Small switch-mode PSUs - Poor PF or something else?

Most small switched mode power supplies have a poor power factor. And unlike "classic" poor power factor there is no simple way to improve this.

A lagging power factor is easily dealt with by adding suitable capacitors in parallel  with the load.

A classic leading or capacitive power factor is rare and is generally tolerated or even welcomed as it helps to correct the generally lagging power factor present on most non domestic installations.

A SMPSU draws current in phase with the mains voltage, it is NOT significantly displaced  with regard to the supply voltage, but current is only drawn when the mains voltage is close to the peak voltage. At other points in the mains cycle no current is drawn.

If such loads are significant then high peak currents at the peak of each sine wave tend to depress the peak voltage and result in the mains voltage becoming a flat topped semi-sinewave. 

A SMPSU draws current in phase with the mains voltage, it is NOT significantly displaced  with regard to the supply voltage, but current is only drawn when the mains voltage is close to the peak voltage. At other points in the mains cycle no current is drawn.

Agree totally with that. And there is an additional problem of the hot neutral with 3 phase supplies- while 3 equal resistive loads, cancel in the neutral, and so do 3 identical capacitors or 3 inductors, the waveforms from 3 identical SMPS, if you have one per phase do not overlap in time and cannot cancel. The neutral sees a 3 times as many pulses of current per cycle than any one phase, and if that is the only sort of load you have then the neutral needs to be sized for 3 times the current of any one line.

These funny waveforms are sometimes called 'triplen' harmonic but really all this means is that a spectrum of continuous  waves and their harmonics is a most awkward way to think about it, and to consider the pulses in the time domain is easier.

  This is one of the reasons that for high power stuff some effort has made to correct the power factor but it involves tricking the electronics to provide a varying load that has a draw current that is more or less proportional to the voltage of the incoming AC, as that is what a resistor does.

In effect there is rectification without smoothing,  a step up circuit whose control follows the input  voltage and then from the DC that generates, a large reservoir cap at perhaps 300-400V a more or less normal step down SMPS.

Mike

A SMPSU draws current in phase with the mains voltage, it is NOT significantly displaced  with regard to the supply voltage, but current is only drawn when the mains voltage is close to the peak voltage. At other points in the mains cycle no current is drawn.

Agree totally with that. And there is an additional problem of the hot neutral with 3 phase supplies- while 3 equal resistive loads, cancel in the neutral, and so do 3 identical capacitors or 3 inductors, the waveforms from 3 identical SMPS, if you have one per phase do not overlap in time and cannot cancel. The neutral sees a 3 times as many pulses of current per cycle than any one phase, and if that is the only sort of load you have then the neutral needs to be sized for 3 times the current of any one line.

These funny waveforms are sometimes called 'triplen' harmonic but really all this means is that a spectrum of continuous  waves and their harmonics is a most awkward way to think about it, and to consider the pulses in the time domain is easier.

  This is one of the reasons that for high power stuff some effort has made to correct the power factor but it involves tricking the electronics to provide a varying load that has a draw current that is more or less proportional to the voltage of the incoming AC, as that is what a resistor does.

In effect there is rectification without smoothing,  a step up circuit whose control follows the input  voltage and then from the DC that generates, a large reservoir cap at perhaps 300-400V a more or less normal step down SMPS.

Mike