Understanding Common Mode Interference

By Dr Jeff Chambers, Westbay Technology Ltd

Introduction

Although the basic mechanisms influencing the flow of interference currents into an equipment are well known to emc engineers, the designer of a digital-based circuit will not have needed such an understanding to develop his, or her, product through to emc evaluation. It may be only at the emc test house, when faced perhaps with a logic device exhibiting random resets during rf immunity testing, that such a clear awareness is suddenly required. This article seeks to set out the basic principles of differential and common-mode current flows, and to explain the possible control measures, in simple terms and without excess recourse to rf theory. The digital designer should thus be better equipped to resolve existing problems, and to anticipate them in future designs.

Origination of Interference Currents

A rectangular loop such as that shown in figure 1, placed in an rf field, will produce a voltage V at the field frequency. The graph in figure 1 shows that the voltage increases with increasing frequency, until a series of maxima and minima are produced. The voltage is also proportional to the height of the loop; a loop height of 1m, in a 1V/m field produces a voltage of 1V at 10MHz, reducing to 0.1V for a 0.1m loop height.

Figure 1: A rectangular loop in an electric field produces a voltage which increases with frequency. The decreasing series of curves are for loop heights of 1m, 0.5m, 0.2m and 0.1m.

Consider now the item of equipment, which has external connections to some other  equipment, shown in figure 2, placed in an rf field. For ease of initial understanding, the equipment is given a metal enclosure, and a ground return path is also shown between the equipment and the source of the external cabling. However, it is important to realise that a ground return from the equipment circuitry will exist at radio frequencies even if a hard wired connection is not provided, as stray capacitance will provide that path. Returning to the concept of loops, it is apparent that three loops can now be defined from the circuit of figure 2, and leading to the current flows shown therein.

Figure 2: An item of equipment with external cables AB and CD, in an electric field.

Loop 1 ABCDA is formed by the external cable and its return, and results in a current from the external field which flows into the equipment input and back by the return. This can be  denoted I_dm, and is the differential mode current.

Loop 2 ABEFA is formed by the external cable and the ground return, and the current flows into the equipment input and out by the ground path. This current may be denoted I_cm, and is the common mode current. Loop3 DCEFD results from the external cable return and the ground return, and as with loop 2, common mode current flows into the equipment and out by the ground path.  As suggested by figure 2, the common mode loop will often be completed by stray capacitance.

In many practical systems, the external cable connection and its return will be physically close together, usually within the same cable sheath. The ground path however, even where it can be defined by the presence of an obvious conductor, will generally be a much greater distance from the external cable connections to the equipment.

Reference to the earlier discussion on the effect of loop size suggests that the common mode current will be much greater in magnitude than the differential mode current.

Whilst the geometry presented above appears straightforward, the actual currents resulting from the radiated field will depend on the rf properties of the circuitry at both ends of the external cables, the distributed capacitance and inductance values of the cable, the nature of the ground paths within the equipment, and of the external ground path. Much of this may be difficult to define.

The Effect of Interference Currents

Reference to figure 2, and Loop 1 in particular shows that differential mode currents will produce a voltage at the input circuitry of the electronics. Its effect here will depend on whether the threshold voltage of the input gate is exceeded, assuming a digital input.

Examination of Loops 2 and 3 shows that the common mode currents seek a ground path. It may be that there is a local well-defined low impedance path to ground close to the input circuitry; if there is not, the current will flow to ground via distributed stray capacitance, which will lead to rf current flow along pcb traces and ground tracks. The resultant voltage drops may result in modulation of intended signal voltages within the electronics, and subsequent disruption.

Control of Interference Currents - First Steps

Attempts to control interference currents may be proactive, during design, or  reactive, when problems during testing occur. The usual reactive starting point is to place a capacitor across the input of the electronics; this will reduce the differential mode voltage developed at the input, as shown in figure 3a, but will not reduce the common mode currents. As discussed above, these are likely in addition to be of both greater magnitude and more insidious in effect.

Figure 3: ‘First aid’ responses to conducted interference. Parallel capacitor at (a) reduces differential mode voltage developed at the input. Series inductance shown at (b) reduce common mode current flow to ground by stray capacitance.

Further attempts during emc testing to eliminate circuit misbehaviour during immunity testing could include the placement of ferrite beads or wound inductors in series with the external connections, as shown in figure 3b. These will reduce common mode current levels, each in different ways. Figure 4 shows two insertion loss plots (made with Westbay Compufilt). A ferrite bead provides a typical impedance of 10’s of ohms up to a few hundred ohms in the 10’s MHz to 100’s MHz range, which is sufficient to provide a few dB’s of insertion loss, as shown by curve 1. The bead impedance consists of both inductive and resistive components, which damps resonance caused by a small parallel leakage capacitance.

Figure 4: Curve 1 is typical for a ferrite bead, producing a few dB loss in a 50 ohm system, part of which however is produced by a resistive component, leading to rf power loss. Curve 2 shows the insertion loss curve for a 10¼H wound inductor, with 5pF self-capacitance between windings.

A wound inductor on the contrary is relatively loss-free, and has a higher self-capacitance between its windings. This produces the type of insertion loss characteristic shown in curve 2 in figure 4, with a high peak at resonance, followed by a rapid decline. It is apparent that a large reduction in circulating current will occur within a limited frequency range, and that if the electronic circuitry is failing immunity testing only within that range, improvements may be obtained.

(A word should be added here on the use of shielded cables; if the external conductors are shielded, and the equipment has a metal enclosure or ground plane, immunity to external fields will be much improved. However in many practical cases the choice of external cabling is not determined by the equipment designer, but by its end user, and for the purposes of this article, shielded cable is not taken to be an option.)

Control of Interference Currents - PCB Assessment

If the equipment is still failing testing, further measures must be taken, and it is here that reactive design improvements tend towards proactive design measures. This ideally begins with an examination of the pcb design; the topic of pcb design for emc has been written on in far more detail than can be covered here, but two provisions can be highlighted:

Provision of adequate decoupling of each device on the board. The size of capacitance used must be sufficient to provide current during the duration of the switching operation, for an acceptable droop in the voltage at the device. This may be estimated from the equation;

     C = (DI . Dt ) / DV  pF

Where DI is the current demand in mA, Dt is the switching time in ns, and DV is the acceptable voltage droop.

Use of a ground plane and power plane. A ground plane will provide some shielding against direct pick-up on the board, although this may not be significant in any case for small boards. The big advantage of a ground plane is its low impedance at high frequencies by comparison with a track. The earlier discussion highlighted voltage differences across the pcb ground system as a source of spurious circuit operations, and it is instructive to compare the rf impedance of a track and a ground plane:

Impedance of a 5mm wide ground trace, 100mm long at 200MHz: 105 ohms

Impedance of a ground plane 200mm wide by 100mm long at 200MHz: 5 mohms/square.

Thus if the circuitry exhibiting emc problems is assembled on a two-layer board, considerable improvements could result from re-design to a four layer board, with ground and power planes.

Control of Interference Currents - Input Filters

After consideration of the pcb design and layout, an input filter design can be addressed. The most effective design will incorporate inductors and capacitors, and will attenuate and decouple both differential mode and common mode currents. However the use of capacitors for common mode filtering does require a conductive ground plane of some description, for connection of the capacitor from line to ground. This can be a metal enclosure, ideally, a metal tray, or the pcb ground plane. What is not going to be helpful is to decouple common mode interference to a 0v track running around the pcb.

Figure 5a below shows two lines entering an equipment protected by inductor and capacitor components. Differential mode currents are attenuated by the L - Cx - L circuits. Common mode currents are attenuated by the L - Cy circuits. This would pertain for signal lines, carrying negligible current; if the conductors were carrying significant current, as in the case for many mains inputs for example, it would be normal to use a bifilar wound inductor to allow flux cancellation by the line current, preventing saturation of the inductors. In this case, the differential mode filter would consist of the Cx capacitor only.

Figure 5: The filter circuit at (a) will provide both differential and common mode attenuation. The insertion loss curves at (b) show the behaviour of a 1nF chip capacitor (1), 0.47uH chip inductor (2) & the L-C combination of the two (3).

Examination of the modelled behaviour of practical L, C and L-C filters is of interest. Wound inductors and chip capacitors possess parallel self-capacitance and series self-inductance respectively, and this produces the insertion loss curves 1 and 2 in figure 5b. Each show a sharp peak in insertion loss at resonance, followed by a sharp decline. These curves are modelled for a 50 ohm source and load impedance, which will not necessarily be true in practice. Further modelling with the Westbay Compufilt software shows that insertion loss decreases for the capacitor filter with decreasing system impedance, and increases with increasing system impedance. The inverse applies for an inductor filter.

Curve 3 in figure 5b depicts the loss curve of an L-C filter (1nF, 0.47uH); this has a higher loss than that shown by the individual components, and has a wider effective frequency range. Examination of the effect of varying system impedance (which is generally not known in practice) from 50 ohms, shows that the L-C filter is much less dependent compared to the C or L circuits alone.

Finally, filter circuits should be carefully placed, as close to the physical input point as possible, and ideally on their own pcb. If the equipment has a metal enclosure or tray, the filter circuit should be connected to it by a short low-inductance connection.

Summary

This article has set out to explain the differences between differential mode and common mode interference currents, and some of the approaches in eliminating their disturbing effect on electronic circuitry. A non-analytical approach has been pursued as far as possible, to ensure a basic understanding is acquired. There is no implied difference in approach whether the external cabling connected to the equipment is carrying mains or signal voltages, or a combination of both.

This is not to say that more detailed analysis cannot be undertaken; Westbay Tools for example provides some illuminating models which examine the input voltages produced by common mode fields in single ended and differential input circuits. Nonetheless exact analysis will rarely be possible due to the large number of variables involved.

More information on Westbay EMC Design Software can be found at: http://www.westbay.ndirect.co.uk/