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Physics Based Modelling for Power Electronic System Design and Validation Using Ansys

Power Electronics is defined as the study of conversion of electric power from one form to another. The power generated is processed through several power electronics components converting AC to DC or DC to DC etc. With the increasing demand in power conversion, miniaturization and need for extended life of the components, the challenges faced by the designers in the process of design and validation increases exponentially.

When we specifically talk about the automotive system, which once upon a time was dominated by the mechanical components, it can be said that it is getting more and more electrified. IC engines are being replaced by electric motors, with greater focus on safety and electronic controls being incorporated into the two, three or four wheelers have increased tremendously [Figure 1]. All these are possible with the introduction of power electronics into the automotive system.

autompbile
Figure [1] Applications of Electronics in an automobile

Trends and Challenges in Power Electronics for Electric Vehicle

Electric vehicles come with their own design challenges and innovations are pertinent to overcome these challenges. The arrival of wide band gap devices such as Silicon Carbide (SiC) and Gallium Nitride (GaN) have resulted in increased operating frequency range, increased power handling capacity, and reduction in size of electronic circuits [1]. One of the biggest roadblocks in EV adoption is charging downtime. With fast charging and Level 2 charging (Charing at high Power rating), this has been minimized. However, these charging techniques require careful design of Power Electronics to support the charging rates. The above trends pose engineering challenges, which either relate to component sizing and validation, or system integration and performance. The reliability challenges such as thermal and structural integrity, while minimizing electromagnetic emissions, can significantly impact the design and development cycle.

If we consider the customer’s perspective, the need for an improvement in the performance, safety and reduction in cost, becomes priority. In line with this, the designer has to focus on design refinement to improve mileage/range of vehicle, increased life of the battery, better comfort, confirm that the model complies with the various EMI-EMC standards, design thermally and structurally reliable system. Simulation through virtual modelling helps in addressing all these challenges prior to entering the prototype building stage.

ComponentPower electronic system design workflow

The different phases involved in the power electronic system design follows the V curve, which includes component design and characterization, reliability study, final system integration and validation with functional safety norms applicable at every single step.

Component Design & Characterization

The components of an electric powertrain system are the high frequency electronic transformers, switching devices like IGBT’s, MOSFETS, FET’s etc., control logics, traction motors, busbars and cables.

About the author:

Reshmi Raghavan,
Reshmi Raghavan, Senior Electrification Specialist, Ansys

Reshmi Raghavan, is working as a Senior Electrification Specialist with Ansys for more than 9 years. She has extensive experience in working with global Automotive OEMs on the subjects such as Power Electronics Design, vehicle electrification and EMI EMC. She has also developed several workflows for Multiphysics modelling, virtual validations, and Machine Learning

Let us focus on the challenges and design methodologies followed in the characterization of the electronic transformers and switching devices in this section.

The characterization of the high frequency transformers comes with its own challenges, where the functioning of the model is quite different from the low frequency transformers. Understanding the relationship between the available output power and transformer parameters, such as core area product, peak flux density, operating frequency, and coil current density are needed for the core selection. Operating a magnetic core at high frequency causes increase in core and copper losses, with the increased losses temperature rise in the model becomes an issue [2]. At Ansys, we use physics-based simulation to design and optimize the various parameters of the transformer like size, turns, material of the core and winding. Extraction of the magnetic parameters like impedance over a frequency range, losses in the core, temperature rise in the windings, structural integrity of the model are analyzed using electro-thermal and structural modelling.

The selection of the switching devices is based on the operating frequency, current rating, and operating temperatures. Prediction of the switching and conduction loss and extraction of the power module parasitic are needed to precisely estimate the characteristics of the electronic circuit. Semiconductor characterization is one of the methods of extracting the required parameters of the switching components to be used for modelling the power electronic circuits. Using Ansys Twin Builder, we extract the model of the switches based on the data provided in the datasheet as shown in Figure [3].

Characterization
Figure [3] Characterization based on datasheet inputs

Why reliability?

Why do we have to focus on the reliability study, be it electromagnetic, thermal or structural? With increasing feature integration and reduction in size of the electronic components, the need for efficient power management, compliance with the emission norms, thermal and structural integrity of the system has become key challenges. The need of the time is to rectify the issues at an early stage, before getting into prototype building or testing, resulting in saving costs and time to market.

Electromagnetic reliability: Compliance with the various EMI EMC standards (CISPR, SAE, ISO, IEC, AIS [3]) is mandatory for each entity, ranging from the component to the vehicle level. Simulation helps in early detection of the emissions and optimizing the designs. At Ansys, using the electromagnetic suite of products, we carry out conducted-radiated emissions, power integrity analysis and provide users with options to tune the design to overcome the design failures. Figure [4] shows the output of the LISN model for the conducted emission test of an inverter model being compared with the measured value and the standard.

LISN
Figure 4. LISN output for conducted emission

Thermal reliability: Increasing the power density increases the challenges in designing thermally reliable components. Controlling the temperature distribution, selection of right cooling topologies, heat sink designs, selection of the correct switching components (SiC or GaN) and more come under this. Using Ansys thermal solutions we can design and analyze the structures and mechanisms that efficiently remove performance degrading and unwanted heat from the system. Figure [5] displays the temperature profile inside the electronic package with the heat sinks and cooling methodologies incorporated.

electronic
Figure5. Temperature profile inside the electronic package<//center>

Structural Reliability: How catastrophic would be a condition where the automotive electronic components lose its structural integrity while the vehicle travels over a bumpy road? This could lead to the failure in the functioning of control systems, leading to an accident amounting to the death of passengers. Virtual modelling helps us in estimating the structural reliability of the component under various situations. Figure [6] shows the deformation in the PCB model under the impact of the electro-thermal load.

electro
Figure 6 Deformation of the electronic package under the impact of an electro-thermal load

System integration and validation

In order to guarantee the functional efficiency of the system, verification and confirmation on the performance of the system post integration of the multiple components is crucial. Ansys system solution solver, Ansys Twin builder, provides us with the opportunity to integrate the various components and carry out the system level closed loop simulation to measure the functioning of each component of the electronic powertrain post integration. Figure [7] highlights the integrated system model for a hybrid electric vehicle with battery management system, motors and controls in place.

hybrid
Figure 7. Closed loop control system of a hybrid electrical vehicle

Conclusion

 Introduction of power electronics into the automotive system has led to electrification of the original mechanical system with improved performance and better safety measures at an affordable price. It is simulation that helps in the early assessment of the design, to understand the failures and to find solutions and incorporate the same in the design before moving on to a prototype building and testing phase.

References:-

[1]https://www.infineon.com/cms/en/product/ wide-band-gap-semiconductors-sic-gan/

[2]https://core.ac.uk/download/pdf/147124489. pdf

[3]https://en.wikipedia.org/wiki/List_of_common_ EMC_test_standards

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