G
Grant
On Mon, 19 Jul 2010 03:19:26 -0700, [email protected] wrote:
On Fri, 16 Jul 2010 10:44:40 -0500, "Tim Williams"
A small part like this doesn't conduct heat into pcb pours very well.
If you stick a part to a relatively thin thermally conductive sheet,
theta goes up as the part footprint area goes down [1].
<snip>
[1] anybody know the exact relation?
Easy to approximate. Assume a circular footprint (cf. spherical chicken). Assume heat dissipation at the center is zero (fair for an infinnitessimal segment, blatantly false for an infinite number of them). If heat diffuses through copper out to infinity, temperature drops inversely with distance (because cross sectional area increases linearly). It looks like a point charge in space, and the temperature is defined by Gauss' law.
Of course, heat diffuses through two or three means, with strange temperature-dependent coefficients besides. So it's not at all true that, the device itself, and the little bit of copper surrounding it that doesn't have a quite circular temperature profile, isn't dissipating any power. In fact, it could be dissipating a considerable amount of power. If the heat source were an infinnitessimal point, it would have infinite temperature, and therefore radiate infinite power density (power can still be finite, since the area is infinnitessimal).
However, it is true that heat diffuses out, one way or another, so maybe the power dissipation is just a little higher in the center, and spreads out in a slightly-steeper-than-inverse relationship, eventually going to zero at infinity all the same. The trouble is deriving the exponent and coefficient of that power law.
Tim
Nice rant, but still no answer.
Given a perfectly thermally conductive puck attached to an infinite
sheet of thin [1] finite-thermal-conductivity material, and assuming
conduction cooling only, what is the relationship of puck theta to
puck diameter?
This is relevant to situations where you have a choice of, say, SOT89
versus DPAK versus D2PAK and you're heatsinking to copper foil.
PC with Core2 series CPU is running >100A regulator 3, 4, or 6 phase
around the CPU area, lots of cooling air flow. But they're not trying
to get that current off board, just direct it through a couple in^^2
of CPU chip.
Really? If the chip requires only 65 Watts how do you get to 100 A
even at 1.2 V?
Q6600 is 105W, I got one
Grant.
Does it take the rather typical 3 or 4 supply voltages at various
currents? A lot of the high power is getting on/off chip (high speed
busses).
I'm sure you can go find and read the specs as easily as I can.
Grant.
Oops, my bad. I guessed from the way you were talking you already had
the datasheet and knew already.
Nah, I read from the other PoV, some regulator chip datasheets for
the CPUs' power supply, but long enough ago to forget the details
Those CPUs have nasty power requirements, stop, start ~100A and don't
dare produce any glitches!
Grant.
