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Posted on 11 June 2019

Efficient 3D Modeling Tool for Computation of Inductive Coupling Effects

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Nowadays 3D modeling to compute electromagnetic fields is widely disseminated. It is used to evaluate electrical properties of components, assemblies and whole systems. In most cases it is a very time consuming process. Scientists of the Fraunhofer IZM developed a new tool for magnetic coupling computation with an effort reduction of 80% compared to available programs. The graphical geometry input effort could be drastically reduced and the functionality of the tool could be restricted to necessary functions.

By Eckart Hoene, Bernd Stube, Stefan Hoffmann and Bernd Schröder, Fraunhofer IZM

Technological progress in semiconductor switching velocity allows significantly higher switching frequencies and consequently a reduction in cost and size of power electronic devices. However, EMC requirements are rising because of this. The smaller the components for switching, energy storage and cooling become, the higher the percentage especially that of device volume is needed for filtering electromagnetic interferences. In most cases, to meet the limits for conducted noise or electromagnetic fields the generated disturbances need to be filtered. This task is mostly solved with passive components. In practice, EMI filter design is mainly carried out by trial and error in many companies due to the complexity of the topic. For accurate filter performance prediction, besides the nominal values of the components, parasitic properties like equivalent serial resistances ESR and inductances ESL of capacitors have to be considered. Additionally, coupling effects between the passive components gather influence when placed close to each other. Often the filter attenuation is almost exclusively determined by inductive coupling between components with high and low interference levels from frequencies of a few hundred kilohertz.

Over the last few years the group Power Electronics Systems of the Fraunhofer Institute for Reliability and Microintegration (IZM) has successfully solved many of its costumers’ EMC issues in which inductive couplings played a key role. Because of its wide experience and available theoretical knowledge the team is often contacted by companies to solve difficult EMC problems. A typical performance requirement of the specialists is the optimization of component placement in EMC filters for power electronic devices to adhere to the limits for conducted noise. A further issue is the detection of critical current paths in hardware designs that affects the functionality. What’s more the evaluation of occurring magnetic fields caused by interference currents in cables, inductors and so on in power plants, which have to comply with limits for magnetic fields, are included. The costumers come from varied branches, especially from the automotive sector, railway technology and the renewable energy industry.

The effect of magnetic coupling is illustrated in figure 1 and can be explained with the transformer principle. A current i1 via a conductor 1 (primary side) causes a magnetic flux Φ1 around it. A part of the flux Φ12 penetrates the adjacent conductor 2 (secondary side) and when there is a temporal alteration in this flux a voltage u2 is induced. This physical law can be described with the mutual inductance M and numerically calculated with the Partial Element Equivalent Circuit (PEEC) method.

Effect of inductive coupling

What is the current procedure to compute inductive couplings in customer projects? Since there are many different layout tools on the market, customers usually provide their layout to the group in the form of a big pile of papers or files. The central task now is to analyze the specific layout and to detect the EMC relevant current paths. These critical paths can be the source of coupling effects among the component system and strongly influence the EMC performance. To quantify their influence, a combined circuit and field simulation is carried out. The layout of the critical paths including the full 3D dimensions of the components is remodeled. An automatic interface for this work is in our experience not helpful, as manual reworking of the geometry and assigning the crossing nodes is more time consuming than creating a new construction which contains the relevant elements only. This construction is subsequently used as the input file for the PEEC solver Fast Henry to compute and detect coupling effects.

In order to reduce this time-consuming procedure significantly, employees of the institute have developed a new software tool to specify critical EMC paths and to create related input files for the Fast Henry solver more efficiently. The primary idea to reduce the three dimensional geometry input effort is that in PCB layouts the currents mainly flow in the plane of the PCB that is defined as XY plane and therefore the geometry input can be reduced to two dimensions. The third geometry dimension is created by assigning the z coordinate and the height to each segment in a GUI parameter sheet. Due to the rectangular current path structure of many components, for example foil capacitors, current shunts, contactors and fuses, their geometry input can also be conducted two-dimensionally while employing simple retrospective assignment of the height parameters. The model creation of inductive components can be executed by macros. The symmetrical structure of CM chokes, cylindrical coils and so on are utilized to generate component models with only a few input parameters via a GUI parameter sheet. A further benefit of the software program is that its functionality and handling is focused on the Fast Henry model creation. The geometry philosophy is based on the geometry characterization of the Fast Henry input file syntax.

With this new tool the user can input customer layouts that are given as pdf-files or as pictures (e.g. in jpg-format). A high level of smart functionality is provided for segment creation of critical paths in the PCB plane using the computer mouse. The user interface is based on the modern look and feel of many programs. It is shown in figure 2. The input data is automatically saved into a Fast Henry input file. In addition the Fast Henry solver can be called directly via the tool. Then the calculated data can be processed further by following programs. In this way it can be ascertained very quickly whether a device layout is sufficient to fulfill the EMC requirements.

Modern user interface of the software tool

An exemplary illustration of how a DC-DC converter’s filter attenuation can be influenced by the component placement is described in the following. In the left picture of figure 3 the filter components are placed adversely and the PCB traces are executed inconveniently. Due to the high magnetic couplings between filter capacitors, coils and PCB traces the conducted noise limits according to CISPR 25 class 5 cannot be adhered to (right graph in figure 3). The influence of inductive couplings on the filter attenuation is dominant at frequencies from 400 kHz.

Component placement picture and measurement result of the adverse layout

By favorable adjustment of the component arrangement using identical parts as depicted in figure 4 the DM noise level decreases greatly. It can be seen that the measured interference voltage values lie more than 10dBμV below the limits. Therefore it is possible to decrease the nominal component values. In this way the costs for filter elements can be minimized.

Component placement picture and measurement result of the favorable layout

As is shown in the comparison between the graph in figure 4 and figure 5 the noise level can be predicted with a very high accuracy with the use of simulation. For this purpose the inductive couplings have to be considered. The Fast Henry model to calculate the self and mutual inductances is imaged in the left picture of figure 5. With this methodology the optimal component arrangement can be found before a practical setup is constructed. Consequently unnecessary expensive recursions can be avoided. The developed software tool allows a very fast simulation model creation. Therefore system design costs can be reduced.

Component placement model and simulation result of the favorable layout

With the aid of the new tool the working group has already successfully solved costumers’ EMC problems and the effect is significant. Mainly by restricting the functionality of the tool to necessary functions the effort for modeling is reduced by 80% compared to the tools available on the market.

 

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