CADDOCK
Applications Engineering Note: AEN-0102
Release Date: 9/20/02, Rev. A, Rev. Date: 9/20/02
Page 1 of 2
Understanding the Power Rating
of Caddock MP9100 Resistors
Understanding the Power Rating
The maximum power rating of the Caddock MP9100
(TO-247 type) resistor is specified with the ceramic
surface maintained at 25擄C using the same method
established and proven by power semiconductor
manufacturers over many years.
Caddock鈥檚 careful design practices and thorough re-
liability testing assure the user that the part will per-
form to all published specifications as long as its case
temperature (T
C
) does not exceed that specified in
Figure 1. Surge ratings must be observed.
MP9100 Derating Curve
g
100 %
terial characteristics and ambient temperature, (T
A
). The
maximum continuous power rating (100 Watts) for the
Caddock MP9100 resistor is based on a case tem-
perature (T
C
) of 25擄C measured at the center of the
exposed ceramic surface. The Caddock MP9100 re-
sistor has a thermal resistance of 1.5擄C/watt (R
脴JC
),
from the junction (resistor film) to the case (exposed
ceramic case interface). For a case temperature (T
C
)
above 25擄C, derating is necessary.
Allowable power dissipation (P
D
, watts) may be deter-
mined by the following equation.
Equation 1
P
D
=
T
J
- T
A
(R
脴JC
+ R
脴CS
+ R
脴SA
)
% Rated Power
80 %
60 %
40 %
20 %
0
25
100
Case Temperature (T ,
擄C)
C
Figure 1
175
In high power applications, the actual power dissipa-
tion capability of the resistor will be greatly depen-
dent on the heat sink, mounting method, interface ma-
Using the information provided in Fig 2, allowable
power dissipation and temperature gradients can be
estimated. Typical thermal resistance of various ma-
terials can be found in Table 1 on the next page. It
should be noted that material characteristics are those
provided by the manufacturers and these can vary
greatly if careful manufacturing processes are not ob-
served. Caddock suggests that actual temperature
measurements should be made under worst case con-
ditions to assure maximum reliability of a design.
Mounting Screw
Figure 2
MP9100 with Mounting Screw
and Belleville Washer
Center Line through Center
of Ceramic Element
Belleville Washer
T
J Resistor Film Temperature (Max. 175擄C)
R
脴JC
Thermal Resistance - Resistor Film
to Exposed Ceramic Case Interface
Detail of
Cross Section
Thermal Grease
or Pad
T
C
Case Temperature (Exposed Ceramic Surface)
R
脴CS
Thermal Resistance - Case to Heat Sink
Surface (Pad or Thermal Grease)
T
S
Heat Sink Temperature
R
脴SA
Thermal Resistance - Heat Sink to
Ambient Air or Reference Temperature
T
A
Ambient or Reference Temperature
Heat Sink
(mounting surface)
P
D
CADDOCK
漏 Caddock Electronics, 2002
A copy of this Application Note can be obtained at www.caddock.com
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