Patent Publication Number: US-2023163681-A1

Title: Electromagnetic interference suppression circuit

Description:
TECHNICAL FIELD 
     This invention relates to suppression of conducted electromagnetic interference (EMI) signals in circuits. 
     BACKGROUND 
     Electromagnetic interference (EMI) suppression is a commonly-used technique for suppressing conducted EMI signals present on power or signal lines of a circuit, e.g., during operation of an electrical device. These conducted EMI signals can be classified into differential mode EMI signals, which are conducted on the power supply and return lines in opposite directions, and common mode EMI signals, which are conducted on the power supply line and the power return line in generally the same direction. 
     In some applications, the conducted EMI signals generated may have a relatively large magnitude, which can be difficult to suppress or eliminate. 
     SUMMARY 
     In a first aspect, there is provided an electromagnetic interference (EMI) suppression circuit, in particular an active EMI suppression circuit, for an AC power circuit. The EMI suppression circuit includes a sensing capacitor circuit arranged to sense an EMI signal on a power line in the AC power circuit; an amplifier circuit operably connected with an output of the sensing capacitor circuit and arranged to provide an EMI compensation signal based on the sensed EMI signal for reducing, or substantially eliminating, the EMI signal on the power line; and a coupling capacitor circuit arranged to be connected between an output of the amplifier circuit and the power line. The EMI suppression circuit further includes a stabilization circuit connected in series with the coupling capacitor circuit and (together) arranged to be connected between the output of the amplifier circuit and the power line, for reducing instability caused by interaction of the EMI suppression circuit with inductance in an application or operation environment associated with the EMI suppression circuit. The application or operation environment may be, e.g., a switching power converter circuit. The stabilization circuit may prevent unwanted resonance caused by interaction between the EMI suppression circuit and inductive components in the application or operation environment. The stabilization circuit may facilitate control of the loop gain of the EMI suppression circuit so that the loop gain does not exceed a predetermined value. In one example, the stabilization circuit is arranged to control the loop gain to be within 1. 
     Optionally, the EMI suppression circuit further includes an input filter circuit connected, preferably in series, with the sensing capacitor circuit to at least partly filter AC power frequency and associated harmonics. The arrangement of the input filter circuit in the EMI suppression circuit can help to attenuate, reduce, or substantially eliminate the AC power frequency and associated harmonics, thus preventing the AC power frequency and associated harmonics from reaching the amplifier circuit to undesirably affect its operation (e.g., saturating the amplifier). 
     The EMI suppression circuit may be arranged to suppress differential mode EMI signals and/or common mode EMI signals, e.g., depending on its connection with the AC power circuit. 
     The power line to which the sensing capacitor circuit is arranged to connect is where EMI signals may appear, and may be a supply line or a return line (the live line, the neural line, or the earth line). The power line to which the coupling capacitor circuit is arranged to connect is where EMI signals may appear, and may be a supply line or a return line (the live line, the neural line, or the earth line). The power line to which the sensing capacitor circuit is arranged to connect and the power line to which the coupling capacitor circuit is arranged to connect is the same power line. 
     The sensing capacitor circuit comprises or consists of one or more capacitors. Optionally, the sensing capacitor circuit is connected in series, and optionally directly, between the input filter circuit and the amplifier circuit. 
     The amplifier circuit comprises or consists of an operational amplifier, which may include an inverting input node, non-inverting input node, output node. Optionally, the sensing capacitor circuit may be connected directly with the non-inverting input node. 
     The coupling capacitor circuit comprises or consists of one or more capacitors. 
     In one example, the input filter circuit comprises or consists of a notch filter. In another example, the input filter circuit comprises or consists of a high pass filter. The high pass filter may be first order high pass filter, second order high pass filter, or even higher order high pass filter. Optionally, the high pass filter comprises a first order high pass filter comprises a capacitor circuit, which may comprise or consist of one or more capacitors, and a resistor circuit which may comprise or consist of one or more resistors. 
     In one example, the stabilization circuit comprises or consists of a resistor circuit, which comprises or consists of one or more damping resistors. Optionally, the resistor circuit is arranged between the output of the amplifier and the coupling capacitor circuit. 
     In another example, the stabilization circuit comprises or consists of a non-linear blocker circuit. Optionally, the non-linear blocker circuit comprises a resistor circuit and a non-linear circuit connected in parallel, and the non-linear circuit is arranged to conduct (e.g., enable current flow) when a voltage across it is above a predetermined conductive voltage and is arranged to be non-conducting (e.g., prevent current flow) when a voltage across it is below the predetermined conductive voltage. The resistor circuit of the non-linear blocker circuit comprises or consists of one or more resistors. Optionally, the non-linear circuit comprises or consists of a diodic circuit. The diodic circuit comprises or consists of two or more diodes, which may be implemented using two or more rectifier diodes, Schottky diodes, zener diode, diode-connected bipolar transistors, MOSFETs, etc. 
     In one example, the diodic circuit comprises or consists of a first diode circuit and a second diode circuit connected in parallel, with the first diode circuit selectively permitting current flow in one direction and the second diode circuit selectively permitting current flow in another direction. The first diode circuit comprises or consists of one or more diodes; the second diode circuit comprises or consists of one or more diodes. Optionally, the first diode circuit comprises or consists of a first diode, the second diode circuit comprises or consists of a second diode, and a cathode of the first diode connected to an anode of the second diode and an anode of the first diode connected to a cathode of the second diode. Optionally, the first diode and the second diode have substantially the same forward voltage, which may correspond to (i.e., equals to) the predetermined conductive voltage of the non-linear circuit. 
     In another example, the diodic circuit comprises or consists of a first Zener diode and a second Zener diode connected back-to-back and in series. The back to back connection may be formed by connecting the anode of the first Zener diode with the anode of the second Zener diode or by connecting the cathode of the first Zener diode with the cathode of the second Zener diode. Optionally, the first Zener diode and the second Zener diode have substantially the same forward diode voltage. Optionally, the first Zener diode and the second Zener diode have substantially the same Zener voltage. Optionally, the sum of one Zener voltage of one of the first Zener diode and the second Zener diode and one forward diode voltage of another one of the first Zener diode and the second Zener diode may correspond to (i.e., equals to) the predetermined conductive voltage of the non-linear circuit. 
     Optionally, the non-linear blocker circuit further comprises an active circuit for boosting a current drive capability of the non-linear blocker circuit, in particular the non-linear circuit or one that includes a diodic circuit. The boosting may improve current drive capability for a given voltage condition. The active circuit may comprise or consist of a transistor circuit operably connected with the diodic circuit. The transistor circuit may comprise two or more transistors, e.g., BJT or MOS transistors, implemented using/in an emitter follower circuit, a source follower circuit, etc. The active circuit may improve the current driving capability of the amplifier circuit and may improve the slew rate. 
     Optionally, the EMI suppression circuit further comprises: a resistor circuit connected between an input of the amplifier circuit and an output of the amplifier circuit. The resistor circuit comprises or consists of one or more resistors. The resistor circuit and the sensing capacitor circuit may together define a filter circuit that cooperates with the input filter circuit to provide a filtering effect better than the filtering effect of the input filter circuit alone. 
     In a second aspect, there is provided an electromagnetic interference (EMI) suppression circuit, in particular an active EMI suppression circuit, of an AC power circuit. The EMI suppression circuit in the second aspect may include the above feature(s) of the EMI suppression circuit of the first aspect. 
     In a third aspect, there is provided an AC power circuit comprising one or more electromagnetic interference (EMI) suppression circuits of the first aspect. 
     Optionally, the AC power circuit comprises: an AC power source connector for connecting with an AC power source; and a switching converter for connecting with a load. At least one of the one or more EMI suppression circuits is connected between the AC power source connector and the switching converter. 
     Optionally, the AC power circuit further comprises a rectifier circuit arranged between the AC power source connector and the switching converter; and/or the AC power source connected at the AC power source connector. 
     Optionally, at least one of the one or more EMI suppression circuits is connected between the AC power source connector and the rectifier circuit. Optionally, at least one of the one or more EMI suppression circuits is connected between the rectifier circuit and the switching converter. 
     Optionally, the one or more EMI suppression circuits comprises at least one EMI suppression circuit of the first aspect, connected in the AC power circuit, for suppressing differential mode EMI signals, and at least one EMI suppression circuit of the first aspect, connected in the AC power circuit, for suppressing common mode EMI signals. 
     In a fourth aspect, there is provided a power circuit comprising one or more electromagnetic interference (EMI) suppression circuits of the first aspect. 
     In a fifth aspect, there is provided an electrical device comprising one or more electromagnetic interference (EMI) suppression circuits of the first aspect or the AC power circuit of the third aspect. 
     In a sixth aspect, there is provided a method for suppressing EMI signals in an AC power circuit. The method comprises: sensing, using a sensing means of an EMI suppression circuit, an EMI signal on a power line; and providing, using a compensation means of the EMI suppression circuit, an EMI compensation signal based on the sensed EMI signal for reducing, or substantially eliminating, the EMI signal on the power line. The EMI suppression circuit comprises a stabilization circuit for reducing instability caused by interaction of the EMI suppression circuit with inductance in an operation environment associated with the EMI suppression circuit. The sensing means may be a sensing circuit, a sensing capacitor, etc. The compensation means may be an amplifier circuit, an amplifier, etc. The stabilization circuit may comprise a non-linear blocker circuit, which may include a resistor circuit and a non-linear circuit (e.g., diodic circuit) connected in parallel, and the non-linear circuit is arranged to conduct when a voltage across the non-linear circuit is above a predetermined conductive voltage and is arranged to be non-conducting when a voltage across the non-linear circuit is below the predetermined conductive voltage. The stabilization circuit may prevent unwanted resonance due to interaction of the EMI suppression circuit and inductive components in the application or operation environment. 
     Optionally, the method further comprises filtering, using a filtering means of the EMI suppression circuit, AC power frequency and associated harmonics. The filtering means may be a filter circuit. 
     Optionally, the EMI suppression circuit is the EMI suppression circuit of the first aspect. 
     In a seventh aspect, there is provided a method for making an EMI suppression circuit. The method comprises: providing means for filtering AC power frequency and associated harmonics; providing means for sensing an EMI signal on a power line; providing means for providing an EMI compensation signal based on the sensed EMI signal for reducing, or substantially eliminating the EMI signal on the power line; and providing means for reducing instability caused by interaction of the EMI suppression circuit with inductance in an operation environment associated with the EMI suppression circuit. The means for filtering AC power frequency and associated harmonics may include a filter circuit. The means for sensing the EMI signal may include a sensing circuit. The means for providing EMI compensation signal may include an amplifier circuit. The means for reducing instability may include a stabilization circuit (e.g., a non-linear blocker circuit). Optionally, the EMI suppression circuit is the EMI suppression circuit of the first aspect. 
     Unless otherwise specified, the terms “connected”, “coupled”, or the like, are intended encompass both direct and indirect connection and coupling, and may cover mechanical connection and coupling, electrical connection and coupling, or both, whether direct or indirect. 
     Other features and aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. Any feature(s) described herein in relation to one aspect or embodiment may be combined with any other feature(s) described herein in relation to any other aspect or embodiment as appropriate and applicable. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which: 
         FIG.  1    is a circuit diagram of a DC/DC switching power converter circuit including an active EMI suppression circuit. 
         FIG.  2    is a circuit diagram of an AC/DC switching power converter circuit with a passive EMI suppression circuit. 
         FIG.  3 A  is a circuit diagram of an active EMI suppression circuit arranged to suppress differential mode EMI signals. 
         FIG.  3 B  is a circuit diagram of an active EMI suppression circuit arranged to suppress differential mode EMI signals. 
         FIG.  3 C  is a circuit diagram of an active EMI suppression circuit arranged to suppress common mode EMI signals. 
         FIG.  4 A  is a circuit diagram of an active EMI suppression circuit arranged to suppress differential mode EMI signals in one embodiment of the invention. 
         FIG.  4 B  is a circuit diagram of an active EMI suppression circuit arranged to suppress common mode EMI signals in one embodiment of the invention. 
         FIG.  4 C  is a circuit diagram of a portion of an active EMI suppression circuit in one embodiment of the invention. 
         FIG.  4 D  is a circuit diagram of a portion of an active EMI suppression circuit in one embodiment of the invention. 
         FIG.  5 A  is a circuit diagram of a portion of an active EMI suppression circuit illustrating a construction of the non-linear blocker circuit in one embodiment of the invention. 
         FIG.  5 B  is a circuit diagram of a portion of an active EMI suppression circuit illustrating a construction of the non-linear blocker circuit in one embodiment of the invention. 
         FIG.  6 A  is a circuit diagram of a portion of an active EMI suppression circuit illustrating a construction of the non-linear blocker circuit in one embodiment of the invention. 
         FIG.  6 B  is a circuit diagram of a portion of an active EMI suppression circuit illustrating a construction of the non-linear blocker circuit in one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    shows a DC/DC switching power converter circuit  100  with an active electromagnetic interference (EMI) suppression circuit  120  arranged between a DC power source  101  and a DC/DC switching converter  109  that is connected with a load  110 . The active EMI suppression circuit  120  is arranged to suppress conducted differential mode EMI signals across DC power lines  102  and  103  supplied by the DC source  101 . The active EMI suppression circuit  120  includes a sensing capacitor  104 , a feedback resistor  105 , an operational amplifier  107 , and a coupling capacitor  106  operably connected with each other. The positive DC power line  102  provides an EMI signal sensing and EMI compensation signal coupling node. The negative DC power line  103  of the DC source  101  provides a reference node. The active EMI suppression circuit  120  is connected across these two nodes. 
     The sensing capacitor  104  is connected between the EMI signal sensing and EMI compensation signal coupling node and the inverting input of the operational amplifier  107 . The resistor  105  is connected between the inverting input and the output of the operational amplifier. The operational amplifier  107  is connected to a power supply, with a positive power supply side Vcc, and a negative power supply side connected to the negative DC power line  103 . The non-inverting input of the operational amplifier  107  is connected at ½Vcc. The coupling capacitor  106  is connected between the output of the operational amplifier  107  and the EMI signal sensing and EMI compensation signal coupling node. An inductor  108  is connected on the positive DC power line  102 , between the EMI signal sensing and EMI compensation signal coupling node and the DC/DC switching converter  109 . 
     In operation, EMI signal on the positive DC power line  102  is sensed by the sensing capacitor  104  and is coupled to the inverting input of the operational amplifier  107  via the sensing capacitor  104 . Based on the sensed EMI signal the operational amplifier  107  provides, at its output, a compensation signal to drive the positive DC power line  102  via the coupling capacitor  106  to cancel the EMI signal at the positive DC power line  102  such that the EMI signal across the positive DC power line  102  and the negative DC power line  103  (or across the DC source) is substantially eliminated. The resistor  105  provides a DC feedback path between the inverting input and the output of the operational amplifier  107  for setting the DC operating point. 
     In this example, the active EMI suppression circuit  120  can be considered as a capacitor having a much larger value compared with the original value of the coupling capacitor  106  at the EMI signal band. 
     In some other implementations, the active EMI suppression circuit  120  may instead be connected to circuit  100  using the positive DC power line  102  as reference node and the negative DC supply line  103  as the EMI signal sensing and EMI compensation signal coupling node. In some other implementations, in the active EMI suppression circuit  120 , the original positive supply side V cc  of the operational amplifier  107  may be selected as the reference node while the negative power supply side of the operational amplifier  107  may be connected to a -V cc . In such case, the non-inverting input of the operational amplifier  107  is connected to -½V cc . 
       FIG.  2    shows an AC/DC switching power converter circuit  200  with a passive EMI suppression circuit  230  arranged between an AC power source  201  and a switching converter  216  that is connected to a load  217 . More specifically, the passive EMI suppression circuit  230  is arranged between the AC source  201  and a rectifier circuit (diode bridge)  213  (which is located between the power source  201  and the switching converter  216 ). The passive EMI suppression circuit  230  is arranged to suppress conducted differential mode EMI signals across the AC power lines (live line and neutral line)  202 ,  203  supplied by the AC source  201  as well as common mode EMI signals across the live line  202  and the earth line  204  and across the neutral line  203  and the earth line  204 . 
     The passive EMI suppression circuit  230  includes two differential mode filter capacitors (“X-capacitors”)  205 ,  208 , a common mode choke (CMC)  220 , and two common mode filter capacitors (“Y-capacitors”)  209 ,  210 . In  FIG.  2   , the CMC  220  is modelled and illustrated as an ideal CMC  206  connected in series with an inductor  207  that represents the leakage inductance of the CMC  220 . 
     In  FIG.  2   , the differential mode filter capacitor  205  is connected across the live and neutral lines  202 ,  203  between the AC source  201  and the CMC  220 . The differential mode filter capacitor  208  is connected across the power lines  211 ,  212  to the diode bridge  213  between the CMC  220  and the diode bridge  213 . The CMC  220  is connected between the two differential mode filter capacitors. The common mode filter capacitor  209  is connected between the power line  212  and the earth line  204 . The common mode filter capacitor  210  is connected across the power line  212  and the earth line  204 . 
     In operation, conducted differential mode EMI signals at the input to the diode bridge  213  of the AC/DC switching power converter circuit  200  are suppressed by a PI-type filter, which is formed by the differential mode filter capacitors  205 ,  208  and the series-connected inductor  207  (of the CMC  220 ). As a result differential mode EMI signals at the power lines (live and neutral lines)  202 ,  203  can be suppressed, e.g., to a level within compliance limit. 
     In operation, conducted common mode EMI signals between both the power lines  211 ,  212  at the input to the diode bridge  213  of the AC/DC switching power converter circuit  200  and an earth line  204  of AC/DC switching power converter circuit  200  are suppressed by a L-type filter, which is formed by the common mode filter capacitors  209 ,  210  and the CMC  206  (of the CMC  220 ). As a result, common mode EMI signals between the power lines (live and neutral lines)  202 ,  203  and the earth line  204  can be suppressed, e.g., to a level within compliance limit. 
     In some other implementations, the passive EMI suppression circuit  230  can instead be connected between the diode bridge  213  and the switching converter  216 . In some other implementations, the passive EMI suppression circuit  230  can be modified to include additional filter section(s). This may be useful in cases where the EMI signals are strong and that the passive EMI suppression circuit  230  alone is unable to satisfactorily reduce the EMI level, e.g., to bring the EMI level to within compliance limit. 
     The active EMI suppression circuit  120  in  FIG.  1    and the passive EMI suppression circuit  230  in  FIG.  2    are not part of the present invention. 
     The inventors of the present invention have realized, through research, experiments, and trials, that the EMI suppression performance in the AC/DC switching power converter circuit  200 , or more generally in some AC power circuits, can be improved by using an active EMI suppression circuit. In particular, the inventors of the present invention have devised that the use of an active EMI suppression circuit can boost the capacitance of differential mode filter capacitors and common mode filter capacitors, such as those in the AC/DC switching power converter circuit  200  in  FIG.  2   . In case for a differential mode filter capacitor, the use of an appropriate active EMI suppression circuit may help to reduce the require capacitance value of the differential mode filter capacitor, hence reducing size and cost of the circuit in which the differential mode filter capacitors is arranged and improving the frequency response (as a smaller differential mode filter capacitor has higher self-resonant frequency (SRF)). In case for a common mode filter capacitor, safety compliance requirement may limit the maximum capacitance value of common mode filter capacitor in switching converters. The use of an appropriate active EMI suppression circuit may help to increase the equivalent capacitance value of the common mode filter capacitor, hence reducing the inductance value of the CMC. Reducing the inductance value of the CMC helps to reduce size, weight, cost and to improve the frequency response (as a smaller CMC has a higher self-resonant frequency (SRF)). 
     However, the inventors of the present invention have also devised that the active EMI suppression circuit  120  cannot be directly used to replace differential mode filter capacitors and common mode filter capacitors, such as the differential mode filter capacitors  205 ,  208  and the common mode filter capacitors  209 ,  210  in  FIG.  2   , due to the presence of large AC line voltage (e.g., up to  270  VAC) of the AC power source (typically 50/60 Hz), which can reduce the dynamic range of, or even saturate, an amplifier circuit, such as the operational amplifier  107 . If the active EMI suppression circuit  120  is directly applied to the circuit  200 , then the high AC voltage on the power line of the circuit  200  may also be sensed by the sensing capacitor  104 . In one example, the voltage of the AC source  201  can be 265 volts at 50 Hz at the power line. If the active EMI suppression circuit  120  is directly applied to the circuit  200 , the high pass filter formed by sensing capacitor  104  and DC stabilizing resistor  105  will attenuate this line frequency signal, and its level of attenuation depends on the cut-off frequency chosen for such high pass filter. Since the compliance requirement of conducted EMI typically starts at 150 kHz, the frequency response of the high pass filter needs to provide near zero phase shift at 150 kHz, which means the cut off frequency is approximately 2 decades below, or around 1.5 kHz. By selecting 2 kHz as the cut off frequency, the suppression at 50 Hz for a first order filter is 32 dB, or the 50 Hz AC signal is reduced by 40 times. However, for a  265  VAC at 50 Hz, the peak voltage is approximately 375 Volts, and the peak-to-peak voltage is approximately 750 Volts. Reduction of the AC signal by 40 times means 18.75 Volts peak to peak. This value exceeds the typical operating voltage of operational amplifier  107 . Placing the active EMI suppression circuit  120  between the diode bridge  213  and switching converter  216  will not alleviate the problem because while the peak-to-peak voltage is reduced to half of the original value, the 50 Hz fundamental frequency is doubled to 100 Hz, which may worsen the situation. The inventors of the present invention have also realized that the replacement of the differential mode filter capacitors  205 ,  208  and/or the common mode filter capacitors  209 ,  210  with the active EMI suppression circuit  120  in the circuit of  FIG.  2    will make the active EMI suppression circuit unstable due to potential resonance as a result of the presence of the inductance  207  in differential mode, the CMC  206  in common mode, and the equivalent resistance of the line impedance stabilizing network (LISN) in EMI measurement set up. 
       FIG.  3 A  illustrates an AC power circuit  300 A including an active EMI suppression circuit  330 A in one embodiment of the invention. The active EMI suppression circuit  330 A may be used to replace each of the differential mode filter capacitors  205 ,  208  in the circuit  200  of  FIG.  2   , or more generally, be used as a differential mode filter capacitor in power circuits. 
     In  FIG.  3 A , the AC power circuit  300 A includes, between the AC source side and the switching converter side: an inductor  304 A that generally corresponds to the leakage inductance  207  in  FIG.  2   , a differential mode filter capacitor  303 A that generally corresponds to the differential mode filter capacitor  205  in  FIG.  2   , and an active EMI suppression circuit  330 A that replaces the differential mode filter capacitor  208  in  FIG.  2   . The active EMI suppression circuit  330 A generally corresponds to the active EMI suppression circuit  120  in  FIG.  1   , but with the addition of an input filter circuit  320 A. 
     More specifically, the active EMI suppression circuit  330 A includes an input filter circuit  320 A, a sensing capacitor  307 A, a feedback resistor  308 A, an operational amplifier  310 A, and a coupling capacitor  309 A operably connected with each other. The power line  311 A provides an EMI signal sensing and EMI compensation signal coupling node. The power line  302 A provides a reference node. The active EMI suppression circuit  330 A is connected across these two nodes or the two power lines  311 A,  302 A. 
     The input filter circuit  320 A is a first order high pass filter and includes a capacitor  305 A and a resistor  306 A. The input filter circuit  320 A is connected, in series, between the EMI signal sensing and EMI compensation signal coupling node and the sensing capacitor  307 A. The input filter circuit  320 A is also connected between the EMI signal sensing and EMI compensation signal coupling node and negative power supply side of the operational amplifier  310 A which serves as the reference node. The sensing capacitor  307 A is connected between the input filter circuit  320 A and the inverting input of the operational amplifier  310 A. The resistor  308 A is connected between the inverting input and the output of the operational amplifier  310 A. The operational amplifier  310 A is connected to a power supply, with a positive power supply side Vcc, and a negative power supply side connected to the power line  302 A. The non-inverting input of the operational amplifier  310 A is connected at ½Vcc. The coupling capacitor  309 A is connected between the output of the operational amplifier  310 A and the EMI signal sensing and EMI compensation signal coupling node. 
     In operation, the input filter circuit  320 A reduces or attenuates AC line frequency to prevent the high AC line voltage from entering the operational amplifier  310 A to saturate it. More specifically, the input filter circuit  320 A, which is a first order high pass filter in this example, along with the sensing capacitor  307 A that senses the EMI signal on the line  311 A, and the resistor  308 A, provide a second order high pass filter that sufficiently attenuate the high AC line voltage and associated harmonics from the AC source to prevent saturating the operational amplifier  310 A. In one example, selecting the cut off frequency of the filter at 2 kHz will offer 64 dB attenuation for the line frequency and therefore the AC high voltage appears at the operational amplifier  310 A can be reduced to less than 0.5-volt peak to peak. The sensing capacitor  307 A senses the EMI signal on the line  311 A. Based on the sensed EMI signal the operational amplifier  310 A provides, at its output, a compensation signal to drive the power line  311 A via the coupling capacitor  309 A to cancel the EMI signal at the line  311 A such that the EMI signal across the line  311 A and the line  302 A (or across the AC source) is substantially eliminated. The resistor  308 A provides a feedback path between the inverting input and the output of the operational amplifier  107  for setting of DC operating point. 
     In some other implementations, the active EMI suppression circuit  330 A may instead be connected to circuit  300 A using the power line  311 A as reference node and the power line  302 A as the EMI signal sensing and EMI compensation signal coupling node. In some other implementations, in the active EMI suppression circuit  330 A, the original positive supply side Vcc of the operational amplifier  310 A may be selected as the reference node while the negative power supply side of the operational amplifier  310 A may be connected to a -Vcc. In such case, the non-inverting input of the operational amplifier  310 A is connected to -½ Vcc. 
       FIG.  3 B  illustrates an AC power circuit  300 B including an active EMI suppression circuit  330 B in one embodiment of the invention. The active EMI suppression circuit  330 B may be used to replace each of the differential mode filter capacitors  205 ,  208  in the circuit  200  of  FIG.  2   , or more generally, be used as a differential mode filter capacitor in power circuits. The active EMI suppression circuit  330 B is substantially the same as the active EMI suppression circuit  330 A in  FIG.  3 A , except that the active EMI suppression circuit  330 B is located before the inductor  304 B, across the lines  301 B and  302 B. The structure of the active EMI suppression circuit  330 B is substantially the same as the active EMI suppression circuit  330 A in  FIG.  3 A . 
     In the circuits  300 A,  300 B, an LC circuit formed by the inductor  304 A and the capacitor  303 A, and by the inductor  304 B and the capacitor  303 B, is inductive in nature above its series resonant frequency whereas the active EMI suppression circuits  330 A,  330 B are capacitive in nature. As a result, a parallel resonant circuit may be formed between the power lines  301 A and  302 A, or between the power lines  301 B and  302 B. In most applications the resonance effect produced by such parallel resonant circuit may cause instability hence may be undesirable. 
     It should be noted that the circuits in  FIGS.  3 A and  3 B  may include a CMC corresponding to the CMC  206  in  FIG.  2   . However, since the CMC has little effect for differential mode EMI signal, it is not shown in  FIGS.  3 A and  3 B . 
       FIG.  3 C  illustrates an AC power circuit  300 C including two active EMI suppression circuits  350 C,  360 C in one embodiment of the invention. The active EMI suppression circuits  350 C,  360 C may be used to replace the common mode filter capacitors  209 ,  210  in the circuit  200  of  FIG.  2   , or more generally, be used as common mode filter capacitors in power circuits. 
     In  FIG.  3 C , the AC power circuit  300 C includes, between the AC source side and the switching converter side: a line impedance stabilizing network (LISN)  370 C connected across the live and neural lines  301 C,  302 C, two differential mode filter capacitors  308 C,  310 C that correspond to the differential mode filter capacitors  205 ,  208  in  FIG.  2   , a common mode choke (CMC)  309 C arranged between the two differential mode filter capacitors  308 C,  310 C and that generally corresponds to the CMC  220  in  FIG.  2   , and two active EMI suppression circuits  350 C,  360 C that correspond to the common mode filter capacitors  209 ,  210  in  FIG.  2   . Each of the active EMI suppression circuits  350 C,  360 C generally correspond to the active EMI suppression circuit  120  in  FIG.  1   , but with the addition of an input filter circuit  330 C,  340 C. 
     More specifically, the active EMI suppression circuits  350 C,  360 C each include an input filter circuit  330 C,  340 C, a sensing capacitor  314 C,  320 C, a feedback resistor  315 C,  321 C, an operational amplifier  317 C,  323 C, and a coupling capacitor  316 C,  322 C operably connected with each other. For the active EMI suppression circuits  350 C, the power line  303 C provides an EMI signal sensing and EMI compensation signal coupling node. The power line  324 C provides a reference node. The active EMI suppression circuits  350 C is connected across these two nodes or the two power lines  324 C,  303 C. For the active EMI suppression circuits  360 C, the power line  303 C provides an EMI signal sensing and EMI compensation signal coupling node. The power line  311 C provides a reference node. The active EMI suppression circuits  360 C is connected across the two power lines  311 C,  303 C. 
     The input filter circuit  330 C is a first order high pass filter and includes a capacitor  312 C and a resistor  313 C. The input filter circuit  330 C is connected, in series, between the EMI signal sensing and EMI compensation signal coupling node on line  303 C and the sensing capacitor  314 C. The sensing capacitor  314 C is connected between the input filter circuit  330 C and the inverting input of the operational amplifier  317 C. The resistor  315 C is connected between the inverting input and the output of the operational amplifier  317 C. The operational amplifier  317 C is connected to a power supply, with a positive power supply side Vcc1, and a negative power supply side connected to the power line  324 C. The non-inverting input of the operational amplifier  317 C is connected at ½Vcc1. The coupling capacitor  316 C is connected between the output of the operational amplifier  317 C and the EMI signal sensing and EMI compensation signal coupling node on line  303 C. 
     In operation, the input filter circuit  330 C reduces or attenuates AC line frequency to prevent the high AC line voltage from entering the operational amplifier  317 C to saturate it. More specifically, the input filter circuit  330 C, which is a first order high pass filter in this example, along with the sensing capacitor  314 C that senses the EMI signal on the line  303 C, and the resistor  315 C, provide a second order high pass filter that sufficiently attenuate the high AC line voltage and associated harmonics from the AC source to prevent saturating the operational amplifier  317 C. The sensing capacitor  314 C senses the EMI signal on the line  303 C. Based on the sensed EMI signal the operational amplifier  317 C provides, at its output, a compensation signal to drive the power line  303 C via the coupling capacitor  316 C to cancel the EMI signal at the line  303 C such that the EMI signal in the line  303 C is substantially eliminated. The resistor  315 C provides a feedback path between the inverting input and the output of the operational amplifier  317 C for setting of DC operating point. 
     The construction of the active EMI suppression circuit  360 C is substantially the same as the construction of the active EMI suppression circuit  350 C. 
     In the circuit  300 C of this embodiment, Vcc1 and Vcc2 are at different levels (e.g., by using different power sources for the active EMI suppression circuits  350 C and  360 C respectively since they have different reference nodes). The nodes on lines  311 C and  324 C are effectively AC shorted by the capacitor  310 C hence in some implementations the use of only one of the active EMI suppression circuits  350 C,  360 C in the circuit  300 C is sufficient to achieve the intended effect. 
     In the circuit  300 C, the line impedance stabilizing network (LISN)  370 C connected to the live line  301 C, the neutral line  302 C and the earth line  303 C is used for EMC compliance test. The LISN  370 C comprises a capacitor  304 C and a resistor  305 C connected in series, with the capacitor terminal of the series RC circuit connected to live line  301 C and resistor terminal of the series RC circuit connected to the earth line  303 C; and a capacitor  307 C and a resistor  306 C connected in series, with the capacitor terminal of the series RC circuit connected to neutral line  302 C and resistor terminal of the series RC circuit connected to the earth line  303 C. This provides return path for the CMC  309 C to earth line  303 C that serves as the sensing node of common mode EMI. LISN is not shown in the circuits  300 A,  300 B in  FIGS.  3 A and  3 B  because it may be ignored for circuit analysis - the equivalent impedance between live and neutral lines of the LISN is around 100 Ohms, which is typically orders of magnitude higher than the impedance of the differential mode filter capacitor connected across the LISN in the conducted EMI compliance measurement frequency band. 
     In the circuit  300 C, a series LCR circuit formed by the single sided inductor of the CMC  309 C at the neutral line  302 C side, and the capacitor  307 C and the resistor  306 C of the LISN  370 C, is inductive in nature above its series resonant frequency whereas the active EMI suppression circuit  350 C is capacitive in nature. As a result, a parallel resonant circuit is formed between circuit nodes on lines  324 C and  303 C. In some applications the resonance effect produced by such parallel resonant circuit may be acceptable whereas in some other applications the resonance effect produced by such parallel resonant circuit may be undesirable. Similarly, a series LCR circuit formed by the single sided inductor of the CMC  309 C at the live line  301 C side, and the capacitor  304 C and the resistor  305 C of the LISN  370 C, is inductive in nature above its series resonant frequency whereas the active EMI suppression circuit  360 C is capacitive in nature. As a result, a parallel resonant circuit is formed between circuit nodes on lines  311 C and  303 C. In some applications the resonance effect produced by such parallel resonant circuit may be acceptable whereas in some other applications the resonance effect produced by such parallel resonant circuit may be undesirable. 
     It should be noted that the circuit in  FIG.  3 C  may include differential inductance corresponding to inductor  207  in  FIG.  2   . However, since the differential inductance has little effect for common mode EMI signal, it is not shown in  FIG.  3 C . 
     As discussed above, the application of active EMI suppression circuits  330 A,  330 B,  350 C,  360 C to replace the differential mode filter capacitors and the common mode filter capacitors may introduce instability due to resonance effect, mainly LC parallel resonance, produced by the equivalent circuit. 
     The inventors of the present invention have realized, through research, experiments, and trials, that in the frequency response of typical parallel LC circuit, peaking effect occurs for gain around the resonant frequency, and a sudden phase change of 180 degrees occurs at the parallel resonant frequency that generally corresponds to the change over from inductive at frequencies below resonance to capacitive at frequencies above resonance. The inventors have thus realized that it would be difficult to achieve stability in the circuit unless some means of gain and/or damping control is included. The instability near parallel resonance may also be considered as over-compensation in time domain for signals near the resonance. This means the output of the operational amplifiers in the active EMI suppression circuits  330 A,  330 B,  350 C,  360 C at frequencies near the resonance provides a compensation signal larger than necessary, and this effect propagates round the circuit loop to create instability. 
     To suppress this instability, the inventors of the present invention have devised a stabilization circuit, as detailed below. 
     In one embodiment, the stabilization circuit is a resistor circuit with one or more damping resistors of appropriate resistance value, is included between the output of the operational amplifier and the coupling capacitor in the active EMI suppression circuits  330 A,  330 B,  350 C,  360 C. In some applications, however, the introduction of the resistor circuit will introduce a penalty of reducing the effectively driving capability of the operational amplifier. This may cause a reduction, e.g., a substantial reduction, of slew rate and hence reduce the ability of the circuit to follow the fast-changing input EMI signal. 
     To address this potential issue, in another embodiment, the stabilization circuit is a non-linear blocker circuit. The non-linear blocker circuit can provide damping for output swing equal or slightly (e.g., 10%) higher than the maximum over-compensation amount V c  while enabling conduction of output signal higher than V c  at both directions. In one example, the non-linear blocker circuit is arranged such that the loop gain of the active EMI suppression circuit from EMI signal sensed from the power line to the compensation signal coupling back to the power line will not exceed 1. 
       FIG.  4 A  illustrates an AC power circuit  400 A including an active EMI suppression circuit  430 A in one embodiment of the invention. The active EMI suppression circuit  430 A may be used to replace each of the differential mode filter capacitors  205 ,  208  in the circuit  200  of  FIG.  2   , or more generally, be used as a differential mode filter capacitor in power circuits. 
     In  FIG.  4 A , the AC power circuit  400 A includes, between the AC source side and the switching converter side: an inductor  404 A that generally corresponds to the inductor  304 A in  FIG.  3 A , a differential mode filter capacitor  403 A that generally corresponds to the differential mode filter capacitor  303 A in  FIG.  3 A , and an active EMI suppression circuit  430 A that replaces the active EMI suppression circuit  330 A in  FIG.  3 A . The active EMI suppression circuit  430 A is substantially the same as the active EMI suppression circuit  330 A in  FIG.  3 A , but with the addition of a non-linear blocker circuit  440 A between the output of the operational amplifier  410 A and the coupling capacitor  409 A to stabilize the circuit  430 A. For simplicity, a detailed description of the active EMI suppression circuit  430 A, in particular the parts same as the active EMI suppression circuit  330 A, is not provided here. 
     In this embodiment, the non-linear blocker circuit  440 A comprises a resistor circuit with a damping resistor  412 A and a non-linear circuit  413 A connected with the resistor circuit in parallel. The non-linear circuit  413 A is open (e.g., not conducting) when voltage across it is below a conduction voltage V c  and closed (e.g., conducting) when voltage across it is above a conduction voltage V c  in both directions. The damping resistor  412 A provides a conduction path when the signal voltage across the non-linear blocker circuit  440 A is below the conduction voltage V c . 
     In the circuit  400 A of  FIG.  4 A , the feedback resistor  408 A may be connected between the inverting input of the operational amplifier  410 A and the output of the operational amplifier  410 A. In another embodiment, the feedback resistor  408 A may be connected between the inverting input of the operational amplifier  410 A and the series connecting node that is between the non-linear blocker circuit  440 A and the coupling capacitor  409 A. 
     It should be noted that the circuit in  FIG.  4 A  may include a CMC corresponding to the CMC  206  in  FIG.  2   . However, since the CMC has little effect for differential mode EMI signal, it is not shown in  FIG.  4 A . 
       FIG.  4 B  illustrates an AC power circuit  400 B including an active EMI suppression circuit  440 B in one embodiment of the invention. The active EMI suppression circuit  440 B may be used to replace each of the common mode filter capacitors  209 ,  210  in the circuit  200  of  FIG.  2   , or more generally, be used as a common mode filter capacitor in power circuits. 
     In  FIG.  4 B , the AC power circuit  400 B includes, between the AC source side and the switching converter side: a LISN circuit  430 B (with resistor  404 B and capacitor  403 B) that generally corresponds to the LISN  370 C in  FIG.  3 C , an inductor  405 B that generally corresponds to the inductor (of the CMC  309 C) in  FIG.  3 C , and an active EMI suppression circuit  440 B that generally corresponds to the active EMI suppression circuit  360 C in  FIG.  3 C . The active EMI suppression circuit  440 B is substantially the same as the active EMI suppression circuit  360 C in  FIG.  3 C , but with the addition of a non-linear blocker circuit  450 B between the output of the operational amplifier  411 B and the coupling capacitor  410 B to stabilize the circuit  440 B. For simplicity, a detailed description of the active EMI suppression circuit  440 B is not provided here. 
     In this embodiment, the non-linear blocker circuit  450 B comprises a resistor circuit with a damping resistor  412 B and a non-linear circuit  413 B connected with the resistor circuit in parallel. The non-linear circuit  413 B is open (e.g., not conducting) when voltage across it is below a conduction voltage V c  and closed (e.g., conducting) when voltage across it is above a conduction voltage V c  in both directions. The damping resistor  412 B provides a conduction path when the signal voltage across the non-linear blocker circuit  450 B is below the conduction voltage V c . 
     In the circuit  400 B of  FIG.  4 B , the feedback resistor  409 B may be connected between the inverting input of the operational amplifier  411 B and the output of the operational amplifier  411 B. In another embodiment, the feedback resistor  409 B may be connected between the inverting input of the operational amplifier  411 B and the series connecting node that is between the non-linear blocker circuit  450 B and the coupling capacitor  410 B. 
     It should be noted that the circuit in  FIG.  4 B  may include differential inductance corresponding to inductor  207  in  FIG.  2   . However, since the differential inductance has little effect for common mode EMI signal, it is not shown in  FIG.  4 B . 
       FIG.  4 C  illustrates a simplified circuit diagram showing only a portion of a circuit  400 C with a non-linear blocker circuit  410 C (with a resistor  403 C and a non-linear circuit  404 C). 
     In  FIG.  4 C , the operational amplifier  401 C generally corresponds to operational amplifier  410 A in  FIG.  4 A  and the operational amplifier  411 B in  FIG.  4 B , the resistor  402 C generally corresponds to the resistor  408 A in  FIG.  4 A  and resistor  409 B in  FIG.  4 B , non-linear blocker circuit  410 C generally corresponds to non-linear blocker circuit  440 A in  FIG.  4 A  and non-linear blocker circuit  450 B in  FIG.  4 B , and the coupling capacitor  405 C generally corresponds to the coupling capacitor  409 A in  FIG.  4 A  and the coupling capacitor  410 B in  FIG.  4 B . 
     In some embodiments, the location or connection of the non-linear blocker circuit  440 A and the coupling capacitor  409 A may be exchanged. In some embodiments, the location or connection of the non-linear blocker circuit  450 B and the coupling capacitor  410 B may be exchanged. 
       FIG.  4 D  illustrates a simplified circuit diagram showing only a portion of a circuit  400 D with a non-linear blocker circuit  410 D (with a resistor  404 D and a non-linear circuit  405 D). 
     In  FIG.  4 D , the operational amplifier  401 D generally corresponds to the operational amplifier  410 A in  FIG.  4 A  and the operational amplifier  411 B in  FIG.  4 B , the resistor  402 D generally corresponds to the resistor  408 A in  FIG.  4 A  and the resistor  409 B in  FIG.  4 B , the non-linear blocker circuit  410 D generally corresponds to the non-linear blocker circuit  440 A in  FIG.  4 A  and the non-linear blocker circuit  450 B in  FIG.  4 B , and the coupling capacitor  403 D generally corresponds to the coupling capacitor  409 A in  FIG.  4 A  and the coupling capacitor  410 B in  FIG.  4 B . 
       FIG.  5 A  illustrates an embodiment of the non-linear blocker circuits  440 A,  450 B,  410 C,  410 D. In  FIG.  5 A , the non-linear blocker circuit  510 A comprises a first diode  504 A and a second diode  505 A and a resistor  503 A connected in parallel with each other. The first and second diodes  504 A,  505 A provide a non-linear circuit whereas the resistor  503 A provides a resistor circuit. In  FIG.  5 A , the anode of the first diode  504 A is connected to the cathode of a second diode  505 A, and the cathode of the first diode  504 A is connected to the anode of a second diode  505 A. The forward voltages for both diodes  504 A and  505 A is equal and is selected to be equal to the conduction voltage V c  of the non-linear circuit. In some implementations, if the forward voltage cannot be obtained by a single diode, then it may be obtained by series connection of one or more diode(s) with same or different individual forward voltage(s). 
       FIG.  5 B  illustrates an embodiment of the non-linear blocker circuits  440 A,  450 B,  410 C,  410 D. In  FIG.  5 B , the non-linear blocker circuit  510 B comprise a first Zener diode  504 B in series with a second Zener diode  505 B in a back-to-back manner (cathode-to-cathode), and a resistor  503 B connected in parallel with the back-to-back-connected Zener diodes. The back-to-back-connected Zener diodes  504 B,  505 B provide a non-linear circuit whereas the resistor  503 B provides a resistor circuit. The Zener voltages for both Zener diodes  504 B and  505 B may be equal while the sum of the Zener voltage and forward voltage of the diode is selected to be equal to the conduction voltage V c  of the non-linear circuit. In some implementations, the Zener diodes  504 B and  505 B may be connected in another back-to-back manner (anode-to-anode). 
       FIG.  6 A  illustrates an embodiment of the non-linear blocker circuits  440 A,  450 B,  410 C,  410 D. In  FIG.  6 A , the non-linear blocker circuit  610 A is a modified version of non-linear blocker circuit  510 A, with the use of an additional active circuit. The active circuit can also be referred to as an active current buffer, and is for boosting a current drive capability of the non-linear blocker circuit at a given voltage condition. In this embodiment, the active current buffer is implemented by two emitter follower circuit transistors  606 A,  607 A. The transistors  606 A,  607 A are connected across power supply Vcc and negative power supply of the operational amplifier  601 A. The transistor  606 A is connected with diode  604 A whereas the transistor  607 A is connected with diode  605 A for respectively controlling the operation of the diodes  604 A,  605 A. In some implementations, the active current buffer may be implemented by source follower circuit using MOS transistors. 
       FIG.  6 B  illustrates an embodiment of the non-linear blocker circuits  440 A,  450 B,  410 C,  410 D. In  FIG.  6 B , the non-linear blocker circuit  610 B is a modified version of non-linear blocker circuit  510 B, with an additional active circuit. The active circuit can also be referred to as an active current buffer, and is for boosting a current drive capability of the non-linear blocker circuit at a given voltage condition. In this embodiment, the active current buffer is implemented by two emitter follower circuit transistors  606 B,  607 B. The transistors  606 B,  607 B are connected across power supply Vcc and negative power supply of the operational amplifier  601 B. The transistors  606 B,  607 B together control the operation (e.g., conduction) of the Zener diodes  604 B,  605 B. In some implementations, the active current buffer may be implemented by source follower circuit using MOS transistors. 
     The use of active current buffer in the embodiments of  FIGS.  6 A and  6 B  may increase in current driving capability over the original driving of the associated operation amplifier (at the same voltage condition) hence improving the slew rate. 
     As will be readily understood by the person skilled in the art, as used herein, common mode choke (CMC) may be referred to as common mode inductor or common mode inductance; X-capacitor may be referred to as differential mode filter capacitor; Y-capacitor may be referred to as common mode filter capacitor, etc. 
     It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments to provide other embodiments of the invention. The described embodiments of the invention should therefore be considered in all respects as illustrative, not restrictive. 
     For example, the EMI suppression circuit of the invention, such as the embodiments in  FIGS.  3 A to  6 B , can be incorporated into the AC power circuit of  FIG.  2    or other AC power circuits not specifically illustrated or described. The EMI suppression circuit of the invention, such as the embodiments in  FIGS.  3 A to  6 B , may also be suitable for use or be used in DC power circuits, AC-DC power circuits, etc. In various EMI suppression circuit embodiments, a resistor may be implemented using a resistor circuit having among other circuit components one or more resistors; a capacitor may be implemented using a capacitor circuit having among other circuit components one or more capacitors; an inductor may be implemented using an inductive circuit having among other circuit components one or more inductors. In various EMI suppression circuit embodiments, a diode may be implemented using a diodic circuit having among other circuit components one or more diodes, which may be implemented using, e.g., rectifier diode, Schottky diode, Zener diode, diode connected bipolar transistor, and/or MOSFET, as appropriate and applicable; an amplifier may be implemented with an amplifier circuit having, among other circuit components one or more amplifiers such as operation amplifiers. The input filter circuit in the EMI suppression circuit of the invention, such as in the embodiments in  FIGS.  3 A to  6 B , can be high pass filter (any order), notch filter, etc., implemented using appropriate circuit components.