Brochure

ESG
8.538.53
8.538.53
8.53
In the examples (a) through (c) SSR operating conditi-
ons are determined as they relate to ambient air tempe-
rature using a heat sink. Similarly, conditions can be
determined for an SSR operating in free air without a
heat sink, provided that a value is given for the radiating
characteristics of the package (RθCA). This value is rarely
given and when it is, it is more commonly combined with
(RθJC) and stated as (RθJA). The equation would appear as
follows:
T
J
- T
A
= P (R
θJC
+ R
θJA
)
T
J
- T
A
= P (R
θJA
)or
where:
(RθCA) = Thermal resistance, case to ambient, [°C/W]
(RθJA) = Thermal resistance, junction to
ambient, [°C/W]
The equation can be used to calculate maximum load
current and maximum ambient temperature as before.
However, the resultant values are inclined to be less
precise due to the many variable that affect the case to
air relationship (i.e., positioning, mounting, stacking, air
movement, etc).
Generally, free air performance is associated with PCB or
plug-in SSRs of 5 amps or less, which have no metallic
base to measure. The question is often raised as to
where the air temperature is measured. There is no
clear-cut answer for this. Measurement is made more
difficult when the SSRs are closely stacked, each crea-
ting a false environment for ist neighbour. One sugges-
ted approach is to place a temperature probe or thermo-
couple in the horizontal plane approximately 1 inch away
from the subject SSR. This technique is reasonably
accurate and permits repeatability.
Ratings
The free air performance of lower powered SSRs is
usually defined in the catalogue by means of a single
derating curve, current versus ambient temperature
based on the foregoing formulas, which is adequate for
most situations.
35 80
Max. ambient temperature [°C]Load current [A
eff
]
Power dissipation [W]
Max. allowable case temperature [°C]
25 / 0
30
25
20
15
10
5
85
90
95
100
105
110
10 20 30 40 50 60 70 8020151050
F
B
LJ
IADG
HK
E
C
No heat-sink
3,0 C/W
2,0 C/W
R
0CS
+ R
0SA
=
1,0 C/W
0,5 C/W
Fig. 10: Thermal derating curves