Temperature rise of a resistance...

[John Larkin]

On Fri, 18 Nov 2022 08:44:57 +0000, piglet <erichpwagner@hotmail.com>
wrote:

On 17/11/2022 20:53, John Larkin wrote:
On Thu, 17 Nov 2022 11:07:18 -0500, legg <legg@nospam.magma.ca> wrote:

On Wed, 16 Nov 2022 10:58:16 -0800, John Larkin
jlarkin@highland_atwork_technology.com> wrote:

On Wed, 16 Nov 2022 10:16:40 -0500, legg <legg@nospam.magma.ca> wrote:

On Tue, 15 Nov 2022 19:30:32 -0800, John Larkin
jlarkin@highlandSNIPMEtechnology.com> wrote:

On Tue, 15 Nov 2022 19:51:43 -0500, legg <legg@nospam.magma.ca> wrote:

On Tue, 15 Nov 2022 16:34:21 -0800, John Larkin
jlarkin@highland_atwork_technology.com> wrote:

On Tue, 15 Nov 2022 08:18:23 -0500, legg <legg@nospam.magma.ca> wrote:

On Tue, 15 Nov 2022 08:31:01 +0100, pozz <pozzugno@gmail.com> wrote:

Suppose I have a resistance with a zero or very low temperature
coefficient (its value stays constant with temperature variation).

At time zero the temperature is T0=20°C and a constant power P is
applied (i.e. a voltage V=sqrt(P*R)). We know that at steady state the
temperature rises from T0=20°C to Ts where Ts depends on many factors:
mechanical charateristics of the resistance package and the capacity to
dissipate electric heat with the air. Anyway at steady state an balance
is reached and Ts is reached.

I\'m interested in the function of T over time. I suppose it\'s an
exponential function, but what is the time constant? And what\'s
important for me: does this time constant depend on resistance value?

It will depend on the thermal capacity (specific heat) of the
materials involved, and the thermal resistance of the structure
to the surrounding environment (linearly dependent on surface area
of the boundary). In electronics RC or RL.

Cooling is not linear on surface area. If you heat sink to an infinite
plane, theta is not zero.


It\'s linear over practical ranges, on avaerage, in a defined
environment.

If there\'s a range of conditions, you just pick the worst
(and keep it out of direct sunlight, where permitted).

RL

It\'s common to make a chassis out of 0.062\" thick aluminum. And not
unusual to heat sink a TO-220 transistor to that chassis.

So, what is theta of a TO-220 transistor bolted to such a chassis?
Assume no insulator and an infinitely large chassis.

How about a SOT-89 soldered to an infinitely large 1 oz PCB plane?

Spreading thermal resistance soon makes a bigger sheet stop doing any
good.

I\'m talking boxes, not sheets.

The top or bottom or side of a box is a sheet. If it\'s infinitely big,
the transistor can\'t know if it\'s a box or a sheet.

So what\'s theta?


I treat a U channel or flat plate as 2 dimensuional surfaces
and use the standard surface rise equation, to get the average.
There will be a delta T across the plate. At each spot point,
the surface temperature will tell you how much power is being
dissipated (per sq cm) from that location.

Sure. How many K/W? Theory is fun, until the transistor fails.

Infinite is theory, mw per cm^2 surface area is reality.

The aim is to reduce the thermal impedance to the practical
physical boundary of, in most cases, the external ambient
environment.

RL

So: given an infinite sheet of 0.062 thick aluminum, and a TO-220
bolted to it, what\'s the transistor theta?

And how big a sheet is close enough to infinite as makes no practical
difference?

That is a very real situation and deserves real numbers.

Any guesses?

And a bonus question, how would theta change if we used a TO-247?


You want guesses, I give you guesses. TO-220 about 1K/W (could be 0.8
could be 1.5)? TO-247 a wee bit lower, say 0.6 to 1.1 K/W ?

piglet

That\'s ballpark. I measured 2.3 K/w for a TO220 on a big sheet of .062
aluminum. The sheet was big enough that its edges were close to room
temp, so making it any bigger wouldn\'t have mattered much. Might get
down to 2.

A TO247 has a contact perimeter that\'s only about 25% greater, so
theta would drop by about that amount. On a thin sheet, the 247 is no
great advantage.

This sub-thread spun off a statement that a thermal tau would be
linear on some surface area. Usually it\'s not.

Thermal systems are sufficiently hard to predict that I usually just
build a model and test it. When air flow is involved, analytics are
even harder. We just designed a baffle to distribute air flow from two
fans evenly among 8 PCBs. Envision lots of cardboard and plastic and
tape.