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Posted on 05 July 2019

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MOV planar arrays can help protect sensitive avionics systems from lightning strikes and other transient events

Modern military aircraft are increasingly reliant on electronics for a wide range of avionics, communications, navigation, life support, weapons system and other mission control functions.

By Matt Ellis, Senior Engineer, Syfer Technology

 

This equipment must be protected against a number of threats, including electrical interference, and military grade components, connectors and cables are used wherever possible to maintain integrity. A general increase in on-board electronics systems, means cost-efficient and effective protection is required from spikes, from ligthening strikes for example, from conducting through to other interconnected electronic systems.

UK-based component manufacturer, Syfer Technology, has been manufacturing and supplying a number commercial and military grade RFI/EMI and transient protection solutions for many years. The portfolio ranges from simple decoupling capacitors, surface mount and panel mount RFI/EMI filters, through to planar capacitor arrays and metal oxide varistor (MOV) planar arrays.

Better out than in

The use of filtered connectors on the outside of electronic control units is by far the best protection, by stopping the spike on the outside. It is better to suppress the noise or ground the spike sooner rather than later. Relying on PCB-based transient voltage protection means that the voltage transient is already inside the box.

Ceramic planar capacitor arrays are commonly used in multi-line EMI/RFI filter circuits inserted into filtered connectors for military and aerospace applications, and Syfer is the world’s leading supplier.

The Planar Capacitor Array is a unitary block of ceramic containing capacitors or a combination of capacitors, feedthroughs and ground lines. In operation, incoming signals encounter very low impedances and are presented with multi-directional paths to ground. Typical capacitance value ranges for C0G are 47pF to 4nF and for X7R are 250pF to 600nF. But these EMI filtering devices have little voltage clamping ability to cope with voltage spikes, lightning strikes and other severe transient events. For this, Syfer recommends the MOV (metal oxide varistor) planar array. Designed specifically for connector manufacturers, it is inserted within the shell of a military or aerospace type connector either complementary to, or replacing a capacitor planar array. The same volumetric and weight benefits apply to MOV planars as to capacitor planars, compared to alternative technology solutions.

Fast route to ground

At operational voltages, an MOV acts as a high value resistor with a maximum specified leakage current of 5ìA. Once the voltage reaches a certain value the device becomes highly conductive and provides a path to ground, making it ideal for use as transient protection. “MOVs operate almost like a solid state switch – providing a fast and efficient short circuit route to ground to limit surges or pulses”, Ellis explains.

The varistor offers impressive performance and typical limits are 500A peak current and 3J of energy with a transient. These limitations are dependent on the geometry of the planar. High density and thin varieties may have lower capabilities, for example.

Figure 1 shows the V-I properties of a 47V working component. At 47V, current is approx 5ìA, nominal voltage at 1mA is 63V, clamp voltage at 10A is 90V. In this case the part specification would be: working voltage 47V, nominal voltage 53 to 69V and clamp voltage 100V maximum at 10A.

Current vs Voltage

Figure 2 shows 1mA of current flow at nominal or breakdown voltage, and 5 or 10A of current at clamp voltage. These properties are bi-directional so the MOV will perform equally well for both positive and negative transient events.

Bi directional properties

Transient protection

With a material response time of less than 500ps, no leads/tracks, and a low inductance geometry, MOVs are more than capable of suppressing lightning induced transients, and voltage spikes caused by power supply glitches and noisy switching circuits. However, in isolation, they are not designed to provide continuous over-voltage protection.

With its inherent capacitance, the MOV planar array can be used as a simple low capacitance C filter (Figure 3).

MOV C Filter

But for better noise suppression, it can be combined with capacitor planar arrays to form a high capacitance C filter (Figure 4), or with multiple capacitor arrays and ferrite inductors to form a balanced or unbalanced Pi filter (Figure 5).

MOV C Filter with cap planar

MOV Protected Pi filter

Figure 6 shows the typical format of an MOV protected connector, with the MOV on the left, and the other two capacitor planars with ferrite beads in between forming an unbalanced Pi filter.

Internal configuration

The main alternative to MOV planar arrays is the TVS (transient voltage suppression) diode. Both technologies have their advantages. Diodes are available for lower working voltages and they also have lower leakage and sharper clamping characteristics. MOVs can compete on energy and current capabilities. A key advantage is that MOVs are considerably more volumetrically efficient, as many components are contained within one device. A reduced component count delivers a number of cost saving benefits.

The drawback of TVS diodes can be clearly seen in the images. Not only are extra piece parts required to mount and connect the diodes to the pins but those extra parts add significant bulk and weight. The varistor planar array can be manufactured to the same dimensional specifications and tolerances as the capacitor planars used in the connector. This means that adding transient suppression to a design which already has a capacitor planar array, need not have an impact on the size of the connector. See Figure 6.

In summary, no other transient voltage suppression technology can match the MOV planar when it comes to efficient use of connector real estate. The combination of MOV planar and capacitor planar arrays provides sophisticated filtering for low level interference as well as high voltage spikes. And prices are competitive with alternative technologies too!

Testing an MOV

Syfer MOV arrays have been tested to RTCA DO160-E section 22 waveform 4 level 5 and waveform 5 level 3 (See Figure 10). 47V and 8V parts were tested for leakage current, nominal voltage and clamp voltage. The same parts were then subjected to 500 pulses at 10s intervals and then re-measured. Failure is defined as a greater than 10% shift in parameters. No failures were observed. Testing has also been undertaken in order to demonstrate the speed of response capabilities. Parts were subjected to a 1MHz 175V square wave with a rise time of less than 400ns. Figure 9 shows the response of a 47V working planar. Note there is no voltage overshoot present prior to full clamping.

Supplying an MOV

Syfer works closely with OEM customers and the connector manufacturers in order to provide a bespoke product which meets exact requirements. Up to three voltages can be combined in one array depending on available space and specification requirements. MOV planars are available to suit many military connector sizes, including circular shell sizes 8 to 24, Arinc 600 and 404 series, and rectangular 24308 series. Also available are discoidal MOVs from 4.5mm OD upwards. Syfer has raw materials in stock and a secure supply readily available. Orders are manufactured to demand, with a typical lead-time of 8 weeks.

Waveform 4

Waveform 5

Response of 47V working

RTCADO-160E levels

Making an MOV

Historically, MOVs were high voltage single layer radial leaded components. Today, they are most commonly seen in surface mount form utilising a multilayer construction. Syfer has taken the technology further to produce multilayer MOVs in planar array and discoidal formats. The MOV base material consists of zinc oxide, doped with small quantities of bismuth, cobalt and manganese amongst other metal oxide additives. It is built up from layers of the zinc oxide interleaved with platinum forming the highly conductive electrodes. During firing, the dopants within the dielectric material migrate to the grain boundaries and cause each grain to act as a P-N junction with an activation voltage of approximately 3.6 volts. In order to achieve higher working voltages many layers of ceramic are used, the grains are effectively linked in series and parallel creating multiples of their discrete properties. Syfer's unique ‘wet-stack’ process ensures a stress-free component is produced with mechanical precision.

 

 

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