M3H-SERIES

(+28V Input, Single/Dual Output)

2

2016-05-19

Circuit Description

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.

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

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).