2 Methods to Design a Negative Power Supply from Positive Voltage 

In theory, any isolated power supply can be treated as a negative power supply by wiring the high level output voltage terminal to ground of system and makes the low level voltage output terminal as the negative output power supply. This method sometimes means high cost, a little complex technology, and large volume. The designer has special skills associated with switch mode power supply. This application note discusses small output power, compact size, and low cost methods to obtain a negative power supply from a positive power supply.