By varying the duty cycle of the mosfet we can increase or decrease the DC input voltage by a factor of -D/(1-D) (where D is the duty cycle) but with opposite polarity. In the example we have set the duty cycle at 33.3% so with 10V at the input we expect that after the transients settle out to have -5V at the output.
If you want to change the duty cycle just change the
width of all voltage pulse sources (5% is 50% duty cycle and 10% is 100% duty cycle).
