[![enter image description here](https://i.stack.imgur.com/ZwMAA.png)](https://i.stack.imgur.com/ZwMAA.png)
A step down dc to dc converter converts a high dc input voltage into a low dc voltage. The basic process is

high dc input voltage->oscillator->step-down transformer->rectification->dc-output voltage

As one can see that a circuit of DC to DC buck converter (Vin = 100-120 ,Vout = 5V,Iout = 2.5A) is shown which is obtained from Texas instrument's webench software for schematic .

In the above design consider only the transformer primary part. In the primary side ,it consists of two types of windings 1.primary and 2.auxiliary winding
Auxiliary winding is used to supply power to the UCC28C40 . 

Among the two ends of primary,one end is connected by dc input i.e. Vin (in between is the snubber circuit that control the effects of the leakage inductance and improve the reliability of the power supply) and the other end is from drain of the MOSFET.

**I know that UCC28C40 has an internal oscillator that can convert dc voltage into ac voltage and that voltage is given to one terminal of the transformer But what about second terminal part of primary winding of the transformer (indicated with vertical red arrow )? According to my observation,its a dc voltage .So how transformer could converts the dc voltage to ac voltage?**