M3H280512T Datasheet, PDF (2/8 Pages) International Rectifier – HIGH RELIABILITY RADIATION HARDENED DC/DC CONVERTERS

M3H-SERIES
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
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 two-stage output filter
reduces the typical output ripple to less than 20mV
peak-to-peak.
The main ( +5 volt ) output is regulated by the control
loop and typically exhibits better than 1% regulation.
The auxiliary ( ±12 volt or ±15 volt ) outputs are
maintained through tight coupling in the power
transformer and main output filter inductor and
typically exhibit better than 5% regulation. The main
output and auxiliary outputs are isolated from each
other. Output power 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).
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 ).
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