Conformity Magazine - October 2007 - (Page 38)

been characterized as protection for IEEE/ANSI Category A1 locations, these locations are described as being 60 feet or more from the service entrance. We wanted an electromagnetic interference filter that could be placed anywhere within a building, even close to a branch service panel or the main service entrance wiring. According to C62.41, worst-case surges inside a building (IEEE/ANSI Category B3 locations) could be a pulse as large as 6,000 volts peak, 3,000 amps peak with a half amplitude duration of 50 microseconds. An electromagnetic interference filter designed for this Category B3 location would have to reduce this 6,000 volt pulse to less than 170 volts peak to pull it within the normal dynamic range of 120 Vac power. This level of performance would qualify the electromagnetic interference filter to be used anywhere within the building. A low pass electromagnetic interference filter with 32 dB attenuation could accomplish this task. Since it is desirable to minimize the electromagnetic interference filter “standby” current, the bulk of the filtering is put within a full wave bridge, which offers several advantages. First, it allows the use of large value electrolytic capacitors, needed to operate at the low impedances required. Second, it also peak rectifies on the power wave, and therefore the bulk of the filtering occurs for voltages (surges and noise) that exceed the power wave peak voltage, without adding significant “standby” current flowing from the normal 60 Hz power (which would otherwise be characteristic of a low impedance filter). After several months of reviewing various electromagnetic interference filter configurations and parameters for optimum performance, a “Eureka” moment came when we considered the following: • Many years ago, in the design of an integrated circuit filter, we used phase cancellation to improve a low pass filter design. An anti-phase signal was added to the main signal path to result in significant (20 db) additional rejection of the undesired signals, leading to a significantly more stable integrated circuit. • Audio cancellation using anti-phase paths for noise reduction is fairly common today. Was it possible to apply the same principles to the power line? Computer Analysis to the Rescue Computer analysis showed that the basic low pass filtering removed the dangerous higher frequency components, but there was still a low frequency pulse of about 150 volts above the power wave peak voltage. The issue then became one of deriving a cancelling voltage. The input inductor bears the brunt of the surge voltage and current. Could a cancelling voltage be derived from the input inductor? We found that, by optimizing the turns ratio, coupling, and damping, the input inductor could be tapped and properly phased to provide a 150 volt cancelling phase voltage for the worst-case surge. Indeed, the residual low frequency pulse could be cancelled to below the power wave peak for even worst-case conditions. Since computer simulations verified the complete design, we built a sample unit. Optimizing the input inductor to meet the computer design parameters was challenging, but the first samples were fine tuned to perform even better than the computer simulations, since the computer simulations used worst-case design parameters. As a result, we’ve engineered what we believe is a different approach to surge suppression, one that removes dangerous surge energy from the downstream protected equipment using practical components. The approach utilizes a low pass electromagnetic interference filter that first filters off the higher surge frequencies. Then, the residual low frequency components are cancelled using well-established phase cancellation principles to provide total surge protection from even the worst-case surge energy. The result is a UL 1283 Listed electromagnetic interference filter that cancels out dangerous surge energy, while also filtering annoying powerline noise. How Total Surge Cancellation Works Although the actual circuit is complex, a simplified schematic shown in Figure 1 clearly illustrates the critical functions. (The tapped inductor is shown as an autotransformer, as the function is easier to understand with this configuration.) While functionally described previously in this article, the schematic will clarify the signal flow, as follows: • A surge enters at point A; • The tapped cancelling winding immediately begins to develop a cancelling voltage due to the surge current flow in the primary winding; Figure 1 38 Conformity oCtober 2007

Table of Contents Feed for the Digital Edition of Conformity Magazine - October 2007

Contents Editor’s Note Commission Proposes Changes for 57-64 GHz Spectrum Rules FCC Continues Actions Against Jamming Device Distributors Radio Operator Fined for Failing to Make Equipment Available for Inspection Commission Fines Unsolicited Fax Advertisers Firm Offering Quality Standards Training Also Fined Breaking New Ground with Lithium-Battery Generic Requirements for Use in Telecommunications Facilities ESD Open Forum EMC UK 2007 Conference Set Staying Abreast of U.S. E-Waste Legislation EU Commission Establishes RFID Expert Group Update on China's CCC Compliance Program System Level ESD Testing: The Waveforms An Alternative Approach to Surge Suppression Testing Equipment Buyer's Guide EU Scientific Committee Releases Updated Data on EMF Risks Updated Standards List For The EU’s EMC Directive Updated Standards List For The EU’s Directives on Measuring Instruments Call for Presentations for 2008 International ESD Workshop Announced CPSC Actions In The News IEC Standards Update Product Reviews UL Standards Update New Products From Our “You Can’t Make This Stuff Up” Department Looking Back: Items from Past Issues of Conformity Upcoming Events Advertisers

Conformity Magazine - October 2007

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