Panasonic EURO 4 Chassis Technical Manual page 81

25.1.1.Regulation
Regulation is achieved through the feedback
winding F1.F2 of T2701. This feedback winding
supplies a negative voltage via D2708 and R2705 to
pin 1. This negative supply is used to bias Q1
(IC2701) which in turn controls the base bias of Q3
(IC2701) and hence the current flow through the
collector/emitter junction of Q3.
Therefore if the load on the secondary side
decreased the current through the primary winding
increases, the feedback voltage also increases
which causes Q1 to conduct more reducing the bias
on Q3 hence the current through the primary
winding P2.P1.
Likewise if the load on the secondary side increases
the primary current reduces, the feedback voltage
decreases reducing the bias on Q1 which in turn
increases the base bias of Q3, increasing the
current flow.
25.1.2.Over-Current Protection
If an increase in current occurs the excessive
current flow through the primary winding P2.P1
is
causes a high current flow through Q3 and pins 3
and 4 of IC2701. This causes the voltage across
R2709 to rise to a sufficient level to switch ON
Q2(IC2701), with Q2 ON current flows through the
collector/emitter junction of Q2 and pin 5, which
inturn causes the voltage at pin 2 to rise causing the
zener diode D2708 to conduct removing the base
bias to Q3, switching OFF the power supply.
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