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Singing ceramic capacitor

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I have designed a LED diver board using the TPS40211 boost converter. Everything is working fine except that when I'm doing PWM, there seems to be a singing noise coming from the input capacitor. This is obviously because of the on/off switching sine the noise seems to be of the same frequency as the PWM signal.

The input capacitance consists of a 330 uF Panasonic FK series rated at 35V and two ceramic 10 uF 50V caps. This is what TI have in their reference design.

Does the noise mean that there's something wrong i.e. too much RMS current or something like that? It seems to be a common problem with electrolytic caps. Is there something which can be done about it? I'm thnking I can reduce the frequency to a level where the noise isn't as audible. If I do that, will I be fine or is this a sign of an underlying problem?

Conrad Hoffman:
When voltage is applied to two capacitor plates, force is generated between them. Can't be helped unless you repeal certain laws of physics. It's normal for capacitors to sing if an audio frequency is applied. The amount depends on the rigidity of the dielectric and various other things. Though I haven't tried it, my guess is a solid dielectric like used in an OSCON cap probably makes less noise. Ditto tantalums and, if you can get one big enough, the better films. Hi dielectric constant ceramics are absolutely the worst!

Okay, so at least it doesn't mean that there's anything wrong with the design?

Conrad Hoffman:
That I can't say without seeing it. OK, maybe can't say even if I do see it! If you excite it at an audible frequency, you'll probably hear it. There are also issues with switching supplies where you can get audible frequencies way below the actual switching frequency. It's more an annoyance than serious, but indicates some more work should be done.

The switching itself does not make any audible noise. Without PWM, it's dead silent. I cannot raise the PWM frequency beyond the audible spectrum since there is a minimum on time requirement.

Any ideas that do not require a change in the design and if not, what can I change?


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