6. E
lectronic Circuit Description
SWTCHING MODE POWER SUPPLY SECTION
Features:
1. AC input: 100Vac – 240Vac.
2. When Power saving is activated, secondary feedback will be changed to reduce the output voltage and
shut down time-base as well as video circuit operation. Thus power is reduced.
6-1-1.
Circuit description:
6-1-2-1 EMI Noise Filter
6-1-2-2
C802 and C803 build as Y capacitors, C804 is X capacitor, L801 and L802 are Common Mode Choke,
those build as a two steps Noise filter.
6-1-2-3
Power supply activation:
The rectified voltage across C809 charges C815 through R809, R810, R811, R812, Q810, and D822.
When UC3842 pin 7 rises up to 14V, Q816 turns on and Q810 turns off. C817 charge to 55Vdc approx.
This voltage is ready for B+ of UC3842 at Pin7 during power save Mode. C844 provides a voltage for UVP
(AC input under voltage protection). When C844 voltage is too low, Q814 turns off, voltage at pin 2 of
UC3842 is over 2.5Vdc, and UC3842 will be turned off to protect power supply.
6-1-2-4
Degaussing circuit
Degaussing coil is connected to relay (SR801), and controlled by MCU through Q104 and Q104A.
When power is turned on, SR801 relay close, automatic demagnetization will operate for a few
seconds before SR801 returns to open. Degaussing also can be operated in the OSD menu.
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6-1-2-5
UC3842 PWM controller
UC3842 is a current feedback PWM controller:
Pin1---is feedback positive end.
Pin2---190V and 9V at secondary side of power transformer provide a sampling
voltage to pin 2 of UC3842 via I808, I807.
Pin3---is a current feedback input. When it is over 1V, there is no output at pin6 to drive
output stage.
Pin4---This pin determine the operating frequency.
Pin6---This output drive output power MOSFET(Q801).
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6-1-2-6
Output voltage feedback control theory
Feedback control is sampled from 190Vdc and 9Vdc at secondary side, through VR R827, it feedback to
reference pin of I808, then I807 coupled it to pin 2 of UC3842. When output voltage rise up, I808 reference
voltage (R) rise up too, and I808 cathode (K) voltage goes down. More conduction on I807 occurs and the
voltage across R826 rises. Thus UC3842 pin2 voltage rises to reduce the duty at output pin6. Q801
conduct less to reduce the secondary side voltages, thus to stabilize the DC voltages at secondary side.
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6-1-2-7
Over-voltage protection
When output voltages are too high, voltage across C817 will be high and voltage drop on R872 is also high.
It will force Q808, Q807 to conduct and pin8 of UC3842 to ground. Thus UC3842 stop to operate.
ViewSonic Corporation
13
Confidential – Do Not Copy
A
9
0f+-1
Содержание A90f+ 1
Страница 57: ...12 PCB Layout ViewSonic Corporation 54 Confidential Do Not Copy A90f 1...
Страница 58: ...VIDEO PWB 0414 ViewSonic Corporation 55 Confidential Do Not Copy A90f 1...
Страница 59: ...KEY CONTROL PWB 0488 ViewSonic Corporation 56 Confidential Do Not Copy A90f 1...
Страница 60: ...MAIN BOARD 13 Schematic Diagrams ViewSonic Corporation 57 Confidential Do Not Copy A90f 1...
Страница 61: ...VIDEO ViewSonic Corporation 58 Confidential Do Not Copy A90f 1...
Страница 62: ...KEY CORTROL ViewSonic Corporation 59 Confidential Do Not Copy A90f 1...
Страница 63: ...POWER SUPPLY CKT ViewSonic Corporation 60 Confidential Do Not Copy A90f 1...
Страница 64: ...MCU CONTROL CKT ViewSonic Corporation 61 Confidential Do Not Copy A90f 1...
Страница 65: ...Time Base DeflectionCKT ViewSonic Corporation 62 Confidential Do Not Copy A90f 1...
Страница 66: ...H V DYNAMIC FOCUS CKT ViewSonic Corporation 63 Confidential Do Not Copy A90f 1...
Страница 67: ...14 Exploded Diagram ViewSonic Corporation 64 Confidential Do Not Copy A90f 1...