Tektronix 1740A Series, User Manual

Page: 97 / 368

Circuit Theory
1740A/1750A/1760 – Series Service Manual
3–33
the drain voltage “flies back” in a positive direction. Current now flows in all of
the secondary windings and supplies power.
The input line voltage is filtered by the rear-panel connector to reduce the
electrical noise conducted into or out of the instrument. R123 limits the initial
charging current through the rectifier diodes and C71.
CR32, CR33, CR34, and CR35 form a bridge rectifier. C71 filters the 110 – 350
Vdc rectifier output. L8 filters the switching noise produced by the switcher.
R116 reduces the circulating current in the parallel circuit consisting of L8 and
C52. DS7, R116, and R118 form a line voltage indicator. R120 and R122
charge C62. C62 provides power to U6 until the primary housekeeping winding
provides power through CR22.
VR3 is the source of the +5 V required by the transformer driver to operate the
Power switch. When power is connected to the instrument, it gets enough
current from R119 and R121 to zener and provide the power required to operate
the transformer driver oscillator.
The transformer driver is a Colpitts oscillator whose inductive resonator is the
isolation transformer T2. The front-panel Power switch is a momentary push
button that shorts the secondary of the transformer and causes the oscillator to
stop when it’s pushed. When the secondary shorts, Q13 stops oscillating. Q12
turns off and starts U5A, a one-shot multivibrator, that clocks U8A, which is the
Power switch memory. It changes state every time the front-panel Power switch
is pushed.
If power is removed from the instrument U8A, retains its memory due to the
storage capacitor, C58. C58 is capable of holding its charge for a week or more
at a time. In order to prevent the one-shot multivibrator, U5A, from tripping
U8A if power is lost, a short time constant (C56 and R89) pulls V
CC
for U5A
down before Q13 quits oscillating.
U6 is a current-mode pulse width modulator that uses two feedback loops. The
inner current-feedback loop directly controls the switcher mosfet peak current.
The outer voltage-feedback loop programs the inner loop peak current trip point.
U6 pin 2 is the inverting input of an internal operational amplifier. The
non-inverting input is set to 2.5 V by an internal voltage reference. Current from
the peak detector flows through R87 and R94. R86 provides a 100
A offset.
The voltage at pin 1 will vary in order to maintain pin 2 at 2.5 V.
The voltage at U6 pin 1 is modified by an internal circuit and sets the trip point
of the internal comparator. Pin 3 is the external input to the comparator. Pin 4
sets the internal oscillator to 80 kHz, R92 and C55 determine the frequency.
Line Rectifier and Filter
Transformer Driver
Pulse Width Modulator
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