SLVUCM2A january 2023 – july 2023 TPSF12C3 , TPSF12C3-Q1
CM filters for both commercial (Class A) and residential (Class B) environments typically have limited Y-capacitance due to touch-current safety requirements and thus require large-sized CM chokes to achieve the requisite attenuation. This ultimately results in filter designs with bulky, heavy and expensive passive components. The deployment of active filter circuits enable more compact filters for next-generation power conversion systems.
Figure 3-3 presents typical schematics of equivalent three-phase passive and active filter designs. Terminals designated L1, L2, L3, N and PE refer to the Live inputs, Neutral and Protective Earth, respectively. Comparing the passive and active circuits in Figure 3-3, the CM inductance of chokes LCM1 and LCM2 each reduces by a factor of four to six times by virtue of the higher effective Y-capacitance with the TPSF12C3 circuit.
The AEF circuit uses a capacitive multiplier circuit in place of the set of Y-capacitors normally placed between the CM chokes in a conventional two-stage passive filter design – see Figure 3-3. The TPSF12C3 senses the high-frequency CM disturbance on the two power lines using four Y-rated sense capacitors and injects a noise-canceling current back into the power lines using a Y-rated inject capacitor.
The advantages of AEF with the TPSF12C3 summarize as:
The X-capacitor placed between the two CM chokes provides 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 (neutral in Figure 3-3) using only one inject capacitor.
Figure 2-2 shows the applicable schematics for three-wire passive and active filters when a neutral connection is not present. The active circuit is similar to that of Figure 3-3, except the TPSF12C3 has one of its SENSE pins tied to ground (SENSE4 in this example), and the inject capacitor connects to the floating star-point connection of X-capacitors CX4, CX5 and CX6.