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M3L-SERIES

The M3L-Series converters utilize a single-ended

forward topology with resonant reset. The nominal

switching frequency is 500kHz. 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 and protection against output over

voltage in the event of an internal control loop failure.

This mechanism limits the maximum output voltage

to approximately 20% over the nominal regardless of

the line voltage.

Circuit Description

An internal EMI filter allows the converter to meet the

conducted emissions requirements of MIL-STD-461C

on the input power leads. A single-stage output filter

reduces the typical output ripple to less than 50mV

peak-to-peak.

Output current is limited under any load fault condition

to approximately 125% 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. The current limit point exhibits a slightly

negative temperature coefficient to reduce the

possibility of thermal runaway.

An external inhibit port is provided to control converter

operation. The converter’s operation is inhibited

when this pin is pulled low. It is intended to be driven

by an open collector logic device. The pin may be

left open for normal operation and has a nominal

open circuit voltage of 10.5V with respect to the input

return (pin 2).

The output voltage of all models can be adjusted

using a single external resistor.

Design Methodology

The M3L-Series was developed using a proven

conservative design methodology which includes

selecting radiation tolerance and established

reliability components and fully derating to the

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

Heavy derating of the power MOSFET gate and drain

voltages minimize 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 ),

Stress, Thermal, and Reliability ( MTBF ).