CIE Sept09 P10-11:Layout 1 07/09/2009 11:12 Page 10
EMC & Circuit Protection
Fast grounding
keeps
aircraft flying
MOV planar arrays can help protect sensitive avionics systems from
lightning strikes and other transient events. CIE talks to Matt Ellis, Senior
Engineer at Syfer
M
odern military aircraft are increasingly
For this, Ellis recommends the MOV (metal oxide
reliant on electronics for a wide range
varistor) planar array.
of avionics, communications,
Designed specifically for connector
navigation, life support, weapons
manufacturers, it is inserted within the shell of a
system and other mission control functions.
military or aerospace type connector either
This equipment must be protected against a
complementary to, or replacing a capacitor planar
number of threats, including electrical interference,
array.
and military grade components, connectors and
The same volumetric and weight benefits apply
cables are used wherever possible to maintain
to MOV planars as to capacitor planars, compared
integrity. A general increase in on-board
to alternative technology solutions.
electronics systems means cost-efficient and
Figure 2
effective protection is required from spikes,
Fast route to ground
lightning strikes for example, conducting through
At operational voltages, an MOV acts as a high
to other interconnected electronic systems.
value resistor with a maximum specified leakage
Figure 2 shows 1mA of current flow at nominal
The range of commercial and military grade
current of 5 A. Once the voltage reaches a certain
or breakdown voltage, and 5 or 10A of current at
RFI/EMI and transient protection solutions
value the device becomes highly conductive and
clamp voltage. These properties are bi-directional so
available are varied and range from simple
provides a path to ground, making it ideal for use
the MOV will perform equally well for both
decoupling capacitors, surface mount and panel
as transient protection. “MOVs operate almost like
positive and negative transient events.
mount RFI/EMI filters, through to planar capacitor
a solid state switch – providing a fast and efficient
Figure 2
arrays and metal oxide varistor (MOV) planar
short circuit route to ground to limit surges or
arrays.
pulses”, Ellis explains.
Transient protection
The varistor offers impressive performance and
With a material response time
Better out than in
typical limits are 500A peak current and 3J of
of less than 500ps, no
According to Matt Ellis, Senior Engineer with Syfer,
energy with a transient. These limitations are
leads/tracks, and a low
the use of filtered connectors on the outside of
dependent on the geometry of the planar. High
inductance geometry, MOVs
electronic control units is by far the best
density and thin varieties may have lower
are more than capable of
protection, by stopping the spike on the outside.
capabilities, for example.
suppressing lightning induced
Figure 3
“It is better to suppress the noise or ground the
Figure 1 shows the V-I properties of a 47V
transients, and voltage spikes
spike sooner rather than later,” he says. “Relying
working component. At 47V, current is approx 5 A,
caused by power supply
on PCB-based transient voltage protection means
nominal voltage at 1mA is 63V, clamp voltage at
glitches and noisy switching
10A is 90V. In this case the part specification would
circuits. However, in isolation,
that the voltage transient is already inside the box”.
be: working voltage 47V, nominal voltage 53 to
they are not designed to
Ceramic planar capacitor arrays are commonly
69V and clamp voltage 100V maximum at 10A.
provide continuous
used in multi-line EMI/RFI filter circuits inserted over-voltage protection.
into filtered connectors for military and aerospace
With its inherent
applications.
capacitance, the MOV planar
The Planar Capacitor Array, for example, is a
array can be used as a simple
Figure 4
unitary block of ceramic containing capacitors or a
low capacitance C filter
combination of capacitors, feedthroughs and
(Figure 3). But for better noise
ground lines. In operation, incoming signals
suppression, it can be
encounter very low impedances and are presented
combined with capacitor
with multi-directional paths to the ground. Typical
planar arrays to form a high
capacitance value ranges for C0G are 47pF to 4nF
capacitance C filter (Figure 4),
and for X7R are 250pF to 600nF.
or with multiple capacitor
But these EMI filtering devices have little voltage
arrays and ferrite inductors to
clamping ability to cope with voltage spikes,
form a balanced or
lightning strikes and other severe transient events. Figure 1
unbalanced Pi filter (Figure 5).
Figure 5
10 September 2009 Components in Electronics www.cieonline.co.uk
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