EMI EMC Project Report
June 1, 2016 | Author: Ashish Agarwal | Category: N/A
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PROJECT REPORT ON
STUDY OF EMI/EMC ( ELEMENTS, EFFECTS, COMPLIANCE PRACTICES WITH SUPPRESSION TECHNIQUES )
(Code: I-02 B) Submitted in partial fulfillment for award of degree of Bachelor of Engineering (Electronics and Communication Branch) Awarded by:
Maharishi Dayanand University Rohtak During Academic Session 2005-2009
Submitted by: GAURAV TRIVEDI DHRUV ARORA AJAY GARG
( 5 ECE 37) ( 5 ECE 34) ( 5 ECE 08)
Under the guidance of
Dr. D.K. THAKUR, Dean (R&D) Submitted to: Dr. S.V.A.V. Prasad Ms. Pragati Kapoor Mr. Ajay Dagar
(HOD) (Project Coordinator) (Project Coordinator)
Department of Electronics and Communication Engineering
LINGAYA’S INSTITUTE OF MANAGEMENT AND TECHNOLOGY, FARIDABAD
CERTIFICATE This is to certify that project on the topic
STUDY OF EMI/EMC ( ELEMENTS, EFFECTS, COMPLIANCE PRACTICES WITH SUPPRESSION TECHNIQUES )
has been successfully completed and submitted by GAURAV TRIVEDI DHRUV ARORA AJAY GARG
( 5 ECE 37) ( 5 ECE 34) ( 5 ECE 08)
for the fulfillment of award of degree of Bachelor of Engineering (Electronics and Communication Branch) Maharishi Dayanand University Rohtak During the course of the project they have worked sincerely and were good throughout the implementation and presentation of the project undertaken.
Dr. S.V.A.V. Prasad (H.O.D., Electronics and Communication)
Ms. Pragati Kapoor (Project Coordinator)
Mr. Ajay Dagar (Project Coordinator)
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ACKNOWLEDGEMENT
It is our profound privilege to express sense of gratitude to our project guide Dr. D.K. Thakur Dean (R&D), Department of Electronics & Communication Engineering, Lingaya’s Institute of Management & Technology, Faridabad, for his kind support and encouragement throughout this project work. I really cherish the valuable advice and suggestions given to us and the time he spent in the discussions about the minute details of work in completion of the project work.
I acknowledge the valuable contribution of all the scientists and team members of Electromagnetic Interference/Compatibility division at Electronics Regional Test Laboratory (North), New Delhi, who have supported me during my Industrial Project.
I am grateful to Mr. Sulekh Chand, Scientist “E” at ERTL (N) for giving me an opportunity to study & work on the on the project i.e. “ Study of EMI/EMC elements, effect, compliance practices with suppression technique.” My special thanks goes to Mr. A.U. Khan and Mr. Gyan Chand, who taught me the technical aspects of the projects assigned to me and provided ample opportunity for developing the skills on EMI/EMC practices.
I am also thankful to all the staff of Electronics & Communication Department of Lingaya’s Faridabad.
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CONTENTS
Chapter No.
Nomenclature
Page No.
0
ABSTRACT
5
1
INTRODUCTION TO EMI/EMC
6
2
PROBLEM OF EMI/EMC
9
3
OCCURRENCE OF EMI
12
4
TYPES OF INTERFERENCE
16
5
EMI/EMC COMPLIANCE PRACTICES
20
6
EMI/EMC SUPPRESION TECHNIQUES
28
7
EMI/EMC STANDARDS & REGULATIONS
30
8
STUDY OF EMI/EMC PROBLEMS IN SMPS
33
9
A PRACTICAL APPROACH – DESIGN & DEVELOPMENT OF EMI FILTER
37
10
EMI TESTING OF DESIGNED FILTER FOR SMPS
42
11
CONCLUSIONS
54
REFERENCES
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ABSTRACT
The use of electronic equipment is increasing day by day. Each electronic product needs specific circuit & frequency for the operation of the products. Most of electronic products are sensitive to radiated frequency & generates emission to the environment & power line. Due to the use of electronic product in every walk of life, the electromagnetic pollution is increasing day by day, which results in the performance of products by this pollution. Now a days most of electronic products uses the digital technology for the operation. Digital as well as analog circuits are sensitive to radiated field and as well as conducted noise. These noises affect the performance of products like disturbance in TV picture by running grinder, disturbance in telephone line, resetting & hanging of computer due to power line disturbances etc. The operation of equipment is effected by this pollution. It is desired that all products shall work satisfactorily without affecting other & getting affected to others. In view of these, the emission & immunity tests are performed. Product specification described the limit of emission as well immunity. To
verify that a specific products will work satisfactory or not on a specific EM environment, the immunity testing are performed by creating the specific environment & verifying the same. Similarly products introduce certain noise/emission in the environment and as well as in power line. Emissions are of two types, Conducted & Radiated. Conducted emission is propagated by conducting wires etc. However radiated does not need any hard connection & these are radiated through radiation. Considerations of EMI/EMC are crucial in the design of circuits and equipment for use in electrical power systems, computers, telecommunications, controls, industrial, and medical instrumentation, transportation electronics, military equipment, information technology products, consumer electronics, and home electrical appliances. It is well recognized that EMI/EMC aspects must be addressed at the beginning in the design of circuits, including printed circuit boards and packaging of equipment and systems. EMI/EMC problems often cause delays in providing satisfactory field operation of systems. Compliances practices are used to comply the emission and immunity requirements. Various techniques are followed to improve the performance in respect of EMI/EMC such as PCB design, grounding, filtering, shielding and bonding practices. In this project we wish to study all theoretical & practical aspects of EMI/EMC and various suppression techniques to comply the requirements as per national & international specifications. We will also study the compliance practices & will try to identify desired solution to comply the conducted emission requirements as per CISPR22 standard for non-compliance products. We will also discuss the details of filter used for complying the conducted emission requirements of products.
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1. INTRODUCTION TO EMI/EMC
A BRIEF HISTORY OF EMI/EMC Until the early part of the twentieth century, few man-made sources of electromagnetic radiation existed. While the first crude radio receivers tended to be susceptible to interference from natural noise sources, the correction of this problem was usually a relatively simple task. Conflicts between early radio transmitters were easily resolved by changing frequencies or by simply moving the transmitter or receiver. As a result, prior to the 1930's, the designers of electrical circuits and systems typically needed only to insure that their devices would function in the presence of natural noise sources such as lightning or sunspots. Understandably, little if any thought was given to designing systems which were immune from external interference and virtually no effort was made to reduce electromagnetic emissions from electrical systems during this period. In the years that followed, more and more man-made sources of electromagnetic radiation began to appear. At nearly the same time that it became possible to transmit and receive complex, information-carrying signals via radio, television, and telephones, the increased generation and use of electricity caused a proliferation of noise sources such as dc motors, ac power lines, relays, and fluorescent light bulbs. The design of electro magnetically compatible systems was still not a priority during this period, however conflicts between electrical devices became much more common. In 1933, the International Special Committee on Radio Interference (CISPR) was formed and produced a document regarding equipment for measuring EMI emissions. World War II saw the introduction of radar and other remote sensing systems, along with the use of radio communication in combat. Instrumental in the development of radar was the introduction of small microwave sources, such as the cavity magnetron. This and other relatively small electronic devices were incorporated into vehicles such as ships, airplanes, and automobiles. During the war, it became evident that vehicles, which emitted electronic signals, even unintentionally, could be detected at a great distance. Moreover, an enemy could disrupt electrical systems such as radios and navigational devices by intentionally broadcasting electronic noise and false signals. The advent of electronic warfare ushered in the need for electromagnetic immunity and compatibility. After the war, the testing of nuclear weapons reveled that the electromagnetic pulse generated by a nuclear blast could damage or destroy certain types of electronic equipment. As a result the U.S. military became interested in creating systems, which were immune from the effects of external interference. In the early 1960's, MILSTD-461 was imposed, regulating not only electromagnetic emissions, but susceptibility as well. Also in the years following World War II, CISPR produced various publications dealing with recommended emissions limits, which were adopted by some European countries. The U.S. government became involved as the manufacturers of digital computers and related devices began selling large numbers of products. With the proliferation of small, integrated
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circuit devices came a dramatic increase in the number of compatibility problems. In 1979, the Federal Communications Commission (FCC) began regulating the amount of electromagnetic energy that digital devices could emit. Today, the increasing speed, and decreasing size of microelectronic circuits has made electromagnetic compatibility a critical aspect of product design.
ELECTROMAGNETIC EFFECTS (EME) Includes electromagnetic environmental disciplines such as Electromagnetic Compatibility (EMC), Electromagnetic Interference (EMI), and Electromagnetic Pulse (EMP)
ELECTRMAGNETIC INTERFERENCE (EMI) (also called Radio Frequency Interference or RFI) is an unwanted disturbance that affects an electrical circuit due to either electromagnetic conduction or electromagnetic radiation emitted from an external source. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit. The source may be any object, artificial or natural, that carries rapidly changing electrical currents, such as an electrical circuit, the Sun or the Northern Lights. It is electromagnetic energy that adversely affects the performance of electrical/electronic equipment by creating undesirable responses or complete operational failure. The interference sources may be external or internal to the electrical or electronic equipment and they may propagate by radiation or conduction. EMI is usually divided into two general categories to help in analyzing conducted and radiated interference effects: narrowband and broadband. Narrowband Emissions - a narrowband signal occupies a very small portion of the radio spectrum. The magnitude of narrowband radiated emissions is usually expressed in terms of volts per meter (V/m). Such signals are usually continuous sine waves (CW) and may be continuous or intermittent in occurrence. AM, FM and SSB fall into this category. Spurious emissions, powerline hum, local oscillators, and many other man made sources are narrowband emissions. Broadband Emissions - a broadband signal may spread its energy across hundreds of megahertz or more. The magnitude of broadband radiated emissions is usually expressed in terms of volts per meter per MHz (V/m/MHz). This type of signal is composed of narrow pulses having relatively short rise and fall times EMI can be intentionally used for radio jamming, as in some forms of electronic warfare, or can occur unintentionally, as a result of spurious emissions for example through intermodulation products, and the like. It frequently affects the reception of AM radio in urban areas. It can also affect cell phone, FM radio and television reception, although to a lesser extent
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ELECTROMAGNETIC COMPATIBILTY (EMC) Electromagnetic Compatibility (EMC) is the branch of electrical sciences which studies the unintentional generation, propagation and reception of electromagnetic energy with reference to the unwanted effects (Electromagnetic interference, or EMI) that such energy may induce. The goal of EMC is the correct operation, in the same electromagnetic environment, of different equipment which use electromagnetic phenomena, and the avoidance of any interference effects. In order to achieve this, EMC pursues two different kinds of issues. Emission issues are related to the unwanted generation of electromagnetic energy by some source, and to the countermeasures which should be taken in order to reduce such generation and to avoid the escape of any remaining energies into the external environment. Susceptibility or Immunity issues, in contrast, refer to the correct operation of electrical equipment, referred to as the victim, in the presence of unplanned electromagnetic disturbances. Interference, or noise, mitigation and hence electromagnetic compatibility is achieved primarily by addressing both emission and susceptibility issues, i.e., quieting the sources of interference and hardening the potential victims. The coupling path between source and victim may also be separately addressed to increase its attenuation. It is the ability of electrical or electronic equipment/systems to function in the intended operating environment without causing or experiencing performance degradation due to unintentional EMI. It is recommended that the performance be tested or qualified to insure operation within a defined margin of safety for the required design levels of performance. The EMI source minus the coupling mechanism path losses should result in an emission level that is less than the victim's susceptibility threshold minus a predetermined safety margin. The goal of EMC is to minimize the influence of electrical noise. Electronic equipment can malfunction or become totally inoperable if not designed to properly minimize the effects of interference from the internal and external electromagnetic environments. Proper equipment and system designs are also necessary for minimizing potential electromagnetic emissions into the operating environment. It is important that electronic equipment designs ensure proper performance in the expected electromagnetic environment, thus maintaining an acceptable degree of Electromagnetic Compatibility (EMC).
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2. THE PROBLEM OF EMI
The use of electronic equipment has increased day by day. Each electronic product needs specific circuit & frequency for the operation of the products. Most of electronic products are sensitive to radiated frequency & generates emission to the environment & power line. Due to the use of electronic product in every walk of life, the electromagnetic pollution is increasing day by day, which results in the performance of products by this pollution. Now a days most of electronic products uses the digital technology for the operation. Digital as well as analog circuits are sensitive to radiated field and as well as conducted noise. These noises affect the performance of products like disturbance in TV picture by running grinder, disturbance in telephone line, resetting & hanging of computer due to power line disturbances etc.
Cable wiring and harnessing is a significant EMI concern. Cables are required to distribute electrical power and transmit electrical signals for the operation of various systems. Since cables are usually routed to accommodate its function, it is often difficult to quantify its environment and it usually varies over both frequency and electric and magnetic field amplitudes. Cables can be EMI radiating sources if they act as radiating antennas, or be susceptible to EMI if they are receiving antennas. Cables can also be coupling paths. In addition, cables are sometimes harnessed together, so interference can also be between two cables that are close in proximity. Therefore, their performance is very difficult to predict. Many specifications classify wiring or cable types into four to six categories but these classifications are generally qualitative in nature. More quantitative classifications should look at levels of power transmitted, or susceptibility of termination.
Connectors are contacts that either link or separate two cables or other equipments. There may be anywhere from several to hundreds of individual wire-pins or coaxial sheaths making simultaneous contact via a connector. EMI problems from connectors are usually related to poor contact which may result in arcing, or overheating that leads to arcing. Poor contact connections can also result in driven-circuit voltage variations from the contact impedance modulation of the driving-circuit source. Impedance coupling from outside sources can happen in connector grounding paths. Improperly shielded connectors or poor cable-connector-equipment- enclosure contact can cause radiated emission penetration or leaking through apertures.
Grounding is one of the least understood EMC subjects, despite the fact that it seems straightforward. Improper grounding is the source of many EMI problems. Grounding is necessary to prevent shock hazard, which occurs when a wiring or component insulation in an equipment frame or housing breaks down. Grounding also protects against lightning damage. Grounding is also necessary to reduce EMI due to electric field flux coupling, magnetic field flux
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coupling, and common impedance coupling. There are two reasons why grounding is not understood well. One reason is that shock and safety control requirements existed before the electronics and high frequency area, so traditional grounding techniques were developed to satisfy those requirements. A second reason is that sometimes a conflict occurs between requirements for safety grounds and EMI control.
Different considerations must be taken into account with shielding. Shielding is the use of conductive materials to reduce radiated EMI reflection or absorption. Usually, the theoretical attenuation offered by materials to electric, magnetic, and electromagnetic waves does not match that achieved in practice. This is because a shielded enclosure or housing is not completely sealed. Any shielding application has some kind of penetrations and apertures like meter windows, cover plates and access cover members, and push buttons. These apertures cause leakage and therefore compromises the integrity of the shielding material. Shielding integrity can be restored through the use of EMC gaskets, EMC sealants, and conductive grease. Gaskets provide either temporary or semi permanent sealing applications between joints and structures. Sealants include conductive epoxies which are used to join, bond, and seal two or more metallic surfaces, and conductive caulking which is used to shield and seal two or more metallic mating members held together by other mechanical means. Conductive grease provides a low-resistivity contact path between mating members.
There are also EMI control techniques that are applied at the component, circuit, and equipment levels. The problem with resistors, inductors, and capacitors is that they do not behave at their stated values, especially at high frequencies due to the effects of parasitic inductance and capacitance. Under certain conditions, their performance degrades at frequencies as low as 1MHz. Inductive devices like transformers, solenoids, and relays produce low-impedance fields that are sources of EMI if they are uncontrolled. The main techniques available for controlling transient-producing devices involves using diodes and filters, and for controlling magnetic fields involves shielding. Surface tracking is an insulator problem that is a source of EMI. Surface tracking (or leakage) is a condition in which small currents creep across the insulator. It is caused by surface contamination of the insulation by moisture or solid conductive particles. The EMI control technique is to protect from contamination through the use of proper material and proper voltage design. Techniques used to minimize EMI in conductors include coating conductors with a high-permeability material and using hollow conductors at higher frequencies to minimize external fields.
Radio-frequency interference (RFI) is a serious EMI problem today due largely to the large number of radio transmitters that exist. Radio transmitters range from large, high-power transmitters such as broadcast, communications, and radar to small, low-power equipments such as handheld radios and cellular telephones. The problem with radio transmitters is twofold, as equipment can cause interference to nearby radio and television receivers, and equipment can be
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upset by nearby transmitters. Radio and television receivers can be very vulnerable to RFI pollution from nearby computers. Repetitive digital signals contain harmonics that can extend into the GHz range. This unwanted energy can be radiated through cables and wiring acting as antennas, or conducted through the ac power system. If the levels are high enough, the receivers can be damaged. It was this emissions problem that caused countries around the world to pass EMI regulations. In the U.S., complaints from consumers about interference with television disruption in the 70's drove the FCC to initiate mandatory EMI testing of personal and commercial computers in the 1980's. Digital circuits are usually the primary source of emissions, and analog circuits are more vulnerable to RFI than digital circuits. In protecting equipment against RFI, it is important to start at the circuit level. Filters can be used and sometimes multistage filters are needed. Slots and seams cause the most problem in RFI shielding, so highquality shields and connectors are needed for adequate RFI protection.
Electrostatic discharge (ESD) is also an EMI problem. An ESD event starts with a very slow buildup of energy, followed by a very rapid breakdown. It is this fast breakdown that causes EMI problems in modern electronic systems. The energy discharge yields EMI frequencies in the hundreds of megahertz. The high speed and frequency of the ESD energy can damage circuits, bounce grounds, and cause upsets through electromagnetic coupling. The most common method of ESD generation is triboelectric charging which is caused by stripping electrons from one object and depositing electrons on another object. In an insulator, it may be a long time before charge recombination occurs, so a voltage builds. If the voltage becomes large enough, a rapid breakdown occurs in the air, creating the ESD arc or spark. Sources of triboelectric charging includes humans, furniture, and material or device movement. Humidity also affects ESD as the lower the humidity, the higher the likelihood of ESD problems. High humidity is helpful because the moisture reduces surface impedance and allows charges to recombine at a faster rate.
There are also issues concerning EMC when humans are the receptors. A scare that has not yet been proven deals with cellular phone emissions. A source cited that radiation emitted from cellular phones has shown to cause short-term memory loss and lapses in concentration. However, this is not a proven fact yet. There was also an early brain cancer scare with cell phones that actually led to the FCC limiting the transmitting power of handheld cell phones to 0.3 watts.
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3. OCCURRENCE OF EMI
The Electromagnetic Interference (EMI) is produced by a source emitter and is detected by a susceptible victim via a coupling path. The coupling path may involve one or more of the following coupling mechanisms:
• • • •
Conduction - Electric current Radiation - Electromagnetic field Capacitive Coupling - Electric field Inductive Coupling - Magnetic field
Coupling Mechanisms : The basic arrangement of noise source, coupling path and victim, receptor or sink is shown in the figure earlier. Source and victim are usually electronic hardware devices, though the source may be a natural phenomenon such as a lightning strike, electrostatic discharge (ESD) or, in one famous case, the Big Bang at the origin of the Universe. There are four basic electromagnetic interference (EMI) coupling mechanisms: conductive, magnetic or inductive, capacitive and radiative. Any coupling path can be broken down into one or more of these coupling mechanisms working together. For example the lower path in the diagram involves inductive, conductive and capacitive modes. Conductive coupling - Conductive coupling occurs when the coupling path between the source and the receptor is formed by direct contact with a conducting body, for example a transmission line, wire, cable, PCB trace or metal enclosure.
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Conduction modes Conducted noise is also characterised by the way it appears on different conductors: • •
Common-mode or common-impedance coupling: noise appears on two conductors in the same direction. Differential-mode coupling: noise appears on two conductors in the opposite direction to each other.
Inductive coupling - Inductive coupling occurs where the source and receiver are separated by a short distance (typically less than a wavelength. Inductive coupling or magnetic coupling occurs when a varying magnetic field exists between two parallel conductors typically less than a wavelength apart, inducing a change in voltage along the receiving conductor. Capacitive coupling - Capacitive coupling occurs when a varying electrical field exists between two adjacent conductors typically less than a wavelength apart, inducing a change in voltage across the gap. Radiative coupling - Radiative coupling or electromagnetic coupling occurs when source and victim are separated by a large distance, typically more than a wavelength. Source and victim act as radio antennas: the source emits or radiates an electromagnetic wave which propagates across the open space in between and is picked up or received by the victim.
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4. TYPES OF INTERFERENCE
Electromagnetic interference divides into several categories according to the source and signal characteristics. The origin of noise can be man made or natural. Continuous interference- Continuous Interference arises where the source regularly emits a given range of frequencies. This type is naturally divided into sub-categories according to frequency range, and as a whole is sometimes referred to as "DC to daylight". •
•
•
Audio Frequency, from very low frequencies up to around 20 kHz. Frequencies up to 100 kHz may sometimes be classified as Audio. Sources include: o Mains hum from power supply units, nearby power supply wiring, transmission lines and substations. Radio Frequency Interference, RFI, from 20 kHz to a limit which constantly increases as technology pushes it higher. Sources include: o Wireless and Radio Frequency Transmissions o Television and Radio Receivers o Industrial, scientific and medical equipment o High Frequency Circuit Signals (For example microcontroller activity) Broadband noise may be spread across parts of either or both frequency ranges, with no particular frequency accentuated. Sources include: o Solar Activity o Continuously operating spark gaps such as arc welders
Pulse or transient interference- Electromagnetic Pulse, EMP, also sometimes called Transient disturbance, arises where the source emits a short-duration pulse of energy. The energy is usually broadband by nature, although it often excites a relatively narrow-band damped sine wave response in the victim. Sources divide broadly into isolated and repetitive events. •
Sources of isolated EMP events include: o Switching action of electrical circuitry. o Electrostatic Discharge (ESD), as a result of two charged objects coming into close proximity or even contact. o Lightning Electromagnetic Pulse (LEMP) o Nuclear Electromagnetic Pulse (NEMP), as a result of a nuclear explosion. o Non-Nuclear Electromagnetic Pulse (NNEMP) weapons. o Power Line Surges/Pulses
•
Sources of repetitive EMP events, sometimes as regular pulse trains, include: o Electric Motors o Electric Fast Transient/Bursts (EFT)
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SOURCES OF EMI An EMI source can be any device that transmits, distributes, processes, or utilizes any form of electrical energy where some aspect of its operation generates conducted or radiated signals that can cause equipment performance degradation. It could be of two types:
Equipment / Man –Made : EMI sources are A.C high voltage transmission lines, fluorescent lamps, microwave ovens, electric motors, hospital equipments, communication transmitters etc.
Natural Sources : Sources that are associated with natural phenomena. They include atmospheric charge/discharge phenomena such as lightening and precipitation static, and extraterrestrial sources including radiation from the sum and galactic sources such as radio stars, galaxies, and other cosmic sources. As shown in the above diagram, all natural sources are classified as broadband, incoherent, radiated, and unintentional.
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RECEPTORS OF EMI Any EMI situation requires not only an emission source but also a receptor. A receptor is also called a "victim" source because it consists of any device, when exposed to conducted or radiated electromagnetic energy from emitting sources, will degrade or malfunction in performance. Many devices can be emission sources and receptors simultaneously. For example, most communication electronic systems can be emission and receptor sources because they contain transmitters and receivers. Figure below shows taxonomy of different receptors that are susceptible to EMI. Similar to the emission source taxonomy, receptors can be divided into natural and man-made receptors. A brief description of each category is given below.
• •
Natural EMI receptors - Natural receptors include humans, animals, and plants. Man-made EMI receptors - Man-made receptors can be categorized into 4 categories: communication electronic receivers, amplifiers, industrial and consumer devices, and RADHAZ. Communication electronic receivers - These receivers include broadcast receivers, communication receivers, relay communication receivers, and radar receivers. Amplifiers - Amplifiers include IF, video, and audio amplifiers. Industrial and consumer receptors - Industrial receptors include digital computers, industrial process controls, electronic test equipments, biomedical instruments, and public address systems and intercoms. Consumer receptors include radio and TV receivers, hi-fi stereo equipment, electronic musical instruments, and climate control systems. RADHAZ - This category includes radiation hazards to electro-explosive devices and fuels. RADHAZ is an acronym for RADiation HAZards, the name given by the U. S. Department of Defense to the program that is determining the extent of radiation hazards and methods for controlling them.
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POTENTIAL EFFECTS OF EMI • •
Lightening affects telephone systems and electronic equipment. The occurrence of lines across the face of a TV screen when blender, vacuum cleaner, or other household device containing a DC motor. • The use of portable electronic devices such as laptops computers, electronic games, portable CD players, phones, etc. endangers flight due to the risk that RF emissions from carry-on electronic devices will affect avionics. • Interference in printed circuits. • Speaking with mobile phones generate noise on the computer screen. • Airport radars may affect many electronic devices. • Electrostatic discharge. • Secure communication problems. These Electromagnetic Interference effects are prominent in many areas and need to be checked before the functioning, that if the EMI is below the specified value or not as this could cause many serious errors. ELECTROMAGNETIC WAVES Electromagnetic waves consist of both an electric field (E) and magnetic field (H), oscillating at right angles to each other. In fact E & H in free space takes the place of voltage and current at the terminals of a circuit. Electromagnetic Spectrum extends from power frequencies of say 50 Hz through radio waves say at 1 MHz, to X- Rays at 1010 MHz and above. Fig. below shows Electromagnetic Spectrum.
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5. EMI / EMC COMPLIANCE PRACTICES
Electromagnetic Interference/Electromagnetic Compatibility tests are one of the basic requirements for the compliance of most electronic and electrical products. Everything from Mobile phones, Service equipment and Modern technological products go through this process. The purpose of these tests is to ensure that other users are protected from the emissions generated when the product is used in their neighborhood. All commercial products will be tested against the standards which are mostly based on CISPR tests.
One or more of the following types of EMI/EMC Tests are applicable to commercial and military electronic equipment as determined by the intended application: Conducted Emissions (CE), Radiated Emissions (RE), Conducted Susceptibility (CS), and Radiated Susceptibility (RS). The Emissions Tests (CE & RE) record any undesirable emissions from the test article. This data is plotted against the applicable specification limits. The Susceptibility Tests (CS & RS) determine the test article's ability to operate in the typical operating environment. The test article is exposed to electromagnetic signals at the levels and frequency ranges required by the applicable specification.
Figure: EMI/EMC Tests
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There are basically 2 tests for RF Emissions. These are Conducted and Radiated Emission tests. Conducted Emission Test Conducted emissions tests use an artificial mains network which is also known as a Line Impedance Stabilizing Network (LISN) as a transducer between the mains port of the Equipment Under Test ( EUT ) and the measuring receiver. It has 3 major functions as described below. • • •
Provides stable, defined RF impedance equivalent to 50 Ohm in parallel with 50µH (or 50 Ohm/5µH for high-current units) between the point of measurement and the ground reference plane. Couples the RF interference from each of the supply phase lines to the receiver, while blocking the LF mains voltage. Attenuates external interference already present on the incoming mains supply.
Although the LISN will reduce both the noise on the mains supply and variations in the supply impedance, it does not do this perfectly and a permanently installed RF filter at the mains supply to the test environment is advisable. Ambient radiated signals should also be attenuated and it is usual to perform the measurements inside a screened room, with the walls and floor of the room forming the ground reference plane. A typical setup for conducted emission is shown in the figure below.
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Radiated Emission Test The term radiated emissions refers to the unintentional release of electromagnetic energy from an electronic device. The electronic device generates the electromagnetic fields that unintentionally propagate away from the device’s structure. In general, radiated emissions are usually associated with non-intentional radiators, but intentional radiators can also have unwanted emissions at frequencies outside their intended transmission frequency band. The radiated RF measurement according to CISPR 22 and CISPR 11 is usually performed on a closed area test site. A Radiated Emission Test setup is shown in figure below.
Any open area test site is likely to suffer from ambient signals and radiations which are generated in the neighborhood and received on the site, but not emitted from the EUT. These signals can easily exceed both the EUT's emissions and the limit values at many frequencies. An emissions plot which contains ambient is hard to interpret and, more importantly, ambient which mask EUT emissions make it impossible to measure the EUT at these frequencies. Thus it is required to conduct these Radiated emission test in closed chambers i.e. Anechoic Chamber which does not allow any in-out of the ambients. The types of antennas used for the measurement and their frequencies is as below.
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EMC testing is necessary in ensuring that product immunity from several sources of transient phenomena and continuous radio frequency phenomena that are present in the electromagnetic environment. The transient events can be from natural causes - electrostatic discharge (ESD), lightning or man made - fault surges, switching transients. These involve very short duration events in the region of nanosecond or microsecond that have high amplitudes that can disrupt or destroy electronic circuits and components in an electronic device. It is also necessary in ensuring that a product does not create electromagnetic emission that are too high which will cause damage to nearby equipment or device. The limit of emission is used as a standard to ensure that all products that comply to the EMC Directive meets this requirements before it can be marketed to the public in countries where these standards are used. Conducted Susceptibility Test The determination or measurement of a device's capability to function in the presence of undesirable conducted EMI. This usually involves conduction through with the I/O cables, signal leads, or power lines. When equipment is susceptible to electromagnetic interference, it can cause the equipment to operate in an undesirable manner. The most typical problem for a single line telephone set is caused when a modulated RF signal is detected by the equipment and amplified so that the interference is heard as audio interference. Other problems may also occur, including unwanted changes of state and data errors. The degree of susceptibility for a particular device can be evaluated by monitoring the performance of the equipment in a conducted electromagnetic field of known field strength and frequency.
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Radiated Susceptibility Test The determination or measurement of a device's capability to function in the presence of undesirable radiated EMI from external electromagnetic sources and by monitoring the performance of the equipment in the presence of conducted interference signals of known amplitude and frequency. The radiated RF immunity standard used is IEC 61000-4-3. This requires a radiated RF field generated by an antenna in a shielded anechoic enclosure using a precalibrated field, swept from 80MHz to 1000Mhz and provide sufficient time to allow the equipment under test to respond. As shown in the figure below, the equipment under test is placed on the 0.8m high wooden table (for table top devices) with its front face in the same plane as the uniform field area that was previously calibrated. Both the antenna position and the uniform area are fixed with respect to the chamber. The standard requires at least 1m of connected cable length to be exposed to the field, and recommends the use of ferrite chokes to decouple longer cables. The cable layout cannot be generally specified, but at least some of the length should be in the same plane as one of the polarizations of the antenna. The equipment under test is rotated on the table so that each of its four sides, and the top and bottom if it may be used in any orientation, face the antenna in turn, and are coplanar with the uniform area. For each orientation, two sweeps are performed across the frequency range, one in each antenna polarization. If the frequency is swept from 80 to 1000MHz in 1% steps with the conventional minimum dwell time of 3 seconds per step, each sweep should take about 15 minutes, and the whole test should take over two hours.
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Electrostatic Discharge ( ESD ) Electrostatic Discharge (ESD) is probably the most familiar EMC term. However, familiarity is often limited to the term, with most people not knowing much beyond that. The word electrostatic indicates static electricity. For example, if you rub a glass rod with a silk cloth or if you rub a piece of amber with wool, the glass and amber will develop a static charge that can attract small bits of paper or plastic. If you connect this glass rod to some earthen metal object then the accumulated charge will be discharged. This is called Electrostatic Discharge or ESD. The electrical spark and shock you sometimes experience when you touch a metal shelf in a store is another example. A semiconductor device goes through lots of human interaction before it is mounted onto a board. It is possible that a device may be subjected to an ESD event during this period. Almost all CMOS devices tend to have protection circuitry to avoid damage in this condition. As a part of the quality process, all Microchip devices are tested against two standard models. They are: • •
Human Body Model (HBM) Machine Model (MM).
The human body model uses a waveform similar to ESD by human interaction. Typically, the machine model limits are one-tenth of HBM. It is believed that human interaction can generate only one pulse as it takes time for them to accumulate charge. A machine can generate multiple pulses so the peak amplitude is low to keep energy the same (model does not use a series resistor; therefore, pulses depend on the resonance of the system). These devices are not biased (not powered) during this testing. This test verifies the ESD behavior of a device when a human handles it, for example, when you touch a device without wearing an anti-static wristband.
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Electrical Fast Transients( EFT ) / BURST EFT stands for Electrical Fast Transients. As the name suggests, it is one kind of noise signal that is fast in nature. By definition it is, “a burst of interference pulses that simulates inductively loaded switches”. Sounds complicated! Let’s look at a simple example of a power drill. The electrical motor in most simple power drills is an inductive load. When you run your power drill it generates lots of noise on power lines. Any other equipment on the same power line will be subject to that noise. If you have your freezer in your garage, you wouldn’t want it to be affected when you run your drill. Would you keep a freezer if its compressor randomly starts switching or acts strangely when you turn on your drill? If your answer is no, then that’s the reason why appliance manufacturers care about EFT. The appliance industry is not the only industry that cares about EFT. The industrial environment is even worse than a home environment. Therefore, you will find many manufacturers concerned about EFT behavior. The International Electrotechnical Commission (IEC) has defined a system level standard, IEC 61000-4-4(1) to address this issue. This standard defines test waveform, levels, set up, procedure and also the
Fig : EFT Test Setup
requirement for test equipment. Figure below shows a test waveform. The signal can have either positive or negative polarity and it’s asynchronous to the power supply. It can be coupled on Line, Neutral or Power Earth or a combination of all three. The normal practice is to couple noise on power lines; however, the IEC standard does specify a mechanism to couple noise on I/O and communication ports. The IEC-compatible EFT generator provides all these options.
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Fig : EFT Waveform Typically, transformer less power supply and Switch Mode Power Supply (SMPS)-based systems face more EFT issues compared to iron core transformer based systems. Most EFT issues are addressed by using line filters, transient protectors, isolation transformers, voltage regulators and an isolated high-power circuit.
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6. EMI / EMC SUPPRESSION TECHNIQUES
The most common methods of noise reduction include proper equipment circuit design, shielding, grounding, filtering, isolation, separation and orientation, circuit impedance level control, cable design, and noise cancellation techniques. A time-varying magnetic field produces an electric field and a time-varying electric field results in a magnetic field. This forms the basis of electromagnetic .Wave propagation occurs when there are two forms of energy and the presence of a change in one leads to a change in the other. Energy interchanges between electric and magnetic fields as the wave progresses. Electromagnetic waves exist in nature as a result of the radiation from atoms or molecules when they change from one energy state to another and by natural fluctuations such as lightning. The technology of generating and processing electromagnetic waves forms the basis of telecommunications. Grounding: Connecting all grounds in a system in such a manner that all of the objectives are met. What is Ground ? • • • • • • •
Ground Wire Zero Volts Ground Plane Signal Ground Chassis Ground Conductive Paint A Trace on PCB connecting Chassis.
Ground Definitions Based On Purpose: • • • •
General : Equipotential reference surface. EMC : A low effective impedance path for the return. ESD : Surface that can source or sink large amount of charge without changing its potential. Safety : Conductor providing a path for currents to flow during circuit faults.
Ground Design Objectives For EMC: • • •
Minimize Cross-Talk Minimize Emissions. Minimize Susceptibility.
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•
Signal characteristics and allowable noise levels are to be considered when designing a grounding scheme.
Grounding Considerations: • • • • •
System performance: A system should perform reliably. Safety of personnel: minimize electrical shock hazard. AF noise emission & susceptibility. RF noise emission & susceptibility. ESD Immunity.
Electromagnetic Shielding: Electromagnetic Shielding May Have Prevented These Problems: • • •
Shielding Can Reduce Unintentional Radiated Emissions. Shielding Can Reduce Susceptibility to RF Fields. Shielding Can Reduce Unwanted EM Effects by Many Orders of Magnitude.
Shielding Last Defense Against Radiated EMI: • • •
Design Circuits to Minimize Radiated EMI Effects. Use of EMI Suppression Techniques Such as Filters, Ferrites, Isolation Transformers, etc. Apply Shielding to Block Radiated EMI.
Shielding Applies To All Levels: • • • • • • • •
Systems Cables Platforms Buildings Components Circuits Functional Stages Equipments
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7. EMI / EMC STANDARDS & REGULATIONS
The ability to sell electronic products depends upon the products being able to meet the specifications contained in various regulations. Therefore, a brief study of the different emission standards is necessary. In the United States, the Federal Communications Commission (FCC) Rules and Regulations, Part 15 Subpart J deals with unintentional emissions from equipment that use digital techniques and generate or use timing signals or pulses of frequencies. These specifications were first formed when users of television and other radio receivers complained about the interference of radiation from nearby operating digital devices. FCC 15J defines 2 classes of computing devices that must conform to emissions specifications. Computing devices refer to any computer peripheral including modems, printers, and other I/O devices. The 2 classes are defined as follows: •
Class A: "A computing device that is marketed for use in a commercial, industrial, or business environment; exclusive of a device which is marketed for use by the general public, or which is intended to be used in the home."
•
Class B: "A computing device that is marketed for use in a residential environment notwithstanding use in commercial, business, and environmental environments."
A device that passes Class B limits may be used in a Class A environment. There are different tests that are required for FCC compliance. The International Special Committee on Radio Frequency Interference (CISPR) is an organization sponsored by the International Electrotechnical Commission (IEC). CISPR is composed of each of the national committees of the IEC, a United Nations commission, and other international unions, commissions, and committees. It is responsible for setting uniform limits on electromagnetic emissions from equipment so that trade would not be inhibited between member countries as a result of different emissions specifications. CISPR publications deal with interference for the following items: • • • • •
Microwave ovens with power consumption below 5 kW Ignition systems Televisions, FM receivers, and AM receiver power-line susceptibility Conducted and radiated emission of household appliances, portable tools up to 2 kW, office machines, dimmer regulators, and other electrical apparatus Fluorescent lamps
Compliance with CISPR usually varies from country to country and each country has their own regulations regarding enforcement of the limits. Currently, there is worldwide effort towards harmonizing various EMC standards to reduce the trade barriers between countries and various sectors like Defense and Civilian. The lack of harmonization of standards is a great burden on manufacturers of electrical and electronic
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systems because it increases the duration of the product development cycle and the compliance evaluation costs. The increased use of these products has made it absolutely necessary to harmonize various EMC standards. In the Defense sector, the success of military missions is dependent on the trouble-free field performance of the electronic and communication equipments used. In the Civilian sector, it is necessary to protect the radio frequency spectrum from the electromagnetic noise emission of electrical systems. CISPR has provided recommendations for the implementation of EMC. However, each country can choose its own set of test instrumentation, test procedures, and test limits in their own EMC standards. This causes any manufacturer wishing to supply electronic equipments to different countries and the Defense and Civilian sector having to deal with a plethora of EMC standards In the United States, the MIL-STD-461D issued by the Department of Defense represents harmonized standards for military equipments and subsystems for EMI / EMC. In Europe, the market unification of 16 countries to form the European Union has affected the EMC standards scenario. The national EMC standards of these countries are being combined to form a harmonized EMC standard, called European Norms. This became known as the EMC Directive which went into effect January 1, 1996. All products that complied to the EMC Directive would bear a CE marking. This mark was required for any nation that wanted to sell electrical equipment in the European Community. CE Marking ( European Conformity ) The CE mark is a conformity mark valid within the European Economic Area (as formulated in various directives). It declares the conformity of a product to the directives applicable within the single European market. In the first instance, it must be made clear what the CE mark is not: • • • •
The CE mark is not an approval mark. The CE mark is not a certification mark. The CE mark is not a safety mark. The CE mark is not issued by a third independent body.
With a number of exceptions, the CE mark is attached to the product by the manufacturer at there own responsibility after conformity with the protection objectives stipulated by the EC directives has been determined.
CE MARKING
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Safety approval marks Now that the various national standards in Europe have been superseded, filters are only tested to the current European standard for filters. After approval has been assigned by an authorized test center, the filters are automatically approved in the other member states of the EU with no further testing. The filter then bears the safety approval mark issued by the authorizing center. Here are some examples of safety approval marks:
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8. STUDY OF EMI/EMC PROBLEMS IN SMPS
The continuous development of switching power supplies implies more efficient and inexpensive power lines filters. Passive power lines filters are now present in almost all domestics and industrial applications. One major reason for installing a filter directly at the power entry point is the suppression of the conducted emissions that would otherwise be injected directly onto the power lines. Another reason is to suppress noise entering the equipment from the power lines. At ERTL (N) we studied and tested conducted emission tests and now present a detailed description and evaluation of these “ Passive ” Line EMI filters. Introduction Power lines filters, often-called EMI filters, are present in almost all equipment, and serves two major purposes: •
First, to suppress the noise generated by the equipment, conducted emissions, which could otherwise be injected directly onto the power lines. The intensity of such emissions is regulated by government agencies in many countries in order not to cause interference with other equipment. In the USA, the Federal Communications Commission (FCC) sets the limits for various classifications of equipment, as a function of it’s operating environment. The international authority in this filed is the International Electrotechnical Commission – IEC. All manufacturers must respect the rules and standards issued by IEC. The controlled equipment primarily falls into the broad category of digital devices or those that use digital techniques for any purposes.
•
Second, to suppress noise entering in the equipment from the power lines. Such noise can cause malfunction of digital or digitally controlled equipment that may be susceptible to the noise frequencies present on the power lines. Due to its bilateral characteristics, the passive EMI filters serves both of these purposes. The FCC regulates the amplitude of conducted noise frequencies from 450kHz to 30MHz. In Europe, these noise levels are controlled from 10kHz (or 150kHz) to 30MHz. The range of controlled frequencies is broader for devices used in the general market than those used in specific, singular installations.
Since almost all today equipment are powered by switching power supplies operating from 20kHz to near 1MHz, the likelihood of superimposing interfering noise frequencies on the power lines is very great. Therefore, the need for an EMI filter at the power lines entry point is apparent. Although not always recognized, an EMI filter also suppresses noise radiated from the power lines (that acts as an antenna). The performances of most filters are specified only up to 30MHz, but the filter will suppress noise at higher frequencies.
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The performance of a filter in a particular application may be better understood from its Common-mode and Differential-mode equivalent circuits. The inductors and capacitors used in a filter are complex components with their effectiveness being dependent on material properties, construction, placement, and means of connection. Similar filters may not perform the same in a given application because of subtle component differences and parasitic parameters. The method used to install a filter in the equipment can have a significant effect on its performance.
EMI Noise Characterization Any conducted noise may be resolved into two components, common (asymmetrical) and differential (symmetrical) modes. An understanding of these modes will assist in analyzing the performance of an EMI power lines filter. Common mode noise is that noise which is identical on each line with respect to ground. Differential mode noise is that part of the total noise that occurs between the two lines with no reference to ground. The common mode currents – ICM are identical at any one frequency in both amplitude and phase. The differential mode current – IDM is a single current in the loop consisting of the power lines.
FIGURE 1a. Common Mode and Differential Mode Noise Currents The common mode currents are the same in both lines, with their return being the ground connection. The differential mode current does not flow in the ground connection. At any one frequency, the total noise current in one of the lines can be expected to be higher than that in the other line. It depends on the amplitude and phase of the component noise current at that frequency, since the total noise current in one line is the sum of the components in one case and their difference in the other.
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There are several reasons, theoretical and practical, why it is difficult to predict conducted EMI: •
• •
Differential and common mode noises are coupled through different paths to the measured EMI. Equipment package and component layout all affect the coupling paths, but the effects are very difficult to quantify. Often, a small change in layout could lead to significant change in EMI performance. The effectiveness of an EMI filter depends not only on the filter itself but also on the noise source impedance. Beyond a certain frequency, the effect of parasitic elements starts to surface. This frequency is the border between “high frequency” and “low frequency”. High-frequency effects include permeability reduction of choke core, parasitic capacitance effect of the inductor and the parasitic inductance effect of the capacitors.
Low Cost Power Lines Filter A typical power lines filter is a simply low-pass filter that provides no attenuation to the power frequency but provides large (ideal infinite) attenuation to RF energy. Consequently, an EMI power lines filter consists of series of inductors and shunt capacitors. The inductors may take two forms. The most common inductor found in almost all low-cost filters is a single magnetic core structure wound with two coupled windings, one in series in one line and the other in series in the other line. In the case of multiphase or split phase filters, the common core inductors must have identical windings connected in each power current carrying line. Similarly, the independent inductors would appear in each of these lines. The principles will be discussed for single-phase filters only.
FIGURE 8b. A Simple Low Cost EMI Power lines Filter In order to increase the effectiveness, filters often include capacitors connected from line-to- line and others connected from line-to-ground. A simple EMI power filter circuit diagram is shown in Figure 8b:
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One can notice the presence of only one inductor (Lc) and a single line-to-line capacitor (Cx1). Line-to-line capacitors are usually made of metal vaporized film or film and foil. Such capacitors have a relatively high value 0.1µF to 1.0µF (their self-resonant frequency is from (1MHz to 2MHz). Thus, they are more effective against lower frequency, differential-mode noise. Line to ground capacitors must be of very low value, from 1.0nF to 10nF (ceramic capacitors with very short leads that resonate at 50MHz or more). Any capacitor, at a frequency higher than its self-resonant frequency, is an inductor and is, therefore, a less effective EMI filter component. The impedance increases with frequency. This is important in selecting the type of the capacitor and the mounting mode into the filter. Inductors, like capacitors, are not purely inductive. The windings, by their nature, are shunted by distributed capacitance. Depending on the inductance value, the windings geometry and the core material, coil self-resonance frequency typically occurs in the range of 150 KHz to 2 MHz.
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9. A PRACTICAL APPROACH – DESIGN & DEVELOPMENT OF FILTER FOR SMPS
Designing of a filter analytically is extremely difficult and hence a practical approach is generally considered for the same. This approach has the following basic rules : • • •
Conducted Emission Measurement shall be performed without the filter. The differential mode and common mode noise shall be determined separately. In general noise source impedance affects filter attenuation. But if the filter components are arranged and sized in a right manner, the source impedance has little effect. Since it is difficult to predict H.F performance at the design stage, the focus of filter design procedure shall be to meet low frequency requirement. After the filter is designed and fabricated, the H.F. performance can be tuned or readjusted, if necessary.
We made some measurements using a standard forward switching power supply based on a specialized IC. The load was a 10 ohms resistor in series with a LED. For the filter, we used the more complex one (Figure 4). The condition for the filter is to meet the CISPR limit (that is more than 10dB attenuation for 10Hz to 150kHz frequency range and more than 20dB for 150kHz to 30MHz range). A more complex filter is presented below. It is often called the “ Total EMI filter”. The basic structure is similar with the simple EMI filter. There are some extra elements, two inductors, Ld2 and Ld1 and one condenser Cx2 connected in a low pass configuration.
A Complete EMI Power lines Filter The design of the two independent inductors Ld1 and Ld2 must take into account both the saturation characteristics of the core material relative to the rated current and the turns required to achieve the desired inductance. Otherwise, the core would be saturated under normal operating conditions and be ineffective as a filter component. The two windings of such a component are designed with equal number of turns, so that the magnetic forces around the core due to the power currents in these windings cancel. An EMI filter will most often contain a
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bleeder resistor to discharge the line-to-line capacitors when power is interrupted (Rx1 , Ry 1,2 in Figure 4). They have no effect in filter performance. If a ground choke is included in the filter, it will suppress common, not differential, mode noise. The approach for designing a filter is as follows : 1. Measure the common mode EMI Noise and Differential Mode EMI noise without a filter using CE Test Setup. 2. Compare the observed data with the specified limit for the mode. Attenuation requirement for filter shall be determined as per procedure given below. Required attenuation (dB) = Emission observed (in dB) – Limit (dB) +6 (dB) 3. The required attenuation for both common and differential mode shall be determined as per step 2 and frequency versus attenuation required (dB) shall be plotted on the graphs. 4. Draw a 40 dB/decade slope line on Ist peak freq. for attenuation required for both noises and note the frequency where this line intercepts the horizontal line. This frequency serves as the common mode and differential mode filter corner/cut off frequency. 5. According to cut off frequencies for common mode the required filter component shall be determined. 1. Calculation for Common Mode Components : As per equivalent circuit for common mode filter the cut off frequency (Fr.CM) is dependent on LCM Common Mode inductance and capacitor (Cy). This frequency is related to common mode inductance (equivalent) & capacitance(equivalent).
Fr.CM =
1. 2π√LCM CCM
Where, LCM is common mode inductance (Equivalent) CCM is common mode capacitance (Equivalent)
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Fr.CM is Common mode cut off frequency (Determined by drawing 40 dB/decade line on plot frequency vs. the required attenuation). CCM = 2 Cy
2. Calculation of Differential Mode Components : (Ld, Cx1 and Cx2) are the basic differential mode components. Cx1 and Cx2 shall be of same value. The calculation of cut off freq. (corner freq.) are related to CDM and LDM. The equivalent circuit of differential mode is given in fig.No.2(c).
Fr.DM =
1. 2π√ LDM CDM Fr. DM = Differential Mode cut off freq. LDM , CDM = Differential Inductance & capacitance LDM = 2Ld +Leakage Inductance, CDM = Cx1= Cx2 LDM = 1. {2πFr.DM}2. CDM
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Fr.DM value has been found out by step 4 i.e. by drawing 40 dB/decade on after required and frequency plot. Cx1, Cx2 and LDM are unknown. These exist some degree of freedom for trade off. Larger the LDM value selected, smaller the Cx1 and Cx2 are needed and vice versa. Practically, leakage inductance is generally in the range of 0.5 – 2% of Lc value. Value of component shall be selected such that it should not have any negative impact on the SMPS or any other devices. The filter has been designed and fabricated according to above procedure for suppressing both differential and common mode noise. The following values of filter components have been calculated according to above cut off frequency and procedure. A filter has been fabricated by using mentioned components. Lc = 13.4 mH , Cy = 22 nF, Cx1 = , Cx2 = 680nF Imposing the two frequencies, we can calculate the elements of the filter in Figure 2. The attenuation provided by the filter for the differential-mode and common-mode are presented in Figure 3.a) and 3.b).
a) differential-mode attenuation
b) common-mode attenuation
FIGURE 3. The diagram of the differential-mode and common-mode attenuation obtained with the filter in Figure 2.
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Figure: EMI Measurements in Various Configurations Red = no EMI filtering. Black = only X and Y capacitors. Grey = only inductors. Blue = complete filter solution.
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10. EMI TESTING OF DESIGNED FILTER FOR SMPS
A filter was designed using the above formula for a SMPS for compliance purposes. The SMPS
along with the designed filter was tested in the laboratory (ERTL ‘N’). During the testing the following EMI characteristics of the SMPS were measured. 1. Conducted emission initial measurement of SMPS 2. Conducted emission measurement in common mode ( Cm ). 3. Conducted emission measurement in differential mode ( Dm ). 4. Conducted emission measurement in both Cm & Dm mode. 5. Average values of peak at lower frequencies. 6. Conducted emission test for class A category. 7. Radiated Emission Test at lower frequencies with horizontal polarization. 8. Radiated Emission Test at lower frequencies with vertical polarization. 9. Radiated Emission Test at higher frequencies with horizontal polarization. 10. Radiated Emission Test at higher frequencies with vertical polarization.
The observations of measurement of the above tests were compared with the acceptable reference levels as per the CISPR 22 standards. It was found that the electromagnetic noise in the SMPS was reduced after incorporating the designed filter and are in compliance with the CISPR 22 standards.
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Conducted emission initial measurement of SMPS
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Conducted emission measurement in common mode
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Conducted emission measurement in differential mode
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Conducted emission measurement in both common & differential mode
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Average values of peaks at lower frequencies
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Conducted emission test for class A category
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Radiated Emission Test at lower frequencies with horizontal polarization
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Radiated Emission Test at lower frequencies with vertical polarization
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Radiated Emission Test at Higher frequencies with horizontal polarization
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Radiated Emission Test at Higher frequencies with vertical polarization
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CHARACTERISTICS OF HIGH EFFICIENCY PASSIVE EMI FILTER
Fig . A High efficiency filter used to suppress the noise of a 240W forward switching power supply.
An Image of EMI Power Line Filter
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11. CONCLUSION
The performance of a filter, in a particular application, may be better understood from its common-mode and differential-mode equivalent circuits. The inductors and capacitors used in a filter are complex components with their effectiveness being dependent on material properties, placement and means of construction. Similar filters may not perform identically in a given application because of subtle component differences and parasitic parameters. Many parasitic parameters exist in any filter, which are not determined by measurements. All of these, plus the properties of the materials in the components will likely make two apparently identical filters behave differently in any given application. Power lines filters used in switching power supplies are exposed to over-voltages, which can cause damages especially to the filter capacitors. It concludes that the designed filter met the Low Freq. design requirements. On incorporating in SMPS the problem of non-compliance was observed for high frequencies. By rearranging the wiring and using thick wire for grounding inside the SMPS, the problem was later eliminated. It was observed that theoretical and experimental data did match for low frequency it was also noticed that common choke became capacitive for high frequencies due to core permeability reduction and parasitic capacitance and hence the measured data deviated slightly from the predicted data. To achieve the goal of compliance for low and high frequencies the filter should be designed for Low Frequency. In case of problem for high frequency the filter should be modified accordingly. In case of non-compliance in radiated emission, torodial /ferrite bead to be put at the output & input to comply the requirements.
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REFERENCES
•
“EMI Suppression Handbook,” William D Kimmel and Daryl D Gerke, Seven Mountains Scientific, 1998,
•
“Printed Circuit Board Design Techniques for EMC Compliance, 2nd Edition” Mark Montrose, IEEE Press, 2000.
•
J J Goedbloed, Electromagnetic Compatibility, Prentice Hall, 1992.
•
V Prased Kodali and Motohisa Kanda, EMC/EMI Selected Readings, IEEE Press, 1996.
•
Research Paper from International Conference on Electromagnetic Interference & Electromagnetic Compatibility, Bangalore 2003. By Sulekh Chand Scientist ‘E’ ERTL ‘N’ STQC, Dept of Information Technology, New Delhi.
WEBSITES • • •
http://www.emiguru.com General EMC information. http://www.ieee.org/web/publications/home/index.html. IEEE Publications. http://www.fcc.gov.
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