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 reliability testing assure the user that the part will perform to all published specifications as long as its case
temperature (TC) does not exceed that specified in
Figure 1. Surge ratings must be observed.
material characteristics and ambient temperature, (TA).
The maximum continuous power rating (100 watts) for
the Caddock MP9100 resistor is based on a case
temperature (TC) of 25°C measured at the center of
the exposed ceramic surface. The Caddock MP9100
resistor has a thermal resistance of 1.5°C/watt (RØJC),
from the junction (resistor film) to the case (exposed
ceramic case interface). For a case temperature (TC)
above 25°C, derating is necessary.
Allowable power dissipation (PD, watts) may be determined by the following equation.
Equation 1
PD =
MP9100 Derating
Curve
g
Figure 1
% Rated Power
100 %
80 %
60 %
40 %
20 %
0
25
100
Case Temperature (T , °C)
175
C
In high power applications, the actual power
dissipation capability of the resistor will be greatly
dependent on the heat sink, mounting method, interface
Mounting Screw
TJ - T A
(RØJC + RØCS + RØSA)
Using the information provided in Fig 2, allowable
power dissipation and temperature gradients can be
estimated. Typical thermal resistance of various materials 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 observed. Caddock suggests that actual temperature
measurements should be made under worst case conditions to assure maximum reliability of a design.
Figure 2 MP9100 with Mounting Screw
and Belleville Washer
Center Line through Center
of Ceramic Element
Belleville Washer
TJ Resistor Film Temperature (Max. 175°C)
R ØJC Thermal Resistance - Resistor Film
to Exposed Ceramic Case Interface
Thermal Grease
or Pad
Heat Sink
(mounting surface)
Detail of
Cross Section
TC Case Temperature (Exposed Ceramic Surface)
PD
R ØCS Thermal Resistance - Case to Heat Sink
Surface (Pad or Thermal Grease)
TS Heat Sink Temperature
R ØSA Thermal Resistance - Heat Sink to
Ambient Air or Reference Temperature
TA Ambient or Reference Temperature
CADDOCK
© Caddock Electronics, 2002
Obtain pdf copies of AEN documents at www.caddock.com
Applications Engineering Note: AEN-0102
Page 2 of 2 - Rev. A
Calculation Examples
Mounting Configurations
Using Equation 1 and data from Table 1, the allowable power can be determined for given ambient conditions. Assuming a maximum ambient air temperature of 50°C with the part mounted using thermal
grease on an aluminum chassis, the allowable power
can be calculated from Equation 1 as follows:
Mounting pressure, surface cleanliness, surface flatness and surface finish are just a few of the variables
which can have considerable effect on temperature
rise and power dissipation. Caddock recommends an
ideal spring mounting force of 4 to 15 pounds, or a
typical mounting screw torque of 6 to 8 in - lbs which
should provide 150 to 300 pounds of force. If a screw
is used, Caddock recommends the use of a good thermal grease in conjunction with a #6 Belleville washer
compressed to 50% of its normal height. These washers (p/n 04CSB52200F004) are available from Arizona Hydrogen, phone number 602-275-4126.
PD =
T J - TA
(RØJC + RØCS + RØSA)
PD =
175°C - 50°C
(1.5°C/W + 0.5°C/W + 2°C/W)
PD =
125°C
4°C/W
PD =
31.25 watts
The same part, mounted on a water cooled heat sink
providing a constant 50°C reference temperature, will
dissipate more than twice this amount of power.
PD =
175°C - 50°C
(1.5°C/W + 0.5°C/W)
PD =
62.5 watts
Table 1 Typical Thermal Resistance of Various
Materials Used for High-Power Designs with
Caddock MP9100 Resistors
Resistor Type
Caddock MP9100 (junction to case)
Interface Material (thickness)
(°C/W)
ØJC
1.5
R
ØCS
(°C/W)
Solder (0.010”)
0.1
Epoxy (0.010”)
10
Filled Epoxy (0.010”)
Surge and Pulse Ratings
R
Thermal Grease (0.003”)
Q-Pad II (0.006” )
2.5
0.5 to 2
0.9
Surge or pulse voltages can be a factor in design reliability. The following guidelines should be followed.
SIL-PAD 400 (0.007”)
3.1
SIL-PAD 2000 (0.015”)
1.7
1. Do not exceed 750 volts peak.
Thermal Clad ® dielectric (0.003”)
0.4
2. Use the following guidelines for maximum pulse energy of a single pulse.
FR-4 / G-10 printed-circuit
board (0.063”)
Pulse Width
10 μsec
100 μsec
1 msec
10 msec
100 msec
Energy (1 J = 1 Watt second)
0.07 J
0.25 J
0.90 J
3.5 J
15 J
Aluminum (0.063”)
Copper (0.063”)
Steel (0.063”)
96% Alumina (0.030”)
Aluminum Nitride (0.030”)
Beryllia (0.030”)
Typical Mounting Surfaces
to Ambient Air
3. Accumulated pulse energy delivered in each second should not exceed allowable average power. Derate the pulse energy using the Derating Curve (Figure 1) based on estimated case temperature.
7" X 5" X 2" 0.040"
Aluminum chassis
4. Above 100 msec to 5 seconds, the overload rating
is 1.5 times the allowable power of the resistor in the
application.
CADDOCK
© Caddock Electronics, 2002
20 to 65
0.1
0.06
0.5
0.36
0.1
0.05
R ØSA (°C/W)
2
Small Finned Sheet Aluminum
heat sink (convection)
20
Small Finned Sheet Aluminum
heat sink (200 LFM Airflow)
10
4.5" X 3" X 3" Mult-Finned
Extruded Aluminum (convection)
1.5
4.5" X 3" X 3" Mult-Finned
Extruded Aluminum (200 LFM)
0.8
Obtain pdf copies of AEN documents at www.caddock.com