OUTPUT SWITCHING CIRCUIT

TO 31R2-2GRC171-2

TM 11-5820-815-14

NAVELEX 0967-LP-544-5010

A5A1C25 to reverse bias on-time switch A5A1Q10. This

cycle signal, both output transistor switches are off. The

keeps A5A1Q10 turned off. When A5A1Q11 turns on, a

40-kHz timing signal assures both output transistors are

pulse of current (startup pulse current) flows through

turned off for approximately 4 microseconds out of each

winding 4-3 of A5T3 to ground. This induces a positive

switching cycle. This is dead zone time which allows the

voltage across A5T3 winding 7-6 (plus at 7) and a

output switching circuit to settle. The voltage monitor

negative voltage across winding 8-9 (negative at 8). The

output, when logic 0, shuts down the dc-dc converter by

positive-induced voltage turns A5Q2/Q3 on.

The

causing the output of A5A1U5A to remain at logic 1.

negative-induced voltage charges capacitors A5A3C3,

A5A3C13, and A5A3C14 through resistor to develop a

4-204. OUTPUT SWITCHING CIRCUIT. Refer to figure

negative voltage at the base of A5Q4,/Q6. This assures

FO-22. The dc-dc converter output switching circuit

A5Q4/Q6 remains turned off while A5Q2/Q3 is turned on.

consists of the following functional circuits: on-time

When A5Q2,/Q3 turns on, current flow through winding

switches A5A1QO1, A5A1Q11; off-time switch

11-10 of A5T3 induces a positive voltage across winding

A5A1Q17/Q12; current base drive transformer A5T3;

7-6 (plus to 7) to keep A5Q2/Q3 turned on once the

output transistor switches A5Q2/Q3 and A5Q4/Q5; step-

startup pulse terminates. Transistor switch A5Q2/Q3

up autotransformer A5T2, and rectifier/filter A5CR15-

remains on until off-time switch A5A1Q17/Q12 turns it

CR16, A5L1-A10C14 (capacitor A10C14 is located on

off.

the chassis). Because of their interrelationship, these

circuits will be discussed as one unit.

4-207. In the off-time switching circuit, the logic 1 to

logic 0 transition from pulse width gate A5A1U5A that

4-205. In general, the output switching circuit functions

turned A5A1Q11 on, also applies a ground to the

as follows: On-time switch A5A1Q11 controls the turn-

cathode end of diode A5A1CR42. This back-biases

on time of output transistor switch A5Q2/Q3. On-time

diode A5A1CR43 causing A5A1Q17/Q12 to be turned off

switch A5A1Q10 controls the turn-on time of output

as long as the signal is at logic 0. When the output of

transistor switch A5Q4/Q6. Only one transistor is on at

pulse width gate A5A1U5A changes from logic 0 to logic

any one time. Off-time switch A5A1Q17/Q12 controls

1, the ground is removed from A5A1CR42. This results

the turnoff time of both A5Q2/Q3 and A5Q4/Q6. Current

in the reverse biasing of A5A1CR42 and forward biasing

base drive transformer A5T3 couples start and stop base

of A5A1CR43 to turn A5A1Q17/Q12 on. When on,

drive signals from the on-time and off-time switches to

A5A1Q12 shorts winding 1-5 to A5T3 to ground through

the base circuits of A5Q2/Q3 and A5Q4/Q6 to switch

diodes A5A1CR45 and A5A1CR46. The shorted primary

them on and off. In addition, A5T3 couples back a base

winding of A5T3 squelches the induced voltage in the

drive signal to sustain base drive once A5Q2/Q3 or

secondary winding causing A5Q2/Q3 to turn off.

A5Q4/Q6 is turned on. Step-up auto-transformer A5T1

combines the current flow through A5Q2/Q3 and

4-208. This completes one-half cycle of the output

A5Q4/Q6 into a stepped up ac voltage (actually a series

switching sequence. For one complete cycle of the

of variable width, voltage pulses). Rectifier diodes

variable duty cycle waveform (output of A5A1U5A),

A5CR15 and A5CR16 and low-pass filter A5L1-A10C14

output transistor switch A5Q2/Q3 turns on, then off, to

rectify and filter the ac voltage into an average dc output

allow current flow through winding 1-2 of step-up auto-

voltage from the dc-dc converter.

transformer A5T1. This induces a positive-to- negative

voltage pulse across winding 4-2 of A5T1 that appears at

4-206. The variable duty cycle waveform from pulse

low-pass filter A5L1-A10OC14 through forward-biased

width gate A5A1U5A is fed to both the on-time switching

rectifier diode A5CR15.

circuit (A5A1U1B, A5A1Q10, and A5A1Q11) and off-time

switching circuit (A5A1Q17/Q12).

In the on-time

4-209. In a similar manner, the next cycle of the variable

switching circuit, each logic 1 to logic 0 transition applied

duty cycle waveform controls the on time of output

to the clock input of flip-flop A5A1U15 causes its outputs

transistor switch A5Q4/Q6. The logic 1 to logic 0

to change state. This results in a 20-kHz square wave at

transition causes flip-flop A5A1U1B to change state.

the true and false outputs of A5A1U1B. Because of the

The resulting logic 1 to logic 0 transition from the true

frequency division, one complete cycle of the output

output of A5A1U1B turns on-time switch A5A1Q10 on

switching circuit requires two cycles of the variable duty

allowing a pulse of current to flow through winding 2-3 of

cycle waveform. As a starting point, assume that the first

A5T3. The induced positive voltage across winding 8-9

logic 1 to logic 0 transition causes the false output of

(plus at 8) turns A5Q4/Q6 on. Once A5Q4/Q6 turns on,

A5A1U1B to change from logic 1 to logic 0. The

current flow through winding 12-13 of A5T3 induces a

negative-going transition is coupled through capacitor

positive voltage across winding 8-9 to keep A5Q4/Q6

A5A1C26 causing base current to flow in on-time switch

turned on. Off-time switch A5A1Q17/Q12 turns A5Q4/-

A5A1Q11. This turns A5A1Q1 on for the length of time

Q6 off when the variable duty cycle waveform changes

required for A5A1C26 to charge. At the same time, the

from logic 0 to logic 1. The current flow through

logic 0 to logic 1 transition from the true output of

A5A1U1B

coupled

through

capacitor

4-38