10.1.5.1 Heat Sink Selection Example

A TO-220 package is dissipating 5 W. The maximum expected ambient temperature is 40°C. Find the proper heat sink to keep the junction temperature less than 125°C (150°C minus 25°C safety margin).

Combining Equation 2 and Equation 3 gives:

T_J, T_A, and P_D are given. R_θJC is provided in the specification table, 2.5°C/W (DC). R_θCH can be obtained from the heat sink manufacturer. Its value depends on heat sink size, area, and material used. Semiconductor package type, mounting screw torque, insulating material used (if any), and thermal joint compound used (if any) also affect R_θCH. A typical R_θCH for a TO-220 mounted package is 1°C/W. Now we can solve for R_θHA:

Equation 4.

To maintain junction temperature less than 125°C, the heat sink selected must have a R_θHA less than 14°C/W. In other words, the heat sink temperature rise above ambient must be less than 67.5°C (13.5°C/W × 5 W). For example, at 5-W Thermalloy model number 6030B has a heat sink temperature rise of 66°C more than ambient (R_θHA = 66°C / 5 W = 13.2°C / W), which is less than the 67.5°C required in this example. Figure 44 shows power dissipation versus ambient temperature for a TO-220 package with a 6030B heat sink.

Another variable to consider is natural convection versus forced convection air flow. Forced-air cooling by a small fan can lower R_θJCA (R_θCH + R_θHA) dramatically. Heat sink manufactures provide thermal data for both of these cases. For additional information on determining heat sink requirements, consult Application Bulletin SBOA021.

As mentioned earlier, once a heat sink has been selected, the complete design should be tested under worst-case load and signal conditions to maintain proper thermal protection.