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Posted on 01 January 2019

Ground-breaking Software for Power Electronics Design

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InCa3D enables parasitic extraction thanks to PEEC method

It is no news that virtual prototyping helps reducing both development costs and time-to-market and as a matter of fact the demands on simulation software are getting higher and higher. Among the numerous tools that are proposed to power electronics designers, Inca3D stands out from the rest by enabling the fast modelling of parasitics in interconnections. It manages the efficient combination of fast processing, high accuracy and user-friendliness.

By Marie Lionet – CEDRAT Groupe

 

The harmful influence of cabling impedances is particularly critical in power electronics applications where switching phases cause brutal current variation. The parasitic inductance introduced by the cabling is responsible for inopportune voltages overshoots that can have irreversible consequences on semiconductors (component destruction). Another major influence of the cabling appears when component are paralleled: current unbalance can appear and lead to unequal operating, thus disturbing the performance of the whole system. Besides, the resistive contribution of the cabling is responsible for Joule losses that may become critical for high power applications.

The characterization of the stray elements introduced by the layout before the realization of a prototype is the privileged way to reduce costs and delays. Unfortunately this is also the critical point in the design process. Engineers usually rely on their experience and some design rules to build up first designs but, faced to the increasing complexity of power electronics structures, this approach is definitely not sufficient. To reduce the development costs and to improve the efficiency of their work, designers need appropriate tools to extract the parasitic elements induced by connexions. This recurrent demand makes InCa3D software the essential tool for power electronic design teams.

Modeling Method

Finite Elements methods (FEM) are usually the reference for the solving of Maxwell’s equations but are not adapted for calculations on cabling structures. A global method – PEEC, Partial Element Equivalent Circuit - making it possible to attribute to each part of the circuit a contribution to the total impedance was proposed by A. E. Ruehli at the beginning of the 70s. On the basis of Maxwell’s equations, the partial element concept is introduced in order to be able to model connections of an electric structure by using an electric circuit with localized components. PEEC is a method which allows the exact calculation of the resistance, the partial inductance and the mutual partial inductances of rectilinear conductors of rectangular cross-section. This method corresponds to the quasi-static electromagnetic field of magnetic type, where the capacitive effect is neglected.

So, from almost any three-dimensional layout, an electrical equivalent circuit can be obtained and used for subsequent circuit calculation. Compiled within an impedance matrix the equivalent circuit takes into account skin and proximity effects. The addition of the electrical environment – current or voltage sources, load resistors, capacitors or inductances – makes it possible to characterize the electrical behaviour of the cabling. This enables the evaluation of local values such as current distribution and induction or the computation of global values like losses, currents, etc.

FEM and PEEC methods provide very close results but with very different computation times: by avoiding the meshing of the air thanks to its integral formulations PEEC method reveals to be much faster than FEM for the modelling of layouts (in the range of 1 min. vs. 1 day). To take advantage of this method a PEEC solver has been developed and implemented within InCa3D – a completely integrated software package.

InCa3D Software

Co-developed by a team of power electronics researchers from the Grenoble Electrical Engineering Laboratory (G2ELab) and CEDRAT – specialized in the development of software solutions for electrical engineering applications - InCa3D combines a powerful PEEC solver and an advanced Graphical User Interface. Any kind of three di used have a relative permeability equal to 1. Besides, the convenient GUI makes the work of the engineer much easier thanks to numerous functionalities (data acquisition boxes, display options, etc.). Figure 1 represents a view of the Inca3D interface.

InCa3D Interface

Structures studied

The first one is based on a ‘pipe’ approach in which the user defines the average path of the conductor with a rectangular cross section. This enable very quick description of conductors in which the current flow is unidirectional. For conductors of complex shape or in which the current flow direction is unknown, a classical CAD description can be done. In both cases geometric transformations like extrusions, symmetries or propagations can be used and of course both types of conductors can be combined within the same model to generate quickly complex geometries. Besides, InCa3D command language is based on an open source programming language (Python) that makes it possible for the user to program his or her own advanced user command and macros.

So InCa3D is well adapted for the modeling of power electronic structures like internal layouts of power module, connection busbars, IC or PCB, etc. But it can also be used for larger systems like switchboard, switchgears or current distribution busbars.

Towards design optimization

Improving the layout of a power electronics system can be as important as boosting the efficiency of the semiconductors components. To achieve this, InCa3D results on the electrical behaviour of the system under study can be much helpful. Analysis of the current density provides results on current distribution, induction, losses and forces whereas the loop inductance can be obtained thanks to impedance matrix computation. Figure 2 and 3 show some InCa3D results.

InCa3D post-processing – Current density of a current distribution busbar

Multi domain analysis is also possible thanks to couplings to circuit simulators or CFD tools. The automatic export to software like Portunus and Saber or to Spice-like simulators makes it possible to take into account the cabling effects for time or frequency domain analysis.

InCa3D post-processing - Induction emitted by a busbar on a plane

Finally, the use of geometric parameters enables simple and fast changes in the geometry opening thus new perspectives of design optimization.

Application: Study of a Power Module

Having an impedance model of a structure is very helpful to design a power electronic module. The parasitic inductances of cabling are the origin of phenomena like unbalanced currents or voltage overshoot in semiconductors. Moreover, the parasitic resistance involves additional losses, which must be dissipated by the heat spreader.

This investigation has been done on a power module – a complete three-phase inverter - whose internal layout has been modelled thanks to InCa3D. To get the equivalent circuit of the power module studied, all connections of the system that play an electrical role have been modelled within the InCa3D geometry descriptor. So, geometry of pins, wire bonds, DBC substrates, and the heat spreader have been described. The complete structure is represented in Figure 4.

InCa3D model of a power module layout

Loop impedance (see Table 1) can be computed with InCa3D revealing unbalances between the different switches.

Impedance calculated with InCa3D on each switching loop

In order to complete the investigation, a circuit simulation can be done providing results like the voltage overshoot at the opening of the IGBT, current unbalances or even the conducted EMI spectrum as presented in Figure 5.

EMI spectrum of the power module obtained thanks to InCa3D and circuit simulator coupling

New perspectives

By providing results that enlarge the knowledge of the structure, the InCa3D software is a ground-breaking tool for conception time reduction. Benefiting from the latest software development technologies (graphical user interface, command language, solvers, etc.) it is the ultimate software to model any kind of electrical connection, review its performance and assess things on a system level. InCa3D gives the opportunity to take into account the electric behaviour of the layout early in the design process and hence to reduce considerably the developing costs.

 

 

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