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EMI and harmonic filter

how does the differential mode EMI filter differ from normal harmonic
filter? any fundamental difference in design? if yes, why?
 
P

Pooh Bear

Jan 1, 1970
0
how does the differential mode EMI filter differ from normal harmonic
filter?

In the same way as differential is not the same as common-mode.
any fundamental difference in design? if yes, why?

In the same way as differential is not the same as common-mode.

Graham
 
R

redbelly

Jan 1, 1970
0
Pooh said:
In the same way as differential is not the same as common-mode.


In the same way as differential is not the same as common-mode.

Graham

For shame, Graham! You didn't answer the final question:
if yes, why?

Because differential is not the same as common-mode.

Mark
 
J

John Popelish

Jan 1, 1970
0
how does the differential mode EMI filter differ from normal harmonic
filter? any fundamental difference in design? if yes, why?
I think they deal with different frequency ranges. EMI involves radio
wave frequencies (say, 30 kHz and above) while harmonic filters deal
with the lower multiples of the power line frequency that occur when
nonlinear loads like rectifiers are used on sine wave sources. The
first kind are there to reduce radiated or conducted interference,
while the second are there to reduce the distortion of the power line
waveform.
 
P

Pooh Bear

Jan 1, 1970
0
redbelly said:
For shame, Graham! You didn't answer the final question:


Because differential is not the same as common-mode.

Mark

Whoops ! My apologies. You're quite right of course. ;-)

Graham
 
hi John,
Suppose we are talking about the conducted EMI in the range of well
known range 150KHz -30Mhz, how does the differential mode noise differ
from harmonics. Is not the harmonics cause of the differential mode
noise. Since the Differential mode filter aim is also remove high
frequency component from the supply, is not it same as reducing
distortion. This thing is really bugging me. I have read several books
on EMI, and none explains about it. I wish to talk to some of the
authors, unfurtunately easy communication to authors are not mentioned
in books. If they are fundamentally different, whats that? if not why
so huge literature on such differential filter design. and i do not see
any difference from classical filter design.
kristo
 
P

Pooh Bear

Jan 1, 1970
0
hi John,
Suppose we are talking about the conducted EMI in the range of well
known range 150KHz -30Mhz, how does the differential mode noise differ
from harmonics.

Jeez !

Power line frequency harmonics are typically caused by the short conduction
angle of typical capacitor input filters after the rectifier.

EMI filters do not address these 'harmonics' in any significant way *at
all*. The most troublesome frequencies are typically the 3rd and 5th
harmonics of the power line frequency and effectively completely untouched
by an EMI filter.

Is not the harmonics cause of the differential mode
noise. Since the Differential mode filter aim is also remove high
frequency component from the supply, is not it same as reducing
distortion. This thing is really bugging me. I have read several books
on EMI, and none explains about it.

You clearly haven't tried very hard then !
I wish to talk to some of the
authors, unfurtunately easy communication to authors are not mentioned
in books.

I doubt they'd want to talk to someone who hasn't even bothered to learn
the basics !
If they are fundamentally different, whats that? if not why
so huge literature on such differential filter design. and i do not see
any difference from classical filter design.
kristo

The classic EMI filter is there to remove *common mode* noise caused by the
switching in the power supply.

Harmonics are dealt with by using 'PFC' techniques.

Graham
 
J

John Popelish

Jan 1, 1970
0
hi John,
Suppose we are talking about the conducted EMI in the range of well
known range 150KHz -30Mhz, how does the differential mode noise differ
from harmonics.

If they occur at harmonic multiples of the power line frequency, then
they are also harmonics. If they are caused by other sources of
energy (switching regulator pulses, for example) then they are not
harmonics. But most harmonic filters deal with the lower multiples,
because these are what propagate efficiently down the power line and
affect other loads.
Is not the harmonics cause of the differential mode
noise.

Yes, they are one kind of differential mode noise. Any source that
puts a different voltage on the 2 power lines connecting a piece of
equipment to the power lines is a source of differential mode noise,
but not all are harmonically related to the power line frequency.
Since the Differential mode filter aim is also remove high
frequency component from the supply, is not it same as reducing
distortion.

Sure. At the lower harmonics of the power line frequency, you don't
want to cause such distortion, because it affects other loads on the
same line. If you have a sensitive load, you may need to remove some
of this distortion to protect your load. The higher end of the
frequency range (where it is not so important or obvious whether a
frequency is an exact multiple of the line frequency or not) involves
preventing radio frequency radiation leaking out of a piece of
equipment (where the power line acts as a transmission line) or
problems with such energy getting into your load (but usually the
first). Differential and harmonic filters do these things. Most
differential mode filters are either there as part of a spike
suppression system (for line spikes coming in when big load switch off
or for distant lightning disturbances) or are part of a conducted
noise suppression system (to meet standards).
This thing is really bugging me. I have read several books
on EMI, and none explains about it. I wish to talk to some of the
authors, unfurtunately easy communication to authors are not mentioned
in books. If they are fundamentally different, whats that? if not why
so huge literature on such differential filter design. and i do not see
any difference from classical filter design.

The big problem with differential mode filters that must be solved is
that they have to pass a really big differential signal -- the line
voltage and load current. Common mode filters can often be much more
compact, because they can use two winding coupled inductors that
cancel the magnetic field of the load current (that big, required
differential signal), so the core has to deal only with the common
mode signal. So big common inductors in small volumes, that pass big
load currents, are possible.

And, with common mode filters, the energy that gets through does not
affect other loads across the line (except as common mode
interference) but the two power lines act together as a single
radiating antenna element. So the energy doesn't go so far before it
escapes as radiation. And that radiation (or susceptibility to
received radiation) is what most common mode filters are trying to
control.
 
hi,
this discussion was not to annoy anyone. It was merely for learning
purpose. The person who himslef do not know the dobut
runs to beat the other back. It has been clearly seen from the
discussion.
The harmonics i was refering is to switching frequency harmonics is
SMPS.
anyway, A lot of thanks to John for kindly putting his concepts.
 
P

Pooh Bear

Jan 1, 1970
0
hi,
this discussion was not to annoy anyone. It was merely for learning
purpose. The person who himslef do not know the dobut
runs to beat the other back. It has been clearly seen from the
discussion.
The harmonics i was refering is to switching frequency harmonics is
SMPS.

I thought as much initially. To suppress that type of interference (
switching noise ) you use a *common-mode* filter.

But you should be aware that 'harmonics' are another issue entirely. See
IEC 61000-3-2.

I'm not aware of the use of any differential mode filter in typical EMI
applications btw.

Graham
 
L

legg

Jan 1, 1970
0
I thought as much initially. To suppress that type of interference (
switching noise ) you use a *common-mode* filter.

But you should be aware that 'harmonics' are another issue entirely. See
IEC 61000-3-2.

I'm not aware of the use of any differential mode filter in typical EMI
applications btw.

If you are refering to commodity power supply AC line filters, then
perhaps your impression is due to the fact that common mode choke
leakage and loss are also effective on differential mode components,
if the appropriate differential mode capacitors are present.

At the actual conversion frequency, however, these small parasitic
inductive and resistive terms are seldom sufficiently to allow
reasonably sized and priced capacitors to work, without additional
discrete differential mode inductors. I'm suprised you've not noticed
their presence.

Differential filters are everywhere, in general EMI control of signal
lines, on input and output, even if only comprised of a single series
resistor, or feedthrough capacitor. The formation of effective common
mode filters is increasingly difficult or impractical, as the
interconnections involved increase in either quantity or variety of
functions.

RL
 
P

Pooh Bear

Jan 1, 1970
0
legg said:
If you are refering to commodity power supply AC line filters, then
perhaps your impression is due to the fact that common mode choke
leakage and loss are also effective on differential mode components,
if the appropriate differential mode capacitors are present.

They are bound to be. That's not their primary function however.

At the actual conversion frequency, however, these small parasitic
inductive and resistive terms are seldom sufficiently to allow
reasonably sized and priced capacitors to work, without additional
discrete differential mode inductors. I'm suprised you've not noticed
their presence.

Differential filters are everywhere, in general EMI control of signal
lines, on input and output, even if only comprised of a single series
resistor, or feedthrough capacitor.

Yeah I do those too ! I don't see a feedthru cap helping PSU emisions much
though.

Pls don't 'muddy the waters'. The methods for dealing with classic SMPS noise
are well established.

Graham
 
J

John Popelish

Jan 1, 1970
0
Pooh Bear wrote:
(snip)
I'm not aware of the use of any differential mode filter in typical EMI
applications btw.

Then you have missed something.

Any common garden variety EMI filter has both common mode and
differential elements. In this typical example of a commercial EMI
filter, the common mode (coupled inductor) choke and line to ground
capacitors are the (primarily) common mode filter elements while the
individual series inductors and line to line capacitors are the
(primarily) differential mode filter elements.
http://www.cor.com/PDF/Q.pdf
 
P

Pooh Bear

Jan 1, 1970
0
John said:
Pooh Bear wrote:
(snip)

Then you have missed something.

Any common garden variety EMI filter has both common mode and
differential elements. In this typical example of a commercial EMI
filter, the common mode (coupled inductor) choke and line to ground
capacitors are the (primarily) common mode filter elements while the
individual series inductors and line to line capacitors are the
(primarily) differential mode filter elements.
http://www.cor.com/PDF/Q.pdf

But it's called a *common-mode* filter you clot !

Parasitic effects are another thing entirely !

Graham
 
J

John Popelish

Jan 1, 1970
0
Pooh said:
John Popelish wrote: (snip)


But it's called a *common-mode* filter you clot !

Parasitic effects are another thing entirely !

(quote of first paragraph on this data sheet)

"This series of RFI power line filters has been developed
specifically for switching power supplies and is designed to
be all the power line filtering needed to control conducted
emissions all the way down to 10kHz. High attenuation is
provided for both common mode and differential mode
interference throughout the frequency range with no
degradation of performance due to the large peak currents
drawn by switching power supplies."

Did you see somewhere on this data sheet where the filter is "called a
common mode filter"?

I didn't refer to any parasitic effects, but to the purposes of
specific components, included in this typical line filter.

(Hint: Exclamation points and personal slurs do not improve your
arguments.)
 
P

Pooh Bear

Jan 1, 1970
0
John said:
(quote of first paragraph on this data sheet)

"This series of RFI power line filters has been developed
specifically for switching power supplies and is designed to
be all the power line filtering needed to control conducted
emissions all the way down to 10kHz. High attenuation is
provided for both common mode and differential mode
interference throughout the frequency range with no
degradation of performance due to the large peak currents
drawn by switching power supplies."

Did you see somewhere on this data sheet where the filter is "called a
common mode filter"?

I didn't refer to any parasitic effects, but to the purposes of
specific components, included in this typical line filter.

(Hint: Exclamation points and personal slurs do not improve your
arguments.)

The filter you quote talks about going down to 10kHz. One can infer
something about it from that.

The typical 'off the shelf' EMI filter only goes down to 150kHz !

A rather different beast IMHO.

Nor is any filter I've come across been suited to reducing line freq
harmonics of order 3 or 5.

I suspect the OP isn't at all familiar with EMI measures

Graham
 
J

John Popelish

Jan 1, 1970
0
Pooh said:
John Popelish wrote:




The filter you quote talks about going down to 10kHz. One can infer
something about it from that.

The typical 'off the shelf' EMI filter only goes down to 150kHz !

A rather different beast IMHO.

Nor is any filter I've come across been suited to reducing line freq
harmonics of order 3 or 5.

Differential and harmonic are not synonymous. Harmonic distortion is
caused by nonlinear differential loads, but there are non harmonic
differential noise problems as well. This example filter is not made
for low order harmonic differential noise but for higher frequency,
non harmonic, differential noise (like you get when a switch mode
supply draws power in pulses at >10kHz from its storage capacitor,
causing ripple voltage while the line rectifiers are conducting and
connecting that capacitor to the line.
I suspect the OP isn't at all familiar with EMI measures.

Little doubt of that, since his was a very basic question.
 
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