O H M I T E T H E R M A L P R O D U C T G U I D E

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Heatsinkable Part Number Preﬁx Ω Watts

TAH/TBH/TCH TAH20 / TAH25 / TCH35 0.05 - 10K 20 - 35

TDH TDH35 0.1 - 10K 35

TAP650 TAP650 0.25-1M 650

ARF ARF150-ARF600 2.2- 1100 150 - 600

TKH TKH45 0.02-510K 45

TEH TEH100/TEH140 0.05 - 1M 100 - 140

TL TL54 - TL122 0.5 - 51K 27 - 275

TGH/G/L/M/P TGHG 0.0005 - 10K 100 - 600

TAP600/TAP800 TAP600 / TAP800 1 - 10K 600 - 800

BA BA1-BA3 0.5 - 18K 500 - 1000

IS IS175 - IS270 5 - 10K 175 - 270

MetalOhm 20/45M - 85/150M 0.05 - 100K 20 - 150

89 Series 805 - 850 0.10 - 100K 5 - 50

WFH WFH90 - WFH330 0.22 - 39K 90 - 330

Heat Part Number Compatible Thermal Resistance

Sinks Preﬁx Packages (C/W)

C60/B60 B60 - C60 Multiple custom 0.7 - 2

in forced convection

C40 C40 TO-247, TO264 1 - 3

in forced convection

C Series C126 - C264 TO-126, TO-220, TO247, TO-264 6.8 - 13.6

CP4 CP4 TAP600/800, TAP1K0/2K0 0.015 - 0.03

at 0.5 to 2 gpm

D DA-T263, DV-T268 TO-252, TO-263, TO-268 4.2 - 6

with PCB conduction

EV-T220, EA-T220 TO-220 6.2 - 11.4

F and R FA, RA TO-218, TO-220, TO-247 3 - 5

M MA-102, MV302 TO-247, TO-264 7.5 - 15

R2 R2V, R2A TO-220, TO-247, TO-264 10

VM VM1 - VM3 TO-220, TO-247 5

W WA-T220, WV-DT2 TO-220, TO-247, TO-264 15 - 18

WC WC-T247 TO-220, TO-247, TO-264 12

HS AH109-AH506 HS75-HS300 0.34 - 1.3

CR CR101-CR401 TO-247, TO-264 2.6 - 8.1

P PA-PV TO-220, TO-247, TO-264 7.46 - 8.07

1. Thermal resistance for 75˚C mounting surface temperature rise in natural convection unless otherwise noted.

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O H M I T E T H E R M A L P R O D U C T G U I D E

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Selecting a Heat Sink

1. Thermal Resistance Basics

Tc Ts Ta Tc: device case temperature (ºC)

Ts: heat sink temperature (ºC)

Ta: ambient temperature (ºC)

Q: heat dissipated (watts)

Rtheta: thermal resistance (ºC / W)

Basic thermal resistance equation

Rtheta ca = Rtheta cs + Rtheta sa = (Tc – Ta) / Q

Solving for the heat sink thermal resistance

Rtheta sa = ((Tc – Ta) / Q) – Rtheta cs

Or solving for heatsink temperature rise over ambient

(Ts – Ta) = (Tc – Ta) – (Rtheta cs x Q)

2. Calculating the Heat Sink Requirement

Example

Q = 10 watts

Tc = 150º C max

Ta = 40º C max

Rtheta cs = 1.0º C / W

(Varies with device package and interface material, such

as thermal grease, silicone pad, Kapton, phase-change

material, etc. Contact the factory for more details.)

Therefore, the thermal resistance required will be:

Rtheta sa = ((150-40) / 10) – 1 = 10º C / W

or lower value will be acceptable

Or, calculating the heat sink temperature rise:

(Ts –Ta) = (150 – 40) / (1 x 10) = 100º C

or lower value will be acceptable (at 10 W power)

3. Selecting the Heat Sink

3a. Board Mounted Heat Sink

If it is a board (PCB) mounted heat sink, there will be

a graph for the speciﬁc heat sink.

For the natural convection curve: power dissipation

(watts) vs. heat sink temperature rise above ambient

(Ts – Ta).

For the forced-convection curve: thermal resistance

from mounting surface to ambient (Rtheta sa (ºC / W) vs.

air velocity (ft. / min.)

For the speciﬁc example, (Ts – Ta) = 95º C at 10 watts

power, so this heat sink would satisfy the thermal re-

quirement in natural convection.

If the heat sink has a clip to hold the device, then the

heat sink thermal data will be given in terms of case

temperature, not sink temperature. Therefore, in natural

convection the case temperature rise above ambient

(Tc – Ta) will be plotted vs. heat dissipation. With known

values of Tc max, Ta max, and Q (watts), one can go

right to the graph for natural convection and determine

whether the heat sink will work.

For forced convection, calculate the Rtheta from the

equation and go to the graph to determine the airspeed

requirement.

Rtheta ca = (Tc - Ta) / Q

3b. Extrusion Heat Sink

If the device has higher power dissipation and an

extrusion is needed, use this quick sizing guide to ﬁnd

the approximate size (volume) of the heat sink to satisfy

the thermal requirements. Then, using the data sheet

of available Ohmite extrusions, one can select potential

shapes and lengths that will meet or exceed this volume.

Example:

If the Rtheta sa calculation requirement is 1.0º C / W,

then from the chart the heat sink volume would be: For

natural convection approximately 90 cubic inches or

greater. For 500 ft./min. airspeed approximately 15 cubic

inches or greater. 90 cubic inches could be satisﬁed by

9.5” length of extrusion AH13070 (6 x 1.75 x 9.5). 15 cu-

bic inches could be satisﬁed by 3.6” length of extrusion

AH12153 (3.6 x 1.15 x 3.6).

So, 9.5 inches of AH13070 is a starting point to select

a heat sink for natural convection. 3.6 inches of AH12153

will work for 500 ft./min. of air ﬂow. Interpolate for other

air speeds. Other selections can be made based on

available geometry of the space and any mounting con-

siderations. However, this tool is only intended to be a

ﬁrst draft for selecting an extruded heat sink. For further

thermal calculations and selection assistance contact the

Ohmite Applications Engineering Desk.

The available Ohmite extrusions are found at www/

Ohmite.com/cat/sink_ah.pdf.

Sink Temp Rise Above

Ambient in Free Air (ºC)

Thermal Resistance

Sink to Ambient (ºC/W)

Thermal Resistance vs Volume

Thermal Resistance ºC / W

10 1 0.1 0.01 0.001

10000

1000

100

0.1

Heat Sink Volume (cu. in.)

Natural

500 LFM

1000 LFM

Cold Plate

CP4

Water Flow

Rate (gpm)

Rtheta cs Rtheta sa

Interface Heat Sink

Resistance Resistance

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