Developing DC Links Without Prototypes

Complex test cycles on hardware prototypes once measured hotspots, resonances, and current peaks. Today, engineers simulate them. TDK provides free tools like CLARA and CapThermal that engineers can use to virtually examine film capacitors – from individual components to complete DC links. No prototype required. This saves time and money and gives you a competitive edge.

Power electronics engineers must get designs right the first time. Simulation and digital modeling—digital twins—are now essential. For film capacitors—key components in DC links, filters, and inverters—TDK provides a comprehensive suite of web-based design tools like CLARA and CapThermal that help engineers predict performance with high confidence.

These tools are backed by sophisticated models derived from real-world testing. A rigorous process combines electromagnetic and thermal analysis to bridge the gap between physical behavior and digital prediction.

From real-world testing to digital twins

As Industry 4.0 and digital twin concepts advance, engineers get deeper insights into the components they use. Therefore, TDK is digitizing its capacitors through finite element analysis (FEA), capturing electromagnetic and thermal characteristics in great detail.

To ensure these models mirror real performance, TDK uses a "test-simulation-model" approach based on three key steps:

1. Representative testing: Selected samples from the standard series undergo electromagnetic and thermal testing under tightly controlled conditions.
2. Simulation alignment: Simulations replicate the conditions of the representative testing, and models are fine-tuned until test and simulation results align.
3. Model creation: Once validated, the model is expanded to represent the entire series for real application use.

Figure 1 illustrates the complete workflow—from geometry input to simulation output. TDK applies this process not only to standard series but also to customized capacitor designs and customer-specific projects.

Virtual characterization: understanding every detail

The physical parameters of the capacitor—geometry, rated voltage, current, and temperature—drive electromagnetic simulations that map loss distribution in 3D. From this, TDK generates SPICE models that allow engineers to simulate capacitor behavior within a converter circuit.

Thermal simulations build on this data, using computational fluid dynamics (CFD) to evaluate temperature distribution and hotspot formation under realistic boundary conditions.

These simulations ultimately form the backbone of TDK's CLARA (Capacitor Life And Rating Application) platform, which includes practical tools such as Capacitor Banks and CapThermal. Each tool helps engineers visualize and optimize capacitor performance without lengthy test cycles.

Figure 1:

Digitalization flowchart

Figure 2:

Examples of ESR and ESL curves obtained by simulation (virtual characterization) and test

 

 

 

The power of electromagnetic modeling

Electromagnetic modeling is the cornerstone of TDK's digitization process. Unlike thermal modeling, which follows well-established steady-state methods, electromagnetic simulation requires a deep understanding of current flow, parasitic effects, and internal field distribution.

Through virtual characterization, engineers can accurately determine impedance (|Z|), capacitance (C), equivalent series resistance (ESR), and inductance (ESL) across frequencies—all without fabricating a single prototype. Figure 2 compares ESR and ESL curves from virtual and physical measurements, showing near-perfect correlation.

This approach also accounts for manufacturing tolerances and aging effects, allowing users to explore both nominal and worst-case performance scenarios.

Including temperature effects: thermal modeling

Temperature affects a capacitor's electrical performance, mainly through the thermal coefficient of resistance (TCR) in the metallization layers. While the properties of bulk metals are well-known, thin metallized films behave differently.

Figure 3:

Increase in losses caused by the TCR, considering representative application conditions (current spectrums) from ambient temperature to representative application temperatures 

TDK has characterized the TCR across the various elements inside a capacitor, ensuring that virtual models reflect temperature-dependent losses accurately. Depending on the application—DC-link, filter, or PCB-mounted—the TCR influence varies (Fig. 3), helping engineers predict how performance evolves under real operating conditions.

SPICE-equivalent models: bridging physics and simulation

SPICE remains the standard tool for electronic circuit simulation. But accurate results depend on precise component models. TDK's SPICE-equivalent models combine mathematical accuracy with computational efficiency, representing real capacitors across both time and frequency domains (Fig. 4).

Figure 4:

Simplified capacitor model

Each model includes enough elements to mimic real-world behavior without slowing simulations. These models are not limited to single components; they can describe complete DC-link systems, PCB assemblies, or custom configurations (Fig. 5).

TDK provides SPICE models for all standard film capacitor series via its website and CLARA platform. Custom models can also be generated upon request, giving engineers the confidence to simulate real capacitor behavior directly within their converter design.

Figure 5:

Comparison of ESR and ESL curves between measurement, simulation, and equivalent spice modeling

 

 

 

 

Figure 6:

Structure of the analyzed capacitor (left), ESR curve and current analysis (upper right), and thermal simulation results at 10 kHz, 37 kHz, and 60 kHz (lower right)

Revealing hidden interactions

Electromagnetic simulation visualizes hidden interactions inside the capacitor. Engineers can detect phenomena such as skin effects, uneven impedance distribution, or internal resonances—effects that are hard to capture through measurement alone.

For example, Figure 6 illustrates how resonances inside a cylindrical capacitor can cause current peaks in its individual capacitive elements, while the total external current remains unchanged. This kind of insight helps developers to avoid unwanted resonances and improve overall system reliability.

Validating through thermal testing and simulation

Thermal modeling follows the same philosophy depicted in Figure 1: test first, simulate next, then model. Real-world conditions are difficult to reproduce, so thermal simulations are essential. They let engineers analyze internal temperature points that are impossible to measure physically.

TDK's Capacitor Bank Simulator within CLARA combines CFD and empirical data to predict the temperature rise of capacitor arrays based on geometry, cooling airflow, and mission profile parameters. Figure 7 shows a sample output for 5 xEVCap in parallel, where the tool calculates the temperature distribution across all units.

Figure 7:

Capacitor banks simulation result

CapThermal: fast insights without heavy simulation

For engineers who need quick thermal insights, TDK developed CapThermal—a simplified, web-based tool that emulates the results of a full FEA simulation. By entering boundary conditions such as losses, ambient temperature, and cooling, users instantly receive hotspot and surface temperature maps (Fig. 8).

CapThermal makes professional-grade thermal analysis accessible to everyone, allowing designers to select the optimal capacitor for their conditions and even explore ways to extend lifetime through better cooling or placement.

Figure 8:

CapThermal example simulation result

Figure 9:

Thermal integration simulation

Thermal integration: seeing the full picture

In automotive and other high-power applications, capacitors rarely operate in isolation. They share their environment with semiconductors and cooling systems. TDK's thermal integration approach takes this into account by simulating the entire subsystem, for instance, the DC-link capacitor, semiconductor modules, and coolant flow path together.

This approach delivers a more accurate picture of real operating temperatures, since component interactions and heat exchange are directly modeled (Fig. 9). As a result, engineers gain a realistic view of how capacitors behave in full systems, not just in isolation.

The road to fully digital design

TDK's ongoing digitization initiative aims to transform every standard capacitor series into accurate, validated digital models. These resources—available through the CLARA platform—include SPICE models, capacitor bank simulations, and now CapThermal for thermal simulation.

By combining electromagnetic, circuit simulation, and thermal modeling techniques, TDK enables engineers to design smarter, faster, and with greater confidence. The result: fewer prototypes, shorter development cycles, and optimized capacitor use in every application—from industrial drives and renewable energy to automotive power electronics.

Fernando Auñón, Fernando Rodríguez, Sergio Sepúlveda, David Olalla TDK Electronics
 

References

[1] S. Chowdhury, E. Gurpinar, and B. Ozpineci, "Capacitor Technologies: Characterization, Selection, and Packaging for Next-Generation Power Electronics Applications," IEEE Transactions on Transportation Electrification, vol. 8, no. 2, pp. 2710–2720, 2022.

[2] H. Wang et al., "A Thermal Modeling Method Considering Ambient Temperature Dynamics," IEEE Transactions on Power Electronics, vol. 35, no. 1, pp. 6–9, 2020.

[3] V.V.R. Narashimha Rao et al., "Electrical resistivity, CR and thermo electric power of annealed thin copper films," Journal of Physics D: Applied Physics, vol. 9, no. 1, 1976.

[4] M.F. Staniloiu et al., "SPICE model of a real capacitor: Capacitive feature analysis with voltage variation," 2020 International Conference and Exposition on Electrical and Power Engineering (EPE), 2020.