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The current mode control IC UC3842 is used in the switching power supply which function of each
pin described as follows.
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.
R616
10K
TLP721F
R617
1K
C610
3300P
250V
2.2.3.7 Degauss Circuit
2-10
pin 1 : Error amplifier output
pin 2 : Error amplifier reverse input
pin 3 : Current sense
pin 4 : OSC sawtooth
+6.5VA
R738
82
1/2W
4
1
3
2
(SHORT)
IC602
K
IC702
TL431
A
C611
3300P
250V
3
2
C621
2200P
250V
(Y)
4
1
RL601
Q706
TR602
H945
2R9M
B55
B56
2
1
M602
TR601
2P
8D-13
No Copy/Reproduction Allowed
pin 5 : Ground
pin 6 : Output
pin 7 : VCC
pin 8 : Reference Voltage:5V
(OPEN)
C730
+45V
(OPEN)
+14VA
R737
R736
R734
20K
39K
1/4W
C725
1/4W
0.47U
50V
(D)
R
R726
VR701
500
470
R735
1.2K
D709
1N4148
D708
R705
1N4148
(OPEN)
D603
R706
20D6
4.7K
D604
B59
L604
20D6
180UH
D605
20D6
API Confidential
(OPEN)
R766
Q765
(OPEN)
Q766
(OPEN)
R767
(OPEN)
SGND
(OPEN)
D766
(OPEN)
REF
REF
(OPEN)
+12V
DEGAUSS
C708
+
47U
16V
CN602
(EL)
2P
2
1
B60
+6.5VA
R768
Q767
(OPEN)
+
R775
C766
(OPEN)
7277e Monitor Service Guide
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