QC 2.0 voltage output is stepped (5, 9 or 12V) while QC 3.0 is variable and changes smoothly when used with a QC 3.0 device. That is, QC 3.0 is able to dynamically adjust the charging voltage over the battery charging cycle and allows the phone to request just enough voltage to reach the desired charge current, thereby QC 3.0 can be more efficient than QC 2.0. The energy efficiency allows the charger to charge faster than QC 2.0 with the same input power.
