universal input voltage

A universal power supply does not use a transformer to step the voltage down but it uses PWM to do it. Initially, it rectifies the AC input voltage directly to a DC voltage with a bridge rectifier and then it chops this DC voltage with PWM to achieve the desired output voltage. The input voltage it does not really matter (if it is between some minimum and maximum limits of course) because if you plug the power supply into a lower AC voltage then the PWM will adjust the pulse width as to obtain the desirable output voltage. On the other hand, if the AC voltage is higher the PWM will reduce the pulse width as to keep the output voltage constant.