Service manual

The maximum current is determined by the time the FET stays into conductance (t0 -
t1). This time is directly determined by the voltage across R3108//R3118. This voltage is
a measure of the current and if it exceeds 1.4 V, TS7101 will be driven into conductivity
and consequently connect the gate of TS7102 to ground; the FET will block.
The voltage across the secondary winding (Pins 8 & 9) will be negative, diode D6111
will block.
Time interval t1 - t2:
The sudden current interruption in the primary coil will induce a counter-e.m.f. trying to
maintain the current. The voltage on the drain of the FET will increase. The secondary
voltage (8, 9) will become positive and will charge C2104 via D6111. All energy that was
stored in L5101 during t0 - t1 will be transferred into the load. Due to the transformer
principle, a voltage will now be induced in the primary winding (3, 5) and the co-coupled
winding (1, 2). This voltage will be: N * USEC (N = winding ratio).
The voltage across the co-coupled coil will be negative, keeping the FET blocked.
Time t2:
At t2, the current through the secondary coil will be reduced to zero, as C2104 is no
longer charged. Therefore, the voltages will decay and will change polarity. The gate of
the FET will be again made positive, is driven into conductivity and the cycle starts
again.
Feedback, stabilization:
The Standby Power Supply always oscillates at maximum power; the only limiting factor
is the maximum primary current that has been pre-set with R3108//3118//R3119
UOUT is determined by R3114, R3124, and zener diode D6122. If the voltage across
R3114 exceeds the threshold voltage of the diode of the optocoupler 7104 (±1 V) or, in
other words, UOUT exceeds 5.2 V the transistor of the optocoupler will conduct.