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Computer power supplies often have "power factor correction" features which raise the power factor in use to levels close to resistive loads (1). I'm curious what, in the absence of power factor correction, power supply loads would be. (Switching-mode power supplies use both inductors and capacitors; I'm not sure what their load looks like on the AC side though)

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  • \$\begingroup\$ Without the PFC they'd be a completely different power supply entirely, i.e. the PFC is an integral part of the supply, not an enhancement of another design. \$\endgroup\$ – Ignacio Vazquez-Abrams Jul 31 '15 at 2:19
  • \$\begingroup\$ @IgnacioVazquez-Abrams: It isn't just a big capacitor or inductor on the end? \$\endgroup\$ – Billy ONeal Jul 31 '15 at 2:23
  • \$\begingroup\$ No. Power supplies use active PFC, which is a DC-to-DC converter that tracks the AC voltage waveform and makes the load look resistive to the utility by forcing the AC current to be sinusoidal and in phase with the voltage. \$\endgroup\$ – Adam Lawrence Jul 31 '15 at 2:55
  • \$\begingroup\$ I think a good answer to this question should address the difference between the displacement power factor and the distortion power factor. \$\endgroup\$ – Li-aung Yip Jul 31 '15 at 14:43
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It's not so much a case of leading or lagging. On a non-PFC power supply, the circuit consists of a bridge rectifier, followed by a large bulk capacitor. The cap charges and droops between the AC line cycles. During a potentially large portion of the AC line cycle the bridge doesn't conduct because the cap voltage is still above the rectified AC line voltage. Right near the peak of the line the line voltage exceeds the cap voltage, and all of the current flows into the cap during that small conduction angle.

So it doesn't look strictly inductive or capacitive, but it does generate large line harmonics. That's really what "power factor correction" standards regulate. It's not really the power factor, but the harmonics. The huge peak currents compared to the average current draw is the issue for the utilities.

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    \$\begingroup\$ I'd suggest going to wikipedia for a more in depth explanation, they have the formulas and a nice graph. Also, "It's not really the power factor" is kinda wrong: generally, power factor is defined as the conjunction of displacement power factor and distortion power factor. \$\endgroup\$ – FrancoVS Jul 31 '15 at 3:00
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    \$\begingroup\$ Sorry but this answer doesn't describe a PC PSU at all, even one without PFC. It describes a linear PSU. In a SMPS (used in all PC PSUs) the charging and discharging of the bulk capacitance happens at the PSU's switching frequency, not AC line freq. \$\endgroup\$ – Jamie Hanrahan Jul 31 '15 at 4:09
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    \$\begingroup\$ @JamieHanrahan How can charging of the bulk cap possibly occur at the switching frequency? Charging can only happen when the AC line is above the bulk cap voltage. Likewise, discharge occurs when the AC line is below the bulk cap voltage. \$\endgroup\$ – John D Jul 31 '15 at 4:31
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    \$\begingroup\$ @JohnD There is no bulk cap where you think it is! A PC-type SMPS has a full wave rectifier followed by a chopper that turns the unfiltered DC into 30 kHz or so square wave. That's input to a step-down transformer, the result is F-W rectified, THEN comes the filter cap. There is no bulk cap in the line freq section at all. (Yes, the old non-PFC PSUs with a 110/220 switch did use a cap, but this was in a voltage doubler before the FW rectifier. It's bypassed in the 220 position.) Heck, part of the whole point of a SMPS is to avoid the much larger cap that would be needed at 50 or 60 Hz. \$\endgroup\$ – Jamie Hanrahan Jul 31 '15 at 5:49
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    \$\begingroup\$ @JamieHanrahan no PC PSU age was specified in the Q or the A. Every ATX supply I've opened up had bridge rectifier --> bulk caps --> switching part of switching. Some of these had automatic voltage switching, some manual, and some the typical modern 90-260VAC spec. These last of course need over-specced input caps. By chance I don't think I've opened a PFC ATX supply, even to change a fan (meaning that I haven't opened the latest models). \$\endgroup\$ – Chris H Jul 31 '15 at 8:37
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Enter image description here

The above image shows a simple non-PFC switch mode power supply. This one would probably not be used as a computer PSU as it's low power, but it is sufficient to illustrate the point.

The mains is rectified to charge a capacitor, and then this high voltage DC rail is used to power the converter, which in this case is a simple flyback converter, but there are several other designs.

The capacitor is chosen to be sufficiently large that there is little ripple on it: as a result the rectifier can only conduct near the peak of the mains.

The result of this is not that the current leads or lags the voltage, but that the current is non-sinusoidal with peaks of current near the peaks of the voltage. This introduces significant harmonic distortion.

To add PFC on most power supplies, a second stage is added before this capacitor which shapes the current to follow the shape of the mains voltage. This is typically a boost converter as shown below.

Enter image description here

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