BUF634 Heatsink

[Devil_H@ck]

I was wondering what kind of heatsink to use for a BUF634. I calculated this for the heatsink:

Junction-to-case: 6 °C/W
Heatsink thermal resistance (just to be safe): 0.5 °C/W
Max power: 24V (+- 12V) * 0.25A = 6 W
Max junction temperature: 125 °C
Ambient temperature: 25 °C

Therefore: (R + 6 + 0.5) * 6 = 125 - 25
<=> R = 61/6 °C/W
<=> R ~= 10.2 °C/W

Could this be correct? I'd use a heatsink like this in that case: http://www.farnell.com/datasheets/14647.pdf (637 series, the smallest one, which is still 6 °C/W)

Can I let the junction temperature rise to 116 without frying the chip? Then I could use this smaller heatsink: http://www.farnell.com/datasheets/19328.pdf (12.9 °C/W)

Do I even need a heatsink? The intention is to use it with a 50 ohm terminated cable, not to abuse the thing to see if the heatsink's sufficient .

Any help is appreciated.

[PMA]

In praxis, you do not necessarily need heatsink. Use BUF634T preferably. For better feeling I would recommend to use any small heatsink sized for TO-220 package (BUF634T).

[Devil_H@ck]

Well, I was going to use the T version (TO-220 5 legs). So, any heatsink is fine?

I found some really small ones in the Fischer catalog, they are only 11.8 mm wide and 8 mm long. Minimum height is 25 mm and then they're 20 °C/W (model SK 470). There are even small ones available, but they are only 36 °C/W (model SK 95). Would 36 °C/W be enough or should I go for the 20 °C/W one?

I can also get model SK 104 (it's in the Eagle library), but that's a bit big. The rating is pretty good though, 14 °C/W minimum, 8 °C/W max.

So, what do you think? Would the smallest one be enough or is it better to pick a bit better heatsink?

[PMA]

Provided you drive 100 Ohm load (50R + 50R) and Vout is up to some 2Vrms (as a preamp) any small heatsink is fine, even for maximum quiescent current of the BUF.

In case you drive high impedance load you need not care about heatsinking.

[Devil_H@ck]

Ok, then I guess I'll use the tiny 36 °C/W one. Thanks!

[Devil_H@ck]

K, another heatsink question, I guess I might as well ask it here instead of starting a new topic.

In my circuit, power is turned on and off with low Ron MOSFETs. I guess the max current through them won't be more than 0.5A. Do I need heatsinks for them or not?

[MWP]

Originally posted by Devil_H@ck
In my circuit, power is turned on and off with low Ron MOSFETs. I guess the max current through them won't be more than 0.5A. Do I need heatsinks for them or not?

Why dont you just try it?
Turn it on and stick you finger on them.
If they are that hot they leave an burn imprint on your skin, they need a heatsink.

At a guess... i would say no.

[5th element]

I agree with MWP if something is too hot to touch then it needs a sink.

On another thought though, (obviously if the chip leaves a burn there after a seconds touch then thats different) but it doesnt take much for something to be too hot to touch. Somewhere in the region of about 45-65 degrees centigrade is all it takes depending on the person. Now lots of devices are stable working at a temp of greater than 100 oC fets can work higher. So even if its too hot to touch it most probably will be fine. However heatsinking will probably increase the things life.

[garbage]

Originally posted by Devil_H@ck
Ok, then I guess I'll use the tiny 36 °C/W one. Thanks!

i am using some smallish heatsinks for my buf634T.
you can take a look at them here images of heatsink .

i used those heat transfer film thingy between the buf634t and the heatsink. as the heatsink does not have a thru-hole. i just applied glue on the edges of the heatsink and buffer.

temperature is warm to touch at the heatsink after running for more than 8 hours.

[Doovieman]

How did you draw .25A via Buf634? I'm using one in a prototype and it draws less than .1A (maybe .05A max). What circuit are you putting around it?