coolingheatsink
I am considering a thermal design and due to space requirements I was considering different orientation of the heat sink, namely putting it bottom down, obviously, and puttin it on the side as in figures provided. I've seen a similar approach with water cooling but I am wondering if this would work as well and if the cooling effect is the same? Anyone had any experience with this?
NB: I am using forced air cooling.
The second one is often screwed flat on a PCB, with the transistor's legs at 90 degree angles, and then of course you put the transistor on the narrow side.
But otherwise it doesn't matter. The heatsink will receive its heat at the same place and the fins don't really care whether the heat entered from the left or the right.
That appears to potentially be a really good idea - but calculating the effective thermal resistance would be immensely difficult, whereas trying it in practice would be quite easy. I'll try neither at present (the latter is your assignment :-) ) but the following observations may help.
My first reaction was that the thermal transfer from case to spring (Rt_CS) would be very high, but you have added the (reasonable) assumption that enough thermal paste is used to make "pretty good" [tm] contact between case and spring.
Assuming that your spring area calculation is correct, that's the same area as plates of about
20mm x 20mm / 32 x 12.5 / 25.4 x 15.7
~= 0.8" x 0.8" / 1.3" x 1/2" / 1" x 0.6"
That's a useful piece of heatsink compared to a TO92 case.
You state datasheet figures of θJA = 160°C/W and θJC ~= 66 °C/w,
so θCA = 160 - 66 = 94°C/W
Even allowing for the poor thermal conductivity of the spring (steel?) I'd guesstimate a halving in effective θCA so you'd hope for a θJA of around 110°C/W.
That assumes θSA of about 50°C/W
For comparison, Aavid make several TO92 heatsinks of a "more usual" style.
The diaagrams and tables below were taken form page 68 of their excellent catalog - here. Effectiveness of course varies with size and area, but the tables suggest that they approach 10°C/W when used in a hurricane and more like 25°C/W as air speed approaches zero. Area of the smaller 575200 heatsink is somewhat under 400 mm^2 both sides. They have added some punch outs to improve airflow, the material is optimised for high thermal conductivity and there is a broad path for heat energy from TO92 case to the outer upper edges. So superior performance to your spring would be expected, and the original ~~~= 50°C/W is probably an OK starting guesstimate.
Note that the Aavid heatsink is shown in some places as 60 K/W. That's presumably a notional still air value. If the air is still there is no convective transfer, and air convection happens as temperature differential rises, but how well your spring does at encouraging this is TBD.
You could easily wind a "spring" using thick copper wire with a mandrel such that it would slip only a TO92 case and then be able to be lightly crimped in place with thermal paste to suit. If the spring was crimped against the TO92 case with eg pliers and then slightly more tightly just above the case to stop it sliding down onto the PCB then a reasonably stable, cheap and easily made heatsink could result. Or you could slide it right down to board level and solder the end to a pad.
Best of all would be to try it in real life.
Obtain a semi-sacrificial TO92 device - say a cheap bipolar transistor.
Use a consistent environment. Minimise artificial airflow and mount within an open box or whatever that roughly models intended use. Finger access required.
Connect say base to collector directly or via a resistor of choice and use a variable power supply to adjust dissipation until it was about too hot to hold onto for more than a few seconds. That's typically about 55°C. Use a thermometer that is suitable (eg IR non contact) if you have one, but fingers sufficeth.
Measure and save mW.
Now add spring, measuring temperature at TO92 case for similar result. Allow time to stabilise after adjusting power.
Measure and report back.
RELATED:
Excellent Aavid heastink catalog
Best Answer
An orientation that puts the cooling fins vertical will give a better cooling effect if the airflow is not forced (by a fan).
Hot air rises so you get better airflow over the fins if they are arranged vertically,
If the airflow is forced you will get better airflow if the fins are aligned with the direction of movement of the air.
Due to the thermal conductivity of the heatsink there will be very little difference in the vertical or horizontal positioning of components relative to the heatsink position.