Tektronix SPG 1000, Instruction Manual

Page: 102 / 282

8–6
SPG 1000
Input, AC to DC Converter, and Voltage
Doubler
This circuitry filters and rectifies the input AC
voltage, charging capacitors C845 and C865 to
approximately 320 VDC.
The line current passes through line filter LF950,
fuse F940, and power switch S930, and is applied
to rectifier CR820. At the input of CR820, P810
is used to select between 110 V and 220 V opera-
tion. If J810 is placed on P820, for 220 V opera-
tion, CR820 works as a full-wave rectifier and
C845 and C865 act in series, charging to the peak
voltage (approximately 320 VDC) during the first
part of each one-half cycle. They then maintain
that voltage through the rest of the cycle, as the
input voltage and current fall to zero.
If, on the other hand, P810 is placed on J810 (for
110 V operation), CR820, C845, and C865 act as
a half-wave rectifier and voltage doubler. During
the positive half-cycle of the AC input only one
of the diodes within CR820 conducts, charging
C865 to the peak positive voltage. A different
diode within CR820 conducts during the negative
half-cycle, and charges C845 to the negative
peak. The total voltage across C845 and C865 is
then approximately 320 VDC.
E920 and E820 limit voltage surges on the input
that might pass the line filter, while R865 and
R845 discharge C865 and C845 when the power
is off. DS720 and associated parts form a relax-
ation oscillator, so DS720 blinks when the instru-
ment is powered up. L700, C5, C6, and C700
form a filter to keep noise developed by the Pow-
er Supply from getting onto C865 and C845 and
out the line cord.
Kick Starter, Housekeeping Supply
These circuits supply the power to start and
maintain oscillation of the Pulse Width Modula-
tor, so long as the input AC voltage is sufficient
to maintain regulation. When the power supply is
first turned on, C656 charges through R921.
When the charge across C656 reaches approxi-
mately 16 V, U722 begins to switch Q638 on and
off through the base drive circuitry (Q648, and
associated circuits).
The power to maintain the +16 V charge on C656
is now provided by the housekeeping winding of
T1, pins 5 and 6, through CR600. If there is in-
sufficient power to maintain the charge on C656
for any reason, such as the removal of P660, then
the charge on C656 is quickly depleted. U722
will turn off when the voltage on C656 drops to
approximately 10 V. Then, C656 will slowly
charge again through R921, and the kick start se-
quence will be repeated. P660 may be used as a
troubleshooting jumper, to trigger and verify the
kick start sequence before applying power to T1
through P500.
Power Inductor Operation
The heart of this power supply is T1, the
multi-winding power inductor. The operation of
T1 is as follows (see Figures 8–2 and 8–3).
Figure 8–2: Power Inductor Circuit
Inductor T1 is initially uncharged (has zero
magnetic flux). Q638, acting as a switch, is
turned on by the base drive from U722. This
places the charge developed on C545 and C865
(approximately 320 V) across the primary
winding. The polarity of this charge is such that
the voltages induced in the secondaries all reverse