Theory of Operation—2213 Service
Q917 will be biased into its active region and the resulting
collector current will cause a voltage drop across R916. This
voltage drop will bias on Q915, and the positive feedback
through R914 will reinforce the turn-on of Q917. Thus Q915
and Q917 will drive each other into saturation very quickly.
Once Q915 is on, U920 will begin to fucntion.
Pulse-width modulator 1C U920 controls the output
voltage of the Preregulator by regulating the duty cycle of
the pulse applied to the gate of Q933. It utilizes an oscillator
whose frequency is determined by R920 and C920
(approximately 40 kHz) and whose output at pin 5 is a
sawtooth voltage. An internal comparator compares this
sawtooth voltage with the output voltage produced by the
two error amplifiers. Whenever the sawtooth voltage is
greater than the error-amplifier output voltage, Q933 is
biased on to supply current to both C934 and the rest of the
circuitry. The two error amplifiers are used to maintain a
constant output voltage and to monitor the output current of
the Preregulator. One input of each amplifier is connected
through a divider network to the internal +5V reference.
The output voltage of the Preregulator is monitored by the
voltage divider at pin 2. The voltage drop across R933,
produced by the Preregulator output current, is applied to
the current-limit amplifier via R929.
When the instrument is first turned on, the current limit
amplifier controls the conduction time of Q933. While Q933
is conducting, the output current increases until a
sufficiently large voltage drop is developed across R933 to
invoke the current limit mode. The current-limit amplifier
holds the output current to the current-limit threshold at
approximately 1 ampere. When the voltage across C934
reaches approximately 43V, the voltage amplifier starts
controlling the duty cycle of Q933 and the Preregulator will
not limit current unless there is excessive current demand.
With Q933 off, C933 charges to the output voltage of the
Power Input circuit. When Q933 turns on, current through
the FET will come from the winding connected to pins 1 and
2 of T933 and from C933. Current to C933 is supplied by
the winding connected to pins 4 and 5 of T933. When U920
shuts off Q933, the collapsing magnetic field will raise the
voltage at the anode of CR933. This diode then becomes
forward biased and passes current supplied by the winding
connected to pins 1 and 2 of T933. This process will
continue for each period of the oscillator, and the duty cycle
will be altered as necessary to maintain 43V across C934.
To shut off Q933 during each oscillator period, Q931 is used
to discharge the gate-drain capacitance. Pin 10 of U920
goes LO, reverse biasing CR931 and turning on Q931 to
effectively short together the gate and source, thus shutting
off the FET.
Once the supply is running, power to U920 will be
supplied from the winding connected to pins 6 and 7 of
T933. Diode CR913 half-wave rectifies the voltage across
pins 6 and 7 to keep filter capacitor C913 charged and to
maintain Vcc voltage to U920 through Q915.
Instrument protection from excessive output voltage is
supplied by silicon-controlled rectifier Q935. Should the
Preregulator output voltage exceed 51V, zener diode
VR935 will conduct, causing Q935 to also conduct. The
Preregulator output current will then be shunted through
Q935, and the output voltage will very quickly go to zero.
With the Vcc voltage of U920 no longer being supplied by
the winding connected to pins 6 and 7 of T933, the
Preregulator will shut down and Q935 will be reset. The
supply will then attempt to power up, but will again shut
down once the overvoltage condition is reached. This
sequence continues until the overvoltage condition is
corrected.
Inverter
The Inverter circuit changes the dc voltage from the
Preregulator to ac for use by the supplies that are connect
ed to the secondaries of T940.
The output of the Preregulator circuit is applied to the
center tap of T940. Power-switching transistors Q940 and
Q942 alternate conducting current through R941 from the
primary circuit common to the Preregulator output line. The
transistor switching action is controlled by T942, a saturat
ing base-drive transformer.
When the instrument is first turned on, one of the switch
ing transistors will start to conduct and the collector voltage
will drop toward the common voltage level. This will induce a
positive voltage from the lead of T942 which is connected to
the base of the conducting transistor to reinforce conduc
tion. Eventually T942 will saturate, and as the voltage
across T942 (and T940) begins to reverse, the conducting
transistor cuts off because of the drop in base drive. The
other transistor will not start conduction until the voltage on
the leads of T942 reverse enough to bias it on. This process
will continue, and the saturation time of T942 plus the tran
sistor-switching time will determine the frequency of Inverter
operation (typically 20 kHz). After the initial Inverter start up,
the switching transistors do not saturate; they remain in the
active region during switching.
In instruments having the Current Limit board, diodes
CR940 and CR942 serve as a negative-peak detector to
generate a voltage controlling the outputs of both the
Preregulator and the error amplifier. In instruments having
the Preregulator board, diodes CR940 and CR942 serve as
a negative-peak detector to generate a voltage for control
ling the output of the error amplifier. Capacitor C951 will
charge to the peak amplitude of the collector voltage of
Q940 and Q942. This voltage level is applied to the divider
composed of R945, R946, and R947. The error amplifier,
composed of Q948 and Q954, is a differential amplifier that
compares the reference voltage of VR951 with the voltage
on the wiper of potentiometer R946. The current through
Q954 will set the base drive of Q956 and thereby control the
3-24
REV NOV 1982
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