IMPROVEMENTS RELATING TO MAINS RFI FILTRATION

Abstract: A filter circuit for connection between a mains supply line and a load powered by the mains, the filter circuit being capable of shunting to earth, at least within a given frequency band, radio frequency conducted emissions generated by the load, and the filter circuit comprising active shunt means, characterised in that said active shunt means comprises an emitter follower means providing current gain and common base amplifier means connected to control the operation of said emitter follower means.

Description: IMPROVEMENTS RELATING TO MAINS RFI FILTRATION This invention relates to the general field of electromagnetic compatibility (EC), which is concerned with controlling certain unwanted disturbances that can be inserted into the mains electricity supply by electrical circuitry powered by the mains. Such unwanted insertions are commonly referred to as conducted emissions. This is an area of significant concern and much legislation is currently in place or planned in order to ensure that conducted emissions, which can (inter alia) cause inefficiencies and losses in the supply system, are kept within controllable limits. In order to ensure that mains-powered circuitry complies with existing and projected legislation, there is a need to address the principal sources of conducted emissions and to devise circuits that are capable, when interposed between the mains supply and the mains-powered circuitry, of reducing to an acceptable level any conducted emissions attributable to the operation of the mains-powered circuitry. Two significant classes of conducted emission are radio frequency noise (RFI) created by switching phenomena, and harmonic distortion currents resulting from non-linear loads. Whilst both are emission phenomena, they have very different characteristics, and for this reason the test methods and appropriate solutions vary. The present invention relates in particular to the reduction of radio frequency conducted emissions created by load equipment that employs switching (pulse width modulation) techniques, and correlated to the relevant switching frequency, which is typically in the range from 1 kHz to 200kHz, depending upon the application, and is based upon the selective filtering to earth of unwanted RFI. Harmonic distortion noise, on the other hand, is correlated to the fundamental mains frequency (50/60Hz) and is typically addressed by injecting a differential (power mode) current between phases, in an anti-phase fashion, such that offending current harmonics in the power line are nulled. Conducted emission noise appears on the power line in two modes; differential (between the phases) and common (with reference to earth) . The present invention principally addresses common mode noise and as such includes an earth reference circuit and a current shunt circuit providing a return path to earth for the common mode current. By contrast, and as will be deduced from the foregoing, the reduction of mains harmonics concerns only the differential mode, (or more appropriately the power mode). A prior RFI filter circuit, operating in common mode, is disclosed in our International Patent Application No. PCT/GB98/01 897. It employs a unity gain amplifying means whose output, which behaves as a voltage source, is connected directly to either power line and whose input is controlled by a voltage that is substantially near to earth potential over a predetermined frequency band, such that a quasi-earth potential (over the aforementioned frequency band) is achieved on the power line at its point of connection to the unity gain amplifying means. The operation of this circuit is that most of the common mode noise current is shunted from the power line and is returned to earth in such a way as to create a low impedance between power line and earth. The inclusion of a series impedance serves to provide a means of dropping the noise voltage between the noise emission source and the aforementioned connection point of the unity gain amplifying means to the power line. The present invention has similar objectives, and operates upon similar general principles, to the circuit of the aforesaid International patent application, but aims to provide a simpler circuit which is potentially more robust and cheaper to implement in practice. According to the invention there is provided a filter circuit for connection between a mains supply line and a load powered by the mains, the filter circuit being capable of shunting to earth, at least within a given frequency band, radio frequency conducted emissions generated by the load, and the filter circuit comprising active shunt means, characterised in that said active shunt means comprises an emitter follower means providing current gain and common base amplifier means connected to control the operation of said emitter follower means. Preferably the power for said emitter follower means and said common base amplifier means is supplied from said mains supply line through a rectifier device which is also conductive of said RF noise. Preferably further, said active shunt means as a whole presents to said power line an operating characteristic associated with a band pass unity voltage gain amplifying means the output of which is connected to the supply line and the input of which is connected to earth. In order that the invention may be clearly understood and readily carried into effect, one embodiment thereof will now be described, by way of example only, with reference to the accompanying drawings, of which: Figure 1 shows a schematic single-line version of a circuit in accordance with one example of the invention, Figure 2 shows a simplified single-phase schematic of the circuit of Figure 1 , and Figure 3 shows in detail a circuit, in accordance with another example of the invention, suitable for use with three-phase 250 volt AC supplies. Referring now to Figure 1 , a mains supply, schematically shown within a dashed outline 1 , comprises a single ac power line 2 having a line terminal 2a. The mains supply 1 is shown as having an intrinsic common mode RF impedance Zm. Connected to the power line 2, via a load terminal 5a, is a load equipment, schematically indicated by the dashed rectangle 6; the load equipment emitting common mode RF noise with source impedance Zn and source voltage Vn. The RF noise propagates towards the power line 2 from the load equipment 6, but is attenuated, in accordance with this example of the invention, by means of a filter circuit 7 which comprises principally an active shunt module 7a. The filter circuit 7 of this example also incorporates respective line and load series impedances 7b and 7c, connected in series between the terminals 2a and 5a; the point of their interconnection being shown at 8. The components 7b and 7c exhibit impedances Zline and Zload respectively. The active shunt module 7a has two terminals; one connected to the point 8 and the other to earth as shown at 20, and is configured so that its impedance is very low over a specified bandwidth. The overall effect of the shunt module 7a is thus to load the noise emissions of the load equipment 6 such as to create a virtual earth voltage node, over the specified bandwidth, on the power line at the connection point 8. At the same time, the series impedance elements 7b and 7c tend to damp parasitic ringing on the power line, and also provide some further noise attenuation by supplementing the impedances Zm and Zn. The active shunt module 7a comprises two principal active components, namely an emitter follower means 1 2 which exhibits a current gain H2, and a common base amplifier means 1 6 which exhibits a current gain H 1 . Module 7a receives power through a rectifier 9 which is maintained in a state of conduction, and hence has a low impedance, at all times. The configuration of the module 7a is such that the output 1 0 of its emitter follower 1 2 is maintained near to earth potential over the specified bandwidth. Thus, when a voltage excursion, due to noise in the specified bandwidth, appears on the power line at the connection point 8, the emitter follower 1 2 draws a current la through the rectifier 9 and dumps most of that current (lb, which approximately equals la) to earth, via its collector terminal 1 4 and a series component 1 9 having an impedance Zp. The common base amplifier 1 6 has a collector electrode 1 3 which draws a current lc (where lc approximately equals lb/H2) from the base of the emitter follower 1 2 and dumps most of this current (Id, which approximately equals lc) to earth via the emitter 1 7 of the common base amplifier 1 6 and a series component 1 8 having an impedance Ze. Base current le for the common base amplifier 1 6 is derived, via a series component 1 1 having an impedance Zf, from the output 1 0 of the emitter follower 1 2. Given that the base 1 5 and emitter 1 7 of the common base amplifier 1 6 follow one another quite closely (thus V1 5 = V17) then the voltage V10 at the output 1 0 of the emitter follower 1 2 is represented as: V10 = le*Zf + ld *Ze; and, using the approximations given above, V10 can be expressed in terms of la and the transistor current gains as: V10 = la(Zf/H1 H2 + Ze/H2). The impedance between the output 10 of the emitter follower 1 2 and earth is then expressed as: V10/la = Zf/H1 H2 + Ze/H2 The overall impedance Zs of the active shunt circuit 7 between the power line at connection point 8 and earth is thus: Zs = V8/la = Zf/H1 H2 + Ze/H2 + Zr, where Zr is the impedance of the conducting rectifier 9. The noise current lb passed to earth via the component 1 9 of impedance Zp produces a voltage swing equal to Zp*lb. This voltage swing is allowed to be large, allowing Zp to be large, providing the voltage swing does not saturate the emitter follower transistor. The common base amplifier 1 6 provides a very large voltage gain between its emitter (input) and collector (output) terminals, 1 7 and 1 3 respectively. The effect of the component 1 1 of impedance Zf is to close the loop with a negative feedback path such that the voltage gain between the input terminal 17 of the common base amplifier 1 6 and the emitter (output) terminal 10 of the emitter follower 1 2 approaches unity. Referring now to Figure 2, the series impedance components 7b and 7c are formed as common mode chokes, each consisting of one turn through a lossy soft ferrite core. A single phase bridge rectifier 9a replaces the single rectifier diode 9 shown in Figure 1 and provides positive and negative power rails 9p and 9n respectively. As compared with Figure 1 , it will be seen that the active components of the active shunt module 7a are duplicated; and that the module now comprises two complementary halves which operate on alternate polarities of the noise signal. Thus, an emitter follower output is connected to each power rail 9p, 9n with emitter follower 1 2a (comprising transistors Q3 and Q5) being connected to rail 9p and emitter follower 1 2b (comprising transistors Q4 and Q6) connected to rail 9n. Each emitter follower is constructed from a complementary transistor pair to form a quasi Darlington arrangement, which provides an extremely high current gain while imposing only one base-emitter junction. Complementary transistors Q1 and Q2 form respective limbs of the common base amplifier 1 6; their emitters being coupled and connected to the component 1 8 of impedance Ze, and their bases being coupled via a capacitor 22 of capacitance Cb and biassed by respective components 1 1 a and 1 1 b of impedance Zfa and Zfb respectively. These impedances not only provide bias for their respective transistors, but also provide respective feedback paths between the emitter follower outputs from Q3 and Q4 at the rails 9p and 9n and the base electrodes of the common base amplifier transistors. It will be appreciated that the transistors on the positive side of the circuit deal with noise of positive polarity whilst the transistors on the negative side deal with noise of negative polarity. The overall circuit thus works as a push-pull emitter follower, except that the emitters are turned outwards to the dc power rails. It will also be appreciated that the connection of the circuit to the live and neutral ac power rails through the rectifier 9a commutates each half cycle of the mains waveform. For three-phase applications, the low impedance path Zr through the bridge rectifier is maintained at all times since there is always at least one pair of diodes in conduction. In the case of single phase operation, there is a short period, at the mains cross-over point, when the bridge rectifier will present a high impedance and during which the active circuit will cease to operate. This does not compromise the overall effectiveness of the filter, however, since tests have shown that, at the mains cross-over point, no significant common mode noise is emitted by the load equipment. Additionally, a capacitor 21 is utilised to provide some differential mode attenuation, as well as to provide a low impedance path between phases and to provide a path for common mode noise to traverse between phases. Figure 3 shows a three-phase arrangement which is similar in general to that of Figure 2. As will be seen, a three-phase bridge rectifier is used to provide a dc ripple supply voltage to the remainder of the active shunt module circuitry. The diode bridge also provides a low impedance path between the dc voltage rail and the three phases of the power line; the communication pattern of the bridge being such as to ensure that there are always at least two diodes conducting at any one time, one to the positive rail and one to the negative rail. Since the diode conduction is controlled by the mains voltage phase relationship and the same mains voltage is supplied to the emission source (the load equipment), the conduction diodes are always connected to the two working phases; one phase always being redundant at any one time. Typically, transistors Q1 and Q2 have quiescent current of 1 mA, with bias resistors R6 and R7 and diodes D7 - D1 0 providing a diode voltage drop across emitter resistors R3 and R4. Collector resistors R2 and R5 provide a bias voltage of 2.2V to the bases of Q3 and Q4. Emitter resistors R8 and R9 then provide for a 3mA quiescent current in Q3 and Q4. Transistors Q5 and Q6 are biassed off with resistor R1 1 . Resistor divider R10 and R1 2 provides a voltage reference for the collectors of transistors Q3 and Q4 which, being of the nature of current sources, would otherwise allow the voltage to drift. An earth reference is provided through series elements R1 and C4. The RF current drawn from here is fed to the emitter of Q1 (or Q2, depending on polarity) via C5 (or C6 as appropriate). This current passes through Q1 or Q2, and is fed across the parallel combination of the collector impedance R2 (or R5) and the impedance presented at the base of Q3 or Q4, as appropriate. An open loop voltage gain is thus achieved between the emitter and collector of Q1 (or Q2), which is set by the ratio of this parallel impedance and the earth reference elements R1 and C4. Transistors Q3 and Q5; and Q4 and Q6 are connected to form respective, quasi Darlington pairs which work as emitter followers, such that the positive rail follows the base of Q3 and the negative rail follows the base of Q4. The open loop voltage gain achieved at Q1 or Q2 is negated by feed back through bias resistors R6 or R7, such that the positive and negative rails attempt to follow the emitters of Q1 and Q2 respectively. The current drawn by the emitter followers is passed via the emitter of Q5 or Q6 to a capacitor C1 0 which is connected to earth. These emitters simulate a current source since they follow the collectors of Q3 and Q4. Small impedances R1 3 and R14 are included in the emitter paths to provide some local negative feedback. X mode capacitors C1 to C3 are included to provide some differential mode attenuation, as well as to provide a low impedance path for the common mode noise to traverse between phases. Zener diodes D 1 1 and D 1 2 are included to prevent damage to transistors Q5 and Q6 in the event of an excessive voltage swing. Referring back to Figure 1 , the insertion loss is a measure of the attenuation performance of a filter. In decibels, this is given by: IL (dB) = 20 log(Vo/Vo'), where Vo is the noise voltage at the mains impedance Zm before filter insertion and Vo' is that after filter insertion, and logs are taken to base 1 0. The noise voltage Vo (without filter) is: Vo = Vn *(Zm/(Zm + Zn)), but since Zn is usually much larger than Zm this approximates to: Vo = Vn *Zm/Zn. Assuming Zs to be much smaller than all other impedances, the noise voltage Vo' (with filter) is: Vo' = Vn*{Zs/(Zload + Zn)} *{Zm/(Zm + Zline)}. If Zline and Zload are now assumed to be smaller than Zm and Zn respectively, to the extent that it is reasonable to ignore them, a conservative approximation for Vo' is: Vo' = Vn *Zs/Zn. Dividing through gives a conservative approximation for the insertion loss: IL (dB) = 20 log(Zm/Zs). Use of the circuits shown herein has given rise to measured attenuations exceeding 40dB at 1 50kHz, with an active shunt module impedance Zs of around 0.1 ohms. The circuit thus provides an extremely low shunt impedance to ground in its specified frequency band. By comparison, a conventional filter would require a shunt capacitance of 10uF to achieve a comparable shunt impedance, and the presence of such a capacitance between the power line and earth would impose an intolerable earth leakage current of around 1 amp at 250V ac; 50Hz. Furthermore, such a mains class Y capacitor is bulky and costly. The circuit of the invention achieves appreciable attenuation with small series components Zline and Zload. By comparison, conventional filtering normally requires inductors of many turns which exhibit the disadvantage of heat dissipation from core and copper losses as well as bulk and cost. Furthermore, in the circuit of the present invention, one current shunt element acts on all phases simultaneously, so there is no need for multiple elements where more than one phase exists. By comparison, the number of components for conventional filters tends to increase with the number of phases. Although the invention has been described herein by way of reference to specific examples, its scope is not intended to be limited thereby and various modifications of a detailed and/or implementational nature will be evident to those skilled in the art.

Claims: CLAIMS: 1 . A filter circuit for connection between a mains supply line and a load powered by the mains, the filter circuit being capable of shunting to earth, at least within a given frequency band, radio frequency conducted emissions generated by the load, and the filter circuit comprising active shunt means, characterised in that said active shunt means comprises an emitter follower means providing current gain and common base amplifier means connected to control the operation of said emitter follower means. 2. A circuit according to claim 1 wherein said emitter follower means comprises at least one transistor device configured as a current amplifier. 3. A circuit according to claim 2 wherein said emitter follower means comprises a complementary pair of transistor devices connected to form a quasi Darlington arrangement. 4. A circuit according to claim 2 or claim 3 wherein said common base amplifier means comprises a transistor device connected to control the base electrode of a transistor of said emitter follower means. 5. A circuit according to claim 4 wherein said common base amplifier means constitutes a primary amplifier and said emitter follower means comstitutes a secondary amplifier, and wherein the collector of a transistor device of said primary amplifier is coupled to the base electrode of a transistor device of said secondary amplifier and a feedback connection is made from the emitter electrode of said transistor device of the secondary amplifier, constituting the output thereof, to the base electrode of said transistor device of the primary amplifier. 6. A circuit according to any preceding claim wherein the power for said emitter follower means and said common base amplifier means is supplied from said mains supply line through rectifier means which is also conductive of said RF noise. 7. A circuit according to claim 6 wherein said rectifier means comprises a diode bridge arrangement capable of supplying positive and negative power rails, and wherein respective and complementary emitter follower means and common base amplifier means are coupled between each of said rails and earth to accommodate positive-going and negative-going components respectively of said conducted emissions. 8. A circuit according to any preceding claim wherein said active shunt means as a whole presents to said power line an operating characteristic associated with a band pass unity voltage gain amplifying means the output of which is connected to the supply line and the input of which is connected to earth. 9. A mains radio frequency filter circuit substantially as herein described with reference to the accompanying drawings.

Inventors: TUCKER ANDREW CECIL, TUCKER, ANDREW, CECIL
Applicants: ROXBURGH ELECTRONICS LTD, TUCKER ANDREW CECIL, ROXBURGH ELECTRONICS LIMITED, TUCKER, ANDREW, CECIL
Application year: 2000
Main classification: H03H11
IPC codes: H03H11