EMI EMC

Introduction :

• Widespread use of electronic circuits for communication, computation, automation, and other purposes makes it necessary for diverse circuits to operate in close proximity to each other

• These circuits affect each other adversely.

EMI EMC

• EMI …..A major problem.

• Large no of electronic devices.

• ICs and LSI to reduce size.

• More circuits crowded in less space.

• Increase in clock freq. upto 1 Ghz.

EMC EMI

• Not merely to operate in lab condition.• To work in real world environment.• Comply with EMC regulations of Govt.• This means that equipment should not be

affected by external electromagnetic sources and should not itself be a source of electromagnetic noise that can pollute the environment.

• Electromagnetic compatibility should be a major design objective.

• An electronic system that is able to function compatibly with other electronic systems and not produce or be susceptible to interference is said to be electromagnetically compatible with its environment.

• The objective is to learn how to design electronic systems for electromagnetic compatibility (EMC).

• A system is electromagnetically compatible with its environment if it satisfies three criteria:

• 1. It does not cause interference with other systems.• 2. It is not susceptible to emissions from other systems.• 3. It does not cause interference with itself.

Noise & Interference

• Noise is any electrical signal present in a circuit other than the desired signal.

• This definition excludes the distortion products produced in a circuit due to nonlinearities. Although these distortion products may be undesirable, they are not considered noise unless they are coupled into another part of the circuit.

• It follows that a desired signal in one part of a circuit can be considered to be noise when coupled to some other part of the circuit.

Noise

• Noise sources can be grouped into the following three categories:

• (1) intrinsic noise sources that arise from random fluctuations within physical systems, such as thermal and shot noise;

• (2) man-made noise sources, such as motors, switches, computers, digital electronics, and radio transmitters; and

• (3) noise caused by natural disturbances, such as lightning and sunspots.

Noise..Contd.

• Interference is the undesirable effect of noise.

• If a noise voltage causes improper operation of a circuit, it is interference.

• Noise cannot be eliminated, but interference can.

• Noise can only be reduced in magnitude, until it no longer causes interference.

EMC

• Electromagnetic compatibility (EMC) is the ability of an electronic system to

• (1) function properly in its intended electromagnetic environment and

• (2) not be a source of pollution to that electromagnetic environment.

• The electromagnetic environment is composed of both radiated and conducted energy.

• EMC therefore has two aspects, emission and susceptibility.

• Susceptibility is the capability of a device or circuit to respond to unwanted electromagnetic energy (i.e., noise).

• The opposite of susceptibility is immunity.• The immunity level of a circuit or device is the

electromagnetic environment in which the equipment can operate satisfactorily, without degradation, and with a defined margin of safety.

• One difficulty in determining immunity (or susceptibility) levels is defining what constitutes performance degradation.

Definitions

• Electromagnetic Compatibility : (EMC)

The capability of electrical and electronic systems, equipment, and devices to operate in their intended electromagnetic environment within a defined margin of safety and at design levels or performance without suffering or causing unacceptable degradation as a result of electromagnetic interference [American National Standards Institute (ANSI) C64.14-1992)].

Electromagnetic Interference : (EMI) :• The process by which disruptive

electromagnetic (EM) energy is transmitted from one electronic device to another via radiated or conducted paths (or both).

• In common usage, the term refers particularly to RF signals; however, EMI is observed throughout the EM spectrum.

• Radiated Emissions. • The component of RF energy that is emitted through a

medium as an EM field. Although RF energy is usually emitted through free space, other modes of field transmission may be present.

• Conducted Emissions. • The component of RF energy that is emitted through a

medium as a propagating wave generally through a wire or interconnect cables. Line-conducted interference (LCI) refers to RF energy in a power cord or alternating-current (AC) mains input cable. Conducted signals propagate as conducted waves.

• Susceptibility. • A relative measure of a device or a system’s

propensity to be disrupted or damaged by EMI exposure to an incident field. It is the lack of immunity.

• Immunity. • A relative measure of a device or system’s ability

to withstand EMI exposure while maintaining a predefined performance level.

• Radiated Immunity. • A product’s relative ability to withstand EM

energy that arrives via free-space propagation.

• Conducted Immunity. • A product’s relative ability to withstand EM

energy that penetrates through external cables, power cords, and input–output (I/O) interconnects.

EMC CONTD

• Control emission

• Raise the level of immunity ( Less susceptible)

• EMC regulations of the Govt. (control allowable emissions and define the degree of immunity required)

• Crisis approach & Band aid approach.

• System approach….Not an afterthought.

FCC

• FCC Regulations

• In the United States, the Federal Communications Commission (FCC) regulates the use of radio and wire communications. Part of its responsibility concerns the control of interference.

FCC..Contd

• If a product uses digital circuitry and has a clock greater than 9 kHz, then it is a digital device under the FCC definition. This definition covers most digital electronics in existence today.

• Computer terminals and peripherals, which are intended to be connected to a computer, are also considered to be digital devices

FCC contd

• Digital devices covered by this definition are divided into the following two classes:

• Class A: A digital device that is marketed for use in a commercial, industrial or business environment

• Class B: A digital device that is marketed for use in a residential environment, notwithstanding use in commercial, business, and industrial environments

• Because Class B digital devices are more likely to be located in closer proximity to radio and television receivers, the emission limits for these devices are about 10 dB more restrictive than those for Class A devices.

Mechanism of Interference

Mechanism …contd

Mechanism..contd

Transfer of EM energy into 4 subparts :

• Radiated emission ( RE)

• Radiated susceptibility ( RS).

• Conducted emission (CE)

• Conducted Susceptibility ( CS).

CE

• Conducted emissions are those currents that are passed out through the unit’s ac power cord and placed on the common power net, where they may radiate and thus cause interference with other devices.

• The frequency range for conducted emissions extends from 150 kHz to 30 MHz

RE

• Radiated Emission concern the electric and magnetic fields radiated by the device that may be received by other electronic devices, causing interference in those devices.

• FCC, as well as other regulatory agencies, requires measurement of the radiated electric field, and the regulatory limits are given in terms of that field in dBmV/m.

• The frequency range for radiated emissions begins at 30 MHz and extends to 40 GHz

FCC regulations

• The FCC specifies that for digital devices, measurements to show compliance with Part 15, must be performed following the procedures described in measurement standard ANSI C63.4–1992 ( 9 Khz to 40 Ghz)

FCC reg.on CE & RE

• The FCC Part 15 EMC Regulations limit the maximum allowable conducted emission, on the ac power line in the range of 0.150 to 30 MHz, and the

• maximum radiated emission in the frequency range of 30 MHz to 40 GHz.

Canadian Standards

EU standards

• The EU’s conducted emission requirements are now the same as the FCC’s (see Tables 1-5 and 1-6 as well as Fig. 1-4).

• The Radiated emission standards are similar but not exactly the same. Table 1-8 shows the European Union’s Class A and Class B radiated emission limits when measured at 10 m.

CISPR RE limits

RF & Transient Immunity

Transient Immunity

• There are basically three types of high voltage transients that electronic equipment designers need be concerned about.

• ESD

• Electrical fast transient (EFT)

• Lightning surge

Transient Voltage Suppression Diode (TVS Diode)..Signal line

suppression.• A TVS diode is similar to a zener diode but with a

larger p-n junction area that is proportional to its transient power rating

• The three most important parameters of TVS diodes are as follows:

• Reverse standoff voltage• Clamping voltage• Peak pulse current

.

Power Line Transient Suppression

• Power-line filters can handle the low energy transients such as ESD and EFT.

• Additional EFT or ESD suppression, if required, can often be obtained on the power line by the addition of a common-mode ferrite choke on the input cable to the power-line filter or power supply.

• Surge, however, has a pulse energy level that is a thousand or more times greater than that of ESD or EFT and is another problem altogether.

• Transient protection for these high-energy pulses is often required prior to the power-line filter. Three types of nonlinear transient protectors are commonly used for high power transient.

• TVS diodes• Gas discharge tubes• Metal oxide varistors (MOVs)

• Transient voltage suppression diodes and MOVs are voltage-clamping devices. They operate by limiting the voltage to a fixed level. Once they turn on, they must dissipate the transient pulse energy internally.

• Gas discharge tubes, however, are crowbar devices. Once they turn on, the voltage across them drops to a very low value; therefore, their power dissipation is much reduced. They can handle extremely large currents.

ESD control

Human body model as a prime source of discharge

• Electrostatic discharge is normally a three-step process as follows:

• 1. A charge is generated on an insulator.

• 2. This charge is transferred to a conductor by contact or induction.

• 3. The charged conductor comes near a metal object and a discharge occurs

• Decay time…The time it takes for the charge to be reduced to 37% of its initial value. The decay time (sometimes called the relaxation time) is equal to

I/O Cables Treatment

Approx values

• L1 + L2 = 0.7 micro

• R1 + R2 = 1 to 30 K

• C1 + C2 = 150 pf.

Biological effects