DC-DC CONVERTER MODULE A5.

TO 31R2-2GRC171-2

TM 11-5820-815-14

NAVELEX 0967-LP-544-5010

converter. In addition, the 40-kHz timing signal provides

allows adjustment of the receive audio output from the

switching voltage to the -5.1-V dc converter to convert

radio set. The variable output of the headset volume

+12 V dc into a -5.1-V dc power supply voltage. When

control is applied to the headset gain input (A4P1-

the radio set is turned off (remote on/off line at ground),

22/A10J13-22) of audio module A4. The variable output

the dc- dc converter is in the standby state (startup

of the receive audio level control is applied to the main af

regulator turned off and the dc-dc converter output at 0

gain input (A4P1-T,/'A10J13-T) of audio module A4.

volt). Removing the ground from the remote on/off line

turns on the startup regulator and causes the voltage

4-180. HEADSET POWER AMPLIFIER. The headset

monitor output to enable the pulse width gate. This in

power amplifier (U8A, Q16, and Q17 of figure FO-21)

turn enables the dc-dc converter to provide +26-V dc

amplifies the headset gain input signal to provide a 100-

regulated output voltage at its output. Once this occurs,

milliwatt minimum headset audio output signal to the

the 26-V dc output voltage is fed back (boot-strapped) to

headset when the headset volume control is set at

the dc power supply to develop the internally-used 25-V

maximum volume. Capacitor C55 couples the input

dc output voltage.

signal through resistor R98 to audio amplifier U8A that

drives complementary output transistors Q16 and Q17.

4-185. The following discussion assumes the dc-dc

Resistors R98 and R104 determine the power amplifier

converter has been on long enough to reach steady-

gain and improve crossover distortion of Q16 and Q17.

state regulation. Refer to the functional diagram of figure

Resistors R102 and R103 in the emitter circuit of Q16

FO-13. Regulation is achieved by remote sensing the

and Q17 provide thermal stability. Resistor R105 and

output voltage at the load.

The error amplifier

capacitor C57 improve high-frequency stability.

(A5A1V4A) compares the remote sense voltage to a

Transformer T3 matches the output impedance of the

reference voltage to produce an output voltage sense

power amplifier to the 600-ohm load of the headset.

level (e). Fluctuations in output voltage at the load

appear as changes in the voltage sense level (∆e error

4-181. MAIN AUDIO POWER AMPLIFIER. The main

voltages). The voltage comparator (A1U3B) compares

audio power amplifier (U8B, Q14, and Q15 of figure FO-

the voltage sense level to a ramp voltage generated by

21) amplifies the main af gain input signal to provide a

the reference ramp circuit (A5A1U5B, A5A1Q13,

100-milliwatt minimum main audio output signal (into 600

A5A1VR11). To develop the ramp voltage, the reference

ohms) when the receive audio level control is set at

ramp circuit processes the 40-kHz timing signal and the

maximum level. The circuit functions identically to the

input voltage level to provide a ramp voltage with 40-kHz

headset power amplifier. Diodes CR12 and CR13

timing and a slope that is a function of the input voltage

provide transient protection to the power amplifier

level. This provides for input regulation of the dc-dc

against high-voltage noise spikes that may be present on

converter. By comparing the voltage sense level to the

the receive audio output lines from the radio set.

reference ramp voltage, the voltage comparator

generates a 40-kHz output switching voltage. The duty

4-182. DC-DC CONVERTER MODULE A5.

cycle of the switching voltage is a function of changes in

the dc-dc converter output voltage or changes in input

4-183. GENERAL. Dc-dc converter module A5 (figures

voltage. Refer to the function diagram of figure FO-13

FO-8 and FO-22) contains two dc-dc converters. One

for effects that changes in input or output voltage have

dc-dc converter is a switching regulator type converter

on the duty cycle. The output of the voltage comparator

that remote senses its output voltage to provide a

is inverted and applied to the pulse width gate

regulated +26-V dc voltage to power amplifier module A8

(A5A1U5A) where it is NANDed with the 40-kHz timing

and to other circuits of the receiver-transmitter. The

signal and the logic 1 voltage monitor (A5A1U4B) output.

second dc-dc converter is a nonregulating, saturating

The positive edge of the resulting 40-kHz output signal

core type converter that provides +10 and -17 V dc to

moves right or left with respect to changes in dc-dc

voltage regulator module A6, and 100 V dc to rf filter

converter input or output voltages. Approximately the

module A7.

last 4 microseconds of the positive half of the signal is

always held in logic 1. This is dead zone time which will

4-184. The regulating dc-dc converter (figure FO-8)

be discussed later. The output of the pulse width gate is

functions as follows: When input voltage (+25 to +50 V

applied to the on-time switches (A5A1U1B, A5A1Q10,

dc unregulated) is first applied to the converter, the

A5A1Q11) and off-time switch A5A1Q17/Q12). The

startup regulator develops a regulated voltage of about

negative edge of the signal controls the on-time

21 V dc from the unregulated input. This voltage is

switches. The moving positive edge controls the off-time

applied to the dc power supply which develops +20 V dc,

switch. The moving positive edge, therefore, controls the

(goes to +25 V dc once dc-dc converter output goes to

on time to off time ratio of the output switching circuit

26 V dc), +12 V dc, and +5.1 V dc to power circuits

4-186. To understand how regulation is achieved, the

internal to the dc-dc converter module. With voltage

switching action of the output switching circuits

applied, the 40-kHz multivibrator generates a 40-kHz

timing signal to provide basic timing for the dc-dc

4-34