ARH28XXXXS SERIES

(28V Input, Single Output)

8

2016-06-29

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

(

T) above ambient temperature is given by the following expres-

sion:

Where

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

and the required heat sink area is

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.

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T

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Case temperature rise above ambient

Device dissipation in Watts

1

1

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30

1

80

130 0 25

7 5

.

.. W

A

=

35

80 7.5

in

HEAT SINK

0.85

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2

594 318

.

..

Inhibiting Converter Output

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 he converter down. An open collector

device capable of sinking at least 100 µA connected to

enable pin 3 will work well in this application.

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

ARH28XXXXS 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.

When an external frequency source is not available, an

internal clock signal is provided through appropriate buffering at

the sync out port. This port can drive a minimum of 3

ARH sync in ports thereby allowing all to operate at the

same clock frequency.