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.
