Figure 9-2 shows a schematic of a conventional two-stage passive EMI filter 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).

Figure 8-1 Circuit Schematic of a Single-Phase Passive Filter and Corresponding Active Filter Solution for CM Attenuation

Also included in Figure 9-2 is the corresponding active filter design. The active circuit replaces the two Y-capacitors positioned between the CM chokes with a single-phase AEF circuit using the TPSF12C1 to provide a lower impedance shunt path for CM currents. The sense pins of the TPSF12C1 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.

The X-capacitor placed between the two CM chokes in Figure 9-2 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 (usually neutral or the power line routed closest to the IC) using only one inject capacitor.