Power Supplies - Tektronix 7D01 Instruction Manual

Theory of Operation---7D01 (SN B020000 & up)
A holdoff signal to the mainframe is produced by U1410B,
01404, and 01406. The frame pulse toggles the output of
U1410B . The output of U1410B is buffered by 01404 and
01406 and connected to the mainframe.
The 7D01 uses the regulated +15, -15, and +5-volt power
supplies from the oscilloscope mainframe, and derives the
--2 and --4 .8-volt supplies from the +15 and -15-volt
power supplies .
DC-DC Inverter
POWER SUPPLIES
The DC-DC Inverter converts power from the +15 and --15-
volt power supplies to a high-frequency (approximately
25 kHz) square wave . This square wave is applied
to the primary winding of a stepdown transformer .
The outputs of the transformer are full-wave rectified and
filtered to provide the -4 .8-volt supply and a regulated
--2-volt supply .
The 2.5-kHz drive for the inverter is provided by U918 . The
frequency and duty cycle are determined by R918, C918,
and R919 . The output of U918 is coupled through R921
3- 1 2
and C921 to the primary winding of T922 . The two secon-
dary windings of T922 are connected to the inverter power
transistors, Q926 and Q928 which drive the center-tapped
primary winding of inverter transformer T942 .
The four secondary windings of 7942 are connected to-
gether to form two center-tapped outputs. The outputs are
then full-wave rectified and filtered . The filtered do output
from one pair of secondary windings provides the --4.8-volt
supply . The filtered do output from the other pair of secon-
dary windings is regulated to --2 volts by Q962, Q964, and
0968 .
Overload protection for the -2 and -4 .8-volt supplies is pro-
vided by 0903, Q910, and multivibrator U912 . If the -2 or -
4.8-volt supply falls toward ground, 0903 or 0910 turns on,
turning Q908 off. With Q908 off, multivibrator U912 disables
U918 for approximately 1/2 second, then allows it to run for
approximately 10 milliseconds . (Under normal load condi
tions, the -2 and -4 .8-volt supplies will stabilize in less than
10 milliseconds .) When the loads on the -2 and -4.8-volt
supplies are returned to normal, Q903 and Q910 are both
off, and 0908 is conducting . With Q908 conducting,
multivibrator U912 is disabled and U918 is allowed to run
normally .
REV OCT 1982