COMMON MODE AND DIFFERENTIAL MODE – DOES IT MATTER
EMC Engineers and test labs toss around the terms Common Mode and Differential Mode casually, but may not explain what they mean, how they are generated, or the impact each has on the test results. The way common mode and differential mode energy is generated is different, and how they impact test results is different. And how they are controlled is significantly different. The way to control one does not often control the other.
Differential Mode currents are those that are equal and opposite in adjacent wires, or if shared among several wires, the net current of all wires is zero. Power lines may be the easiest to understand – current which flows in the phase or hot lead is equal to and in the opposite direction to the return or neutral current in the adjacent lead. For three phase power, the currents may be shared among three phases, and may include a neutral current as well, but the total current of all lines at some location (e.g., at the input connector) adds to zero.
Common Mode currents are those that flow in adjacent lines in the same direction and tend to be in phase. They are often measured in microamperes, whereas differential mode is measured in amperes. The return currents of common mode currents are by remote paths, such as a ground or chassis current, or by radiation back to the source. Placing a current probe around the cable which includes multiple wires will show that a non-zero current is present in that cable.
The reason this is an issue is that there tends to be a larger loop created by this current path. The common mode current may flow from the equipment to some remote location, where the energy can be coupled to ground, which then flows back to the source through the equipment chassis and back to the source. This large loop can generate many orders of magnitude, possibly over 106 times more, more radiated emissions than differential mode energy for the same amount of
Figure 1 - Common Mode verses Differential Mode loop areas
OVERVIEW OF DIFFERENTIAL MODE CURRENTS
Differential mode currents may be easier to understand in the sense that power lines and many signals use differential mode current paths. Each current carrying conductor has an adjacent and parallel return path which carries an equal and opposite current. How power is used may also create additional differential mode currents. Motors can contribute to this. Switch mode power supplies will also generate differential mode currents at the fundamental switching frequency, and at least a few harmonics of that frequency. Since most standards measure conducted emissions in each power line individually, differential mode noise can be a source of significant challenges.
Many of the examples to follow will refer to switched mode power supplies, mostly because they are used almost everywhere, and because they always seem to be a challenge meeting emissions standards.
Assume a certain unit uses DC power which is routed to a switch mode power supply, as the switching circuit opens and closes the current path, the current in the return line will also start and stop at the same time. The switch closes and the current flows from line to return. The switch opens, and the current stops in both. However, to make the power supply efficient, the speed the switch transitions must be as fast as possible, causing the current to flow and stop abruptly. These crisp edges contribute to the creation of high frequency harmonics of the switching frequency. For a 100 kHz switching frequency, it is common to see conducted emissions above 10 MHz, and over 100 MHz is not unusual.
Figure 2 - Differential Mode Filter Concept
To control this, a local source of charge to the switcher should be supplied. The easiest method is to supply a capacitor from line to return at the input side of the switching circuit. The capacitor should be large enough to supply a significant charge, or current, to the circuit. However, some capacitor technology is not designed for high-speed transitions. Although excellent for bulk storage of charge, electrolytic capacitors have high equivalent series resistance (ESR) and are often limited in their ability to efficiently supply current above 100 kHz. Switching power supplies typically operate above this frequency. Thus, ceramic and film capacitors may be employed for the highest speed transitions. In addition, series inductors can be used, and it is best to have an inductor in both the input power line as well as the return line.
Signal lines are typically configured for differential mode operation, where a signal is sent down one wire, and the return signal is in an adjacent wire. These wires are often a twisted pair, or possibly coaxial using a shield return. Most standards do not perform conducted emissions on these lines or will perform the test around both the signal and return. This should produce low conducted emission measurement since the sum of the currents should be zero. However, there is the common mode current to consider.
OVERVIEW OF COMMON MODE CURRENTS
So how are common mode currents generated? They are caused by the parasitic aspects of the circuit’s electronics, cables, traces, and components. They may be generated by voltage caused by capacitance of components to other components or metallic struc- ture, or by magnetic field of inductive components and cables.
Referring again to the switch mode power supply. These use power transistors, such as MOSFETs which have heat sink tabs, as shown in the TO-220 drawing. These transistors handle significant power, and can generate heat. To keep temperatures low enough, they are mounted to larger heat sinks, which are typically referenced or bonded to chassis. The transistors tend to have significant induced voltages as a result of the switching of the power. Being very close to the larger heat sink can drive a displacement current to that heat sink. Remember that currents must flow in complete loops, in other words, the induced current in the heat sink must find a path back to the source. If it cannot do so locally, inside the equipment, it will find a remote path to do so.
Figure 3 - MOSFET in a TO-220 Package, Cutaway
This is the nature of common mode currents. The net current on a wire pair or bundle which is non-zero must be created in some manner, typically in parasitics such as this capacitance, and must return to that source of the energy to complete the circuit. These currents can be nearly the same on all lines, but not always. Common mode currents can result in a current on a single wire or few wires in a cable bundle.
Magnetic sources can also produce similar results. If an uncontrolled magnetic field is allowed to couple into adjacent conductive metal, chassis or otherwise, it will often generate a current there. These currents must also complete a circuit. The currents may simply circulate in the metal and not propagate outside the chassis, or it may require a return path on some wire or cable. One example is the placement of high current wires next to chassis or an unassociated circuit or wire bundle, which shall be called the victim circuit. If these source wires have significant radio frequency (RF) energy, the induced current will try to find a path back to the source wire, which creates common mode energy in that source wire, as well as victim circuit.
Not all common mode currents are generated inside the equipment. If the cables of the equipment are exposed to a radiated electromagnetic field (or electric, or magnetic), this can create a common mode current in that cable. These currents can create upsets in the equipment through induced currents and voltages in sensitive circuits. Again, these currents must find a path back, which can be by coupled energy to the chassis.
ASPECTS OF FILTERS FOR EACH MODE OF CURRENT
As seen earlier, differential mode filtering typically involves line to line capacitance, or series inductance, or both. When there is a demand of current, if the current must come from the power line, there can be significant voltage swings due to the inductance of the wiring and circuit. However, if the current can in part be supplied locally from a line-to-line capacitor, this can reduce voltage on the power line due to reactance. If additional inductance is placed on the outboard side of the capacitor, the impedance of the inductor in each leg will further encourage charge draw from the capacitor.
Figure 4 - Differential Mode Current Paths
When dealing with common mode energy, the path the currents are traveling must be considered. For example, if the MOSFET is driving a current into the heat sink and the heat sink is bonded to chassis, then the task is to locally return the current from the chassis back to the power being supplied to the transistor. In this case, line to chassis capacitors is employed. This provides the high frequency path for the current to move from chassis to the power lines.
Figure 5 - Common Mode Current Paths
The presence of the capacitor in the above two examples does not eliminate the path out of the equipment. Current will flow in the paths of least impedance, but higher impedance paths will still have some current flow, akin to a resistive divider network. The goal is to reduce the current leaving the equipment to a level which is below the required standard. In the case of common mode currents, the use of series inductors will help, but may not have an optimal amount of inductance. However, the line-to-line capacitor used in the differential mode filter will have little or no benefit in reducing common mode noise. Line-to-line capacitors will only make common mode noise more common mode if that were possible.
Further common mode reduction may be obtained using a common mode inductor, or balun. Baluns are inductor cores, typically toroidal formats, with all the wires wound in the same direction with the same number of turns. This can be seen in Figure 6, where two windings are wound around a single toroidal inductor core, shown on opposite sides of the core for clarity. The common mode current is shown in blue and is in both lines in the same direction. The differential mode current is in both lines but in opposite directions. Notice that using this method, the magnetic flux developed in the core by the differential mode currents will cancel, whereas the common mode currents will add.
Remember that common mode currents tend to be much lower than differential mode currents. As a result, a high permeability core can be used, and will not saturate as it would if used as a differential mode core. Having higher permeability means that this inductor can have very high values of inductance and still function well. However, if the same core was used as a differential mode inductor, the core would saturate, and the performance of the inductor would suffer.
Figure 6 - Common Mode Inductor
Grounding of equipment and filters may be important for safety reasons, but often have little actual benefit in the control of energy. Ground is not an open hole to pour noise into. Instead, what needs to be determined is how the currents flow from the source of noise, and back. This may be into a grounding system, and if so, the equipment and filters must be bonded to that ground to provide a low impedance return path.
MEASUREMENT AND FREQUENCY RANGE
To determine if the issue is common mode or differential mode is relatively easy when using a spectrum analyzer and a RF current probe which is designed to work across the frequency range in question. With the equipment energized, start by clamping the current probe around one wire of the cable in question, for example the input power line. Make a measurement across a frequency range in question. For this example - 1 to 10 MHz. Store the readings or note the peak values. Then add the remaining power and return or neutral lines to the current probe. If it is a two-wire input, then power and return or neutral only. Re-scan the same frequency range and look for changes. If the analyzer is capable of multiple traces, it is helpful to store the first trace or freeze it on the display, and then use a second trace for the second reading.
If you have differential mode, peak values measured will decrease when placing the additional line or lines inside the current probe. Remember that with two wires the total of the differential currents will cancel – 1 ampere going left, and 1 ampere going right equals a total of 0 amperes measured.
But for readings which are the same or have increased, these are common mode currents. If you add additional lines and each line has common mode current on it, the additional lines will add to the reading. Even if there is no increase, the line originally measured has a current on it which does not have a return current in the remaining lines being measured. The return is by some other path. The cable should still be considered to have common mode current and should be addressed with common mode solutions.
Differential mode energy is typically created through the processing of power or energy delivered to a circuit. These tend to be lower frequency phenomena, mostly in the kilohertz up to a few megahertz. On the other hand, common mode energy is created by parasitics of circuits – the capacitance of devices to metal, and the magnetic fields of current paths. Since parasitics are more prominent in the high frequency, common mode issues are also high frequency. It is typical to find conducted emissions issues made up of differential mode energy below 5-10 MHz, and common mode energy above 1-5 MHz, with the 1-10 MHz range being the transition zone. Radiated emissions, especially below 300 MHz, is almost always a common mode energy problem.
CONCLUSION
Understanding the causes of common mode and differential mode energy is very important in the process of knowing how to control that energy. Knowledge of the circuit and how the energy is being use and routed is also important. When performing testing, it is easy to understand what the significant contributor is by observing the frequency range of the energy, or by a simple conducted emissions measurement on the cable in question.