How to Choose a Heatsink

In power electronics, we use a lot of current. Unfortunately, a lot of power is also dissapated as heat. Depending on how much power you are using, you may find that your system is producing too much heat, and causing the destruction of your ICs. Linear regulators are especially vunerable to overheating, because they dissapte a very large amount of heat while in use. But how can we combat the heat, and allow our ICs to continue functioning even with the heat? Well, we can use a heatsink. Heatsinks basically increase the area in which the disapated heat can go and decrease the overall temperature, saving our circuit. So, in this video I will explain the best way to determine which heatsink your project needs.

First, let’s look at an example IC that will generate the heat we need to demonstrate the ability of a heatsink. I will be using the LM317 linear regulator because it makes it easy for us to understand the power losses that will be in effect. The best place to start is to find the maximum temperature in the datasheet. As we can see, the datasheet recommends that we stay within the 0 to 125 degree celcius range for this device. Now that we know the maximum temperature, we need to know how much the temperature will increase given a power loss. In this example, I will be giving the LM317 a 12 volt input, and have it output 5 volts at 1 amp. Remember, the equation for power is voltage multiplied by current, so we can expect a power loss of 7 watts. But, how do we know how much heat that will generate.

Well, that brings us into the concept of thermal resistance. You can think of it sort of like electrical resistance, in that with a higher resistance, less heat will be able to flow. So in other words, a higher thermal resistance leads to a higher temperature. Ideally, we would have as low of a thermal resistance as possible to keep our circuits cool, but that isn’t always possible. So, how do we find said thermal resistances, and how can we use them to find our resulting temperature? Well, we can find all of our information in the device datasheet thankfully.

We can already see that the datasheet has a large table with different thermal metrics, and there are several different columns for each option. That brings us to the package that your IC is using. The LM317 I am using is a TO-220 package, however we can see the other packages around the table. The important metric that we are looking for is the Junction-to-ambient thermal resistance. This basically is the resistance from the inside of the IC, to the exposed area. The LM317 has a 37.9 degress celcius per watt thermal resistance. Now that we know these important values, we can take a look at an important equation that will allow us to calculate the expected heat.

Going back to the example earlier, we already know that our expected power loss is 7 watts. We can use this equation: Junction temperature equals the power multiplied by the junction to case resistance, plus the case to surface resistance, plus the surface to air resistance finally, plus the ambient temperature. For our bare LM317, we can replace all of these resistances with the 37.9 degrees celcius per watt that we got from the datasheet, because the datasheet already made these calculations for us. Now, plugging in all of the values, with the ambient temperature being estimated at about 25 degrees celcius, we calculate that we will get a junction temperature of 290 degrees. So, we clearly need a heatsink for this application because 290 is a lot larger than 125. If we changed the load, however, to only 1 watt of loss, we would get a resulting temperature of 63 degrees. So, if your load is small enough, you can get away without using a heatsink. But let’s continue with the first example and select an appropriate heatsink.

If we are using a heatsink, we need to select a different value from the datasheet. This time we are looking for the junction to case thermal resistance. For the TO-220, we have a value of 4.2 degrees per watt. We just need two more values, the case to surfance and the surface to air resistances. The case to surface resistance depends on your setup with the heatsink, and is a bit more difficult to get. Its value has a small range, from about 0.5 to 2 degrees per watt, depending on how the heatsink is mounted. Direct mounting is about 1 to 1.3 degrees per watt. Whereas mounting with a mica insulator gives a resistance of about 1.6 degrees per watt. These figures, by the way are specific to the TO-220 package. If you are ever unsure or just don’t want to run the calculations, you can likely safely pick a case-to-surface resistance of about 1.6 considering that you are using proper mounting methods. Finally, we need to find the surface to ambient resistance. This is actually very easy if you have access to the heatsink datasheet. I have a few heatsinks here that range from 24 degrees per watt all the way to 3 degrees per watt.

The way we can tell whether a heatsink is suitable is simply run the equation. Repeating that same 7 watt example, we find that the small heatsink will give us a temperature of 233 degrees. While this is better than the 290 on the bare IC, it still is far too much. The large heatsink, on the other hand, gives us a temperature of 86.6 degrees. This is a lot lower than the 125 degree limit, so this heatsink is more than enough. It is also a good idea to aim for a maximum temperature lower than the datasheet recommends. For example, we should aim for 105 degrees maximum even though the datasheet says that we can go up to 125 degrees. This is just good engineering practice to leave room just in case. Another thing to keep in mind is the ambient temperature. Sure, we can say that it is 25 degrees while it is in open air, but in closed cases, it will be higher. One more note, adding a fan can greatly improve the thermal resistance of a heatsink, so if you don’t mind the audible noise generated by a fan, it is a good option.

Now, I will show you the proper way to mount an IC onto a heatsink. I will give both a TO-220 and a TO-3 example. Starting with the TO-220 package, we first need the heatsink itself, a screw, an insulating shoulder washer, an isolating pad, and a nut. First we will put the washer into the TO-220’s hole. Then we will fit the screw into that hole. The isolating pad will go through the other side of the screw. Finally, we can locate the hole on the heatsink and screw the whole assembly into place. Make sure that is tightly fit. No matter the size of heatsink, this method should work. Next is the TO-3, which is a bit more tricky, but the process overall is the same. First, take your insulating pad and put it onto the bottom of the IC. Then, fit it onto the heatsink, check the other side to make sure that you can see all the way through. Next, stick a screw through one side and place an insulating washer on the other side. This washer makes sure that no contact is made with the heatsink. Next put on a nut and tighten it. Repeat on the other side. Now pick one side and put on a solder tab and another nut to hold it in place.

Now you know all about heatsinks, from how to pick one all the way to mounting it to your IC. If you enjoyed this video please consider subscribing so that you can see my other videos. Have a good one.