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

8 The Concept of Capacitive Amplification

An AEF circuit for CM noise mitigation either amplifies the apparent inductance of a CM choke or the apparent capacitance of a Y-capacitor over the frequency range of interest. A VSCI AEF configured for CM attenuation uses an amplifier stage as a capacitive multiplier of the injection capacitor, CINJ. It is this higher value of active capacitance that supports lower values for the CM chokes to achieve a target attenuation.

Looking at Figure 7-1, Equation 1 shows that the injection capacitance is effectively multiplied by GAEF, the CM voltage gain from the power lines to the amplifier output:

Equation 1. v C I N J = 1 - G AEF ( f ) v AEF
i C I N J = C INJ d v C I N J d t = 1 - G AEF ( f )   C INJ d v AEF d t
⇒ C INJ,active ( f ) = 1 - G AEF ( f )   C INJ

Figure 8-1 shows a simulated plot of an injection network impedance when a FB-VSCI AEF circuit is enabled and disabled. The lower impedance above 2 kHz (and especially above 100 kHz) is caused by the capacitive amplification by the active circuit of a 4.7-nF injection capacitor and its associated damping network.

GUID-20230223-SS0I-G2QQ-NNR5-ZFGFNKMTMMMD-low.png Figure 8-1 Example of Inject Branch Impedance ZINJ With AEF Enabled vs. a Conventional Y-Capacitor, Showing the Boosted Equivalent Capacitance at Higher Frequencies by Active Feedback Action