Sony S75F Service Manual page 37

Protection circuit:
1. Over-voltage protection: D203 or D204 is broken down when the output of L201 or L202 exceeds
11V, which increases the voltage applied across R222 or R223.It results in rising the DTC value of
U201.When the value falls downs to be lower than surge voltage, the output is shut. The whole process
acts as protection.
2. Over-current protection: when the output connects to ground and the inside reference voltage
charges the capacitance connected to SCP to 1V, then PWM is shut. In addition, when the load is unloaded,
it also means that there is no feedback voltage applied to PIN14 and PIN3 of U201, the inside circuit will
work to protect the circuit
DC-DC Circuit: The DC-DC Circuit is made up of MOSFET Q203, energy-storing inductor L201 and D201,
The pulse output of BA9741A is magnified by the amplifier made up of Q205, Q207 to drive the MOSFET.
It performs charge to L201 when the high level coming, thus the following waveform is produced at the
other terminal of L201, as follows:
L201 performs discharge when the low level coming and D201 and L201 form durative current loop.
Because the current of pulse output is so weak that it can't drive MOSFET Q203, thus a circuit containing
Q205, Q207 used for amplifying should be employed.
PIN9 of U201
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S75F Sony