After the DC voltage is input, it is connected to the drain of the switching tube inside the chip through the primary winding of the transformer, and the voltage source inside the chip obtains power from the switching tube drain, and provides the starting voltage to the chip after voltage reduction.
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After the chip is started, the internal switch tube will produce a high-speed switch, and the input direct current will be converted into pulsating DC and then input to the primary of the switching transformer. The voltage induced by the transformer secondary is divided into two ways, one is 18V voltage, and the other is 5V voltage after 7805 voltage regulation. The 18V voltage also returns to pin 4 of the chip to supply power to the chip.