Theory of Operation—2465B/2467B Service
The Preregulator Control circuit consists primarily of
control IC U1030, its switching buffers, and its power sup-
ply components. The control IC senses voltage on the
pri-
mary winding of T2060 and varies the "on time" of a
series-switching transistor, depending on whether the
sensed voltage was too high or too low. The switching
transistor Q1050, rectifier CR1050, choke T1050, and
capacitor C1050 form a buck-switching regulator circuit.
The output voltage at W1060 is proportional to the pro-
duct of the rectified line voltage on C1020-C1022 and the
duty cycle of Q1050. In normal operation, Q1050 is on
about one-half the time. When Q1050 is off, current flows
to W1060 and T1060 through CR1050.
PREREGULATOR CONTROL POWER SUPPLY. Since
the Preregulator Control network controls supply startup
and preregulates the secondary supplies, an independent
power source must be established for it before any of the
other power supplies will operate. The independent power
supply for the control circuitry is composed of
Q1021,
Q1022,
and associated components.
Initially, when instrument power is applied, the positive
plate of capacitor C1025 is charged toward the positive
rectified line voltage through R1020. The voltage at the
base of Q1022 follows at a level determined by the voltage
divider composed of R1022, R1024, CR1023, and the load
within U1030. When the voltage across C1025 reaches
about +21 V, the base voltage of Q1022 reaches +6.8 V
and Q1022 turns on, saturating
Q1021.
The +21 V on
the emitter of Q1021 appears at its collector and estab-
lishes the positive voltage supply for the Preregulator IC.
With Q1021 on, R1024 is placed in parallel with R1022,
and both Q1022 and Q1021 remain saturated.
The +21 V level begins to drain down as the control IC
draws current from C1025. If the Preregulator Control IC
doesn't start the switching supply (and thus recharge
C1025 and C1023 via CR1022) by the time the voltage
across C1025 reaches about +8 V, Q1021 will turn off.
Resistor R1024 pulls the base of Q1022 low and turns
that transistor off also. (Capacitor C1025 would only
discharge low enough to turn off the transistors under a
fault condition.) In this event, C1025 would then charge
again to +21 V, and the start sequence would repeat.
Normally, the control IC will start Inverter action before the
+
8
V level is reached, and current is drawn through
T1050 via Q1050. This induces a current in the secondary
winding of T1050 via Q1050. This induces a current in the
secondary winding of 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 will be
charged up to that level and held there.
PREREGULATOR START-UP. As the supply for the
Preregulator Control IC 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 primarily by R1032 and C1032. The
simplified schematic of Figure 3-12 illustrates the voltage
control functions of U1030.
As the Preregulator power supply turns on, capacitor
C1034 charges from the +5 V reference level toward
ground potential through R1034 and R1037. As it does,
the voltage at pin 4 (one input of Dead-Time Comparator
U1) will pass through the positive-peak value of the
tri-
angular waveform on the other input of the Dead-Time
Comparator. The comparator will then begin outputting
narrow pulses that become progressively wider as the
voltage on pin 4 settles to zero volts. These pulses drive
switching transistor Q1050, and their slow progression
from narrow to wide causes the various secondary sup-
plies to gradually build up to their final operating levels.
The slow buildup prevents a turn-on current surge that
would cause the current-limit circuitry to shut down the
supply.
During startup, capacitor C1072 acts as a substantial
load,
and a relatively large current flows in the windings of
T1050 for the first few cycles of Preregulator switching.
These strong current pulses ensure that storage capacitor
C1066 becomes charged sufficiently to start the Inverter
Drive circuit. Once the Inverter Drive stage is operating,
the normal switching current through T1050 maintains the
required charge on C1066. (The Inverter Drive power sup-
ply is discussed later in this description.)
Dead-Time Comparator U1 is referenced at approxi-
mately 0.1 V above the ground level at pin 4 (established
when C1034 becomes fully charged) and outputs a nar-
row, negative-going pulse that turns off switching transis-
tor Q1050 for a portion of each switching cycle. This off
time ensures that flip-flop U1064B in the Inverter Drive cir-
cuit toggles every cycle (thereby maintaining the proper
duty cycle), independent of the voltage conditions being
sensed by the remainder of the voltage control circuitry.
PREREGULATION. Once the initial charging at power-
up is accomplished, as just described, the voltage-sensing
circuitry begins controlling the Inverter switching action.
The actual voltage sensing is done by error amplifier U2.
The level at the center tap of output transformer T1060 is
applied to pin 1 and is compared to the reference
established by R1045 and R1046 at pin 2. If the sensed
level at pin 1 is lower than the reference level (as it will
always be for the first few switching cycles), the of error-
amplifier U2 will be LO. The LO, applied to the inverting
input of U3, results in a long-duty-cycle drive signal to
3a-47
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