TTI TSX Series Service Manual page 9

Auxiliary Supplies - Several control auxiliary supplies are derived from 50/60 Hz auxiliary
transformer T1 secondaries:
(i) +5V 270mA DC via BR2,C27,IC4 for linear control, ADC, (DACs) and drive for the two
meter LED displays.
(ii) –5V 50mA DC via D2,C28,IC3 for linear control, ADC (and DACs).
(iii) 3V 1.5A DC floating, unstabilised via D18,D42,D26 for driver Q4 collector.
(iv) 10V 0.9A AC via PJ2 for the CPU board.
(v) 12V DC via BR1, C25 and IC2, supplies IC1, IC11 and Q20.
Only programmable versions incorporate CPU board and DACs. Reference diodes D35 and D36
provide +2.45V and -2.45V reference voltages respectively. R80 bleeds current from +5V to the
negative output terminal to provide a small output current sink capability.
Inadequate auxiliary control supplies are detected by positive and negative voltage detectors
R42,D51,R48,Q23 and R43,R50,Q23 respectively via on delay Q21,C48,IC12-B. Status lines
AVAUXT and AVAUXT bar go high and low respectively only when IC4 input, IC4 output and IC3
output voltage have maintained about +7, +4 and -3.5 VDC respectively for about 150 msec.
Linear Regulator Collector Voltage Control System - Virtual earth error amplifier IC6-B current
drives OPTO1 input via D49,Q24 so as to maintain low collector-emitter voltage across the linear
regulator power transistors. At zero collector current the collector voltage sets to about +1.85V
determined by the -5V supply and R127,R136. At higher currents increasing voltage drops across
emitter sharing and current sense R54 resistors and, in 35V/10A supplies current feed-forward
via R126, give a linear reduction in collector- emitter voltage to about 1.2V at the maximum output
current of 5A per device. This arrangement provides increased Vce "headroom" at low output
currents to accommodate the larger transient dip in switchmode output voltage that follows
maximum current demand without increasing maximum dissipation. R129,R130,R11,C38 assist
stability while D31,D39,Q28 speed amplifier overload recovery. R128 is selected to normalise
control loop gain.
Switchmode Shutdown Control - In normal operation ENABLE and ENABLE bar are high and
low respectively. Even if IC6-B output goes transiently negative, reverse biassing D49, OPTO1
input is still maintained at about 300 microamps by R133,Q24; low enough for PWM output to
cease although Q13 in the switchmode section remains on and IC1 active.
The switchmode section is shut down by driving ENABLE and ENABLE bar low and high
respectively; turning off Q24 and driving OPTO1 input and output currents to zero. Q13 in the
switchmode section then turns off inhibiting PWM output and holding soft start capacitor C18
discharged. IC6-B output is also driven negative via D62 to avoid switchmode surging on re-
enablement.
Switchmode Maximum Current Limiting - The maximum current demanded from the
switchmode section will be the sum of the maximum DC output current and the maximum
transient current required to charge switchmode filter capacitor(s) C73,(C74). The former is
limited to 10/20A, depending on the model while the latter is proportional to the maximum positive
rate of change of switchmode output voltage.
Diode D30 limits the current from IC6-B input via R132 to about 2 microamps. This is balanced by
the current from feedback capacitor C39 limiting the rate of increase of power transistor collector
voltage to about +1.3 V/msec, corresponding to a peak current into the filter capacitors of about
1.3/6.3A in the 35V/10A and 18V/20A units respectively provided the output voltage remains
constant.
Protection outside constant voltage output mode is provided by circuitry around Q19. If the rate of
change of output voltage approaches about +6/+3V per sec for 35V/10A and 18V/20A units
respectively differentiating network C52,R124 begins to turn off Q19 injecting current into the
amplifier input via R123,D17 to limit the switchmode section output. This mechanism protects
even against the unlimited rate of rise of output voltage following connection of batteries to the
output terminals.
8