30-08-2014, 10:40 AM
crydom
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ABSTRACT
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. This paper discusses “Why Heat Sinks
are Required for Reliable Solid State Relay
Operation”, how the minimum required Heat Sink
thermal impedance rating is calculated based
upon application operating conditions, and
includes an example calculation.
INTRODUCTION
All SSRs in their conduction state create thermal energy in the output semiconductor at the rate
of approximately 1 to 1.5 watts per ampere of load current for AC output SSRs, and 0.2 to 1.5
watts per ampere of load current for DC output SSRs depending upon their design. This power
dissipation raises the Solid State Relay’s operating temperature above the surrounding ambient.
Solid State Relays can operate reliably without heat sinks up to approximately 5 amps of load
current depending upon model, duty cycle and ambient temperature. Free air ratings of
traditional panel mount SSRs may be as high as 8 to 10 amps, but this rating relies on the SSRs
exposed metal base plate acting as a heat sink, meaning that it must be exposed to ambient air.
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.
CONCLUSIONS
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 and using the formulas noted in
the text above.
Note: Crydom offers technical assistance selecting a heat sink for any given SSR application
through it Applications Engineering Department and on its web site. A selection “tool” to
calculate a heat sink rating based upon load current and ambient temperature is available at
www.crydom.com.