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ARH28XXS Series

trade name of Sil-Pad® 4001. This particular product is an

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 sometimes messy to use.

A conservative aid to estimating the total heat sink surface

area (AHEAT SINK) required to set the maximum case

temperature rise (DT) above ambient temperature is given

by the following expression:

∆T

PP

Eff

OUT

=

==

−

⎧

⎨

⎩

⎫

⎬

⎭

Case temperature rise above ambient

Device dissipation in Watts

1

1

where

≈ ⎧⎨

⎩

⎫

⎬

⎭

−

−

∆T

P

80

594

085

143

.

.

.

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

case temperature of an ARH2815S 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

.

..

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.

The user should remember that when operating in the

vacuum of space, this method does not apply and all heat

must be conducted away from the attaching surface.

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

As an alternative to application and removal of the DC

voltage at 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.

Inhibiting Converter Output

A benefit of utilization of the enable input is that following an

initial charge of the input capacitor, subsequent turn-on

commands will induce no uncontrolled current inrush.

Figure II.

Enable Input Equivalent Circuit

Synchronization

When using multiple converters, system requirements may

dictate operating several converters at a common system

frequency. To accommodate this requirement, the

ARH28XXS type converter provides a synchronization

input port.

The sync input port permits synchronization of an ARH

converter to any compatible external frequency source

operating in the band of 225 to 310 KHz. In the event of

failure of an external synchronization source, the converter

will revert to its own internally set frequency. When external

synchronization is not desired, the sync in port may be left

open (unconnected) permitting the converter to operate at

its own internally set frequency.