Conducted Emissions Basics

What are conducted emissions? Why are conducted emissions important? Conducted emissions limits. What causes conducted emissions? Conducted Emissions Analysis & more topics about conducted emissions e

What are conducted emissions?

Conducted emissions are nothing more than ripple current or ripple voltage on equipment cables that is produced as a byproduct of active electronic circuit operation.

Steady state conducted emissions, are most commonly described in terms of their frequency spectrum, while conducted emission transients produced by equipment turning on, turning, off, or changing mode are usually described in terms of the time domain waveforms they induce.

The nature of conducted emissions is that they are conducted on electrical cables. That is, current and voltage induced on electrical cables by circuit operation are conducted emissions.

Equipment containing active circuitry will conduct audio frequency and radio frequency current and voltage on its electrical cables. The amplitude and frequency content of the emissions is a function of the circuit design and packaging.

Why are conducted emissions important?

Equipment connected to an electrical cable is subjected to conducted emissions current and voltage induced by all other by the equipment connected to the cable. If there are numerous pieces of equipment sharing the cable, as for example in the case of a power bus, each equipment contributes its own conducted emissions to the bus noise and each is subjected to the noise contributed by conducted emissions from all other equipment on the bus.

A secondary, but very important, byproduct of conducted emissions is radiation. Conducted emissions on cable conductors cause the cables to radiate. This topic is beyond the scope of this article, but suffice to say that excessive conducted emissions are a major cause of excessive radiated emissions.

Conducted emissions limits

In order to reduce the likelihood that a piece of equipment will interfere with other equipment due to its conducted emissions, and potentially cause excessive radiated emissions, limits are placed on allowable conducted emissions levels. That is, each equipment is required to control its conducted emissions to levels lower than specified by the conducted emissions limit.

An example of two conducted emissions limits are shown in the figure below, taken from RTCA/DO-160G. These limits are applicable to commercial aircraft equipment. Each industry has different conducted emissions limits that are established by cognizant governing agencies.

The most common conducted emissions limit is the limit imposed on equipment input power lines. Nearly every product produced for public, private, government, medical, automotive, or industrial use must control conducted emissions to within specified limits.

Figure 1 Sample Conducted Emissions Limits

The most common conducted emissions limit is for equipment input power lines. Nearly every product produced for public, private, government, medical, automotive, or industrial use must control conducted emissions to within specified limits.

What causes conducted emissions?

Any circuit that actively switches or modulates current or voltage as it operates will produce conducted emissions. For most equipment the dominant source of conducted emissions on input power lines is a switch mode power supply (SMPS). Switching power supplies convert input voltage to output voltage needed by the equipment's circuitry.

SMPS topologies are numerous, providing conversion from ac to dc, high-to-low voltage, or low-to-high voltage. SMPS operation is beyond the scope of this article, but all switching power supplies have one thing in common: they draw time varying current from the input power bus. Current waveform characteristics vary with SMPS design and application, but all induce ripple current and voltage on the input power lines. That is they all produce conducted emissions.

Ripple current and voltage are synonymous, related to one another by the power source impedance. Since power source impedance is non-zero, changes in current drawn from the power lines induces voltage disturbances; rapid current fluctuations induce rapid voltage fluctuations, and vise-versa.

Switching Power Supply Example

In the world of electromagnetic interference, conducted emissions are usually expressed in the frequency domain. Ripple voltage and current in the time domain directly translates to conducted emissions in the frequency domain.

SMPS Circuit

Take for example the switching power supply shown in the simplified schematic below. It switches at 200 kHz so it draws current pulses from the power lines 200,000 times per second.

Figure 2 Switching Power Supply Simplified Schematic

Time Domain Waveform

The waveform of one input current pulse is shown below. On each switching cycle the input current rises from 0 to about 2.1 amps in 42 nsec, the time is takes the FET to turn on. The current then ramps up to 3.5 amps over the remainder of the 1.5 sec conduction cycle. The current drawn from the input power lines then drops back to zero in 40 nsec, the turn off time of the FET. At the end of the 5 sec period (reciprocal of 200 kHz) the waveform repeats.

Figure 3 Input Current Waveform

Frequency Domain Spectrum

A Fourier transform is used to convert the time domain waveform to its equivalent frequency domain spectrum. The Fourier transform decomposes the waveform into the frequencies it contains. In this case the spectrum is comprised of frequencies that are multiples of 200 kHz. The amplitude of each frequency component is determined by the shaped of the waveform. If sine waves at 200 kHz, 400 kHz, 600 kHz, etc., having the corresponding amplitudes shown in the graph below, were superimposed on one another the resulting wave would be the time domain waveform shown in the graph above.

Figure 4 Input Current Specturm

How does this spectrum relate to conducted emissions?

If we assume for a moment that the power supply has no filtering, the current spectrum shown above will be present on the input power lines.

Conducted emissions limits are generally specified in decibels, with units of dBµA (decibels referenced to 1 A) or dBµV (decibels referenced to 1 V). For this example, the limit units are dBμA (see y-axis in Figure 1.) To

convert from amps to dBµA use the following equation:

Unfiltered Conducted Emissions

The current spectrum in the graph below is the same as the current spectrum in the graph above, except that current units have been changed from amps to dBμA and the conducted emissions limit has been overlaid.

Figure 5 Input Conducted Emissions with Limit Overlay

Filtered Conducted Emissions

As shown in the graph above, conducted emissions are more than 70 dB over the conducted emissions limit at 150 kHz. EMI filtering is needed to attenuate conducted emissions level to below the limit.

An EMI filter consisting of the components and values shown in the figure below will be added to the input power lines to bring conducted emissions down to acceptable levels.

Figure 6 Input EMI Filter Schematic

When placed on the switching power supply input lines the circuit will be as shown in the schematic below.

Figure 7 Filtered Switching Power Supply Simplified Schematic

Analysis of the switching power supply equipped with the input EMI filter shows that conducted emissions are now attenuated to levels below the conducted emissions limit, as required.

Conducted Emissions Analysis

Conducted emissions produced by switching power supplies are perhaps the most common cause of conducted emissions test failures. Conducted emissions are best controlled by proper packaging and EMI filtering early in the design cycle.

Analysis software is invaluable for taking the guesswork out of conducted emissions control. For the analysis example above, EMI Analyst™ software from EMI Software LLC, Sedona, Arizona, USA was used to model the active circuit waveforms, synthesize and optimize EMI filtering, and simulate the conducted emission test setup. For more information, visit the EMI Analyst™ website at .