APPNOTESMP9100 Datasheet, PDF (1/2 Pages) List of Unclassifed Manufacturers – Understanding the Power Rating of Caddock MP9100 Resistors

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°C using the same method
established and proven by power semiconductor
manufacturers over many years.
Caddock’s 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.
Figure 1
100 %
80 %
60 %
40 %
20 %
0
MP9100 Derating Curve
g
25
100
175
Case Temperature (T_C, °C)
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-
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°C measured at the center of the
exposed ceramic surface. The Caddock MP9100 re-
sistor has a thermal resistance of 1.5°C/watt (R_ØJC),
from the junction (resistor film) to the case (exposed
ceramic case interface). For a case temperature (T_C)
above 25°C, derating is necessary.
Allowable power dissipation (P , watts) may be deter-
D
mined by the following equation.
Equation 1
T- T
P=
J
A
D _{(RØJC + RØCS + RØSA)}
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
Belleville Washer
Thermal Grease
or Pad
Heat Sink
(mounting surface)
CADDOCK
© Caddock Electronics, 2002
Figure 2 MP9100 with Mounting Screw
and Belleville Washer
Center Line through Center
of Ceramic Element
T_J Resistor Film Temperature (Max. 175°C)
R _ØJC Thermal Resistance - Resistor Film
to Exposed Ceramic Case Interface
T_C Case Temperature (Exposed Ceramic Surface)
P
D
R _ØCS Thermal Resistance - Case to Heat Sink
Surface (Pad or Thermal Grease)
T_S Heat Sink Temperature
R _ØSA Thermal Resistance - Heat Sink to
Ambient Air or Reference Temperature
T_A Ambient or Reference Temperature
Detail of
Cross Section
A copy of this Application Note can be obtained at www.caddock.com