ViewSonic P90F Service Manual download pdf (Page 17)

ELECTRONIC CIRCUIT DESCRIPTION

8-1 SWTCHING MODE POWER SUPPLY SECTION

8-1-1 Features

1. AC input: 100Vac – 240Vac.

2. When Power Saving activated, change feedback to reduce the output voltage and shut down

timebase and video circuit operation, thus power reduce to be less than 5W.

8-1-2 Circuit description:

8-1-2-1 EMI Noise Filter

8-1-2-2 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.

8-1-2-3 Power supply activation:

The rectified voltage across C809 charge C815 through R809, R810, R811, R812, Q810,

and D822, When UC3842 pin 7 rise up to 14V, Q816 on and Q810 off. C817 charge to

55Vdc, this voltage is ready for B+ of UC3842at Pin7 during Suspend, or Off Mode( > 10V).

C844 also provides a voltage for UVP (Vac under voltage protection). When C844 voltage is

too low, Q814 turns off, voltage at pin 2 of UC3842 is over 2.5Vdc, UC3842 will be turned off

and protected.

8-1-2-4 Degaussing circuit

Degaussing coil is connected to relay (SR801), and controlled by MCU through Q856 Q857,

When power is turned on, SR801 close, and automatic demagnetization operate for a few

seconds, then SR801 open. Degaussing also can be operated by pressing degaussing key

on the front bezel.

8-1-2-5 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, no output at pin6 to drive

output stage.

Pin4---This pin determine the operating frequency which is trig by a sync. signal

coming from I810.

Pin6---this output drive power MOSFET (Q801).

8-1-2-6 Output voltage feedback control theory

Feedback control is sampled from 190Vdc and 9Vdc at secondary side, through VR R827

2.5V feedback to reference pin of I808( R ), then I 807 coupled to pin 2 of UC3842. When

output voltage rise up, I 808 Reference voltage(R) rise up too, Cathode( K) voltage goes

down. More conduction on I807 in turns cause voltage across R826 to rise. UC3842 pin2

voltage rise 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.

8-1-2-7 Over-voltage protection

When output voltage 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.

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ViewSonic P90F Service Manual Page 17

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