Figure 9-3 and Figure 8-2 show schematics for conventional two-stage passive EMI filters with and without neutral, respectively, in kilowatt-scale, grid-connected applications. L, N and PE refer to the respective live, neutral and protective earth connections. Multistage filters as shown provide high roll-off and are widely used in high-power AC line applications where CM noise is often more challenging to mitigate than DM noise. The low-order switching harmonics usually dictate the size of the reactive filter components based on the required corner frequency (or multiple corner frequencies in multistage designs).

Also included in Figure 9-3 and Figure 8-2 are the corresponding active filter designs. The active circuit replaces the bank of Y-capacitors positioned between the CM chokes with a three-phase AEF circuit using the TPSF12C3 to provide a lower impedance shunt path for CM currents. The sense pins of the TPSF12C3 interface with the power lines using a set of Y-rated sense capacitors, typically 680 pF, and feed into an internal high-pass filter and signal combiner. The IC rejects both line-frequency (50-Hz or 60-Hz) AC voltage as well as DM disturbances, while amplifying high-frequency CM disturbances and maintaining closed-loop stability using an external tunable damping circuit.

Figure 8-1 Circuit Schematic of a Three-Phase, Four-Wire Passive Filter and Corresponding Active Filter Solution for CM Attenuation

Figure 8-2 Circuit Schematic of a Three-Phase, Three-Wire Passive Filter and Corresponding Active Filter Solution for CM Attenuation

The X-capacitors placed between the two CM chokes effectively provide a low-impedance path between the power lines from a CM standpoint, typically up to low-MHz frequencies. This allows current injection onto one power line, typically neutral, using only one inject capacitor. If the three-phase filter is a three-wire system without neutral as shown in Figure 8-2, the SENSE4 pin of the TPSF12C3 ties to ground and the inject capacitor couples through a star-point connection of the X-capacitors.