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M3G-SERIES

(28V Input, Single/Dual Output)

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 two-stage output filter

reduces the typical output ripple to less than 20mV

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 under-voltage lockout circuit prohibits the

converter from operating when the line voltage is too

low to maintain the output voltage. The converter will

not start until the line voltage rises to approximately

16.5 volts and will shut down when the input voltage

drops below 15.5 volts. The one volt of hysteresis

reduces the possibility of line noise interfering with

the converter’s start-up and shut down.

An external inhibit port is provided to control converter

operation. The nominal threshold relative to the

input return (pin 2) is 1.4V. If 2.0 volts or greater are

applied to the Inhibit pin (pin 3) then the converter

will operate normally. A voltage of 0.8V or less will

cause the converter to shut-down. The pin may be

left open for normal operation and has a nominal

open circuit voltage of 4.0V.

Design Methodology

The M3G-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 ceramic capacitors, a capacitor with 50V

rating is used for in-circuit voltage stress of less than

10V. Careful sizing of decoupling capacitors and

current limiting resistors minimizes the possibility of

photo-current burn-out. 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 and RadSPICE were 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,

Failure Modes and Effects (FMEA) and Reliability

(MTBF).

Synchronization input and output allow multiple

converters to operate at a common switching

frequency. Converters can be synchronized to one

another or to an externally provided clock. This can

be used to eliminate beat frequency noise or to avoid

creating noise at certain frequencies for sensitive

systems.

Remote sense is provided on the single output

models to compensate for voltage drops in the

interconnects between the converter and the load.

The output voltage of dual output models can be

adjusted by a single external resistor.

The M3G-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.