This the Simple Isolated Converter Circuit Diagram. In this Simple Isolated Converter Circuit Diagram a negative output voltage de-de converter generates a -5 V output at pin A. In order to generate -5 Vat point A, the primary of the transformer must fly back to a diode drop more negative than -5 V. If the transformer has a tightly coupled 1/1 turns ratio, there will be a 5 V plus a diode drop across the secondary.
The 1N5817 rectifies this secondary voltage to generate an isolated 5-V output. The isolated output is not fully regulated since only the -5 V at point A is sensed by the MAX635.