Comparatively simple measures can be taken
to enhance EMC if a circuit is to be used in a
well-known environment. But this becomes
more difficult if the module is to be used as
universally as possible in different applications.
Disturbance fields may cause problems, particularly
with high integration levels. The aim
of the current test procedure is to develop a
temperature monitoring system controlled by
a microcontroller which can be used in greenhouses,
for example.
The module will be offered without a package
and should be a genuine all-round device. A
number of potential circuit environments thus
have to be taken into account. Is the printed
circuit board housed in a metal or plastic
package? Is the circuit ground well connected
to the package in the metal version? Is the circuit
operated in the vicinity of other metal
conductors such as a top-hat rail, 230V mains
lead, etc? The module has to be protected
against all possible interference mechanisms
since its future environment is unknown.
We will initially consider conducted interference:
disturbances may enter the module via
the power supply socket (a switched-mode
power supply unit, for example) or the peripheral
interface (a temperature probe, for
example). The magnetic fields caused by disturbance
currents flowing through the board
may induce voltages in the conductor loops.
Two problems have to be taken into account
with regard to safeguarding the module functionality:
the induced voltage may either be
treated as a logic signal by the integrated
circuit input, or it may drive a disturbance
current, which causes problems in other parts
of the integrated circuit. All conductor runs
have been relocated in the printed circuit
board intermediate layers to prevent this. Critical
signal pins of the controller such as the
reset pin and the sockets which connect the
printed circuit board to the outside are fitted
with filter elements.
The same correlations apply to disturbances
which enter the module via fields. Magnetic
field vortexes may penetrate the circuit and induce
a voltage in the conductor loops, which
in turn drives a disturbance current through
the module and causes the aforementioned
problems. Interferences are also caused by electric
coupling. Electric fields capacitively couple
into the circuit board line networks, or even
components. The resulting displacement current
may cause a voltage drop at a resistor
(against Vss or Vdd), which in turn is recognised
as a logic signal, or induce voltages in
other parts of the circuit. The bottom of the
printed circuit board that is only populated
on one side is provided with a continuous
GND layer as a counter-measure. The top is
also GND-flooded to minimize the disturbing
influence of magnetic fields. Both capacitive
and magnetic coupling also have to be considered
on the integrated circuit level. H-field
coupling causes a magnetic field vortex to
penetrate the IC. A disturbance voltage is induced
in the IC current loops. The induced
voltage may interfere with signals or the supply
voltage in the IC and cause faults or drive a
disturbance current through the conductor
loop and thus interfere with the integrated circuit.
During E-field coupling, a voltage which
is present between the IC and field source generates
an electric field depending on the respective
IC-to-field source distance. The electric
field lines end on the metal parts (pad of the
IC pin, bond wire, die) of the IC. They conduct
a displacement current into this surface.
Since the EMC of the integrated circuit itself
cannot be influenced, a controller has to be
found with the highest possible immunity for
the application. A number of integrated circuits
with a comparative range of functional features
are potential candidates for this application.
The manufacturer data sheets, however, do
not reveal the respective immunity parameters.
A new criterion has thus to be found to
evaluate the immunity with E and H field coupling.
The aim is to evaluate/compare the potential
integrated circuits in terms of their immunity
to disturbances coupled in via fields.
Either one or both coupling mechanisms (Hfield/
E-field) can cause faults depending on
the IC design. An objective immunity evaluation
thus has to subject the integrated circuits
to disturbances via both coupling mechanisms.
The chosen approach is shown in the following
diagrams.
Separate coupling circuits were designed for
both coupling mechanisms. An EFT/burst generator
(burst generator according to IEC 61000-
4-4) was used as a disturbance source. This
generator was connected to the coupling waveguide
via a 50 ohm high voltage cable for Hfield
coupling. The wave guide had a 50 ohm
input to ensure that the burst reaches the
device under test without distortion. An additional
measuring shunt monitored the generated
disturbance pulses. The wave guide was
arranged above the devices under test at the
defined angle and distance. This guaranteed
that all ICs were subjected to a comparable
disturbance field with an identical EFT/burst
generator setting. The pulse shape generated
by the waveguide (measured via the shunt) is
shown in the following diagram. Apart from
the generator setting, the magnetic field angle
also had to be taken into account since it is related
to the waveguide orientation and has a
direct influence on the interference effect
achieved. A similar set-up was chosen for Efield
coupling.
The EFT/burst generator was connected to a
coupling electrode instead of a waveguide.
This electrode was arranged at a defined distance
above the device under test. The voltage
between the coupling electrode and the device
under test generated an electric field proportional
to the burst voltage amplitude. The devices
under test could thus be subjected separately
to E-field and/or H-field disturbances.
The subsequent measurement was expected
to show that the individual integrated circuits
fail completely or cause faults at different
EFT/burst generator voltages, coupling mechanisms
(E-field/H-field) and field angles. Or
in other words: the integrated circuit immunity
to E/H-field disturbances differed from manufacturer
to manufacturer and the measured results
let the engineer choose a suitable IC for
the described application. 80C51 microcontrollers
from three manufacturers were examined
as potential candidates for the application
in the course of the measurements described
below. The integrated circuits were not tested
in the application but on customised test
adapters to create reproducible conditions and
prevent parasitic effects by other parts of the
circuit. The following parameters applied for
the measurement: identical package pin-out
(VQFP44), comparable functionality – all three
are 8051-compatible, identical test adapter
(packaged with the same filter elements), and
Figure 2. H-field coupling
Figure 3. Burst H-field source
Figure 4. E-field coupling
Figure 5. Burst E-field source
Figure 6. Device under test and waveguide
P1202-4
identical test program and/or firmware The
integrated circuit was tested during operation.
The test program was selected so that each
component in the integrated circuit
(timer/UART/watchdog, etc) was used and the
corresponding test signals on the pins provided
information about their functionality. A pin
was continually toggled (heartbeat signal) and
a static signal sent to the outside in the present
example. An oscilloscope was sufficient to
monitor this test set-up. In addition, the outputs
were connected to LEDs to receive visual
feedback about the operating state of the
device under test. The individual operating
states of the IC were controlled by a PC via a
test adapter-to-PC connection.
The test program ran in the following way:
LED_01 (heartbeat) flashed slowly while
LED_02 came on permanently during the start
of the IC. Depending on its firmware, the IC
changed over to another operating mode which
caused LED_01 (heartbeat) to flash faster and
switched LED_02 off should a crash and subsequent
reset occur. Irregularities of the heartbeat
signal indicated an internal program sequence
problem.
It comprised the following components:
EFT/burst generator with a maximum generator
voltage of 4.4 kV, base plate for the test
adapter with an integrated IC-to-PC interface,
device under test in the test adapter, H-field
source/E-field source with a 3 mm spacer, oscilloscope
and oscilloscope adapter, and power
supply for the PC interface and IC. The IC
was connected to the PC interface via the test
adapter. This allowed the engineer to monitor
and control the IC. The measurement set-up
shown in the figure also included an oscilloscope
adapter which made it easier to connect
the oscilloscope’s scanning heads and did not
affect the measurements. A controlled switchedmode
power supply unit with an internal current
limiting function supplied the measurement
set-up with power and was intended to
protect the IC from destruction in the event
of a malfunction. The field sources were connected
to the EFT/burst generator. The 50
ohm measurement output of the field source
was connected to the oscilloscope to monitor
the injected pulses.
The measurement procedure was as follows:
first of all, the IC program was started. Its
proper functioning was monitored by the oscilloscope.
The field source was placed over
the centre of the integrated circuit, starting
with the H-field source. It was important to
adjust the field source relative to the device
under test when this was subjected to an Hfield.
This ensured that the results are comparable
since the interference effect depends
on the field angle. The field angle did not
have to be adjusted if an E-field was coupled
in. Field coupling was started at the EFT/burst
generator lowest amplitude value and a positive
polarity. The severity was then gradually
increased up to a maximum generator voltage
of 4.4 kV or until a fault occurred. The polarity
was then switched over and the measurement
repeated. Several measurements had to
be taken at different field angles under the
influence of H-field. The integrated circuits
were subjected to the disturbance for one
minute in each of the test runs. Tables 1 and
2 summarise the results and show at which
voltage amplitude, polarity and field angle a
reset occurred for the different ICs.
Only three field angles were chosen for an
initial test of the ICs under the influence of a
magnetic field. A second test run with a finer
resolution can be carried out to pinpoint any
functional faults that occur.
The different immunity levels of the integrated
circuit become visible straight away. None of
the tested integrated circuits was susceptible
to E-field.
A crash could only be invoked in the 80C51
IC from Manufacturer 2 at 4 kV while the IC
from Manufacturer 3 carried out a reset at a
value as low as 1 kV when subjected to magnetic
field. Since all of the test conditions (test
set-up, interconnection, test program, etc)
were identical, the differences must be inherent
to the integrated circuits themselves. The measurements
at a field angle of 180° provided the
same results as the measurements at a field
angle of 0° with the opposite generator polarity.
The deviations which occurred and are clearly
visible in the table can be explained by variations
within the generator. These can be verified
on the basis of the pulse shape generated at
the oscilloscope’s measurement output. None
of the integrated circuits could be influenced
at a field angle of 90°. The heartbeat signal
was not influenced during any of the measurements,
i.e. the ICs were functional until the
reset. In view of these findings it seems reasonable
to assume that the integrated circuits’
power supply was disturbed. The device under
test is located in a spacer. The spacer has a degree
scale where the position of the waveguide
(H-field) and thus the field angle can be read.
The Vcc and Vss pins are on opposite sides of
the IC package in the present example. A maximum
voltage is induced in this loop at a field
angle of 0° and/or 180°, leading to an IC
power supply failure and thus a reset. Since no
other faults occurred and none of the integrated
circuits was susceptible to E-field, the generator
voltage at which the ICs failed when subjected
to H-field was used as a comparison criterion.
As a result, the 80C51 from Manufacturer 2
was chosen for the application since it has the
highest immunity level of the ICs measured.
After this EMC assessment the engineers can
proceed to the development of the modular
electronic switchgear.
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