Pioneer S-W1EX Service Manual page 46

1
Explanation on switching of the power supply
A
1. When the AC power is applied, a starting voltage is applied to the Vin terminal (Pin 4) of IC1 via the starting resistor, and the MOSFET
located in IC1 (STR-F6467) starts switching.
In this case, be especially careful, because the voltage between the drain and the source of the MOSFET becomes very high (about 600 V).
2. Through the switching operation, electric charges are stored in the electrolytic capacitor in the reference-voltage section of the secondary
circuit via the transformer.
3. The error amplifier detects the voltage. The photocoupler gives a feedback to IC1, and switching is performed so that the reference
voltage becomes 47 V.
4. When the reference voltage reaches 47 V, the voltages of all the windings are stabilized, and each winding outputs the set voltage.
B
5. Switching has become stabilized.
6. In a stabilized condition, the starting voltage is supplied from the auxiliary winding.
7. If the output load is high, changes in the reference voltage are detected by the error amplifier and are transmitted from the photocoupler
to IC1. Switching is then controlled acccordingly.
8. If load is small, the switching frequency becomes higher, and if load is greater, it becomes lower.
9. If switching is performed normally, the voltage at the Vin terminal (positive-side voltage of C17) becomes about 18 Vdc.
C
10. If there is any abnormality with the power, at the Vin terminal of IC1 a sawtooth waveform at a frequency of about 0.1 sec/dev. is
observed as the voltage waveform.
11. When supply of AC power is stopped, the starting voltage is stopped to be applied to IC1, switching is stopped, and the power is shut
off.
D
E
F
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