Konka LC-TM2018 LCD TV switching power supply circuit

Konka LC-TM2018 LCD TV switching power supply circuit

Shown is the switching power supply circuit of Konka LC-TM2018 LCD TV . Since the LCD TV is equipped with high-voltage and high- current devices, the performance of the circuit is also different from that of the picture tube TV. The circuit only outputs one voltage, that is, +12 V voltage.     The AC input and rectifier filter circuit converts the AC 220 V voltage into a DC voltage of about 300 V through the mutual inductance filter L901 and the bridge rectifier stack BD901. The 300 V DC voltage provides bias voltage for the drain of the switching field effect transistor through the primary winding ①~③ of the switching transformer T901 , and at the same time provides the starting voltage for the switching, oscillation, and voltage stabilizing control integrated circuit N901 pin ⑤.     After the power is turned on, the starting voltage causes the oscillation circuit in N901 to start working. The N901@ pin outputs a driving pulse to make the switching field effect transistor V901 work in the switching state, so a switching current is formed between the drain and source of the field effect transistor. The secondary @ ~ ⑥ windings of the switching transformer are positive feedback windings, and the @ pin is externally connected to a rectifier diode D903. The voltage after starting the oscillation is added to the ⑦ pin of N901 as a positive feedback voltage to maintain the oscillation of N901.     The output of the secondary ⑧, @~⑧, and @ of the switching transformer is rectified and filtered by D904 and D905 (dual rectifier tubes) to form a +12 v voltage. In order to reduce the ripple, an LC pin-shaped filter is added to the output end.     The voltage stabilization control circuit is mainly completed by the photoelectric coupler N902. The +12 V voltage forms a voltage dividing circuit through R915, R914, and R913. R913 is used as a sampling point to capture the error sampling voltage for N903 (TL431). N903 is the error amplifier and controls For the light-emitting diode in the photocoupler N9Q2 , the +12 V voltage fluctuation will cause the luminous intensity of the light-emitting diode in the photocoupler to change. This change is fed back to the ② pin of the N901 through the transistor in the photocoupler, forming a negative feedback loop for voltage stabilization control.