HP WF1907 Service Manual page 2

1. Power Board Operation Theory
1.1 Line filter consists of C801, T801, C802, C803, C804, C808, C809, C815, C819, C820, C825. It
eliminates high frequency interference to meet EMI's requirement.
1.2 Rec & Filter
Bridge diode D812, D813, D814, D815 converts AC source into pulsed DC. This pulsed DC is
smoothed and filtered by C805. R802 is an NTC (negative thermal coefficient ) resistor, used to
reduce inrush current to be within safe range.
1.3 Power transformer :
T802 converts energy for square wave from power source C805 to secondary side to generate +5V,
+12V and +22V.
1.4 Output :
The square wave from T802 is rectified by D808 D809, D810, then filtered by C817, C818, C824, C823,
C830, C833, C822 to generate +22.5V and +12V, +5V respectively.
1.5 Driver :
Q803 drive T802 from PWM control of I801 for power converted.
1.6 FB :
Negative feedback CKT consists of photo coupler I802 and adjustable regulator I803. It can
maintain output voltages +22.5V and +12V, +5V at a stable level.
1.7 PWM :
1.7.1 Start : When power is turned on. C807 is charged a 11V and a starting current above 40uA
to pin 1 of I801. I801 starts to oscillate and outputs a pulse train through pin 6 to drive Q803.
1.7.2 OPP : When Q803 turns on, C805 supplies a linearly increasing triangle current through the
primary inductance of T802 to the driver Q803, once the peak value of this current multiplied
by R811 exceeds 0.52V, pulse train will be turn off immediately to protect Q803, T802 from
being burned out.
1.7.3 Regulation : If output voltage +22V goes up, the R terminal of I803 gets more bias,
accordingly photo transistor and photo diode flows more current. The voltage of pin 2 goes
up too, making the pulse width of pin 6 to become narrower. So the output voltage +22V will
be pulled down to a stable value.
1.7.4 OVP : If +22.5V goes up too much, the induced voltage on pin 4 of T802 becomes large also.
Suppose that it is over 20V. The pulse train at pin 6 goes down to zero, shutting Q803 off
immediately.
1.7.5 SCP : If output terminal is short to ground, photo transistor does not conduct, hence I802
does not conduct either. Then oscillation of I801 is stop, shutting Q803 off immediately.
HP W1907& WF1907 Service Manual
Page 1