Normally though an additional winding on the transformer. Switch mode transformers have three or four sets of windings. The primary (high voltage), secondary (

Normally though an additional winding on the transformer. Switch mode transformers have three or four sets of windings. The primary (high voltage), secondary (low voltage) a bootstrap winding (for the chip) and sometimes a flyback winding. The bootstrap winding takes off a variable voltage depending on the local supply and uses a linear regulator to produce the required voltage. For a better explanation than I can manage in text the EEVBlog channel goes in to all sorts of regulators and how they work: https://www.youtube.com/watch?v=cM7t1Mpu7s4&list=PLn-ZIFhWxhWpbwaHZNTBWoOmeyeVPCq_i https://www.youtube.com/watch?v=cM7t1Mpu7s4&list=PLn-ZIFhWxh...