COMMON-MODE AND DIFFERENTIAL-MODE CURRENTS

In any circuit, there are both common-mode (CM) and differential-mode (DM) currents, both of which determine the amount of RF energy that is developed and propagated. There is a significant difference between the two. Given a pair of wires or traces and a return path, one or the other mode will exist, usually both. Differential-mode signals carry data or a signal of interest (information). Common mode is a side effect, or a

byproduct of differential-mode transmission, and it is most troublesome for EMC compliance. Common-mode and differential-mode currents are

illustrated in Fig. 2.18.

Figure 2.18: Common- and differential-mode current

configurations.

2.7.1 Differential-Mode Currents

Radiated emissions from differential-mode (DM) current is the component of RF energy that is present on both the signal and return paths that are opposite to each other. If a 180° phase shift is established precisely, differential-mode current will be canceled. Thus, EMC is assured. In

contrast to common-mode (CM) currents, which are usually high-frequency signals, the DM signal may very well not be at RF frequencies. In fact, a DC power line carrying differential-mode DC current (non-RF signal) may be carrying RF common-mode current at the same time.

Differential-mode signals

1. Convey desired information, since most signal traces are a single-ended route (source-to-load). This is the differential mode of data transmission.

2. Cause minimal interference, as RF fields generated oppose each other and cancel out if properly set up, by having the RF return path physically close to the routed trace.

2.7.2 Common-Mode Currents

Common-mode (CM) current is the component of RF energy that is present on both the signal and RF return path, usually in common phase. The

measured RF field resulting from common-mode current will be due to the sum of the currents that exist in both the signal trace and the return trace.

This summation may be substantial and is the major cause of RF emissions, especially from I/O interconnects. Common-mode current develops through a lack of differential-mode cancellation, or in the presence of poor CM rejection. DM current will exist when there is poor balance in the circuit. This poor cancellation is due to an imbalance between two transmitted signal paths. If the differential signals are not

exactly opposite and in phase to each other, their currents will not cancel out. The portion of the RF field that is not canceled is "common-mode"

current. Common-mode effects, however, may also be created as a result of ground bounce and power plane fluctuations caused by components drawing current from a power distribution network.

Common-mode signals are

1. The major source of RF radiated energy.

2. Contain no useful information.

Common mode begins as the result of currents mixing in a shared metallic structure, such as the power and ground planes. Typically, this happens because currents are flowing through undesirable or unintentional return paths. Common-mode currents develop when return currents lose their pairing with their original signal path (e.g., splits or breaks in planes) or when several signal conductors share common areas of the return plane.

In Fig. 2.18, current source, I1, represents the flow of current from voltage source, E, to load, Z. Current flow, I2, is current that is observed in the return system, usually identified as an image plane, ground plane, or 0V-reference. The measured radiated electric field of the common-mode current is caused by the summed contribution of both I1 and I2.

A simple analogy that helps explain one way that common-mode energy is developed is to analyze Fig. 2.18 in very simple terms. Assume 1 watt of power is sent from the source to the load. The load dissipates one-half watt of power. This means that current in path I2 is one-half watt. Under this situation, violation of Ampere's law occurs, which states that the sum of the current in a circuit must equal zero. Visualize now that one-half watt travels toward the source. At the same time, one-half watt travels back from

source to load. Mathematically, if we take the limit of the current to zero at any particular point of time, we observe that half of the energy is traveling left, while the other half is traveling right. The summation of these two currents at any particular point of time equals the source current, I1; hence Ampere's law is satisfied. This residual one-half watt of power is added to the 1 watt in the source trace. This means that a total of 1-1/2 watts of power is present across the load. This energy is common-mode and is significantly greater than differential-mode current.

With differential-mode currents, the RF energy developed is the difference between I1 and I2. If I1 = I2 exactly, there will be no radiation from

differential-mode currents that emanate from the circuit (assuming the distance from the point of observation is much larger than the separation between the two current-carrying conductors), and hence, no EMI. This occurs if the distance separation between I1 and I2 is electrically small.

Design and layout techniques for cancellation of radiated fields emanating from differential-mode currents are easily implemented in a PCB with an image plane or RF return path, such as a ground trace. On the other hand, RF fields created by common-mode currents are harder to suppress.

One design and layout technique to reduce common-mode currents is to decrease the distance spacing between the signal trace and RF current return path, or image plane (power or ground). In most cases, this is not fully possible because the spacing between a signal plane and image plane must be at a specific distance to maintain constant trace impedance of the PCB.

An RF current return path is best achieved with a solid plane for multilayer PCBs or a ground trace for single- and double-sided boards. The RF

current in the return path will couple with RF current in the source path (magnetic flux lines traveling in opposite direction to each other). This coupling provides for flux cancellation or minimization, as seen in Fig. 2.19.

Figure 2.19: RF current return path and distance

spacing.

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Table of Contents

2.3: MAGNETIC FLUX AND CANCELLATION

2.10: GROUND AND SIGNAL LOOPS (EXCLUDING EDDY 2.13: SLOTS WITHIN AN IMAGE PLANE

Chapter 2 - Printed Circuit Board Basics

Printed Circuit Board Design Techniques for EMC Compliance: A Handbook for Designers, Second Edition by Mark I. Montrose

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