Electronics > Projects, Designs, and Technical Stuff

Buzzing Ceramic Caps

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hardsoft:
Hi All,

I have a problem I'm sure others must have run into.  I have a LED boost circuit that creates a ~28V LED back light current drive for an LCD from a 5V rail.  It works great except when I use my PWM dimmer the ceramic cap buzzes (and it's really annoying).  Fun fact - ceramic caps are piezoelectric devices.

Anyways, I've found a massive metal film capacitor that buzzes much less, takes up way more space but if it is what I have to do I'll deal with it.  Anyone else familiar with some alternate cap types for this situation?  I think the voltage range is too high for tantalum.

Another option is to increase the PWM frequency outside the audible range (20kHz+) but my LED booster chip recommends 2kHz max.  It's a simple chip with multi sources so I'd prefer not to change it.  I could always try to lower the frequency which would at least get the noise into a less annoying range.  But that brings me to my real question, does anyone have any knowledge of long term reliability of surface mount ceramic caps (or anything for that matter) in this type of situation?  Raising the frequency might prevent us from hearing anything but it doesn't mean the caps aren't vibrating.  I can't imagine the vibration is good for the solder connections.  Am I just being paranoid about this?  I can still hear a small amount of buzzing with my metal film cap which leaves me a bit worried.

alm:
Would using a better dielectric, like X7R or NP0/C0G, help? I believe that the piezoelectric effect is mainly an issue with cheap crap like Z5U.

mikeselectricstuff:
Yes - X7R/X5R's don't buzz as much, however some noise can come from the inductor, and I've even heard noise come from LEDs themselves, albeit only just noticeable on a board with several hundred of them.
It may also be worth trying a  ceramic cap with a higher voltage rating.
For a backlight it doesn't matter if there is a fair amount of ripple, so you may not need as much capacitance as you would for a conventional PSU.
 

NiHaoMike:
Couldn't you embed the capacitors in epoxy or electrical grade silicone to dampen the vibrations?

mikeselectricstuff:
encapsulating is a major PITA  do do in production, and should be considered a last resort.

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