Tektronix 1705A Line Rectifier and Filter, PulseW idth Modulator

Theory of Operation
1705A Spectrum Monitor 4-15
The input line voltage is filtered by the rear-panelconnector to reduce the
electricalnoise conducted into or out of the instrument. R89 limits the initial
chargingcurrent through the rectifier diodes and C54.
CR21, CR22, CR23, and CR24 form a bridge rectifier. C54 filters the 110 to
350 Vdc rectifieroutput. L4 filters the switchingnoise produced by the
switcher. R102 reduces the circulating current in the parallel circuitconsisting of
L4 and C44. DS4,R93, and R94 form a line voltage indicator. R91and R92
chargeC42. C42 provides power to U5 until the primaryhousekeeping winding
provides power through CR17.
U5 is a current-modepulse width modulator (PWM). A current-mode PWM
uses two feedbackloops. The inner current-feedbackloop directly controls the
switcher mosfetpeak current. The outer voltage-feedback loop programs the
inner loop peakcurrent trip point.
U5 pin 2 is the inverting input ofan internal op-amp. The non-inverting input is
set to 2.5 V by an internalvoltage reference. Current from the peak detector
flows through R83 and R79. R84 provides a 100 A offset. The voltage at U5
pin1willvaryinordertomaintainU5pin2at2.5V.
The voltage atU5 pin 1 is modified by an internal circuit and sets the trip point
of the internalcomparator. U5 pin 3 is the external input to the comparator. R 88
and C52, connected to U5 pin 4, set theinternal oscillator to 80 kHz.
The circuitworks as follows: The oscillatorresets the latch and U5 pin 6 goes
high, turning the switcher mosfet on. The current through the switcher mosfet
Line Rectifier and Filter
PulseW idth Modulator