AVERAGE CURRENT COMPARATOR

TO 31R2-2GRC171-2

TM 11-5820-815-14

NAVELEX 0967-LP-544-5010

output current limit point will lower and cause the actual

voltage varies as a function of input voltage applied to

current limit to be about 12 amperes.

the dc-dc converter and the voltage sense level varies as

a function of the output voltage from the converter, the

4-201. INSTANTANEOUS CURRENT COMPARATOR

duty cycle of the voltage comparator output is a function

The instantaneous current comparator (A5A1U3A of

of variations in both input and output voltages. The

figure FO-22) compares a current sense voltage derived

variable duty cycle signal, when applied to the output

from actual output current flow from the dc-dc converter

switching circuits, controls switching times to counteract

to a reference voltage that determines the instantaneous

the voltage variations that originally caused the change in

(peak) current limit of the dc-dc converter.

The

duty cycle.

instantaneous current reference voltage (about 2 V dc) is

developed through voltage divider action of resistors

4-200. AVERAGE CURRENT COMPARATOR. The

A5A1R46, A5A1R47, A5A1R48, and A5A1R70 and is

average current comparator (A5A1U6 of figure FO-22)

applied to the inverting input of A5A1U3A. The voltage

compares a current sense voltage derived from actual

divider sets the instantaneous current limit (about 20

output current flow from the dc-dc converter to an

amperes peak) higher than the average current limit. To

average current reference voltage that determines the

develop the instantaneous current sense voltage, current

average current limit of the dc-dc converter. The

sense transformer A5T2 senses current flow through the

average current reference voltage (about 1.5 V dc) is

turned-on output transistor switch (A5Q2/Q3 or

developed through voltage divider action of resistors

A5Q4/'Q6). The resulting ac voltage from A5T2 is full-

A5A1R46, A5A1R47, A5A1R48, and A5A1R70 and is

wave rectified by diodes A5A1CR31 through A5A1CR34

applied to the noninverting input of A5A1U6. Resistor

and applied through diode A5A1CR53 to the noninverting

A5A1R47 is test selected to set the average output

input of A5A1U3A. When the instantan-eous current

current limit of the dc-dc converter module to about 15

sense voltage (peak output current) exceeds the current

amperes.

To develop the average current sense

reference voltage (peak current limit), the output of

voltage, current sense transformer A5T2 senses current

A5A1U3A goes positive. Transistor A5A1Q14 inverts the

flow through the turned-on output transistor switch

positive voltage and applies a logic 0 voltage to the input

(A5Q2/Q3 or A5Q4/Q6). The resulting ac voltage from

of pulse width gate A5A1U5A. This causes the output of

A5T2 is full-wave rectified by diodes A5A1CR31 through

the pulse width gate to go to a logic 1 to turn off the

A5A1CR34 and applied to a fast attack-slow release

output transistor switch that is on at that instant. This

peak voltage detector composed of diode A5A1CR13,

circuit action will continue until the overload condition

resistor A5A1R67, and capacitor A5A1C39. The dc

goes away and the peak output current becomes less

voltage developed across resistor A5A1R67 and stored

than the current limit.

by capacitor A5A1C39 is the average current sense

voltage and is proportional to the level of current flow

4-202. To prevent the voltage and current comparators

through the output transistor switches. This voltage,

from tripping more than once per period of the 40-kHz

when applied through resistor A5A1R68 to the inverting

timing signal, diode A5A1CR47 makes the comparator

input of A5A1U6, is compared by A5A1U6 to the average

circuit self-latching. When the output of either the

current reference voltage. When the average current

voltage comparator or current comparator goes positive,

sense voltage (output current) is less than the average

the positive voltage is fed back through A5A1CR47 to the

current reference voltage (output current limit), the output

noninverting input of A5A1U3A. This latches the output

voltage from A5A1U6 is positive enough to reverse bias

of A5A1U3A positive. The latch is reset by the 40-kHz

diode A5A1CR14. With A5A1CR14 reverse biased, the

timing signal during dead zone time. Application of the

output of A5A1U6 has no effect on the input voltage to

0-voltage portion of the 40-kHz timing signal to the

A1U3B. As the output current increases and approaches

strobe input of A5A1U3 (pins 2 and 8) forces the output

the current limit point, the increase in average current

of A5A1U3 to 0 volt independent of its inputs.

sense voltage causes the output of A5A1U6 to become

less positive and forward biases A5A1CR14. With

4-203.

PULSE WIDTH GATE.

Pulse width gate

A5A1CR14 forward biased, the output of A5A1U6 takes

A5A1U5A NANDs the inverted (by A5A1Q14), variable

control of the voltage sense level input to voltage

duty cycle signal from the output of the voltage/current

comparator A5A1U3B. When this occurs, output current

comparator (A5A1U3) with the 40-kHz timing signal and

flow, as opposed to output voltage level, controls the duty

the voltage monitor output signal. Whenever the output

cycle of the voltage com- parator output. Any additional

of the voltage monitor is valid (logic 1), A5A1U5A applies

increase in output current flow causes the duty cycle (on

the variable duty cycle signal to the output switching

time of output switching transistors) to decrease. The

circuit. During the logic 0 portion of the variable duty

reduction in duty cycle lowers the output voltage and

cycle signal, one of the output transistor switches

causes the average output current to start limiting at

(A5Q2/Q3 or A5Q4/Q6) is on the length of time

approximately 15 amperes. Due to resistor A5A1R75

determined by the width of the logic 0 pulse. During the

feeding back output voltage to the current reference

logic

portion

the

variable

duty

input

A5A1U6,

the

4-37