The working principle is: 220V AC voltage enters the interference suppression filter composed of C101 and L101 through the power switch and safety tube, and then gets a DC voltage of about 300V after bridge rectification and filtering.
When the power supply is switched on, 300V voltage through the starting branch resistance R102, R103 to IC101 (5L 0380R) 3 pin one - one pulse, its internal switch tube is in a micro conduction state, at this time, there is current through the 5L038OR 2 pin. The ①-② winding of the switching transformer generates induced electromotive force, and the induced electromotive force is coupled to the positive feedback winding ③-④, and the induced voltage is injected into the ③ foot of 5L.0380R after rectification and filtering, so that the switching tube enters the saturated conduction state and completes the starting process of the switching power supply.
The secondary windings of the switching transformer output different DC voltages through their own rectification and filtering circuits to supply the decoder motherboard.