Very odd USB charger SMPS circuit

Today, let's take a look at a very strange schematic of a 5V USB charger switching power supply. It contains a Poweric PI751 chip and it has no optocoupler, no auxiliary winding, high side switching, floating GND pin of the chip and voltage sensing connected to the primary winding! It's apparently a 1A design (the voltage plummets at about 1.1A), but they marked it as 2A. You can support me on Patreon:   / diodegonewild