Heat Sinks for Switching Power Supplies
2.2 Heat-Sink Mounting
Following are some examples of how power dissipation can be calculated. As
sume that we wish to find the maximum power dissipation of a transistor (in watts).
The following conditions are specified: a maximum junction temperature of 200°C (typical for a silicon power transistor), a junction-to-case thermal resistance of 2°C/W, a heat sink with a thermal resistance of 3°C/W, and an ambient temperature of25°C.
First, find the total junction-to-ambient thermal resistance: 2°C/W + 3°C/W = 5°C/W. Next, find the maximum permitted power dissipation (200°C - 25°C =
175°C; 175/5 = 35 W, maximum). This is well above the 27 W calculated for the preceding example.
If the same transistor is used without a heat sink, but under the same condi
tions and with a TO-3 case, the maximum power can be calculated as follows. First, find the total junction-to-ambient thermal resistance: 2°C/W + 30°C/W = 32°C/W.
Next, find the maximum permitted power dissipation (200°C - 25°C = 175°C;
175/32 = about 5.5 W, maximum). This is well below the 27 W, and a heat sink is required.
Some power-transistor data sheets specify a maximum case temperature rather than a maximum junction temperature. Assume that a maximum case temperature of 130°C is specified instead of a maximum junction temperature of 200°C. In that event, subtract the ambient temperature from the maximum permitted case tempera
ture: 130°C - 25°C = 105°C. Then divide the case temperature by the heat-sink thermal resistance: 105°C/3°C = 35 W maximum power.
Table 2-4. Interface thermal resistance for a number of diode and transistor cases
Package type and data
JEDEO outline number
D 0 4
D 0 5
D 0 2 1
T 0 3
T 0 6 6
T 0 8 3
Description
10 32 stud 7/16 hex 1/4 28 stud
11/16 hex Pressfit,
1/2
Diamond
Diamond
1/2 20 stud
Recommended hole and drill
size 0.118.
No. 12 0.25.
No. 1
0.14, No. 28
0.14, No 28
0.50, 0.50
Interface thermal resistance (°C/W)
Torque in lb
15
30
•
•
•
130
Metal to metal Dry
0.41
0.38
0.15
0.20
- -
Lu bed
0.22
0.20
0.10
0.10
0.50
0.10
With insulator Dry
1.24
0.89
-
1.45 0.8 0.4
Lubed
1.06
0.70
-
0.8 0.4 0.35
Type
3-mil mica
5-mil mica
-
3-mil mica 2-mil mica Anodized aluminum
'Can be tapped for 1C 24 machine screw
2.2.1 Interface Thermal-Resistance Values
Table 2-4 shows the interface thermal resistance (in °C/W) for a number of diode and transistor/IC cases. Table 2-4 also shows recommended hole and drill sizes as well as torque for the mounting nuts or screws.
Notice that the interface thermal resistance changes quite drastically for dif
ferent mounting conditions. For example, assume that a TO-3 case is involved. If the case is mounted (on the heat sink or metal surface) with an insulator but no ther
mal compound or lubrication, the interface thermal resistance is 1.45°C/W. If a ther
mal compound is used, the resistance drops to 0.8°C/W. If circuit conditions make it possible to eliminate the insulator, the thermal resistance drops to 0.1°C/W (with thermal compound) or 0.2°C/W (without compound).
2.2.2 Fastening Techniques
The various types of packages shown in Table 2-\ require different fastening techniques. Mounting details for stud, flat-base, press-fit, and disk-type components are shown in Figs. 2-2, 2-3, 2—4, and 2-5, respectively. The following notes supple
ment the illustrations.
With any of the mounting schemes, the screw threads should be free of grease, to prevent inconsistent torque readings when tightening nuts. Maximum allowable torque should always be used to reduce thermal resistance. However, care must be ex
ercised not to exceed the torque ratings of parts. Excessive torque applied to disk- or stud-mounted parts (Figs. 2-2 and 2-5) could cause damage to the semiconductor die.
To prevent galvanic action from occurring when components are used with aluminum heat sinks in a corrosive atmosphere, many devices are nickel- or gold- plated. Take precautions not to mar the surface.
Suitable compound applied for aluminum, abrade surface
with coating applied
\ \
Heat sink mounting
plate ^ Mounting f^T
surface — — ^ ? finish
(see text)
Fastening hardware:
Flat washer (optional) (a) Belleville nut
combination (shown) or
(b) Nut plus locking washer \ 1 I l X
or locking nut |
Figure 2-2. Mounting details for stud-mounted components
With press-fit components (Fig. 2-4), the hole edge must be chamfered as shown to prevent shearing off the knurled edge of the component during press-in.
The pressing force should be applied evenly on the shoulder ring to avoid tilting or canting of the device case in the hole during the pressing operation. Thermal com
pound should also be used to ease the component into the hole.
Typically, the pressing force varies from 250 to 1000 lb, depending on the heat-sink material. Recommended hardness for typical heat-sink materials are as follows: copper, less than 50 on the Rockwell F scale; aluminum, less than 65 on the Brinell scale. A heat sink as thin as 1/8 inch may be used, but the thermal resistance increases in proportion to the reduction in contact area. A thin mounting surface re
quires the addition of a back-up plate.
With the disk-type mounting (Fig. 2-5), a self-leveling type of mounting clamp is recommended to ensure parallel contacts and even distribution of pressure on each contact area. A swivel-type clamp or narrow leaf-spring in contact with the heat sink is usually acceptable.
The clamping force should be applied smoothly, evenly, and perpendicular to the disk-type package to prevent deformation of the device or of the sink-mounting surfaces, during installation. The spring used should provide a mounting force within the range recommended by the component manufacturer. Typical clamping forces for disk-type components are 800 to 2000 lb.
Chamfer edge 0.01-in.
radius to remove burrs Flat bearing surface
Sheet meta I screws
Semiconductor
Insulator
Chassis or heat sink
Socket
When not using a socket, machine screws tightened to torque limits produce lowest thermal resistance
— Clearance holes
Insulating bushings
Screws or rivets
Figure 2-3. Mounting details for flat-base-mounted components
Installing a disk-type device between two heat sinks should be done so as to permit one heat sink to move with respect to the other. Such movement prevents stresses from developing because of thermal expansion, which could damage the component.
When two or more components are to be operated electrically in parallel, one of the heat sinks can be common to both (or all) components. Individual heat sinks must be provided against the other mounting surfaces of the components so that the mounting force applied in each case is independently adjustable.
2.2.3 Preparation of Mounting Surface
In general, the heat sink should have a flatness and finish comparable to that of the component. For the typical experimenter or hobbyist, the heat-sink surface is satisfactory if it appears flat against a straightedge and is free of any deep scratches.
Of course, in commercial power supplies, it is necessary to measure the actual flat
ness with special tools and indicators.
0.01 nominal
/ (
Chamfer 0.501
0.505
n . ^nam
Ah,
0.24 | | Q - U " * ~ Heat sink Heat sink mounting
Rivet.
X—i
nzzzL
Additional heat-sink plateJ *
X
Intimate -'
contact Complete knurl contact area
Figure 2-4. Mounting details for press-fit components
Figure 2-5. Mounting details for disk-type components
Leaf spring clamp
Most commercially available case or extruded heat sinks require spot-facing.
In general, milled or machine surfaces are satisfactory if prepared with tools in good working condition.
The surface must be free from all foreign material, film, and oxide (freshly bared aluminum forms an oxide layer in a few seconds). Unless the heat sink is used
immediately after machining, the mounting area should be polished with No. 000 steel wool, then rinsed with acetone or alcohol, and coated immediately with ther
mal grease or compound.
Many aluminum heat sinks are black-anodized for appearance, durability, performance, and economy. Anodizing is an electrical and thermal insulator that offers resistance to heat flow. As a result, anodizing should be removed from the mounting area.
Another aluminum finish is irridite (chromate acid dip), which offers low resis
tance because of the thin surface. For best performance, the irridite finish must be cleaned of oils and films that collect when the heat sinks are manufactured and stored.
Some heat sinks are painted after manufacture. Because paint of any kind has a high thermal resistance (compared to metal), paint must be removed from the heat- sink surface where the component is attached.
2.2.4 Thermal Compounds
Thermal compounds (also called joint compounds or silicon greases) are used to fill air voids between the mating surfaces so as to improve contact between the component and heat sink. A typical compound has a resistivity of about 60°C/in./W, compared to about 1200°C/in./W for air. As a result, the thermal resistance of voids, scratches, and imperfections filled with a compound or grease is about one-twenti
eth of the original resistance.
Thermal compounds are a formulation of fine zinc particles in a silicon oil that maintains a greaselike consistency with time and temperature. There are two com
mon methods for applying the compounds. With one technique, the compound is ap
plied in a very thin layer with a spatula or lintless brush, and then the surface is wiped lightly to remove excess material. The other technique involves applying a small amount of pressure to spread the compound. After the mounting is complete, any excess is wiped away using a cloth moistened with acetone or alcohol.
Some recommended thermal compounds follow:
Astrodyne—Conductive Compound 829
Dow Corning—Silicon Heat Sink Compound 340 Emerson & Cuming, Inc.—Eccotherm, TC-4 General Electric—Insulgrease
George Risk Industries—Thermal Transfer Compound XL500 IERC—Thermacote
Wakefield—Thermal Compound Type 1201