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.

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
[e-mail]

Sales Support: [phone]
Technical Support: [phone]
Fax [phone]

Mexico
Tel: [phone]
Fax: [phone]

South America
Tel: [phone]
Fax: [phone]

EUROPE
United Kingdom

Tel: [phone]
Fax: [phone]

Germany
Tel: [phone]

Italy
Tel: [phone]
Fax: [phone]

France
Tel: [phone]
Fax: [phone]

ASIA
China

Tel: [phone]
Fax: [phone]

Taiwan
Tel: [phone] x130
Fax: [phone]

India
Tel: [phone]
Fax: [phone]

OTHER REGIONS
Tel: [phone]

Fax: [phone]

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

general a thermal compound will give the
lowest thermal impedance, but it is very
important to use a minimum of thermal
compound – too much is almost as bad as
none at all. One widely used technique ch
is to apply a thin layer of compound and
then apply pressure to the relay while rotat­ing it back and forth to squeeze any excess
compound out before attaching the relay to
the mounting surface.

In some applications the relay will be
mounted directly to a panel. This technique
will work up to load currents of 7 to 8 Arms
depending on the panel material, ambient
temperature, etc. In these cases it is abso­lutely essential to mount the relay to an un
painted surface and use a good thermal
compound. As a rule, the minimum per­relay panel area should be 25 square
inches.

Above these current levels some form of
heatsink will be required. Determine heat­sink thermal impedance using rating
curves like those shown in Fig 1.

60

40

50A

1°C/W

1.5ºC/W

2ºC/W

.5ºC/W

90

100

approximately 115 degrees Centigrade, a
heatsink of about 1.5 degrees Centigrade
would be more suitable. If the point of 31
Watts dissipation is read all the way to the
right, then them aximum allowable base­plate temperature is shown for a junction
temperature of 125 degrees Centigrade. In
the example discussed, this is 106
degrees Centigrade. However, to allow for
the 10 degrees Centigrade safety factor,
the baseplate temperature should not
exceed 96 degrees Centigrade.

3.0

C/W)
øsa–°

2.5

2.0

1.5

THERMAL RESISTANCE (R

1.0

HS-1

HS-2

10 1505 253020

DISSIPATION (W)

35

Fig 2

20

Power Dissipation

0

10 20 40 5030 20 40 60 80

Load Current [Arms] Max Ambient Temp. [ºC]

NO HEATSINK

110

120

Base Plate Temp [ºC]

Fig 1

The figure shown is for a 50 Arms rated
relay. Assume the load current is 30Arms,
then in the left hand side of the curve the
power dissipated is seen to be 31 Watts.
Reading across to the heatsink versus
ambient temperature curves shows that an
ambient of 40 degrees Centigrade requires
a heatsink with a thermal impedance of 2
degrees Centigrade per Watt. However,
toreduce the junction temperature to

As shown by the thermal impedance
curves for HS-1 and HS-2 heat sinks in Fig
2, not only is the surface area of the heat­sink a factor but also the power being dissi­pated. There are many sources for heat
sinks, enabling designers to select one
most suitable thermally and mechanically
for the application.

Page 2 of 2

© 2007Crydom, Inc. All Rights Reserved.