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ARH28XXS Series

28V Input, Single Output

Circuit Operation and Application Information

The ARH28XXS series of converters have been designed

using a single ended forward switched mode converter

topology. (Refer to Figure I.) Single ended topologies enjoy

certain advantages in radiation hardened designs in that

they eliminate the possibility of simultaneous turn on of

both switching elements during a radiation induced upset.

In addition, single ended topologies are not subject to

transformer saturation problems often associated with

double ended implementations.

Figure I. Block Diagram

The design incorporates a two-stage LC input filter to

attenuate input ripple current. A low overhead linear bias

regulator provides both a bias voltage for the converter

primary control logic and a stable, well-regulated reference

for the error amplifier. Output control is realized using a

wide band discrete pulse width modulator control circuit

incorporating a unique non-linear ramp generator circuit.

This circuit helps stabilize loop gain over variations in line

voltage for superior output transient response. Nominal

conversion frequency has been selected as 250 KHz to

maximize efficiency and minimize magnetic element size.

Output voltages are sensed and fed back to the controller

using a patented magnetic feedback circuit. This circuit is

designed to be relatively insensitive to variations in

temperature, aging, radiation and manufacturing tolerances

making it particularly well suited to radiation hardened

designs. The control logic has been designed to use only

radiation tolerant components, and current paths include

series resistances to limit photocurrents.

Other key circuit design features include output short circuit

and overload protection, input undervoltage lockout and

an external synchronization input port, permitting operation

at an externally set clock rate. Alternately, a synchronization

output is provided to lock frequencies with another

converter when using more than one converter in a system.

The ARH series of converters is capable of providing

relatively high output power from a package of modest

volume. The power density exhibited by these devices is

obtained by combining high circuit efficiency with effective

methods of heat removal from the die junctions. Good

design practices have effectively addressed this

requirement inside the device. However when operating

at maximum loads, heat generated at the die junctions

depends upon minimally restricted thermal conduction from

the base plate for that heat to be carried away. To maintain

case temperature at or below the specified maximum of

125°C, this heat can be transferred by attachment of the

ARH28XXS to an appropriate heat dissipater held in intimate

contact with the converter base-plate.

Thermal Considerations

Effectiveness of this heat transfer is dependent on the

intimacy of the baseplate to heatsink interface. It is therefore

suggested that a heat-transferring medium possessing

good thermal conductivity be inserted between the

baseplate and heatsink. A material utilized at the factory

during testing and burn-in processes is sold under the

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Sample

& Hold

+Vout

Output

Return

Overload

& Short

Circuit

Pulse Width

Modulator

Primary Bias

& Reference

EMI

Filter

Under-Voltage

Detector

1

3

4

2

+Input

Enable

Sync In

Input

Return

Level

Translator

& Buffer

Error

Amp

+

-

10

9

+Sense

-Sense

5

Sync Out

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