Tektronix 1740A Series, Руководство пользователя

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Circuit Theory
3–34
1740A/1750A/1760 – Series Service Manual
The pulse width modulator works as follows: The oscillator resets the latch and
U6 pin 6 goes high, turning the switcher mosfet on. The current through the
switcher mosfet increases, causing the voltage across R90 to increase. This
voltage is divided across R91 and R92, to input to the comparator (pin 3). When
the voltage at pin 3 reaches the comparator trip point, the latch toggles and the
switcher mosfet is turned off. This process is repeated at an 80 kHz rate.
Switching the mosfet oscillator on and off drives the power transformer, T3.
C53 increases the noise immunity by rolling off the internal operational amplifier
frequency response. R97 holds the switcher mosfet off as the circuit is powering
up. R93 slows the turn-on of the switcher mosfet while CR26 speeds up the turn
off.
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.
Rise time 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.
Snubber
Output Filters
Error Amplifier
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