Tektronix 2445, Instruction Manual

Page: 83 / 351

Theory o f Operation—2445 Service
The +21-V level begins to drain down as the control
1C draws current from C1025. If the Preregulator Control
1C doesn't start the switching supply (and thus recharge
C l025 and C l023 via CR1022) by the time the voltage
across C1025 reaches about +8 V, Q1021 w ill turn off.
Resistor R1024 pulls the base of Q1022 low and turns
that transistor o ff also. (Capacitor C l025 would only
discharge low enough to turn o ff the transistors under a
fault condition.) In this event. C l025 would then charge
again to +21 V, and the start sequence would repeat.
Normally, the control 1C w ill start Inverter action before
the +8-V level is reached, and current is drawn through
T1050 via Q1Q50. This induces a current in the secondary
winding o f T1050 and charges C1025 positive via diode
CR1022. The turns ratio of T1050 sets the secondary
voltage at approximately +15 V; and, as long as the supply
is being properly regulated, C1025 w ill be charged up to
that level and held there.
PREREGULATOR START-UP. As the supply fo r the
Preregulator Control 1C is established, an internal switching
oscillator begins to run. The oscillator generates a repetitive
triangular wave (as shown in Figure 3-11) at a frequency
determined prim arily by R1032 and Cl 032. The simplified
schematic of Figure 3-12 illustrates the voltage control
functions o f U1030.
3-41
Figure 3-11. Timing relationships o f the Inverter Drive signals.