EMC filter design tutorial
a tutorial or summary about the basics of EMC filter design and the use of
RFI EMC filters to ensure that circuits meet the electromagnetic compatibility
EMC requirements and pass the EMC testing that is required today.
In order that an item of electronics equipment can pass its
EMC testing and gain its EMC compliance, it is necessary to incorporate various
elements into the design. By designing the circuit to meet the electromagnetic
compatibility, EMC requirements it is possible to significantly reduce the
levels of unwanted signals entering and leaving the unit. One of the major ways
in which this can be done is to use an EMC filter or a series of filters.
There are many ways in which EMC filters can be incorporated
into a unit from a mechanical viewpoint. They may exist as stand alone EMC
filters to be fixed near to the extremities of the unit. They may be mounted on
the edge of the electronics board. However one popular method of incorporating
an EMC filter into a unit is to incorporate the filter into the connector
itself. This has many advantages in terms of convenience and performance.
However whatever the method used, a filter is often necessary if the
electromagnetic compatibility, EMC requirements are to be met.
EMC filter methodology
Although circuits may be well screened to prevent any signal
radiated or being picked up by the circuit itself, there are always
interconnections to and from the electronics circuit. These wires themselves can
conduct unwanted signals into and out of the unit. If the unit is to be able to
meet its electromagnetic compatibility, EMC requirements and pass its EMC
testing, it is necessary to reduce the levels of unwanted signals that can enter
or leave the unit via its interconnections.
In order to enable the unwanted signals to be removed, EMC
filters need to be placed in the various lines. The idea is that the interfering
signals generally have a frequency above that of the signals normally travelling
along the wire or line. By having what is termed a low pass filter as the EMC
filter, only the low frequency signals are allowed to pass, and the high
frequency interference signals are removed.
These EMC filters can be in one of a variety of formats.
Often they may be as simple as a resistor or a ferrite placed around a wire or
cable. For more exacting requirements, these EMC filters may need to be made up
from a number of components.
The EMC filters may categorised into two main types. One is
where the unwanted energy is absorbed by the EMC filter. The other type of
filter rejects the unwanted signal and in this case it is reflected back along
the line. For EMC filtering applications, the absorptive type is preferred.
EMC filter application
When developing filters for use in electromagnetic
compatibility, EMC applications, the EMC filters are nearly always low pass
filters, although on occasions bandpass filters may be used. The reason for
using low pass filters is that typically interfering signals, i.e. ones that are
easier to pick up or radiate tend to be at higher frequencies. These can be
filtered by allowing the low frequencies through and rejecting the high
frequencies.
The cut-off point for the low pass filter used as the EMC
filter has to be chosen so that it rejects the unwanted frequencies, but does
not have any undue effect on the wanted signal. Unfortunately this choice is not
always easy and it may require some degradation of the wanted signal.
The EMC filter placing is of importance. EMC filtering can be
placed at any or every level of assembly between segregated areas of circuitry.
EMC filters may be placed between segregated areas of a printed circuit board.
They may be placed between different boards within a module or sub-assembly, and
an EMC filter may be placed between different modules or subassemblies. However
a particularly important place for EMC filters is between the equipment and its
external environment. An EMC filter placed here is particularly effective as it
will prevent unwanted signals even entering the equipment. Once they enter they
are more difficult to contain.
EMC filter design
The EMC filter design is critical to the electromagnetic
compatibility, EMC performance. The EMC filter must be capable of providing the
required level of attenuation of the unwanted signals while allowing through the
wanted signals. In addition to this the EMC filter design must match both the
source and load impedances.
Typically for a high impedance circuit, a capacitor connected
between the line and ground provides better results, while for low impedance
circuits a series inductor placed within the line provides the best results.
Often a single component like this designed to have a reactance with little
effect at frequencies appropriate to the wanted signals, but a much higher
effect at the higher frequencies of the unwanted signal can provide levels of
attenuation of up to 30 dB or 40dB in some cases. To improve the performance of
one of these basic filters, further components can be added to make
multi-component EMC filters. However to give the required performance they must
be configured correctly. One precaution to ensure that inductors face a low
impedance sink or source and capacitors face a high impedance.
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