Choosing the Right Heatsink for Your Project

In summary, the individual is seeking help in choosing the appropriate heatsink for their project. They are unsure about the values for Thermal Resistivity and Power Dissipation on Digikey and are wondering if they only need to be concerned about Thermal Resistivity. They have found a suitable heatsink but it has a low power dissipation value. Another person advises them to stick with the thermal resistivity figure and mentions the importance of ambient temperature and the need for an adequate gate drive circuit. The individual realizes they forgot to include ambient temperature and plans to add a fan to decrease the thermal resistance. They also mention using a MOSFET as a starter relay and driving it fully on.
  • #1
Wetmelon
154
1
Hello again

I'm trying to decide on the correct heatsink for my project, but I'm not sure exactly what I'm looking at. On Digikey, each heatsink has values for Thermal Resistivity and Power Dissipation. This is the datasheet information for the MOSFET:

Tj = -55 to +175 C
Rds(on) = .02 ohm
Rjc = .75 C/W
Rcs = .5 C/W
Rja = 62 C/W

I'm driving 40A at 12V, so my power = 40*40*.02 = 32 W

Temperature Rise (Rjc) = .75*32 = 24 C
Temperature Rise (Rcs) = .5*32 = 18 C
Total Temp Rise (Rjc+Rcs) = 18+24 = 42 C

Available Temperature Rise (150-42) = 108 C

Max Thermal Resistivity of Heatsink = 108/32 = 3.75 C/W


Is the Thermal Resistivity the only thing I have to be worried about? I found a suitable heatsink, but it says its power dissipation is only 9W @ 60C.
 
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  • #2
What do you think? Do I need to worry about the "Power Dissipation" value, or just Thermal Resistivity?
 
  • #3
Wetmelon said:
What do you think? Do I need to worry about the "Power Dissipation" value, or just Thermal Resistivity?
Stick w/ the thermal resistivity figure. The sink you've selected is insufficient in ambient, unforced, air. If you want to operate the FET w/ a max junction of 150C in up to, say, 30C ambient then you need an effective thermal resistance of ~2.4 C/W. I suspect a common PC fan will move your sink's resistance down a degree C per Watt, just guessing.

Also, since you're running close to the margin, the above applies if the FET is rarely switched. If you are switching w/ any frequency at all you should include the increased power dissipation during the switch time. And of course be sure you have an adequate gate drive circuit, since if the FET switches slowly for some reason you'll need spares. :tongue2:
 
Last edited:
  • #4
mheslep said:
Stick w/ the thermal resistivity figure. The sink you've selected is insufficient in ambient, unforced, air. If you want to operate the FET w/ a max junction of 150C in up to, say, 30C ambient then you need an effective thermal resistance of ~2.4 C/W. I suspect a common PC fan will move your sink's resistance down a degree C per Watt, just guessing.

Also, since you're running close to the margin, the above applies if the FET is rarely switched. If you are switching w/ any frequency at all you should include the increased power dissipation during the switch time. And of course be sure you have an adequate gate drive circuit, since if the FET switches slowly for some reason you'll need spares. :tongue2:

Crap, I forgot to add in ambient temp. Depending on where this goes, it could be up to 35 or 40C ambient.

Damn.

I'm adding a fairly high-speed fan to the case, giving me nearly 600 LFM of flow. From the charts & graphs I've seen, it should bring my value down a pretty decent amount (5-6 C/W?). I simply didn't have room on the board for the nice big 2.4C/W heatsink, so I'm going to have to give it what I can and blow a lot of air over it. It's also only going to be switched on for a second or two to start the motorcycle's engine then switched off. Yes, I'm using a MOSFET as a starter relay? Got a problem!? ;D The other one I need to sink is going to be driving up to 30A the entire time the car is powered on.
 
  • #5
Wetmelon said:
Crap, I forgot to add in ambient temp. Depending on where this goes, it could be up to 35 or 40C ambient.

Damn.

I'm adding a fairly high-speed fan to the case, giving me nearly 600 LFM of flow. From the charts & graphs I've seen, it should bring my value down a pretty decent amount (5-6 C/W?).
You stated the thermal resistance of the heat sink was 3.75 C/W. I assumed that was still air. Is it? If so, it can not decrease by 5-6C/W.

I simply didn't have room on the board for the nice big 2.4C/W heatsink, so I'm going to have to give it what I can and blow a lot of air over it. It's also only going to be switched on for a second or two to start the motorcycle's engine then switched off. Yes, I'm using a MOSFET as a starter relay? Got a problem!? ;D The other one I need to sink is going to be driving up to 30A the entire time the car is powered on.
No problem, just make sure you drive it fully on to the rated Vgs.
 

1. What is a heatsink and why is it important for my project?

A heatsink is a device that is used to dissipate heat from electronic components such as processors, transistors, and integrated circuits. It is important for your project because these components generate heat during operation, and if not properly cooled, can malfunction or even fail.

2. How do I know what size heatsink to use for my project?

The size of the heatsink you need will depend on the amount of heat generated by your electronic components. It is important to calculate the thermal design power (TDP) of your components and then choose a heatsink with a higher thermal resistance (measured in degrees Celsius per watt) than the TDP of your components.

3. What type of material should I look for in a heatsink?

Aluminum and copper are the most commonly used materials for heatsinks. Aluminum is lightweight and inexpensive, but has a lower thermal conductivity compared to copper. Copper, on the other hand, is more expensive but has a higher thermal conductivity, making it more efficient at dissipating heat. Ultimately, the choice will depend on your budget and the cooling needs of your project.

4. Do I need to use thermal paste or a thermal pad with my heatsink?

Yes, thermal paste or a thermal pad is necessary to ensure optimal heat transfer between the electronic component and the heatsink. These materials fill in any microscopic gaps between the two surfaces, allowing for better heat conduction.

5. Can I use a fan with my heatsink for better cooling?

Yes, using a fan in conjunction with a heatsink can provide better cooling for your project. The fan helps to increase airflow, which can improve the heat dissipation process. However, it is important to choose a fan that is compatible with your heatsink and does not create excessive noise or vibrations.

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