A common mode EMI noise suppression method combining a passive filter and a modulation strategy

Abstract

The application discloses a common-mode EMI noise suppression method combining a passive filter and a modulation strategy, and belongs to the technical field of power electronics and electrical engineering. The method uses an improved random switching frequency SVPWM modulation method and measures a common-mode voltage spectrum without adding an EMI filter, reduces the common-mode voltage amplitude from the source, and then realizes the reduction of the common-mode electromagnetic interference spectrum, and determines the required insertion loss according to the EMC standard, to the intersection point of the minimum frequency, and uses the corresponding frequency of the point as the cutoff frequency of the EMI filter, and then determines the filter element parameters according to the cutoff frequency and verifies them. Through the use of the common-mode interference comprehensive suppression means combining the filter and the random PWM strategy, the design margin of the filter can be reduced, the cutoff frequency of the EMI filter can be increased, the weight and volume of the inductor can be reduced, the lightweight and miniaturization of the filter can be realized, and the improved random switching frequency SVPWM is also a guarantee for the EMI filter to achieve the filtering effect.

Description

Technical Field

[0001] The present invention relates to the fields of power electronics and electrical engineering, and in particular to a common-mode EMI noise suppression method combining a passive filter and a modulation strategy. Background Art

[0002] The rapid switching of power converters in motor control systems can cause electromagnetic interference (EMI), which can be categorized as differential-mode EMI and common-mode EMI. Common-mode EMI is caused by common-mode current. This common-mode current generates a common-mode voltage, forming a loop between the phase line and ground. This current flows through the motor's internal coupling capacitance path through the bearings, inducing high shaft voltages on the motor shaft and generating bearing currents. These currents discharge through the oil film, continuously damaging the bearing surface and potentially causing motor aging and damage.

[0003] At present, electromagnetic interference suppression methods for motor control systems can be mainly divided into two categories: one is to reduce the emission intensity of EMI interference sources; the other is to cut off the EMI conduction path. Among them, the EMI suppression technologies that have received more attention mainly include: EMI filter technology, switching waveform optimization technology and improved modulation strategy.

[0004] EMI filter technologies can be categorized into three types: active, passive, and hybrid filters. All of these filters suppress electromagnetic interference currents through the conduction path. Passive EMI filters block EMI conduction paths using passive components such as inductors and capacitors, but their low-frequency performance is poor and they require the addition of larger and heavier inductors and capacitors. Active EMI filters detect EMI interference signals using active components such as operational amplifiers and power amplifiers and inject opposing compensating signals. However, these filters are constrained by high-frequency parasitic parameters, compensation response speed, and system stability requirements. Hybrid EMI filters combine the advantages of both but are complex to design. Switching waveform optimization techniques, including soft switching and active gate drive technologies, suppress EMI by controlling the voltage and current change rates of switching devices, but this can affect system switching losses and efficiency. The improved modulation strategy starts with pulse width modulation, with the goal of reducing the amplitude of the noise source, and conducts a series of improvements and innovations in the modulation method. Related methods include random pulse width modulation (RPWM) and variable switching frequency pulse width modulation (VSFPWM). Reducing the common-mode voltage amplitude at the source is beneficial to improving the electromagnetic interference situation, but it cannot guarantee that the EMI current of the entire system will be reduced. At the frequency point with worse resonance, the impedance value is very small, and the EMI current does not decrease but may increase in amplitude.

[0005] Therefore, in order to better suppress common-mode electromagnetic interference at high frequencies, it is necessary to further study a common-mode EMI noise suppression method that can reduce the filter size without affecting the suppression effect.

[0006] This patent addresses the problem that traditional passive filter methods lead to increased core size and filter weight and volume, and proposes a common-mode EMI noise suppression method that combines passive filters and modulation strategies. Summary of the Invention

[0007] In response to the deficiencies of the above-mentioned prior art, the purpose of the present invention is to provide a common-mode EMI noise suppression method that combines a passive filter and a modulation strategy. By adopting a comprehensive common-mode interference suppression method that combines a filter and a random PWM strategy, the design method of the common-mode EMI filter is improved on the basis of the random PWM strategy. After adopting the improved random switching frequency SVPWM, the design margin of the filter can be reduced and the cutoff frequency of the EMI filter can be increased, which is conducive to reducing the weight and volume of the inductor, achieving lightweight and miniaturization of the filter, and at the same time achieving electromagnetic interference suppression of the servo drive system at a lower cost. This method effectively solves the problems of excessive core size, weight and volume of the filter caused by traditional passive EMI filters, and ensures the suppression effect of common-mode electromagnetic interference.

[0008] The present invention adopts the following technical solutions to achieve the above-mentioned purpose:

[0009] A common-mode EMI noise suppression method that combines passive filters and modulation strategies uses the following common-mode EMI noise suppression design process:

[0010] Step 1.1: Use the improved random switching frequency SVPWM instead of the traditional random switching strategy SVPWM, and measure the common-mode voltage spectrum without adding the EMI filter, thereby reducing the common-mode voltage amplitude at the source and reducing the common-mode electromagnetic interference spectrum;

[0011] Step 1.2: Determine the required insertion loss according to the EMC standard and find the intersection point where the two frequencies are minimum. The frequency corresponding to this point is used as the corner frequency value required for the filter component parameter design.

[0012] Step 1.3: Design an LCL EMI filter and obtain the key component parameters: common mode capacitance C Y and common mode inductor L CM The parameter values ​​are obtained and verified. If they meet the standards, the overall design of the filter is completed.

[0013] Step 1.4: If the standard is not met, the cutoff frequency is changed from nf s Reduced to (n-1)f s , repeat the above design steps to obtain the cutoff frequency (n-1)f s The corresponding LCL type EMI filter component parameters are checked again and the adjustment is repeated until the final EMC standard is met.

[0014] Furthermore, the random PWM strategy adopted by the method is an improved random switching frequency SVPWM. Under the condition that the spread spectrum range is 5% to 10% of the switching frequency, this method can further disperse the high-frequency harmonics and reduce their peak values, thereby achieving the purpose of suppressing electromagnetic interference; the improved random switching frequency SVPWM strategy divides the original spread spectrum range into n frequency bands, each frequency band corresponds to a new average switching frequency, and sets the switching frequency switching period T b , every time after T b The switching frequency change band is switched once every 1 / n time, thereby reducing the distribution range of the switching frequency value differences between adjacent switching cycles while keeping the overall spread spectrum range and the average switching frequency unchanged.

[0015] Furthermore, the original noise of the converter is measured using the LISN circuit. The noise attenuation required by the system is obtained by measuring the noise before the EMI filter is added. The common-mode and differential-mode interference calculated by the measurement are compared with the limit values ​​required by the electromagnetic compatibility standard to obtain the insertion loss IL required by the designed common-mode EMI filter. REQ The formula is as follows:

[0016] IL REQ (dB)=20lgV measure +120+IL Margin (dB)-A Limit (dBuV)

[0017] Where V measure That is, the common mode voltage spectrum size after modulation by the improved random switching frequency SVPWM strategy, IL Margin The safety margin is increased appropriately based on the actual situation. According to GJB 151B-2013 Military Equipment and Subsystems Electromagnetic Emission and Sensitivity Requirements and Measurement CE102 Standard, A Limit It is a limit value in dBμV (decibel microvolts) that is used to specify the maximum allowable conducted emission level of a device within a specific frequency range to ensure that the device does not generate excessive electromagnetic interference during operation and affect the normal operation of other devices.

[0018] Furthermore, the calculated insertion loss can be used to obtain the minimum frequency point where the system common mode noise intersects the peak standard line given by the EMC standard. The switching frequency multiple k closest to it is assigned to n, and f is req_min With nf s In comparison, the definition of the turning frequency f c =nf s -Δf / 4, thus obtaining the switching frequency multiple k and the transition frequency f c The relationship between the RSF-SVPWM spread spectrum range Δf is used to determine the component parameters of the common-mode EMI filter.

[0019] Furthermore, under the condition of a certain transition frequency, according to the capacitance requirements under the power level specified in "GJB151B-2013", and taking into account the reduction of the volume of the entire EMI filter, a Y capacitor close to 0.075μF / kW is selected, and the typical value of the Y capacitor is 330nF.

[0020] Furthermore, the common mode inductor L CM The value of the common mode capacitor C Y and the transition frequency f c The values ​​of are related to the common mode inductance L CM The value of can be given by the following formula:

[0021]

[0022] Compared with the prior art, the technical solution adopted by the present invention has the following beneficial effects:

[0023] (1) By using an improved random switching frequency SVPWM strategy, this method can further disperse high-frequency harmonics and reduce their peak values ​​under the condition of a small switching frequency variation range. Compared with the random PWM strategy, it solves the problem of being unable to balance the high-order harmonic dispersion effect and system control performance.

[0024] (2) In order to meet EMI standards, traditional common-mode EMI filters increase the number of inductor turns, which leads to an increase in the core size, weight, and volume of the filter. By improving the design method of common-mode EMI filters based on the use of random PWM strategies, it is beneficial to reduce the weight and volume of the inductor, thereby achieving lightweight and miniaturization of the filter.

[0025] (3) In order to solve the problem that the traditional modulation method cannot guarantee the reliability of the system and the size of the filter core is too large, the modulation method and the EMI filter method are combined by using a common mode interference comprehensive suppression method that combines the filter and the random PWM strategy. The additional attenuation provided by the modulation method can avoid the increase in the weight and volume of the filter inductor to a certain extent, and can also provide an additional guarantee for the EMI filter to achieve the filtering effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a flow chart of a common-mode EMI noise suppression method combining a passive filter and a modulation strategy in the present invention.

[0027] Figure 2 It is a schematic diagram of a circuit for measuring interference voltage on LISN in the present invention.

[0028] Figure 3The figure is a circuit diagram of an LCL type EMI filter used for suppressing common mode EMI noise in the present invention.

[0029] Figure 4 This is a schematic diagram of system EMI common mode noise when no EMI filter is used for suppression in the present invention.

[0030] Figure 5 It is a schematic diagram of the system EMI common mode noise after being suppressed by the EMI filter in the present invention. DETAILED DESCRIPTION

[0031] The technical solutions of the present invention are further described below based on the embodiments, but the present invention is not limited to the specific embodiments. In the following detailed description of the present invention, some specific details are described in detail. Those skilled in the art can fully understand the present invention without the description of these details.

[0032] The technical solution of the present invention is further described in detail below with reference to the accompanying drawings:

[0033] A common-mode EMI noise suppression method combining a passive filter and a modulation strategy. The method utilizes an improved random switching frequency SVPWM modulation method and measures the common-mode voltage spectrum without an EMI filter. This method reduces the common-mode voltage amplitude at the source, thereby reducing the common-mode electromagnetic interference spectrum. The required insertion loss is determined according to EMC standards, and the frequency corresponding to the minimum intersection of the two frequencies is used as the EMI filter's cutoff frequency. The filter component parameters are then determined based on this cutoff frequency. By combining a filter and a random PWM strategy to achieve comprehensive common-mode interference suppression, the filter's design margin can be reduced and the EMI filter's cutoff frequency increased, which helps reduce the weight and volume of the inductor, achieving lightweight and miniaturized filters. The improved random switching frequency SVPWM also provides an additional guarantee for the EMI filter's effective filtering.

[0034] The common mode EMI noise suppression design process used in the method is as follows Figure 1 As shown in the figure, it includes using the improved random switching frequency SVPWM instead of the traditional random switching strategy SVPWM to obtain a certain amount of common mode EMI spectrum reduction instead of the original unsuppressed common mode noise; determining the required insertion loss according to the EMC standard, and obtaining the minimum intersection point of the two frequencies to obtain the corner frequency value required for the next step of filter component parameter design; then designing the LCL type EMI filter, obtaining the key component parameter C Y With L CM The parameter values ​​are determined and verified to ensure the rationality and reliability of the filter design.

[0035] To obtain a reduced common-mode noise spectrum, the random PWM strategy adopted in the method is an improved random switching frequency SVPWM. Under the condition of a small switching frequency variation range, this method can further disperse high-frequency harmonics and reduce their peak values, thereby achieving the purpose of suppressing electromagnetic interference. The variation of the random numbers taken from adjacent switching cycles is used to reflect the variation level of the random sequence, and the random number R of the kth switching cycle value is determined. k for:

[0036]

[0037] Furthermore, the high-order harmonic amplitude and the random number R that randomly modulates the switching frequency are used. k By analyzing the influence of random number sequence characteristics on the amplitude of high-order harmonics, it can be found that the random number difference sequence R taken from adjacent switching cycles i+1 -R i The smaller the distribution range of , the smaller the amplitude of the higher harmonics. Therefore, the improvement of the traditional RSF-SVPWM strategy can be achieved by reducing the distribution range of the random number difference sequence taken from adjacent switching cycles while keeping the spread spectrum range the same. The original spread spectrum range is evenly divided into n frequency bands, each frequency band corresponds to the new average switching frequency, and the switching frequency switching period T is set. b , every time after T b The switching frequency change band is switched once every 1 / n time, thereby reducing the distribution range of the switching frequency value difference between adjacent switching cycles while maintaining the overall spread spectrum range and average switching frequency unchanged, further optimizing the high-order harmonic dispersion effect. Figure 4 and Figure 5 It can be seen from the system EMI common mode noise that the peak value of the system common mode noise under the traditional SVPWM strategy is significantly reduced compared with the improved random switching frequency SVPWM strategy. That is, based on this, the problem that the traditional RSF-SVPWM is difficult to balance the high-order harmonic dispersion effect and system control performance is improved. Under the same spread spectrum range conditions, better harmonic dispersion effect is obtained, which can also improve the system EMC performance without sacrificing the comprehensive control performance of the PMSM servo system.

[0038] Furthermore, before selecting the EMI filter topology, it is necessary to first measure the interference noise when no EMI filter is added to the inverter. Figure 2The figure shows how to use LISN to measure the original noise of the converter. That is, by measuring the noise before adding the EMI filter, the noise attenuation required by the system is obtained. The common-mode interference calculated by the measurement is compared with the limit value required by the electromagnetic compatibility standard. A 6dB margin is added to obtain the insertion loss required by the designed common-mode EMI filter. Before adding the EMI filter, the interference voltages V1 and V2 at the two test ports are measured simultaneously. The time domain signal measured on the LISN is then separated into differential mode and common mode. The common-mode voltage V flowing through the LISN is CM It can be calculated by the following formula:

[0039]

[0040] According to GJB151B-2013, electromagnetic emission and susceptibility requirements for military equipment and subsystems, and the EMC peak standards specified in CE101 and CE102, the CE101 standard applies to common-mode currents from 25Hz to 10kHz, and the CE102 standard applies to common-mode voltages from 10kHz to 30MHz. The required insertion loss IL can be calculated by subtracting the specified limit from the common-mode noise. REQ , where V measure That is, the common mode voltage spectrum size after modulation by the improved random switching frequency SVPWM strategy, IL Margin The safety margin value is appropriately increased according to the actual situation, IL Margin Taking 6dB, the insertion loss in the present invention is calculated to be 50dB.

[0041] IL REQ (dB)=20lgV measure +120+IL Margin (dB)-A Limit (dBuV)

[0042] IL REQ (dB)=20lgV measure +120+IL Margin (dB)-A Limit (dBuV) = 44dB + 6dB = 50dB

[0043] Further, such as Figure 4 The system EMI common-mode noise shown in the figure shows that compared with the traditional RSF-SVPWM strategy, the system common-mode EMI noise peak obtained by the improved random switching frequency SVPWM strategy has been significantly reduced. Under the same spread spectrum range, better harmonic dispersion effect is obtained. The insertion loss can be used to obtain the minimum frequency point where the system common-mode noise intersects with the peak standard line given by the EMC standard. The switching frequency multiple k closest to it is assigned to n, and f is assigned to n. req_min With nfs In comparison, the turning frequency f c = nf s - Δf / 4, where Δf represents the size of the RSF-SVPWM spread spectrum range, and Δf can be half of the switching frequency multiple k, used to determine the element parameters of the filter. The filter selects the LCL type filter, and when designing the EMI filter, it is usually designed for the common mode and differential mode components in the interference signal respectively, so the parameter value C X of the differential mode filter is selected as 2 μF, which is not described here, and only the element parameter value C Y of the common mode EMI filter is designed below. CM .

[0044] Further, the volume of the EMI filter mainly depends on the volume of the common mode inductor, so under the condition of a certain turning frequency, a larger Y capacitor should be selected as possible to get a smaller volume of the common mode inductor, thereby reducing the volume of the entire EMI filter. The size of the Y capacitor is restricted by the leakage current to ground, and the size of the leakage current is closely related to the safety of human life. The leakage current of all power electronic devices is limited. According to the requirements of the safety capacitor in “GJB151B-2013”: for 50 Hz AC power supply equipment, it should be less than 0.1 μF; for 400 Hz AC power supply equipment, it should be less than 0.02 μF; for DC power supply system, the capacitance of each polarity power line to ground at the interface should not exceed 0.075 μF / kW of the connected load. For a DC load less than 0.5 kW, the filter capacitor should not exceed 0.03 μF. Therefore, C Y should not be greater than 375 nF. Therefore, C Y is selected as 330 nF.

[0045] Further, the winding direction of the two coils of the common mode inductor on the magnetic ring is consistent, and the magnetic field direction generated by the common mode current in the two coils is the same, which has a strong suppression effect on the common mode current. The value of the common mode inductor L CM can be given according to the following formula.

[0046]

[0047] In theory, the larger the common mode inductor is selected, the better. However, when designing the EMI filter, the volume of the filter must be considered, so the common mode inductor cannot be selected too large. At the same time, considering that the high frequency characteristics of the common mode inductor may affect the performance of the EMI filter, combined with the values of the common mode capacitor C Y and the turning frequency f c given above, the turning frequency of the LC common mode filter should not be higher than 13.9 kHz, so LCM =220μH, at this time the LC common mode filter corner frequency is 13.21kHz.

[0048] Further, such as Figure 5 The figure shows the result of common-mode noise suppression after adding the common-mode EMI filter. The common-mode noise of the system after adding the filter is measured again and compared with the peak standard line given by the EMC standard to determine whether it passes the EMC standard. If it meets the standard, the overall design of the filter is completed; if it still does not meet the standard, the cutoff frequency is increased from nf s Reduced to (n-1)f s , repeat the above design steps to obtain the cutoff frequency (n-1)f s The corresponding LCL type EMI filter component parameters are verified again and adjusted repeatedly until the final EMC standards are met to ensure the rationality and reliability of the filter design. Figure 5 It can be seen that the common-mode noise is suppressed to below the EMC standard value after using the improved random switching frequency SVPWM strategy, while the traditional RSF-SVPWM strategy has less than satisfactory suppression effect in the low-frequency band, exceeding the standard value by up to approximately 16dB. The use of the improved random switching frequency SVPWM can reduce the design margin of the filter and increase its cutoff frequency value, which is beneficial for reducing the weight and volume of the inductor, achieving lightweight and miniaturized filters. At the same time, it can achieve electromagnetic interference suppression of the servo drive system at a lower cost. Combining active and passive suppression methods, the additional attenuation provided by the active suppression method can, to a certain extent, avoid further increases in the weight and volume of the filter inductor, and also provide an additional guarantee for the EMI filter to achieve the filtering effect.

[0049] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A common-mode EMI noise suppression method combining passive filters and modulation strategies, characterized in that: The common-mode EMI noise suppression design process used in the method is as follows: Step 1.1: Use the improved random switching frequency SVPWM instead of the traditional random switching strategy SVPWM, and measure the common-mode voltage spectrum without adding the EMI filter, reduce the common-mode voltage amplitude from the source, and achieve the reduction of the common-mode electromagnetic interference spectrum; Among them, under the condition that the spread spectrum range is 5%~10% of the switching frequency, this method can further disperse the high-frequency harmonics and reduce their peak value, thereby achieving the purpose of suppressing electromagnetic interference; the improved random switching frequency SVPWM strategy divides the original spread spectrum range into n frequency bands, each of which corresponds to a new average switching frequency, and sets the switching frequency switching period T b , every time you pass T b / n The switching frequency change band is switched once every 24 hours, thereby reducing the distribution range of the switching frequency value differences between adjacent switching cycles while maintaining the overall spread spectrum range and average switching frequency unchanged. Step 1.2: Use the LISN circuit to measure the original noise of the converter. By measuring the noise before adding the EMI filter, the required noise attenuation of the system is obtained. The common-mode and differential-mode interference calculated by the measurement are compared with the limit values ​​required by the electromagnetic compatibility standard to obtain the required insertion loss of the designed common-mode EMI filter. IL REQ The formula is as follows: ， in, V measure That is the common mode voltage spectrum size after modulation by the improved random switching frequency SVPWM strategy, IL Margin It is the safety margin value appropriately increased according to actual conditions; A Limit It is a limit in dBμV (decibel microvolt) that specifies the maximum permissible conducted emission level of equipment within a specific frequency range; Step 1.3: Insertion loss calculated according to step 1.2 IL REQ , find the minimum frequency point where the system common mode noise intersects the peak standard line given by the EMC standard as f req_min , the nearest switching frequency multiple k Assign to n ,Will f req_min and nf s In comparison, the definition of the turning frequency f c = nf s −Δ f / 4, thus obtaining the switching frequency multiple k , transition frequency f c The magnitude of the RSF-SVPWM spread spectrum range Δ f The relationship between the transition frequency f c As a basis for designing filter component parameters; Step 1.4: Design an LCL EMI filter and obtain the key component parameters: common mode capacitance C Y With common mode inductor L CM If the parameters are met, the overall design of the filter is completed; if not, the cutoff frequency is changed from nf s Reduced to ( n -1) f s , repeat the above design steps and get the cutoff frequency ( n -1) f s The corresponding LCL type EMI filter component parameters are checked again and the adjustment is repeated until the EMC standards are finally met.

2. The common-mode EMI noise suppression method combining a passive filter and a modulation strategy according to claim 1, characterized in that: For the selection of safety capacitors, under the condition of a certain transition frequency, according to the capacitance requirements under the power level specified in "GJB151B-2013", and taking into account the reduction of the volume of the entire EMI filter, a Y capacitor with a capacitance close to 0.075μF / kW is selected. The typical value of the Y capacitor is 330nF.

3. The common-mode EMI noise suppression method combining a passive filter and a modulation strategy according to claim 1, characterized in that: The common mode inductor L CM The value of common mode capacitance C Y and the turning frequency f c The values ​​of are related to the common mode inductance L CM The value of can be given by the following formula: 。

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