Transcript EMI EMC pt 4

EMC Design of PCBs
Contents
•
•
•
•
•
•
•
Introduction
Component selection and mounting
PCB trace impedance
PCB layer stackup
Crosstalk control
Power distribution
Decoupling
Contents
•
•
•
•
Zoning
Grounding
VIAs connection
Termination
Introduction
Introduction
Introduction
• EMC Control Measures
can be incorporated at
three levels.
– Primary level (PCB, Circuit
etc)
– Secondary level (Cables,
connector etc)
– Tertiary level (Shielding)
Introduction
Introduction
• Primary Level
– Circuit design measures, Board layout
grounding
• Secondary Level
– Cables & Connectors
• Tertiary Level
– Apertures ,Gaskets etc
Component Selection & Mounting
• Digital circuits
– Digital circuits are generator of EMI,high
frequency square-waves are rich in harmonics
are distributed throughout the system.
• Analog circuits
– Analog circuits are much quieter because
high frequency square waves are not normally
the feature.
Component Selection & Mounting
• Fourier transform
– Basic to understanding of why switching circuit cause
interference is the concept of time domain /frequency
domain transform
– Switching waveform can be represented as
trapezoidal with defined rise and fall time .The
harmonic amplitude content of a trapezoid decreases
from the fundamental at a rate of 20dB per decade
until a breakpoint is reached at 1/ptr, after which it
decreases at 40dB/decade
Component Selection & Mounting
Component Selection & Mounting
• It is advisable to use the slowest logic
family that will do the job; don’t use fast
logic when it is unnecessary. Treat with
caution any proposal to substitute devices
from a faster logic family, such as
replacing 74HC parts with 74AC.
Component Selection & Mounting
• Harmonics contents of different logic families
Component Selection & Mounting
• Some IC manufacturers are addressing the problem of RF
emissions at the chip level.
• By careful attention to the internal switching regime of VLSI
devices, noise currents appearing at the pins can be
minimized.
• The transition times can be optimized rather than minimized
for a given application .
• Revised package design and smaller packages can allow
the decoupling capacitor to be placed as close as possible
to the chip, without the internal lead frame’s inductance
negating its effect;
Component Selection & Mounting
• From the point of
view of immunity, a
slow logic family will
respond less readily
to fast transient
interference
Component Selection & Mounting
• The high frequency spectral content of the
component signal are gauged by the signal
speed :
where f0 is the fundamental frequency (inverse
of repetition rate) of its signal, I0 is the
magnitude of its drive current, and tr is its
rise/falltime
Component Selection & Mounting
• In a digital system we know that the key
component of a signal that dictates its highfrequency spectral content is its rise/falltime. In
fact we can say that a digital signal has a
“bandwidth” that is essentially the reciprocal of
its risetime, BW ¼ 1/tr . Hence a clock having a
500 ps rise/ falltime has significant spectral
content up to 2 GHz.
Component Selection & Mounting
• In the initial layout of the PCB, a spreadsheet
should be compiled of every part to be placed on
the PCB according to its “speed”.
• This priority list should be given to the layout
personnel with instructions to apply the greatest
care to placement of the highest-speed
components at the top of the list and to place the
components first according to the highest speed
and next to the next lowest speed, and so on.
Component Selection & Mounting
• Numerous automated layout software provide for
“autorouting” so that the schematic is laid out and parts
connected in a rather automatic manner. This can and
usually does create severe EMC problems unless it is
used judiciously.
• It is best to place the highest-speed components
manually and to manually route their connection lands
with the previous concepts kept in mind
Component Selection & Mounting
• As a general principle, recall that the higher the spectral
content of a signal, the greater the ability of that signal to
couple inadvertently to other conductors and parts.
• Hence the highest speed components and the lands they
connect should be positioned on the PCB well away
from off board connectors and lands as shown below.
Component Selection & Mounting
Component Selection & Mounting
Anticipating that the rise/ falltimes of this clock may need to be
increased to reduce its high-frequency content, pads should be
placed so that
(a) a series resistor can be inserted in the lands and
(b) a capacitor can be inserted across the lands forming a lowpass
filter.
Component Selection & Mounting
• In the beginning, the capacitor pads would not be populated, and a
0-V surface-mount resistor could be placed in series with the clock
lands.
• Later in the design, if problems are uncovered necessitating
increasing the clock rise/falltimes, then the capacitor can be added
and the value of the series resistor can be changed.
• Why is this significant? The answer is that no time-consuming
repeated PCB layout would be required; only the bill of materials
(parts list) need be changed. This is a virtually cost-free solution.
Component Selection & Mounting
• Once again, it is a good practice to place all the
components, or at least the higher-speed ones, by hand
and not allow automatic software to do this.
• Perhaps in the future EMC layout rules will be
incorporated into these layout tools, but today it seems
better to place these components by hand.
Component Selection & Mounting
• Illustration of the unintentional coupling of signals between chip
bonding wires, causing supposedly quiet module pins to have highfrequency spectral content. A processor pin, the reset pin, was
thought to be “quiet” and hence was connected to a long PCB land.
Unsuspected coupling caused it to carry the clock signal, thereby
creating large radiated emissions. A series inductor was added to
prevent this from occurring.
PCB Trace Impedance
• Track impedance is dominated by
inductance at frequencies higher than a
few kHz
PCB Trace Impedance
• Where L is the inductance and di/dt is the rate of
change of current. For example, consider the
case where the power-supply wiring has an
inductance of 50 nH. If the transient current,
when a logic gate switches, is 50 mA and
thegate switches in 1 ns, the noise voltage
generated across the power-supply wiring when
this one gate changes state will be 2.5 V
PCB Trace Impedance
• Trace inductance of a connection can be reduced in two
ways:
– minimizing the length of the conductor, and if possible increasing
its width
– running its return path parallel and close to it.
• The inductance of a PCB track is primarily a function of
its length, and only secondarily a function of its width.
For a single conductor of diameter d and length l inches,
following equation gives the self-inductance
PCB Trace Impedance
PCB Trace Impedance
• Two conductors with current in opposite directions (e.g., a signal and
its ground return trace) have a total loop inductance Lt, equal to
where L1 and L2 are the partial self-inductances of the individual
conductors and M is the partial mutual-inductance between them. I
To minimize the total loop inductance, the partial mutualinductance between the conductors should be maximized.
Therefore, the two conductors should be placed as close together
as possible
PCB Trace Impedance
• Another method for decreasing the inductance of a circuit
is to provide alternative paths for current flow. These
paths must be electrically, but not necessarily physically,
parallel. If two equal inductances are paralleled, then the
equivalent inductance will be one half that of one inductor.
• On many low-cost products, inner plane boards cannot be
afforded. Hence the important effects of having a ground
plane (and a power plane that can act like a ground plane
in returning currents) are not present
PCB Trace Impedance
• On low-cost products where ground plane cannot be
afforded, a simple way of reducing the inductance of
grounding system is with a gridded ground system.
PCB Trace Impedance
• The limiting case of a gridded ground is when an infinite number of
parallel paths are provided and the ground conductor is continuous,
and it is then known as a ground plane
• The main EMC purpose of a ground plane is to provide a lowimpedance ground and power return path to minimize induced ground
noise
PCB layer stackup
• PCB layer stackup (the ordering of the layers and the
layer spacing) is an important factor in determining the
EMC performance of a product
• The following four factors are important with respect to
board stackup:
–
–
–
–
The number of layers
The number and types of planes (power and/or ground)
The ordering or sequence of the layers
The spacing between the layers
PCB layer stackup
• In deciding on the number of layers, the following should
be considered:
– The number of signals to be routed and PCB cost.
– Clock frequency.
– Will the product have to meet Class A or Class B emission
requirements?
– Will the PCB be in a shielded or unshielded enclosure?
– The EMC engineering expertise of the design team.
PCB layer stackup
• One- and Two-Layer Boards:
– The main EMC concern here is to keep loop areas as small as
possible.
– One- or two-layer boards should only be considered when clock
frequencies are less than 10 MHz
– On one- or two-layer boards all critical signals should be routed
first, in order to guarantee optimum routing. Critical signals should
be routed as short as possible, with an adjacent ground return
trace
– On two-layer digital boards, the ground and power should be
routed so as toform a grid (see Section 10.5.3). It is not unusual to
see a 10- to 12-dB decrease in emissions when a ground grid is
added to a two-layer digital board that previously did not have one
PCB layer stackup
• Multilayer Boards:
– Multilayer boards (four or more layers) using ground and/or power
planes provide significant reduction in radiated emissions over
two-layer
– A often–used rule of thumb is that a four-layer board will produce
20 dB or more, less radiation than a two-layer board—all other
factors being equal
– Boards containing planes are much better than those without
planes for the following reasons:
– The planes allow signals to be routed in a microstrip (or stripline)
configuration. These configurations are controlled impedance
transmission lines that produce much less radiation than the
random traces used on one- and two-layer boards
PCB layer stackup
• Multilayer Boards:
– When the return current is on the adjacent plane, the loop area is
reduced.
– The ground plane significantly decreases the ground impedance
and hence the ground noise
PCB layer stackup
Multilayer Boards Objective :
– A signal layer should always be adjacent to a plane.
– Signal layers should be tightly coupled (close) to their adjacent
planes.
– Power and ground planes should be closely coupled together.*
– High-speed signals should be routed on buried layers located
between planes. The planes can then act as shields and contain
the radiation from the high-speed traces.
– Multiple-ground planes are very advantageous, because they will
lower the ground (reference plane) impedance of the board and
reduce the common-mode radiation.
– When critical signals are routed on more than one layer, they
should be confined to two layers adjacent to the same plane. As
discussed, this objective has usually been ignored.
PCB layer stackup
Four layer Board :
A common four-layer board configuration, consisting of two signal layers
and two planes, is shown in Fig.
It consists of four uniformly spaced layers with internal power and ground
planes. The two external trace layers usually have orthogonal routed
PCB layer stackup
Four layer Board :
• If the power plane shown in Figs. 16-12 or 16-13 is split to
accommodate different dc voltages, it is important to restrict routing
on the bottom signal layer so that traces do not cross the split in the
plane.
• If some traces must cross the split, stitching capacitors should be
located close to where the traces cross the split to provide a lower
impedance return current path.
PCB layer stackup
Six layer Board :
• Most six-layer boards consist of four signal routing layers and two
planes.
• From an EMC perspective, a six-layer board is preferred over a fourlayer board because it is easy to shield high-frequency signals by
placing them on buried layers between planes, or to provide for
orthogonally routed signal layers that are referenced to the same
plane.
PCB layer stackup
Six layer Board :
PCB layer stackup
Six layer Board :
PCB layer stackup
Eight layer Board :
• Most eight-layer board stackups are used to improve
EMC performance rather than add additional routing
layers.
• Most eight-layer boards (and all the ones that we will
concentrate on here) consist of four signal-routing layers
and four planes.
PCB layer stackup
Eight layer Board :
Crosstalk control
Crosstalk is one of the several important PCB design that must be
considered during any design cycle.
Crosstalk refers to the unintended electromagnetic coupling between
traces, wires to traces, cable assemblies.
Crosstalk between cables, wires and traces affects intrasystem
performance.
Crosstalk is generally considered to be functionality concern (signal
quality)
Crosstalk control
• To cope with this problem, it is essential to have
suitable circuit models to simulate complicated
structures.
• Simplified but accurate models are of great help
in understanding the crosstalk mechanism.
Lumped model of two coupled lines that can be
easily implemented in SPICE-like circuit
simulators. By virtue of its simplicity, this is the
first approach for inexperienced users when
dealing with crosstalk and using a commercial
circuit simulator that usually does not offer
efficient coupled line models in its library.
Crosstalk control
Crosstalk control
Crosstalk control
Crosstalk control
Crosstalk control
Near end and Far end Crosstalk
• Near-End Crosstalk (NEXT), defined as the
voltage VNE at point D of the victim line
near to the source;
• Far-End Crosstalk (FEXT), defined as the
voltage VFE at point E of the victim line far
from the source.
Crosstalk control
Since the overall coupling effect is
the sum of capacitive and
inductive crosstalk, near-end
crosstalk is of greater magnitude,
and hence, of greater concern for
a PCB designer.
Crosstalk control
• Techniques to Reduce Crosstalk
1. Use wide traces.
2. Locate traces near to the reference plane.
3. Increase the spacing between traces.
4. Match the lines.
5. Reduce the coupling length.
6. Provide other traces for shielding.
7. Avoid locating critical traces near to the edge of the PCB.
8. Ensure a solid ground plane without cuts.
Power Distribution & Decoupling
The characteristics of an ideal direct current (dc) power distribution
system are as follows:
To supply a constant dc voltage to the load
Not to propagate any ac noise generated by the load
To have a 0 O ac impedance between power and ground
Power Distribution & Decoupling
There are two primary purposes for using decoupling between power
and ground-reference planes.
1.Functionality:
Power Distribution & Decoupling
Role of decoupling capacitor:
Power Distribution & Decoupling
• Effective power supply decoupling has become increasingly more
difficult to achieve as the result of increasing clock frequencies and
faster rise times. Ineffective decoupling can lead to excessive power
bus noise as well as to excessive radiated emission.
• It is important to understand that decoupling is not the process of
placing a capacitor adjacent to an IC to supply the transient switching
current rather it is the process of placing an L–C network adjacent to
an IC to supply the transient switching current .
Power Distribution & Decoupling
2. To reduce the noise injected into the power and ground reference
plane
Power Distribution & Decoupling
• All decoupling capacitors have inductance in series with them.
Therefore, the decoupling network is a series resonant circuit. The
–
–
–
–
inductance comes from three sources, as follows:
The capacitor itself
The interconnecting PCB traces and vias
The lead frame inside the IC
Power Distribution & Decoupling
• The internal inductance of a surface mount technology (SMT)
capacitor itself is typically 1–2 nH, the interconnecting PCB traces
and vias add 5 to 20 nH or more according to the layout, and the
internal lead frame of the IC may have 3 to 15 nH of inductance
according to the type of IC package. The inductances of the
interconnecting PCB traces, however, are usually the only
parameters under the system designer’s control.
• From the above, we observe that the total inductance can vary from a
low of about 10 nH to a high of 40 nH. Typically it is in the range of 15
to 30 nH. It is this inductance that limits the effectiveness of the
decoupling network. It is very important to remember this fact—we
are placing an L–C network between the power and ground, not a
capacitor