Power Board Operation Therory - HP VF15 Service Manual

1. Power Board Operation Therory

1.1 Line filter consists of C801, T801, C802, C803, C804, C837, C838. It eliminates high frequency interference to meet
EMI's requirement.
1.2 Rec & Filter
Bridge diode D801 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 +12V and +15V.
1.4 Output :
The square wave from T802 is rectified by D809, D810, then filtered by C817, C822 to generate +15V and +12V
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 +15V and +12V at a stable level.
1.7 PWM :
1.7.1 Start : When power is turned on. C807 is charged a 15 volt and a starting current above 40uA to pin 7 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 exceeds1 volt, pulse train will
be turn off immediately to protect Q803, T802 from being burned out.
1.7.3 Regulation : If output voltage +15V 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 +15V will be pulled down to a stable value.
1.7.4 OVP : If +15V goes up too much, the induced voltage on pin 4 of T802 becomes large also. Suppose that it is
over 18 volts, ZD801 conducts, pin 3 of I801 is pulled up over 1 volt. 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 Q806 does not conduct
either. Then oscillation of I801 is stop, shutting Q803 off immediately.
HPD-K15AA Power Board Block Diagram
Line
Filter
Rec. &
Power
Filter
Transformer
PWM Driver
FB
vf15 / FP15 / f1523 / FP5315
Outpur Rec. & Filter
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