M3G-SERIES

(70V Input, Single/Dual Output)

2

2016-06-30

Circuit Description

The M3G-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 reduces the conducted emissions to

less than 5mArms 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 58 volts and will shut down

when the input voltage drops below 54 volts. The four

volts 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

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.

Design Methodology

The M3G-Series was developed using a proven conserva-

tive design methodology which includes selecting radia-

tion tolerant and established reliability components and

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

MIL-STD-1547 except for the CDR type ceramic capaci-

tors, a capacitor with 50V rating is used for in-circuit volt-

age stress of less than 10V. Careful sizing of decoupling

capacitors and current limiting resistors minimizes the

possibility of photo-current burn-out. Heavy d-erating 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 Rad SPICE

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).