In electronics, things have a nasty tendency to get hot, and knowing how to manage that heat is an important part of the design process. I want to step you through a very simple real-world example I recently encountered to show you some thermal basics.
I was building a power supply around the classic LT3080 from Analog Devices (it will always be a Linear Technology part in my mind). This a programmable low drop out regulator with some unique features. In my design, the regulator could put out a current of up to 1A and, under worst case conditions, see a voltage drop of just under 6V, meaning it could be dissipating up to 6W. Would I need a heatsink (probably yes) and, if so, what heatsink will I need?
I was using the TO220 package, and a quick look at the data sheet (Figure 1) shows that the maximum junction temperature—TJMAX —is 125°C, and the thermal resistance between the junction and the ambient temperature—ϴJA—is 40°C/W. This means the silicon die inside must run at less than 125°C, and that temperature difference between the die and the air around the device will be 40°C for every watt dissipated.
The easiest way to think about this is as a simple analogy with electrical circuit theory. Temperature is equivalent to voltage, power is equivalent to current, and thermal resistance is equivalent to electrical resistance as per Figure 2. In our case 6W across a 40°C/W thermal resistance will result in a junction temperature rise of 240°C above ambient! Clearly a heatsink will be required.
So how do we choose a heatsink? We work backwards and calculate the junction to ambient thermal resistance we need to achieve a manageable junction temperature given a particular ambient temperature. My advice is to always be conservative with thermal calculations—the hotter things are, the shorter will be the life of your components.
I assumed a maximum junction temperature of 100°C and a maximum ambient of 50°C. this means I can afford a 50°C temperature differential between ambient and junction. With 6W being dissipated I need a thermal resistance of no more than 8.3°C/W. So, can I now go and find a heatsink meeting these specs? Not so fast. We have to take the thermal resistance between the junction and the case, ϴJC, and that between the case and the heatsink, ϴCH, into account. See Figure 3.
The first is easy. The device data sheet (Figure 1) shows us that ϴJC is 3°C/W. This means we have 5.3°C/W left for the thermal resistance between case and ambient. How do we find the thermal resistance between the case and the heatsink? With metal-to-metal contact and heatsink compound, a TO220 package has a thermal resistance of about 1°C/W. This would leave 4.3°C/W for the heatsink.
I looked at quite a few heatsinks and landed on the RA-T2X25E from Ohmite which has a thermal resistance of 4.8°C/W. This was a fraction higher than I would have liked, but its big brother, the RA-T2X38E at 3.8°C/W was too tall for my application. A quick calculation shows the maximum junction temperature will be 103°C at 50°C ambient. I can live with that.
“LT3080 – Adjustable 1.1A Single Resistor Low Dropout Regulator.” Analog Devices, https://www.analog.com/media/en/technical-documentation/data-sheets/3080fc.pdf.
Roehr, Bill. “Mounting Considerations for Power Semiconductors,”, On Semiconductors. https://www.onsemi.com/pub/Collateral/AN1040-D.PDF
“RA-T2X-25E – Ohmite Mfg Co.” https://www.ohmite.com/catalog/f-and-r-series-heatsink/RA-T2X-25E.