Electromagnetic compatibility filter

Abstract

A two stage tuneable notch filter is presented for removing unwanted noise signals in high power systems. The filter comprises a pass stage and a stop stage, either or both of which may contain tuneable elements. A variety of tuning mechanisms are presented. The system may be controlled dynamically so as to tune the system in real time as a function of detected noise signals by a controller incorporated in the system.

Description

[0001] Electromagnetic Compatibility Filter

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a filter for Electromagnetic Compatibility Filtering on power supply systems, in particular for high current applications.

[0004] BACKGROUND PRIOR ART

[0005] The constant advancements of power electronics have led to the widespread introduction of an increasing variety of non-linear loads. Together with increased efficiency, this technological progress has led to a significant increase in the level of conductive and radiated emissions, including in the 2-150 kHz range. This electromagnetic interference (EMI), in turn, adversely affects sensitive equipment, which can result in operation and communication problems. Using passive low-pass EMI filters can significantly reduce the noise level within the system. However, there is a shortage of EMI filters on the market for noise suppression in this frequency range because existing civil EMI standards regulate the level of accepted emission starting at 150 kHz, and only limited standards require the regulation from 2 kHz. Furthermore, the key issue of low-pass filter design for the low-frequency range is that to provide a low cut-off frequency the inductor and / or capacitor require high values. Most modern electronic devices include a certain amount of filtering to comply with Electromagnetic Compatibility (EMC) regulations. Although a lot of research has been conducted on the design and optimization of Electromagnetic Interference (EMI) filters, there is still no golden rule for designing or applying a filter for each specific case. Thus, trial and error methods are often used to select a filter that will provide a desirable attenuation. In this case, the inductor will become large and heavy, and the capacitor will have a high parasitic resistance and hence faster aging.

[0006] It is accordingly desired to develop new power EMC circuits offering satisfactory suppression characteristics while offering improvements in, in particular, cost, bulk or weight. SUMMARY OF THE INVENTION

[0007] According to the present invention in a first aspect there is provided a notch filter comprising a Band Stop section and a Band Pass section, wherein said Band Stop section comprises a first component grouping comprising a first capacitance component and a first inductance component in parallel, in line with a filtered line, and wherein said Band Pass section comprises a second component grouping comprising a second capacitance component and a second inductance component in series, coupling said filtered line and a reference line, and wherein a capacitance or an inductance of a respective said component of at least one of said component groupings is tunable under the control signal received from a controller.

[0008] According to a development of the first aspect, the notch filter is adapted to filter a line carrying one ampere or more of current.

[0009] According to a development of the first aspect, one said component in said first component grouping is continuously tunable under the control of a controller, and one said component in said second component grouping is tunable by being switchable to one of a plurality of discrete values, or wherein one said component in said second component grouping is continuously tunable under the control of a controller, and one said component in said first component grouping is tunable by being switchable to one of a plurality of discrete values.

[0010] According to a development of the first aspect, said first capacitance component and said second capacitance component are tunable.

[0011] According to a development of the first aspect, said first inductance component and / or said second inductance component is tunable by being switchable to one of a plurality of discrete values.

[0012] According to a development of the first aspect, the first inductance component and / or said second inductance component is switchable to one of a plurality of discrete values by switchably coupling one or more selected inductance sub components in a subcomponent bank such that the cumulative induction of the switched devices in said subcomponent bank provides the desired overall value.

[0013] According to a development of the first aspect, one or more said inductive components comprises a multi tap inductance component comprising a plurality of windings with a plurality of tap connections at respective windings such that the induction of a said multi tap inductance component can be determined by switchable coupling said multi tap inductance components via a selected pair of said taps to provide a desired inductive value.

[0014] According to a development of the first aspect, the first capacitance component and / or said second component is tunable by being switchable to one of a plurality of discrete values

[0015] According to a development of the first aspect, the first capacitance component and / or said second capacitance component is switchable to one of a plurality of discrete values by switchable coupling one or more selected capacitance sub components in a subcomponent bank such that the cumulative capacitance of the switched devices in said subcomponent bank provides the desired overall value.

[0016] According to a development of the first aspect, the first capacitance component and / or said second capacitance component is tunable by adjusting a DC bias voltage thereof.

[0017] According to a development of the first aspect, the first capacitance component and / or said second capacitance component comprises a dielectric material exhibiting a self polarization behaviour reversible by the application of said DC bias voltage.

[0018] According to a development of the first aspect, the first capacitance component and / or said second capacitance component comprises a dielectric material comprising BaTiOa- According to a development of the first aspect, the first inductance component and / or said second inductance component is tunable by controlling the degree of saturation in a core thereof by modulation of the current in a secondary coil thereof.

[0019] According to a development of the first aspect, the notch filter further comprises a controller adapted to receive a signal characteristic of a signal on said filtered line, and to determine one or more control signals suitable to tune each said tunable capacitance component or inductance component so as to achieve a desired modification in said signal characteristic, and to emit said one or more control signals to said respective said tunable capacitance component or inductance component.

[0020] According to a development of the first aspect, the signal characteristic is a frequency response.

[0021] According to a development of the first aspect, the steps of receiving, determining and emitting are performed continuously and in real time.

[0022] In accordance with the present invention in a second aspect there is provided a controller for a notch filter for performing EMC filtering, said controller being adapted to receiving a signal on a filtered line, determine control signals suitable to tune each tunable capacitance component or inductance component of a notch filter comprising a Band Stop section and a Band Pass section, wherein said Band Stop section comprises a first component grouping comprising a first capacitance component and a first inductance component in parallel, in line with a filtered line, and wherein said Band Pass section comprises a second component grouping comprising a second capacitance component and a second inductance component in series, coupling said filtered line and a reference line, and to emit said one or more control signals to said respective said tunable capacitance component or inductance component.

[0023] In accordance with the present invention in a third aspect there is provided computer implemented method of performing EMC filtering comprising receiving a signal on a filtered line, determining control signals suitable to tune each tunable capacitance component or inductance component of a notch filter comprising a Band Stop section and a Band Pass section, wherein said Band Stop section comprises a first component grouping comprising a first capacitance component and a first inductance component in parallel, in line with a filtered line, and wherein said Band Pass section comprises a second component grouping comprising a second capacitance component and a second inductance component in series, coupling said filtered line and a reference line, and emitting said one or more control signals to said respective said tunable capacitance component or inductance component.

[0024] In accordance with the present invention in a fourth aspect there is provided a computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the method of the third aspect

[0025] In accordance with the present invention in a fifth aspect there is provided computer- readable medium comprising instructions which, when executed by a computer, cause the computer to carry out the method of the third aspect.

[0026] BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The invention will be better understood and its various features and advantages will emerge from the following description of a number of exemplary embodiments provided for illustration purposes only and its appended figures in which:

[0028] Figure 1 shows a filter architecture as known in the art;

[0029] Figure 2a shows a first tunable filter variant;

[0030] Figure 2b shows a second tunable filter variant;

[0031] Figure 2c shows a third tunable filter variant;

[0032] Figure 2d shows a forth tunable filter variant;

[0033] Figure 3a shows a fifth tunable filter variant;

[0034] Figure 3b shows a sixth tunable filter variant;

[0035] Figure 3c shows a seventh tunable filter variant;

[0036] Figure 3d shows an eighth tunable filter variant;

[0037] Figure 4 presents an example of a non-continuously tunable capacitance sub-circuit;

[0038] Figure 5presents an example of a non-continuously tunable Inductance sub-circuit;

[0039] Figure 6 shows an alternative, continuously tunable capacitance configuration; Figure 7 shows the behaviour of the tunable capacitor circuit of figure 6;

[0040] Figure 8a shows an alternative continuously tunable inductance configuration;

[0041] Figure 8b represents the behaviour of the tunable inductance of figure 8a;

[0042] Figure 9 presents a method in accordance with an embodiment;

[0043] Figure 10 presents a complete system according to an embodiment; and Figure 11 presents a complete system according to an embodiment.

[0044] DETAILED DESCRIPTION OF THE INVENTION

[0045] A notch () filter for EMI filtering is proposed in divergence from most prior art solutions. As such, narrow-band emissions may generally be target, for example at 100 kHz, as opposed to the broad spectrum approach of general low pass filters. Embodiments offer high attenuation can be provided with very small and lightweight components.

[0046] Notch filters provide high attenuation for narrow specific frequency bands, making them very application-specific. The expected interference may be determined to adapt the filter to these conditions in the design stage. With proper component selection, the weight and volume of such a filter can be reduced several times compared to the use of a conventional lowpass filter, and a higher effective attenuation value for the detrimental interference can be achieved.

[0047] Figure 1 shows a filter architecture as known in the art.

[0048] As shown in figure 1 , there is provided a two-stage filter, comprising a combination of a stop-stage 120 (band-stop filter) and a pass stage 130 (band pass filter). As shown, the stop stage comprises an inductance 121 and a capacitance 122 in series between the filtered line input 101 and a reference line 102, which may for example be a ground line or another line. Meanwhile, the Pass Stage comprises an inductance 131 and a capacitance 132 in a parallel configuration, the two components in parallel being situated in line in a phase in between the filtered line input 101 and the filtered line output 103. As shown, a load is represented schematically by load impedance 140 coupled across the filtered line output 103 and the reference line 102. As shown, a signal source representing the time variant noise signal to be filtered is represented schematically power source 111 and input impedance 112 coupled in series across the filtered line input 101 and the reference line 102.

[0049] It may be borne in mind that the circuit of figure 1 is a schematic representation of an ideal circuit, in which parasitic effects may be disregarded, or lumped together with the components shown.

[0050] It may be borne in mind that the circuit of figure 1 is a particular schematic representation of a circuit, and that there may exist many different representations that are electrically equivalent.

[0051] The use of the notch filter is a good solution when the unwanted noise is expected only at a single frequency because it can provide higher attenuation with lower costs and size.

[0052] The frequency response of the filter is defined by specifying suitable values for the frequency dependent components. 121 , 122, 131 , 132. The total performance of such a filter consists of the performances of each stage. The filter performance is typically characterized by a Q-factor that depends on the square root of the ratio of inductance to capacitance and the active resistance. For stop-stage, the Q factor is given by

[0053] Where Css is the capacitance value of 122, and Lssis the inductance value of 121. For stop-stage, the Q factor is given by

[0054] Where CPs is the capacitance value of 132, and Lssis the inductance value of 131. The impedance R presents an equivalent impedance within the system, including the parasitic impedance of elements and load / source impedances. From the perspective of the filter performance, the Q-factor defines the sharpness of the LC circuit resonance and can be defined by where BW is the bandwidth of the band-stop and fris the resonant frequency of the resonant circuit. The resonant frequency for both stages is defined by an identical equation

[0055] Therefore, to enable adaptation of the filter performance to changing conditions (in particular, the resonant frequency), some filter elements should be adjustable or, in other words, tunable. For the best performance on the notch filter, both stages should be adjusted at the same frequency.

[0056] In view of the fact that a Notch filter is limited to filtering out EMI in a narrow frequency band, it is an object of the invention to provide for a circuit adapted to be tunable to a desired frequency.

[0057] To this end, there is provided a notch filter comprising a Band Stop section and a Band Pass section, wherein said Band Stop section comprises a first component grouping comprising a first capacitance component and a first inductance component in parallel, in line with a filtered line, and wherein said Band Pass section comprises a second component grouping comprising a second capacitance component and a second inductance component in series, coupling said filtered line and a reference line (a return or other line), and wherein a capacitance or an inductance of a respective said component of at least one of said component groupings is tuneable under the control signal received from a controller.

[0058] The controller may intervene to tune the filter once or at a limited number of times, e.g. in the factory. The controller need not be automatic in all embodiments, but may alternatively comprise an interface to control the tuning settings.

[0059] Figures 2a, 2b, 2c, 2d present possible implementations of such a filter. In particular, figure 2a shows a first tunable filter variant.

[0060] As shown in figure 2a, the filter comprises the same components of figure 1 , with corresponding components bearing like references, the exception being the replacement of fixed value inductance 121 with tunable inductance 221 .

[0061] Figure 2b shows a second tunable filter variant.

[0062] As shown in figure 2b, the filter comprises the same components of figure 1 , with corresponding components bearing like references, the exception being the replacement of fixed value capacitance 122 with tunable capacitance 222.

[0063] Figure 2c shows a third tunable filter variant.

[0064] As shown in figure 2c, the filter comprises the same components of figure 1 , with corresponding components bearing like references, the exception being the replacement of fixed value capacitance 132 with tunable capacitance 232.

[0065] Figure 2d shows a forth tunable filter variant.

[0066] As shown in figure 2d, the filter comprises the same components of figure 1 , with corresponding components bearing like references, the exception being the replacement of fixed value inductance 131 with tunable inductance 231 .

[0067] Preferably, at least one component in each component grouping may be tunable, so as to maintain a common resonant frequency between the two groupings.

[0068] Figures 3a, 3b, 3c, 3d present possible implementations of such a filter.

[0069] In particular, figure 3a shows a fifth tunable filter variant.

[0070] As shown in figure 3a, the filter comprises the same components of figure 1 , with corresponding components bearing like references, the exception being the replacement of fixed value inductance 121 with tunable inductance 321 a, and of fixed value inductance 131 with tunable inductance 331 a.

[0071] Figure 3b shows a sixth tunable filter variant.

[0072] As shown in figure 3b, the filter comprises the same components of figure 1 , with corresponding components bearing like references, the exception being the replacement of fixed value capacitance 122 with tunable capacitance 322a, and fixed value capacitance 132 with tunable capacitance 332a.

[0073] Figure 3c shows a seventh tunable filter variant.

[0074] As shown in figure 3c, the filter comprises the same components of figure 1 , with corresponding components bearing like references, the exception being the replacement of fixed value inductance 131 with tunable inductance 331 and fixed value capacitance 132 with tunable capacitance 332b.

[0075] Figure 3d shows an eighth tunable filter variant.

[0076] As shown in figure 3d, the filter comprises the same components of figure 1 , with corresponding components bearing like references, the exception being the replacement of fixed value capacitance 122 with tunable capacitance 322b, and of fixed value inductance 131 with tunable inductance 331 b.

[0077] It will be appreciated that still further variants are possible, in which any three of the four frequency dependent components 121 , 122, 131 , 132 are replaced with tunable components, or indeed where all four frequency dependent components 121 , 122, 131 , 132 are replaced with tunable components, The proposed filter is particularly suited to high power applications (e.g. where the current carries is greater than or equal to one ampere), in particular because the number of frequency dependent components may be less than required for high performance band pass filters, and the values of these components may be in more convenient ranges, and that these considerations, together with the greater cost and bulk of corresponding components for high current applications are correspondingly greater, all other things being equal, this approach makes for a lighter, smaller and cheaper circuit for equivalent performance. In such applications, additional adaptations in terms of component selection, circuit insulation, system cooling and the like may be expected.

[0078] The embodiments of figures 2 and 3 call for one or more tunable components. While certain tunable components are known per se, it cannot be assumed those tunable components that are known in one field of application are necessarily applicable in the field of the present invention. For example a tunable capacitor in a radio tuner will not necessarily be appropriate for the high power EMC application of the present invention. Indeed most electrically variable capacitors and inductors are designed for high-frequency, low-voltage, and low-current applications only.

[0079] A number of examples of tunable devices suitable for incorporation in embodiments will now be presented.

[0080] Generally speaking, two classes of tunable devices may be envisaged- devices may be continuously tunable to any arbitrary value within a given range, or may be tuned to one of a number of discrete predefined values. Capacitors and inductors of each of these classes will be presented by way of example below.

[0081] Figure 4 presents an example of a non continuously tunable capacitance sub-circuit. One of the most accessible ways to change the capacitance value is to switch with capacitors in series and in parallel, for example as shown in figure 4. Specifically as shown, there is provided a pair of capacitors 411 and 412 connected in series between an input rail 401 and an output rail 402. A first switch 421 is connected in parallel with capacitor 411 , such that capacitor 411 can be effective short circuited leaving only capacitor 412 as effective in the series. Meanwhile there is provided a capacitance 413 connected in parallel with the first pair of capacitors 411 , 412 across the input rail and output rail, but in line with a second switch 422, such that capacitor 413 may be selectively switched into or out of the circuit depending on the position of the switch. Meanwhile there is provided a capacitance 414 connected in parallel with the first pair of capacitors 411 , 412 across the input rail and output rail, but in line with a third switch 423, such that capacitor 414 may be selectively switched into or out of the circuit depending on the position of the switch.

[0082] Difference combinations of switch settings may provide a wide range of resulting effective capacitance. By suitable selection of the values of capacitors 411 , 412, 413 and 414, the set of possible values is defined.

[0083] It will be appreciated that any number of capacitors and switches may be provided, in serial or parallel configurations, or combinations of each, so as to provide and desired number of possible configurations and possible values.

[0084] The switches may be implemented in any convenient manner, including conventional manual mechanical switches, switchable microelectromechanical systems (MEMS), solid state switches (transistors...), addressable fuses, etc.

[0085] Accordingly, the notch filter as presented herein may comprise the said first capacitance component and / or said second capacitance component as switchable to one of a plurality of discrete values by switchably coupling one or more selected capacitance sub components in a subcomponent bank such that the cumulative capacitance of the switched devices in said subcomponent bank provides the desired overall value.]

[0086] Although not shown in a figure, an equivalent approach may be used to define an inductance component switchable to one of a plurality of discrete values by switchably coupling one or more selected inductance sub components in a subcomponent bank such that the cumulative induction of the switched devices in said subcomponent bank provides the desired overall value.

[0087] Figure 5 presents an example of a non continuously tunable inductance sub-circuit. The inductance of a conductive coil is defined by the equation where pO is a permeability of free space, p is a relative permeability of the used magnetic material, N is a number of turns, Aeis an effective core area, and le is an effective magnetic path length based on the core sizes.

[0088] Thus the inductance changes as the square of the number of turns, such that an inductive coil may be provided with a plurality of tap connections at respective windings such that the induction of the component can be determined by switchable coupling the multi tap inductance components via a selected pair of the taps to provide a desired inductive value.

[0089] An example of such a component is presented in figure 5. As shown, a coil 501 is provided with connections at each extremity and four intermediate taps as positions along its length. One of the extremity connections constitutes an input for the device as a whole (reference 500), and the remaining five connections are switchable connectable to the output by respective switches 521 , 522, 523, 524. By closing one of these switches while leaving the others open, any of five different inductance values may be obtained. By defining the number of loops, the positions of the taps, and the other physical characteristics as discussed above, the desired number of possible values, and the values themselves may be chosen.

[0090] Figure 6 shows an alternative, continuously tunable capacitance configuration.

[0091] Capacitors come in many types, each suited to a particular use. Multilayer Ceramic Capacitors (MLCCs) are one such category. These capacitors are used in particular because of their small size, high capacitance density, low equivalent series resistance (ESR) and low cost and especially suited for high power (high current and high voltage) applications. These capacitors are usually used as decoupling capacitors and in smoothing filters, to remove ripple and noise. MLCCs of certain types may suffer from the issue that an applied DC bias voltage can drastically decrease its capacitance. This issue is usually mitigated by the manufacturer by providing a larger capacitance at no bias voltage, such that the required capacitance will be assured at the operating voltage.

[0092] As such, there may be provided a notch filter wherein the first capacitance component and / or the second capacitance component comprises a dielectric material exhibiting a self-polarization behaviour reversible by the application of said DC bias voltage.

[0093] A dielectric material used in these kinds of capacitors is BaTiO3, which has a very high dielectric constant at room temperature. This can be mainly attributed due to the so called self-polarization that this material exhibits. This material’s electrons align themselves when an external electric field is applied. When an AC voltage is presented, this means that the polarization flips with a period equal to that of the signal. This behaviour means that the apparent dielectric constant of the material is very high. The reason that the capacitance goes down when a DC voltage is applied is not because of this spontaneous polarization that this specific dielectric material exhibits, but because this DC bias voltage can reverse this polarization. This bias voltage limits the amount of polarization of the dipoles, which means that the apparent dielectric constant of the material will go down, which in turn reduces the capacitance.

[0094] In accordance with embodiments meanwhile, this characteristic may be used as a mechanism for implementing capacitor tuning, offering a large tuning range, a big range of possible capacitance values and a higher Q-factor in the filter, and can possibly handle a lot of power.

[0095] As shown in figure 6, a capacitor of a type exhibiting a change in capacitance in response to a DC bias voltage 611 is provided, optionally in association with a further offset capacitor 610 of fixed value serving to set the lower limit of the effective overall capacitance of the circuit. Bias circuitry 620 is provided coupled across the tunable capacitor 611 , which as shown schematically comprises a variable power supply 621 and load resistance 622, such that a desired DC voltage may be established across capacitance 611 by appropriate setting of the power supply. It will be appreciated that real DC bias circuits may be of any design as will readily occur to the skilled person.

[0096] Figure 7 shows the behaviour of the tunable capacitor circuit of figure 6.

[0097] Figure 7 plots the relationship between the capacitance of capacitor 611 on the y axis as a function of the Bias voltage provided by bias circuit 620 on the x axis.

[0098] As shown, the capacitor exhibits a hysteresis, with a rising bias voltage giving higher overall values than a falling bias voltage. The relationship nevertheless shows strong linearity, and a tunable range in the order of 100% (form 40nF to 90nF for a bias voltage from 0 to 40 volts in the particular example shown), albeit with degraded linearity at extremes.

[0099] In the field of variable inductors, electrostatically controlled magnetoelectric inductors are often considered to be relevant for electromagnetic compatibility (EMC) applications. However, the required high electric field value is up to 14 kV / cm. may constitute a disadvantage for some implementations.

[0100] Figure 8a shows an alternative continuously tunable inductance configuration.

[0101] A class of tunable inductor circuits may be based on a saturation process. An example of such a circuit is shown in figure 8a. As shown, there is provided a pair of coils 811 , 813, with a common core element 812. The first coil 811 is connected as the tunable inductor component, for example as shown in any of figures 2 or 3, while the second inductor 813 serves to control the saturation of the core 812 by DC bias current, and thereby influence the effective inductance of the first coil 811 .

[0102] As shown, the bias circuit 820 comprises DC power supply 821 and programmable load 822, used to inject the current through the DC coil. The parasitic resistance of the coil is assumed to be negligibly low. An inductor with a nanocrystalline core may be used to reduce the influence of the impedance of the bias circuit on the output impedance of the tunable inductor. For operation, the inductor must be characterized in terms of the inductance variation due to the saturation of the core on which it is wound. For this purpose, a DC bias current is injected into the DC coil via a DC power supply, and its magnitude is adjusted using a programmable electronic source.

[0103] As such, the first inductance component and / or said second inductance component may be tunable by controlling the degree of saturation in a core thereof by modulation of the current in a secondary coil thereof.

[0104] Figure 8b represents the behaviour of the tunable inductance of figure 8a

[0105] In Figure 8b, the effective inductance of inductor 811 is plotted on the y axis, against the DC bias current imposed in inductor 813 by bias circuit 820.

[0106] As shown, in this particular scenario the inductance varies between 100 and 400 pH, with a saturation current between 0 and 2 Amperes.

[0107] This particular example is based on the use of a nanocrystalline core allowing the achievement of inductive behaviour of the inductor at frequencies close to 100 kHz.

[0108] The inductance values are obtained through the measurement of the impedance of the secondary inductor and its phase.

[0109] The dashed line indicates that the inductance has decreased by 30% of the initial value, which is generally considered to mean that the core has reached the saturation point.

[0110] As the saturation current decreases, the change in inductance will follow a different trajectory due to the hysteresis effect.

[0111] Accordingly, various tunable frequency dependent component implementations have been presented. The skilled person may select any of these, or others as may be known in the art, for incorporation in any of the filter circuits envisaged herein, in any combination. For example one said component in said first component grouping may be continuously tunable under the control of a controller, or in a closed loop automatic system, for instance based on a current measurement in the notch section, and one said component in said second component grouping be tunable by being switchable to one of a plurality of discrete values, or one component in the second component grouping may be continuously tunable under the control of a controller, and one said component in said first component grouping be tunable by being switchable to one of a plurality of discrete values.

[0112] Dynamic tuning

[0113] As discussed generally above, there is provided a tunable notch filter.

[0114] The use of a notch filter has significant advantages in cases where EMI is expected at a certain frequency and EMC has to be achieved. Notch filters provide high attenuation for a narrow specific frequency band.

[0115] The advantage of the notch filter is also directly associated with its main disadvantage. Due to the very narrow operation frequency band of the notch filter, it ceases to be effective if the emission frequency of the noise source is time-variant. The conducted emission spectrum can change during the operation time, and these variations can be either periodic changes or permanent variations of the dominant emission frequency.

[0116] A permanent shift in the dominant emission frequency may be the result of temporary or permanent degradation of the converter components during operation. In addition, during the operation of the filter, the condition of its components may change such as the capacitors that tend to degrade due to aging. That in turn will lead to a change in their values and, accordingly, to a shift in the resonance frequency thus lowering the filter performance. It will be appreciated that while embodiments include circuits where tunable elements are adjusted manually, other embodiments may advantageously include an automated control of the tunable component values.

[0117] The ability of the filter to adjust itself to changing operating conditions will, therefore, significantly increase its efficiency.

[0118] This approach is applicable to any of the tunable circuits and tunable component types presented above.

[0119] On this basis, there may be provided a controller adapted to receive a signal characteristic of a signal on the filtered line, and to determine one or more control signals suitable to tune each said tunable capacitance component or inductance component so as to achieve a desired modification in the signal characteristic, and to emit said one or more control signals to the respective tunable capacitance components and / or inductance components.

[0120] The received signal characteristic may comprise an instantaneous measurement of the frequency response of the filter system.

[0121] The received signal characteristic may comprise an instantaneous measurement of the signal frequency profile of the input or output of the filter system.

[0122] The received signal characteristic may comprise an instantaneous interference signal frequency as extracted from, for example the signal frequency profile of the input or output of the filter system. This instantaneous interference signal frequency may be derived by the controller itself.

[0123] The controller may comprise a model (that is to say for example, a mathematical or algorithmic definition of the notch filter such that necessary component values can be programmatically resolved on the basis of an input of current and desired system conditions) of the filter system, so as to determine the necessary value of each tunable component for a desired notch filter resonant frequency (corresponding for example to the instantaneous interference signal frequency), and to emit control signals to each tunable component accordingly.

[0124] The controller may comprise a lookup table of precalculated tunable component values, so as to determine, by reference to the table, the necessary value of each tunable component for a desired notch filter resonant frequency (corresponding for example to the instantaneous interference signal frequency), and to emit control signals to each tunable component accordingly.

[0125] In either case, the controller should be able to determine component values taking into account time variant effects such as the hysteresis effects mentioned above, the settle times of tunable devices, etc. as applicable.

[0126] The operations of receiving signal characteristics, determining tunable component values and emitting control signals may be performed continuously and in real time, so the notch filter can, for example continuously adapt to changing interference characteristics, such that whatever those characteristics may be at any given time, the resulting signal is always clean at the tuned frequency.

[0127] The controller may be implemented in one or more hardware components. Such hardware components may include analogue elements such as a phased-locked- loop, and or logic elements such as field-programmable gate arrays (FPGAs), program- and application-specific integrated circuits (PASIC / ASICs), program- and application-specific standard products (PSSP / ASSPs), system-on-a-chip (SOC), and complex programmable logic devices (CPLDs), for example.

[0128] Figure 9 presents steps of a method of operation of a controller according to an embodiment.

[0129] As shown, the method starts at step 900, before proceeding to step 910 at which signal characteristics as described above are received. The method then proceeds to step 920 at which suitable control signals suitable to tune each tunable capacitance component or inductance component of a notch filter as presented above are determined, and then proceeds to step 930 at which the one or more control signals are emitted to the respective said tunable capacitance component or inductance component. The method then terminates at step 940 as shown, although the method may preferably continually loop back to step 910 for continuous real time operation.

[0130] The operation of determining may be performed with reference to a model or look up table for example as discussed above.

[0131] The method may be implemented by a suitably programmed computer, and may be embodied in a computer program for such a computer, or a computer readable medium bearing such a computer program.

[0132] The term “program” may be used to describe an aspect of computing system implemented to perform a particular function. In some cases, a program may be instantiated via logic device executing machine-readable instructions. It will be understood that different modules may be instantiated from the same application, service, code block, object, library, routine, API, function, etc. Likewise, the same program may be instantiated by different applications, services, code blocks, objects, routines, APIs, functions, etc. The term “program” may encompass individual or groups of executable files, data files, libraries, drivers, scripts, database records, etc.

[0133] A computer readable medium may comprise for example one or more types of storage device including optical memory (e.g., CD, DVD, HD-DVD, Blu-Ray Disc, etc.), semiconductor memory (e.g., FLASH, RAM, EPROM, EEPROM, etc.), and / or magnetic memory (e.g., hard-disk drive, floppy-disk drive, tape drive, MRAM, etc.), among others. Storage device may include volatile, non-volatile, dynamic, static, read / write, read-only, random-access, sequential-access, location-addressable, file- addressable, and / or content-addressable devices.

[0134] It will be appreciated that a storage device may include one or more physical devices, and excludes propagating signals per se. However, aspects of the instructions described herein alternatively may be propagated by a communication medium (e.g., an electromagnetic signal, an optical signal, etc.), as opposed to being stored on a storage device. As such, the controller may be wholly or partially implemented in software.

[0135] By this means, a relatively inexpensive, low bulk circuit can achieve the same benefits of a much larger and more expensive band pass filter.

[0136] It may be imagined that in scenarios where interference is expected at multiple separate frequencies, multiple notch circuit filters as discussed herein may be provided in series, with each circuit being controlled to address a particular interference frequency, so that the combined effect is to remove all interference bands. Multiple notch circuit filters may be controlled by a single controller as presented above. The multiple notch circuit filters may be of different respective configurations selected from the various options presented herein as may be more suited to particular frequencies for example. On this basis, the controller may preferentially control respective notch circuit filters to address the interference frequencies for which they are best suited, so as to achieve the best possible overall system performance.

[0137] Figure 10 presents a complete system according to an embodiment.

[0138] Of the four filter elements, the most suitable for tuning may be the capacitor in the stop-stage CSS and the inductor in the pass-stage LPS. The reason for this is that in the pass-stage, a high-frequency current with low value flows through LPS, and the changing of the CPS value results in a change of the leakage current at low frequency, which may adversely affect the performance of the equipment as a whole and the safety of the installation. On the other hand, in the stop-stage, the inductor LSS is constantly exposed to the flowing mains current, and its switching or continuous tuning causes an effect on this current. Meanwhile, the capacitor CSS is not under high voltage and can be used for switching.

[0139] As shown in figure 10, the circuit of figure 1 is essentially reproduced with corresponding elements bearing like reference numbers. As shown, on the basis of the preceding discussion, fixed capacitance 122 is replaced with a non-continuously tunable capacitor 1022 as discussed with respect to figure 4, and fixed inductance 131 is replaced with a continuously tunable inductor 1031 as discussed with respect to figure 8a. The specific circuit shown for elements 1022 and 1031 is the same as for the corresponding figures 4 and 8a, but as discussed above it will be appreciated that any suitable implementation of the circuits of corresponding principle of operation may be envisaged as a function of the desired operating frequency range, desired Q factor, etc.

[0140] As shown, the system also comprises optional controller 1000 as discussed above.

[0141] Figure 11 presents a complete system according to an embodiment.

[0142] While the embodiment of figure 11 may represent an ideal implementation in certain cases, in other cases practical consideration may lead to the adoption of an alternative implementation. As shown in figure 11 , for the bandpass section the high Q fixed capacitor and continuously tunable inductor may be discarded in favour of a fixed inductance and acontinuously tunable capacitor. The reason is that the bandpass is preferable be high-Q, and therefore narrowband, and therefore sensitive to variation. A tunable capacitance may be preferred over a tunable L, insofar as the inductance tuning uses saturation, which may in some case have unpredictable effects. As regards the Bandstop section, a tunable capacitor, and a fixed inductor may be preferred, and a switchable or stepped capacitor implementation may be selected due to its simpler implementation.

[0143] As a variant, element 121 may additionally be replaced with a tunable element in a continuously or stepped configuration e.g. as discussed above, capacitor.

[0144] As shown in figure 11 , the circuit of figure 1 is essentially reproduced with corresponding elements bearing like reference numbers. As shown, on the basis of the preceding discussion, fixed capacitance 122 is replaced with a continuously tunable capacitor 1122 as discussed with respect to figure 6, and fixed capacitor 132 is replaced with a non-continuously tunable capacitor 1132 as discussed with respect to figure 4. The specific circuit shown for elements 1132 and 1122 is the same as for the corresponding figures 4 and 6, but as discussed above it will be appreciated that any suitable implementation of the circuits of corresponding principle of operation may be envisaged as a function of the desired operating frequency range, desired Q factor, etc.

[0145] Performance may be further improved by additionally replacing inductor 121 with a tunable inductor, of a continuous or non-continuous type as discussed above.

[0146] As shown, the system also comprises optional controller 1100 as discussed above.

[0147] Accordingly, a two stage tunable notch filter is presented for removing unwanted noise signals in high power systems. The filter comprises a pass stage and a stop stage, either or both of which may contain tuneable elements. A variety of tuning mechanisms are presented. The system may be controlled dynamically so as to tune the system in real time as a function of detected noise signals by a controller incorporated in the system.

[0148] It will be understood that the configurations and / or approaches described herein are exemplary in nature, and that these specific embodiments or examples are not to be considered in a limiting sense, because numerous variations are possible. The specific routines or methods described herein may represent one or more of any number of processing strategies. As such, various acts illustrated and / or described may be performed in the sequence illustrated and / or described, in other sequences, in parallel, or omitted. Likewise, the order of the above-described processes may be changed.

[0149] The examples described above are given as non-limitative illustrations of embodiments of the invention. They do not in any way limit the scope of the invention which is defined by the following claims.

Claims

24CLAIMS1 . A notch filter comprising a Band Stop section and a Band Pass section, wherein said Band Stop section comprises a first component grouping comprising a first capacitance component and a first inductance component in parallel, in line with a filtered line, and wherein said Band Pass section comprises a second component grouping comprising a second capacitance component and a second inductance component in series, coupling said filtered line and a reference line, and wherein a capacitance or an inductance of a respective said component of at least one of said component groupings is tunable under the control signal received from a controller.

2. The notch filter of any preceding claim adapted to filter a line carrying one ampere or more of current.

3. The notch filter of claim 1 or 2 wherein one said component in said first component grouping is continuously tunable under the control of a controller, and one said component in said second component grouping is tunable by being switchable to one of a plurality of discrete values, or wherein one said component in said second component grouping is continuously tunable under the control of a controller, and one said component in said first component grouping is tunable by being switchable to one of a plurality of discrete values.

4. The notch filter of any preceding claim wherein said first capacitance component and said second capacitance component are tunable.

5. The notch filter of any preceding claim wherein said first inductance component and / or said second inductance component is tunable by being switchable to one of a plurality of discrete values.

6. The notch filter of claim 5 wherein said first inductance component and / or said second inductance component is switchable to one of a plurality of discrete values by switchably coupling one or more selected inductance sub components in asubcomponent bank such that the cumulative induction of the switched devices in said subcomponent bank provides the desired overall value.

7. The notch filter of claim 5 or 6 in which one or more said inductive components comprises a multi tap inductance component comprising a plurality of windings with a plurality of tap connections at respective windings such that the induction of a said multi tap inductance component can be determined by switchable coupling said multi tap inductance components via a selected pair of said taps to provide a desired inductive value.

8. The notch filter of any preceding claim wherein said first capacitance component and / or said second component is tunable by being switchable to one of a plurality of discrete values.

9. The notch filter of claim 8 wherein said first capacitance component and / or said second capacitance component is switchable to one of a plurality of discrete values by switchable coupling one or more selected capacitance sub components in a subcomponent bank such that the cumulative capacitance of the switched devices in said subcomponent bank provides the desired overall value.

10. The notch filter of any preceding claim wherein said first capacitance component and / or said second capacitance component is tunable by adjusting a DC bias voltage thereof.11 . The notch filter of claim 9 wherein said first capacitance component and / or said second capacitance component comprises a dielectric material exhibiting a self polarization behaviour reversible by the application of said DC bias voltage.

12. The notch filter of claim 11 wherein said first capacitance component and / or said second capacitance component comprises a dielectric material comprising BaTiOa-13 The notch filter of any preceding claim wherein said first inductance component and / or said second inductance component is tunable by controlling the degree of saturation in a core thereof by modulation of the current in a secondary coil thereof.

14. A filter system comprising a notch filter as defined in any preceding claim and a controller adapted to receive a signal characteristic of a signal on said filtered line, and to determine one or more control signals suitable to tune each said tunable capacitance component or inductance component so as to achieve a desired modification in said signal characteristic, and to emit said one or more control signals to said respective said tunable capacitance component or inductance component.

15. The filter system of claim 14 wherein said signal characteristic is a frequency response.

16. The filter system of claim 14 or claim 15 wherein said steps of receiving, determining and emitting are performed continuously and in real time.17 A controller for a Notch filter for performing EMC filtering, said controller being adapted to receiving a signal on a filtered line, determine control signals suitable to tune each tunable capacitance component or inductance component of a notch filter comprising a Band Stop section and a Band Pass section, wherein said Band Stop section comprises a first component grouping comprising a first capacitance component and a first inductance component in parallel, in line with a filtered line, and wherein said Band Pass section comprises a second component grouping comprising a second capacitance component and a second inductance component in series, coupling said filtered line and a reference line, and to emit said one or more control signals to said respective said tunable capacitance component or inductance component.

18. A computer implemented method of performing EMC filtering comprising receiving a signal on a filtered line, determining control signals suitable to tune each tunable capacitance component or inductance component of a notch filter comprising a Band Stop section and a Band Pass section, wherein said Band Stop section comprises a first component grouping comprising a first capacitance component and a first inductance component in parallel, in line with a filtered line, and27 wherein said Band Pass section comprises a second component grouping comprising a second capacitance component and a second inductance component in series, coupling said filtered line and a reference line, and emitting said one or more control signals to said respective said tunable capacitance component or inductance component.

19. A Computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the method of claim 17.

20. A computer-readable medium comprising instructions which, when executed by a computer, cause the computer to carry out the method of claim 17.

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