A heat sink is a piece of metal engineered to dissipate the maximum thermal energy into the ambient surroundings. It assists a component to remain below its maximum operating junction temperature by drawing this energy away, thereby preventing damage through excessive temperatures. All electronic components dissipate heat, and usually their package (body) is sufficient to dissipate it into the surroundings, however voltage regulators such as a 7805, 7812, LM317T, require assistance if they are to operate to their extreme limits.
This article is not about how to make a heat sink for a voltage regulator; rather, it is about how to determine if a heat sink is required, and how to choose one with the proper size. There are many manufacturers, and many electronics shops supplying them, however their use tends to be sporadic. Most of the time engineers and students overlook their requirement. Part of the reason for this is that there is nothing on the internet to show how to calculate the required size and consequently, it takes less time to buy and fit it, then do the mathematics for its requirement!
This multi-page article shows how to determine if a heat sink is required, how to calculate its size, and, how to select one using the parameters given in the documentation sheet of the component.
Thermodynamics is a completely different field of science with all sorts of strange terms such as thermal circuit, and thermal resistance. It is a craft known only to a small group of select bearded men with open toe sandals, who are usually extremely clever. However, it all boils down to one word known as thermal resistance.
The amount of heat energy that flows between two bodies is proportional to the difference in temperature between them. Thermal resistance is inversely proportional to this heat flow because it is a resistance. In everyday language, you would say that polystyrene is a poor conductor of heat energy and has a high thermal resistance. However, copper is a good conductor of heat energy and has a low thermal resistance.
This is analogous to Ohm’s Law where one might say that wood is poor conductor of electricity and has a high electrical resistance, however copper is a good conductor of electricity and therefore has a low electrical resistance.
Thermal resistance has the units °C/W, or temperature divided by watts in simple terms. When you go to the shops to buy a heat sink, it will have a rating value in these units. Obviously, this value reflects its ability to dissipate heat energy into the surroundings.
Do I need a Heat Sink?
To determine if a heat sink is required you need to find the following two parameters from the documentation sheet of the device.
- θJC = Junction-to-case thermal resistance
- θJA= Junction-to-ambient thermal resistance
All voltage regulators have these parameters in their documentation. Here is a small list of regulators from different manufacturers and their stated parameters for a TO-220 case.
|TI||μA7805||19 °C/W||17 °C/W|
|National Semiconductor||LM7805||50 °C/W||4 °C/W|
|ST Microelectronics||L78||50 °C/W||5 °C/W|
|On Semiconductor||MC7805||65 °C/W||5 °C/W|
|Fairchild||KA7805||65 °C/W||5 °C/W|
|Fairchild||LM7805||65 °C/W||5 °C/W|
To determine if you need a heat sink for your voltage regulator there are some very simple parameters that you need to compare.
θJA(TOTAL) is a value that we have to calculate, which is shown in the following sections of this article, however assume you have it, then these are the conditions shown in the form of inequalities that helps to determine if a heat sink is required.
Get a higher wattage voltage regulator
If the junction-to-ambient (total) thermal resistance is less than junction-to-case thermal resistance, then you need to look for a higher wattage regulator.
Heat sink not required
If the junction-to-ambient (total) thermal resistance is greater than junction-to-ambient thermal resistance, then a heat sink is not required.
Heat sink required
If the junction-to-ambient (total) thermal resistance is less than junction-to-ambient, but more than junction-to-case thermal resistance, then a heat sink is required.
- θJA(TOTAL) = Total junction-to-ambient thermal resistance
- TJ = Junction Temperature
- TA = Ambient Temperature
- PD = Power Dissipation
The maximum operating junction temperature is usually 150 ºC for 7805 voltage regulators, and operating at the maximum will certainly affect the reliability of the device. The recommended operating junction temperature is usually 125 ºC. This figure tends to be the same for the majority of silicon based voltage regulators in a TO-220 package.
The ambient temperature is the maximum temperature of the surrounding area that the device will encounter. It is usually 25 ºC; however, you have to consider the worst-case scenario. In the summer, the temperature can go as high as 30 ºC, and if the airflow was poor then a figure as high as 60 ºC is more appropriate.
Calculating Power Dissipation
The power dissipation of the regulator is very simple to calculate using the standard formula for current multiplied by voltage.
This formula describes the power dissipation, which is the difference in input voltage to the output voltage, multiplied by the output current. As you can see, increasing the input voltage increases power dissipation. In addition, increasing the amount of current drawn also increases dissipation.
Calculating Heat Sink Size
The heat sink-to-ambient thermal resistance is θSA, which the following formula calculates, and since we have all the pertinent values, we can plug them in. This is the value you need when selecting a heat sink.
It is a nice sunny day and the temperature is the hottest it has ever been at 27 ºC, and you are building a power supply for a Raspberry Pi. Your power supply is providing 15 V DC. You expect the RPi to draw no more than 700 mA current with all the peripherals attached. You decide to use the Fairchild LM7805 voltage regulator in your circuit. Calculate if a heat sink for the voltage regulator would be required.
Looking at the documentation sheet of the LM7805, you discover the values of the following parameters.
- θJC = 5 °C/W
- θJA = 65 °C/W
Since θJA(TOTAL) > θJC, this voltage regulator will be sufficient for the purpose.
Since θJA(TOTAL) < θJA, a heat sink will definitely be required.
Calculating Heat Sink Size
The heat sink-to-ambient thermal resistance is θSA, which the following formula finds, and since we have all the pertinent values, we can plug them in.
Therefore, a heat sink with a rating close to 9 ºC/W will be sufficient.
Looking through catalogues, the Aavid Thermalloy model 6100BG has a rating of precisely 9 °C/W, which is sufficient. There are many cheap heat sinks on eBay, Maplin, RS, and any heat sink with a close value should work.
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