Given that the converter of interest is an AC/DC converter, the input is naturally from an AC power supply. Fortunately, the AC power for homes and offices is governed by established nominal voltages. While in Japan it is 100VAC, worldwide we must address at least a range encompassing 100VAC to 240VAC. Moreover, since these numbers are nominal values, including tolerances, in many cases a lower limit of -15%, which is 85VAC, and an upper limit of +10%, or 264VAC may have to be accommodated.
Since in some countries electrical power is poorly regulated or delivered, establishing adequate tolerances requires considerable experience and an understanding of the actual conditions that prevail in a given country. In a nutshell, the input voltage range for the power supply device being designed is determined by the conditions in the country to which the systems are shipped incorporating the power supply device.