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S-SERIES

(28V Input, Single/Dual Output)

Circuit Description

Design Methodology

The S-Series converters utilize a single-ended

forward topology with resonant reset. The nominal

switching frequency is 525KHz. Electrical isolation

and tight output regulation are achieved through the

use of a magnetically coupled feedback. Voltage

feed-forward with duty factor limiting provides high

line rejection.

Output power is limited under any load fault condition

to approximately 135% for singles and 145% for

duals of rated. An overload condition causes the

converter output to behave like a constant current

source with the output voltage dropping below

nominal. The converter will resume normal operation

when the load current is reduced below the current

limit point. This protects the converter from both

overload and short circuit conditions. There are no

latching elements included in the load fault protection

circuits to eliminate the possibility of falsely triggering

the protection circuits during single event radiation

exposure.

An under-voltage protection circuit prohibits the

converter from operating when the line voltage is

too low for safe operation. The converter will not

start until the line voltage rises to approximately 12V.

An external enable port is provided to control

converter operation. This input is intended for

operation with an open collector transistor drive or

a relay closure to the input return. The pin may be

left open for normal operation and has a nominal

open circuit voltage of 10.5V.

The S-Series was developed using a proven

conservative design methodology, which includes

selecting radiation tolerant, and established

reliability components and fully derating to the

requirements of MIL-STD-975 and MIL-STD-1547

except for the CDR type of capacitors, a capacitor

with 50V rating is used for in-circuit voltage stress of

less than10V. Heavy derating of the radiation-

hardened power MOSFET virtually eliminates the

possibility of SEGR and SEB. A magnetic feedback

circuit is utilized instead of opto-couplers to minimize

temperature, radiation and aging sensitivity. PSPICE

was used extensively to predict and optimize

circuit performance for both beginning and end-of-

life. Thorough design analyses include Radiation

Susceptibility (TREE), Worst Case, Stress, Thermal

and Reliability (MTBF).