Switch Mode Power Supply Circuit Description And Schematic - Ceronix 1493 Service Manual

SWITCH MODE POWER SUPPLY CIRCUIT DESCRIPTION.
Oscillator waveform without sync:
Oscillator waveform with sync:
FET drive, C5184 pin 10:
The series regulator IC
current to the monitor GND by pulse width
modulation. A PNP transistor
emitter current, that is directly proportional to
the 127V line voltage due to resistor
adjustment resistors
J13
J13
current is transmitted to the power supply V-
line, and is applied to a resistor
. The voltage across these resistors is
J16
J16
compared to a reference voltage by the error
amplifier. If the +127V line goes up the output
of the error amplifier voltage goes up.
The pulse width modulation, which controls the
+ 127V line voltage, is accomplished by turning
the FET drive on at some particular voltage
along the rising slope of the oscillator waveform.
This particular voltage is the error amplifier
output voltage. See waveforms above.
The FET drive is always off during the
negative slope of the oscillator, or just after the
sync pulse. Since the FET drive pulse is started
by the error amplifier voltage and terminated by
the end of the oscillator cycle, a control system
via pulse width modulation has been established.
The oscillator waveform is produced by charging
capacitor with a constant current set by
102 102
resistor to a voltage of 5V and then
J7 J7
discharging the capacitor with double the
charging current to 2.5V. Adding the flyback
pulse, via capacitor
123
123
synchronizes the oscillator, since the oscillator
frequency is set below the horizontal frequency.
Resistors , and capacitor
J2 J2 J4 J4
error amplifier's AC gain, to hold the control loop
stable. Capacitor
107 107
stable. Capacitor
110
110
noise, but, if too large, will cause the power
supply to be unstable. The 127V line is adjusted
by making solder connections on the J PRA
. Solder connections
to page 65 for the layout)
and are used to raise the 127V line up to 4.5
JD
JD
volts in steps of 1.5 volts. Connections
lower the 127V line as much as 4.5V.
The FET works together with the
136 136
transformer to provide a low resistance
166
166
current path from V- to GND. This low
resistance coupled with no large voltage times
current products is what makes the power
supply efficient. Resistor
means for sensing the FET current. In the low
current mode, it is used to set the 300mA current
and in the full on mode it is used to
90
, controls
115
115
, has an
112
112
and
J1
J1
& . This
J14
J14
, , &
J5 J15
J5 J15
to this waveform
limit the
108 108
holds the error amplifier
reduces power supply
(refer
JC JC
and
JA JA JB
provides a
137 137
sense the max. current. Resistors
and capacitor
electrical noise. Transistor
short the FET drive to V- when the
116 116
monitor is turned off to protect the FET from
conducting current with a still large drain
voltage. Resistors
provide a means for checking flyback diode
conduction via a comparator. If the
142 142
comparator measures low flyback diode voltage
the FET is turned on to the .3A low current
mode. This mode is necessary for power up,
since initially the +127V line is 0V and no
reverse diode voltage exists. The over voltage
protect circuit, at pin 14, has a trip voltage of
8V and when it is activated, it shuts down the
power supply. The EHT is measured by
rectifying the flyback pulse, with diode
from a secondary winding of the FBT.
Capacitors
125 125
are connected as a low pass filter to smooth out
the simulated EHT voltage which is then
applied to the C5184 at pin 14. Resistor
protects the IC current sense input from
voltage spikes and resistor
PNP transistor from momentary overvoltage
damage due to line spikes. Zener diode
protects the horizontal and video circuits from
overvoltage due to power supply failure. If the
+127V line exceeds 160V, the zener diode
shorts to GND the +127V line.
At the input to the power supply is a
voltage doubler which outputs between 240 to
425VDC depending on the AC line voltage. It
has a three amp fuse
traces, an inrush current limiter
protect the rectifier diodes
Capacitors
150
150
diode noise from the monitor to the AC input.
For 220VAC operation the voltage doubler is
replaced by a full wave rectifier by adding
JB
diodes ,
151 151 154 154
removing the jumper at
are the raw DC filter capacitors. Resistor
supplies the power supply start current and
resistors
143A 143A
connected filter capacitors for 220VAC
operation.
Caution! When working on a monitor with a
degaussing relay,
to avoid causing the residual current relay to
close on a cold posistor. This can happen if the
24V line is energized by a external power supply.
Error Amp. V.
Fet Drive
With Sync
140 140
reduce power supply
138 138
and diode
127 127
, , and
J10 J10 J11 J11 J12 J12
, and resistors
124 124 126 126
protects the
113
113
to protect the PCB
146 146
to
159 159
&
148 148 156 156
and are used to reduce
155
155
, capacitors ,
152 152 153 153
. &
152 144
152 144
and balances the series
143B 143B
unplug the degaussing coil
468 468
,
133 133
134 134
,
130
130
,
J9 J9
J8
J8
181
181
181
181
.
and
163
163
J6
J6