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M3H-SERIES

The M3H-Series converters utilize a single 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

provides high line rejection.

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

17 volts and will shut down when the input voltage

drops below 16 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 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).

Single output models feature remote sense to

automatically correct for voltage drops between the

converter output anf the load. The output voltage of

dual output models can be adjusted using a single

external resistor.

Design Methodology

The M3H-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. 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 ).