Definition and measurement of IC parameters in PCB practice D T

Definition and measurement of IC parameters in PCB practice D T
DEVELOPMENT TOOLS
Definition and measurement of IC
parameters in PCB practice
by Gunter Langer, Langer EMV-Technik
The EMC development
process can be greatly
simplified and accelerated, if
sufficient information on
potential disturbance sources
(ICs) is available right at the
beginning. This article shows
how EMC disturbances
propagate from ICs.
Figure 1: Measurement on pins
in a near short-circuit and
open-circuit condition
Today’s interference suppression strategies
in PCB development have reached the limits
of their performance capability. ICs are only
exposed as potential disturbance emission
sources after a first development sample has
been built and near-field probes used to
search for RF sources in the electronics.
Often it is not the disturbance source itself
that is found in this procedure but the conducting networks to which it feeds disturbance currents and voltages. The electronics
are then mostly modified with suppression
components, copper foil and the like. Finally,
a compliance measurement is performed to
confirm successful suppression. We cannot
make this procedure any faster but industry
now calls for quicker and more efficient
suppression methods.
I
Only a deep intervention into the emission
mechanisms can lead to a noticeable acceleration. Countermeasures can be taken earlier and
more efficiently if the ICs are identified as
potential disturbance sources. EMC-relevant
IC parameters that make suitable tools in
EMC-compliant PCB design are needed from
the very beginning of the development process.
This new approach will determine electronics
development in compliance with EMC
requirements over the next ten years. However,
the theoretical basis still has to be formulated
and the relevant tools developed for EMC
practice to tackle the tasks that are assigned by
industry in the future. Due to their internal
switching processes integrated circuits emit
electric and magnetic near fields. These fields
are generated in the interior of the IC by currents and voltages. The complex networks generate a multitude of such fields. The largest surfaces which can emit E-fields and the biggest
current loops which can induce excitation voltages are located in the pin, lead frame and bonding wire areas. It has to be taken into account in
this consideration that even pins where static or
quasi-static signals are present can carry RF
through internal parasitic coupling.
RF voltages can exist between the inner metal
parts of an IC and ground. These voltages generate electric fields between the relevant metal
part and the ground system of the PCB. Most of
the field lines take the shortest way to the
ground system. A few exit vertically from the
top far into the environment. These field lines
are the exciter field lines which generate a displacement current. The charging current that
can be derived therefrom may couple over to
neighbouring metal parts and excite the relevant metal system as an antenna.
Magnetic fields generated in IC current loops
such as Vdd - Vss loops are divided into two
parts, H1 and H2. The H2 field is generated on
the IC’s internal current conductor and closes
in the space above the IC. Its effective range is
10 to 15 cm. The H2 field is much stronger than
29
H1 which develops around the metal plate of
the PCB. A knowledge of the strength and direction of the magnetic fields has to be taken
into account in PCB design according to EMC
principles.
Disturbances are only emitted on the two following conditions: there must be an RF source,
and a transmitting antenna connected to it. ICs
are the RF sources of a PCB. The transmitting
antenna can be the metal of the module
(>300MHz) or connected metal parts (such as
a cable harness) depending on the frequency.
Ideal conditions for the emission of disturbances are obtained if the transmitting antenna extends freely into the environment and its
resonance length remains in the range of the IC
disturbance frequencies. Two basic physical
principles responsible for the transfer of RF
from the IC up to the antenna elements are
relevant in this set-up.
The first basic principle is coupling via magnetic near fields. The exciter field H1 induces a disturbance voltage in its own ground system (selfinduction) which feeds the connected cable
harness like an antenna. The question is: how
strong are the disturbances emitted on the basis
of this principle in practice? A 100 x 100 mm
circuit board with a microcomputer (microcomputer example) mounted in the middle was
used as an example (TEM cell print). The microcomputer examined is widely used in the
May 2007
DEVELOPMENT TOOLS
as in the aforementioned examples was used in
a practical examination. The results show the
coupling to neighbouring metal parts was
stronger for the H-field than for the E-field in
the microcomputer examined. Other IC types
can show a completely different behaviour with
regard to intensity of and relationship between
E and H fields. The problems that are encountered in automotive electronics also arise in
industrial electronics. The limits are much
higher there but the ICs used can also cause
much stronger fields.
There are two ways in which an IC can cause
disturbance emissions: 1) via near fields which
emerge from its surface, 2) via RF current and
voltage which penetrate the PCB from its pins.
RF current generates magnetic near fields and
RF voltage electric near fields via the PCB’s conductor runs. These near fields and special
mechanisms of action are responsible for the
emission of disturbances. The stronger the near
fields, the more intense the disturbance emission. The IC’s potential to cause disturbance
emissions is determined by the maximum RF
voltage and maximum RF current that it can
emit. The real conditions depend on the impedance of the PCB conductor networks. The
maximum potential is reached under short-circuit and open-circuit conditions of the IC.
Figure 2: Open-circuit voltage measurement on IC 02 in 3D and 2D
automotive industry and represents a typical
automobile application. Two metal bars were
connected to the ground plane of the TEM cell
print as a transmitting antenna. The metal bars
simulate connected cable harnesses. Subsequently, the antenna’s half-wave excitation was
considered. The length of the antenna elements
was mechanically changed to correspond to
several different frequency measuring points
between 70 MHz and 250 MHz. The respective
emissions were measured at a distance of 3 metres using a broadband antenna. These emission
values are critical when the IC is used in automobile electronics. The emission intensity can
be further increased by reducing the width of
the module.
Figure 3: Measurements of the applied test IC
on the board mains simulation and checking
the countermeasures
May 2007
The second basic principle is coupling via
electric near fields. Part of the E-field lines cause
a displacement current to flow which in turn
excites the connected cable harness like an antenna. The same circuit board as for the magnetic near field excitation is used to measure the
effect on emissions. The antenna element is
only connected to one end of the TEM cell
print. The other procedures are identical.The
results of measurement show that this emission
mechanism is relevant in practice.
In addition to the basic principles of coupling
out near fields there are two other possibilities.
The H2 magnetic field cannot directly cause
emissions. However, a voltage is induced (mutual induction) if the field surrounds a metallic conductor (cable harness, structural part).
This induction voltage can use the conductor as
an antenna and excite it to emit disturbances.
The same TEM cell print as above was used in
the practical examination of the effect. The
metal bar had a constant length of 2 metres in
the experiment. The distance between the
metal bar and the microcomputer was changed.
The range of the H2 magnetic field and the associated emission intensity are shown in a critical situation for the microcomputer example.
Cable harnesses, cable pulls, metal braces, seat
frames, steering columns, screen connecting cables etc can be such metal parts. Capacitive
coupling to neighbouring metal parts can
cause similar effects. The same TEM cell print
30
The open-circuit voltage and short-circuit current can be seen as pin-related IC parameters.
These parameters are determined in a measurement under near open-circuit and near shortcircuit conditions (figure 1).
Both these parameters depend on inner functions of the IC and the signal status of the pin.
The number of spectra per pin should be limited to a sensible number. Nevertheless, large
amounts of data will be created that cannot be
visualized by conventional means (display of a
spectrum analyzer, Excel-spreadsheet on a PC,
for example), particularly for practical use. A
3D presentation is a solution which allows the
EMC engineer to quickly recognise very active
pins (figure 2) using special measuring software
on a PC. This software also offers a 2D alternative. The EMC engineer can thus assign corrective measures to the critical pins on which the
PCB developer can rely in the run-up to the actual development process.
The IC parameters thus make the development
process smoother and pave the way even for the
use of ICs which may be problematic in terms
of EMC. A measurement on a vehicle component showed that the limit value was exceeded
by 24dB at 120 MHz (figure 3). This problem,
however, was only discovered when a development sample was tested. Near-field probes
identified an E-field excitation as the cause of
this problem. The responsible IC was separat-
DEVELOPMENT TOOLS
Figure 4: Measuring system for pin current and pin voltage
ed and its open-circuit voltage determined. The
measuring set-up for determining the IC parameters is shown in figure 4. The test IC
(DUT) is mounted on an adapter circuit board
which is embedded in a ground plane. The resulting ground area is a prerequisite for performing measurements up to the GHz range.
A measuring probe can be placed on the
ground plane to determine the RF voltage and
RF current. This probe is mobile and can be applied to every IC pin. The measuring path (IC
– pin contact – probe) is only a few millimetres
so that the measurement can be taken over an
electrically short distance. The IC is supplied
and controlled from the connection board via
filters.
Extraordinarily high voltages were found in a
40 MHz array on the IC’s pins for the quartz
(shown black in figure 2). All leads that are connected to these pins and metallic parts emit an
electric field. The electric field is very strong and
excites the module with the cable harness to
emit disturbances. The electric field is coupled
out via:
I the bond wire and lead frame of the IC quartz
pin,
I the 15mm lead to the quartz,
I the quartz enclosure and SMD components
connected to the quartz.
This type of problem can be diminished by
reducing the surface of these metallic parts,
shortening a lead or better embedding in
ground and by using smaller quartz enclosures.
This was however not sufficient in this practical example. The RF voltage of the IC was too
high. The metal areas of the bond wire and lead
frame were already large enough to cause the
limit values to be exceeded when the components were measured. Corresponding capacitors cannot be used to reduce the voltage on
quartz. A relocation of the IC or an electrolytic capacitor as a screen would have been a suitable remedy to the problem. But at this level of
development, it was too late for such a corrective measure. The use of a screening plate was
the last resort. The positive results are shown in
figure 3. The limit values were no longer
exceeded.
The right ways can be followed, and late-stage
suppression measures that absorb a lot of time
and money avoided, if sufficient information
on potential disturbance sources (ICs) is available right at the beginning of the development
process. Even ICs that are complex in terms of
EMC technology can be safely mounted. The
EMC development process is thus greatly simplified and accelerated. This in turn means industry’s demand for shorter development times
can be met and EMC reworking reduced. This
vision can be gradually put into practice in the
years to come. I
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF

advertisement