*3.15mm):
**broken link removed**

…So why has an big X2 capacitor been used here, instead of a small ceramic? (there is no price difference)

 

[smijesh]

The purpose of C4 capacitor is emi suppression and also it is placed across the line/high voltage. So x2 type capacitors are recommended, Box type capacitor having UL approval. If you want UL marking on the PCB You should use UL approved component.

 

[dick_freebird]

Different cap types have different failure modes,
maybe this one is especially benign (UL likes that).

 

[treez]

thanks, but C4 is not across the lines. C4 is a primary DC bus capacitor. I cannot understand why the alternative 630V rated , smaller ceramic capacitor (shown in top post) is not acceptable?
I am sure that UL does not insist that DC bus caps have to be X2 type?
DC bus capacitors are very often electrolytic.

 

[smijesh]

thanks, but C4 is not across the lines. C4 is a primary DC bus capacitor. I cannot understand why the alternative 630V rated , smaller ceramic capacitor (shown in top post) is not acceptable?
I am sure that UL does not insist that DC bus caps have to be X2 type?
DC bus capacitors are very often electrolytic.

C4 Capacitor is not placed directly across the line. But effectively it is across the high voltage. Any insulation failure in the capacitor may causes fire. So we should use X2 type capacitor for safety. UL is more concern about human safety. C4 capacitor is a part of differential mode filter, used in the circuit for EMI suppression.

 

[treez]

yes I agree but there is no regulatory requirement to put x2 caps there, -usually people just put electrolytics there. I cannot understand why they put an x2 cap there when there's no need, and the x2 cap is twice the size of an equivalent voltage rated ceramic.

 

[mtwieg]

How much capacitance do you think that 0.1uF rated capacitor has when biased at 340VDC?

 

[godfreyl]

So why has an big X2 capacitor been used here, instead of a small ceramic? (there is no price difference)

I guess saving a couple of square mm of space was not their No1 top priority when selecting that capacitor. Others have mentioned good resons for preferring the X2 cap.

Can you show the suggested PCB layout with the X2 cap, and an alternative layout with the other cap, to demonstate the overall PCB size reduction achievable?

 

[treez]

LED bulb circuits are always in tiny spaces and need the smallest component space.

 

[Dan Mills]

Not really, the elephant in the room with LED bulbs is essentially always led die temperature.

If the thermal management is imposing a packaging volume lower limit for heat dissipation then there is no reason to try to make the electronics more compact then that lower limit on system volume.

Note also that this is not a DC link cap in the classic sense operating more as a high frequency bypass cap, in that this circuit delibrately operates in constant on time mode where the voltage on this capacitor approximately tracks the absolute value of the mains (Thus improving the power factor), nobody is going to use an electrolytic for 100nF of capacitance.

The X2 cap might simply be a BOM reduction excersize (Makes C1 & C4 the same part), in volume production this might actually be a bigger win then going to a ceramic father then a film part

Regards, Dan.

 

[treez]

Ok but as we see in the bom (below), C4 and C1 are not the same part...

bom:
**broken link removed**

...I agree it shouldn't be a 'lytic cap, but why not the much smaller ceramic in the top post? in truth the driver pcb is likely to be potted and stuffed away in the base, and should be as small as possible no matter what volume for the leds in their cooling.

 

[Dan Mills]

Could be as simple as being a part the guy doing the app note allready had in his component library....

Given that it is just an app note, and that almost nobody takes them strictly verbatim into products, if I was writing it and could save myself a few minutes (Plus a few minutes for someone to review the footprint I created) by using something allready in my parts libraries then the thinking goes something like this: "I Need 100nf at greater then 400V, lets see what we have in the library... That one will do".

Sometimes the decision making really is that simple.

Regards, Dan.

 

[schmitt trigger]

My guess is safety and CYA.

Let's say the app had included a properly rated ceramic cap.

If an incompetent engineer just copies and pastes this design, but carelessly decides to replace this capacitor with a lower rated one and an accident occurs, they cannot blame TI.
TI will show that the app note originally indicated a safety cap.