I'm planning to make a power supply with the LM2596 (adjustable voltage variant). In the datasheet I read that efficiency for VIN=12V, VOUT=3V, ILOAD=3A is 73%. That means, if I'm not mistaken, that 3 × 3 / 73 × 27 ≈ 3.33W will be wasted as heat.
When a heatsink is rated as 20K/W, does that mean that for every watt that is dissipated as heat, the temperature of the heatsink rises with 20K? When the heatsink is connected properly, is this the value that I should calculate with, or is the temperature of the component always higher than that of the heatsink, s.t. I should actually calculate with for example 22K/W?
In this example then, the temperature rise with a 20K/W would be 3.33 * 20 = 66.7K, which is still OK because the chip can handle up to 125°C according to the datasheet.
Is this correct? I'm a little bit worried about being OK with my component getting around 90°C, even though the datasheet says it's fine.
Best Answer
Just for reference, I'm going to use °C/W because it's what I'm used to.
In the datasheet for your regulator under the thermal characteristics there are typically two values.
\$R_{\theta JA}\$ - Junction to ambient
\$R_{\theta JC}\$ - Junction to case
If not using a heatsink then the \$R_{\theta JA}\$ is the value you use to calculate how warm the regulator is going to be.
When using a heatsink you take your \$R_{\theta JC}\$ and add this to your heatsink rating (20°C/W in this case) also remembering to add in any other thermal resistances. The thermal pad between the device and the heatsink is one such example.
Now this 22°C/W is in a perfect world where it has a super perfect thermal connection so take your result as an estimate rather than a "It will always be 60°C"
When you calculate the temperature, you are correct in thinking this will be the temperature of both the component and heatsink. However, remember that there's a lot of difference between a TO-220 package and a block of aluminium and it will take time for both items to reach the same temperature.
Edit: Because I remembered I had it to hand, here's a thermal image of a PCB I'm working on. This is about 5 minutes after switch on, notice the component itself is 43°C and yellow on the thermal image but the surrounding large copper heatsink pad for the device is a purple and closer to 20°C.
P.S - Please ignore the white hot supernova approaching from the right, this is a work in progress board