Today I made some research on the heat sink that we’ll need to use for our SSR. Basically, heat sinks are required to insure the proper operation and long term reliability of Solid State Relays because they provide a means to dissipate the power that is normally developed by the SSR into the surrounding ambient air and maintain a safe operating temperature. Selecting the correct Heat Sink for any given SSR application involves coordinating form factor, size, mounting and thermal impedance rating.
Heat Sinks are made of high thermal conductive material such as aluminum. Not only is aluminum’s thermal conductivity high, its cost is relatively low. Although other materials such as steel may provide a limited measure of heat sinking, thermal conductivity of these materials are relatively low compared to aluminum and therefore far less effective as a Heat Sink. Coatings also tend to diminish the thermal dissipation effectiveness of most materials and except anodizing, should be avoided.
Heat Sink performance is rated by thermal impedance, measured in °C per watt (°C/W) (thermal impedance is the inverse of thermal conductivity). Thermal impedance represents the resistance to the transfer of thermal energy, therefore lower numerical thermal impedance ratings mean more efficient heat transfer.
Thermal Impedance ratings of Heat Sinks are based upon natural convection air flow. To achieve this performance, the Heat Sink must be oriented such that air flow over its finned surface is maximized. Generally this means the fins should be orientated vertically.
Solid State Relays controlling loads rated at more than 5 amps require a heat sink for reliable operation. The size and thermal rating of the heat sink increases as the load current carried by the SSR increases, or as the operational ambient temperature increases. The Heat Sink thermal impedance rating necessary to maintain a safe SSR operating temperature can be calculated by knowing several SSR and operational parameters.
There is a common method that many people use to calculate and select an appropriate heat sink for a given solid state relay application. The method is described below:
http://www.digikey.com/Web%20Export/Supplier%20Content/Crydom_172/PDF/Crydom_WP_HS.pdf?redirected=1
A typical Heat Sink
