EMI Filtering for High-Reliability Applications

Effective EMI filtering is necessary for almost every modern electronic device, including devices that generate their own EMI, as well as devices that are sensitive to EMI within their environment, and is especially important in high-reliability applications that utilize lower-power signals and have strict signal fidelity demands. High-reliability EMI filters are designed to consistently meet or exceed performance requirements and are vitally important to any industry with a high cost of failure. For example, many medical, military, and defense applications depend on high-reliability filters for the accurate transmission and reception of signals required to ensure patient and soldier safety or achieve mission success. High-reliability filters also play a crucial role in a number of commercial applications, such as instrumentation, where sensitivity to low-power signals in a noisy environment is a requirement.

Filtering Properties

The primary purpose of an EMI filter is to suppress the transmission of selected frequencies of a given signal. In most cases, a simple low-pass filter, which passes a low-frequency signal while blocking out unwanted higher frequency noise, will be sufficient. The cutoff frequency is the frequency at which a filter begins to attenuate part of the signal, and is typically defined as the point at which the signal amplitude is 3 decibels (dB) below the nominal passband value. A filter’s capacitance and inductance values are carefully selected to achieve the desired cutoff frequency and frequency response.

Circuit Selection

There are several different circuit configurations utilized for EMI filtering, ranging from a single grounded capacitor to a circuit with up to three elements. Ideal selections depend on the unique characteristics and properties of the device the filter will be employed in, such as device impedance.

Table 1: EMI filter circuit selection options.

Figure 2: Insertion loss vs. frequency for multiple filter configurations with a full load in a balanced 50Ω system that accounts for component and circuit resonances.

EMI filter performance must be matched to capacitor performance for effective filtering. Ceramic capacitors, which use very thin layers of a ceramic dielectric to separate conductive layers from one another and prevent the free flow of electricity, can be designed to withstand voltages in excess of 1,600VDC. So, filter designers must choose the appropriate dielectric materials and dimensions best suited for each capacitor’s unique set of performance requirements.

Filters must also be designed and tested for each circuit’s AC or DC voltage conditions. DC filter ratings are not the same as AC filter ratings, so a filter’s ability to successfully withstand high DC voltage does not necessarily predict its performance when exposed to high AC voltage and vice versa. For instance, AC-rated filters will often have lower capacitance values to prevent overheating and damage resulting from reactive current heating, while DC-rated filters expected to experience short, irregular bursts of voltage, like those employed in implantable cardiac defibrillators, are likely to undergo a pulsed voltage test to ensure capacitor integrity.

In addition to being able to withstand the appropriate voltages, well-designed filters also resist the leakage of current across the capacitor. It is impossible to construct a capacitor that completely prevents this flow of electricity, but filter designers can mitigate leakage current through the proper material selection to minimize its impact on device performance.

Multiple Filtered Lines

Complex electronic devices often require filtering on multiple signals or power lines. One way to isolate them from one another is to use a specialized metal ferrule that contains and acts as a common ground for multiple filters, connecting their cases together while the pins remain isolated.

Figure 3: From left to right (Mobile: Top to Bottom), a discoidal capacitor, a metal ferrule containing multiple filters, a filtered feedthrough array, and capacitor arrays.

A single EMI feedthrough filter is typically constructed as a donut-shaped capacitor called a discoidal, the inside and outside diameters of which act as connection points for the case and the lead and serve as the poles of the capacitor. In applications where space is at a premium, multiple discoidal capacitors can be integrated into a single piece of ceramic referred to as a capacitor array and installed in feedthrough for device integration. These types of filtered feedthrough assemblies provide the highest concentration of filters in the smallest physical volume and are often used in implantable medical devices, in which high density and minimal volume offer a real competitive advantage, as well as for commercial and military connector filtering.

Experienced filter manufacturers can also develop fully tested and assembled feedthrough and capacitor arrays in custom shapes and sizes to facilitate seamless, single-step integration into more challenging device designs to reduce the possibility of component damage or installation errors. There is no hard limit on the number of capacitors that can be integrated into an array, but cost increases with quantity.

System Integration and Installation

A variety of case sizes and lead lengths are available to suit specific applications. For instance, miniature filters are available for applications with space and weight restrictions, and custom designs are available when existing case sizes are not an ideal fit. Some systems also require filters to exhibit rugged resistance against moisture and other contaminants, to employ airtight seals that prevent the leakage of gasses, or to account for environmental variables like the normal reduction in insulation resistance that occurs as ceramic heats up. Thoughtful materials selections and installation methods can achieve these characteristics and help prevent performance degradation over the component’s lifetime.

Figure 3: From left to right(Mobile: Top to Bottom), solder-in, bolt-style, and cylindrical EMI filters.

Although filters must be installed in an EMI shield, such as a well-grounded conductive enclosure, for optimal performance, device designers can choose from a variety of installation options. Solder-ins are one of the most compact options, are designed to withstand the heat of being soldered into a final assembly, and can also provide excellent levels of hermeticity. Other options include bolt-style filters, which are threaded and may have a hexagonal head; cylindrical filters, which have the largest internal volume and can accommodate complex circuits in a single package, such as a Pi or T filter; and screw-style filters. All filter styles have the option for hermetic sealing on both ends of the component to prevent outgassing.

Additional measures can be taken to protect the integrity of the filter both during and after the installation as well. For example, preheating a solder joint before exposing it to the full soldering temperature can avoid the application of unnecessary thermal stress, and gently handling filter cases can help prevent mechanical deformation.

Testing and Inspection

When filter performance is mission critical, filter selection should be limited to those that have been 100% tested for relevant properties. Standard high-reliability parts undergo electrical and visual inspections and additional testing sequences to further ensure filter reliability. There are also several US-MIL standards that provide extremely specific requirements for electrical, mechanical, and thermal testing procedures and results. Since most ceramics have a high propensity for crack propagation, ceramic components with any hint of mechanical instability will generally fail early on. Stress-testing high-reliability filters and capacitors eliminate any inferior parts that may cause trouble in the future.

AVX Corporation offers several different levels of testing to provide the required reliability for numerous applications. R-level testing is more stringent than standard commercial testing and includes a variety of tests designed to simulate filter performance under stress. B-class and S-class testing include more comprehensive tests than R-level, such as a radiographic inspection — a process in which a part is imaged with X-rays in order to non-destructively examine its internal structure. Group A and Group B refer to different test levels within a lot. Group A testing is performed on all lots produced and includes some tests that are performed on 100% of parts and others that are only performed on a sample. Group B testing is generally performed on a sample and includes extensive life testing. Space-level parts, or S-class parts with Group B testing, represent one of the highest reliability levels achievable for an EMI Filter.

Table 2: AVX’s R-level testing procedures subject 100% of parts to these tests in the order shown. Summary data for R-level screening is shared with customers and verification is included with product shipments.

Table 3: AVX’s Group A testing sequence for MIL-PRF-28861 certification. These tests are performed on a 100% basis when customers specify Class S or Class B reliability.

Actual testing sequences may vary slightly depending on the filter manufacturer and any special device requirements, but often include tests outlined in MIL-PRF-28861, MIL-PRF-31033, MIL-STD-202, MIL-STD-220, and other specifications. Experienced filter manufacturers will identify and execute the best testing regimens for each application.

Cost and Manufacturing Considerations

Filter specifications including circuit complexity, voltage and current ratings, size, and temperature resistance, as well as the desired reliability level and amount of testing required to achieve it, all impact filter cost to varying degrees. While many of these factors are governed by device-level requirements, there are some aspects of filter design in which experienced manufacturers may be able to reduce costs without notably affecting filter function. Any cost/performance tradeoffs should be thoroughly discussed during the design phase to ensure that a quality product can be created at a reasonable cost.

Additionally, seemingly minor changes to capacitor designs can have hugely adverse effects on the manufacturing and device design processes. For example, small changes in capacitance can sometimes require complete capacitor redesigns. While many devices have unique requirements that demand completely custom filters, some new devices can utilize an existing template to achieve shorter lead times and lower qualification and per-piece part costs for initial volumes.

Commercial Filters

Regulations for commercial filters and devices can vary based on the country in which they are sold. For example, some countries restrict the concentration of hazardous materials in electronics and may demand that manufacturers comply with RoHS or similar standards. Experienced filter designers are well versed in these requirements and can discuss various options, such as the usage of conductive polyimide instead of solder and cadmium-free terminations, with their customers.

Rules and regulations regarding the emission of electromagnetic radiation can drive the decision to use a filter as well. In addition to keeping harmful EMI out of a device, filters can also prevent EMI from escaping and interfering with other sensitive equipment in the vicinity. Almost every country has regulations that determine the approved type and amount of EMI that devices may generate.

Aerospace and Defense Filters

Manufacturing products for aerospace and defense companies requires compliance with strict rules and regulations that govern almost every aspect of device performance, manufacturing, and testing. Defense companies must often follow strict regulations regarding how they select and audit their suppliers. There may have a specific list of suppliers that have received accreditation to certain US military standards and/or other countries’ requirements that must be met in order to sell parts there. For example, the United States requires that companies comply with applicable export compliance regulations, such as Export Administration Regulations (EAR) and International Traffic in Arms Regulations (ITAR). Aerospace and defense suppliers must also follow certain regulations regarding the security and confidentiality of the products they are manufacturing, as well as any information regarding the product’s intended use or user. AVX is included on the Qualified Products List (QPL) for the Defense Supply Center Columbus, which is a part of the US Department of Defense.

Medical Device Filters

Medical device technology has advanced at an astounding pace in recent years, with devices becoming ever more compact, complex, and, in some cases, even less intrusive for patients. Due to the increasingly connected world we live in, these and other sensitive devices are subjected to growing levels of electromagnetic interference from our environment. Any unfiltered medical device is at risk for harmful interference from various sources of EMI. To maintain high levels of precision and reliability, many medical devices employ the use of EMI filters to minimize noise and maintain signal fidelity. Medical device filters often have some of the most stringent spatial requirements, and thus commonly use filtered feedthrough arrays or miniature discoidal capacitors, which deliver high-density performance in low-volume packaging.

Since a large majority of implantable medical devices are battery powered, it is important to minimize filters’ power to prolong device life. Measuring and minimizing equivalent series resistance (ESR) or dissipation factor (DF) can help keep unnecessary battery drain to a minimum, and high insulation resistance (IR) can help minimize current leakage across the capacitor. Modern implantable medical devices typically use primary cell, non-rechargeable batteries that are designed to last 10 or more years. However, since high-reliability implant battery technology has not kept pace with the miniaturization of many other electronics employed in these devices, minimizing power consumption is crucial to achieving target battery and device life expectancies.

In addition to requirements for greater functionality, several global regulatory bodies also impose strict regulations, ranging from design to audits to documentation, on medical device manufacturers and their suppliers in an attempt to further ensure patient safety. As such, it is critical that medical device designers select filter manufacturers with a proven track record of compliance with regulations and reliable products. Medical device designers can also benefit from utilizing existing filter designs where possible, as doing so can result in qualification time and cost savings for implantable devices.

Source: AVX technical paper

by Amanda Ison, AVX Corporation, Sun Valley, CA