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Main Power Supply
Figure 20b
Most of the DC voltages used throughout the unit are derived from this power
supply. The voltage divider circuit consisting of resistors R551, R553 and R556
reduces the output of the PFC circuit to create the startup voltage. This voltage
enters pin 4 of IC552 to begin the oscillation. The startup voltage is also applied
to the voltage feedback circuit consisting of the optical coupler D566 and Diode
D554. The PWM output at pin 3 of IC552 is applied to the primary of the
transformer T502. As current passes through the transformer, magnetic field
builds up in its core. The primary current passes through the transistor located
inside the IC and the two source resistors R569, and R576. The voltage at the
transistor's Source increases at a proportional rate to that of the current increase.
This voltage is applied to the non-inverting input of the op-Amp at pin 1 of IC552
for over-current protection (OCP).
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