Electrolytic SMD caps getting too hot to touch 1

[daxliniere]

Hey gang,

I have tested two different brands of PCI-e riser boards with an audio DSP card containing 8x SHARC 21469 ICs. Both of the risers seem to work perfectly, but their capacitors get too hot to touch within about a minute.

The max current draw for that card is allegedly 8W, which seems to check out reading the Analog Devices datasheets. These are the kinds of cards that are used in cryptocurrency mining, so they normally have a whopping big GPU card plugged into them, not a little 8W DSP card. The risers have a 12V ATX connector (6-pin) for power input and send the data signals across a USB3 cable (not USB3 protocol, of course) to a dummy card that plugs into the motherboard's PCI-e slot. One of the riser cards I've tested has 4 SMD caps the other has 8 caps (4x2 in parallel).

My questions are:

1) Why would this be happening?
2) What could I do to mitigate it? I'm happy to change or add caps.

Thanks in advance to anyone who can shed some light on this strangeness.


All the best,
Dax.

 

[itsmoked]

There are several criteria for specing electrolytic capacitors.

The Voltage. Obvious.
The Temperature. Obvious
The MAXIMUM RIPPLE CURRENT. Not obvious.

Google "electrolytic ripple current".

Too much and the caps heat up and die very quickly. You have a that happening.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[daxliniere]

Hi Keith,
Thanks so much for the reply and the information. That's really great.

I am going to source some low ESR caps capable of handling higher ripple current and see if that solves the problem.

All the best,
Dax.

 

[itsmoked]

There you go!

More capacitance can reduce the ripple current, or rather you're using less of the cap's ripple budget. However that can also cause problems with the circuitry if it requires a specific capacitance, so lower ESR caps would be a good first stab at it.

See Panasonic OS-CON caps.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[daxliniere]

Hey Keith, thanks again.

I looked up those Panasonic OS-CON caps. Seems like 16v is about as good as I can find in 270uF. I suppose that'll be fine for 12v considering it's a computer PSU that is supplying it, therefore it should never really stray near 16v, right?

And what if I didn't care about maintaining form-factor? As in, I am happy to use much larger caps not specifically designed for tight spaces like SMD PCBs.
I'm thinking of using Panasonic FM series 470uF @ 35V, 0.019Ω ESR (EEUFM1V471)

Thanks again,
Dax.

 

[MacGyverS2000]

I agree with Keith... self-heating from ripple. They likely put a 12V -> 3.3V SMPS on there and didn't take into account the amount of ripple (looks like a relatively small inductor, which equals high ripple).

Caps lose capacitance as you raise their DC voltage level... getting that close to the cap's 16V limit is going to limit your actual capacitance to probably <<20% of its original value (i.e., that 270uF cap is going to look like a 27uF cap). Add in some MLCCs to provide a low-resistance path to kill a good chunk of the ripple... you don't necessarily need more capacitance, you just need to turn a god chunk of the existing capacitance into very-low-ESR, and a number of MLCCs will do that for you.



Dan - Owner

http://www.Hi-TecDesigns.com

 

[daxliniere]

Hey everyone, if you are interested, I ended up solving this with some help of a forum member here:

https://www.eevblog.com/forum/proje...tting-too-hot-to-touch/msg3712207/#msg3712207

 

[itsmoked]

That was quite the ride!
Thanks for the heads-up.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[MacGyverS2000]

That article doesn't discuss anything useful about capacitors, and certainly nothing about ripple current. Spam.

Dan - Owner

http://www.Hi-TecDesigns.com