Z28 Datasheet, PDF (2/7 Pages) International Rectifier – LOW VOLTAGE HIGH EFFICIENCY RADIATION HARDENED DC/DC CONVERTER

Z-SERIES
Circuit Description
The Z-Series converters utilize a single ended forward
topology with resonant reset. The nominal 250kHz switch-
ing frequency has been selected to optimize magnetic
element size and switching loss. Electrical isolation be-
tween the primary and secondary sides of the converter
is assured through exclusive use of magnetic coupling for
all signals crossing the primary/secondary barrier. Volt-
age feed-forward is utilized to provide high line rejection.
To achieve the high efficiencies characterizing these con-
verters, synchronous rectifiers have been used in place
of rectifying diodes thus minimizing the conduction losses
associated with those elements.
An internal EMI filter reduces the conducted emissions on
the input power leads. A two-stage output filter reduces
the typical output fundamental 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 pro-
tects the converter from both overload and short circuit
conditions. The current limit point exhibits a slightly nega-
tive 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. This hys-
teresis voltage reduces the possibility of line noise inter-
fering with the converter’s start-up and shut down. An
input overvoltage circuit is also in place that will shut down
the converter if the input voltage exceeds approximately
60 V, thereby precluding the possibility of exceeding the
voltage rating of the synchronous rectifiers.
An external inhibit port is provided to control converter
operation. The nominal threshold relative to the input re-
turn (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.
A synchronization input is provided allowing operation of
the converter synchronously with a user provided fre-
quency source. This input permits synchronization of the
converter to any compatible external frequency source
operating in the band of 225 to 300 KHz and is edge
triggered with synchronization initiated on the negative
transition. This signal should be a negative going pulse
referenced to the input return and have a 20% to 80%
duty cycle. Compatibility requires the negative transition
time to be less than 100 ns with minimum pulse amplitude
of +4.25 volts referred to the input return. In the event of
failure of an external synchronization source, the con-
verter will revert to its own internally set frequency. When
external synchronization is not desired, the sync input
may be left open (unconnected) permitting the converter
to operate at its’ own internally set frequency.
Remote sense is provided to compensate for conduction
losses in the connections between the converter outputs
and the load. The use of this sense feature permits com-
pensating for as much as 250 mV in both the output and
return lines. (500 mV each leg, round trip)
An output adjust pin is provided permitting the user to
adjust the output voltage by approximately ± 5%. Adjust-
ment is accomplished by connecting a resistor brtween
the adjust pin (Pin 10) and +Sense (Pin 9) or –Sense (Pin
11). The direction of the output change is opposite the
sign of the sense pin to which it is connected, that is
connecting to the – Sense pin causes an increase and
connecting to the +sense pin causes a decrease in output
voltage.
Design Methodology
The Z-Series was developed using a proven conserva-
tive design methodology that includes selecting radiation
tolerant and established reliability components and fully
derating to the requirements of GSFC PPL-21, MIL-STD-
975 and MIL-STD-1547. Heavy derating of the radiation-
hardened power MOSFET virtually eliminates the possi-
bility of SEGR and SEB. A magnetic feedback circuit is
utilized instead of opto-couplers to minimize temperature,
radiation and aging sensitivity. PSPICE was used exten-
sively to predict and optimize circuit performance for both
beginning and end-of-life. Thorough design analyses in-
clude Worst Case, Stress, Thermal and Reliability (MTBF).
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