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Improve Power Quality With EMI Filters for Power Supplies

Key Takeaways

  • EMI filters for power supplies are generally called EMI power line filters.

  • EMI power line filters protect equipment from the harmful effects of electromagnetic interference. 

  • Third-order EMI filters are the most popular EMI filter topology for AC power supplies.

  EMI filters for power supplies

EMI filters for power supplies are made up of inductors and capacitors

One of the inevitable parts of any electrical or electronic equipment is its power supply. Irrespective of the type of power supplied (either AC or DC), the quality of the power matters when considering the performance of the load connected to it. Power quality is an especially critical parameter to look for when supplying sensitive loads. 

In modern electronics, electromagnetic interference from power supplies is a major threat to power quality. Conducted and radiated emissions from power supplies are strong enough to damage sensitive loads. EMI filters for power supplies are a viable solution that can help prevent electromagnetic noise from damaging sensitive equipment. These filters also protect the power supplies from switching transients and high-frequency electromagnetic spikes.

Let’s take a look at the different types of EMI filters and how they can protect your power supply.

EMI Filters for Power Supplies

Almost all power supplies produce electromagnetic interference. According to regulatory bodies, like the FCC or IEC, conducted and radiated emissions from power supplies need to be within their set limits. To adhere to the regulatory limits and protect sensitive equipment, EMI filters are employed. 

EMI Power Line Filters

EMI filters for power supplies are generally called EMI power line filters. They protect the equipment placed downstream of the filter from the harmful effects of EMI. EMI power line filters are connected between the power supply and the equipment. They accept the unfiltered input signal from the power supply and deliver a filtered voltage supply to the equipment. These filters only allow desirable currents into the sensitive equipment by extracting any unwanted currents. EMI power line filters also reduce common-mode and differential mode noises from the power supply and suppress electromagnetic noises from reaching the load.

The reverse conduction of electromagnetic noises—from the load side to the utility side—is prevented by incorporating EMI filters for power supplies. For example, in the absence of EMI filters, switching transients affect the input power supply. In the case of DC-AC inverters, DC input in the oscilloscope is full of switching transients due to the high-frequency switching of inverter switches. The continuous occurrence of these transients above the tolerance limit can lead to problems in the DC power supply. By incorporating EMI filters, both the power supply and the load equipment are protected from electromagnetic noises.

When supplying AC power, extra care is required, as the EMI generated in the load can flow towards the utility mains and pollute the power grid. That is why EMI filters are so critical, especially for AC power supplies.

EMI Filters for AC Power Supplies

Compared to the fundamental frequency of AC power supplies, electromagnetic noises are high-frequency signals. To suppress high-frequency noises from entering a sensitive load,  low-pass filters are used as EMI filters for AC power supplies.

Low-pass EMI filters for power supplies can be either active or passive filters, and they divert noise from the sensitive load to the ground. Active filters are systems that analyze the noise present in the waveform of interest and inject the anti-noise generated to the voltage or current waveform so that noise gets canceled and noise-free outputs are achieved. In passive EMI filters, inductors and capacitors are arranged in different fashions according to filter requirements. The internal configuration of passive EMI filters determines up to what frequency they can suppress high-frequency noise.

According to the internal configuration of the inductor-capacitor arrangement, EMI filters for AC power supplies are classified into the following types:

  1. First-order filters - Placing a single reactive element between the power supply and the load forms a first-order filter. To make this filter a first-order, low-pass filter, there needs to be a single capacitor placed in parallel or an inductor in series between the power supply and the load. If the first-order filter is capacitive, it extracts the noise currents from reaching the load, whereas an inductive filter allows only the low-frequency signals to pass through it. To achieve a high attenuation rate, second-order EMI filters are chosen. 
  2. Second-order filters - Second-order EMI filters consist of two reactive components (a capacitor and inductor) and are called L-filters due to their L-shaped arrangement. The impedance of the power supply, load, and highest frequency to be suppressed determine the values of the inductor and capacitor in the L-filter. 
  3. Third and higher-order filters - A third-order EMI filter is the most popular EMI filter topology for AC power supplies. It constitutes three or more reactive elements and, according to the arrangement, is referred to as either a  ‘pi’ filter or a ‘T’ filter. The physical size of an EMI filter increases with the increase in its order. Third-order filters’ footprints are greater than first and second-order EMI filters. 

The amount of electrical and electronic products connected to utility mains is increasing, making EMI filters for power supplies required for quality systems designs. The inclusion of EMI filters is beneficial in limiting the EMI from degrading power supplies and protecting the utility mains from noise pollution. Cadence’s suite of design and analysis tools can help you in configuring the best EMI filter for the electrical or electronic system you are designing.

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