Electromagnetic radiation is generated by any change in the flow of electric current. It’s produced by natural phenomena (lightning, solar flares, the aurora) and manmade electrical and electronic devices—radio transmitters, power tools, computers, relay contacts, lamp dimmers—just about anything that uses or controls electrical energy.
When electromagnetic energy enters where it isn’t wanted, it can interfere with a device’s operation or use. Managing electrical noise, both the production of and susceptibility to it, is therefore an important part of product and system design. Failure to anticipate noise sources and minimize susceptibilities during initial design can require expensive and time-wasting fixes later on.
The first step in noise management is understanding the vocabulary, origins, and effects of noise.
Table of Contents
- EMI/RFI/EMC
- Ripple Current
- Switching, Capacitance, And Inductance
- Grounding
- Compliance
- Measurements
-
References
EMI/RFI/EMC
In the context of power-system design, noise—commonly called electromagnetic interference (EMI) or radio frequency interference (RFI)—appears as electrical currents. These currents degrade the performance of susceptible devices. Electromagnetic Compatibility (EMC) is the ability of the source and the receptor to operate properly in a given electromagnetic environment.
Noise currents can flow both to and from the power supply’s input, along the wiring that connects the converter to the energizing source (the ac line or a battery, usually). Currents exiting the supply are called conducted emissions. Radiation from this path can cause interference with other devices, which is why power cables are often routed through ferrite chokes.
Currents entering the supply are called conducted susceptibilities (Fig. 1). Susceptibilities aren’t necessarily “swallowed up” by the converter, and they can make their way into the device being powered. Specifications for conducted susceptibility are most often required for military equipment, not so often for industrial or consumer products.
The current flow can be common mode or differential mode. In common mode, the emission has the same polarity on both conductors and travels to and from ground. In differential mode, the emission has opposite polarity on the conductors, without any reference to ground (Fig. 2).
2. In the same basic package, conducted emissions form at the ac or dc input and generate both differential- and common-mode noise.
Radiated EMI is electromagnetic radiation leaving or entering the product enclosure. Waves leaving are radiated emissions. Unwanted waves entering are the source of radiated susceptibility.
Ripple Current
A not-necessarily obvious source of conducted EMI is ripple in the converter’s output. The ripple generally appears at harmonics of the ac line frequency or switching-converter rate. As ripple is most likely to affect components within the power system, it’s up to the designer to determine how much is acceptable. There are no third-party standards.
Switching, Capacitance, And Inductance
Electrical devices switching on and off create almost all electrical noise. (A steady dc current doesn’t generate noise.)
Even a passive device such as a rectifier can generate noise. Rectifiers don’t turn on and off (that is, conduct and stop conducting) instantaneously. Common rectifiers have a response time of about 1 µs, generating harmonics not only at multiples of the ac frequency being rectified, but also in the megahertz region. These harmonics can find their way to both the input and output of the power supply.
The active devices in a switching power supply (commonly MOSFETs) switch more rapidly (~20 ns), generating higher frequencies, up to 30 MHz, which is the highest frequency specified in commercial standards for measuring conducted interference. (This doesn’t mean higher frequencies that could cause interference should be ignored.)
If the front end includes power factor correction (PFC) circuitry, there will be additional EMI from the PFC’s MOSFETs and diodes.
Multiple conduction paths created by parasitic capacitances within the power supply and the system itself permit high-frequency EMI currents to enter almost any part of the system. These stray capacitances appear not only between parallel wires, but also between conductive surfaces. The parasitic inductance of wiring and board traces contributes to voltage “kicks” that can propagate through the parasitic capacitances.
Grounding
Proper grounding reduces the effects of all noise sources, both emissions and susceptibilities, by providing a low-impedance path to ground for EMI currents. There are four significant “ground” points in a power system (Fig. 3).
3. Good grounding minimizes all noise sources by presenting noise currents a low-impedance path to earth. The four places in a power system that are of concern are the front end input and output (points 1 and 2), which present different potentials, and the dc-dc converter input and output (points 2 and 3).
Whether the input is ac or dc, points 1 through 4 are not guaranteed to be at the same potential. The front-end input and output (1 and 2) are at different potentials also, due to interconnect resistance and the inductive elements used in filtering. The converter’s input and output (2 and 3) can be at different potentials, due to the transformer “floating,” without a common input-output ground.
Usually, only the chassis (4) is connected to ground (earth). Obviously, the lower the value of Z for the EMI frequencies of interest, the more effective the grounding. (Z is presumably a complex value that varies with frequency.) It might be useful to look at a block diagram and decide which paths the high-frequency noise currents are likely to take based on how these points are selected and how their connections are implemented.
The ideal situation is for Z to be “zero,” with all chassis current passing to earth. A non-ideal situation would be noise currents travelling from the front end to the equipment being powered. During EMI qualification testing, it’s important that all noise currents go to earth, and not to the measurement equipment, as that would affect both common- and differential-mode measurements.
Compliance
A third-party specification usually enforces compliance with standards for conducted and radiated emissions. There are three applicable specifications for conducted emissions (Fig. 4):
4. The chart shows relevant conducted emission limits using the LISN method.
- FCC part 15, Level A or level B (United States)
- EN 55022, Level A or Level B (Europe)
- MIL-STD-461 for military applications
There are significant differences at low frequencies. FCC part 15 starts at 450 kHz.
EN 55022 starts at 150 kHz. MIL STD 461 (not shown) starts at 10 kHz for CE 102. The designer should check the revision level for the spec of interest and state it in any specification.
Measurements
There are two commonly used measurement procedures for conducted emissions: quasi-peak and average. The spec limits for each are given in the EN 55022 specification. Both measurements may be required.
The FCC and EN specs have two acceptance levels. Level A is for business and industrial equipment. Level B requires a lower emission level for consumer products.
Output ripple is usually specified as a percentage of the output voltage and given in millivolts, peak-to-peak. Ripple is usually measured with an oscilloscope having a 20-MHz bandwidth.
Most EMI radiates from the input and/or output cables, which don’t benefit from any shielding the enclosure might provide. Their emission levels might vary, depending on the location of the cables, and how they are dressed.
References
- FCC, Part 15, Level A or Level
- EU standard 55022 (CISPR22)
- MIL-HDBK-241B, Design Guide for Electromagnetic Interference (EMI) Reduction in Power Supplies, 1983
- MIL-STD-461E, Requirements for the Control of Electromagnetic Interference Emissions and Susceptibility, 1999
- EDN’s Designer’s Guide to Electronic Compatibility, Daryl D. Gerke and William D. Kimmel, Cahners Publications (supplement to EDN magazine), January 1994
- Controlling Radiated Emissions by Design, Michael Mardiguian, Van Nostrand Reinhold, 1992
- EMC for Product Designers, Tim Williams, Butterworth-Heinemann Ltd., 1992
- Noise Reduction Techniques in Electronic Systems, Henry W. Ott, John Wiley & Sons, 1988
- Printed Circuit Board Design Techniques for EMC Compliance, 2nd Edition, Mark Montrose, IEEE Press, 2000