Tektronix WFM601, Service Manual

Page: 84 / 304

Theory of Operation
WFM 601 - Service Manual
3–27
The primary winding (pins 1 and 2) has a shadow winding (pins 3 and 4) with
one end connected to ground. The B+ end of the primary winding is in phase
with the grounded end of the shadow winding. The signal ends of both windings
are connected together through C51. Because both windings have the same
number of turns and their signal ends are connected, the signal voltages on the
windings are forced to be the same regardless of stray inductance in the
transformer. CR23 ties the signal end of the shadow winding to the B+ end of
the primary winding to prevent it from having a peak signal voltage greater than
B+. This also prevents the primary winding from having a peak signal voltage
greater than B+.
The signal end of the primary winding can go no more that 2 x B+ (about 700 V
with a 250 V mains) because the other end of its winding is at B+. Holding the
signal voltage at 700 V or less protects the switching transistor, Q14.
Risetime snubbing is done with CR25, R88, and C59. Slowing the risetime of
Q14 reduces RF interference.
The three output windings supply four output voltages. Each output is rectified
by a single diode and filtered by an LC pi filter.
The error amplifier regulates the +5 V output by feeding an error signal to the
pulse width modulator. U3A is a voltage reference that outputs 2.5 V for the
operational amplifier, U3B. R71 and R73 provide a feedback voltage for the
error amplifier. C42 and R77 form a frequency-dependent network for loop
stabilization. The output of the error amplifier operational amplifier drives the
light emitting diode input stage of the optoisolator, U4.
An optoisolator consists of a light emitting diode, as a transmitting device, and a
light-sensitive transistor as a receiver. When the intensity of the LED changes
the base current in the receiver changes equivalently to alter the pulse width
modulator feedback voltage.
U7 is a quad comparator, whose outputs are open collectors. All four comparator
outputs are connected in parallel and under normal operating conditions are high.
Whenever the output of any one of the comparators goes low, Q15 will turn off
and the pulse width modulator current sense line will go high and shut down the
power supply.
U7B is the comparator for the +16 V supply. U7C is the line voltage compara-
tor, sensing the rectified ac primary. If either output is low, Q15 turns off to shut
down the power supply. U7A senses the power switch status from the Q output
of U8A. Whenever the output of Q8A is low, the output of U7A will also go
low.
Snubber
Output Filters
Error Amplifier
Shutdown Logic