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    Common mode noise and earth connections in offline SMPS

    We have an offline, isolated LED driver in a plastic enclosure. Wires from the enclosure feed the LEDs which sit on MCPCB which is mounted on a big earthed heatsink.
    Unfortunately this big LED heatsink has to be earthed for electrical safety reasons…but of course, this is a nuisance because it means that common mode noise can then more easily pervade due to the accessible earth near the LED driver.
    I am wondering if the best way to counter this is to also have an earth connection to the offline LED driver PCB…and then have a common mode filter incorporating Y caps from live and neutral to this earth connection on the PCB?

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    Re: Common mode noise and earth connections in offline SMPS

    I think that would be the preferred way. However, you have to think of the unit as a whole, it may generate noise but how does it get out. If you ignore magnetic radiation which will be very short ranged, you are left with conduction through the power wires. If they are all at the same noise potential there is really no problem. What you should try to achieve is minimum differential between L, N and E as close to the source as possible. Capacitors between them seems the logical solution.

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    Re: Common mode noise and earth connections in offline SMPS

    Thanks,
    As we know, Common Mode noise in SMPS is always the hardest one to minimise. We try to understand it better so as to be able to better combat against it….
    Our power supply will sit at the top of a pole in a parking lot…….now, common mode noise involves noise flowing between live and earth , or between neutral and earth.
    So we are thinking, can we take the earth out of this situation (or make it less significant), such that we reduce common mode noise like that?
    For example, what if we used a plastic, non-conductive, 3 metre pole to mount the lamp on (admittedly a strong plastic) …..and that we did not run an earth wire up to the lamp at the top of the pole…then presumably this would in itself reduce common mode emissions?...since there would be no earth for the common node noise to flow into?
    (Thanks Betwixt ...i always "thank" your posts, as they are always extremely informative, but it often tells me "you may not vote on any more threads today")



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    Re: Common mode noise and earth connections in offline SMPS

    Hi,

    As we know, Common Mode noise in SMPS is always the hardest one to minimise
    Is it? A common mode choke should work. Or a ferrite ring around the supply wires...

    So we are thinking, can we take the earth out of this situation
    You talk about common mode noise current.

    Indeed it can be minimzed by disconnecting EARTH.

    By why and how is EARTH involved now in noise current?
    * either just by stray capacitance
    * or by installed noise filter capacitors?

    If you now installed filter capacitors (usually "Y" type safety capacitors), then they can not work anymore, thus instead of noise current you get noise voltage....which again may cause radiated noise.

    Klaus

    Added:
    You say it's a power supply installed at a light pole.
    Is the light pole a metal pipe?

    If so, the pipe should be earthed in either case and it will act as a shield and will reduce radiated noise.

    Additionally one can expect that no critical device will be operated in close proximity to the supply.
    Thus I recommend to check if you are using correct EMI limits. Maybe a more "realistic" EMI measurement may be used (higher distance of "receiver antenna", maybe DUT installed in a metal pipe during test.
    If you do this, then you might add these "operating conditions" in the manual of the supply/light.
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    Re: Common mode noise and earth connections in offline SMPS

    The situation is similar to your crystal set thread where it is aptly described as like a one handed clap. A noise generator has to be between two places for it to have effect, there will inevitably be some capacitive coupling on one side, even if there is no voltaic connection but in essence a signal source with only one connection is ineffective. So answering your question, if there was no connection to Earth through the pole and there was no signal between N and L there would be no interference. I don't know your exact product specifications or requirements but I'm guessing if it is up a pole, there is little chance of someone tampering with it while in operation and it should be safe, as long as it is enclosed, to omit the Earth connection altogether. You still have to take care of capacitance to Earth as currents through it would re-introduce a loop and hence common mode noise but you should be able to eliminate 99.99% of it with careful board layout.

    Think of domestic LED lighting in homes, it only has two connections and it passes EMC tests without sophisticated filtering (if any at all!). It is considered safe because it isn't possible to touch anything live. Moving it from ceiling height to the top of a pole should make it even safer.

    Brian.
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