Power line noise is a common source of frustration since it degrades device performance, reduces dependability, and may even cause unexpected system behavior. One must ask themselves two questions while building an electrical gadget.

· How will my gadget react if there is an abnormally high amount of noise on the power source it is plugged into?

· Is it possible for my gadget to generate noise that might interfere with other gadgets using the same power source?

There are two main categories of noise: common-mode and differential-mode. Common-mode noise refers to a particular kind of noise that occurs on two conductors and has the same polarity, frequency content, and amplitude. This noise may be muffled with the use of a common mode choke.

How do you define common-mode noise?

Large discharge currents coupled onto lengthy cables, the existence of undesirable radio frequency emissions, or switching devices like inverters and motors linked to the power supply are the usual culprits for common-mode noise on power supplies entering a device. Common-mode noise may also be generated inside the device itself by use of switching components and unshielded circuits. Switched-mode power supplies have a serious challenge from common-mode noise introduced through the power lines. Protecting equipment that draws electricity from these sources and fulfilling regulatory standards both need careful management of these emissions.

Observing a noise signal on two wires provides a visual representation of common-mode noise. Common-mode noise has a few consistent features regardless of its origin:

· The noise on both conductors will be in phase with one another, and the noise will have the same polarity when measured in the time domain.

· When measured by a receiver, the level of noise on each conductor is found to be the same.

In practice, noise is seldom only common-mode, and in most cases, common-mode noise will coexist with differential-mode noise. From an electromagnetic interference (EMI) and electromagnetic compatibility (EMC) standpoint, common-mode noise is of more concern due to its potential "loudness" (intensity). As a result, it will contribute more to conductive and radiated EMI issues. Since common-mode currents are the origin of radiated common-mode noise, filtering or otherwise suppressing common-mode noise is necessary to guarantee minimal noise is emitted and propagated in a PCB design.

Choosing a Common Mode Choke to Suppress Noise

Electromagnetic interference and switching transients are problematic because they damage both incoming and outgoing power lines equally and are unaffected by differential shielding or filtering. The front-end low-pass filter may be implemented using discrete inductors and capacitors rather of a common-mode choke. While this may be a cheap solution on its own, main transformer flux bands or shields may be needed to prevent parasitic capacitance from causing common-mode currents inside the transformer. This may increase the price, increase the complexity, and reduce the dependability.

A common-mode choke may be used instead; it filters out the unwanted DC component of a power line while attenuating the high-frequency noise that is shared by several power lines. To eliminate the conducted switching and RF noise generated by switched-mode power supply is the primary use for common-mode chokes. A common-mode choke is able to cancel out the energy from common-mode noise on a power supply by using voltage fluctuations to generate opposite magnetic fields inside a core, which then radiate out as heat.  

Choosing the Appropriate Choke

A choke is a magnetic inductor that filters out or dampens high-frequency noise without interfering with direct current. There are three things to keep in mind before deciding on a choke.

· Is there a minimum level of noise suppression that must be met? The impedance is set by this factor.

· Find out what the lowest frequency is that the choke can block out. The frequency range is established by this factor.

· Just how much electricity will the choke have to withstand? This will restrict the kinds of systems where a choke may be employed and the locations inside those systems.

Generally, of thumb, the greater the physical size of the choke, the lower the frequency that it must filter. Choke parts may be available in either surface-mount packages or more conventional through-hole wire coils and encapsulated components, all of which have their advantages and disadvantages depending on the choke's primary properties. With isolation of up to 1500 V and current ratings of up to 15 A, surface mount versions are available and are well suited for easy board building by eliminating AC line-conducted common-mode noise across a wide frequency range. Because of this, switch-mode power supply can make excellent use of them.

The two most common kinds of common mode chokes are RF (radio frequency) and alternating-current (AF) (audio frequency). One key distinction between the two is the choke's inner core's construction material. The core of an RF choke is either powdered iron or ferrous beads, whereas that of an AF choke is solid magnetic iron. Both the saturation flux density and the current rating of solid ferrite cores tend to be greater than those of powdered iron cores. Which one you need depends on the lower frequency range of the noise; however, AF chokes are often recommended for DC power supply.

To acquire the greatest possible choking impedance and hence the most suppression of common-mode noise, it is recommended to pick the largest choke that will physically fit the available board space even if you are unclear of the actual filtering qualities you require.

Because of their optimal magnetic coupling between windings and low leakage inductance, ferrite toroid cores are the most efficient core form. A higher price tag is to be expected due to the plastic mounting base, the large number of windings, and the core material. Moreover, the isolation between windings limits their maximum voltage rating to less than 1500 V. For applications with voltages up to 3000 V, ferrite cores with an 'E' or 'U' shape provide superior isolation between windings. If a choke can become too hot, it will exceed the parameters specified in the data sheet. Keep heat management in mind if the choke will be subjected to high common-mode noise levels.

Choke Limitations

Saturation effects in the ferrite core determine the level of performance achieved by any given ferrite choke. High-voltage, high-energy common-mode noise or surges may cause a chock's core to saturate, allowing the surge voltage to bypass the device. How well the choke handles a surge is determined by the maximum current it can safely handle and the form of the surge. As well as the common mode chokes, additional surge protection should be built if the item in issue will be subjected to such spikes.