Genius 1493, ISO XFR-75W, 2093, 2793, 3693, 1793, 1993 Switch Mode Power Supply Circuit Description

SWITCH MODE POWER SUPPLY CIRCUIT DESCRIPTION.

Oscillator waveform without sync:

Oscillator waveform with sync:

Error Amp. V.

FET drive,

C5184 pin 10:

Fet Drive

With Sync

The series regulator IC

, controls

sense the max. current. Resistors

,

115

140

133

current to the monitor GND

by

pulse width

and capacitor

reduce power

supply

138

modulation.

A PNP transistor

, has an

electrical noise.

Transistor

and diode

112

127

emitter current, that is directly

proportional to

short the FET drive to

V-

when the

116

the 127V line voltage due to resistor

and

monitor is turned off to protect the FET

from

J1

adjustment resistors

&

. This

conducting current with a still large drain

J13

J14

current is transmitted to the power supply V-

voltage. Resistors

,

,

and

J10

J11

J12

134

line, and is applied to a resistor

,

, &

provide a means for

checking flyback diode

J5

J15

. The voltage across these resistors is

conduction via a comparator.

If the

J16

142

compared to a reference voltage by the error

comparator measures low flyback diode voltage

amplifier.

If the +127V line goes up the output

the FET is turned on to the .3A low current

of the error amplifier voltage goes up.

mode. This mode is necessary for power up,

The pulse width modulation, which controls the

since initially the +127V line is 0V and no

+ 127V line voltage, is accomplished by turning

reverse diode voltage exists. The over voltage

the FET drive on at some particular voltage

protect circuit, at pin 14, has a trip voltage of

along the rising slope of the oscillator waveform.

8V and when it is activated, it shuts down the

This particular voltage is the error amplifier

power supply.

The EHT is measured by

output voltage. See waveforms above.

rectifying the

flyback pulse, with diode

,

130

from a secondary winding of the FBT.

The FET drive is always off during the

Capacitors

,

and resistors

,

125

124

126

J9

negative slope of the oscillator, or just after the

are connected as a low pass filter to smooth out

sync pulse.

Since the FET drive pulse is started

the simulated EHT voltage which is then

by the error amplifier voltage and terminated by

applied to the C5184 at pin 14.

Resistor

J8

the end of the oscillator cycle, a control system

protects the IC current sense input from

via pulse width modulation has been established.

voltage spikes and resistor

protects the

113

The oscillator waveform is produced by charging

PNP transistor from momentary

overvoltage

capacitor

with a constant current set by

damage due to line spikes. Zener

diode

102

181

resistor

J7

to a voltage of 5V

and then

protects the horizontal and video circuits

from

discharging

the capacitor with double the

overvoltage due to power supply failure. If the

charging current to 2.5V.

Adding the flyback

+127V line exceeds 160V, the zener diode

181

pulse, via capacitor

123

to this waveform

shorts to GND the +127V line.

synchronizes the oscillator, since the oscillator

frequency is set below the horizontal frequency.

At the input to the power supply is a

Resistors

J2

,

J4

and capacitor

108

limit the

voltage doubler which outputs between 240 to

error amplifier's AC gain, to hold the control loop

425VDC depending on the AC line voltage. It

stable.

Capacitor

107

holds the error amplifier

has a three amp fuse

to protect the PCB

146

stable.

Capacitor

reduces power supply

limiter

to

110

traces, an inrush current

159

noise, but, if too large, will cause the power

protect the rectifier diodes

&

.

148

156

supply to be unstable.

The 127V line is adjusted

Capacitors

and

are used to reduce

150

155

by making solder connections on the J PRA (refer

diode noise from the monitor to the AC input.

to page 65 for the layout).

Solder connections

JC

For 220VAC operation the voltage doubler is

and

are used to raise the 127V line up to 4.5

replaced by a full wave rectifier by adding

JD

volts

in

steps of 1.5 volts.

Connections

JA

and

JB

diodes

,

, capacitors

,

and

151

154

152

153

lower the 127V line as much as 4.5V.

removing the jumper at

.

&

152

144

163

are the raw DC filter capacitors.

Resistor

J6

supplies the power supply start current and

resistors

and

balances the series

The FET

works together with the

143A

143B

connected filter capacitors

for 220VAC

136

transformer

to provide a low resistance

operation.

166

current path

from

V- to GND. This low

resistance coupled with no large voltage times

Caution! When working on a monitor with a

current products is what makes the power

supply efficient.

Resistor

provides a

degaussing relay,

468

unplug the degaussing coil

137

means for sensing the FET current. In the low

to avoid causing the residual current relay to

current mode, it is used to set the 300mA current

close on a cold posistor.

This can happen if the

and in the full on mode it is used to

24V line is energized by a external power supply.

90