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8.
Power Circuit
Outline
Fig 8.0
8.1
Start-Up Circuit
1. A start-up circuit is used to start and stop operations of the control IC IC801, by
detecting the voltage appearing at Vin terminal pin 4 IC801.
2. At start-up C816 is charged via the start-up circuit consisting of R818, which causes
the voltage at pin 4 of IC801 to increase. Once the Vin terminal pin 4 voltage reaches
approximately 18.2V, IC801 begins to operate and drive the internal power MOSFET
transistor. This causes current to flow through the primary winding P1 and P2 and the
drain/source terminals at pin 1 and 3 of IC801.
3. Once IC801 begins to operate, the supply voltage at pin 4 is supplied via the
rectifying diode D817 which is fed from the driving winding of the switching
transformer T801.
4. The drive winding voltage of T801 is initially unable to meet the supply voltage
demanded and so the voltage at pin 4 decreases.
5. The charge held by C816 however slows this decreasing voltage at pin 4 long enough
to prevent the shut down voltage of 9.7V being reached, thus allowing the drive
winding voltage at V1/V2 to become established.
6. If voltage at pin 4 falls below 9.7V, IC801 stop functioning.
8.2
Output Voltage Control
1. When the internal MOSFET transistor of IC801 conducts, the current flows via the
primary winding P1 and P2 of T801 and IC801 Pin 1 (Drain) and pin 3
(Source/Ground) causing a voltage drop across R820, R821 to develop.
2. The voltage drop across R820, R821 is then fed back to pin 7 of IC801 via R830.
3. This feedback voltage at pin 7 is then fed to internal comparator which is used to
detect when the voltage at pin 7 exceeds the internal generated 0.94V reference
The supply voltage for the main
power supply circuit is fed to the
bridge rectifier D803 where the
AC voltage is fully rectified and
smoothed by C810. [Fig 8.0].
This smoothed DC voltage then
feeds the supply to the drain
terminal of IC801 pin 1 via the
winding P1–P2 of transformer
T801. Here the voltage is held at
the
drain
of
MOSFET transistor of IC801.
29
the
internal
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