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Using EMI Filter Beads for Noise Suppression

Published Date
Author Cadence PCB Solutions

Key Takeaways

  • EMI poses a challenge to the reliable operation of electronic circuits.  

  • Ferrite beads are a type of ferrite inductor with a low inductance value and minimal parasitic capacitance. When used for EMI filtering, they are called EMI ferrite beads. 

  • The fundamental working principle of an EMI filter ferrite bead is that it behaves like a resistor at high frequency, and the power loss in the ferrite beads prevents EMI from reaching other sections of the circuit.  

 EMI filter beads

EMI filter beads are commonly used in passive EMI filters

Electromagnetic interference (EMI) is one of the main causes of maloperation in electrical devices and electronic circuits. EMI can originate from natural sources or can be manmade. Solar flares, electrical storms, and lightning are some examples of natural EMI sources. Non-linear electronic circuits such as computers, microwaves, heaters, and lamps generate residential EMI. Power supplies also play a significant role in generating EMI in electronic circuits. Switching converters such as rectifiers, AC-DC converters, or DC-DC converters on the power supply side of electronic devices transmit EMI and radio frequency interference (RFI) to the remaining circuit sections.

It is necessary to limit EMI to minimize the failure of electrical and electronic systems. To accomplish this, EMI filtering can be used to minimize the effects of EMI. Passive EMI filtering consisting of capacitors and inductors is effective at filtering EMI. EMI filter beads are commonly used in passive EMI filters and help attenuate interference. 

Switching Power Supplies and EMI Generation

Most electronic devices work on DC power, and DC power is converted with a combination of a rectifier and a DC-DC converter. This works by plugging the electronic device into the grid power, which is alternating in nature. The AC power is rectified, and the rectified DC power is converted into the required voltage levels for the electronic device using DC-DC converters.

On the power supply side of electronic devices, there are switching systems and rapid current variations. The current variation and switching devices often cause electromagnetic interference. This EMI interferes with other circuit sections on the same board and can cause issues in the device’s operation.

EMI appears as a current disturbance to the circuits downstream of the power supply section. The EMI can be either conductive or radiative. The EMI generated from the power supplies of electronic circuits can also travel upstream and pollute grid power as well. In such cases, EMI travels to other devices connected to the same grid, interfering with their functioning.

If not addressed properly, the EMI generated from the power supply of an electronic circuit can cause detrimental effects on a much larger scale. Employing EMI filters to prevent EMI from entering adjacent circuits or other devices connected to the same grid power is recommended. To achieve EMI compliance from regulatory bodies like the FCC or IEC, EMI filters are a necessary part of the power supply design of electronic circuits. 

EMI Filters

EMI filters are employed to protect the circuits downstream of the power supply from the effects of EMI. EMI filters connected between the power supply circuit and the device divert EMI from reaching the adjacent circuits by filtering out unwanted noises in the power supply voltage and supplying the filtered voltage to the device or the downstream circuits. Unwanted currents are extracted by the EMI filter as well. These filters are equally effective at mitigating common-mode noise and differential-mode noise.

EMI filters are constructed using passive elements–capacitors and inductors. Depending on how the capacitors and inductors are connected, they are classified as:

  1. Series - The inductor and capacitor combination is in series with the power supply.
  2. Parallel - The filter is connected parallel to the power supply.
  3. Hybrid - The filter components make use of series and parallel connections.

Using EMI Filter Beads as LC Filter Inductors

The EMI from switching power supplies can be common-mode or differential-mode. The passive EMI filters, otherwise called LC filters, suppress the EMI generated from switching power supplies. In EMI filter design, the core of materials used in the inductor design matters. Ferrite beads are commonly used along with capacitors to suppress the noises generated from switching power supplies. Ferrite beads are the type of ferrite inductors with a low inductance value and a minimum parasitic capacitance.

Ferrite beads can be featured as lossy inductors with low-quality factors and high self-resonant frequency, and are free from unwanted resonances. EMI ferrite beads attenuate and dissipate EMI from the switching power supply circuits in the form of heat energy. The fundamental working principle of an EMI filter ferrite bead is that it behaves like a resistor at high frequency, and the power loss in the ferrite beads removes EMI noises from reaching other sections of the circuit. 

Compared to traditional inductors, EMI filter beads offer a higher noise-suppression effect, as they absorb EMI energy and dissipate heat in turn. To suppress EMI from switching power supply sources in PCBs, an LC filter with EMI filter beads and capacitors works best. For specific filtering requirements in a given application, you may find multiple design options. Cadence software offers a full suite of design tools that can help you develop, simulate, and analyze your electronic circuit with different filter options.

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