2-10
API Confidential
No Copy/Reproduction Allowed
7277e Monitor Service Guide
The current mode control IC UC3842 is used in the switching power supply which function of each
pin described as follows.
pin 1 : Error amplifier output
pin 5 : Ground
pin 2 : Error amplifier reverse input
pin 6 : Output
pin 3 : Current sense
pin 7 : VCC
pin 4 : OSC sawtooth
pin 8 : Reference Voltage:5V
When power is initially applied to the circuit, capacitor C607 charges through C604, R623, D601,
D605. When the voltage across C607 reaches a level of 16V, IC601 is turn-on the +5Vdc will be set
up at pin8 then R613, C615 generate a fixed frequency sawtooth wave to pin4, at this time
MOSFET will be driven by pin6 with square wave the pulse width of square wave is decided by pin2
, pin3 is current feedback control, It will to sense MOSFET current. The D611, D612, R614, C617
are soft start components to avoid the duty too large when power starts up.
2.2.3.6 Feedback Circuit
This power supply adopt feedback circuit of +45V and +15V. It used IC702 for voltage regulation
and IC602 for primary-secondary isolation. The output voltage differential signal will be detected
and sensed to the pin2 of UC3842 for comparison then the duty cycle of MOSFET will be decided to
control the output voltage.
+45V
3
4
2
1
R
K
A
+
IC602
C611
C610
R737
Q767
IC702
Q765
Q766
C766
VR701
C725
C730
R768
R736
R735
R734
R767
R617
R738
R616
R726
R766
D766
R775
TLP721F
250V
3300P
250V
3300P
1/4W
20K
(OPEN)
TL431
(OPEN)
(OPEN)
(OPEN)
500
(D)
50V
0.47U
(OPEN)
(OPEN)
(SHORT)
1.2K
1/4W
39K
(OPEN)
1K
(OPEN)
1/2W
82
10K
470
(OPEN)
(OPEN)
(OPEN)
REF
REF
+14VA
+6.5VA
+6.5VA
SGND
2.2.3.7 Degauss Circuit
B59
B56
B60
B55
+12V
2
1
2
1
3
4
+
1
2
D709
M602
C621
RL601
TR602
L604
TR601
D708
D603
D604
D605
R706
R705
C708
Q706
CN602
1N4148
2P
250V
2200P
(Y)
2R9M
180UH
8D-13
1N4148
20D6
20D6
20D6
4.7K
(OPEN)
16V
47U
(EL)
H945
2P
DEGAUSS