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EMI/EMC filters and filtering: EMI power filters crush the noise.

Electromagnetic Compatibility (EMC) standards specify how much interference a piece of electrical equipment can "inject" into the wires that connect it to the power mains. To keep that type of conducted electromagnetic interference (EMI) within the limits specified by government agencies, engineers rely on good design techniques and power-line filters. A typical filter removes unwanted signals by providing them with a low-impedance path to ground, or it blocks them with a high-impedance connection and "forces" them into a lower impedance path. The wanted signals pass through the filter with slight effect.

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Conducted emissions come in two forms--differential mode and common mode. Differential-mode emissions appear primarily on one input-power lead (line or neutral), whereas common-mode emissions appear on both input-power leads (line and neutral). In general, differential noise occurs below 1 MHz to 2 MHz, and common-mode noise prevails at higher frequencies. (1) Power supplies, for example, usually require differential-emission filters, and motor-drive circuits need common-mode filters.

Each type of noise requires its own filter, as shown in Figure 1. A differential-mode filter provides independent chokes for the line and neutral connections. A common-mode filter employs one choke core, wound so currents in the line and neutral circuits cancel one another and do not saturate the core material. Commercial filters and modules often combine both filter circuits in one package.

Filter from the Start

Brian Jones, Marketing Engineer, at Schurter explained, "If engineers have experience with system design, they know to include a power-line EMI filter in a circuit when they start a design rather than waiting for their design to fail EMI tests and then trying to squeeze in a filter. If their design builds on a previous one, the engineers usually know from the start the filter characteristics they will need. But if they plan a new project that involves different types of high-frequency switching circuits and higher operating frequencies, they can face new EMI problems that require analysis." Often, engineers know whether they have differential-mode or common-mode conducted emissions. And, they may have run tests so they know how much attenuation they need over a given frequency range.

In some cases, though, engineers leave filter specifications to the latter stages of product development. "Before you can specify an EMI filter, you must know how much noise your equipment produces and at what frequencies," explained Ken Pagenkopf, Engineering Manager at Curtis Industries. "But you usually cannot determine the noise output until after you have built and tested the equipment. People familiar with EMI filtering requirements usually leave space for a filter because they know a design will need one, and they know from past experience where to locate it." That approach avoids trying to squeeze in a filter module as a product moves from a prototype to a production-ready design.

Pagenkopf suggests engineers use the following to specify their preliminary requirements for filter manufacturers: current and voltage, frequency of operation (50 Hz, 60 Hz, 400 Hz), size (dimensions), ambient operating temperature ([degrees]C), special environmental needs, termination types (spade lugs, solder eyes and wires) and frequency versus attenuation information (insertion loss).

Design It Right

"Engineers should try to design circuits and equipment so it will not need a filter," said Dave Armitage, Engineering Manager at Schaffner. "Then, ideally a filter cleans up the last bits of EMI the engineers could not 'design out.' But engineers under pressure have to limit what they worry about, so sometimes they cross their fingers and hope to not encounter EMI problems."

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In a design, problems with grounding typically cause trouble later on. "If you use an ohmmeter to verify conductivity and see a 0[ohm] resistance, you think the circuit provides a good ground," said Armitage. "But from an EMI perspective, it can be a poor ground." Filters in a metal housing--a typical configuration--require solid ground connections, and engineers should not rely just on metal screws or rivets that fasten a filter to a chassis or panel. Filters should have a continuous run of grounded metal beneath them. The more metal-to-metal contact, the better. "A good connection between the filter case and ground should not have intervening protective coatings such as paint or zinc," stressed Armitage.

Ken Pagenkopf of Curtis Industries said, "Engineers should ensure their designs provide solid, low-impedance, short ground connections. We see bad designs that mount filters on insulated standoffs or that use long ground leads." Pagenkopf also recommends engineers be wary of designs that use a chassis as a current-return path; keep noise off heat sink and ground planes; and place a filter as close as possible to a noise source.

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Designers also must consider the layout and routing of cables. "You do not want long cables nor do you want wires and cables to pass close to noisy components," noted Dave Armitage of Schaffner. "Engineers should position a power-line filter so incoming power lines connect to it with short leads. Any excess wire between the power-entry point and the filter could act as an antenna that picks up noise, thus bypassing--and defeating--the filter."

"Keep in mind, the lower the frequency of the EMI you must get rid of, the bigger the filter," said Pagenkopf of Curtis Industries. Engineers may think they will have difficulty removing low-frequency EMI on a PCB, so they design in a power-line filter. "But the magnetic components get much larger when you have EMI at 150 kHz or below, so control low-frequency EMI at its source."

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Engineers may forget to budget for the direct and indirect costs of power-line filters. Say engineers replace an electromechanical relay with a less-expensive solid-state relay. They may not consider the cost of the power-line filter needed to make their new design pass testing requirements. The cost of the added filter and the solid-state relay may exceed the cost of the original mechanical relay. So, keep filter "budgets" in mind as you design equipment.

What is Your Rating?

Engineers understand the need to derate the performance of ICs and other circuit components. Power-entry modules and discrete filters also come with derating information that influences how engineers can apply them. In the case of filtered power-entry modules, the graph in Figure 2 shows how the admissible operating current decreases from 100 percent to 0 percent over a 45[degrees]C span. "The data shows when you operate a filter at 80[degrees]C, you should draw only about 10 percent of its rated current if you want the filter to operate properly," said Brian Jones of Schurter. "Engineers may forget to derate filters, and some suppliers do not talk about derating, which is dangerous."

"An IEC standard specifies a maximum rating of 70[degrees]C for the pin temperature for a standard power inlet," explained Diane Cupples, Vice President of Marketing at Schurter. "We provide filter derating information for power-entry modules up to a maximum operating temperature of 85[degrees]C, but only to point out that the derated current becomes 0A at that high temperature. Customers should use the derating plot to better understand the decrease in admissible operating current at ambient temperatures above 40[degrees]C."

Customers can operate their equipment anywhere within the derating "envelope," but according to Jones, Schurter suggests a maximum operating temperature of about 50[degrees]C for filtered power-entry modules and 70[degrees]C for discrete filters. At that temperature, a circuit should draw only about 60 percent to 70 percent of a filter's rated maximum current, which places operating conditions well within the graph's envelope.

Acknowledgment

Thanks go to Ron Taylor and Ralph Bright at Interpower for providing information for this article.

References

(1) "DC Power Input Considerations," Modular Devices, Inc. www.modev.com/content/ApplicationNotes/html/dcpower06.htm.

(2) "Filter selection for mains conducted emissions," Laplace Instruments. www.laplaceinstruments.com/EMCIS/ESArticle1.htm.

For Further Reading

* A sidebar to this article, titled "Test Your Equipment," can be viewed at www.ecnmag.com. Click on "Current Issue."

* "Choosing and Installing Mains Filters," Compliance Engineering. www.ce-mag.com/archive/2000/janfeb/armstrong.html.

* "Interference Immunity in Consumer Products," Conformity. March 2006. www.conformity.com/0603/0603interference.html.

by Jon Titus, Senior Technical Editor

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Author:Titus, Jon
Publication:ECN-Electronic Component News
Date:Jan 1, 2007
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