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7

ART28XXT Series

Effectiveness of this heat transfer is dependent on the

intimacy of the baseplate-heatsink interface. It is therefore

suggested that a heat transferring medium possessing

good thermal conductivity is inserted between the

baseplate and heatsink. A material utilized at the factory

during testing and burn-in processes is sold under the

insulator but electrically conductive versions are also

available. Use of these materials assures optimum surface

contact with the heat dissipater by compensating for minor

surface variations. While other available types of heat

conducting materials and thermal compounds provide

similar effectiveness, these alternatives are often less

convenient and are frequently messy to use.

A conservative aid to estimating the total heat sink surface

temperature rise (

∆T) above ambient temperature is given

by the following expression:

≈ ⎧⎨

⎩

⎫

⎬

⎭

−

−

∆T

P

80

594

085

143

.

.

.

∆T

PP

Eff

OUT

=

==

−

⎧

⎨

⎩

⎫

⎬

⎭

Case temperature rise above ambient

Device dissipation in Watts

1

1

where

As an example, assume that it is desired to maintain the

case temperature of an ART2815T at +65°C or less while

operating in an open area whose ambient temperature

does not exceed +35°C; then

∆T = 65 - 35 = 35°C

From the Specification Table, the worst case full load

efficiency for this device is 80%; therefore the maximum

power dissipation at full load is given by

()

P

=•

−

⎧

⎨

⎩

⎫

⎬

⎭

=•

=

30

1

80

130

0 25

7 5

.

.. W

and the required heat sink area is

A

=

35

80 7.5

in

HEAT SINK

0.85

•

⎧

⎨

⎩

⎫

⎬

⎭

−=

−143

2

594

318

.

..

1Sil-Pad is a registered Trade Mark of Bergquist, Minneapolis, MN

Thus, a total heat sink surface area (including fins, if any)

of approximately 32 in2 in this example, would limit case

rise to 35°C above ambient. A flat aluminum plate, 0.25"

thick and of approximate dimension 4" by 4" (16 in2 per

side) would suffice for this application in a still air

environment. Note that to meet the criteria, both sides of

the plate require unrestricted exposure to the ambient air.

Inhibiting Converter Output

As an alternative to application and removal of the DC

voltage to the input, the user can control the converter

output by providing an input referenced, TTL compatible,

logic signal to the enable pin 3. This port is internally pulled

“high” so that when not used, an open connection on the

pin permits normal converter operation. When inhibited

outputs are desired, a logical “low” on this port will shut the

converter down.

An open collector device capable of

sinking at least 100 µA connected to enable pin 3 will work

well in this application.

Figure II. Enable Input Equivalent Circuit

A benefit of utilization of the enable input is that following

initial charge of the input capacitor, subsequent turn-on

commands will induce no uncontrolled current inrush.

118K

2N2907A

150K

Enable

Input

Input

Return

150K

2N2222A

2N2222A

V

in

64K

186K

150K

5.6 V

Converter inhibit is initiated when

this transistor is turned off

65K

CR2