1-1. Introduction
An EMI suppression filter (EMIFIL
) is an electronic component for providing electromagnetic noise
suppression for electronic devices and is used in conjunction with
shields and other protection. This filter only extracts and removes
components that can cause electromagnetic noise from electric
currents that are conducted through wiring. Chapter 1 describes the
reasons for using EMI suppression filters (EMIFIL
) in electronic devices and also provides an overview of the
operations of shields and filters that are typical parts used for
electromagnetic noise suppression.
1-2. What is electromagnetic noise interference?
When electronic devices receive strong electromagnetic waves,
unwanted electric currents can be induced in the circuit and thus
cause unintended operations or interference with the intended
operations. If the energy applied from the outside is too powerful,
electronic devices can be damaged. Even if the energy applied from
the outside is small, if it is mixed with the radio waves used for
broadcasting and communication, it can cause loss of reception,
abnormal noise in sound, or disrupted video at places where the
radio waves for broadcasting and communication are weak. Such
interference caused by external electromagnetic waves is called
electromagnetic noise interference, and the electromagnetic waves
that cause interference are called electromagnetic noise
(hereinafter, noise).
Noise can cause interference with various electronic devices. The
source of noise also varies. Noise that does not cause any
interference with particular appliances (e.g. washing machines and
refrigerators) can seriously affect other devices (e.g. AM radios).
Therefore, there are rules to suppress the noise generated from
electronic devices to a certain level and to make sure that
electronic devices operate correctly under a certain noise level so
that we can use electronic devices with security. These rules are
called noise regulations.
If an electronic device is considered a source of noise, the
occurrence of noise is called emission (emission of noise). In
contrast, if an electronic device is considered a victim of noise,
the tolerance to noise is called immunity (noise tolerance). Noise
regulations specify the emission and immunity of electronic devices.
(Immunity is also referred to as EMS: ElectroMagnetic
Susceptibility)
1-2-1. Classification of electromagnetic noise
Electromagnetic noises can be classified into natural noises and
artificial noises based on the source origin of electromagnetic
noise as shown in the figure.
Natural noises are those that existed before the existence of
electronic devices, for example, lightning and static electricity.
Electronic devices are required to have immunity to natural noises.
Artificial noises are noises that came to be after electronic
devices started to be used and are dealt with both emission and
immunity. As electronic devices have become more commonly used, the
interference caused by artificial noise has increased. This point
will be described in detail in the next section.
1-2-2. Change in noise issues due to congestion of electronic
devices
The congestion of electronic devices used around our vicinity is
increasing, and the content and degree of noise interference are
changing as the performance of each electronic device increases. For
example, before 1970 (before digital circuits became popular), we
were concerned about issues of interference (radio interference)
between wireless radios. However, as household digital devices such
as personal computers became popular, we became more concerned about
receiving interference to the reception of radio and TV by the radio
waves generated by those devices.
In general, as the congestion of electronic devices increases, the
distance between source and victim is reduced, resulting in an
increasing degree of noise interference. In addition, as the
performance of electronic devices increases, the operating circuit
frequency increases and noises of higher frequencies are generated,
resulting in an expanding affected frequency range. Furthermore, due
to the power saving capability of electronic devices, more circuits
can operate with a lower voltage, resulting in more cases of being
affected by noise with less energy.
With the further increase in congestion and performance and the
downsizing of electrical devices, the issue of noise interference is
expected to become more serious.
1-2-3. "Intra-system EMC", the autointoxication of an electronic
device
Noise interference can occur to electronic devices without any
external noise. Noise generated from a circuit inside an electronic
device can cause interference with another circuit in the same
electronic device. This is called intra-system EMC. For example, if
a mobile phone has a built-in digital circuit, noise from the
digital circuit can degrade the receiver performance of the mobile
phone (reducing the receiver sensitivity) as shown in the figure
below. In such a case, the distance between the noise source origin
and the victim is significantly smaller than those of general noise
interferences, causing more serious interference. Depending on the
case, noise suppression is provided on a level far stricter than the
limit of noise regulations.
1-3. Noise suppression
Noise interference occurs when three factors (noise source, victim
and transmission path) exist as indicated by the principle diagram
in Fig. 1-7. Noise interference can be eliminated if you can
eliminate one of these factors.
Therefore, you can take measures on the noise source origin side or
on the victim side. For example, if you do not use digital circuits,
switching power supplies or transmitters (e.g. incandescent lamps),
the noise generated from the electronic devices will be very small.
Another example would be setting up redundancy processing in
software on the victim side.
So, even if the information is slightly altered, the signal can be
recovered. These measures can be fundamental solutions. But, many
such cases can cause large secondary effects like significantly
reducing the performance of electronic devices or increasing their
size, making such measures unrealistic.
Usually, noise is shut out in the transmission path as shown in Fig.
1-8. There are two types of noise conduction (spatial conduction and
conductor conduction). As shown in the figure, the spatial
conduction is dealt with by shields, while the conductor conduction
is dealt with by filters.
As shown in Fig. 1-7, spatial conduction and conductor conduction
have a tendency of mutual transformation through a wire that works
as an antenna. Therefore, even if conductor conduction is only a
problem at one location, you cannot completely ignore the
possibility of spatial conduction.
1-3-1. Shields
Shields refer to surrounding electromagnetic fields being shut out
by enclosing the target object with a metal plate or other
protection as shown in Fig. 1-9.
Although the effects of shields generally depend on the
conductivity, magnetic permeability and thickness of the material
used, the noise suppression for general electronic devices can
achieve sufficiently large effects with a very thin metal plate such
as aluminum foil. You must be aware that the effects of the noise
suppression for electronic devices often depend on the connection
method for forming an enclosure (gaps, contact resistance etc.)
rather than the material specifications.
When making openings in the shield for heat release, limiting the
maximum size of each opening is more important than limiting the
total area of the openings. As shown in Fig. 1-10, if there is an
elongated opening or slit, this part can work as a slit antenna
(especially for high-frequency range where the length l in the
figure exceeds 1/2 of the wave-length) and the radio waves can go in
and out the shield. In order to prevent this, the individual
openings should be kept small. From this viewpoint, plate materials
with many small holes such as punched metal and expanded metal are
good materials for both ventilation and shielding.
1-3-2. Filters
Filters refer to a part or function that can let the necessary
components through and remove unwanted components among the electric
current flowing in conductors. Although the noise is diverted to
ground in Fig. 1-12, the noise energy can be alternatively absorbed
inside the parts or can be returned to the noise source origin
(increasing the impedance).
Since noise tends to be distributed more in a relatively higher
frequency range as shown in Fig. 1-13, the noise suppression for
electronic devices usually uses low-pass filters that remove
high-frequency components. You can use general-purpose parts such as
inductors (coils), resistors and capacitors for this low-pass
filter. However in order to completely shut out noise, a dedicated
component, an EMI suppression filter, is used. EMI suppression
filters will be described in detail in Chapter 3 of this document.
Apart from these filters that take advantage of uneven frequency
distribution of noise, there are filters that take advantage of the
voltage difference (varistors etc.) and filters that take advantage
of the conduction mode difference (common mode choke coils etc.).
In addition to these filters, transformers, optical cables or
optical isolators may be used as a kind of filter. Although these
components can achieve exceptional denoising effects in some cases,
the applicable situations are limited.
1-4. How to use shields and filters
1-4-1. Use shields and filters at one point
Filters are used for noise that conducts through conductors, while
shields are used for noise that conducts through space. However,
since the conductor through which noise conducts can also work as an
antenna, these two types of conductions are mutually transformed
into each other via the conductor as an antenna. Therefore, in order
to completely shut out noise, both filters and shields need to be
used at one location.
For example, when a shield is used for shutting out spatial
conduction, if there is a conductor that penetrates through the
shield as shown in Fig. 1-14, this conductor picks up and draws the
noise inside the shield to the outside of the shield causing noise
emission. As a result, spatial conduction cannot be completely shut
out solely with a shield.
Likewise, when a filter is used for shutting out conductor
conduction, the wires before and after the filter may be coupled
with each other via spatial conduction as shown in Fig. 1-15. As a
result, the conductor conduction cannot be completely shut out
solely with a filter.
When using both a shield and filter together at one location as
shown in Fig. 1-16, both spatial conduction and conductor conduction
are completely shut out allowing complete noise elimination.
If the length of the conductor between the noise source and filter
is significantly short as shown in Fig. 1-17, the effect of the
conductor as an antenna can be ignored and the noise can be thus
eliminated only with a filter to a certain extent. Therefore, if you
can use a filter at a close vicinity to the noise source, the noise
suppression can be achieved only with a filter.
1-4-2. Filters and ground
In order to effectively use filters and shields, it is generally
necessary to have a connection with good ground.
If there is a built-in bypass capacitor inside the filter, ground
becomes a route to return the noise current back to the noise source
as shown in Fig. 1-18. You need to consider keeping the impedance of
this part very low.
If the impedance to ground is large as shown in Fig. 1-19(a), some
voltage appears at ground due to the noise current interfering with
good noise elimination. If this ground is shared with another wire
attached to another filter, the voltage appeared at ground can go
back to other wires through the filter capacitor.
This type of noise coupling through the impedance of ground is
called common impedance coupling. This status of having noise on
ground is also referred to as the occurrence of common mode noise.
Common mode noise will be described in a later chapter. The common
impedance coupling is one of the mechanisms that cause common mode
noise.
Since the effect of filters that have built-in capacitors are highly
susceptible to the conditions of the connected ground, you need to
use a stable ground with a low impedance.
1-4-3. Shields and ground
Shield also need ground.
Static shields must be connected to ground that is in principle an
external earth (zero volts). Since an electrical current flows on
the wire connected to ground in accordance with the change in the
electric field to be shielded, the wire needs to have a low
impedance.
In many cases, when using shielded cables, the shield also works as
a return pathway for the current that has gone through the inner
conductor (such as the outer conductor of coaxial cable for
example). Therefore it needs to be connected to ground that can
return this current (when shielding a signal, connect to the circuit
ground).
In cases where the noise has been guided to ground as is the case of
Fig. 1-19, if shields are connected to this ground, the shield draws
out and then emits the noise from ground working just like as an
antenna, which could increase the noise. When connecting shields,
you need to choose a low impedance ground with a stable potential.
Enclosure shielding cases practically work as a relatively good
ground. If there is a shielding case that covers the entire device,
this shielding case itself can be a good ground for the purpose of
noise suppression even though it is not connected to earth (if the
discharge current needs to be drained to earth for the purpose of
suppressing static electricity or other currents, it needs to be
connected to earth). Here, we call this ground an enclosure shield
ground.
This enclosure shield ground can also be used as ground for the
shielded cables. However, in order to make this shield work as a
return pathway for signals as described above, it also needs to be
connected with the circuit ground. Therefore, if the enclosure
shield ground and the circuit ground have been separated, the
connection gets complicated.
1-4-4. Reinforcement of ground
As described above, connecting to a stable ground is necessary for
enhancing the effect of filters and shields. In addition, when using
a shielding case that covers the entire device, this shielding case
itself can be used as a stable ground. Therefore, shields generally
have a function to stabilize ground.
If there is a wire that goes through this shielding case, you are
providing a hole that allows noise going in and out of the shield as
shown in Fig. 1-14 making the shielding case ground unstable. In
such a case, you can use filters in this wire to stop noise from
going in and out so that the shielding case ground can be
stabilized.
Since appropriately applied shields and filters will work for
stabilizing ground as described above, there is a mutually helpful
relationship between shields, filters and ground.
Circuit ground is also a type of ground apart from the
above-described shielding case ground and often induces more noise
voltage than the shielding case ground does. This kind of ground is
called "dirty ground." In contrast, a ground where noises are not
induced is called "clean ground."
It is desirable that ground to which shields or filters are
connected is a clean ground. However, a circuit ground connection is
required to return the signal return current or to return the noise
current back to the noise source origin. If the circuit ground is
dirty, the noise voltage should be minimized by reducing the
impedance of the circuit ground, providing a ground plane along the
circuit board, or connecting with the enclosure shield ground.
The operation of stabilizing the ground noise by reducing the
voltage in this manner is referred to as "ground reinforcement."
Covering a part of the circuit board with a shielding case helps the
ground reinforcement. Fig. 1-21 shows some of the methods for ground
reinforcement.
1-4-5. Filters and ground
When connecting cables to the shielding case, filters are attached
to prevent noise from going in and out through the cables. Ground of
this filter is to be formed on the circuit board. However, in order
to stabilize ground, it is often connected to the shielding case
ground instead of the circuit ground. Therefore, ground for filters
that are connected to the shielding case ground is often created at
the section where the cables are attached. Here, we call this ground
a "filter ground."
In general, a filter ground is not only connected to a shielding
case ground but also connected to a circuit ground in order to
return the noise generated inside the circuit to the noise source.
In this case, it also works for ground reinforcement for the circuit
ground at the same time. When using shielded cables, the shield can
be connected to the filter ground. In this case, it must be
connected to the shielding case ground at a very low impedance as
the effectiveness of shielded cables depend on the quality of the
filter ground.
Fig. 1-22 shows an example of a filter ground. It is important to
keep the filter ground at a very low impedance with reference to the
shielding case ground.
Although Section 1-4-2 has explained that ground of the filter is to
be connected to the noise source at a low impedance (with reference
to the circuit ground), Fig. 1-22 shows that the connection to the
shielding case ground is given a priority. This is because it is
practically difficult to return to the noise source at a low
impedance as the connection point of cables are usually far from the
noise source. It is also because it is hard to get a good result by
connecting the filter ground at a low impedance as the circuit
ground is often dirty due to noise from other circuits.
Therefore, when using a filter for a single circuit at a close
vicinity to the noise source, the filter is connected to the circuit
ground as explained in Section 1-4-2. However, this connection is
not easily achieved when the noise source is far away (such as a
case of cable junction) and two or more noise sources need to be
considered. As a practical technique, when using filters at a cable
junction, you can find a stable ground such as a shielding case
ground as shown in Fig. 1-22 and connect the filter ground.
Chapter 1 - Key points
-
An electromagnetic wave that interferes with the operation
of electronic devices is called noise.
-
The means to shut out the noise in the transmission path
include shields and filters.
-
In order to make shields and filters work effectively,
grounding is important.
Next: Chapter 2 Mechanism of Causing Electromagnetic Noise