WO2006008322A2

WO2006008322A2 - Emc filter

Info

Publication number: WO2006008322A2
Application number: PCT/EP2005/053613
Authority: WO
Inventors: Norbert Haeberle; Peter Arthur Kull; Andrew Cecil Tucker
Original assignee: Schaffner Emv Ag
Priority date: 2004-07-23
Filing date: 2005-07-25
Publication date: 2006-01-26
Also published as: EP1619768A1; EP1774652B1; WO2006008322A3; EP1774652A2; US7667988B2; US20070159860A1

Abstract

EMC filter, for connection between a mains supply network and a mains -operated appliance to reduce conduction noise between said supply network and said appliance, comprising a voltage divider (1030) connected to said mains network, for generating a voltage lower than a voltage of said mains network; rectifying means (1031) connected to an output of said voltage divider, for generating a DC voltage (+Vs, -Vs) ; and an electronic active circuit (201), supplied by said DC voltage (+Vs, -Vs) , for absorbing a noise current transmitted between said supply network and said appliance.

Description

Filter

ReId of the invention

This patent application relatesto electrical f ilters for f iltering an unwanted noise component f rom an electrical or electronic circuit or line and more particularly, but not exclusively, to electromagnetic compatibility filtersf or mains linesor distribution lines at mainsf requency.

Description of related art

Electromagnetic compatibility (EMC) is an increasingly critical factor in the electric and electronic industry nowadays. A large number of electric circuits and appliances exist, which are liable to generate unwanted electrical noise, or to suff er f rom noise generated by other circuits or appliances.

The conducted noises and disturbances are generally dealt with by inserting a low-pass LC f ilter on the mains supply line of the noise-generating devices or of the noise-sensitive devices. Such filters attenuate the unwanted f requency componentsto a harmless level. Many filter topologies, including the classic " L, " T' and " pi" f ilter topologies, can be employed.

European Patent application EP1069673 shows an example of a three-phase noise suppression f ilter comprising passive elements.

Passive EMCf ilters have been proven effective in a number of applications. A shortcoming of thistechnique, however, isthat in order to attain the required attenuation level, high-value capacitors and inductances are needed for this application. The size and cost of the resulting filter are mainly determined by these large components, in particular when high attenuation of common mode noise is needed. Moreover, the leakage current isdirectly proportional to the capacity of the filter capacitors It is also known to employ active elementsin noise suppression filters, f or example asin European patent EP0995266. The use of active elements allowsthe use of smaller inductances and capacitors, thereby providing more compact f ilters The existing active filters, however, remain less reliable than their traditional passive homologues, can fail due to overvoltage or overtemperature breakdown, or may exhibit instability and oscillations.

It is an aim of the present invention to provide an EMCf ilter which is more compact and reliable than the known devices.

It is a f urther aim of the present invention to provide an EMC filter combining a high attenuation and a low leakage current.

It is also an aim of the present invention to provide an EMCf ilter with a production cost lower than the known devices

Brief summary of the invention

These aimsare attained by the device which isthe subject of the appended independent claims, optional and additional usef ul features being introduced in the dependent claims

Brief Description of the Drawings

Figure 1 shows a schematic of an EMCfilter according to a f irst aspect of the invention.

Figure 2 shows a schematic of a portion of an EMCf ilter according to another aspect of the invention.

Figures 3a to 3d show equivalent circuits usef ul to the stability analysis of the circuit of the invention. Figure 4 shows, in bloc diagrammatic form, an EMC active filter according to the invention.

Figure 5 shows, in bloc diagrammatic form, an EMC active filter according to an aspect of the invention.

Figure 6 shows a variant of the filter of figure 5 according to another aspect of the invention.

Figure 7 shows an active shunt module of the filter of f igure 5.

Figure 8 shows, in bloc diagrammatic form, a modular EMC active filter according to an aspect of the invention.

Figure 9 displays, in diagrammatic form, a choke module of the filter of figure 8.

Figure 10 displays, in diagrammatic f orm, a capacitor module and a power supply module of the f ilter of f igure 8.

Figure 1 1 displays, in diagrammatic form, an amplif ier module of the filter of figure 8.

Detailed Description of the Invention

Ref erring first to Figure 4, the f ilter circuit of the invention comprises at least one inductive part L1 , L3 in serieswith each power line 11 , I2, I3, 11 ', I2', I3' and one active shunt circuit 10 between each line and a current sink, f or example the earth. The current to be filtered f lows through each inductance f rom the mains supply line, on the left of Figure 4, to the load on the right of Figure 4.

The embodiment illustrated on Figure 4 is built as a T-filter and comprises two current compensated inductances L1 and L3 and an active shunt between the middle point of the inductances and the earth. The active shunt 10 of the invention may however be used in other filter arrangements, f or example in f ilter arrangementscomprising only one inductance in serieswith each power line. The inductive part may comprise any combination of current compensated and not current compensated inductancesin each line.

The active shunt 10 actsas a capacitive bypassf or attenuating high f requency voltage variations, notably noise, between the inductances L1 and L3. It comprises an amplifier 100, preferably a high-voltage; high- power operational amplif ier, a passive network 101 , a star-point circuit 102 and a DCf loating power supply 103.

The star-point circuit 102 includes passive elements, f or example resistors and/or capacitors, connected between each line and a star-point 1010. The voltage at 1010 isfed to the passive network 101 which may comprise, f or example, an arrangement of capacitors and/or resistors The passive network isalso connected to the reference point, for example to the earth point PE, and to the output of the operational amplif ier 100. The passive network 101 isarranged so asto reference one input of the amplifier 100 to the earth potential and the other input to the star-point 1010; the output of the operational amplifier isfed back to the inverting input of the amplif ier 100 and to the capacitor C11.

In thisway the operational amplifier 100 continuously triesto keep the input 1010 at the potential of the earth over the filter bandwidth. The current driven by the amplif ier 100 in order to compensate f or voltage variationsat 1010 isfed f rom the power lines 11 -I3 through the DCf loating power supply 103. Thus any voltage variation at 1010 due to noise in the filter bandwidth will drive current f rom the power linesto earth through the power supply and the capacitor C11.

The DC power supply 103 of the invention comprisesa rectifier 1031 for converting the AC power voltage available on lines L1 -I3 into the DCsupply voltages +Vs, -Vs, available to the operational amplifier 100. The DC power supply 103 of the invention f urther comprises a voltage reducing circuit 1030 f or reducing the DC voltage supplied to the amplifier 100 to a suitable level, and to reduce the power lossthrough this amplifier.

In a preferred embodiment, the DC power supply f urther comprises security elements 1032 for protecting the amplifier against overvoltage, fast or important du/dt or di/dt variations, and/or for detecting and transmitting anomaly conditions

The embodiment of the invention shown on Figure 4 filters common mode noise on the star point 1010. In a various embodiment (not shown), three active filters are used for f iltering diff erential mode noise between each pair of power lines supplies. Each differential mode active filter preferably includes its DC power supply with a rectifier and a voltage reducing circuit, its own operational amplifier 100, and its own passive network 101. The differential mode active filters may f urther include common and/or independent anomaly conditions detections. The three common mode active filters may be used instead or in addition to the differential active filter of Figure 1. Voltage reducing circuit may be provided in the differential mode active filters but not in the common mode active filter, or just the opposite, on in both types of filters

Figure 1 represents a more detailed schematic diagram of an example of EMC active filter according to the invention. Power is fed through the f ilter f rom each line 11 -13, 11 '-13' to load via two inductances, in this case via f errite beads U and L2. A star-point system 102 comprises an X capacitor network C1 - C3 and bleed resistors R1 -R3 to provide diff erential mode attenuation with assistance f rom stray line inductance. The X capacitor network also servesto make a solid RF connection to share common mode current between phases The capacitor network comprises one capacitor C1 - C3 and one resistor between the connection point of the two inductances and one common point 1010 which also serves as an input for the active portion of the f ilter. C4 - C9 form a voltage divider 1030 which servesto reduce the mainsvoltage to a level suitable f or the active elements and which will also reduce power loss. The divider 1030 will also supply only a limited amount of power, for example 290 VAC on the output lines 1041 -1043 instead of 400 VAC, with a limited maximum power so that in the event of a very large noise signal appearing, the voltage will collapse and thusthe circuit will be protected f rom power overload. The voltage divider is advantageously made of capacitive elements for acting mainly on the higher f requency voltage components, for reducing the power losses, and for attenuating high f requency symmetric noise voltages. Other types of voltage dividers, including voltage dividers made of resistors or of other passive or active components, are possible but less advantageous for most applications.

The capacitive elements of the illustrated voltage divider 1030 are connected in delta; a star connection, comprising one capacitor C4-C6 in serieswith each line 1041 -1043 and a second capacitor between each line and a common point, is also possible.

In the illustrated example, the voltage divider 1030 dividesthe ACvoltage bef ore rectification. Voltage dividers acting on the rectified DC voltage are also possible, although thiswould require, among others, larger components in the voltage divider.

Other types of voltage reducing circuits 1030 may be used for supplying the active portion of the f ilter 1 with an adequate voltage. The voltage reduction circuit may also be combined with the rectifier 1031.

Advantageously, the circuit of the invention does not include f uses. The capacitive divider 1030 comprising, in this case, the capacitors C4- C9 limitsthe current to harmless levels, even in the case of a short circuit. By avoiding f uses, the stray inductance of the noise current path can be reduced. A rectifier 1031 comprising a bridge D1 -D6 providesafull rectification of the 3 phase mainsand isused both for providing a DC supply +Vs, -Vsfor the active filter circuit, and asa current path for the noise.

In the presented embodiment, the DC supply for the active part of the filter circuit isobtained by the capacitive divider 1030 and the bridge of diodes D1-D6. Other rectifying means, for example an AC-DC converter, could also be used for thisf unction and are comprised in the scope of the present invention.

The active portion of the filter 1, which we will describe now, is arranged in order to continuously try to keep the input 1010 at the potential of the earth. The current path includesthe amplifier supplying lines+Vs, -Vsand the earth-connected capacitor C11, which actsin the bandwidth of the filter like a very low impedance element due to the active circuit.

R4 and D7 form a voltage reference for the current source Q3. Thisprovidesfor example 1mA bias current to the differential pair Q1 and Q2 whose base terminals form the differential input to the complete amplifier. Q1 and Q2 share the current for current source Q3. The collector of Q1 isa current source which controlscurrent to the positive rail. The collector of Q2 controlscurrent to the negative rail via the current mirror Q4 and Q5.

Q6 and Q9 form a complementary Darlington to drive positive current through C11 while Q7 and Q10 do the same for negative current. Q6 and Q7 are biasto about 1mA, for example. Q9 and Q10are biasfor example to about 10mA by Q8 whose collector-emitter voltage can be set to just about two diode drops. Q1 input isbiased via R23to earth and R11/C10 isa feedback impedence. The circuit is arranged in such a way that C11 followsearth potential at low frequencies (say below 1kHz for example). Resistor / capacitor chains R19,R20,R21 ,R22,C12,C13,C14 and R24,R25,R26,C15,C16 are lag-lead networkswhich ensure a maximum leading base current of 45 degrees (see open loop analysis). C10 provides a dominant pole roll off at high f requencies (say above 50OkHz). Note that the amplifier elements are all de coupled and no bypass capacitors are used. This helpsto ensure no unforeseen phase shifting which may induce resonance.

The circuit 201 f ollowing the rectifier stages 1031 is equivalent, in this embodiment, to the combination of elements l OO and 101 of f igure 4.

The stability of the circuit of f igure 1 will now be analyzed and discussed; the circuitsshown on figures 3a to 3d illustrate this explanation.

Open loop

As visible on figure 3b, representing the open loop behavior of the circuit, a voltage V1 is applied to amplif ier input RC chain. The resulting base current (i) in Q1 and Q2 will be limited to a maximum of 45 degrees leading. The output current H^* i is in phase with the input current, where H isthe overall current gain. This output current passesthrough C1 1 resulting in V2 and effective impedance L of the power input, which includes U and the supply impedance (LISN) resulting in V3. If we ignore V2 for the moment, the stability criterion saysthat V3 must not be in phase with V1.

The maximum phase shift of V3 with respect to the output current (in the power input) will be 90 degrees leading (where L1 and LISN are purely inductive). Theref ore, in the worst case V3 will lag V1 by 45 degrees (i.e. the phase margin issafe at 45 degrees). The voltage V2 always lagsthe output current by 90 degrees It is only important at low f requencieswhere it follows earth potential. At higher f requencies it should be isolated f rom the base inputs

Closed Loop

Ref erring now to the closed-loop equivalent circuit of figure 3a, a noise current I is applied to l_2. This current f lows almost entirely via the amplif ier output (H^* i) and to earth via C1 1. The resulting input current (i) produces a shunt voltage V₁ = Z_in ^* i , where Z_in isthe impedance of the input RC chain. Put in terms of output current V1 = Zj_n ^* I / H.

Turning this around, Zs = V1 / l_noiSΘ = Z_in / H, where Z_s isthe active shunt impedance. So the shunt impedance can be controlled by the ratio of

Figures 3c and 3d represent the behavior of the amplif ier circuit of this embodiment in the case in which the internal f eedback, respectively the external feedback are open.

Typically a shunt impedance of lessthan 10OmR can be created. L1 servesto provide a f urther attenuation by forming a divider with the LISN.

Figure 2 represents another embodiment of a f ilter according to the invention. In this embodiment, only portions of the circuit downstream f rom the points +Vs and -Vs are changed; the voltage divider 1030 and rectifier 1031 used f or supplying a DC voltage +Vs, -Vs are similar to the corresponding elementsshown on Figure 1.

In the embodiment of Figure 2, the gain stages provided by the transistors Q1 -Q10 are replaced by a high-voltage integrated, monolithic operational amplifier 100. A fast, high-voltage and high-power operational amplifier is required for this application, preferably the operational amplif ier should be based on the M OSFET tech no logy. Bipolar circuits, however, are also possible within the f ramework of the present invention. The capacitive divider provided by C4-C9 on f igure 1 limitsthe risk of amplifier f ailure.

The inverting input -Vs of the operational amplif ier 100 is connected, for the f requency interval where the noise is expected, to the star-point 1010. The feedback acts in the sense of maintaining this point to a fixed potential, close to the reference potential. The current path is provided by the operational amplifier 100 and the certified Y capacitor C11 connected to itsoutput. In thisway, the common -mode noise current circulates f rom the lines 11-13, II '- 13' via the impedances L1 or 12, the voltage dividing capacitors C4-C9, the bridge diodes D1 -D6 on figure 1 , the power operational amplif ier 100 and the capacitor C11.

The effect of the active filter circuit isthat the value of the capacitor C11 appears, at noise f requency, many times larger than the impedance of the capacitor itself , f or example 100 times larger. A relatively small capacitor C11 , with a little leakage and volume, can be employed.

According to one aspect of the invention the amplifier 100 and the network 101 of the shunt module 10 may be seen asa transconductance amplifier, which has an output current dependent f rom the input voltage at the star point 102. The transf er f unction isso chosen that very little current isdrawn in the shunt module at DCor at mains f requency, while the shunt module has low impedance in the f requency range where noise attenuation isdesired, typically between 130 kHz and 1 M Hz.

Optimally the impedance in the attenuation range is real, in order to dissipate rapidly the energy of the noise component, without unwanted oscillations. Typically impedancesof 1 ohm or less are obtained.

Due to the very low impedance in the active f ilter, the impedances L1 , L3 can be much lower than in a passive filter of equivalent performances, and these coilscan be kept simple and small, f or example they can consist of a straight wire section in a f errite bead or in a suitable magnetic core, or can contain only a limited number of turns, for example only one or two turns, around a magnetic element, f or example around a ring core. Current-compensated impedances, in which several phases are wound around a common core, are pref erred.

The voltage-clamping transzorbs D7 and D8 provide overvoltage and polarity-reversal protection, whereasthe low-capacitance diodes D11 - D13 are used for input protection, with a minimal power loss. Overloading or oscillation of the operational amplifier 100 automatically implies a drop in itssupply voltage, due to the power limitations imposed by the voltage divider C4-C9, thereby limiting the possible damages. The capacitor C11 is dimensioned to withstand the eventuality of a short circuit of the amplifier output, without the need of security f uses. The T-arrangement of the inductances L1 and l_2 f urther reduces passive du/dt and di/dt values at the input of the amplifier.

The active circuit section 1233 of this embodiment, as visible on figure 23, is equivalent to the combination of elements 101 , 102, and 1032 of the circuit of figure 1.

Other security features can be added in order to f urther protect the active filter of the invention against external disturbances, like f or example lightning, RFdisturbances, electrostatic discharges, electromagnetic pulses, and also against powerline anomalies, like overvoltages, missing phases etc.

A f urther advantage of the device of the invention isthat it does not include transformers at mainsf requency or electrolytic capacitors, which are notoriously unreliable and subject to aging.

The active filter of the invention preferably also comprises elements 1032 for monitoring and detecting anomalous conditions, f or example the collapse of the rail-to-rail voltage at the output of the bridge of diodes D1 -D6, can be used to signal an anomalous condition like, for example, an oscillation or a component f ailure. An anomalous offset at the output or at the input of the amplifier could also be monitored for detecting a failure of a f ilter component.

In the event of an anomaly, detected f or example by a variation in the supply voltage or in the output voltage of the operational amplif ier, an appropriate signal may be sent out in order to trigger corrective actions. The circuit of the invention may comprise a light transducer, f or example a LED, or an acoustic transducer, for example a loudspeaker or a buzzer, for emitting a luminous alarm signal or an acoustic alarm signals.

The active f ilter can optionally include a signal output connection, f or transmitting a status signal to a remote receiver. Such statussignal may comprise, in a simple case, a simple " OK" f lag, indicating that all the monitored parameters are within their respective safe ranges, or more elaborate data, providing detail on various operating parameters like current, power, voltage, noise level and temperature.

Preferably, the signal output connection comprises an insulated output, for example a relay output, a photocoupler output, a light fiber output, or an interf ace f or sending messages over a telecommunication network, f or example over a wireless data network, a field network or a power line communication network. The remote receiver hasthusthe possibility to verify the correct f unctioning within prescribed operating limits of a number of EMC filters according to the invention.

The remote receiver may be a part of a control system, which is set up to take adequate safety measures, in the event of an anomalous condition, like overload, overtemperature, or overvoltage, either according to an automatic program, or at the initiative of an operator.

Optionally, the f ilter of the invention can provide means, f or example a relay, to switch itself off autonomously in case of a detected anomaly, for example if power or temperature admissible f unctioning limits are exceeded, or in case of failure.

Optionally, the DC voltage generated by the voltage divider CA- C9 and the bridge diodes D1 -D6 can be used to generate an auxiliary voltage which is employed for supplying some of the filter's components, for example the monitoring and detection system described above.

Advantageously, the auxiliary voltage so generated may be made available f or powering components external to the f ilter, f or example lamps, batteries, or other electronic circuitry. Preferably, the optional supply is made safe and stabilized to some standard value, like 5, 12, 24 or 48 V, by means of an appropriate DC-DC converter. Snce the auxiliary voltage is galvanically isolated f rom the mains lines, thanksto the voltage divider C4-C9, the risk of f ailures and breakdowns is avoided.

The f ilter of the invention may advantageously be employed as EM C su p pressor at the input or output of f requency converters, as a network f ilter, or for any other device connected to a powerline.

The illustrated embodiment shows a 3-phase filter circuit. Active monophase f ilters including a f requency divider and a rectif ier according to the invention may also be built.

The filter of the invention may also be employed on DC power lines, f or example after the power supply in an electronic appliance, or in automotive applications.

According to another aspect of the invention, represented in figure 5 the filter comprises a first and a second inductive elements L1 , L3, analogically to the previous embodiments, placed in series along the power lines 11 , I2, I3, 11 ', I2\ I3'. In addition to that, the f ilter of the invention also comprises additional inductive elements, L10, L20, whose f unction isto inductively couple the power lines and the shunt circuit, constituted by the power supply 710, by the active modules 702 and 703, which may be identical, and by the capacitor bank 720. To this purpose the inductive elements L10 and L20 have secondary windings inductively linked to the power lines.

To simplify the realization, the turn ratio of L10 and L20 can be

1 :1 , which meansthat the emf voltage of the secondary windings equals that of the primary windings The invention however is not limited to this particular turn ratio. In the example of f igure 5 the inductive element L10 hasa common magnetic circuit to which all the power lines and secondary winding are coupled and is arranged f or maximal transfer of a common mode signal f rom the power linesto the shunt active modules 702 and 703.

At the same time inductive element L20 hasa separate magnetic circuit for each power line and is arranged to provide a shunt path f or differential mode signalstowardsthe capacitor bank 720. Other disposition are however possible and comprised in the scope of the present invention.

Inductive elements L1 , L3, L10 and L20 may be realized by means of magnetic cores, f or example made of ferrite, permalloy, sintered metal or laminated steel, according to a variety of shapes and dispositions available in the art. In a particularly advantageous realization the inductive devices are obtained by passing the power cablesinto magnetic beads

The f ilter represented in figure 5 isa two-stage low-passf ilter with active shunt elements constituted by amplif iers 702 and 703. In fact connecting the second amplif ier 702 behind the inductive device L10 is equivalent to connecting it between L1 and L10 via another bank of 'X' capacitors like in f igure 6a, because the voltage drop on the secondary windingsof L10 isequal or proportional to that present in the power conductors 11 , I2, I3 when traversing L10. The second shunt module 702 thusseesthe same noise that it would experience if connected between L1 and L10 (with a multiplicative factor to the value of L10 in the case of a turn ratio different f rom unity).

In the same way connecting the capacitors 720 at the secondary windingsof the inductive device L20 isthe same, in term of filter response, than placing the capacitors bank 720 between L10 and L20.

The inductive device L10 create virtual shunt nodes H1 , H2, H3, which are electrically equivalent to the section of the phase conductors 11 , I2, I3, respectively, between the chokes L1 and L10. The inductive device L20 create virtual shunt nodes H4, H5, H6, which are electrically equivalent to the section of the phase conductors 11 , I2, I3, respectively, between the chokes L20 and L10.

By 'virtual shunt nodes' are intended, in this application, nodes of an electric network of a power filter, which are equivalent, f or the purpose of connecting shunt elements, to a node of the power line to which they are inductively coupled.

It will be appreciated that the inductively coupled shunt of the invention requires only one galvanic connection point G1 , G2, G3 for each phase of the power cable, and only one bank of " X" capacitors 750 for the coupling of the common mode f ilter stages provided by amplifiers 702 and 703.. This allows for a simpler and more reliable realization, particularly in the case of multi-stage f ilters, like the filter presented in this example.

Figure 7 shows an example of realization of the power supply 710 and of the active module 703. The power supply 710 comprises, like in the previous case, a voltage divider 711 , capacitively realized, f or reducing the voltage and current supplied to the active modules 712 and 703.

The active module 703, which is hasthe same layout asthe active module 702, comprise a power M OSFET output stage

The active modules 702 and 703 are transconductance amplif iers, with an output current dependent f rom the input voltage at the input point SP. The transfer f unction of the active modules 702 and 703 isso chosen that very little current isdrawn at DC or at mains f requency, while the shunt module presents a low impedance in the f requency range where noise attenuation isdesired, typically between 130 kHz and 1 M Hz.

Optimally the impedance in the attenuation range is real, in order to dissipate rapidly the energy of the noise component, without unwanted oscillations. Typically impedances of 1 ohm or less are obtained. Due to the very low impedance module in the active f ilter, the impedances L1 , l_2 can be much lower than in a passive filter of equivalent performances, and these coils can be kept simple and small. The active modules 702 and 703 also induce a much lower leakage current than a conventional capacitor bank.

With ref erence to f igure 6, according to a variant of the invention the active modules 702, 703 and the power supply have been replaced by a f urther capacitor bank 730. This variant of the invention only includes passive elements and could be advantageouswhere a higher leakage current istolerated.

According to another aspect of the invention, represented in figure 8, the f ilter is realized in a modular fashion, comprising a choke module 2001 , a capacitor module, 2002, an amplif ier module 2003 and an optional power supply module 2004. The capacitor module 2002 and the amplif ier module 2003 correspond to the active shunt module of the previous embodiments

With reference to the f igure 9, the choke module 2001 comprises a first and a second inductive elements L1 , L3, analogically to the previous embodiments, placed in series along the power lines 11 , I2, I3, 11 ', I2', I3'. In addition to that, the choke module 2001 also comprises additional inductive elements, L10, L20, L30, whose f unction isto inductively couple the power lines and the shunt circuit, constituted the capacitor module 2002 and by the amplifier module 2003.

In the example of f igures δ and 6, the inductive element L10 and L30 are arranged f or maximal transfer of a common mode signal f rom the power linesto the shunt circuit, whereas inductive element l_2 doesthe same f or differential mode signals Other disposition are however possible and comprised in the scope of the present invention.

As before, the inductively coupled shunt of the invention requires only one galvanic connection point G1 , G2, G3 f or each phase of the power cable. Thisallowsf or a simpler and more reliable realization, particularly in the case of multi-stage filters, like the f ilter presented in this example. Advantageously the choke module 2001 can adopt solid busbar conductors or insulated cablesof adequate gauge for the phases 11 , I2, I3.

Figure 10 showsa capacitor module adapted for coupling with the choke module of f igure 9. The capacitor module includesthe necessary coupling capacitorsf or transferring the noise signal to the amplif ier module, aswell as a bank of capacitor in star conf iguration for the attenuation of the differential mode noise. The capacitor module comprises, in thisvariant, a power supply module 2004 comprising a transformer whose output 2010 isavailable for the supply of the amplifier modulesor of other circuits.

In thisvariant of the invention the supply of the amplifier is provided by a separate transformer 2004 instead of a capacitive divider. Thissolution is preferable when the power dissipated in the active shunt modulesis larger, for example in EMCfiltersf or motorsdriveswith long cables, which have to cope with large noise spikes.

Preferably the transf ormer used is a step-down transformer, which reducesthe line voltage to same convenient value, in order to limit the voltage and power requirement to the active modules.

In power supply module the +Ve and -Ve are imposed between two star pointsof two identical RC star networks, the two networks forming a three-phase bridge. Thisway the DC voltage is completely isolated f rom the ACand vice versa. Also the DC module plays no role in the filtering f unction of the circuit, the noise being separately transmitted to the SPinputsof the amplifiers

Preferably the capacitor module is realized on a PCB (Printed Circuit Board) for a simple and economical assembly by conventional soldering techniques. It should be noted that the capacitor module does not have to handle the rated currentsat mainsf requency. The connection between the choke module 2001 and the capacitor module can be realized by soldering, crimping, or by an electric connector of adequate voltage and current capability. Given the relatively low level of current involved, this connection poses little reliability concerns

Figure 1 1 displays an amplifier module 2003 adapted for coupling with the capacitor module of figure 10. The amplifier module 2003 comprises two amplifiers, 2002 and 2007, of similar construction, for dissipation of the common mode noise component. These amplif ier are preferably realized as hybrid circuitswith SM D components on a AI2O3 substrate. The connection between the amplifier module and the capacitor module isdone by a conventional electrical connector, or by other appropriate connection means

Claims

1. EM C f ilter, for connection between a supply network and a electric operated appliance to reduce conduction noise between said supply network and said appliance, comprising: a rectif ier, for converting an ACvoltage supplied by said mainssupply network into a DC voltage, a voltage reducing circuit, for reducing said DC voltage, an electronic active circuit, supplied by said reduced DC voltage, f or absorbing a noise current transmitted between said supply network and said appliance.

2. The f ilter of the preceding claim, wherein said voltage reducing circuit is connected to said supply network and suppliesto said rectifier an ACvoltage lower than a voltage of said supply network.

3. The f ilter of one of the preceding claims, wherein said voltage reducing circuit comprises a voltage divider.

4. The f ilter of the preceding claim, wherein said voltage reducing circuit is made up of capacitors.

5. The f ilter of one of the preceding claims, wherein said voltage reducing circuit is made up of delta-connected passive elements

6. The f ilter of one of the preceding claims, wherein said rectifier comprises a bridge of diodes.

7. The f ilter of one of the claims 1 to 5, wherein said rectif ier comprises an AC-DC converter.

8. The f ilter of one of the preceding claims, wherein said active circuit comprises an integrated operational amplifier.

9. The filter of the preceding claim, wherein said operational amplifier is a M OSFET amplifier.

10. The f ilter of one of the preceding claims, f urther comprising security elements f or protecting the amplifier against overvoltages, fast or important du/dt or di/dt variations, and/or for detecting and transmitting anomaly conditions.

1 1. The f ilter of one of one of the preceding claims, f urther comprising monitoring means for detecting a malf unctioning of said filter.

12. The filter of the preceding claim, wherein said monitoring means are arranged to detect a variation in said DCvoltage.

13. The f ilter of claim 1 1 wherein said monitoring means are arranged for detecting a variation in an output terminal of an amplif ier comprised in said active circuit.

14. The f ilter of one of the claims 1 1 to 13, wherein said monitoring means generate a statussignal and wherein said status signal is connected to the input of an acoustic or an optical transducer of said filter.

15. The f ilter of one of claims 1 1 to 14, wherein said monitoring means generate a statussignal and f urther comprising a status out put connection, f or transmitting said statussignal to external components.

16. The f ilter of one of the preceding claims, wherein said operational amplifier is a MOSFET amplifier, f urther comprising a supply output connector, connected to said DC voltage.

17. The filter of the preceding claim, f urther comprising an isolated DC-DC converter, said supply output connector being connected to an output of said DC-DC converter.

18. The f ilter of one of the preceding claims, wherein said operational amplifier is a MOSFEET amplif ier, f urther comprising at least one inductor, connected between said supply network and said electric operated appliance, wherein said inductor comprises an essentially straight conductor section in a magnetic core.

19. The f ilter of one of the preceding claims, wherein said operational amplifier is a MOSFEET amplif ier, f urther comprising at least one inductive means, connected between said supply network and said electric- operated appliance, wherein said inductor comprises one or two turns, around a magnetic element.

20. The f ilter of one of the preceding claims, f urther comprising a star-point circuit including passive elements connected between each power supply line of said supply network and a star-point, wherein the current f lowing through said electronic active circuit depends on the potential on said star-point circuit.

21. The f ilter of one of the preceding claims, comprising at least one inductive device (L10, L20) connected to the power supply lines in a manner to transfer inductively the noise current to the electronic circuit.

22. The filter of the preceding claim, in which said inductive device (L10, L20) comprises secondary windings, inductively coupled to the power lines.

23. The filter of the preceding claim, in which said inductive device (L10, L20) comprises a common magnetic circuit coupled to all phases of the power line, for coupling a common mode noise current.

24. The filter of claim 22, in which said inductive device (L10, L20) comprises independent magnetic circuits individually coupled to each phase of the power line, for coupling a differential mode noise current.

25. The f ilter of one of the preceding claims, wherein the electronic circuit is connected to a virtual shunt node (H1 -H6) inductively associated to a node of the conductors lines (11 , I2, I3) of the supply network.