How to verify the proper Heat Sink
For additional information:
AMERICAS
United States and Canada
2320 Paseo de las Americas,
Suite 201, San Diego, CA 92154
sales@crydom.com
Sales Support: (877) 502 5500
Technical Support: (877) 702 7700
Fax (619) 710 8540
Mexico
Tel: +52 (222) 409 7000
Fax: +52 (222) 409 7810
South America
Tel: +55 (11) 3026 9008
Fax: +55 (11) 3026 9009
EUROPE
United Kingdom
Tel: +44 (0) 1202 606030
Fax: +44 (0) 1202 606035
Germany
Tel: +49 (0) 180 3000 506
Italy
Tel: +39 02 66599260
Fax: +39 02 66599268
France
Tel: 0 810 123 963
Fax: 0 810 057 605
ASIA
China
Tel: +86 (21) 6249 0910
Fax: +86 (21) 6249 0701
Taiwan
Tel: +886 2 8751 6388 x130
Fax: +886 2 2657 8725
India
Tel: +91 (80) 329 02 245
Fax: +91 (80) 412 38 066
OTHER REGIONS
Tel: +44 (0) 1202 606030
Fax: +44 (0) 1202 606035
In certain instances, once the heat sink requirements for a SSR in a particular application
have been determined and installed, (see the Crydom paper entitled SELECTING A
SUITBLE HEATSINK), 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 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,
(SCR’s or Triac), within the SSR and then comparing that temperature with a “standard”
absolute maximum temperature that the SSR power devices must never exceed. The
maxium power device temperature is generally considered to be 125°C but for an added
safety margin, 1 15°C should be used. (Of course for the truest indication, the entire system
being evaluated should be stable and operating at its maximum rated parameters including load currents, ambient temperatures, and with doors and access panels in their normal
operating positions.)
There are three additional pieces of data needed to perform this evaluation. They are the
actual load current switched by the SSR, the specified “Thermal Resistance – Junction to
Case” - Rθjc of the SSR, and the measured temperature of the SSR base plate. Ideally , the
temperature measurement of the SSR base plate should be taken directly from the bottom
center of the SSR. However, since in most installations this is not practical since the SSR
is mounted to a heat sink surface, the next best accessible location is on the top surface of
the base plate near the mounting screw holes at the junction of the plastic case to the base
plate surface. (To compensate for this measurement location, it is a good practice to add 3
to 5 degrees to the actual measurement.)
Using the above data, and an estimated power drop of 1 Watt for every 1 Arms of load
current, the total internal dissipation in Watts can be calculated. (e.g. 35 Arms load = 35
Watts of internal dissipation.) Next, multiply the internal dissipation in Watts by the Rθjc
value, (in °C/W), to determine the internal temperature rise. This temperature value is
added to the measured base plate temperature to arrive at the calculated temperature of the
internal power devices. If this value is less than the 115°C “standard maximum with safety
margin” value, then the heat sink is adequate.
In some cases, the heat sink information and derate curves provided within the SSR specification sheets may include expected base plate temperatures, but generally do not consider
the safety margin values.
© 2007Crydom, Inc. All Rights Reserved.
Page 1 of 1
Selecting a Suitable Heatsink
For additional information:
AMERICAS
United States and Canada
2320 Paseo de las Americas,
Suite 201, San Diego, CA 92154
sales@crydom.com
Sales Support: (877) 502 5500
Technical Support: (877) 702 7700
Fax (619) 710 8540
Mexico
Tel: +52 (222) 409 7000
Fax: +52 (222) 409 7810
South America
Tel: +55 (11) 3026 9008
Fax: +55 (11) 3026 9009
EUROPE
United Kingdom
Tel: +44 (0) 1202 606030
Fax: +44 (0) 1202 606035
Germany
Tel: +49 (0) 180 3000 506
Italy
Tel: +39 02 66599260
Fax: +39 02 66599268
France
Tel: 0 810 123 963
Fax: 0 810 057 605
ASIA
China
Tel: +86 (21) 6249 0910
Fax: +86 (21) 6249 0701
Taiwan
Tel: +886 2 8751 6388 x130
Fax: +886 2 2657 8725
India
Tel: +91 (80) 329 02 245
Fax: +91 (80) 412 38 066
OTHER REGIONS
Tel: +44 (0) 1202 606030
Fax: +44 (0) 1202 606035
other documents:
> Why Use Solid State Relays?
> SSRs - The Inside Story
> Selecting a Solid State Relay
> DC Output Solid State Relays
> SSR Overvoltage Protection
Due to the forward voltage drop of the
output SCRs, solid state relays generate
an internal power loss. The amount of
power generated is afunction of the load
current. The manufacturer provides
power loss curves, as shown in Fig 1. At
normal load currents the power loss can
be estimated at 1 Watt for every 1 Arms
of load current.
Power
Switch
Junction
Thermal Impedance
Junction to Baseplate
Relay
Baseplate Heatsink Air
Thermal Impedance
Baseplate to Heatsink
In order to maintain an acceptable power
switch junction temperature, some form
of heatsink must dissipate the heat
generated by the power loss. For most
printed circuit board types, the relay
current rating is established by measuring the thermal impedance, from the
dissipating elements to air, using the
relay package as the heat sink. Some
printed circuit board types are available
with an integral heatsink; their ratings
reflect the additional effects of the
integral heatsink.
Panel mount relays usually require an
external heatsink. The electrical analogy
shown below identifies the primary
thermal impedances in the path from
junction to ambient air:
Obviously the junction temperature Tj can
be calculated if the power dissipation is
known. The normal maximum allowable
junction temperature is 125 degrees Centigrade. Most designs are based on providing a 10 degree Centigrade safety margin
and use a heat sink to keep the junction
temperature to 115 degrees Centigrade.
Thermal Impedance
Heatsink to Air
Tj = Power x (sum of thermal impedances)
The relay manufacturer provides the
thermal impedance junction to baseplate,
and the heatsink manufacturer provides
the thermal impedance heatsink to air.
However, the thermal impedance from
baseplate to heatsink is determined by the
assembly procedure used. It is important
that the surface to which the relay is being
assembled is clean, flat, bare metal (NOT
PAINTED). If an anodized aluminum heatsink is used, the thermal impedance of the
anodized surface may be acceptable,
depending on the thickness of the anodize.
A thermal compound (or thermal pad)is
needed to minimize the baseplate-toheatsink thermal impedance. In
© 2007Crydom, Inc. All Rights Reserved.
Page 1 of 2