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3.1. Input voltage range

Since the mains voltage changes according to the country, and even within the same country a means must be provided to adapt the power supplies to those different voltages. Under this criterion, there are three types of switched power supplies: those that can only operate at the rated voltage, those that allow the user to set the operation voltage and those with a universal input which can operate at any voltage within a wide range ( typically 90 to 260 V) without any setting been required. This last system is normally used for low-range power supplies, since for the same output data it requires the use of primary components with a higher performance than in the two others systems. It is also usual in power supplies provided with power factor correction (PFC), in which case the correction circuit is the one absorbing the input voltage changes (please see section 7.1.5.). In power supplies provided with a voltage select switch, the circuit commonly used is the one shown on figure 13. When the power supply is connected to 115V, the input voltage switch (W) is on and the circuit behaves as a voltage doubler. For a voltage of 230 V, the switch W is left open, and the circuit provides a full-wave rectifier. Thus, the C1 and C2 charging voltage, which is applied to the converter is in both cases approximately 320 V, i. e. the ac voltage peak value is 230 V. In some cases W is replaced with a triac controlled by a circuit that automatically selects the input voltage range.

Figure-13

Figure-13

Figure-14

Figure-14