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Nuclear Fundamentals


TO 31R2-2GRC171-2
TM 11-5820-815-14
NAVELEX 0967-LP-544-5010
will be discussed. Refer to figures FO-13 and FO-22.
receives the instantaneous current sense voltage and
Without getting into detailed circuit theory at this time,
compares it to an instantaneous current reference
on-time switch A1Q11 turns output transistor switch
voltage that determines the maximum peak output
A5Q2/Q3 on. This allows current from the unregulated
current limit (about 20 amperes). When the peak output
input  to  flow  through  the  top  half  of  step-up
current exceeds the current limit, the output of the
autotransformer A5T1.  Off-time switch A5A1Q17/Q12
current comparator goes positive to turn off the output
turns output transistor switch A5Q2/Q3 off to disrupt the
transistor switch (A5Q2/Q3 or A5Q4/Q6) that is on at that
current flow. For one cycle of the pulse width gate output
instant.  The instantaneous current limit is set higher
signal, output transistor switch A5Q2/Q3 turns on and
than the average current limit so that transitory
off, the length of on time being determined by the duty
overcurrent conditions that are greater than the average
cycle of the signal. In a similar manner at the beginning
current limit but less than the instantaneous current limit
of the next cycle, on-time switch A5A1Q10 turns output
do not affect the dc-dc converter output.
transistor switch A5Q4/Q6 on.  This allows current to
flow through the lower half of step-up autotransformer
4-188.  The voltage monitor (A5A1U4B) monitors for
Off-time switch A5A1Q17/Q12 turns output
invalid power supply voltages, excessive input voltage, or
transistor switch A5Q4/Q6 off. For this cycle of the pulse
a grounded remote on/off line. Should a power supply
width gate output signal, output transistor switch
voltage be invalid, or the input voltage exceed about 64 V
A5Q4/Q6 turns on then off. This alternating action of
dc, or the remote on/off line go to ground, the output of
turning A5Q2/Q3 on then off and then A5Q4/Q6 on then
the voltage monitor will go to 0 volt to inhibit the pulse
off develops a variable pulse width voltage across step-
width gate. This shuts down the dc-dc converter output
up autotransformer A5T1.  The induced ac voltage
by turning both output transistor switches (A5Q2/Q3 and
across A5T1 is rectified and filtered into an average dc
A5Q4./Q6) off.
voltage. To summarize the regulating process of the dc-
dc converter, variations in output voltage are remote
The following describes individual circuit
sensed and used to vary the duty cycle of the 40-kHz
functions of the dc-dc converter module. Refer to figure
switching voltage that controls the on time of the output
transistor switches.
The output transistor switches
alternately turn on and off to apply voltage to the load in
4-190. STARTUP REGULATOR. The startup regulator
the form of voltage pulses that vary in width as a function
(A5Q5, A5A1Q4, and A5A1Q15 of figure FO-22)
of the remote sensed voltage. By filtering the voltage
produces a regulated output voltage (about 21 V dc) for
pulses, the output voltage at the load appears as dc with
input voltages that vary from 25 to 50 V dc. Removing
the level being a function of the voltage pulse width. As
the ground from the remote on/off line allows a reference
load voltage variations are sensed, the pulse width is
voltage (about 12 V dc) to be developed across zener
varied to counteract the voltage change and maintain a
diode A5A1VR3 and applied to the base of transistors
nearly constant dc voltage level at the load.
A5A1Q15.  Transistor A5A1Q15 functions as an error
amplifier by comparing the reference voltage to a sample
The  instantaneous  current  comparator
of the output voltage developed across resistor A5A1R11
(A5A1U3A) and average current comparator (A5A1U6)
of voltage divider A5A1R63-A5A1R11.
The output
provide overcurrent circuit protection (current limiting).
voltage at the emitter of A5Q5 increases until the voltage
Output current from the dc-dc converter is sensed by
drop across A5A1R11 is one base-to-emitter drop less
current sense transformer A5T2.
The induced ac
than the reference voltage (about 11.5 V dc across
voltage is rectified, filtered, and applied to the two
A5A1R11 or about 21 V dc at the emitter of A5Q5). At
comparators as an average current sense voltage and
this point, A5A1Q15 controls the base current to
an instantaneous current sense voltage. The average
A5A1Q4 and A5Q5 to hold the output at about 21 V dc.
current comparator receives the average current sense
The output of the startup regulator is applied to the dc
voltage and compares it to a current reference voltage
power supply and to the voltage monitor.
that determines the average output current limit of the
dc-dc converter (about 15 amperes).  Whenever the
4-191.  DC POWER SUPPLY.  The dc power supply
output current exceeds the average current limit, the
(A5Q1 and A5A1VR5 of figure FO-22) receives voltage
average current sense voltage exceeds the average
from the startup regulator through diode A5A1CR53 to
current reference voltage. When this occurs for longer
produce +5.1-, 12-, and 25-V dc regulated output
than several cycles of the switching frequency, the output
voltages. The 25-V dc output is approximately 20 V dc
of the average current comparator decreases toward
when being supplied from the startup regulator. Once
zero which causes the voltage sense level to decrease.
the dc-dc converter provides a regulated output, 26-V dc
This in effect reduces the output voltage of the dc-dc
output voltage is applied through diode A5A1CR22 and
converter (26 volt) such that the average current remains
choke A5A1L2 to back-bias diode A5A1CR53 and to
at the average current limit point. In a similar manner the
supply input voltage to the dc power supply

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