Figure 1: Active EMI filter for single-phase system (FB-VSCI topology)

An active EMI filter is a type of EMI filter that uses active devices such as op-amps in conjunction with passive components to attenuate the high-frequency noise currents (called conducted EMI) present on the power supply lines and signal lines, thereby ensuring protection and reliable operation of electrical and electronic devices. 

Conducted electromagnetic interference (conducted EMI) refers to the high-frequency noise signals generated during normal operation of electrical and electronic devices (such as power electronic converters, motor drives, and other electronic circuits) and travel through power and signal lines and will interfere with the operation of other connected equipment on the same lines. Furthermore, when the high-frequency noise currents flow through the power lines, it will transform the power line into an unintended antenna, emitting radiated EMI signals that may interfere with nearby electronic devices or AM radio signals.  

The active EMI filter can suppress common mode (CM) and differential mode (DM) noise voltage or current (i.e., conducted EMI) on power supply lines, thereby preventing conducted disturbance from entering or leaking out from a filtered device, keeping the device working smoothly without disturbing other connected and nearby devices. This helps to improve the Electromagnetic compatibility (EMC) of the device. The active EMI filter attenuates conducted noise over the frequency range of 150 – 30MHz required by conducted emissions standards such as EN55022 (CISPR22). 

A key benefit of active EMI filters is their ability to reduce the choke size needed in common mode EMI filter design compared to passive EMI filters. This reduction helps to decrease the size, weight, and cost of EMI filter implementation. Active EMI filters are used for EMI filtering in both DC and AC (single-phase and 3-phase) applications, including HVAC motor control, welders, inverters, telecom AC/DC rectifiers, high-density server rack power supplies (PSUs), automotive applications, aerospace, defense, and other industrial systems.

Construction and working principle of active EMI filter:

An active EMI filter circuit consists of a sensing circuit, an amplifying circuit (gain stage), and an injection circuit. The filter first senses high-frequency noise signals (voltage or current) present on the supply lines. Then, it inverts and amplifies this noise signal to create an anti-phase noise-cancellation signal (voltage or current), which is finally injected back into the system to suppress or cancel out the conducted EMI on the lines. The active EMI filter circuit can use a feedback (FB) or feedforward (FF) approach (Figure 2). 

 In the feedback (FB) approach, the filter senses the noise signal at the EMI victim's end and injects a noise cancellation signal at the EMI source side. In the feedforward (FF) approach, the filter senses the noise signal at the EMI source end and injects a noise cancellation signal at the EMI victim's end.  

Figure 2: Feedback and Feedforward control approach shown

Also, based on the FB or FF control approach, and the kind of noise sensing signal (voltage or current) and noise cancellation signal (voltage or current), the active EMI filter can use any one of the following topologies.  Depending upon the application requirements, a suitable topology is used. 

• FB-CSVI - Feedback-Current sensing and Voltage injection
• FB-CSCI - Feedback-Current sensing and Current injection
• FB-VSVI - Feedback-Voltage sensing and Voltage injection
• FB-VSCI - Feedback-Voltage sensing and Current injection
• FF-VSVI - Feedforward-Voltage sensing and Voltage injection
• FF-CSCI - Feedforward-Current sensing and Current injection

Note: Voltage can be sensed with sensing capacitors, while the currents can be sensed using a current transformer. 

Note: Voltage can be injected with a series transformer, while the currents can be injected using injection capacitors. 

Figure 1 shows the FB-VSCI topology active EMI filter for a single-phase system. This circuit uses an active EMI filter IC with sensing capacitors (for voltage sensing) and an injection capacitor (for current injection) to suppress conducted EMI on the power lines.  The IC features a signal combiner, filter circuit, op-amp circuit, compensation, and protection (like UVLO and thermal shutdown protection) circuitry.