Crydom all products Instruction Sheet

How to verify the proper Heat Sink
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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.
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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.
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