Capacitor noise in switch mode power supply

[shadewind]

I'm building a power supply circuit using a National Semi LM2676 switching regulator. The output capacitor is this one:
http://se.farnell.com/jsp/search/productdetail.jsp?SKU=9692150

When I load the power supply down pretty hard (2.7A or something like that) I get a weird whining noise which drops in pitch from the output capacitor. It seems to be the output capacitor because doing the same thing with the capacitor removed (it produces horrible output of course but it's just for testing) makes no noise.

What is causing this? That kind of load is outside of the specs for my component choices of course but I'm still curious of the exact cause.

[tecman]

It is not that unusual for caps to "sing".  I am not sure, but it could be due to magnetostriction.  (Just got a trade journal at work today with an article on cap noise in power supplies, focused mainly on ceramics which have piezo characteristics).  Usually the solution is to parallel a number of caps to attain lower esr and handle higher ripple currents.  Also your cap seems to be a bit light on its ripple current capability.  With the current you are drawing, this can lead to short like of the cap.  At those power levels, I would recommend paralleling 3 or 4 of those caps.

paul

[shadewind]

I'm not going to draw that much current later, that was just for testing. Is the noise because of the higher ripple current when drawing more output current? According to the data sheet, the ripple current is about 30% or the load current.

[Franki]

Have you checked the voltage on the cap @ 2.7A? I shuldn't wonder if it sings if voltage is above or a little below 25V when you are having ripples in there.

[shadewind]

I'm outputting 5V so I seriously doubt that.

[shadewind]

I finally have a Rigol DS1052E on the way so hopefully, my switcher design will be much less of a guessing game.