Principle 1. STRW6556A startup and power supply
After the AC220V mains passes the anti-interference circuit composed of the interface CN601, F601 and LC, the rectifier filter obtains about 300V voltage through the switching transformer TOOO primary winding L1 to the IC601(STRW6556A)① pin; After the 220V mains is rectified by half wave, the starting resistance ROOO(120kΩ\/lW) charges the capacitor C609(l0μF\/50V) and supplies it to the IC601④ pin. When the 4V voltage rises to about 15V, the oscillating circuit in the IC601 starts to work, forming a changing current on the transformer TOOO primary winding L2. The induction pulse of the transformer T000Ll winding is rectified by the diode D602 and filtered by the capacitor C609 to form 18V voltage, which is added to the IC601④ pin, then the switching power supply is stable and continues to work normally.
2. Overvoltage and undervoltage protection
IC601④ foot is connected with overvoltage protection and undervoltage protection circuit. When the voltage of the IC601④ foot rises to 25.5V for some reason, the internal overvoltage circuit starts and overvoltage lock, and the internal oscillation circuit stops output and enters the overvoltage protection state. At this time, the voltage of the IC601④ foot fluctuates greatly and is unstable, and can only be unlocked when the voltage of the IC601④ foot drops to 7.2V. Or it can only be unlocked after the faulty component is removed. When the IC601④ pin voltage drops to 9.7V for some reason, the internal undervoltage circuit starts, undervoltage lock, and the internal oscillation circuit stops the output. That is to say, the IC601 should work normally, and its ④ pin voltage should be above 15V and below 18V.
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3. Overcurrent detection and protection
IC601 7 foot is the overcurrent detection end. When some reason causes the current of the switching transformer T000 winding L2 to increase, the current passes through the resistor R607, voltage regulator diode ZD602, R604(6.8kΩ), R601(220Ω) to the ground to form a loop, and the voltage is formed at both ends of the R601. When the voltage is 1.45V, That is, when the IC601 7 pin voltage rises to 1.45V, the working state of the internal circuit is flipped, and the overcurrent detection protection lock circuit is started, forcing the oscillation circuit to stop output, and playing the role of overcurrent detection protection.
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4. Standby control and voltage regulator circuit
The standby control circuit is composed of Q610, Q501, R577 and R576. To make the TV in standby, the power control terminal of the CPU needs to output a low level, and the ON\/OFF signal voltage from the CPU circuit of the signal board is added to the Q610 base through R612(l0kΩ) to make it cut off. After the Q610 is cut off, Q50l is switched on. Optocoupler OT601 internal light emitting diode light enhancement, ③, ④ the photosensitive transistor in the foot of the internal resistance is reduced, so that the IC601⑥ foot voltage rise, through the internal circuit into the control voltage, so that the internal circuit work in standby state.
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