M3L-SERIES

(+28V Input, Single/Dual Output)

2

2016-09-21

Circuit Description

The M3L-Series converters utilize a single-ended forward

topology with resonant reset. The nominal switching frequency is

500 kHz. 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.

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

de-rating to the requirements of MIL-STD-975 and MIL-STD-

1547. Heavy de-rating 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).