Electrical code Compliance - conducted EMI basic question

Author

conducted EMI basic question

krishmaniac@hotmail.com

2007-04-08, 3:25 am

I am wondering what may be the real reason behind limiting the
conducted EMI from a switch mode power supply connected to a utility.
I was just thinking initially that the noise of the SMPS will be
radiated through the power cord and affect the near by applicanes, but
recently i read that it may get conducted to the system and coupled to
others. The system is a high voltage power system, the noise level is
so minute compared to the power level, and most often other system
connected to the utility will have a noise bypass capacitors
connected. So what is conducted EMI everyone talking about. Is it
routing around the whole system or just being radiated through the
supply cord?

Paul E. Bennett

2007-04-08, 9:25 am

krishmaniac@hotmail.com wrote:

> I am wondering what may be the real reason behind limiting the
> conducted EMI from a switch mode power supply connected to a utility.
> I was just thinking initially that the noise of the SMPS will be
> radiated through the power cord and affect the near by applicanes, but
> recently i read that it may get conducted to the system and coupled to
> others. The system is a high voltage power system, the noise level is
> so minute compared to the power level, and most often other system
> connected to the utility will have a noise bypass capacitors
> connected. So what is conducted EMI everyone talking about. Is it
> routing around the whole system or just being radiated through the
> supply cord?

Many power utilities use the power lines for data transmission purposes to
enable them to monitor the state of their sub-stations, switching centres
and network synchronisation. Interference to those low level signals could
upset quite a lot of people.

--
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Paul E. Bennett ....................<email://peb@amleth.demon.co.uk>
Forth based HIDECS Consultancy .....<http://www.amleth.demon.co.uk/>
Mob: +44 (0)7811-639972
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krishmaniac@hotmail.com

2007-04-08, 8:25 pm

Thanks. So is all these conducted EMI stuffs just to avoid the
interference to PLC(power line communications). But its a huge buzz
for domestic,industrial users as well where the power line is not used
for communication purposes. If its problem a hughe filter can be
located at the service mains of each customer to avoid all these
conducted emi issues. I am still not clear about the conducted EMI
issues.
> ********************************************************************

sdemc@canada.com

2007-04-09, 3:25 am

On Apr 8, 1:12 am, krishman...@hotmail.com wrote:
> I am wondering what may be the real reason behind limiting the
> conducted EMI from a switch mode power supply connected to a utility.
> I was just thinking initially that the noise of the SMPS will be
> radiated through the power cord and affect the near by applicanes, but
> recently i read that it may get conducted to the system and coupled to
> others. The system is a high voltage power system, the noise level is
> so minute compared to the power level, and most often other system
> connected to the utility will have a noise bypass capacitors
> connected. So what is conducted EMI everyone talking about. Is it
> routing around the whole system or just being radiated through the
> supply cord?

Conducted EMI for the US, is regulated mainly by the FCC (in
particular 15.107). It used to be that FCC required measurements 450
kHz and up. It is likely no coincidence that the FCC, who is in
charge of broadcast, started the limit at the begining of the AM
broadcast range. The technical requirements are about protecting
'other systems' particularly if those other systems are receivers (AM,
Shortwave, Amateur RF, Citizens Band) and also data line receivers
like the one already mentioned. Although the noise level is minute
in comparison to the overall system power, the requirement is about
the noise level at particular frequencies, where signals are low and
sensitivities high. This means it really doesn't matter what your 60
Hz power is. The combination frequency/power requirement keeps in
check unintentional interference with licensed broadcast services or
intentional transmissions. Conducted issues can come from a variety
of areas within a system and can 'leak' out the power supply cord.
SMPS problems can be challenging to fix due to the relatively high
switching frequencies, but there is some good hints on the internet
which can help.

Hope this helps,
Scott.

krishmaniac@hotmail.com

2007-04-10, 9:25 pm

On Apr 9, 11:32 am, s...@canada.com wrote:
> On Apr 8, 1:12 am, krishman...@hotmail.com wrote:
>
>
> Conducted EMI for the US, is regulated mainly by the FCC (in
> particular 15.107). It used to be that FCC required measurements 450
> kHz and up. It is likely no coincidence that the FCC, who is in
> charge of broadcast, started the limit at the begining of the AM
> broadcast range. The technical requirements are about protecting
> 'other systems' particularly if those other systems are receivers (AM,
> Shortwave, Amateur RF, Citizens Band) and also data line receivers
> like the one already mentioned. Although the noise level is minute
> in comparison to the overall system power, the requirement is about
> the noise level at particular frequencies, where signals are low and
> sensitivities high. This means it really doesn't matter what your 60
> Hz power is. The combination frequency/power requirement keeps in
> check unintentional interference with licensed broadcast services or
> intentional transmissions. Conducted issues can come from a variety
> of areas within a system and can 'leak' out the power supply cord.
> SMPS problems can be challenging to fix due to the relatively high
> switching frequencies, but there is some good hints on the internet
> which can help.
>
> Hope this helps,
> Scott.

Thanks for the clarifications. I appreciate that. My only questions
was suppose a SMPS inject the noise goes into the system, other system
connected to will be affected by it. The noise injected has to find
its path back to the source, how it is completed. I undertsand its a
big mesh of connecttion in a system, somewhere in the netwrok will be
the ground path. I have these doubts because i have seen in IEEE
papers and practice a DC power supply without any ground connection
having bulky common mode filters at the input. It is explained Common
mode noise is the one flowing in the ground wire and returns via both
the supply lines; when there is no ground wire where this CM problem
comes from.
Thanks for explanations.

Ned Forrester

2007-04-11, 1:25 pm

krishmaniac@hotmail.com wrote:
> On Apr 9, 11:32 am, s...@canada.com wrote:
>
> Thanks for the clarifications. I appreciate that. My only questions
> was suppose a SMPS inject the noise goes into the system, other system
> connected to will be affected by it. The noise injected has to find
> its path back to the source, how it is completed. I undertsand its a
> big mesh of connecttion in a system, somewhere in the netwrok will be
> the ground path. I have these doubts because i have seen in IEEE
> papers and practice a DC power supply without any ground connection
> having bulky common mode filters at the input. It is explained Common
> mode noise is the one flowing in the ground wire and returns via both
> the supply lines; when there is no ground wire where this CM problem
> comes from.
> Thanks for explanations.
>

There is always a return path through the stray capacitance to earth of
both devices.

Conducted emissions on the mains are regulated, not only because they
might conduct directly to, and interfere with, another connected device,
but also because once conducted onto the mains, the mains wiring acts as
an antenna to radiate those emissions and thus they might interfere with
radio services.

--
NOTE: to reply, remove all punctuation from email name field

Ned Forrester n_f_orrester@whoi.edu 508-289-2226
Applied Ocean Physics and Engineering Dept.
Oceanographic Systems Lab http://adcp.whoi.edu/
Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA

w_tom

2007-04-12, 9:25 am

On Apr 10, 10:04 pm, krishman...@hotmail.com wrote:
> I have these doubts because i have seen in IEEE papers and
> practice a DCpower supply without any ground connection having
> bulky common mode filters at the input. It is explained Common
> mode noise is the one flowing in the ground wire and returns via
> both the supply lines; when there is no ground wire where this CM
> problem comes from.

Common mode currents can enter on AC mains and leave via data lines,
antenna wire, chassis, or any other conductors. Common mode does not
necessaryily mean a ground wire

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