Addressing Differential and Common Mode Noise With Innovative Dual-Mode Choke Technology

Lazaro Rodriguez Standard Product Engineering Manager Triad Magnetics


Circuit designers must deal with many types of noise

CMF Series Dual Mode Chokes

Circuit designers must deal with many types of noise: internal, external, RF, line frequency and more. Noise can be a limiting factor in system performance and so must be addressed and minimized. The question is “at what cost and effort?”  Is it better to sacrifice performance or cost?  In either case, the trade-off is not very appealing in today’s competitive electronics industry.

Even the ubiquitous switched-mode power supply (SMPS) has noise issues. SMPS units are subject to differential mode (DM) noise and common mode (CM) noise, both of which must be suppressed.

Noise Mechanisms and Solutions

DM noise and CM noise have different causes and thus different solutions.  Differential mode noise is noise that is conducted on the line and neutral in opposite directions (Figure 1). Common mode noise is conducted on the line and neutral in the same direction returning through ground (Figure 2).



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Figure 1. DM  Noise




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Figure 2. CM Noise

The basic DM filter uses a single-winding choke (inductor) inserted in series with the line path, along with a capacitor from line to neutral, thus blocking noise propagation through the system. Since the DM inductor is in series with the line path, it handles the noise and DC offset current being supplied to load. Therefore, it must be designed to provide the needed inductance, but do so with low DC resistance to handle the RMS current and peak line current without saturating (Figure 3).

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Figure 3. DM Noise Suppression

The basic CM filter uses a dual-winding inductor in the line and neutral paths, plus a capacitor from line to ground (Figure 4). The current that flows in one winding (line) creates a magnetic field that is opposed by the flux created by the current flowing on the other winding (neutral); thus cancelling most of the flux in the core, preventing any saturation and allowing differential current to flow unimpeded. Conversely,



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Figure 4. CM Noise Suppression

Common Mode noise flows on both the line and neutral lines in the same direction, creating equal in phase flux in each winding which adds together. This acts as a high impedance to the common mode noise to help attenuate it. Although typically not stated, all common mode chokes have some leakage inductance.

This leakage inductance can be used to attenuate differential mode noise. The problem with this leakage inductance is that it exists as a byproduct of the mechanical separation of two coupled windings and not as a designed parameter. The CM filter choke needs only to have the required inductance along with a sufficiently low DCR for the RMS current.

Solving DM and CM Noise Issues

Through advanced technology and applications expertise, Triad Magnetics has developed a new, better solution to the problem of differential and common mode noise. As the DM and CM noise mechanisms are largely unrelated, their solutions typically require two different chokes (inductors).

It would be fortunate and much more convenient if the two noise-suppressing approaches could be implemented by one choke. Such an approach would save space and increase reliability by reducing component count, would simplify the bill of materials (BOM) and lower overall cost. Fortunately, this new solution from Triad Magnetics combines both chokes into a dual-function, open-frame design that provides the features of both chokes in a single, smaller, more cost effective package.

The CMF Series Dual Mode Chokes (Figure 5) from Triad are more than a simplistic co-packaging of two distinct devices into a single package. Instead, their design has been delicately balanced between differential and common mode noise inductance. Too much differential mode noise inductance and the core can saturate and not provide any common mode filtering benefits. On the other hand, too much common mode inductance, no differential mode inductance will be available for filtering differential mode noise.  The CMF Series Dual Mode Chokes provide both exceptional common-mode noise suppression and are highly effective in suppressing differential mode noise.



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Figure 5. DM and CM Noise Suppression

The two-in-one CMF Series Dual Mode Chokes solve both the DM and CM noise problems.  They are ideal for suppressing both types of noise in a wide range of applications that includes switch mode power supplies, LED lighting, electronics ballast and other devices (Figure 6).  They are compact in size and come in either a horizontal package (where height clearance is a concern) or a vertical package that requires minimal board space.

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Figure 6. CMF Series Dual Mode Chokes

There are 21 CMF models designed with precision wound coil bobbins in an open frame package. The current ratings range from 0.45 to 2.3A; inductances are from 10 to 100mH; differential inductances from 200 to 2100mH. DC resistances are 188 to 2930mΩ, depending on the model.

They’re available in horizontal packages (13.5h × 15 × 24.5mm to 14h × 25 × 29mm) and vertical packages (27h × 15 × 29mm) to fit tight clearance situations.  Their compact package makes them especially useful where higher density circuit boards are desirable to create smaller, more compact end products (Figure 7).



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Figure 7. CMF Installed On Circuit Board

Despite their small size, the creepage and clearance spacing is greater than 3mm, and they are rated for 300VAC operation. They’re an excellent solution for most designs, including switch mode power supplies, LED lighting, electronic ballast and more.

Designed for general purpose applications, the CMF Series Dual Mode Chokes operate over a temperature range of (-40 to 105°C).  They are also RoHS compliant to the 2011/65/EU standard, which is intended to prevent lead contamination of the environment.


When both differential and common mode noise are an issue, there is a new technology solution available from Triad Magnetics.  With over 70 years of electronics industry experience, Triad’s engineers have solved some of the industry’s most difficult filtering and power issues and the CMF Series Dual Mode Chokes are another example of this dedication to innovation.  If you’re experiencing a design dilemma that requires magnetic components, chances are that the engineers at Triad have already solved the problem for someone else.

In addition to the CMF Series, Triad Magnetics offers more than 1,000 standard part numbers to help electronics designers find the right magnetic component solution available off-the-shelf.  If not, the creative thinkers of Triad Magnetics can offer powerful custom solutions. Whether it's switchmode/high frequency, wall plug-In, power transformers, inductors or audio transformers, each product is backed up by the industry's most resourceful and organized magnetic supplier organization.

Triad Magnetics is dedicated to quality and reliability.  Producing products for long-run reliability requires a higher level of personal commitment. For example, each manufacturing team is empowered with the responsibility for quality and reliability through a highly-evolved system of critical self-inspection. The Triad Magnetics system of in-process inspection, pre-ship audits, and failure analysis has allowed many of our customers to eliminate their incoming inspection process while our continuous improvement protocol provided the highest levels of product quality and reliability.

The global manufacturing and distribution system capabilities of Triad Magnetics supports responsive customer service. Our manufacturing facilities in the U.S., China and the Philippines support just-in-time delivery capabilities, as well as our large network of distributors with thousands of part numbers ready to ship overnight.

Triad Magnetics