Isolated type active EMI filter having no additional elements on power line
The present disclosure relates to an electromagnetic interference (EMI) filter for preventing noise emitted from a power line cable, and an isolated type active EMI filter having no additional elements on a power line. The EMI filter includes a common-mode (CM) choke disposed on a power source side, a Y-cap disposed on an EMI source side, and a sensing winding configured to sense a current. The EMI filter also includes an amplifier configured to amplify the noise current, and a transformer configured to inject a signal of the secondary coil into the Y-cap as a compensation signal.
1 . An isolated type active electromagnetic interference (EMI) filter having no additional elements on a power line, the filter comprising:
a common-mode (CM) choke having a form in which a winding is wound on each of a live line and a neutral line connected to an EMI source;
a Y-cap including two capacitors connected in series, wherein the two capacitors are connected between the live line and the neutral line and are commonly connected to the ground;
a sensing winding wound on the CM choke and configured to sense a noise current of the CM choke;
an amplifier configured to amplify the noise current sensed by the sensing winding; and
a transformer installed in front of the Y-cap, the transformer comprising a primary coil configured to receive a signal amplified by the amplifier and a secondary coil isolated from the power line by being connected to the ground connected to the Y-cap, the transformer configured to inject a signal of the secondary coil into the Y-cap as a compensation signal,
wherein, when a capacitance of a parasitic circuit of the CM choke is referred to as C cm and a capacitance of a parasitic circuit of the sensing winding is referred to as C sen , the number of turns (N sen ) of the sensing winding is less than a square root of 2×C cm /C sen.
2 . The filter of claim 1 , further comprising a low-pass filter located at an input terminal of the amplifier.
3 . The filter of claim 2 , wherein the low-pass filter comprises:
a filter resistor (R f ) comprising a first end connected to the sensing winding and a second end connected to the amplifier; and
a filter capacitor (C f ) comprising a first end connected to the second end of the filter resistor (R f ) and the amplifier, and a second end connected to the ground.
4 . The filter of claim 2 , wherein a first impedance (Z in, AEF ) viewed from the input terminal of the amplifier toward the low-pass filter is set greater than a second impedance (Z sen, para ) of parasitic RC components of the sensing winding.
5 . The filter of claim 2 , wherein a cutoff frequency of the low-pass filter is greater than a maximum operating frequency (f op,max ) determined by an inductance part of the transformer and a capacitance (C d ) of the bypass branch.
6 . The filter of claim 5 , wherein the cutoff frequency of the low-pass filter is less than a resonance frequency determined by a coupling coefficient (K sen ) of the sensing winding, the number of turns (N sen ) of the sensing winding, an inductance (L cm ) of the CM choke, and the capacitance (C sen ) of the parasitic circuit of the sensing winding.
7 . The filter of claim 6 , wherein an effective capacitance of the Y-cap is configured to be determined at least by the number of turns (N sen ) of the sensing winding, a turns ratio (N in ) of the primary coil and the secondary coil of the transformer, and a gain of the amplifier.
8 . The filter of claim 1 , wherein the CM choke is disposed on a power source side from which power is supplied, and the Y-cap is disposed on an EMI source side at which EMI occurs.
9 . The filter of claim 1 , wherein the CM choke is disposed on an EMI source side at which EMI occurs, and the Y-cap is disposed on a power source side from which power is supplied.
10 . The filter of claim 1 , further comprising a phase compensator connected in parallel between an input and an output of the amplifier.
11 . The filter of claim 10 , wherein the phase compensator comprises:
a compensation resistor (R c ) and a compensation capacitor (C c ) connected in parallel to each other,
wherein each of the compensation resistor (R c ) and the compensation capacitor (C c ) comprises a first end and a second end opposing each other,
wherein the first end of the compensation resistor (R c ) and the first end of the compensation capacitor (C c ) are connected to the input of the amplifier, and wherein the second end of the compensation resistor (R c ) and the second end of the compensation capacitor (C c ) are connected to the output of the amplifier.
12 . A method of reducing EMI noise using the filter of claim 1 , the method comprising:
sensing the noise current of the CM choke via the sensing winding;
amplifying the sensed noise current by the amplifier;
receiving, by the primary coil of the transformer, the signal amplified by the amplifier; and
injecting, by the secondary coil of the transformer, a transformed signal into the Y-cap as the compensation signal.