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#129384 - 02/16/05 10:01 AM SMPS= Harmonics  
Trumpy  Offline


Member
Joined: Jul 2002
Posts: 8,211
SI,New Zealand
OK guys,
Getcha thinking caps on!. [Linked Image]
XYZ454 (Brian) and I were talking about this in the Chat room tonight.
Now we all know that an SMPS (Switch-mode Power Supply) creates a lot of harmonics in it's Neutral.
Question is why?.
My understanding of it is, when the Switch-mode chip converts the incoming 50 or 60Hz Sine wave into HF AC and then to DC at a lower voltage.
All of this Phase conversion (modification of the original Sine wave) cause a spike to be transmitted down the Neutral.
Sure the SMPS has made computers a lot lighter and to a certain extent cheaper too, but my understanding is of the SMPS, the heavier you load it, the worse the Harmonics get?.
Can anyone give me a definite answer?.


Let's face it, these days if you're not young, you're old - Red Green grin

Tools for Electricians:

#129385 - 02/17/05 09:33 PM Re: SMPS= Harmonics  
Bob  Offline
Member
Joined: Feb 2002
Posts: 182
Mobile, AL, USA
Trumpy
This information may answer your question http://www.ecmweb.com/mag/electric_fundamentals_harmonics/


#129386 - 02/17/05 11:39 PM Re: SMPS= Harmonics  
winnie  Offline
Member
Joined: Sep 2003
Posts: 649
boston, ma
Trumpy,

You left out a step. A switch mode power supply works by taking AC, rectifying it to DC, chopping the DC at high frequency (thus high frequency AC, usually with a substantial DC component), using enery storage (usually an inductor), and producing the necessary low voltage DC.

The source of the harmonics is the _low frequency_ rectification. Basically you see a pulse of current during the peak of the AC voltage waveform, when the AC voltage is greater than the rectified DC voltage. At other portions of the AC cycle, the current flow is zero.

This pulsing current has a very strong harmonic component. This isn't sending pulses down the neutral per-se, but instead creating harmonics, some of which can only flow down the neutral.

If you design your SMPS properly, with a 'power factor correcting' front end, then you can cause the input current to flow in phase with the supply voltage, and virtually eliminate harmonic issues. You still will have switching noise issues, however.

-Jon


#129387 - 02/20/05 04:44 PM Re: SMPS= Harmonics  
pdh  Offline
Member
Joined: Jan 2005
Posts: 354
To see what kind of current waveform you get from a computer power supply, scroll down to picture 8 on this page:

http://www.zlan.com/waveforms.htm

A rough analogy of how this works is the thermostat in a heating/cooking system. But instead of heat, the power supply is re-filling capacitors. And instead of doing so only when the caps are below a certain level, it just tops them off at a constant rate, e.g. 100 Hz or 120 Hz depending on where in the world you are. The waveform will change depending on how loaded the P/S is.

This waveform really plays hell with a three phase neutral, and also makes voltage drop worse (e.g. heats up wires, CB thermal elements, etc) due to I-squared-R. I'm now doing some investigation into running computers P-P (e.g. on 208 volts) to help mitigate some of this (still inconclusive).


#129388 - 02/21/05 04:31 AM Re: SMPS= Harmonics  
marcspages  Offline
Member
Joined: Oct 2004
Posts: 48
London, UK
Trumpy (and others),

There is a lot of bad teaching going round regarding harmonics. One of them is "harmonics are created" by the likes of SMPSs etc. If harmonics were 'created', don't you think some smart alec would have found out how to take that energy and power the computer with it by now?

It is far better to say that the waveform of an SMPS has a "high harmonic content".

As for "only on Neutral". This has been confused with running many such devices in a 3-phase system. The Live and Neutral carry the same current waveform so are therefore subjected to the same harmonic content. When placed in a 3-phase system, then Neutral is then subjected to the sum of the harmonics of all three phases - in some instances the sum causes cancellation, in others, addition (read up on triple-n - or 'triplen' as it is now called - harmonics, but also be warned about the bad teaching that says "all triplens add on the Neutral"!).

Also, strangely enough, the heavier you load the SPMS, the lower the relative harmonic content (not amount, as this is relative to the current drawn, but content as a percentage).

PDH,

You will most certainly rid the Neutral of harmonics running your hi-tech load L-L, but you're going to increase the harmonic content on the phases. However, I do believe the total harmonic content on the supply side of the transformer will be reduced as you're creating 6-pulse rectification. I'd be very interested in your results.

M.


[This message has been edited by marcspages (edited 02-21-2005).]


#129389 - 02/22/05 04:47 AM Re: SMPS= Harmonics  
Trumpy  Offline


Member
Joined: Jul 2002
Posts: 8,211
SI,New Zealand
Marc,
Quote
However, I do believe the total harmonic content on the supply side of the transformer will be reduced as you're creating 6-pulse rectification.

What transformer?.
SMPS's don't use a transformer, at least not like a traditional Power supply.
Quote
There is a lot of bad teaching going round regarding harmonics. One of them is "harmonics are created" by the likes of SMPSs etc. If harmonics were 'created', don't you think some smart alec would have found out how to take that energy and power the computer with it by now?

How come then, that we have EMC laws that pertain to SMPS's?.
I guess that the "smart-alec's" are still working that one out.
And also, I'm a Radio Ham, I can't use my Radio gear on the 2 metre (144MHz) band and my HF set on 20 metres (14MHz) for hash coming from the SMPS on my computer.


Let's face it, these days if you're not young, you're old - Red Green grin

#129390 - 02/22/05 05:57 AM Re: SMPS= Harmonics  
pdh  Offline
Member
Joined: Jan 2005
Posts: 354
Figures there will be some hams here.

I find more of the QRM (ham code for man made noise) on the ham bands when near a computer is due to the main board digital circuitry, than any other part. The add-on boards, power supply, disk drives, and monitors, do add their own. Good grounding, quality feed-line shielding, and locating the antenna well above the computer area, do all help in reducing the problem.

I do worry about these high frequency electronic fluorescent ballasts getting to be more popular. And as discussed in another thread, you're going to see a lot more of these in California in the coming years.

For more fun try 160 meters where the harmonic power is stronger and the antenna is less directional.

de KA9WGN

P.S. Check the quality of your GFCI receptacles by transmitting your 2m HT on full power next to one. Have a fire extinguisher handy in case you have a bad one.


#129391 - 02/22/05 05:59 AM Re: SMPS= Harmonics  
marcspages  Offline
Member
Joined: Oct 2004
Posts: 48
London, UK
Trumpy,

For "transformer", read "utility transformer" i.e. the xfos used to supply the site with power. Harmonic content that adds on the Neutral creates circulating currents in the utility delta-star (delta-wye) xfos. PDH spoke of supplying the hi-tech load phase-phase so I assumed he had a transformer giving him a 3-phase supply (and also assumed this to be a delta-star). Sorry for the confusion!

The EMC laws pertain to any equipment actually but specifically SMPSs as SMPSs transformers are driven by high frequency oscillators with very sharp switching edges (and the sharper the edge, the more efficient the SMPS). These sharp edges, using Fourier's idea, are full of high frequency content and it is this content that must be kept in check (hence the EMC filters that exist on both the input and output of the power supply). And...

[Linked Image]

This is the typical input to an SMPS and, as can be seen, is asymmetrical i.e. Live and Neutral have exactly the same components. Their purpose is to keep the noise inside the power supply (shown in red arrows) – but some will leak out, however, the noise of the oscillator will emit on both the Live and Neutral, not just the Neutral alone.

I'm going to throw a little curve-ball here; I actually believe the noise you are experiencing from your PC is not the SMPS (or only a small percentage is the SMPS), but rather the motherboard and other associated peripherals such as keyboard, monitor, etc. My PC is a Dell and they are constructed in shielded cases (the edging uses a finger type material that ensures, once the case is closed, it is like a solid impenetrable block). I can operate all bands without an issue. There are plenty articles on the 'Net about shielding a PC for radio use.

Hope I've cleared up any confusion I created?

M.



[This message has been edited by marcspages (edited 02-22-2005).]


#129392 - 02/27/05 01:08 AM Re: SMPS= Harmonics  
Trumpy  Offline


Member
Joined: Jul 2002
Posts: 8,211
SI,New Zealand
pdh,Marc,
Thanks a lot for your thoughts on this.
Come to think of it pdh, I reckon that the EMI from my Computer PSU, could be the result of buying a cheapo PSU (as in lack of sheilding).
I've been doing a fair bit of reading up on the subject of the SMPS in the last week or so and as far as I can tell, a good SMPS should not cause any problems due to it's operation at all.
I guess that you get what you pay for in this respect.
They even make SMPS units to power Ham gear here, so I have no idea what I'm talking about!. [Linked Image]

de ZL3TPQ


Let's face it, these days if you're not young, you're old - Red Green grin


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