Hi Muriel,
I think the answers so far have given you some good ideas. There are
number of different things that can happen. Filter design should not be
considered as completely separate from the system design especially in a
power supply.
First, by providing a low impedance to the power supply switching
circuitry a primary goal is to minimize the magnitude of the noise
voltage generated. Less noise means less extra filtering will be
required. Second, high impedance components in series, reflect the high
frequencies back into the system.
The goal of filtering is to mismatch the noise source to the load. This
generally reflects the energy back to the source where it is dissipated.
In reality the losses can be anywhere in the loop from source to filter,
but remember we are looking at minute power levels.
It must also be realized that an improper choice of filter can increase
the risk for emissions. If the source has a low impedance, and it is
filtered with a shunt capacitor, more current will flow which might
increase switching currents and in turn, increase noise in the power
system. If a series ferrite or small resistor is used the switching
current will be the same or less than for the unfiltered case.
Reflecting the energy back usually entails a very short distances (put
the filter by the problem) so the antenna effect of the associated
etches will be minimal. The same noise currents on a long I/O or power
cable will be much more easily radiated.
So look at what needs to be filtered and how its impedance varies across
the frequency range of interest. This will give you an idea of how you
will be able to filter it most effectively.
Fun stuff isn't it - hope this is helpful,
Colin..
-----Original Message-----
From: Muriel Bittencourt de Liz
[mailto:mur...@eel.ufsc.br]
Sent: Monday, December 03, 2001 4:00 PM
To: EMC-PSTC List
Subject: physics behind EMI powerline filter
Hello Group,
I have a "long-time" question, concerning the energy
issues in a EMI powerline filter.
I'll put a case, and ask the question after.
This is the case:
- When trying to minimize the conducted emissions from a
electrical equipment / circuit, one of the things to do is to put a EMI
filter at the power entrance. This filter can be from a manufacturer
(ready filter) or you can make one (with common mode inductors,
capacitors, inductors).
This are the questions (they arearelated, i.e., complete
each other):
- What, physically speaking, happens to the EMI energy
that leaves the equipment when I add a filter??? When there is no
filter, I understand that the energy goes to the mains?? Does the filter
reflect the EMI energy, keeping it "arrested" inside the equipment?
- Thinking under the light of the principle of energy
conservation, what happens to the EMI energy when I add a filter?? It
cannot be lost...
- ... And, supposing that the energy keeps arrested
inside the equipment, isn't it worse for radiated emissions?? i.e., it
can increase the level of radiated emissions??
Thanks in advance for your attention.
Regards,
Muriel Bittencourt de Liz
Ph.D. Student
Interest Areas: EMC for power electronics, RF measures,
EM interference
Federal University at Santa Catarina State
Florianópolis, Santa Catarina, Brazil
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