Switching power supply circuit of Tsinghua Tongfang Computer DVP-P636 DVD player

Switching power supply circuit of Tsinghua Tongfang Computer DVP-P636 DVD player

The switching power supply circuit of Tsinghua Tongfang DVP-P636 DVD player is shown in the figure.     The AC 220 V voltage is rectified and filtered to obtain a DC voltage of about 300 V. After passing through the ①~@ winding of the switching transformer Tl, it is added to the catching pole of the field effect switch vrri (2N60B) to provide working power for the power circuit: at the same time, the AC After the 220V voltage passes through the starting resistors R15 and R12, it charges c8. When the voltage at both ends of C8 reaches 16 V, the Ul (UC3842)@ pin generates a +5 V reference voltage, which is added to the ④ pin of Ul through Rl, so that The oscillator inside the @ pin starts working, and the PWM pulse generated is output from the ⑥ pin and sent to the gate of VTI through R19, so that VT1 works in the switching state, and pulse current flows through the primary winding of Tl. According to the principle of electromagnetic induction. The positive feedback winding of the ◎～④ pins of Tl will generate induced pulse voltage. This voltage is rectified by D5. R12 limits the current, and after filtering by C8 and C9, a DC voltage of about 17 V is obtained, and is added to the Ul⑦ pin to provide Ul with the operating voltage required for normal operation.     At the same time, other windings of Tl will also generate corresponding induced pulse voltages, and after processing by corresponding rectifier and filter circuits, the voltages at all levels required for the normal operation of the whole machine will be obtained.     When the output voltage increases for some reason, the 5v voltage increases accordingly, the potential of the voltage dividing point of R6 and R7 (the output voltage setting end of U2) rises, and the potential of the K pole of U2 drops, flowing through the photocoupler ① , The current of pin ② increases, and the luminescence of the internal omen diode increases; the internal resistance of the photosensitive transistor ce junction becomes smaller. The potential of PC1④ pin decreases. The internal control circuit of Ul is activated, which reduces the duty cycle of the pulse signal output by the U1@ pin, shortens the conduction time of VT1, and reduces the voltage of the secondary output of Tl.