For the global automotive sector to meet its ambitious electrification goals, inverters and their capacitors must undergo continuous improvement and optimization.

Decarbonization goals across the globe are leading to an increased adoption of electric vehicles. With electric car sales soaring, even the most successful manufacturers must adapt to changing conditions. The German automotive sector, along with its global counterparts, is doing so by developing elektrische autos. Electric cars are an important focus of Robert Bosch — a leading automotive company founded in Stuttgart. Today, Bosch supplies electric powertrains, systems, and components to automakers worldwide.

As the automotive industry races toward an electrified future, Bosch is accelerating its R&D into the essential building blocks of electric drivetrains. One of these components is the inverter, which changes DC current from the car’s batteries into AC current to power its drive motor. The inverter’s ability to provide a smooth flow of current depends on its integral DC link capacitor. “The capacitor is one of the most expensive components of the inverter. Its performance has a direct impact on the performance and reliability of the inverter, which is fundamental to the operation of the drivetrain,” explained Martin Kessler, Bosch’s senior expert for automotive electronics.

For the global automotive sector to meet its ambitious electrification goals, inverters and their capacitors must undergo continuous improvement and optimization. Kessler and his team rely on multiphysics simulation to test and refine Bosch’s DC link capacitors. Their simulation-enabled predictive analysis complements and optimizes the live prototyping of new designs. “It is simply not possible to predict potential problems with testing alone; we need both simulation and prototyping working hand in hand,” said Kessler.

Indispensable Inverter, Crucial Capacitor

Figure 1. A diagram of a Bosch three-phase inverter’s core circuitry. The battery provides DC current, which is converted to a three-phase AC current by the action of three sets of transistors. By switching on and off in a precise sequence, the transistors produce alternating current in three distinct phases, causing the car’s drive motor to rotate. To make the motor’s performance as smooth as possible, the DC link capacitor helps manage the input current that is fed to the transistors.

To make a fully electric car, it is not enough to replace the engine with an electric motor and the gas tank with a battery. Such familiar devices are only parts of a larger system, which helps deliver smooth, reliable performance by adjusting to the constantly varying conditions under which every vehicle must operate.

The role of the inverter in an automotive drivetrain is simple in concept, but complex in practice. The inverter must satisfy the AC demands of the motor with the DC provided by the battery, but it must also adjust to ongoing fluctuations in load, charge, temperature, and other factors that can affect the behavior of each part of the system. All of this must occur within tight cost and spatial constraints, and the component must sustain this performance for years to come (Figure 1).

To understand the inverter’s function, consider what a three-phase AC motor needs in order to operate. If connected to DC current, the motor simply will not rotate. Instead, it must be provided with alternating current with three distinct but complementary waveforms, enabling the motor’s three-part field coil to magnetically attract the segments of its rotor in a sequential pattern. “To control the activity of the motor, we must control the amplitude and frequency of the inverter’s current output,” explained Kessler. “The speed of the motor is proportional to frequency, while amplitude helps determine its torque.”

“The desired current waveform through the transistors has a relatively steep gradient. The only way to achieve switch-mode current with this high gradient is to have very low inductance in the source path,” he said. Inductance is the force opposing changes in current flow. Every slight change in current will be limited by an induced counteracting voltage, which will disrupt the desired waveform — and the smooth rotation of the motor.

Figure 2. DC link capacitors are made from metallized polypropylene film, which is wound into an elongated canister shape.

To reduce the inductance in the source path of the transistors, a capacitor is placed in parallel across the input lead from the battery, which is called the DC link. The DC link capacitor (Figure 2) is placed in direct proximity to the transistors and provides the desired current waveforms through the transistors. The low impedance of the capacitor minimizes any remaining ripple voltage on the battery side.

A typical capacitor consists of two electrodes separated by an insulating gap, which may simply be airspace or some kind of material. In this application, Bosch uses capacitors made with metallized polypropylene film. A thin coating of metal (forming the electrodes) is sprayed on each side of the film, which provides the necessary dielectric gap. The metallized film is then wound tightly into a canister shape. As with the inverter itself, the capacitor’s conceptual simplicity conceals a multifaceted engineering design problem.

Capacitors are widely available components that are installed in countless electronic devices, but they can’t be just picked up from the marketplace. “There are multiple interdependent factors at work. First, we have high demands for performance and reliability. Second, there are very tight spatial requirements. Third, we face difficult thermal constraints, as the polypropylene film in a capacitor can only withstand temperatures up to around 105 °C. This issue is compounded by the interaction of electromagnetic and thermal activity throughout the inverter. And finally, the capacitor is relatively expensive,” Kessler said.

Simulation Solves the Black Box Problem

Figure 3. 3D model image showing simulation of EM effects inside a DC link capacitor design.
Figure 4. A model of the electromagnetic field generated by the capacitor, which aids the calculation of loss distribution in the unit.

To meet the design challenges of a DC link capacitor, Kessler developed a process that combines experimental testing with multiphysics simulation. As an example of why simulation-based analysis is a necessary part of his work, he cites the difficulty of finding and measuring potential hot spots, where high heat and coupled effects can cause failures. “We try to locate hot spots by placing a lot of thermocouples inside prototypes and measuring temperatures at various load points,” Kessler said.

“A simple 2D model of a capacitor is also insufficient,” Kessler said. “The inverter is a distributed system with internal resonances and a complex loss distribution. Our coupled EM and thermal analysis must account for skin effects and proximity effects. We cannot calculate an absolute value for peak temperatures without a 3D finite element approach, which also enables us to model the spatial distribution of coupled EM and thermal effects. This is an ideal task for the COMSOL Multiphysics® software,” he added. (Figures 3, 4, 5)

Figure 5. A 3D model showing simulation of thermal effects inside a DC link capacitor design, and a cutaway view showing the hotspot location in the capacitor.

Kessler’s design process validates simulation models against measured results, where possible, and then uses the validated models to pinpoint potential problems. “By helping us locate hot spots in the model, the simulation helps us avoid issues that would have appeared late in the development process, or even after production had started,” he said. “Instead, we can get specific results and make adjustments early in the process.”

Figure 6. A plot of the ESR curve, as calculated in the simulation, compared with ESR values derived from measurement of a live prototype. Alignment of these curves helps validate the model for further analysis.

“We perform EM modeling and validation of every new design. We compare the calculated equivalent series resistance (ESR) curve with the ESR curve as measured from a prototype (Figure 6). If these curves are aligned, we can set up boundary conditions for stationary and transient heat calculations,” said Kessler. “We can compare the temperature curves from our thermocouples with the results of probes in the COMSOL Multiphysics® model. If they match, we can then simulate all the critical points where we must keep temperatures within limits.” The curve data is put into the COMSOL Multiphysics® software via the LiveLink for MATLAB® interfacing product.

“Before we can do this, we have to think about which factors should be incorporated into the model. Some of the variables we receive from the OEM, such as maximum DC link voltage, are not very relevant to our simulation,” said Kessler. “But the current, switching frequency, e-machine values, and modulation schemes all help define a current spectrum. We need to calculate the current spectrum for all three phases of our output to establish power losses. Once we have this, we can do the harmonic analysis with COMSOL Multiphysics® for the frequencies of the current spectrum. Then we sum up our losses for every harmonic.”

Findings from their analyses can then lead to design changes. Kessler explained that each new capacitor design typically undergoes three rounds of testing. “With simulation, the improvement curve gradient is much steeper from one phase to the next. Our knowledge grows quickly, and this is reflected in the final product.” The latest generation of Bosch inverters promises 6 percent greater range and a 200 percent jump in power density compared to previous designs.

Electrification Shifts into High Gear

As automakers convert more of their product lines to electric propulsion, Kessler believes that the need for rapid, cost-conscious R&D will also increase. “Electric mobility is growing up now. We expect that the OEMs will come to us with more varied needs, for inverters in different power classes and that meet tighter spatial constraints,” said Kessler. “I do think that the number of products that require new capacitor designs will keep expanding. With our simulation-driven development methods, we are confident that we can keep up with this growth,” he added.

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This article was written by Alan Petrillo, content writer, COMSOL (Burlington, MA). For more info visit here .