Conducted Immunity Filtering and EMI Filters

Key Takeaways

• An overview of conducted emissions.

• The types of conducted emissions.

• An overview of conducted immunity filtering.

In the field of electromagnetic interference, there are four phenomena: conducted emissions, conducted susceptibility, radiated emissions, and radiated susceptibility. Conducted emissions are disturbances affecting the operation of electrical equipment through electrical wiring. The switching actions in the equipment are the main cause of conducted emissions. The capability of the equipment to withstand conducted emissions is called conducted immunity.

Conducted immunity filtering is a filtering-based mitigation method used to prevent conducted emissions. There are several types of EMI filters used in industrial, commercial, and residential equipment for conducted immunity filtering. In this article, we will discuss the types of conducted emissions as well as some recommended filtering techniques to mitigate their harmful effects. 

Conducted Emissions

The switching of logic gates and variations in current or voltage are common in any circuit operation. As a circuit operates, fluctuations in the current or voltage cause disturbances called conducted emissions. Conducted emissions are within the frequency range of 150kHz and 30MHz. These emissions are detrimental to a device’s performance. 

Types of Conducted Emissions

Power Line Conducted Emissions

The conducted emissions seen in power lines are called power line conducted emissions. Power line conducted emissions are additive; the conducted emissions from various devices connected to the same power line get added up and the intensity of the emissions multiplies. Power line conducted emissions limits are imposed to protect the equipment connected to these power lines. 

Signal Line Conducted Emissions

When conducted emissions interfere with signal lines, they form signal line conducted emissions.  Cables are the main carriers of this type of conducted emission, and they carry low-frequency electromagnetic interferences, which disturb signal integrity.

Devices, cables, and associated circuits must be protected from conducted emissions. This can be achieved by increasing the conducted immunity of these devices. The conducted immunity can be increased by incorporating filters in power lines and device interfacing points. 

Conducted Immunity Filtering 

The conducted immunity limits of each class of devices are prescribed by the appropriate regulatory bodies. From the product development stage, manufacturers must take the necessary measures to meet these stated limits. Designers usually rely on conducted immunity filtering methods to meet these standards; utilizing EMI filters for conducted interference mitigation. These EMI filters are designed to protect the device from conducted emissions and fast transients.

Ideally, EMI filters de-amplify the amplitude of frequency signals that are greater than the cut-off frequency. By designing the filter with consideration to the frequency of interest in the system, the EMI filters become more effective. 

Power Line Conducted Immunity Filtering 

Power line conducted emissions are limited using EMI power line filters or mains EMI filters, which are low pass filters. Low pass EMI filters allow the flow of power frequencies and block the other higher frequency components from reaching the equipment. Power line EMI filters are placed at the point of common coupling where the devices are connected to the power lines. They prevent noise from entering and exiting the device toward the power lines. 

Conducted Immunity Filtering Components

Passive EMI filters used in conducted immunity filtering consist of two components: inductors and capacitors. There are several types of passive EMI filters available for conducted immunity filtering. Depending on the number of passive components, EMI filters are classified into first-order filters, second-order filters, third-order filters, and higher-order filters.

The simplest EMI filter consists of only one passive component, either an inductor or capacitor.   Inductors allow only low-frequency components through, whereas capacitors allow high-frequency components. Single-component filters are not preferable due to their low attenuation.

Second-Order EMI Filters or LC Filters

LC filters are the most popular second-order EMI filter used. They consist of an inductor and capacitor arranged in a series-parallel fashion. The inductor allows the low frequency or fundamental frequency components, whereas the shunt-connected capacitor bypasses high-frequency components. The combined filtering effect of inductors and capacitors allows a reduction of conducted emissions, affecting the equipment connected downstream. Thus, we can conclude, LC EMI filters increase the conducted immunity and are a vital part of imbibing conducted immunity filtering.

Conducted immunity filtering using EMI filters is critical to limiting conducted emissions and increasing the conducted immunity of a device. It is important for designers to note that achieving conducted immunity using filters is highly dependent on the filter topology design, cut-off frequency, source impedance, load impedance, etc. Cadence’s suite of design and analysis tools can help you in designing EMI filters to reduce conducted emissions.

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