So far, we have discussed board level EMI suppression solutions, which are necessary to meet EMC standards. However, they may not be sufficient for applications in which the enclosed system is either not immune to, or even emits, EMI. Applications like these — including medical, space, aerospace, and other mission critical systems — require box level EMI filtering.
Box level refers to the level within the system that encloses an entire electronic circuit, or set of circuits. Typically, the subsystem is enclosed in a metal box to physically shield it from the surrounding environment and filter out conditions like shock, vibration, temperature, radiation, transients, and EMI.
Box level EMI filtering follows the same EMI rules discussed in parts one through three of this series; so, there will still be a source, a path, and a victim. The path must be determined, keeping in mind that current flows through the path of least impedance. However, box level filtering components differ, as they are typically screwed, bolted, or soldered to the actual metal box (Figure 1), whereas board level filtering devices are typically surface mount.
Reliability is of the utmost importance in mission critical applications. Requirements differ from industry to industry, customer to customer, and application to application, which means that custom EMI filters are often required. Suppliers like AVX frequently develop completely custom components, designed and manufactured to meet exact customer specifications, as well as semi-custom filters, which tailor existing filters to meet customers’ performance requirements and any special qualifications required for the application.
Some applications are more stringent than others. For example, medical implantable applications have some of the most stringent qualification requirements that exist today, which is understandable since the performance and reliability of implantable devices directly impacts patients’ well-being and quality of life. Factories that supply implantable medical devices are routinely audited to ensure that both the industry’s and the customer’s standards are met. Additionally, in this industry, suppliers have to obtain customer approval before making any changes to currently approved processes, materials, or designs.
Furthermore, government restrictions — for example, the International Traffic in Arms Regulations (ITAR) — may require even more stringent quality control measures to guarantee the protection of government and military-related projects.
Box level EMI filters are typically constructed using a discoidal capacitor inside a tubular sleeve, as this forms a feedthrough capacitor, which exhibits extremely effective high frequency EMI filtering capabilities. Inductive elements may also be utilized to realize filter topologies, including Tee, pi, and LC circuits (Figure 2); although, it should be noted that inductors may alter device performance due to saturation of the ferrite material. However, there are several ways that filter circuit and physical properties can be modified to meet the demands of a given application.
A “C” circuit consists of a capacitor inside a solder type, screw-in, or cylindrical package; and this configuration exhibits a feedthrough construction that effectively attenuates high frequency noise.
LC circuits consist of a capacitor and an inductor, and are typically used in systems with a low impedance source and a high impedance load, or a high impedance source and a low impedance load. The values of the L and C elements are modified to tune the circuit and filter out specific frequencies.
Tee filters consist of two series inductors, which present high impedance to both the source and the load, and one parallel capacitor; and both the capacitance and inductances can be tuned to filter out specific frequencies.
Pi filters consist of two parallel capacitors and one series inductor; and, due to the extra capacitance, provide more effective high frequency attenuation than LC filters. Their capacitance and inductances can be tuned to filter specific frequencies as well.
Lastly, filter plates designed for use in connectors can also filter EMI at every I/O pin (Figure 3).
Box level EMI solutions effectively attenuate high frequency noise, are uniquely constructed, and are unique to medical, space, aerospace, and other mission critical applications in which the enclosed system is either not immune to, or even emits, EMI. Designs are typically either wholly or partially customized to fit customers’ requirements, and filters are put through rigorous qualification tests for optimal performance in their intended operational environment prior to being shipped due to the stringent requirements that typically govern the high-reliability applications into which box level EMI solutions are designed.