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3DQDVRQLF
3.4.
Start Up
A start-up circuit is used to start and stop the
operations of the control IC IC801 (STR-F6523), by
detecting the voltage appearing at the V
operations of the control IC IC801 (STR-F6523), by
detecting the voltage appearing at the V
IN
terminal,
pin 4.
At start-up, capacitor C810 is charged via R804,
which causes the voltage at pin 4 of IC801 to
increase. Once V
which causes the voltage at pin 4 of IC801 to
increase. Once V
IN
terminal pin 4 voltage reaches
approximately 16V, IC801 begins to operate and drive
the internal power MOSFET, causing current to flow
through the drain/source terminals at pins 3 and 2,
and to the winding B1-B2 of switching transformer
the internal power MOSFET, causing current to flow
through the drain/source terminals at pins 3 and 2,
and to the winding B1-B2 of switching transformer
T802 via L803 and R809. The current at terminal B1
is split into two paths.
is split into two paths.
The first path follows the current being fed to the
winding B1-B2 and back to pin 4 of IC801 via R811
and D805. Once the control circuit starts operation,
the voltage at the V
winding B1-B2 and back to pin 4 of IC801 via R811
and D805. Once the control circuit starts operation,
the voltage at the V
IN
terminal pin 4 of IC801 starts to
decrease. However, the drive winding voltage
reaches the set value before pin 4 voltage drops to the
shutdown voltage of 10V. Hence the voltage supply to
pin 4 is maintained.
reaches the set value before pin 4 voltage drops to the
shutdown voltage of 10V. Hence the voltage supply to
pin 4 is maintained.
The second path is connected from terminal B1 to P2
of the FBT. This causes current to flow via the winding
P2-P1, which provides the +B supply to the FBT T552
pin 9.
of the FBT. This causes current to flow via the winding
P2-P1, which provides the +B supply to the FBT T552
pin 9.
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