How this circuit start the oscillate

[sfiber]

Hi friends I have a circuit that use TOP221 8 pin IC and circuit is similiar with datasheet's sample circuit.But I dont understand how this circuit start the oscillate.Could you explain it please.

[sameos]

If you look at the functional block diagram in the datasheet, you'll find that the TOP221 chip has an internal oscillator. 

[sfiber]

but this IC wants control signal.If DC applied to flyback transformer first,how can control signal come to IC.

[kony]

RTFM.

[Zero999]

Refer to page two of the data sheet.
https://www.power.com/sites/default/files/product-docs/top221-227.pdf

Note that the example circuits, shown on the data sheet, are not isolated from the mains, so the DC side needs to be treated with the same precautions as the mains.

[Wimberleytech]

On startup, current flows through the transformer primary winding to the drain pin through an internal shunt path to Control pin which charges the capacitor attached thereto.  Power to the chip is then provided from the Control pin.  Once the voltage rises to an internal threshold, the internal shunt path is disabled and charge to the Control-pin capacitor is refreshed via the secondary auxiliary winding under control of the opto.

[sfiber]

yeah,thats what I wanted,thanks.