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THEORY
1. LINE INPUT
The input rectifier section converts the AC line
voltage into a crudely filtered and unregulated DC
voltage, powering the switching regulator. The input
section is a full-wave bridge.
1.1. FILTER
To reduce noise from the power supply, a low-pass
filter isolates the switcher.
The conducted noise is
filtered by X and Y capacitors and a common mode
transformer.
2. POWER SUPPLY
The design uses a discontinuous flyback topology
operating in current-mode resulting in a multiple
output switcher with outputs which track well. There
are no output filter chokes. Slower diodes are used.
The fast transient response of the control loop
maintains picture integrity. Very fast current limiting
protects the switcher against short circuits.
2.1. P.W.M. INTEGRATED CIRCUIT
The 3842 is an integrated current mode pulse width
modulator.
It
consists of an oscillator, error
amplifier, current sense comparator, under-voltage
lockout, and an output MOSFET driver stage.
2.2.
UNDER-VOLTAGE LOCKOUT
This circuit insures that V cc is adequate to make
the 3842 fully
operational before
enabling the
oscillator, voltage reference, and before turning on
the output stage.
The turn-on/off thresholds are at
l6v and lOv respectively.
2.3.
OSCILLATOR
The oscillator consists of a pull up resistor from the
Sv reference to pin 4 of IC601 and a timing capacitor
to ground. When the voltage ramps up to about 2.8v
OF
10
OPERATION
on pin 4 of IC601 an internal current source pulls
down, discharging the timing capacitor to a l.Ov level.
This level releases the current source and starts the
next cycle.
Oscillator frequency is roughly equal to
l/.5SRC.
Synchronization is achieved by feeding timing pulses
from a reference into the oscillator.
2.4.
OUTPUT STAGE
The 3842 has a single totem-pole output capable of
operating to l.0 A peaks and a 200mA average cur
rent.
2.5. CURRENT SENSE COMPARATOR
Current-mode controllers inherently keep close
watch over the pass transistor's current.
Pin 3 of
IC601 is connected to a voltage comparator which
shuts off the output when current reaches the desired
level, as prescribed by the error amplifier. This way
the controller will only allow the needed amount of
power into the output transformer.
This method
differs from most pulse width controllers which
compare the error amplifier's output against the
oscillator's voltage ramp.
This results in control of
on-time, which does not necessarily mirror the power
stored in the transformer.
This comparator also serves the dual purpose of
monitoring current limit.
Any time pin 3 of IC601
rises
above
1 volt the
output
will
terminate.
Output -short
circuits
and
core
saturations
are
detected before they destroy the pass transistor.
2.6.
ERROR AMPLIFIER
Voltage on pin 2 of IC601 is compared with a 2.Sv,
2% source.
Errors in output voltage are amplified
and fed to pin 1 of IC601 where they are frequency
compensated by an RC back to pin 2 of IC60l. This
error voltage is dropped by l.4v and divided by 3
before being fed to the current comparator.
In this application, the error amplifier is on the
secondary
side
of
the
switcher.
Inside
the
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