Active noise filter

Abstract

An active feed-forward noise filter for reducing noise signals in an electronic circuit comprises an amplifier between a sensor circuit and an incoupling circuit, the amplifier being wired in a mixed feedback.

Description

[0001]202305052 1 Description Active interference filter The invention relates to an active interference filter in feedback topology for reducing interference signals in an electronic circuit, comprising a sensor circuit, an amplifier and a coupling circuit. EMC interference generated by converters and switching power supplies must be attenuated so that standards for electromagnetic compatibility can be met. The attenuation can be achieved with passive components. However, these are typically heavy and large. An alternative to purely passive filters are active EMC filters (AEF). Active filters use active components such as operational amplifiers, and thus the interference is actively suppressed by suitable wiring.Operational amplifiers generally have a very high open-loop gain, which is reduced via external circuitry and adjusted to the required gain in the circuit. The required gain is usually in the range of up to a maximum of 20 dB, and the external circuitry, which acts as feedback, is spatially connected very close to the operational amplifier, resulting in short delays and low parasitic inductances. For the best possible operation of an active noise filter in feedback topology, however, very high gains > 60 dB are advantageous, and the output of the operational amplifier is fed back via the passive filter of the surrounding circuit. This results in significantly longer delays and higher inductances than with purely external circuitry for the operational amplifier.This significantly reduces the stability reserve 202305052 2 at high frequencies, and instabilities can occur. To avoid this, operational amplifiers can be used which are not inherently fast and are therefore no longer active at high frequencies. However, since the operational amplifier must not be too slow at the same time in order not to render the active interference filter ineffective, the disadvantage is that only a very small selection of possible operational amplifiers remains. Alternatively, complex low-pass filters can be introduced into the inner feedback loop. The disadvantage of low-pass filters in the feedback loop is that they introduce an additional phase shift in the system's transfer function. The object of the invention is to specify an active interference filter in feedback topology in which oscillations, i.e. instabilities, are avoided, particularly for high frequencies above 1 MHz.This object is achieved by an active interference filter having the features specified in claim 1. The active interference filter according to the invention in feedback topology for reducing interference signals in an electronic circuit comprises a sensor circuit for detecting an electrical interference signal in a line of the circuit. It further comprises an amplifier circuit with an amplifier, wherein the sensor circuit is coupled to a first input of the amplifier, and a coupling circuit for coupling an output signal of the amplifier circuit, which reduces the interference signal, into the line. The amplifier is connected with mixed feedback. Advantageously, by connecting it with mixed feedback, the gain of the amplifier can be limited at high frequencies, in other words, its bandwidth can be restricted.The reduction in gain compared to open loop gain is determined by the choice of resistors used in the amplifier's circuitry and can thus be adjusted to avoid oscillation, particularly at high frequencies above 1 MHz. For mixed feedback, the amplifier output is expediently connected to a first amplifier input via a first feedback resistor and to a second amplifier input via a second feedback resistor. This refers in particular to direct feedback, i.e. without any further elements in the connection between output and input apart from the designated resistors and electrical lines. In the following text, this is also referred to as the internal feedback loop. The amplifier can be an amplifier circuit with discrete transistors or an integrated operational amplifier.In an operational amplifier, the first input is its inverting input and the second input is its non-inverting input. Operational amplifiers are advantageously available in many variations of properties. A maximum gain of at least 40 dB is advantageous for the operational amplifier. The coupling circuit preferably comprises a coupling capacitor. This is expediently arranged between the output of the amplifier and the line. If the circuit has an inductance connected in series with the coupling capacitor, referred to below as a converter inductance, then the coupling capacitor results in a passive filter, which is referred to below as an LC filter. 202305052 4 Further advantageous embodiments of the interference filter according to the invention can be found in the dependent claims.The embodiment of the independent claims can be combined with the features of one of the subclaims or, preferably, with those of several subclaims. Accordingly, the following additional features can be provided: In an advantageous embodiment of the invention, the sensor circuit comprises a first and a second node, between which a series circuit with a sensor capacitor, a sensor resistor, and a sensor inductance is arranged. The first node is connected to the line, and a third node between the sensor capacitor and the sensor resistor is connected to the first input of the amplifier via an input resistor. It is understood that the order of sensor resistor and sensor inductance is interchangeable, and the sensor resistor can also be designed in the form of a parasitic resistance of the sensor inductance.The sensor circuit represents a damped second-order high-pass filter. This high-pass filter largely excludes low-frequency signals, particularly in an AC circuit such as a converter, the useful signals at the mains frequency from amplification and feedback, from being passed on to the amplifier circuit. The values ​​for R. S and L S are preferably chosen so that the cutoff frequency f S = R S / (2πL S) is significantly above the frequency of the useful signals in the circuit, for example, at 20 kHz or 100 kHz. Furthermore, it is advantageous if the values ​​are selected so that the cutoff frequency is above that of the LC filter, as this avoids oscillations in the frequency range below the cutoff frequency of the LC filter. In a setup with a coupling capacitor and a sensor capacitor, it is advantageous if the capacitance of the sensor capacitor is at most one-third of the capacitance of the 202305052 5 coupling capacitor. This minimizes the influence of the sensor capacitor on the resulting resonance frequency through a converter inductance of the circuit and the coupling capacitor.If the electronic circuit includes a converter inductance connected in series with the coupling circuit, it is advantageous to select the values ​​for the sensor capacitor and the sensor inductance such that the cutoff frequency of the sensor circuit is at least ten times, in particular at least 50 times, the cutoff frequency of the LC filter formed by the converter inductance and the coupling capacitor. This distance ensures that the sensor circuit can form a range with a gradient of 40 dB per frequency decade in the amplifier's transfer function. An electronic circuit in which the interference filter is preferably used is, for example, a power converter such as an inverter or DC / DC converter, or a switching power supply.The electronic circuit comprises at least one phase conductor and one neutral conductor, and the sensor and the coupling circuit are expediently connected to the phase conductor. If multiple phase conductors are present, the designated elements are expediently connected to exactly one phase conductor. Further similar active interference filters can then be included in the circuit, each connected to one of the other phase conductors, so that interference in each of the phase lines is reduced. In this case, the second input of the amplifier is connected to the neutral conductor via a second input resistor. In this case, the second node of the sensor is also connected to the neutral conductor. The neutral conductor can be a ground potential.202305052 6 Elements of the electronic circuit, such as the switches of an active rectifier, form an interference source, and the sensor circuit is connected to the phase conductor with its first node relative to the interference source behind the coupling circuit. Depending on the application, the first node can be arranged towards a mains connection, as seen from the interference source, or towards a load, but further away from the interference source than the coupling circuit. This creates a feedback topology for the active interference filter, in which the result, i.e. the reduced interference signals, are fed back into the sensor circuit, i.e. the interference filter, as an input signal. In addition to the internal, mixed feedback near the amplifier, there is also an external feedback loop that includes the coupling circuit and the sensor circuit or parts of the sensor circuit.If the power converter is multi-phase, for example three-phase, and accordingly comprises three phase conductors, then it preferably comprises an active interference filter of the type described for each phase conductor. Each of the active interference filters is then connected to one of the phase conductors and the neutral conductor or earth. The active interference filter is advantageously modular, i.e. it can also be retrofitted into existing circuits. The existing components of the circuit, e.g. the converter, do not have to be modified. The components of the active interference filter should be adapted to the existing components of the circuit as described; for example, the capacitances of the coupling capacitor and the sensor capacitor should be adapted to an existing converter inductance. It is advantageous to carry out the adaptation using the following steps.If the converter inductance is not already determined, it can be selected based on the current ripple requirements. The size of the coupling capacitor is then determined to achieve a suitable cutoff frequency for the resulting LC filter. This frequency must be well above the frequencies for useful signals. The higher it is, the smaller and lighter the components used can be. At the same time, however, this influences the filter range of the active noise filter, which, for stability reasons, cannot start below the cutoff frequency of the LC filter. A trade-off must therefore be made here. The next step is to select a suitable operational amplifier that can output the necessary voltages and currents. The capacitance of the sensor capacitor should be no more than one-third of the capacitance of the coupling capacitor.The next step is to determine the maximum closed-loop gain of the operational amplifier. This is achieved by selecting the resistors of the inner feedback loop. Since the lowest possible gain should be maintained at low frequencies, it is advantageous to maintain a maximum gain of 40 dB. The cutoff frequency of the sensor circuit can then be determined. This is preferably selected about one and a half decades higher than the cutoff frequency of the LC filter. The first decade is used to attenuate the gain by 40 dB from the maximum gain of 40 dB to approximately 0 dB. The remaining half decade is used to reduce the gain at the cutoff frequency of the LC filter by a further 10 dB. The sensor inductance can be determined according to L. S = 1 / ((2πf S ) 2 C S). The sensor resistance can be determined based on the peak that occurs at the frequency 202305052 8, at which the active noise filter begins to work. Higher values ​​reduce the 40 dB range, while lower values ​​increase the resulting peak. A favorable value is obtained according to R S = f20dB,limit / 2πL S Finally, it can be determined how much the open-loop gain of the amplifier should be reduced at high frequencies to ensure stability at high frequencies. For this purpose, the feedback resistors of the inner feedback loop are suitably selected according to R FB+ = R FB- ≈ 100 * 2π‧f S ‧L S4 is a diagram showing the filtering effect of the active interference filter in comparison to a passive interference filter. Figure 1 shows a simplified representation of a DC / DC converter 10 in which an active interference filter 50 is arranged according to an exemplary embodiment of the invention. The DC / DC converter 10 comprises a DC voltage source 12 which serves as the input voltage. The DC voltage source 12 is connected via a forward conductor 14 and a return conductor 15 to a load 16 which is shown as a resistor in this example. A changeover switch 18 is arranged in the forward conductor 14.The changeover switch 18 is arranged so that it can break the connection between the forward conductor 14 and the DC voltage source 12, thereby establishing a direct connection between the forward conductor 14 and the return conductor 15, thus bridging the DC voltage source 12. Following the changeover switch 18 is a converter inductance 17 of a size of, for example, L. CONV= 100 µH. During operation, the switch 18 is switched at a switching frequency of, for example, 10 kHz and thus ensures a resulting voltage at the load which has a DC component that is lower than the voltage of the DC voltage source 12 and also has an AC voltage component whose primary frequency corresponds to the switching frequency. Since the resulting voltage waveform is not sinusoidal, further frequency components arise up to very high frequencies. The active interference filter 50 is connected to the forward conductor 14 at a first and second node 20, 21 and to the return conductor 15 at a third node 22. The active interference filter 50 is an interference filter in feedback topology. Therefore, the first and second nodes 20, 21 cannot be combined; rather, the order in which the internal elements of the active interference filter 50 are connected is important.The structure of the active noise filter 50 is shown in Figure 2. The active noise filter 50 comprises a sensor 51, an injector 52, and an amplifier circuit 53 connected between the sensor 51 and the injector 52. The sensor 51 is a series circuit comprising a sensor capacitor C. S , a sensor resistance R S and a sensor inductance L S This series circuit is connected between the second node 21 and the third node 22, i.e. between the forward conductor 14 and the return conductor 15. The amplifier circuit 53 comprises an operational amplifier 54. Its inverting input is connected via a first amplifier resistor R 1- with a node between the 202305052 10 sensor capacitor C S and the sensor resistance R S It is also connected via a second amplifier resistor R FB-connected to the output of operational amplifier 54, which represents a first internal feedback path. The non-inverting input of operational amplifier 54 is connected via a third amplifier resistor R 1+ connected to the third node 22, in this case the return conductor 15. It is also connected via a fourth amplifier resistor R FB+ connected to the output of operational amplifier 54. This provides a second internal feedback path. In this embodiment, the injector comprises an injector capacitor C I . The injector capacitor C I is connected between the first node 20 and the output of the operational amplifier 54. The operational amplifier 54 is therefore connected with a mixed feedback or in a subtractor circuit. With this topology, the voltage of the injector capacitor C Imeasured, filtered with a high-pass filter, and then fed into the circuit. For this purpose, sensor 51 is designed as a damped second-order high-pass filter. The injector capacitor C I is connected between the high-pass filter, i.e. the sensor 51, and the non-inverting input of the operational amplifier 54. The amplifier circuit 53 constructed in this way is stable when the feedback resistance of the positive feedback circuit, i.e. the third amplifier resistance R 1+ is smaller than the negative feedback resistance, i.e. the first amplifier resistance R 1- . For this purpose, the third amplifier resistor R 1- be chosen to be at least 0.2% smaller than the first amplifier resistance R 1+ . In this embodiment, the following values ​​are used: 202305052 11 R S : 0.1 Ω L S : 1 µH C S : 100 nF R FB- = R FB+ : 3 kΩ R 1- : 47 kΩ R 1+: 41.07 kΩ L CONV : 100 µH R L : 500 Ω C I : 300 nF Parasitic elements also influence the resulting transfer function, especially the parasitic inductance L I in the injector capacitor C I and the parallel capacitance C par in the converter inductance 17. Their values ​​in this example are L I : 3 nH C par : 50 pF With the values ​​mentioned, an active noise filter 50 can be built in a DC / DC converter 10. The open-loop gain A ol and the transfer function A cl of the interference filter 50 are shown in Figure 3. Five areas of the transfer function A cl are highlighted by the markings (1) … (5). The range marked with (1) can be adjusted via the mixed feedback. The gain can be set to any value compared to the open-look gain A olThis results in a roughly constant reduction in gain compared to the open gain of operational amplifier 45 starting at a frequency of between 1 MHz and 10 MHz. The reduction in gain is approximately -24 dB in Figure 3. This very effectively prevents oscillations at frequencies above 1 MHz. The reduction at high frequencies is: 202305052 12 With the exemplary values ​​given above, A cl / A ol = 3 / (3 + 47) = 0.06. The area marked with (2) in Figure 3 represents the maximum gain of the amplifier that can be achieved. This can be adjusted by choosing the values ​​for the amplifier resistors R FB- , R FB+ , R 1- and R 1+ be set and results in ^^ ^^ ^^ ^ ^ ^ ^^ ^ ^ ^ ^^ ൌ ା ା ^^^ ା ^^ ^ െ ^^ ^ ^^^ ^ ^^ ^^^ ^ ^^^The maximum gain, apart from the peak that occurs at the sensor's cutoff frequency of approximately 500 kHz, with the exemplary values ​​for the resistors A given above, is cl ≈ 7.4. The ranges marked (3) and (4) concern the behavior at low frequencies. Here, it is advantageous to achieve a low gain, since the active noise filter 50 should only be active above a certain cutoff frequency, which in this embodiment is 200 kHz. This range can be adjusted by the design of sensor 51, which acts as a high-pass filter and thus keeps low-frequency (noise) signals away from the amplifier. A suitable adjustment is therefore achieved by selecting the values ​​for C S , L S and R S . A range with a 40 db / dec gradient results from f S = R S / (2πL S). Overall, the structure created for the active noise filter 50 is stable, thus allowing no oscillation in the considered frequency range between 0 Hz and 10 8 Hz. Figure 4 shows the resulting filter effect, i.e. the attenuation of signals of a particular frequency. 202305052 13 shows the curve A LC the attenuation by an exclusively passive filter, which consists of the components inductance 17 = L CONV and the injector capacitor C I is formed. The course A AEF shows the attenuation by the active noise filter 50. The two curves overlap below a frequency of about 100 kHz and at very high frequencies to a large extent, as can be expected from the curve shown in Figure 3. At the resonance frequency f LC a peak is visible: Between f LCand 200 kHz, the passive LC filter dominates the transfer function, resulting in an attenuation curve of 40 dB / dec. From the zero crossing of the transfer function A cl From Figure 3, at approximately 200 kHz up to a frequency of approximately 20 MHz, the active noise filter 50 reduces the signals, i.e., the noise, significantly better than the passive LC filter. The improvement compared to the passive filter averages approximately 20 dB in the specified frequency range. Between 200 kHz and 500 kHz, an attenuation curve of 80 dB / dec results. Between 1 and 10 MHz, resonance points can be seen, which are caused by parasitic elements, specifically the inductance in the injector capacitor C Iand the parallel capacitance in the converter inductance 17. 202305052 14 Reference numeral 10 DC / DC converter 50 Active noise filter 12 DC voltage source 14 Forward conductor 15 Return conductor 16 Load 17 Converter inductance 18 Unswitch 20, 21, 22 Node 51 Sensor 52 Injector 53 Amplifier circuit 54 Operational amplifier L S Sensor inductance R S Sensor resistance C S Sensor capacitor C I Injector capacitor R FB+ , R FB- Feedback resistors R 1+ , R 1- Resistors A LC , A AEF Signal attenuation A cl Transfer function

Claims

202305052 15 claims 1. Active interference filter (50) in feedback topology for reducing interference signals in an electronic circuit (10), comprising - a sensor circuit (51) for detecting an electrical interference signal in a line (14) of the circuit (10), - an amplifier circuit (53) with an amplifier (54), wherein the sensor circuit (51) is coupled to a first input of the amplifier (54), - a coupling circuit (52) for coupling an output signal of the amplifier circuit (53) that reduces the interference signal into the circuit (10), - wherein the amplifier (54) is connected with a mixed feedback.

2. Active interference filter (50) according to claim 1, wherein the output of the amplifier (54) is connected via a first feedback resistor (R FB- ) is connected to a first input of the amplifier (54) and via a second feedback resistor (R FB+) is connected to a second input of the amplifier (54).

3. Active noise filter (50) according to claim 1, wherein the sensor circuit (51) comprises a first and a second node (21, 22), between which a series circuit with a sensor capacitor (C S ), a sensor resistor (R S ) and a sensor inductance (L S ), wherein the first node (21) is connected to the line (14), a third node between the sensor capacitor (C S ) and the sensor resistance (R S ) via an input resistance (R 1- ) is connected to the first input of the amplifier (54).

4. Active noise filter (50) according to claim 1, wherein the amplifier (54) is an operational amplifier (54) and the first input is its inverting input and the second input is its non-inverting input. 202305052 16 5. Active noise filter (50) according to claim 1, wherein the coupling circuit (52) comprises a coupling capacitor (C I ) between the output of the amplifier (54) and the line (14).

6. Active noise filter (50) according to claim 5 and claim 3, wherein the capacitance of the sensor capacitor (C S ) at most one third of the capacitance of the coupling capacitor (C I ).

7. Active noise filter (50) according to claim 5 and claim 3, wherein the electronic circuit (10) comprises a converter inductance (17) connected in series with the coupling circuit (52) and the cut-off frequency of the sensor circuit (51) is at least ten times the cut-off frequency of the filter formed by the converter inductance (17) and the coupling capacitor (C I) is formed.

8. Electronic circuit (10) with an active interference filter (50) according to one of the preceding claims, wherein the electronic circuit (10) comprises a phase conductor (14) and a neutral conductor (15) or ground conductor and the sensor circuit (51) and the coupling circuit (52) are connected to the phase conductor (14).

9. Electronic circuit (10) according to claim 8, wherein elements of the circuit (10) form an interference source and the sensor circuit (51) is connected to the phase conductor (14) behind the coupling circuit (52) with respect to the interference source.

10. Electronic circuit (10) according to claim 8 or 9, wherein the second input of the amplifier (54) is connected via a second input resistor (R 1+ ) is connected to the neutral conductor (15) or earth conductor.

11. Electronic circuit (10) according to one of claims 8 to 10, wherein the second node (22) of the sensor circuit 202305052 17 device (51) is connected to the neutral conductor (15) or earth conductor.

12. Power converter (10) or switched-mode power supply according to one of claims 8 to 11.

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