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  1. #21
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    Re: Please help clarify this doc on Earthing and EMC of Offline SMPS

    ever tried to build an effective broadcast antenna at 531kHz..?
    ...here's one at 200kHz
    https://en.wikipedia.org/wiki/Droitw...itting_Station

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    I would like to relate this statement to the common mode choke pictured along with it. It has unlikely relevant common mode suppression above 10 or 20 MHz. If your device is able to radiate RFI, this kind of common mode choke will hardly hinder it.
    …Yes but common mode noise above 20MHz can occur in pulses, and those pulses can be at a frequency of 150khz or so….and then that common mode noise shows up on the Quasi-Peak plot at 150khz…….also, those common mode chokes will offer some attenutation at >20MHz….if only better than not having that common mode choke there and just having bare wires.
    The chokes shown are admittedly for lower frequencies…….but in the near field, even lower frequencies can couple to adjacent things and jump out of a PCB where not intended….so those chokes aren’t entirely great at lower frequencies.

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    The question is whether common mode chokes are needed when there is no wired earth conenction........and the fact is that they are.....as the post of #12 shows...thats from one of the worlds best SMPS companies in China......they didnt put those chokes in there by mistake.

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    Another point is the PI Expert Design software by Power.com…if you do a offline flyback design in it, (with no earth connection, only Live and neutral) , it always puts a large common mode choke at the input…and has done for at least the last decade....this is not a mistake.

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    I’ve seen mains connected LED drivers failing badly on common mode noise at around 150khZ…….when you remove their earth wire connection, you do not suddenly pass the conducted EMC scan.



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  2. #22
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    Re: Please help clarify this doc on Earthing and EMC of Offline SMPS

    ...here's one at 200kHz
    So you agree that your device should not be considered as an antenna in the frequency range of interest?

    You can ignore EM effects if device dimensions are << wavelength and refer to a electro-/magnetostatic model. The problem of common mode choke effect for a device without earth and other external connections reduces to

    Click image for larger version. 

Name:	cm choke.PNG 
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  3. #23
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    Re: Please help clarify this doc on Earthing and EMC of Offline SMPS

    not clearly visible in the photo of the 200kHz antenna is the 4 wires going off to the other tower some 100's of meters away ... this is the structure needed to convert ( or match ) oscillating currents at low frequencies to the impedance of free space and get the energy to be radiated ... such that the antenna now looks like a resistance ...


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