Design of filter connector for multilayer varistor array

"Connectors contribute nothing to electronic equipment" is an old and controversial statement that caused some controversy when it was written, but is now undoubtedly outdated and wrong! Currently, a single connector can achieve equipment protection against continuous or transient noise. Of course, the cost is immediately obvious. Add filtering, and a $10 connector becomes a $100 connector. Add protection against voltage spikes, and you are looking at a $1,000 connector.

Fortunately, recent innovations have changed this confusing situation. Multilayer planar arrays of filter capacitors can now perform functions such as transient protection. Filter connectors that use multilayer varistor planar arrays for transient protection provide an alternative solution for product design.

Varistor

A varistor is a variable resistor. At low applied voltages, a varistor acts as a conventional high-value resistor and obeys Ohm's law. Above a certain threshold voltage, the device becomes highly conductive, exhibiting low impedance at high voltages. When the varistor is conductive, it clamps the applied voltage to a specified maximum value that the device can withstand. With these properties, varistors are used in electronics to protect circuits from transient overvoltages. At low voltages, a varistor is similar to a ceramic capacitor, and because of this, it can be used as part of a filter to handle continuous noise. So why aren't varistors more often used in filtering applications, where they can fulfill their dual role - continuous noise attenuation and transient voltage suppressor?

Varistor pulse level

One of the limitations of varistors when used for filtering applications is degradation of their protection capabilities, which may be the result of repetitive impacts of voltage surges.

Varistors are made of ceramic materials. The main component of most varistors is zinc oxide (ZnO). Small amounts of other oxides such as bismuth, cobalt, manganese, etc. are also added. Therefore, varistors are sometimes called metal oxide varistors or MOVs.

During the manufacturing process, the raw ceramic powder is mixed, formed, and then fired. Metallization is used to make electrical connections. During the firing of the ceramic, a polycrystalline structure is formed (Figure 1). The added metal oxide moves to the edge of the crystal, forming a semi-conducting layer PN junction. This means that the average grain size is determined by the original powder recipe and the firing temperature. When the applied voltage per unit grain boundary is less than 3.6 volts, the grain boundary exhibits high resistance. Above the above threshold, it will switch to high conductivity.

Figure 1

The switching voltage of the varistor itself depends on the average number of grains between the electrodes.

The crystal structure of varistors has no directionality, and therefore, varistors are bipolar devices. They exhibit electronic properties, such as symmetry and spike voltage breakdown characteristics, similar to back-to-back Zener diodes. Conventional wisdom holds that varistors suffer from degradation of electrical performance under high current repetitive pulses (particularly reduced clamping performance and increased leakage current). For a while, varistors manufacturers required the use of ceramic compositions with large grains. Large grain size results in fewer crystal boundaries per unit area of ​​electrodes and longer current paths between electrodes. The series resistance per unit area of ​​electrodes is relatively high, resulting in a proportionally reduced peak current capability.

For varistors that operate at low voltages, surface mount chips require the use of fine-grained ceramic components, which can be achieved with a multilayer structure. When this is accomplished, the small and uniform grains connected to the large electrode area relative to the component surface provided by the multilayer structure allow a significant increase in the peak current capacity within the unit component volume.

The current to energy ratio of multilayer varistors (MLVs) is very stable compared to other varistors types, and it has now been demonstrated that MLVs can withstand tens of thousands of full-rated current peak surges without performance degradation.

Speed ​​and Overshoot

Component manufacturers of various transient protection technologies want to adopt their favorite products, and response speed, or not, is often a consideration for varistors. The response time of varistor substrate materials is much less than 500 picoseconds.

The main factor that slowed down the response time of some early varistors was the parasitic inductance introduced during the finished package. 25 to 50 mm leads were used in the construction, which exhibited an inductance of 0.6 nH/mm, and the high self-inductance formed the characteristics of radial wire varistors.

Now, the multilayer structure has eliminated the leads, and the response time of a 1206MLV chip with a typical inductance of 1200pH is no more than 1 nanosecond. The configuration of other components, if it becomes a filter capacitor structure, is suitable for varistors, and its equivalent series inductance (ESLs) should be as low as 30pH. These provide feasibility for the response time to drop to tens of picoseconds.

Another problem caused by the inherent self-inductance (L) in the varistor structure is voltage overshoot. The changing current (di/dt) caused by the varistor self-inductance (L) will generate a voltage of -Ldi/dt. At the voltage spike, an overshoot will appear on the varistor (the sum of the clamping voltage and the induced voltage caused by the varistor self-inductance). Reducing the self-inductance in the MLV filter structure to tens of pH can eliminate the worried voltage overshoot.

EMI filter

The effective measure for radiation is shielding. The radiated noise energy is absorbed by the shield and dissipated in the form of heat. The effective measure for conduction is EMI filter, which transfers the conducted energy from the protected system to the ground. (Figure 2). EMI filter is composed of capacitors and inductors, and uses their different impedance characteristics to selectively reduce unnecessary signals.

Figure 2

The filter is bidirectional. It blocks unwanted signals from leaving the system, and it also blocks the system from emitting noise outward. A common practice now is to install the filter on the device's circuit board, but it is most effective to place the filter at the exit and entrance of the device shield. Connectors can bring power and signal cables together so that they enter the device shield at only one point. Every contact point in the connector can form a filter, which can be a C, T, L or Pi type structure depending on the system requirements. Because the use of filter connectors can remove the filter circuit at the circuit board level, the size and weight of the entire system can be reduced. At the same time, the reliability of the system is improved due to the reduction in the number of solder joints.

Multilayer Planar Capacitor Array

Multilayer planar arrays are a specific component design for EMI filtering connectors. Multiple capacitors are contained within a ceramic block. Individual wires are connected to each capacitor through a through hole and are connected to ground within the device. Very low impedance is exhibited when the signal follows multiple paths to ground (Figure 3). Each contact of the connector is connected to one or more holes in the array. In each hole there is a capacitor - the 'hot electrode' is connected to the surrounding whole, and the ground electrode covers the entire plane and is connected to the connector shell through the perimeter of the plane (Figure 4).

Figure 3

Figure 4

Planar-based filter connectors are available for all MIL-STD connector designs. The connector shape (the shape of the planar array must be consistent) is either round or rectangular. Commonly used rectangular designs include D-Sub and High Density D-Sub and Micro Ds, Arinc404s and Arinc600. Irregular shapes are also available. The corresponding planar sizes are: from 5 mm square to 75 mm diameter.

Contact counts range from 2 to over 200. Standard contact ranges from 0.3mm diameter up to coax - all filter capable. Standard contact depths start at 0.63mm.

Within a planar array, up to 6 different capacitance values ​​with no ratio relationship can be placed on the layout. Each plane can be specified with a different operating voltage, and a typical 300V DC rated plane can withstand transient peaks of up to 750V. Transient capabilities up to 3000V need to be specified. Individual holes may be isolated (feed points) or grounded. A maximum ground plane resistance of 10MΩ can be specified, and crosstalk capacitance can be limited to 10pF or less.

The complexity of planar arrays lies not in the complex electrical requirements to form a single component, but in the mechanical precision of manufacturing the device. Typically, the pin position accuracy of the connector must be better than ±0.05 mm, and the planar array must have the same or better tolerance. The planes must be completed (shaped and drilled) before the ceramic is fired, and they shrink during firing, generally about 20 percent. The position of the pins of a 30 mm diameter plane after firing moves at least 2.5 mm relative to the center reference point, which is 50 times the allowable value of the pin tolerance!

Planar arrays are a state-of-the-art passive device. Each device has multiple capacitors, multiple capacitance values, and multiple electrical functions. It is an original integrated passive component.

Multilayer Varistor Array

At low voltages, multilayer varistors behave like capacitors. The boundaries of the crystal grains are insulating, exhibiting the properties of a dielectric material. The effective dielectric constant of the MLV is about 800, which is one-quarter to one-third of the dielectric constant of a typical X7R multilayer capacitor dielectric. The capacitance values ​​obtained from varistors are lower than those of conventional capacitors. Given that these are the low end of what is available with conventional multilayer capacitor technology, the filtering performance of the MLV is indistinguishable from that of a capacitor in terms of total value.

When used in filters, varistors provide additional transient protection. They dissipate the energy contained in the transient voltage pulse as heat (Figure 5). The highly conductive zinc oxide particles act as a heat sink to ensure rapid and even heat dissipation throughout the device and minimize temperature rise (however, varistors can only dissipate a small amount of average power and are not suitable for continuous power applications).

Figure 5

It has been shown that it is possible to build a multilayer structure that does not have a multilayer capacitive element structure and can therefore not be reused as a varistor. These complex components are incorporated into protective EMI filter connectors (both plugs and sockets) and filter adapters. They can be used to replace or supplement capacitors in C, L, T or Pi filter structures.