STARTUP REGULATOR

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

A5T1.

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

4-189.

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

FO-22.

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

4-187.

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

instantaneous

current

comparator

4-35