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Passive EMI Power Filter Classifications

Author Cadence System Analysis

Key Takeaways

  • EMI power filters are introduced between the point of common coupling (PCC) and a device to mitigate power line EMI.

  • EMI power filters are classified into different types based on parameters such as topology, number of phases, and connection. 

  • Depending on the system requirements, an appropriate EMI power filter type is chosen.

 Power line

Electromagnetic interference in power lines is a pervasive problem in electrical and electronic devices

Electromagnetic interference in power lines is a pervasive problem in any electrical or electronic device. If you consider the power supply part of electrical and electronic devices, it will be either standalone, supported by battery, or plugged into the supply mains. In all cases, there will be EMI issues in the power lines. In utility-connected devices, a rectifier and DC-DC converter combination is used for transforming the AC power mains into the DC input requirements of a device. Eliminating noises in the power lines of equipment is an important part of any product design. The electromagnetic interferences in the power line need to be under control so they do not interrupt the operation of the equipment. EMI power filters are one way of doing this; they are introduced in the product development stage to mitigate electromagnetic disturbances. 

EMI in Power Supplies

The electromagnetic noises from a power supply interfere with almost all electrical and electronic devices connected to the power supply. Electromagnetic interference from the supply side appears in the form of current disturbances to the devices downstream. The noises in the electrical current affect the reliability and integrity of the device. 

EMI is bidirectional; it flows to the device from the power supply side and enters the utility side or grid-side from the device. In both cases, the EMI affects the normal functioning of the connected devices.

Emissions that flow via wires are called conducted EMI. The input currents that are drawn by the device from the power supply typically contain conducted EMI issues. The other problematic type of EMI is in the form of radiation from the power supply, which can be destructive to the device itself as well as to the workings of nearby equipment.

Electromagnetic compliance is necessary in cases where the power supply turns out to be an EMI source. 

Using EMI Power Filters to Ensure Electromagnetic Compliance

The electromagnetic compliance of a device is essential when it is plugged into supplies that inject EMI to the power line. EMC-compliant devices operate normally under electromagnetic environments and don't interrupt the operation of nearby devices.

There are various techniques and mitigation methods employed in the supply side of electrical and electronic devices to protect them from EMI and to make them electromagnetically compliant. One such method is to introduce EMI power filters between the point of common coupling (PCC) and the device. 

What Are EMI Power Filters?

EMI power filters are electromagnetic interference mitigating components used to reduce the disturbances in a power line. They are commonly made of inductors and capacitors and are collectively called passive EMI power filters.

EMI power filters are generally classified into different types based on three parameters: topology, number of phases, and connection.

Topology

EMI power filter classifications based on topology are broadly divided into:

  • Shunt
  • Series
  • Hybrid

Each of these three topologies can be further classified into tuned and damped filters. 

Number of Phases of the AC Input Power Supply

The power supply given to a device can be a two-wire or single-phase, three-phase three-wire, or three-phase four-wire connection. The main classification of EMI power filters is based on the number of phases, which is again subdivided into three types: shunt, series, and hybrid.

Connection

We have seen the different types of connections of passive EMI power filters—shunt, series, and hybrid. In shunt-connected EMI power filters, the filter is connected parallel to the EMI source, that is the power supply. In series-connected EMI power filters, the inductor and capacitor combination is in series with the power supply and precedes the device of interest. Hybrid passive EMI power filters make use of shunt and series connections. 

Depending on the system requirements, an appropriate EMI power filter type is chosen. EMI power filters should be designed considering the whole system and the EMC compliance required for the load to be connected. Nowadays, off-the-shelf filters are also available. Factors such as rated voltage, rated current, isolation voltage, operating temperature, and leakage current play a vital role in determining the best EMI power filter. Cadence’s suite of design and analysis tools can help you design and validate EMI power filters for your electrical and electronics products. 

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