APPNOTESMP9100 datasheet(2/2 Pages) ETC

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Calculation Examples

Using Equation 1 and data from Table 1, the allow-

able power can be determined for given ambient con-

ditions. Assuming a maximum ambient air tempera-

ture 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:

D =

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.

D =

62.5 watts

Surge and Pulse Ratings

Surge or pulse voltages can be a factor in design reli-

ability. The following guidelines should be followed.

1. Do not exceed 750 volts peak.

2. Use the following guidelines for maximum pulse en-

ergy of a single pulse.

Pulse Width

Energy (1 J = 1 Watt second)

µsec

0.07 J

100

µsec

0.25 J

1 msec

0.90 J

10 msec

3.5 J

100 msec

15 J

3. Accumulated pulse energy delivered in each sec-

ond should not exceed allowable average power. De-

rate the pulse energy using the Derating Curve (Fig-

ure 1) based on estimated case temperature.

4. Above 100 msec to 5 seconds, the overload rating

is 1.5 times the allowable power of the resistor in the

application.

Table 1 Typical Thermal Resistance of Various

Materials Used for High-Power Designs with

Caddock MP9100 Resistors

Applications Engineering Note: AEN-0102

Page 2 of 2 - Rev. A

A copy of this Application Note can be obtained at www.caddock.com

Resistor Type

(

°C/W)

Caddock MP9100 (junction to case)

Interface Material (thickness)

(

°C/W)

Solder (0.010”)

Epoxy (0.010”)

Filled Epoxy (0.010”)

Thermal Grease (0.003”)

Q-Pad II (0.006” )

SIL-PAD 400 (0.007”)

SIL-PAD 2000 (0.015”)

Thermal Clad ® dielectric (0.003”)

FR-4 / G-10 printed-circuit

board (0.063”)

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

(

°C/W)

7" X 5" X 2" 0.040"

Aluminum chassis

Small Finned Sheet Aluminum

heat sink (convection)

Small Finned Sheet Aluminum

heat sink (200 LFM Airflow)

4.5" X 3" X 3" Mult-Finned

Extruded Aluminum (convection)

4.5" X 3" X 3" Mult-Finned

Extruded Aluminum (200 LFM)

1.5

0.1

2.5

0.5 to 2

0.9

3.1

1.7

0.4

20 to 65

0.1

0.06

0.5

0.36

0.1

0.05

1.5

0.8

ØJC

ØCS

ØSA

ØJC + RØCS + RØSA)

J - TA

(1.5

°C/W + 0.5°C/W + 2°C/W)

175

°C - 50°C

°C/W

125

°C

(1.5

°C/W + 0.5°C/W)

175

°C - 50°C

Mounting Configurations

Mounting pressure, surface cleanliness, surface flat-

ness 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 ther-

mal grease in conjunction with a #6 Belleville washer

compressed to 50% of its normal height. These wash-

ers (p/n 04CSB52200F004) are available from Ari-

zona Hydrogen, phone number 602-275-4126.

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