When it comes to switched power supplies, is the power supply efficiency affected by the presence of a power factor correction circuit? In my understanding power supply efficiency is estimated by measuring the ratio of power drawn from the electrical outlet when the power supply works under several loads, for instance 20,50 and 100% loads are analyzed in computer PSUs to give the 80 plus certificate, but is the presence of a power factor correction (PFC) circuit improving the efficiency? Do they take under account the reactive power drawn from the outlet to estimate the ratio to compute the efficiency or just real part and then the PFC circuit has no influence?

  • \$\begingroup\$ In my opinion they only take real power into account \$\endgroup\$ – Claudio Avi Chami Apr 16 '16 at 6:28

In the field of engineering, efficiency means thermodynamic efficiency, real output power divided by real input power. A power factor correction circuit reduces the efficiency of the equipment that contains the circuit but improves the overall system efficiency by reducing the RMS current required to transmit a given amount of real power.

In the case of switched power supplies, the power factor correction circuit improves the total power factor by reducing the harmonic distortion of the input power waveform. The benefit is partly reducing the current required, but perhaps more importantly, preventing the power supply from contributing to distortion of the voltage of the source power system.

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  • \$\begingroup\$ What do you mean by "reduces the efficiency of the equipment", that after introducing the PFC circuit the amout of real power consumed is higher? How is this related to the efficiency when working under several loads, i.e the efficiency of the switched power supply? From what you're saying can I imply that it decreases as now the denominator of the efficiency ratio is higher? \$\endgroup\$ – VMMF Apr 16 '16 at 15:26
  • \$\begingroup\$ Some power is lost (converted to heat) in the power factor correction circuit. That means that the total input power to the power supply is increased by the addition of power factor correction. Power losses are reduced in the power distribution wires, transformers etc. all the way to the power generating station. The user of the power supply gains mostly a "cleaner" AC source and lower AC circuit current allowing. \$\endgroup\$ – Charles Cowie Apr 16 '16 at 16:28
  • \$\begingroup\$ Thanks, I understand what PFC is. My question is, how it is related to the switched power supply efficiency? \$\endgroup\$ – VMMF Apr 16 '16 at 23:56
  • \$\begingroup\$ And I'm talking about this efficiency http://www.hardwaresecrets.com/understanding-the-80-plus-certification/ \$\endgroup\$ – VMMF Apr 17 '16 at 0:10
  • \$\begingroup\$ I understand. That is the efficiency of the power supply. Adding a PFC circuit inside the power supply adds to the losses inside the power supply and reduces its efficiency. In a SMPS, the PFC is not simply a capacitor, but even if it was it would use some power because it has internal resistance it is not an "ideal" capacitor. \$\endgroup\$ – Charles Cowie Apr 17 '16 at 1:03

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