Buzzing Ceramic Caps

[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.

[TopherTheME]

I've seen this problem solved with hot glue but never seen a more elegant solution. I wouldn't be worried about the long term stability of the joints since the vibrations are small and solder is relatively ductile. A more likely failure mode would be the destruction of the capacitor itself.

[hardsoft]

Thanks for the feedback.  I'm using X7R ceramics rated at a 100V.  They're 1206 size, and I've read that using smaller size caps decrease the noise level, which makes sense, though I'm not sure there would be that great a difference.  I'm just glad not to see any responses about horror stories involving caps vibrating off a board after a year of use.  Using some type of epoxy sounds like a nice idea and may give it a shot.

[djsb]

This link suggests the use of polymer capacitors (whatever they are).

http://powerelectronics.com/power_systems/dc_dc_converters/power_buckconverter_design_demystified/index2.html

David.

[mikeselectricstuff]

Polymer caps are like tantalums on steroids - insanely low ESR (<10mR) but somewhat expensive at the moment. Slightly higher capacitance density than ceramics.

[RayJones]

The important thing to be aware of is your annoyance of buzzing caps being audible will also play out in reverse.

These caps will also be highly microphonic and in RF oscillators especially this can be very much a bad thing.

[mikeselectricstuff]

The important thing to be aware of is your annoyance of buzzing caps being audible will also play out in reverse.

These caps will also be highly microphonic and in RF oscillators especially this can be very much a bad thing.

Maybe, but in an RF application you're not likely to be using a cap with a value large enough to be using an X7R or similar class of dielectric.
Obviously this type of cap should  be avoided in low-level signal-path applications.

[RayJones]

The OP did say piezoelectric and by jingo you can pay heaps for some equipment that just loves a bit of tapping to set it off.
Usually hidden inside a sealed unit will be a dodgy ceramic cap, insisted upon by the bean counters no doubt.

I made a special point of RF oscillators, other equipment at the lower end of the spectrum can also be equally affected by these cheap and nasty ceramic caps.

If it can vibrate with signal, a vibration can create a signal, that's the point I was making.

[Zero999]

You can probably make a cheap FM transmitter with a ceramic capacitor doubling as the micropohone.