How to Select a Suitable Heat Sink

Any semiconductor device under operation produces heat. The heat originates at its junction and travels outwards. The longevity and reliability of the semiconductor device is inversely proportional to the square of the change in its junction temperature. Therefore, if the temperature change of the junction can be halved, you'll get an approximate quadrupling of the expected life of the component. The opposite is also true. Therefore, a notable increase in the life and reliability of the component may be arrived at through a relatively small lowering of the operating temperature, as these parameters increase exponentially when operating temperature is reduced.

The die, or junction, of the semiconductor is the primary heat source. The heat is transferred to the case when the semiconductor is in operation. The semiconductor is usually mounted on a heat sink, which carries the heat away from the case to the ambient air via conduction and convection.

The whole process of removing heat from the junction of a semiconductor device involves numerous independent thermal transfers. The entire heat flow is established from observing two basic elements: one, the heat input value, and two, the ambient air temperature. The complete process can be simulated by an electrical analog. Here, a thermal generator acts as the current source, thermal resistances are represented by resistors and the thermal inertia of the different material in the path can be represented by capacitors. Any material absorbs a quantity of heat during a short period, and that constitutes its transient capacity. After this absorption, the temperature rises in the same way that voltage rises across a capacitor provided the supply current is maintained.

We can model the entire heat flow process with the heat sourced from the junction, which has a small thermal inertia (C_junction), and a thermal resistance to the case (Rth_jn-case). The case has its own thermal inertia (C_case), again small, and a thermal resistance to the heat sink (Rth_hs-amb). The heat sink has a somewhat larger thermal inertia (C_{heat sink}), depending on its size, and a corresponding thermal resistance (Rth_hs-amb) to the ambient air. Since the ambient air is considered infinite, it has zero thermal inertia.

The small thermal inertia values can be safely ignored as they will be overcome very quickly. The heat sink will take some time before it reaches a steady temperature, as the heat flow in equals the heat flowing out. The steady-state temperatures are as shown.

With the ambient temperature assumed to be 25°C (T_amb), we will strive to keep the junction temperature at 125°C (T_jn). The Rth value for jn-case for a specific device is given by the manufacturer. For example, for a transistor MJE3055T, which comes in a TO-220 pack, Motorola specifies the thermal resistance from the junction to case to be Rth_jn-case =1.67°C/W. If the power to be dissipated from the transistor is Pd = 30W, the temperature difference between the junction and case will be Rth_jn-case x Pd, or 1.67 x 30 = 50°C, which means the case temperature will be T_case = 125-50 = 75°C.

It is typical to use a thin material between the transistor case and the heat sink to act as an electrical insulator. Since heat has to be transferred through it, the material must have a good thermal conductivity. One of the materials used is a silicon pad, which is soft and fills in the microscopic irregularities between the surface of the transistor and the heat sink. The thermal conductivity of silicon pads is typically 1°C/W. Therefore, a power flow of 30W across the silicon pad will cause a temperature drop of 1 x 30 = 30°C, which sets the heat sink temperature to be T_hs = 75-30 = 45°C.

Now, we require a heat sink that must dissipate 30W at 45°C to the ambient temperature of 25°C. Therefore, you must look for a heat sink that has a thermal conductivity of Rth_hs-amb = (T_hs – T_amb)/Pd, or (45-25)/30 = 0.67°C/W. Manufacturers usually provide information about the thermal resistance of the heat sinks they make, so making a well-informed choice should not be difficult.

Even if the information regarding the thermal resistance is not provided by the manufacturer, all is not lost. Using a heat source such as a power resistor mounted on the heat sink, and fed with a specific power, it is possible to calculate the thermal resistance from the temperature drops produced by the arrangement.

This article is contributed By J.Peteroff

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