Basics of Noise Countermeasures
Lesson 14: Using Common Mode Choke Coils for Power Supply Lines

10/28/2014
Application

Category: Noise suppression filter Room
This article provides information on the use of common mode choke coils for power supply lines.

1. Care must be taken for magnetic saturation according to the type of common mode choke coil.
An earlier article explained that the magnetic flux produced by the differential mode current in the coils of a common mode choke coil cancel each other out, so there is no need for concern over magnetic saturation. However, care should be taken as some common mode choke coils are designed to also enable some reduction of differential mode noise. Fig. 1 describes the structure of this type of common mode choke coil.

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Normally, common mode choke coils are designed so that the magnetic flux produced in the two coils by the differential mode current cancel each other out as much as possible. However, some common mode choke coils are designed as shown in the figure above so that not all of the magnetic flux cancels each other out, and some magnetic flux leaks.
  
This means that some magnetic flux remains, resulting in differential mode impedance. Common mode choke coils designed in this manner effectively reduce both common mode noise and differential mode noise. In addition, high-speed signals are not passed through power supply lines, so there is no need to take into account the effect on signals, enabling effective removal of power supply line noise.

However, differential mode magnetic flux remains in this type of common mode choke coil, so increasing the differential mode current also increases the magnetic flux leakage and can easily result in magnetic saturation. Large differential mode currents flow in power supply lines, so the effect of magnetic saturation increases.
Therefore, this type of common mode choke coil should be used within the range that does not result in a drop in performance due to magnetic saturation.

In addition, when the use of a common mode choke coil does not noticeably reduce power supply line noise, it may be the case that components with large differential mode impedance are used, resulting in magnetic saturation, so this point should be checked. Note that large differential mode impedance is not necessarily bad, and that as long as components with large differential mode impedance are used within the range where magnetic saturation does not occur, they can be very effective, so components should be chosen wisely. Figure 2 shows an example of the characteristics of a common mode choke coil with large differential mode impedance. (Note: A good characteristic diagram showing the characteristics of a product for power supply lines could not be found, so this characteristic diagram shows the characteristics of a product for signal lines.)

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2. Common mode choke coils for AC power supply lines
The common mode choke coils described thus far are used in relatively low-voltage locations. However, many common mode choke coils are actually used in circuits connected to AC power supply lines (commercial power supply lines), such as the primary side of switching power supplies. Common mode choke coils used in these locations are often referred to as “line filters.”
These common mode choke coils are used together with across-the-line capacitors (X-capacitors) and line bypass capacitors (Y-capacitors), and are used to prevent noise generated by the secondary side circuit from leaking to the primary side, and to prevent noise generated by the power supply circuit from escaping via the power cord. 

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Across-the-line capacitors remove differential mode noise, and line bypass capacitors and common mode choke coils remove common mode noise. Increasing the capacitance of a line bypass capacitor enables removal of lower frequency common mode noise, but the risk of current leaking to the ground increases together with the capacitance, so the capacitance must be kept to a certain value or less. For this reason, common mode choke coils are mainly used for the low-frequency range that cannot be covered by line bypass capacitors. In addition, the use of normal mode choke coils may also provide enhanced differential mode noise removal effects in some cases.

Common mode choke coils used in AC power supply lines are used in high-voltage locations, so they must support high voltages. Insulation performance and other characteristics are specified by the set safety standards, so common mode choke coils designed for use in AC power supply lines are designed to satisfy these standards. Products with a rated voltage of AC 250 V satisfy these standards for the most part. However, some sets may have special requirements, so check with the manufacturer in these cases.


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Written by: Yasuhiro Mitsuya, Component Business Unit, Murata Manufacturing Co., Ltd.