SLVAFJ9 March   2023 TPSF12C1 , TPSF12C1-Q1 , TPSF12C3 , TPSF12C3-Q1

 

  1.   Abstract
  2. 1 Table of Contents
  3.   Trademarks
  4. 2 Introduction
  5. 3 EMI Frequency Ranges
  6. 4 Passive EMI Filters for High-Power, Grid-Tied Applications
  7. 5 Active EMI Filters
  8. 6 Generalized AEF Circuits
  9. 7 Selection of the CM Active Filter Circuit
  10. 8 The Concept of Capacitive Amplification
  11. 9 Practical AEF Implementations
  12. 10Practical Results
    1. 10.1 Low-Voltage Testing
    2. 10.2 High-Voltage Testing
  13. 11Summary
  14. 12References

5 Active EMI Filters

There have been numerous publications [4-7] detailing the application of AEF, with results yielding a substantial reduction in filter size and volume relative to a conventional passive-only design. Much like a passive EMI filter, the AEF circuit connects to the lines between the EMI source and the EMI victim, as shown in Figure 5-1. Unlike a passive filter, however, an AEF circuit uses active devices and control to sense the residual (DM or CM) voltage or current disturbance, and injects an opposing signal that directly negates that noise disturbance. Based on the superposition theorem of signals with equal amplitude and opposite phase, the injected voltage or current can theoretically cancel or nullify the incident noise voltage or current contribution from the EMI source – essentially a destructive interference. This strategy is commonly applied in acoustics and successively for EMI.

Figure 5-1 Fundamental Concept of AEF With Sensing, Gain and Injection Stages. The Control Structure can be FB (a) or FF (b)

The expectation is that AEF significantly reduces EMI, resulting in a smaller-size filter versus a traditional passive-only design with equivalent attenuation. Along with AEF, other (smaller) passive components interface to the power stage and improve the overall attenuation – these circuits are known as hybrid EMI filters (HEFs). The design and implementation of AEF and HEF circuits depend on the path of conduction (DM or CM) and the required sensing, gain and injection stages. As shown in Figure 5-1, the cancellation signal is directly generated from a measured signal by a feedback (FB) or feedforward (FF) approach.