Emi filter simulation ltspice


  • Electronic – How to simulate the impedance for this LISN circuit in LTspice
  • Inductor Simulation with LTspice
  • Common Mode Chokes & EMI Filters vs. LTSpice Components library…but which model effectively used?
  • Design and Simulation of EMC filters with LTSpice
  • Optimizing EMI Filters Using Circuit Simulation
  • SPICE Models
  • Electronic – How to simulate the impedance for this LISN circuit in LTspice

    Chris Debraal Milwaukee, WI Sponsor The insertion loss of a power line EMI filter can be accurately predicted over a wide range of frequencies through circuit simulation. While analyzing the circuit, it is important to consider test modes and impedances, basic component values, and a variety of parasitic elements representing each component, as well as construction and layout.

    Formulas for determining equivalent series inductance and resistance of capacitors, as well as inter-winding capacitance and core roll-off, will help create component models. Other elements concerning lead length, proximity effects, and crosstalk will help define the system parasitics. Manipulation of the basic and parasitic component values will lead to performance enhancements and will suggest design improvements.

    A comparison of simulation and actual test data will demonstrate how accurate results can be. With complete circuit representations, performance of EMI filters can be correctly characterized and can be optimized for efficiency. EMI Electromagnetic interference EMI is a type of undesirable electromagnetic emission that causes various levels of noise response, malfunctioning, or degradation in the performance of electrical equipment. This noise is most often found in the frequency range of 10 kHz to 30 MHz.

    To reduce this noise, an EMI filter is often used. An EMI filter is a passive electronic device used to suppress conducted interference present on any power or signal line. The filter may be used to suppress noise generated by the device itself, as well as noise generated by other equipment, and to improve the functionality of the device within its electrical environment.

    EMI filters can include components to suppress both common-mode line-to-ground and differential-mode line-to-line interference. These components include inductors and capacitors. Assessing these components involves a variety of details that must be addressed to insure proper filtering, such as inter-winding capacitance within the inductor coil, effective series resistance ESR , and effective series inductance ESL or lead inductance within the capacitors themselves. Line-to-ground capacitors are used in conjunction with the inductors to boost common-mode performance, and line-to-line capacitors are used to obtain differential-mode performance.

    The line-to-line resistor provides no help in performance. Figure 1. EMI filter schematic. With a basic understanding of the construction of an EMI filter, a designer is prepared to take the next step by simulating the filter circuit to determine how well the filter will perform over the desired frequency range.

    A number of software programs can be used to simulate circuits. The makers of this type of software often offer evaluation versions easily downloadable from the Internet. These programs are very user friendly and will allow for easy simulation of circuits using AC analysis tools, while setting the desired frequency range and nodes to be analyzed.

    The filter schematic used in this example is shown in Figure 1. These values are used as a standard insertion loss test method; therefore, it is necessary to represent these values in the schematic to simulate real insertion loss testing. Inductor Simulation The first component to be considered is the inductor.

    Most inductors are made up of a toroid core wound with various gauges of magnet wire. The most common core materials used in filter applications are ferrite and iron powder. This roll-off value is almost always specified within the specification of the core material.

    Simply find the f0 within the spec of the core material, and insert it into the formula shown in the circuit schematic.

    Inductors carry a standard inductance value most effective in common-mode performance , as well as a leakage inductance value most effective in differential-mode performance. Consequently, the common-mode circuit will have an inductor model that represents the standard inductance, and the differential-mode circuit will show an inductor model that includes the value of the leakage inductance.

    Both models will also include a roll-off frequency formula to help make the simulations as close as possible to the actual filter. Again, the differential-mode roll-off frequency is approximated at 30 times higher than the common. Figure 2. Inter-winding capacitance in farads : Inter-Winding Capacitance Inter-winding parasitic capacitance is created from the proximity of the winding to the core and to other windings. The space between each turn acts as a capacitor in parallel with the inductor.

    The higher the level of capacitance, the greater reduction there is in the impedance of the inductor at higher frequencies. This reduced impedance will allow unwanted noise to pass more freely through the system. Hence, more parasitic capacitance will lower the upper limit of useful frequency.

    Figure 3. Coil schematic and plots. The number of turns on a coil is limited by the inner diameter of its core and just how tightly the turns have been packed together. For increased inductance, once the maximum number of turns has been reached to wind the coil within a single flat layer, the designer must then create more turns with overlapping windings, thus creating multiple layers.

    While the first layer has capacitance from adjacent turns, the subsequent layers have additional capacitance from the adjacent layers as well, thus vastly increasing the inter-winding capacitance of the inductor. Clearly, with this vast increase, it is vital to include this component value in filter simulations in parallel with the inductor to obtain a more realistic result indicating actual filter performance.

    Figure 2 shows capacitance, as well as a picture of a typical inductor coil and a cross-section of the spacings of each turn and layer of wire. This formula is a simple approximation and a good approach for finding a useable value of inter-winding capacitance for a circuit simulation. However, in the case of this simulation, it is only. Figure 4. Capacitor schematic and plots.

    Now, all the parasitic information can be put into the schematic. Figure 3 shows both the schematic and a plot of a simulated inductor coil as compared to that of a real inductor coil. Clearly, the match in plots is very favorable. Capacitor Lead Inductance Any length of wire carries a certain inductance. Therefore, capacitor leads will also have a small amount of inductance associated with them, which can be easily measured.

    This inductance value will be shown in series with the capacitor within the simulation circuit Figure 4. As the length of a capacitor lead varies, the lead inductance will also vary and so will the resonant frequency of the capacitor. This value is usually specified within the specification of the capacitor and will also appear in series with the components within the filter schematic.

    Figure 5. Differential-mode schematic and plots. Figure 4 shows a picture of a line-to-line film capacitor used for differential-mode performance, as well as the plots of simulated results versus real results.

    Again, as with the inductor, the plots are very similar. Figures 5 and 6 are the simulated schematics for both modes, as well as the filter plots versus actual filter test results. Since the models do not address certain types of parasitics such as grounding, shielding and iron proximity, unrealistic results may be seen well above dB within a software simulation.

    Normally, because of these specific parasitics, the best real performance one might expect out of most EMI filters is 80 dB at any given frequency shown as a dashed line in the differential-mode plots Figure 5. Aside from that limitation, the differential plots match very closely and demonstrate that the models do approximate the insertion loss of the filter. Figure 6. Common-mode schematic and plots. In the common-mode plots Figure 6 , the resonant frequency patterns match, but there is again a shift in magnitude because of the parasitics that were not included.

    Conclusion As shown in the information above, EMI filters can be easily simulated to help save time, money, and effort in resolving electromagnetic interference problems in any application.

    Regardless of the design, these simulations will determine the exact type of filter needed and are ideal for custom designs where time and materials are limited. Rather than building countless prototypes and testing each design, these simulations create the designs on-screen rather than in the screen room. About the Author.

    Inductor Simulation with LTspice

    Introduction LTspice is a very powerful tool for simulating electronic circuits. It can perform simple simulations to verify the functionality of a new design. Besides, complex analyses such as Worst Case Analysis, frequency response, or noise analysis, among others, can be completed in a short time. Filters are critical elements in a circuit for many applications.

    To obtain accurate results in the simulations, as close as possible as under a real environment, real effects need to be taken into account. Parasitics play a key role in filtering since they can provoke the opposite effect and amplify the noise. In this article, we will review the different types of noise that are present in a circuit, as well as how to have an accurate simulation of an EMC filter with LTspice.

    Common mode and differential noise Before designing a good filter, we need to know what kind of noise can be present in a circuit. The following circuit is simple, but it will serve to explain the two types of noise. The current generated by the voltage supply V1 will circulate through R1 and then it will go back to the source, so to ground or the reference voltage.

    In ideal conditions, the only current, and then voltage, present in the circuit will be the one generated by the generator V1. A noise overlapping this signal is named differential noise.

    This noise follows the same direction as the signal, as shown below. There is a second situation, in which common-mode noise appears. In this case, the direction is the opposite from the ground to the load. To be really effective against interferences, the design of filters need to consider both types of noise.

    The type of components and their location varies depending on the type of noise to attenuate. We are going to see its capabilities with a simple low-pass filter: For any AC analysis, we need to define one AC source. Many parameters can be configured in a voltage source at LTspice, but amplitude is enough for our purposes. Scale, start and stop frequency, as well as the number of points to calculate are necessary for the definition of an AC analysis.

    After running the simulation, we can plot the output level relative to the input, i. There are two lines, the continuous one corresponds to the magnitude dB while the discontinuous corresponds to the phase. This type of simulation is suitable to verify the functionality of a circuit, as an analog filter. For EMC purposes, we need to simulate common-mode and differential mode noise, so we can be sure that our filters are good for filtering both noise types.

    The following circuit is an EMC filter comprised of capacitors and a common-mode choke to attenuate common mode noise C4, C1, L3 , and two inductors and capacitors to attenuate the differential noise L1, L2, C2, C3.

    Note: the 3. There is usually a trade-off between filter attenuation and leakage current. To simulate a Protective Earth PE or a chassis connection, we use a potential that is slightly capacitively coupled to the regular ground or negative potential.

    To simulate differential noise, we can superpose a voltage source to the signal generator. For the case of the common-mode noise, it can be simulated adding a voltage source to the negative lead.

    A good filter is good enough when both types of noise are attenuated enough, so never neglect one of the types of noise to focus completely on the other one. Ideal vs real filters Unfortunately, real filters do not behave as well as ideal filters, and they present limitations. If we want simulations close to real results, we need to take into account the parasitic elements of filters, as well as of the board where they will be mounted on.

    Parasitic components create resonances that can modify the cut-off frequency of an EMC filter. Therefore, if we do not consider parasitics, we might observe that adding a filter gets the situation worse. Knowing the exact value of each parasitic element can be difficult. Depending on the manufacturer and the given data, values can be obtained from impedances given at different frequencies. In the case that they are not given, they can be estimated to analyze the worst situation possible.

    The following schematic shows the same EMC filter presented before, with some parasitics added. The comparison between the frequency responses of the ideal filter red and the real filter green is shown below.

    The frequency response of the ideal filter falls smoothly until its cut-off frequency. On the other hand, the real filter is effective until the frequency of approximately 30 kHz. Hence, we can see that behavior changes considerably when we are close to a real environment. Note: when downloading LTspice models, you should double-check what is contained in the models, because sometimes they include already all the parasitic elements, saving a lot of work. Self-resonance and dumping Self-resonances of filters can amplify noise in several dB, provoking an undesired effect.

    There are some methods to avoid it or, at least, to reduce the negative impact as much as possible. One of them is quite simple and consists of adding one resistance in series with the end capacitor. The value of the resistance does not need to be huge, we only need to be careful about its power rating. An aspect of capacitors that is normally negative is the Equivalent Series Resistance ESR , which usually needs to be as low as possible.

    If we want to reduce the dumping of a filter, we can select a capacitor with a high ESR and we could avoid the use of an extra resistor. The following figure shows the transfer function of a filter without dumping resistance green and another one, corresponding to the filter with a resistance blue.

    The difference between them is quite big since the amplitude around the resonance frequency is reduced by 9 dB. The tradeoff of this technique is that the filter slope is less pronounced, so all the frequency behavior has to be analyzed to ensure that all the results are acceptable. Conclusion LTspice is a powerful tool that saves cost and time in many applications.

    EMC filters need to be designed specifically for each application, so simulating them in advance saves a lot of time. LTspice performs frequency analysis, which permits the representation of bode plots, the principal tool to study filters.

    LTspice also can include real parameters such as parasitics to obtain simulations as real as possible. Files If you want to test the circuit on your own, you can download them from the following link EMC filter with LTspice.

    There are tons of white papers. Introduction CMCs are mainly used to limit Common mode emissions. I found few and, being LTSpice libraries of such components encrypted, what I found not providing any information about which theoretical model effectively adopted.

    Moreover I was intended to define a generally applicable model that, using some measurable electrical parameters or data sheet available information and leveraging on the remembering I had of the CMC stray parameters equivalent circuit, could create the condition to deduct, in a simple and straightforward way, an equivalent DIY LT spice model. Such a frequency range allow the usage of lumped parameters circuit models either for standard inductors or CMC. Let me resume assumptions I started from: Low frequency, I mean transmission line theory not required; Lumped circuit parameters NOT dependent from frequency.

    Common Mode Chokes & EMI Filters vs. LTSpice Components library…but which model effectively used?

    All parameters involved are available in the data sheet or easily measurable through a 4 wires LCR meter as per below.

    Lstray : applicabile only in case of power transformers or CMC. Coupling capacitance [F] between the two windings [measured directly between windings] Rdc : series DC resistance [ohm]. Let me call this model Basic DIY. However, in the case of this simulation, it is only. Figure 4. Capacitor schematic and plots. Now, all the parasitic information can be put into the schematic. Figure 3 shows both the schematic and a plot of a simulated inductor coil as compared to that of a real inductor coil.

    Clearly, the match in plots is very favorable. Capacitor Lead Inductance Any length of wire carries a certain inductance. Therefore, capacitor leads will also have a small amount of inductance associated with them, which can be easily measured.

    This inductance value will be shown in series with the capacitor within the simulation circuit Figure 4. As the length of a capacitor lead varies, the lead inductance will also vary and so will the resonant frequency of the capacitor.

    This value is usually specified within the specification of the capacitor and will also appear in series with the components within the filter schematic. Figure 5. Differential-mode schematic and plots. Figure 4 shows a picture of a line-to-line film capacitor used for differential-mode performance, as well as the plots of simulated results versus real results. Again, as with the inductor, the plots are very similar.

    Figures 5 and 6 are the simulated schematics for both modes, as well as the filter plots versus actual filter test results. Since the models do not address certain types of parasitics such as grounding, shielding and iron proximity, unrealistic results may be seen well above dB within a software simulation.

    Normally, because of these specific parasitics, the best real performance one might expect out of most EMI filters is 80 dB at any given frequency shown as a dashed line in the differential-mode plots Figure 5. Aside from that limitation, the differential plots match very closely and demonstrate that the models do approximate the insertion loss of the filter.

    Figure 6. Common-mode schematic and plots.

    Design and Simulation of EMC filters with LTSpice

    In the common-mode plots Figure 6the resonant frequency patterns match, but there is again a shift in magnitude because of the parasitics that were not included. Conclusion As shown in the information above, EMI filters can be easily simulated to help save time, money, and effort in resolving electromagnetic interference problems in any application. Regardless of the design, these simulations will determine the exact type of filter needed and are ideal for custom designs where time and materials are limited.

    There is a second situation, in which common-mode noise appears.

    Optimizing EMI Filters Using Circuit Simulation

    In this case, the direction is the opposite from the ground to the load. To be really effective against interferences, the design of filters need to consider both types of noise. The type of components and their location varies depending on the type of noise to attenuate.

    We are going to see its capabilities with a simple low-pass filter: For any AC analysis, we need to define one AC source. Many parameters can be configured in a voltage source at LTspice, but amplitude is enough for our purposes.

    Scale, start and stop frequency, as well as the number of points to calculate are necessary for the definition of an AC analysis.

    After running the simulation, we can plot the output level relative to the input, i. There are two lines, the continuous one corresponds to the magnitude dB while the discontinuous corresponds to the phase.

    SPICE Models

    This type of simulation is suitable to verify the functionality of a circuit, as an analog filter. For EMC purposes, we need to simulate common-mode and differential mode noise, so we can be sure that our filters are good for filtering both noise types. The following circuit is an EMC filter comprised of capacitors and a common-mode choke to attenuate common mode noise C4, C1, L3and two inductors and capacitors to attenuate the differential noise L1, L2, C2, C3.

    Note: the 3. There is usually a trade-off between filter attenuation and leakage current. To simulate a Protective Earth PE or a chassis connection, we use a potential that is slightly capacitively coupled to the regular ground or negative potential. To simulate differential noise, we can superpose a voltage source to the signal generator.

    For the case of the common-mode noise, it can be simulated adding a voltage source to the negative lead. A good filter is good enough when both types of noise are attenuated enough, so never neglect one of the types of noise to focus completely on the other one. Ideal vs real filters Unfortunately, real filters do not behave as well as ideal filters, and they present limitations.


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