NEC LCD1550X Service Manual page 11

Conventionally, the charging current of the oscillator's capacitor was changed by the
photocoupler current, and the charging time until the threshold voltage Vth (1) $ 0.73[V] was
changed. However, with the STR-G6452, the DC bias is controlled.
(3) Pin OCP/FB terminal and OCP circuit
This is a pulse bypass method overcurrent protection circuit that detects the MOSFET
drain current's peak value every pulse, and reverses the oscillator output.
The MOSFET drain current is detected by connecting R906 across the pin
source terminal and pin
terminal.
The OCP terminal's threshold voltage r+h(1) is approx. 0.76V (typ.) in respect to the
GND. R905, R935 and C909 are the filter circuits used to prevent malfunctions by the
surge current generated when MOSFET turns ON.
When the secondary output drops in the overload state, the primary auxiliary coil voltage
drops in proportion. Thus, the pin
and the operation stops.
In this case, the circuit current drops simultaneously, so the pin
with the charging current from R903 and R934, and intermittent operation that restarts
with the operation start voltage takes place.
0.76
0V
ON width control
GND terminal, and inputting the drop voltage into the OCP/FB
voltage drops to below the operation stop voltage,
1 - 5
MOSFET
voltage rises again