It is common in switching power supply designs to connect a bulk capacitor right after the bridge rectifier. This leads to charging of the filtering capacitor at the peaks of the input sine wave. Apparent power becomes high due to high RMS current and this leads to low PF. Current meters with “True RMS” function are required for measurements of input current. PF for typical S.P.S. falls within the range of 0.4~0.6; for S.P.S. with PFC function, PF can be above 0.95. The relation among input current, output power, input voltage, power factor, and efficiency is as follows:

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