I understood that much...You cannot have an "infinite heatsink". I made them by adding liquid cooling and keeping the temperature at 25C when 1000 watts was added to it. An infinite heatsink does not need fins or airflow. It is stuck at room temperature no matter how much energy you add. You can't have one. It is math not reality.
After saying you can't have one, I build them. I build machines that test power transistors so we can make data sheets. We have a heatsink that can be set to any temperature from -60C to +200C and will hold that temperature even when the power transistor is trying to lift the temperature. Our heating/cooling machine is the size of a washing machine. It takes huge amounts of power. It could heat and cool your house. I am testing transistors that power the drive motors in cars and trucks. I know that before the test the heatsink is 25C and after the test it is at 25C. Most data sheets are made using math not real-world tests. But if you don't trust the math, I can prove the numbers.
I was talking about the math behind real calculations for heatsinks. From what I have read, it's an arcane art.
https://thermtest.com/thermal-resources/thermal-conductance-calculator
Here's a thermal conductivity calculator. According to it, the conductivity of aluminum is 225.94 W/m K. According to it, a 1mm thick piece of aluminum has a conductivity of 225940.00 W/m2 K; which seems too high to me. Note: I don't actually use these values below.
Ohmite's examination of their F and R series heatsinks (pdf).
Here's a work by Ohmite. I'm using this as a basis to judge airflow.
Basics of thermal resistance and heat dissipation (pdf).
I'm working off of the above number provided by the conductance calculator, and the formula on page 4 of this guide.
I'd like to add, that the CFM and the M/S required by that pdf are incompatible. That is to say, air is three dimensional whereas plain old meters is a one dimensional value system. So I have assumed that the author got their value wrong, just like where "Representative length" is specified as meters, but other places I've read it's square meters in the other people's calculations. In fact, how would you choose which dimension to use when measuring a heatsink if you were to use only meters?
Trying to convert CFM to cubic meters a second, doesn't look very promising. Looking at this pdf from ohmite, my own 44CFM fans, would have such a low value on this chart (0.0207656878 CM/S), that even zooming in to get the value would be impossible.
So, I chose the low value from their chart of 300 m/s. Again, I'll have to find some way to test this supposition.
So the calculation ends up where I'd be able to dissipate ~ 61watts of heat with my best plain old aluminum heatsink and fans.
Equation:
6*(300/(((150*69+35*69*26)/1000)/1000)**0.25)**0.8
6 [turbulent flow in the presence of fins. A flat heatsink would not produce turbulent airflow] * (300 [already explained above] / (((150 [mm long] * 69 [mm wide] + (36-1 [mm for height. Subtract 1 mm for height as it was already used just before here.] * 69 [mm wide] * 26 [fins] )/1000 [mm2->meters]) /1000[mm2->meters2]) **0.25[part of the formula] )**0.8[
part of the formula]The result is 970.6Watts; which seems way too high.
OTOH: If I use the 44CFM value and convert it to cubic meters and use that instead of 300m/s, I get 0.22W; which seems way too low for a heatsink that size.
Ok, I'll write every question I can think of about the setup off the top of my head down. Sorry it's so excessive, but what else can I do to understand how these high heat dissipation devices work?It's as big as a large chest freezer, and about as exciting to look at.
And the top of it gets used as a "temporary" repository for all sorts of junk.
What is the fin length? Are the fins all one piece, or are they segmented like long columns as some heatsinks are? Do the fins have parallel gouges in them to increase the surface area? Is there are special design to the fins that allows more surface contact for airflow, or better movement of air, like those special swimsuits in the Olympics that mimic a shark's skin?
Does it use heatpipes? What is the diameter of the heatpipes? What are their lengths? Are the heatpipes flat, or round? Are they sintered heatpipes? Do the heatpipes have wicks? Are the heatpipes grooved? Are they more than one of the above?
Does it use vapor chambers? What are the vapor chamber's dimensions? What is the thickness of the vapor chamber's copper connection to the heatsource (MOSFETs)? What is the spacing of the pillers in the vapor chambers? What do the vapor chambers connect to?
My mistake.Not sure which datasheet you ere looking at, but the IXYS datasheets (attached is that for the device I use) have extended SOA down to DC and that's critical for linear operation. Here are the relevant charts for the IXTX110N20L2 at case temperatures of 25 and 75degC...
That's clear. And thanks for the picture.From the data sheet, the thermal resistance, junction to case Rthjc for the plus247 package is 0.13degC/W, so for a case temperature of 75degC and a junction temperature of 150degC the maximum dissipation is (150 - 75)/0.13 = 576W, ie the same as the SOA chart and the SOA example in the datasheet (200v, 2.88A, the RH limiting line on the SOA chart). But is that case temperature realistic? Well, for an ambient of 25degC the thermal resistance of of the thermal paste and the heatsink together must be less than (75 - 25)/575 = 0.087degC/W. The thermal resistance of a good thermal paste, a few microns of Arctic Silver 5, is around 0.05degC/W so we are looking at a heatsink of around 0.037degC/W! Such heatsinks exist, for a price, but as always there's a catch; heatsinks are rated assuming the heat load is across the whole surface not at one point. The upshot of all this is you actually need 3 devices to hit around 550W.
Here is a CHT simulation of 8 devices on a pair of identical heatsinks, each device handling 150W for a total of 1200W with a junction temperature of around 80degC with an airflow of 7.5m/s.
One question, why the 75C graph? I mean, 75C is a fine assumption for case temperature, but why not 65C or 85C?
Now I need to read Janis59's post; several times and the rest of the pdf...
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