# Revisiting Buck converter switching psu and ripple voltage

Hi guys, I have done some more reading since my last question regarding switching power supplies after some great guidance from the members here. I have decided to build one for my university project, specifically a USB dedicated charging port that runs from a car cigarette lighter socket.

I have been reading chapter 9 of Power Electronics Handbook by Muhammed Rashid, and I am able to make the calculations regarding input/output ratios, switching frequency and the inductor value required to keep the circuit in continuous conduction mode at maximum load. I am struggling to understand the calculation regarding the minimum output capacitor required when related to Vr (ripple voltage). He gives the equation as

``````Cmin = ((1-D)Vo)/(8VrLf^2)
``````

If I specify a ripple voltage of 0.2V, then my simulation in Multisim shows a ripple of +-1V. Now this result happens to be +-20%, suspiciously reminiscent of 0.2 I specified for the ripple voltage.

My assumption is that ripple voltages would be in units of volts, but this is not mentioned anywhere in the part of the book I have (the online library only let me download a single chapter). Can anyone confirm if there is indeed a unit for ripple, and is it in volts, percentage or something else?

Thank you :)