Heatsinks

ECE 2A: Circuits, Devices, and Systems

Fall 2010

HEATSINKS:

Power and heat
Heatsink characteristics
Making heatsinks
Use existing heatsinks
Mounting devices

POWER AND HEAT

Any device carrying a current and simultaneously a voltage drop across its terminals is dissipating power, tending to increase its temperature. How hot the device gets is determined by the package and its environment. In some low-power applications, natural convection to the surrounding air and/or thermal conduction through the device leads may be sufficient to keep the device at a safe operating temperature. For power dissipation of 0.5W or higher, we need to consider the thermal problem carefully. The safe operating temperature of a device and/or maximum power dissipation levels will be specified in the data sheet. Most device are fairly robust in the short term, but long term reliability is adversely impacted by operation at elevated temperatures. Typically a transistor or IC must be operated at an internal temperature of < 150C, which often translates to a case temperature of <100C.

Consider the power-supply that we make in ECE 2. This is designed to put out around 9-12V at 0.2 A. However, the input DC voltage to the regulator circuits (coming from the full-wave bridge rectifier) may be as large as 18V, depending on the transformer. Thus the regulator must drop around 6-9V, so at the maximum current level the device will be dissipating 1.2-1.8 Watts. We can manage this thermal dissipation with a heat-sink.

HEATSINK CHARACTERISTICS

IC packages and heatsinks are characterized by a so-called "thermal resistance", with units of °C/Watt, that essentially reduces a thermal problem to a simple equivalent electrical circuit. The rate of thermal energy dissipation (in Watts) is analogous to a current flow, and the temperature drop is analogous to a voltage drop. A TO5 transistor (for example) without a heatsink has a package-related thermal resistance of somewhere in the region of 220°C per watt. So for every watt dissipated it will increase its temperature by 220°C, or about 240 - 250°C at normal room temperatures. That's hot enough to melt solder! But a clip-on heatsink with a characteristic of 40°C/W will limit that device to 40°C + 25°C (room temperature) = 65°C. With the heatsink the transistor will therefore survive. The heatsink simply conducts heat away from the device and dissipates it into the room, so the larger the surface area, the lower (better) the °C/W rating that heatsink has.

Now lets apply this idea in reverse. Suppose you have a device that is dissipating 1 Watt, and you need to keep the case temperature below 75C. If room temperature is 25°C, that means the allowable temperature rise above ambient is 50°C. So we need a heatsink with a thermal resistance of 50°C/1W=0.5°C/Watt. Most suppliers catalogues give a heatsinks °C/W characteristic.

MAKING HEATSINKS

You can evaluate home-made heatsinks by using the following 'ROUGH' formula:

As an example, let us make a heatsink with 18 SWG aluminum sheet, folded as shown. The heatsink is going to be 20cm wide (W), 10cm deep (D) and 12cm high. Each 'fin' is 10cm x 12cm = 120cm. Each fin also has two sides, area = 240 square cm. There are 10 fins so we have a total of 2400 square cm. The rear plate is also 2 sides x 20cm x 12cm = 480 square cm. Total area = 480 + 2400 = 2880 square cm. Since the square root of 2880 = 53.66 we therefore have 50/53.66 = 0.932°C/watt.

It is also common practice to use the enclosure or chassis of a unit as the heatsink. An aluminum box 5cm x 10cm x 20cm would therefore have an OUTSIDE surface area of 5 x 10 x 20 = 1000 square cm. Square root of 1000 = 31.623 so our equipment case has a thermal dissipation of (50/31.623)°C/W = 1.58°C/W Since we need 1.13 we do not have enough, but we can use the case to cut down the size of the heatsink we need. Notice that the inside of the case was not taken into account. The case is a closed box so without ventilation the inside of the case will dissipate nothing.

MOUNTING DEVICES