What are the harmonic effects of Switch Mode Power Supplies

Discussion in 'Power Electronics' started by TRIAD-ELECTROMOTIVE-LTD, Jun 12, 2019.


    Thread Starter New Member

    Jun 27, 2018
    What are the harmonic effects of Switch Mode Power Supplies?

    I am to believe that SMPS can cause a lot of harmonic distortion on the AC supply. If anyone could explain why this is and what steps should be taken to reduce the negative effects of harmonics I would greatly appreciate it.
  2. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    It's because they're pulsing an Inductor Transformer across the DC rectified mains, this causes large back Emfs which are filtered in the Switchmode psu with caps and dual wound inductors.
  3. BR-549

    AAC Fanatic!

    Sep 22, 2013
    A harmonic current, is a current at an un-wanted time. It's out of phase or mis-matched current. It steals from the wanted current, because it is out of time. This out of time current, causes out of time voltages.....and it causes out of time impedance. It clusters everything up. Causes un-needed waisted heat. And many other effects. Including EM radiation.

    The direction and strength of a magnetic field causes a physical dimensional change in magnetic materials. A harmonic field will further modulate that dimensional change. Adding a vibration to a vibration. The difference of the two, might be within hearing range, driving people, subconsciously insane.

    Magnetic materials stretch and pull in different directions with the magnetic field. They yo-yo in 2 directions. Alternately swell and contract one way, then the other. Like a cartoon object.

    The physical reaction of the material, might even encourage, more harmonics and noise.

    If instead of switching the voltage and current, if we could control the area of current, it might be a little quieter. Using area, instead of rate, might not generate harmonics.

    Any ideas on designing an area oscillator? Or an area valve?
  4. mvas

    Active Member

    Jun 19, 2017
    The typical problem with older Switch Mode Power Supplies is ... The shape of the current wave form is not a sine-wave.
    To fix this, newer Power Supolies use PFC front-ends.

    See figure 3 here ...
    You want the current waveform of your power supply to look like figure 2, not like figure 3.

    a) This problem is not cause by "large back Emf's".
    b) This problem is not cause by "out-of-phase" current.

    It is caused by a non-sinusoidal shaped current waveform, which is fixed by using PFC.