Electronic Circuit Description - Nokia 447ZiPlus Service Manual

6. ELECTRONIC CIRCUIT DESCRIPTION

6-1. Switching Mode Power Supply (SMPS)
The switching mode power supply for this monitor is a full range type.
Universal power input of ac 100V ... 240V is compatible.
The AC input is supplied through the slow blow fuse F801, the first line filter T802, the second line filter
T803 to the rectifier D811 ...D814.
The rectified DC voltage is about 125V ... 335V and is fed to the switching transformer T801 via R808.
Prior to start, small current flows to C866A via R849, R850, Q850.I862 is to operate immediately
when the voltage across C866A reaches 16V. At the same time, Q868 is on, the rectified current
flows through the primary winding of T801 and the energy is stored in it.
When Q868 is off, the energy is transferred to the secondary winding, the auxiliary winding is close -
coupled and in phase with the secondary winding, it will provide a holding voltage on C866A and fed to
pin 7 of I862 via R831 , D831,D870 I862 will keep working provided that voltage across C866A is over
10V.
The oscillating frequency of the switching mode power supply is around 30KHz, which is determined by
the timing components R861, C863.
Capacitor C863 is charged to 2.8V via the 5V reference voltage on pin8 of I862 and discharge to 1.2V,
during the discharge time interval, the oscillator generates a series of blanking pulses which cause the
output at pin6 of I862 to go low.
The fly-back pulse from T440 is buffered by R864 and differentiated by C864, R863. A sharp pulse is thus
obtained to force C863 to discharge, so that the oscillating frequency is synchronized and kept in phase
with that of horizontal frequency to avoid interference on the display screen.
Should the voltage vary due to the charge of input AC or output loading, the auxiliary winding will reflect
these variation to pin2 of I862 via D831, R831, R835. The voltage at pin2 of I862 is compared with the
internal 2.5V reference voltage to produce an error voltage which is limited to 1V by internal zener diode.
The comparator output is then compared with the voltage at pin3 of I862 which is supplied from the
voltage detected across R867, while Q868 is on. Q868 is off only when the voltage drop across R867 has
reached that of the error voltage, or1 V p-p maximum.
The error voltage controls the duty cycle of the pulse width modulator to regulate the output
voltage.
If over-load or short circuit occurs at the secondary, the voltage across R867 reaches 1 V p-p rapidly,
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