Theory of Operation
1735 HD High Definition Waveform Monitor
4–25
The rectifier is a full-wave bridge. DS7, along with R129, C62, and R130, form
a relaxation oscillator, with DS7 flashing whenever line voltage is present. In
addition, C62 serves as a smoothing capacitor for the approximate 110 to 350 V
rectifier output. (Rectifier output voltage is dependent on input mains voltage.)
R124, L8 and C44 provide additional EMI filtering.
Controller.
The controller is a pulse-width modulator which uses U4, a current-
mode controller, to regulate the duty cycle driving the switcher. C45 and R98,
connected to pin 4, are the timing elements for an internal oscillator that sets the
operating frequency to about 80 kHz. The output at pin 6 is an 80 kHz repetition
rate, variable duty cycle, square-wave signal that drives the switcher. Pin 8
supplies a 5 V reference, and pin 3 is the current sensing of the switcher source
current.
When the switcher is running, V
CC
for the controller is supplied by a housekeep-
ing supply. CR20 is the coupling diode for this supply. Before controller start-up,
C42 is charged from the line, through parallel resistors R200 and R201.
Charging current is approximately 2 mA. When the charge on C42 reaches
approximately 16 V, U4 starts and provides the switcher driving waveform. Once
the switcher is operating, the 16 V housekeeping supply is provided from T2,
through CR20.
Switching Transistor.
The low-voltage power supply is of the flyback type. Q16 is
the main switching transistor, switching at 80 kHz. When it is turned on, its
drain voltage is 0 V and the current raises linearly, charging T2 with a current
ramp. When Q16 turns off, the drain voltage “flies back” in a positive direction.
This flyback action on the primary of the transformer drives the secondary
windings with the correct voltage. The amount of power delivered to the
secondaries is dependent on the duty cycle driving the gate of the switching
transistor.
Voltage Snubber.
CR21 and C43 form a voltage snubber to keep the voltage on
the drain of Q16 from getting too high. CR24 and C49 slow the rising edge of
the switching transistor pulse to reduce electrical noise.
When the switcher is running, a voltage ramp representing the source current of
Q16 is applied to pin 3 of U4. Pin 1 is the DC level output of an internal error
amplifier. The internal error amplifier compares the current sense voltage ramp to
the DC level at pin 2. R87 provides a negative feedback voltage from pin 1 to the
current node at pin 2. The current into the pin 2 node is approximately
1 mA/–10 V of DC level. This current is comprised of the power supply
secondary error voltage, coupled through T3, and the negative feedback through
R87. It holds the output of the controller to 5 V.
The timing waveform for the internal 80 kHz oscillator is a sawtooth waveform
whose repetition rate and slope are determined by C45 and R98.
Summary of Contents for 070-8462-03
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Page 19: ...Preface xiv 1735 HD High Definition Waveform Monitor...
Page 20: ...Getting Started...
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Page 35: ...Installation 1 14 1735 HD High Definition Waveform Monitor...
Page 36: ...Operating Basics...
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Page 179: ...Corrective Maintenance 7 20 1735 HD High Definition Waveform Monitor...
Page 181: ...Repackaging 7 22 1735 HD High Definition Waveform Monitor Figure 7 10 Repackaging...
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