40-kHz MULTIVIBRATOR

TO 31R2-2GRC171-2

TM 11-5820-815-14

NAVELEX 0967-LP-544-5010

4-192. 40-kHz MULTIVIBRATOR. The 40-kHz multi-

down the dc-dc converter until the overvoltage condition

vibrator (A5A1U2) generates an asymmetrical, nominal

is removed. Resistor A5A1R41 is test selected to set the

40-kHz output waveform to provide basic timing for the

trip lever at 64 0.5-V dc input voltage.

dc-dc converter.

Resistor A5A1R39 and capacitor

A5A1C29 set the low-level time of the waveform to about

4-196.

ERROR AMPLIFIER.

The error amplifier

4 microseconds. Resistors A5A1R38 and A5A1R39 and

(A5A1U4A of figure FO-22) compares the remote

capacitor A5A1C29 set the high-level time to about 21

sensed dc-dc converter output voltage to a reference

microseconds. Resistor A5A1R38 is test selected for an

voltage developed across zener diode A5A1VR12 (about

output frequency of 39.2 to 40.8 kHz.

6.2 V dc). The amplified difference voltage determines

the voltage sense level at the input to voltage comparator

4-193.

-5.1-V DC CONVERTER.

The -5.1-V dc

A5A1U3.

For nominal input voltage to the dc-dc

converter (A5A1U1A, A5A1Q6, A5A1Q7, and A5A1Q8 of

converter, the voltage sense level determines the

figure FO-22) converts the 12-V dc power supply voltage

quiescent output voltage from the dc-dc converter. Test

to a -5.1-V dc power supply voltage. Flip-flop A5A1U1A

select resistors A5A1R53 and A5A1R54 allow

receives the 40-kHz timing signal to produce a

adjustment of the error amplifier gain to set the voltage

sense level for 26 0.15-V dc at the dc-dc converter

symmetrical, 20-kHz switching voltage to the base of

transistor A5A1Q8. This switches A5A1Q8 on and off at

output. Fluctuations in output voltage appear as error

the 20-kHz rate which, in turn, alternately switches

voltage around the voltage sense level. At the input to

transistors A5A1Q6 and A5A1Q7 on. When A5A1Q6 is

the voltage comparator, diode A5A1CR52 clamps the

on, capacitor A5A1C19 charges through diode

maximum voltage sense level to about +5.7 V dc. Diode

A5A1CR25 to approximately 10 V dc. When A5A1Q7 is

A5A1CR51 clamps the minimum voltage sense level to

on, A5A1C19 discharges through A5A1C20, A5A1R16,

about -0.6 V dc. Resistor A5A1R49 and capacitor

and A5A1CR24. This action causes A5A1C20 to charge

A5A1C34 provide lag for closed-loop stability.

toward -10 V dc. Zener diode A5A1VR9 clamps the

output to about -5.1 V dc.

4-197. VOLTAGE COMPARATOR AND REFERENCE

RAMP. The voltage comparator (A5A1U3B of figure FO-

4-194. VOLTAGE MONITOR. The voltage monitor

22) converts voltage changes at its input into a variable

(A5A1U4B, A5A1Q9, and A5A1VR10 of figure FO-22)

duty cycle waveform that drives the output switching

compares power supply voltages to produce a validity

circuits to maintain regulation. The variable duty cycle

signal (logic 1) to enable the pulse width gate

waveform is generated by comparing the voltage sense

(A5A1U5A). In addition, the voltage monitor monitors

level from error amplifier A5A1U4A to the positive ramp

the remote on/off line and 25- to 50-V dc unregulated

voltage developed across capacitor A5A1C33 of the

input line. With the remote on/off line open and all power

reference ramp circuit (A5A1Q13, A5A1U5B).

supply voltages valid, the output of A5A1U4B is low

(approximately -3.6 V dc). This back-biases diode

4-198. To develop the ramp voltage, A5A1U5B inverts

A5A1CR55 causing transistor A5A1Q9 to be turned off.

the 40-kHz timing signal to provide a nominal 40-kHz

The positive collector voltage of A5A1Q9 back-biases

switch voltage at the base of A5A1Q13. The 40-kHz

diode A5A1CR49 and A5A1CR50 and enables pulse

timing signal causes the A5A1Q13 to turn on for about 4

width gate A5A1U5A.

If a power supply voltage

microseconds and off for about 21 microseconds. When

becomes invalid, the output of A5A1U4B goes high

A5A1Q13 turns on, A5A1C33 discharges rapidly to

(approximately 10 Vdc). This forward biases A5A1CR55

ground through diode A5A1CR48. When A5A1Q13

to turn on A5A1Q9. When on, A5A1Q9 forward biases

turns off, the dc-dc converter input voltage applied

A5A1CR49 and A5A1CR5[0] and inhibits pulse width

through resistors A5A1R44 and A5A1R45 charges

gate A5A1U5A to shut down the dc-dc converter.

A5A1C33 for about 21 microseconds. Since the RC time

Forward-biased diode A5A1CR50 discharges capacitor

constant of the charge path is about 88 microseconds,

A5A1C34 to decrease the voltage sense level to about 0

the voltage developed across A5A1C33 for 21

V dc. Forward-biased diode A5A1CR49 decreases the

microseconds approximates a ramp voltage. The slope

current reference voltage to about 0 V dc. Reducing

of the ramp voltage is a function of the dc voltage level

these voltages to near zero allows the output of the dc-dc

applied to the input to the dc-dc converter. Zener diode

converter to come up slowly once the fault is removed.

A5A1VR11 limits the maximum ramp voltage to about

Capacitor A5A1C23 and resistors A5A1R24 and

6.2 V dc.

A5A1R25 form a fast attack-slow release time for

momentary faults.

4-199. By comparing the voltage sense level to the ramp

voltage, the voltage comparator (A5A1U3B) generates

4-195. Zener diode A5A1VR10 monitors the 25- to 50-V

an output waveform that changes from low level to high

dc unregulated input voltage. Input voltages which

level at the point where the ramp voltage level equals the

exceed about 64 V dc cause A5A1VR10 to conduct and

voltage sense level. Refer to waveforms of figure FO-13.

forward bias A5A1Q9. This turns A5A1Q9 on to shut

Since

the

ramp

4-36