Apparently I have a thing for testing. I rather love to run experiments, even when there’s no immediate need for the results. I guess I just enjoy trying stuff, and hey, maybe even learning a thing or two.
So last weekend I was doing a bit of tinkering and got to wondering about the performance of different heatsinks. Now just intuitively, I know that larger heat sinks tend to dissipate more heat than smaller ones – particularly if they have larger surface areas. But just how much better is a tall, finned heatsink than a small, clip-on device? This is what I wanted to find out. So I gathered up five sinks of varying sizes and started to design my test.
- A small clip-on heatsink, 230-75AB, cost: $0.36
- A medium-sized, bolt-on heatsink, 7-340-2PP-BA, cost: $3.91
- A larger bolt-on heatsink, 531202B00000G, cost: $1.28
- A Pentium 4, Socket 478 CPU cooler, cost: ~$35.00
- A Crydom SSR heatsink, HE-54, cost: ~$25.00
I decided to test each of these sinks with a TO-220 package transistor (since, at least in the case of the first three devices, this is what they were designed to cool). Specifically, I’ve chosen the IRL2703, an N-channel MOSFET rated for 30V and 24A. Now, the TO-220 package is also commonly used with regulators, diodes, etc. So these heatsinks might be found in a number of different applications.
In order to control the power dissipated in the transistor, I’ve built a very simple current-control circuit using an op-amp (LM741) and a 0.1Ω sense resistor (Rs):
I will be using the transistor as, essentially, a variable resistor connected to a 12VDC power supply. The transistor’s drain-to-source resistance varies based on the voltage applied between its gate and source. Control of this voltage is the responsibility of the op-amp shown in the schematic above.
If you can recall the “golden rules” of ideal op-amps, you’ll remember that, with negative feedback (that is, a path between the op-amp’s output and its inverting (-) terminal), the voltage at the op-amp’s inverting (-) terminal will be equal to the voltage at its non-inverting (+) terminal. In the circuit above then, if Vin = 1V, after being passed through the potentiometer’s 10:1 voltage division, the voltage at the op-amp’s non-inverting (+) terminal, and consequently its inverting terminal, will be 0.1V (remember also that no current flows into either of these “ideal” terminals).
So notice that the op-amp’s inverting terminal is connected to one leg of our sense resistor, Rs. So when our input voltage is 1V, the voltage across Rs will be 0.1V (remember the voltage divider). By Ohm’s law then, the current through the sense resistor will be I = V/R = 0.1/0.1 = 1Amp. So an input of 1V will yield 1A through the transistor. With an input voltage of 12V then, we’ll be dissipating approximately 12W in the transistor (actually 11.9W, to be precise; we’ll lose 0.1W in the sense resistor).
For my experiments, I’ll be using a constant current of 0.5A, which will yield a power of 6W. This seems to be a reasonable value to use with all of my heat sinks. And it’s still not quite enough to completely toast a naked (no heatsink) transistor.
In order to measure the temperature of the transistor as the test proceeds, I’ll be using a Kintrex infrared thermometer, model IRT0421, which I’ve had for years. My plan is to simply move the thermometer around the transistor, at close range (1-2″), and record its maximum reading. Admittedly, this isn’t the most precise solution. A better idea would probably be to attach a small thermocouple between the transistor and the sink. But that’s something I don’t have. My method should at least provide a good indicator of relative, if not absolute, performance.
My procedure here is quite simple. First, I will attach a heatsink to the transistor using a small thermal pad. I will then apply a current of 0.5A at 12VDC, and allow the transistor to reach a stead-state temperature. I will then allow everything to cool, remove the thermal pad, and try the same test again. Perhaps my video-self can explain this better:
Before diving into the results, here are a few more pictures of the circuit and setup. By the way, in case you’re wondering what that silver pen-like device is that’s jammed into my protoboard next to the opamp, it’s my home-made scope probe. I took an old BIC pen, removed the guts, and then epoxied in a dulled sewing needle and wire. It’s actually a great little probe; I leave it connected to my IOBoard most of the time.
This is how the transistor was attached to the Crydom heatsink:
And for the Pentium 4 cooling package, a single 4-40 hole was tapped (note that this image has been flipped horizontally, for aesthetic reasons, so ignore the wire colors):
Alright, so I’m sure you can’t wait to learn how things turned out, right? Well, without further ado, I present to you my grand table of results:
|Heatsink Under Test
||Temp @ 10 Mins.
||Temp @ 40 Mins.
(Note that these test were performed at an ambient temperature of 21°C. The “Expected Temp” numbers are calculated by multiplying the manufacturer’s ratings by 6W, and then adding the ambient temperature. Also, the smaller heatsinks were only tested for 10 minutes, as it only took this long for them to reach steady state. The more massive heatsinks were given 40 minutes.)
So now you may be wondering, “Wait, what happened to the tests with and without the thermal pads in place?” Well I’ll tell you: the difference in final temperature with and without the pads was insignificant (perhaps one or two degrees at most). This was a little surprising to me, as I’ve always been told to be liberal with the thermal grease/paste (and these thermal pads serve the same purpose, they’re just cleaner). So I figured the exposed side of the transistor would be somewhat warmer without the pads in place. And yet, that does not appear to be the case. But am I confident enough in this result to stop using thermal paste/pads? Eh, not really. I’d probably still use the pads, and I’d make sure the heatsink was firmly attached (since that makes for better conduction).
Another interesting point to note: the snap-on heatsink I tested here performed little better than operation without a sink (it yielded just a 13% lower temperature rise).
Strangely though, the snap-on heatsink was the only one that seemed to measure up to its manufacturer’s °C/W rating. I calculate a value of (136-21)/6 = 19.2°C/W, which is less than the advertised 28.5°C/W (a good thing). But neither of the bolt-on heat sinks met their advertised ratings. The medium-sized sink reached just 12°C/W while the larger one hit 9.5°C/W. I’m not sure what to make of this. Is there something I’m missing here? Perhaps an error in my calculation? It sure seems straight-forward…
Well finally, I was rather pleased with the performance of the Pentium 4 cooler as well as with the Crydom heatsink. I did expect both to do well though. The Socket-478 Pentiums could produce about 60W, so you’d expect even a stock heat sink to be able to handle one-tenth of that power with little problem. I was amazed at just how quickly turning on the fan brought down the temperature though. Within just a few minutes the heatsink felt cool to the touch (having just been at about 38°C/100°F).
In light of this data, which of these heatsinks would I choose? Well, for this transistor, the datasheet lists a maximum operating junction temperature of 175°C. In the above tests, I’ve measured case temperatures, so we’ll need to factor that in. Again, the datasheet lists a thermal resistance from junction to case of 3.3°C/W. So when dissipating 6W, as in the tests above, the junction temperature will be about 20°C warmer than the temperature of the case. So our maximum allowed case temperature will be 155°C. You’ll notice that with no heat sink, the transistor reached a temperature of 152°C, so in theory, you could safely operate at 6W with no heatsink at all. But should you? No. For one thing, stuff starts to smell nasty at that temperature. Plus, continuously running hot will almost certainly reduce the operating life of your transistor.
In general, I’d suggest keeping transistors well below 100°C. So in this case, I’d be comfortable with either of the two bolt-on heatsinks. Anything more is overkill.
By the way, here’s a neat article on
defacing currency some home-made heat sinks.
So, comments, questions, suggestions? Feel free to leave them below. Thanks!
Christoph kindly pointed out in the comments section (below) that I’d forgotten to incorporate the dissipation of the TO-220 package itself when predicting case temperature values based on the heatsink specs. Although not quite half of the transistor’s surface area is attached to the heatsink, the front is still sitting in open air. Now, the datasheet specifies a thermal resistance of 62°C/W from junction to ambient. I’m measuring case temperatures, so we need the thermal resistance between case and ambient. To get this, I need to subtract out the junction to case resistance of 3.3°C/W, for a case to ambient package resistance of 58.7°C/W.
Now, instead of trying to figure out the thermal resistance for just the front of the heatsink (which would just be a guess, really), let’s see what the effect of adding in the transistor datasheet’s full thermal resistance has on our predicted temperatures.
As with electrical resistances, we can determine the combined thermal resistance of the heatsink and transistor by adding them in parallel, as follows:
R_tot = 1/(1/R_transistor + 1/R_heatsink)
Let’s apply this for the clip-on heatsink:
R_tot = 1/(1/58.7 + 1/28.5) = 19.2°C/W.
That’s a pretty big difference! For 6W, this predicts a temperature of 136.2°C – much less than the 192°C I calculated above, and much closer to my measured value (136°C). Here is a summary of the new predicted resistances and temperatures (I have NOT yet updated the table above with these new values):
Clip-on Sink: 19.2°C/W, 136°C
Medium Bolt-on Sink: 2.9°C/W, 38°C
Large Bolt-on Sink: 6.7°C/W, 61°C
P4 CPU Cooler: N/A
Crydom Sink: No Effective Difference (the supposed resistance of this heatsink is already so low, adding in the dissipation of the transistor itself makes no noticeable difference)
So making this correction helps with the clip-on sink, but it’s made things worse (at least by comparison with the tested value) for the two bolt-on sinks…
The other adjustment we could make would be to add the case-to-sink resistance of the transistor. The datasheet lists this value as 0.5°C/W for a flat, greased surface. Which in this case adds 3°C for 6W. This could account quite well for the difference in measured values for the Crydom heatsink, but it doesn’t make a huge difference for the others.
Again, these are all pretty much approximations, and as I’ve already admitted, my testing procedure is not terribly accurate. However, I do believe that it’s still a fairly good relative comparison of these sinks. If I were measuring the temperatures of the sinks directly, yes, there would be differences in their emissivity, which would affect my IR thermometer. However, I held the thermometer very close to the transistor and measured the temperature of the same point on the case of the same transistor in all tests. So I’m not too worried about this. Again, thanks to all for the comments!