On a common mode choke, the core material keeps the windings coupled together. By contrast, the single choke or single winding inductors have just the one winding on the one core. This is a chart showing the difference of common mode impedance. Obviously, a common mode choke would have common mode impedances to suppress unwanted common mode noise. For a communication or signal application, it would be beneficial to have very low differential noise suppression on the common mode choke Even though we're talking about common mode chokes, every common mode choke will also have some differential mode impedance as well. It is important that the differential suppression is not at the transmitted signal frequency, so as not to distort the signal.

Transformer Construction

AA simple two-winding transformer construction consists of each winding being wound on a separate soft iron limb or core which provides the necessary magnetic circuit.

This magnetic circuit, know more commonly as the “transformer core” is designed to provide a path for the magnetic field to flow around, which is necessary for induction of the voltage between the two windings.

However, this type of transformer construction where the two windings are wound on separate limbs is not very efficient since the primary and secondary windings are well separated from each other. This results in a low magnetic coupling between the two windings as well as large amounts of magnetic flux leakage from the transformer itself. But as well as this “O” shapes construction, there are different types of “transformer construction” and designs available which are used to overcome these inefficiencies producing a smaller more compact transformer.Generally, the name associated with the construction of a transformer is dependent upon how the primary and secondary windings are wound around the central laminated steel core. The two most common and basic designs of transformer construction are the Closed-core Transformer and the Shell-core Transformer.

In the “closed-core” type (core form) transformer, the primary and secondary windings are wound outside and surround the core ring. In the “shell type” (shell form) transformer, the primary and secondary windings pass inside the steel magnetic circuit (core) which forms a shell around the windings as shown below.Amorphous Alloys are metallic glass materials without a crystalline structure. Amorphous-Alloy Cores provide better electrical conductivity, higher permeability and magnetic density, and efficient operation over a wider temperature range than cores made from conventional materials. Smaller, lighter, and more energy-efficient designs are possible for transformers, inductors, invertors, motors, and any device requiring high frequency, low loss performance.

Advantages of using amorphous cores:Common Mode vs. Differential Mode Noise

All noise received in an electrical system is induced into the system in two ways:

Conducted EMI: This type of noise is received from some other component in the system. This noise propagates into the system as a current, either through direct conduction or by capacitive or inductive coupling.

Radiated EMI: Noise is emitted and received radiatively, meaning that radiated EMI is a form of crosstalk. Some of the strategies for reducing crosstalk will also apply to radiated EMI.

Each mechanism can put either type of noise into some portion of an electronic system. Any pair of traces or wires on a PCB, IC, or cable assembly can experience two types of noise: common mode and differential mode noise. Common mode noise will have the same magnitude and polarity on each side of the interconnect, while differential mode noise has opposite polarity. Note that we haven’t considered intrinsic random noise sources like Johnson noise, which does not need an external source.

The image below shows the difference between common mode and differential mode noise. In this image, the noise voltages on each side of the interconnect are V1 and V2. These voltages are measured with respect to the reference plane below the traces, which is assumed to be 0 V everywhere.