This is the circuit diagram of 5V to Isolated 5V Converter, rated at 20mA electric current.
In this converter circuit, a negative output voltage dc to dc converter generates a -5V output at pin A. In order to generate -5V at point A. the primary of the transformer must fly back to a diode drop more negative than -5V. If the transformer has a tightly coupled I : 1 turns ratio. there will be a 5 V plus a diode drop across the secondary. The IN5817 rectifies this secondary voltage to generate an isolated 5V output. The isolated output is not fully regulated since only the -5V at point A is sensed by the MAX635.