Designing With Caddock MP Series

CADDOCK
Applications Engineering Note: AEN-0101
Release Date: 10/23/02, Rev. B, Rev. Date: 2/8/06
Page 1 of 10
Designing With Caddock MP Series
TO-Style Heat Sink Mountable Power Film Resistors
Contents
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
Introduction (Page 1)
Understanding Power Rating (Page 1)
Thermal Design (Page 2)
Electrical Design (Page 3)
Applying Thermal and Electrical Design (Page 4)
Quick Guide for Heat Sink Selection (Page 5)
Assembly Materials and Techniques (Page 7)
Lead Forming (Page 9)
Thermal Design Verification (Page 10)
Figure 1
Temperature Measurement Locations
on a TO-220 Metal Tab Power Package
Tab Temperature
Molded Package Temperature
Lead Temperature
1.0 INTRODUCTION
These notes will provide a design tool for the conservative
and reliable application of Caddock TO-Style Heat
Sink Mountable Power Film Resistors. This family of
resistors includes the familiar TO-126, TO-220 and
TO-247 style packages, which are commonly used for
power semiconductor devices. These resistors allow a
designer to apply his thermal design experience gained
from using these power semiconductor packages, and
utilize off-the-shelf thermal materials, thermal hardware,
and mounting hardware in the application of these
TO-Style resistors.
2.0 UNDERSTANDING POWER RATING
The maximum power rating of these resistors is
specified with the case temperature (TC) at 25°C. This
is the same method established and proven by the
power semiconductor industry. Case temperature is
the temperature measured at the center of the resistor
mounting surface which is in contact with the heat
sink, while the resistor is under electrical load. Case
temperature must not be confused with the molded body
temperature, the tab temperature, the lead temperature
or the ambient temperature (Figure 1).
Using case temperature we can determine the
temperaure of the resistor film (TJ). This is important
since early device failures are usually traced to excessive
temperatures of the resistor film. Excessive film
temperatures will cause a drift of the resistance value
or reduced component life. Proper thermal design,
CADDOCK
© Caddock Electronics, 2002
Film Temperature (TJ)
Case Temperature (TC)
Heat Sink Temperature
followed by temperature measurements to verify the
design, and consistent mounting procedures will avoid
these problems.
The film temperature (T J ) is related to the case
temperature (TC) by the parameter “thermal resistance”
(R θJC). Thermal resistance is expressed in °C/W.
In other words, the thermal resistance (RθJC) is the
temperature rise (°C) between “J” (film) and “C” (case)
per watt applied. Each Caddock TO-Style power film
resistor has a specified thermal resistance (Table 1).
Table 1
Model
MP725
MP825
MP915
MP820
MP821
MP850
MP916
MP925
MP930
MP2060 0.005Ω
MP2060 0.010Ω
MP2060 0.015Ω
MP2060 ≥0.020Ω
MP9100
Package Power Rating RθJC TJ max
25°C Case (°C/W) (°C)
Style
25 Watts
D-Pak
5.00
150
25 Watts
TO-126
5.00
150
15 Watts
TO-126
8.33
150
20 Watts
TO-220
7.50
175
20 Watts
TO-220
7.50
175
50 Watts
TO-220
2.50
150
16 Watts
TO-220
7.81
150
TO-220
25 Watts
5.00
150
TO-220
30 Watts
4.17
150
TO-220
18 Watts
6.94
150
TO-220
36 Watts
3.47
150
TO-220
54 Watts
2.31
150
TO-220
60 Watts
2.08
150
TO-247
100 Watts
1.50
175
A copy of this Application Note can be obtained at www.caddock.com
Applications Engineering Note: AEN-0101
Page 2 of 10 - Rev B
3.0 THERMAL DESIGN
Example:
As stated earlier, the power rating of a TO-Style resistor
is established at 25°C case temperature. However, in
nearly all real-life applications the case temperature will
be higher than 25°C. A typical thermal system has three
thermal resistances, acting in series, which restrict the
flow of heat from the film to the ambient air (Figure 2).
These three thermal resistances are:
An MP9100 resistor is used with thermal grease on a
heat sink. The thermal grease has a thermal resistance
of approximately 1°C/W. The manufacturer of the heat
sink states the thermal resistance is 7.5°C/W. The
maximum ambient temperature is 55°C. What is the
maximum power the resistor can dissipate?
Thermal Resistance 1: Power Resistor Package (RθJC)
taken from Caddock Data Sheet or Table 1.
Thermal Resistance 2: Thermal Interface Material (RθCS)
must be taken from manufacturer’s data sheet. Consider
actual contact area.
Thermal Resistance 3: Heat Sink (RθSA) taken from
heat sink manufacturer. Air flow must be considered.
We can determine the maximum power dissipation using
Equation 1.
Equation 1:
PD TJ TA PD =
(TJ - TA)
PD = Power dissipation (to be determined)
TJ = 175°C (MP9100 from Table 1)
TA = 55°C (Maximum ambient temperature)
RθJC = 1.5°C/W (MP9100 from Table 1)
RθCS = 1°C/W (Thermal grease)
RθSA = 7.5 °C/W (Heat sink)
Using Equation 1:
PD =
(175°C - 55°C)
(1.5°C/W +1°C/W +7.5°C/W)
PD = 12.0 watts (maximum power dissipation)
In this example, the MP9100 which is rated at 100 watts
at 25°C case temperature, can safely dissipate 12.0
watts. The actual power dissipation capability of the
resistor is greatly dependent on the heat sink, thermal
interface material, ambient temperature and proper
mounting.
RθJC + RθCS + RθSA
Power dissipation (watts)
Resistor film temperature (°C)
Ambient temperature (maximum) (°C)
RθJC Thermal resistance of the Power Resistor Package (°C/W)
RθCS Thermal resistance of the Thermal Interface Material (°C/W)
RθSA Thermal resistance of the Heat Sink (°C/W)
TJ Resistor Film Temperature
R θJC Thermal Resistance of the Power
Resistor Package. The thermal resistance
from the resistor’s film (J) to the case (C)
Detail of
Cross Section
Figure 2: Thermal Resistances
in a Typical Thermal System
TC Case Temperature (Measured at the center
of the resistor’s heat dissipating surface)
PD
R θCS Thermal resistance of the Thermal
Interface Material. The thermal resistance from
the case (C) to the heat sink (S)
TS Heat Sink Temperature
R θSA Thermal resistance of the Heat
Sink. The thermal resistance from the
heat sink (S) to the ambient air (A)
TA Maximum Ambient Temperature
CADDOCK
© Caddock Electronics, 2002
A copy of this Application Note can be obtained at www.caddock.com
Applications Engineering Note: AEN-0101
Page 3 of 10 - Rev B
4.0 ELECTRICAL DESIGN
For your application, begin by first determining the average and maximum power dissipation to be applied to the
resistor and the maximum ambient temperature. DC and
standard AC RMS power levels, without surges, require
no further special consideration.
Pulses or transients require special consideration since
they cause instantaneous temperature rise of the resistor
film. Caddock Applications Engineering can guide you
through these considerations.
For applications with transients, pulses or surges the
following must be considered:
1. Do not exceed a peak voltage of twice the normal
rated operating voltage of the device.
MODEL PEAK VOLTAGE (do not exceed)
MP725
400 V peak
MP825
600 V peak
MP915
400 V peak
MP820
600 V peak
MP850
600 V peak
MP925
1000 V peak
MP930
500 V peak
MP2060
500 V peak
MP9100
750 V peak
2.For greatest pulse handling use the MP9100, MP2060,
MP925, MP930 or MP850. These resistors have the
largest resistance film elements.
3.Using Figure 3, estimate the energy (E) and the pulse
duration (t) for a single pulse in your application.
Figure 3: Pulse shapes and energy
V
E=
t
V
t
V
t
V
t
V2t
R
1
CV2
2
t = RC
E=
E=
V2t
3R
V2t
E=
3R
CADDOCK
© Caddock Electronics, 2002
Units
E = Energy
(joules)
t = Time
(seconds)
4.Refer to the Single Event Pulse Chart (Figure 4).
On this chart find the point where the energy (E) and
time (t) coincide. “Qualify” that this point falls below
the maximum pulse energy curve for the product you
have selected. This “qualified” pulse is valid for film
temperatures up to 150°C. “Qualified” single event
pulses determined from the chart do not require additional heat sinking.
5.Multiple pulse applications: When multiple “qualified”
pulses occur at a uniform pulse frequency it is important to determine average power for the selection
of the proper power resistor model, the proper heat
sink and thermal interface material. The average
power dissipation for uniformly spaced pulses can
be calculated from the following formula:
Equation 2: Pave = E f
Pave = PD = average power dissipation (watts)
E = single pulse energy (Joules)
f = frequency (pulses per second)
Select the power resistor model, heat sink and thermal
interface material based on the average power
(PD) and the maximum ambient temperature using
Equation 1. For oddly spaced pulses within a pulse
string consult Caddock Applications Engineering.
6. Longer duration pulses, exceeding 100 milliseconds,
are not shown in Figure 4 (single event pulse chart).
A heat sink will thermally assist pulses of this duration. For reference, the thermal time constant of the
resistance film is approximately 150 µseconds and
the time constant for the entire system (heat flow
from element to mounting surface) is approximately
50 milliseconds. For pulses over 100 milliseconds
refer to the momentary overload specification on the
product data sheet. Derate this overload specification
based on the case temperature.
Always verify designs with pulse and surge conditions
through thorough testing of the design at maximum
operating temperature and maximum pulse loading
(or some margin above maximum pulse loading).
Damage to the resistor by excessive pulse loading
will be indicated by an increasing resistance of the
resistor.
V = Voltage
(volts)
R = Resistance
(ohms)
C = Capacitance
(farads)
A copy of this Application Note can be obtained at www.caddock.com
Applications Engineering Note: AEN-0101
Page 4 of 10 - Rev B
Energy of individual pulse (Joules)
Figure 4: Single Event Pulse For Caddock TO-Style Power Film Resistors
100
10
1
MP916, MP925, MP930, MP850
0.1
0.01
MP9100
MP2060
MP915, MP820, MP821, MP825, MP725
10μs
100μs
1ms
Pulse Width (See definitions in Figure 3)
5.0 APPLYING THERMAL AND ELECTRICAL DESIGN
The following example applies the principles explained
in the prior sections.
Example:
An MP850-300Ω-1% has been selected. A 5µf capacitor charged to 150 volts will be discharged through the
resistor at 100 Hz. The maximum ambient temperature
is 75°C.
1.The MP850 is rated for 300 volts continuous, or 600
volts peak, so the peak voltage level of 150 volts is
acceptable.
2.The energy stored in the capacitor is:
10ms
0.1 sec 3.The average power is:
Pave = E f (Equation 2)
Pave = 0.056 Joules X 100 Hz
Pave = PD = 5.6 watts
4.The heat sink thermal resistance that is required,
using Equation 1:
PD = 5.6 watts
TJ = 150°C (MP850 from Table 1)
TA = 75°C (Ambient temperature)
RθJC = 2.5°C/W (MP850 from Table 1)
RθCS = 1°C/W (Assumed for thermal grease)
RθSA = To be determined
Using Equation 1:
(150°C - 75°C)
5.6 W =
(2.5°C/W + 1°C/W + RθSA)
V2
E = 1/2 C
(from Figure 3)
= (1/2) X (5 E-6) X (150)2
= 0.056 Joules = 56 mJ
RθSA = 9.9°C/W
t = R C (from Figure 3)
= (300) X (5 E-6)
= 0.0015 seconds = 1.5 mS
Therefore the heat sink selected must have a thermal
resistance of 9.9°C/W or less.
From the chart (Figure 4) the MP850 is rated 500 mJ
for a pulse width of 1.5 ms, so the 56 mJ pulse is well
below the maximum rating for the MP850.
CADDOCK
© Caddock Electronics, 2002
A copy of this Application Note can be obtained at www.caddock.com
Applications Engineering Note: AEN-0101
Page 5 of 10 - Rev B
6.0 QUICK GUIDE FOR HEAT SINK SELECTION
Table 2A
The Heat Sink Selection Tables (Tables 2A, 2B and 2C)
are provided as a quick reference for selecting a heat
sink. These tables assume using a thermal interface
material, such as thermal grease, with a thermal resistance of 1°C/W.
Heat sink (Rθsa) 2°C/W
Using Tables 2A, 2B or 2C to select a heat sink
To select a heat sink when power dissipation and ambient
temperature are known, use the following procedure:
1.Select the table (Table 2A, 2B or 2C) with the same or
higher ambient temperature as in your application.
2. Find the resistor model you have chosen.
3.Follow that row across until you find a power dissipation equal to, or slightly greater than the power level
in your application.
4. The number at the top of this column is the thermal
resistance for the heat sink required for your application.
Using Tables 2A, 2B or 2C to determine maximum power
with a known heat sink
To determine the maximum power with a known heat
sink and ambient temperature, use the following procedure:
1.Select the table (Table 2A, 2B or 2C) with the same or
higher ambient temperature as in your application.
2.Along the top row of the chart, find the thermal resistance that is equal to, or slightly higher than the heat
sink you are using.
3.Follow that column down to the row for the resistor
model you have chosen.
4. That power rating from the table represents the maximum continuous power that should be applied.
25°C Ambient Temperature
(Thermal Interface Rθcs=1.0°C/W)
MP820
MP821
MP825
MP850
MP915
MP916
MP925
MP930
MP2060 0.005Ω
MP2060 0.010Ω
MP2060 0.015Ω
MP2060 ≥0.020Ω
MP9100
14.3 W
14.3 W
15.6 W
22.7 W
11.0 W
11.6 W
15.6 W
17.4 W
12.6 W
19.3 W
23.5 W
24.6 W
33.3 W
50°C Ambient Temperature
(Thermal Interface Rθcs=1.0°C/W)
Heat sink (Rθsa) 2°C/W
MP820
MP821
MP825
MP850
MP915
MP916
MP925
MP930
MP2060 0.005Ω
MP2060 0.010Ω
MP2060 0.015Ω
MP2060 ≥0.020Ω
MP9100
4.5 W
4.5 W
4.0 W
4.4 W
3.6 W
3.7 W
4.0 W
4.1 W
3.8 W
4.2 W
4.4 W
4.5 W
5.5 W
11.9 W
11.9 W
12.5 W
18.2 W
8.8 W
9.3 W
12.5 W
13.9 W
10.1 W
15.5 W
18.8 W
19.7 W
27.8 W
5°C/W 10°C/W
9.3 W 6.8 W
9.3 W 6.8 W
9.1 W 6.3 W
11.8 W 7.4 W
7.0 W 5.2 W
7.2 W 5.3 W
9.1 W 6.3 W
9.8 W 6.6 W
7.7 W 5.6 W
10.6 W 6.9 W
12.0 W 7.5 W
12.4 W 7.6 W
16.7 W 10.0 W
15°C/W
5.3 W
5.3 W
4.8 W
5.4 W
4.1 W
4.2 W
4.8 W
5.0 W
4.4 W
5.1 W
5.5 W
5.5 W
7.1 W
25°C/W
3.7 W
3.7 W
3.2 W
3.5 W
2.9 W
3.0 W
3.2 W
3.3 W
3.0 W
3.4 W
3.5 W
3.6 W
4.5 W
Table 2C
75°C Ambient Temperature
(Thermal Interface Rθcs=1.0°C/W)
MP820
MP821
MP825
MP850
MP915
MP916
MP925
MP930
MP2060 0.005Ω
MP2060 0.010Ω
MP2060 0.015Ω
MP2060 ≥0.020Ω
MP9100
© Caddock Electronics, 2002
6.4 W
6.4 W
6.0 W
6.8 W
5.1 W
5.3 W
6.0 W
6.2 W
5.4 W
6.4 W
6.8 W
6.9 W
8.6 W
Table 2B
Heat sink (Rθsa) 2°C/W
CADDOCK
5°C/W 10°C/W 15°C/W 25°C/W
11.1 W 8.1 W
11.1 W 8.1 W
11.4 W 7.8 W
14.7 W 9.3 W
8.7 W 6.5 W
9.1 W 6.7 W
11.4 W 7.8 W
12.3 W 8.2 W
9.7 W 7.0 W
13.2 W 8.6 W
15.0 W 9.4 W
15.5 W 9.6 W
20.0 W 12.0 W
5°C/W 10°C/W 15°C/W 25°C/W
9.5 W 7.4 W
9.5 W 7.4 W
9.4 W 6.8 W
13.6 W 8.8 W
6.6 W 5.2 W
6.9 W 5.4 W
9.4 W 6.8 W
10.5 W 7.4 W
7.5 W 5.8 W
11.6 W 7.9 W
14.1 W 9.0 W
14.8 W 9.3 W
22.2 W 13.3 W
5.4 W
5.4 W
4.7 W
5.6 W
3.9 W
4.0 W
4.7 W
5.0 W
4.2 W
5.2 W
5.6 W
5.7 W
8.0 W
4.3 W
4.3 W
3.6 W
4.1 W
3.1 W
3.1 W
3.6 W
3.7 W
3.3 W
3.9 W
4.1 W
4.1 W
5.7 W
3.0 W
3.0 W
2.4 W
2.6 W
2.2 W
2.2 W
2.4 W
2.5 W
2.3 W
2.5 W
2.6 W
2.7 W
3.6 W
A copy of this Application Note can be obtained at www.caddock.com
Applications Engineering Note: AEN-0101
Page 6 of 10 - Rev B
COMMONLY AVAILABLE HEAT SINK SURFACES
All Caddock TO-Style resistor packages have the resistor
element electrically isolated from the heat sink mounting surface. Therefore, the chassis, metal panels or PC
board metallization can effectively be utilized for heat
sinking. The following are typical thermal resistances
for commonly available heat sinking surfaces.
40°C/W
2.5°C/W
2.0°C/W
Printed Circuit Board (glass epoxy)
1” X 1” pad, 2 oz. Copper
2.9°C/W Aluminum Sheet
6” X 4” X 3/16”
Vertically oriented
6.5°C/W
Aluminum Sheet
2” X 2” X 3/32”
Vertically oriented
3.5°C/W
Aluminum Sheet
6” X 4” X 3/32”
Vertically oriented
Aluminum Chassis
6” x 4” X 2” X 0.040”
Aluminum Chassis
7” x 5” X 2” X 0.040”
Illustration
RØSA Convection RØSA 200 FPM Airflow
Check to see if the clip is made to apply pressure to
the molded body or on a metal tab. Metal tab clips
will work for the MP820 and MP821 only. Make
sure that the power resistor has full contact with the
heat sink, and adequate pressure is applied.
30°C/W to 45°C/W
25°C/W to 30°C/W
25°C/W Typical
8°C/W to 10°C/W
Most of these heat sinks have clips designed
for standard TO-220 with tabs and are good for
MP820 and MP821 only!
20°C/W to 30°C/W
7°C/W to 10°C/W
Stamped heat sinks with nut and bolt for mounting. See pages 8-10 for proper screw mounting
guidelines.
20°C/W Typical
3°C/W to 6°C/W
Pin fin heat sinks similar to those used for microprocessors are now available for TO-220 packages. These provide large surface area for efficient
cooling with airflow.
3°C/W to 7°C/W
Extruded multi-fin heat sinks. Available for nut and
bolt assembly or clips. Generally the entire mounting surface must be in contact with the heat sink.
Resistor models with heavy copper tabs (MP820/
MP821) can be more forgiving since the heavy
copper will spread the heat more effectively.
15°C/W Typical
CADDOCK
© Caddock Electronics, 2002
Comments
A copy of this Application Note can be obtained at www.caddock.com
Applications Engineering Note: AEN-0101
Page 7 of 10 - Rev B
7.0 ASSEMBLY MATERIALS AND TECHNIQUES
Due to variations in the mating surfaces between the
resistor package and the heat sink air voids are created. Since the thermal resistance of air is very high
(1200°C/W/in), these voids will substantially degrade
performance. A 0.001” air gap under a TO-220 device
will cause a 10°C/W rise in resistor temperature. Therefore, it is important to use a thermal interface material
to fill these air voids. Several materials are available
to reduce thermal resistance between the resistor and
heat sink surface.
All Caddock MP Series products have electrical isolation
between the case and the resistor element, therefore,
thermal interface materials do not need to be electrically
insulating.
Thermal grease is a combination of thermally conductive particles combined with a fluid forming a grease-like
consistency. The fluid has typically been a silicone oil,
however there are now very good “non-silicone” thermal
greases. Thermal grease has been used for many years
and typically has among the lowest thermal resistances
of all thermal materials available. There are a number
of things that must be considered to provide optimum
performance and avoid problems.
1.The heat sink surface area must be free of dirt
particles, scratches, dents, voids, and burrs. It is
recommended that the surface flatness be less than
0.001 in/in and surface finish in the range of 15 to 60
microinches.
2. Generally, the entire mounting surface of the Caddock TO-Style Power Resistor must be in thermal
contact with the heat sink. Resistor models with heavy
copper tabs (MP820/MP821) can be more forgiving,
since the heavy copper tab will spread the heat more
effectively.
3.Thermal grease must be applied thinly and evenly
over the entire contact area. Never apply a thick
coating and try to press it flat. Thermal grease usually
flows poorly and this will lead to breakage of the part.
Also a thick grease layer leads to poor heat transfer.
To determine the correct amount and application
procedure, follow the manufacturer’s recommendations. Verify your procedure with actual temperature
measurements.
4. Proper mounting pressure must be maintained. Insufficient mounting pressure can significantly increase
thermal resistance. Mounting torque is addressed
later in this section.
CADDOCK
© Caddock Electronics, 2002
5. At prolonged high temperatures the silicone in silicone
greases can separate and migrate, coating nearby
components. This migration increases the thermal resistance of the remaining grease. New “non-silicone”
thermal greases significantly reduce this problem.
Thermal pads, an alternative to thermal grease, are
available from a number of manufacturers such as
Bergquist, Chomerics, and Aavid/Thermalloy. These
thermally conductive pads are available in sheet form or
in pre-cut shapes designed for various standard packages such as the TO-126, TO-220 and TO-247. These
pads utilize silicone rubber binder combined with a variety of materials such as aluminum oxide, boron nitride, or
magnesium oxide to provide good thermal conductivity.
For specialized applications pads are laminated with
Kapton, fiberglass, or other materials.
Thermal pads are spongy materials and require firm and
uniform pressure to perform properly. Charts are available from the pad manufacturers that show the optimum
mounting pressure and how reduced mounting pressure
will increase thermal resistance.
New materials continue to appear. These include thermal adhesives and tapes, phase change materials, gap
fillers and screen printable materials.
Information on thermal management, thermal interface
materials and heat sinks are available from the following (Figure 5):
Figure 5
Aavid Thermalloy
www.aavidthermalloy.com
AOS Thermal Compounds
www.aosco.com
AI Technology
Balkhausen GmbH
(Germany)
Bergquist
Chomerics
Cool Innovations
www.aitechnology.com
www.balkhausen.de
www.bergquistcompany.com
www.chomerics.com
www.coolinnovations.com
Electronics Cooling
www.electronics-cooling.com
Keramische Folien GmbH
(Germany)
www.kerafol.com
Emerson & Cuming
Fujipoly
Kunze Folien GmbH
(Germany)
Loctite
www.emersoncuming.com
www.fujipoly.com
www.heatmanagement.com
www.loctite.com
PADA Engineering
(Italy)
www.padaengineering.com
Power Devices
R-Theta
www.powerdevices.com
www.r-theta.com
Thermagon
Wakefield Engineering
www.thermagon.com
www.wakefield.com
Thermacore
www.thermacore.com
A copy of this Application Note can be obtained at www.caddock.com
Applications Engineering Note: AEN-0101
Page 8 of 10 - Rev B
Screw Mounting
Hardware Selection
Proper hardware is an extremely important consideration
in a good thermal design. The hardware must maintain
firm even pressure on the device, through thermal cycling, without deforming the heat sink or the device.
Figure 7 shows some typical hardware configurations
for screw mounting a TO-Style package to a heat sink.
When screw mounting a Caddock TO-Style resistor the
following considerations should be observed.
Spring Clips are preferred by many designers in place
of screw assembly for attaching Caddock TO-Style
Power Resistors to the heat sink. These spring clips are
available from several manufacturers. Aavid Thermalloy
offers a number of standard springs and heat sinks
specifically designed for clip mounting TO-220 and
TO-247 packages.
2. Use a good thermal grease or thermal interface material (pad, phase change material, etc) between the
resistor and the heat sink.
Spring clips offer many benefits for ease of assembly,
but their biggest advantage is the consistent application
of optimum force over the center of the power resistor
(see Figure 6). The following are recommendations for
selection of a spring clip.
1.The recommended spring force is 8 to 30 pounds (35
to 130N) applied to the molded package directly over
the center of the resistor element.
2. Caddock’s MP2060 resistor has been specifically
designed for spring clip assembly.
3. Excellent results can be obtained on all Caddock
TO-Style Power Resistors with a spring clip assembly
and a clean, flat and smooth surface with a thin and
evenly spread layer of good quality thermal grease.
Figure 6
Clip Mounting Configurations
MP2060
HEAT SINK
MP2060
HEAT SINK
MP2060
HEAT SINK
CADDOCK
© Caddock Electronics, 2002
1. Make sure the surface of the heat sink is clean, flat
and free of burrs or surface irregularities.
3.A flat washer should be used between the screw and
the package to help spread out the mounting force,
this is especially important on the molded package
styles.
4. Use a spring washer such as a conical washer (Belleville Washer) or similar spring washer to maintain
adequate pressure during the operating life of the
product and through temperature cycling and variations.
Do not overtighten the mounting screw. The maximum
torque for TO-220 style package is typically 8 in-lb or
1.1 n-m. For recommended and maximum mounting
torques please see the data sheet for the Caddock resistor model in questions. CAUTION: Grease on the
threads, variations in thread depth, pitch, and tolerances
can effect torque measurements. We do not recommend
using torque limits without first testing your specific application to assure that proper deflection of the washer
is maintained.
Belleville or Conical Washers used with a screw are
an effective method for attachment to a heat sink. A
Belleville washer is a conical spring washer designed
to maintain constant pressure over a wide range of deflection. The washers withstand long-term temperature
cycling without variation in pressure. The large open end
of the washer should face the device or heat sink that
it contacts. Flat washers, star washers, and most split
lock washers should not be used in place of Belleville
washers since they do not provide a constant mounting
pressure and may cause damage to the resistor.
When screw mounting a TO-220 package, a force of 125
to 350 pounds (550 to 1500 N) is necessary for optimum
heat transfer. Most commonly available Belleville washers cannot provide enough force and will flatten out,
rendering them useless.
It is very important that this force is carefully controlled
and uniformly distributed to avoid lifting the edge of the
resistor or cracking the package. A common cause of
resistor failure is breakage caused by excessive force
applied over a small area. This can happen when proper
mounting torque is not applied.
A copy of this Application Note can be obtained at www.caddock.com
Applications Engineering Note: AEN-0101
Page 9 of 10 - Rev B
Figure 7: Screw and Washer Mounting Techniques
Spring Washer (Belleville or other)
MP Resistor
Thermal Pad or Grease
Heat Sink
Flat Washer
Flat Washer
Improper lead forming may damage the resistor package, but can be done if proper care is taken. The MP820
and MP821 are the most versatile choices due to the
round leads. Lead bending on all other packages, with
flat leads, should be limited to the vertical axis. If SMT
formed leads are required, the MP725 (D-PAK style
package) with SMT formed leads and a solderable tab
is available.
1. Always provide strain relief when lead forming. While
forming, the leads must be supported or gripped between the bend and the package.
MP Resistor
2.The minimum bend radius is 0.050”. Forming a
tighter radius can crack the plating and/or weaken the
lead. Using a mandrel or forming fixture is recommended.
Thermal Pad or Grease
Heat Sink
8.0 LEAD FORMING
3.Do not twist the leads at the body of the resistor.
Spring Washer (Belleville or other)
Assembly Considerations
1.Never let the head of the screw contact the resistor.
Use a flat washer or conical washer to evenly distribute the force.
4.Generally, do not splay the leads (bend outwards,
parallel to the heat sink mounting plane). This may
be done on the MP820 or MP821 only.
2. Do not over-torque. If the screw is too tight, the package may crack or have a tendency to bow up at the
end farthest from the screw (lead end). Pneumatic
tools are not recommended.
3.Avoid sheet metal screws which have a tendency to
roll up the edges of the hole and create damaging
burrs on the heat sink.
4.Rivets are not recommended. With rivets it is difficult
to maintain consistent pressure and they can easily
damage plastic packages.
5.Plastic mounting hardware that softens or creeps at
high operating temperatures must be avoided.
6.Avoid using this TO-Style family of power resistors
for SMT assembly. For surface mount requirements,
use Caddock MP725 D-Pak Style power film resistors,
Type CC or Type CD chip resistors.
CADDOCK
© Caddock Electronics, 2002
A copy of this Application Note can be obtained at www.caddock.com
Applications Engineering Note: AEN-0101
Page 10 of 10 - Rev B
9.0 THERMAL DESIGN VERIFICATION
Example:
Always verify your thermal design with actual temperature measurements of the prototype. These measurements must consider the maximum power dissipation,
maximum ambient temperature, and assembly variables.
The metal tab temperature measures 50°C on an MP820
(RθJC=7.5°C/W) operating at 10 watts at maximum ambient temperature.
Case temperature (Figure 1) is the best measurement
for design verification, since by calculations (Equation 1)
the film temperature (TJ) can be determined. However,
case temperature is often difficult to measure.
Alternate temperature measurements, described below,
can give a general indication of the film temperature.
These alternate temperature measurements will always
be lower than the film temperature (TJ), since the film is
the heat generating source. Therefore, these alternate
temperature measurements must be well below the
maximum rated operating temperature of the resistor.
An alternate temperature measurement can be made
at the leads where they exit the resistor body (lead
temperature see Fig. 1) or on the metal tab adjacent to
the plastic package (tab temperature MP820/821 see
Fig. 1). A measurement of 100°C would be very safe,
when measured at the highest ambient temperature and
highest average power for the design. For temperature
measurements, use a low mass thermocouple probe that
will not sink heat away from the measurement point on
the resistor. A thin layer of thermal grease on the thermocouple will assure good thermal contact.
For temperature measurements other than case temperature, consider the following issues. Refer to Figure
1 for these measurement locations.
Lead temperature measurements can approach the
film temperature, but are always cooler. Some board
materials, a large copper bus or heavy copper pads can
provide heat sinking which lowers temperature of the
leads. Leads should always be measured at the hottest
point closest to the body and this temperature compared
to the “hottest spot” molded body temperature.
TJ = 50°C+(10W x (7.5°C/W+1°C/W)) = 135°C
The film temperature (TJ) is safely below the maximum
temperature rating of 175°C .
Heat sink temperature can provide an indication of
case temperature, but very easily provides false data.
A cool heat sink and a hot resistor can indicate poor
mounting caused by improper mounting force, thick or
poor quality grease, burrs or poor mounting surfaces,
any of which can reduce the flow of heat from the resistor to the heat sink.
Case temperature (TC) provides the most accurate estimate of the resistor element temperature (TJ) but can
be difficult to measure (Figure 1). The resistor element
temperature (TJ) will exceed the case temperature by an
amount which can be calculated using the power applied
(PD) and the thermal resistance (RθJC) of the part.
Example:
The case temperature measures 75°C on an MP930
(RθJC = 4.17°C/W) operating at 6 watts at 25°C ambient
temperature.
TJ = 75°C+(6W x 4.17°C/W) = 100°C
The film temperature is 100°C.
What if the system is to operate at 70°C ambient temperature? This is 45°C above the original ambient
temperature (70°C-25°C). The resistor element will then
be 145°C (100°C + 45°C). The MP930 with a maximum
temperature rating of 150°C is an acceptable but marginal choice, considering the variations of mounting,
thermal grease thickness, etc. The MP2060, MP850 or
MP9100 would be a more conservative design.
Metal “tab temperature” adjacent to the molded resistor body provides a good means of estimating the case
temperature of a metal tab TO-220 resistor. The temperature should be measured at the hottest spot along
the tab/body interface. This temperature will be about
1°C/W cooler than the case temperature (Tc).
CADDOCK
© Caddock Electronics, 2002
A copy of this Application Note can be obtained at www.caddock.com