HOW TO CALCULATE AND SELECT A HEAT SINK FOR A GIVEN SOLID STATE RELAY APPLICATION
The basic structure of a Solid State Relay includes an internal power semiconductor mounted to an electrical insulator which in turn is mounted to the Solid State Relay’s base plate. To form an assembly, the SSR with an accompanying thermal interface material placed on its base plate is then torque mounted to the Heat Sink.
The thermal model representing the above configuration includes the following elements:
A.
The selected SSR with specified thermal impedance (RΘ ssr), forward voltage drop (Vf), and maximum allowed internal operating temperature (Tj).
B.
The thermal interface material placed between the SSR and the Heat Sink and its specified thermal impedance (RΘ tp).
C.
The calculated minimum Heat Sink thermal impedance rating (RΘ hs) required for proper SSR operation.
D.
The operating environment’s max ambient air temperature in °C (TA ).
How to verify the proper Heat Sink
In certain instances, once the heat sink requirements for a SSR in a particular application have been determined and installed, it may be desirable to verify that the system does indeed provide adequate cooling to ensure reliable SSR operation.
The following is a relatively simple method to check this suitability, and essentially uses some of the calculations from SELECTING A SUITABLE HEAT SINK (above) in a reverse manner. This technique may also be used on existing systems in the field that might have been more or less “empirically” designed, to gain information on their performance and potential reliabilty. This method involves determining the temperature of the internal power devices