# CST – Computer Simulation Technology

## Coupled EM-Thermal Modeling of Power Chokes

Power chokes are used together with inverters (e.g. solar inverters as in Figure 1) to reduce EMC issues. In general, one choke is used for each phase of an inverter. Currents flowing through these chokes are in the range of a few amps. Electric and magnetic losses lead to heat generation, and this heat can affect the performance and reliability of inverters. The denser the individual components of an inverter are packed together, the more critical heat management is. CST EM STUDIO® (CST EMS) can be used to calculate the electric losses in the choke winding of a power choke, and these losses can be used as a heat source to simulate temperature distributions using the thermal solvers in CST MPHYSICS® STUDIO (CST MPS).

Figure 1: Prototype of a solar inverter [1]

The virtual prototype of the power choke and its physical counterpart built into the solar inverter are shown in Figure 2. The choke has dimensions of about 56 mm x 54 mm x 83 mm. Its winding material is copper and the core is made from laminated ferromagnetic material....

Figure 2: Virtual (a) and physical [1] (b) model of a power choke

The workflow to solve for the temperature distribution of the power choke is as follows: First, the conductor losses in the choke winding and the magnetic losses in the ferromagnetic material need to be calculated. In Figure 3, the electric loss density in the power choke is shown for an effective input current of 2.21 A at 18.8 kHz. The electric losses were calculated on a tetrahedral mesh using the magnetoquasistatic solver in CST EMS. The total simulated electric losses were calculated as 11.95 W, which is in very good agreement with the measured electric losses of 11.19 W [1]. Magnetic losses were analytically estimated to 11.78 W applying Steinmetz’s formula [1, 2].

Figure 3: Electric loss density distribution

In a second step of the described workflow, the thermal problem is solved usingthe stationary thermal solver in CST MPS. The electric and magnetic losses of the first step serve as heat sources. To consider the interaction of the power choke with the ambient environment, emissivity ε (radiation) and heat transfer coefficient h (convection) are defined for the choke surfaces. For natural convection, h = 5 W/m2/K is a typical value.

Figure 4: Temperature distribution of the power choke, (a) simulated and (b) measured [1]

In Figure 4, the simulated temperature distribution of the power choke is compared with the measured one. An excellent agreement was achieved even without using a more sophisticated computational fluid dynamics-based thermal solver.

References

[1] Courtesy of Fraunhofer IZM.

[2] Steinmetz, C., “Note on the law of hysteresis,” Electrician, no. 26, 1891, pp. 261-262.

Acknowledgments

The work described in this paper has received funding from the Federal Ministry of Education and Research under grant agreement no 16N10943 (Solar project).

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###### Solvers:
• LF-FD
• MPS

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