Heat Transfer

Introduction

The rise in temperature due to core and copper losses within magnetic components is a crucial factor affecting efficiency and lifespan. As temperature rises, the resistance of copper windings increases, which intensifies heat production and further elevates resistance, potentially leading to complete winding failure. Therefore, accurately predicting the hotspot temperature is essential in the design of an inductor to implement effective cooling strategies. This guide offers a comprehensive and systematic approach to utilizing TRAFOLO to determine the temperature distribution within an inductor during operation.

Model description

The model studied in the present analysis comprises a ferrite core with two windings and a dielectric enclosure. To evaluate temperature rises and losses in the inductor, transient heat transfer analysis is employed. The core and coil have been set as heat sources, assuming homogeneous heat loss distribution. Simplifications include the exclusion of dielectric films and small parts to streamline the geometry.

Modelling Instructions

Setup

  1. From the Model Builder, click on the Setup tab.
  2. Locate the General section and set the Domain type as Full.
  3. Since the primary focus in this case is on thermal optimization, solving a coupled electromagnetic and thermal problem may not be the most efficient strategy. Thermal simulations are significantly faster to compute than electromagnetic simulations. Therefore, if the user knows the losses in components (calculated by a separate electromagnetic simulation), running the thermal simulation independently allows for quicker iterations. Thus, in the Simulation type section, set the Heat transfer as Transient and the Electromagnetics as Off.
    Temperature distribution in the inductor.

    Domain type and Simulation type settings for heat transfer analysis.

Materials

  1. Select the Materials tab from the Model Builder.
  2. Select Polyvinyl chloride (PVC) from the Global Material section and click the Copy button.
  3. Copy Copper from the Global Materials section to the Local Materials section similarly.
  4. Click on the Add New Material button. The Add New Material window will open.
  5. In the Name text field, type Ferrite.
  6. Enter the values as per the table below:
    Density [kg/m3] Heat Conductivity [W/m.K] Heat Capacity [J/kg.K]
    4800 4.0 750

    Given that only thermal simulation is being conducted for this model, the electromagnetic material properties can be disregarded. Therefore, only the aforementioned three thermal properties are necessary for consideration.

    Material properties being set for ferrite core. Density, thermal conductivity and heat capacity are required parameters for thermal simulation.

    Dialog box to configure ferrite material properties.

  7. In the Default Materials section, select the material type for each component from the dropdown list as per the table below:
    Component Core Coil Other Bobbin Gaps
    Material type Ferrite Copper Not set Polyvinyl chloride (PVC) Not set
    The "Materials" tab for inductor design displays global materials on the left and selected local materials on the right. Users can copy, add, and set default materials for core, coil, and bobbin components.

    Interface for material selection for different components of the model.

Core

  1. From the Model Builder, click on the Core tab.
  2. Click the New Group button. The Group1core will be added.
  3. Click the Open in new tab icon. The Group1core tab will open.
  4.  From the Source dropdown list, select CAD.
  5. Click the Import button. The Import CAD dialog box will show up.
  6. Select the file to be imported.
  7. From the Scale dropdown list, select mm.
    CAD geometry being imported for the core, with the "Unmodified" geometry handling algorithm.

    Importing CAD geometry with unmodified geometry handling algorithm.

  8. Select Unmodified from the Geometry Handling Algorithm dropdown list to avoid cutting the core by other components and detect intersections while assembling the geometry.
  9. Click the OK button.
    Geometry configuration for importing along with visualization of core structure in 3D.

    3D model visualization of core geometry settings.

Coil

  1. From the Model Builder, click on the Coil tab.
  2. Click the New Coil tab. The Group1coil will be added.
  3. Click Open in new tab icon. The Group1coil tab will open.
  4. From the Source dropdown list, select CAD.
  5. Click the Import button. The Import CAD dialog box will show up.
  6. Select the file to be imported.
  7. From the Scale dropdown list, select mm.
  8. Select Unmodified from the Geometry Handling Algorithm dropdown list to avoid cutting the coil by other geometries and detect overlapping while assembling the geometry.
  9. Click the Ok button.
    Coil geometry imported from CAD and configured for properties such as wire type and scale.

    3D geometry configuration for winding along with 3D visualization for Group1coil.

  10. From the Wire Type dropdown list, select Stranded & Litz wire.
  11. Repeat the process for the second coil, Group2coil.
    Complete coil structure which includes both coil geometries.

    3D model visualization of the entire coil geometry.

Other

  1. If there is any additional body type, such as clamp, dielectric, or shield, it can be included in Other. Here, this step can be skipped.

Bobbin

  1. Click the Bobbin tab from the Model Builder.
  2. Click the New Group button. The Group1bobbin will be added.
  3. Click the Open in new tab icon. The Group1bobbin tab will be added.
  4. Select CAD from the Source.
  5. Click the Import button. The Import CAD dialog box will show up.
  6. Select the CAD file.
  7. From the Scale dropdown list, select mm.
  8. From the Geometry Handling Algorithm dropdown list, select Cut by other objects to allow the cutting of bobbin by other geometries in case of overlapping.
  9. Click the Ok button.
    Bobbin tab configuration for importing and visualization of the 3D geometry.

    Geometry configuration and 3D visualization of bobbin.

Assembly

  1. Click the Assembly tab in the Model Builder.
  2. Click the Assemble Geometry button. The assembled geometry will be displayed.
    The assembled geometry depicts the complete inductor structure.

    Assembled geometry with component details.

Heat

  1. From the Model Builder, click on the Heat tab.
  2. In the Settings section, enter 25 deg C as the Initial Temperature. This serves as the starting point for the transient heating of the components.
  3. In the Timestep [s] text field, enter 1.0 5.0 20.0 50.0 100.0 150.0 200.0 250.0 300.0. We chose smaller time steps at the beginning due to the rapid rise in temperature when power is applied to the component. Heat generation and removal rates balance out as the component heats up, causing the temperature rise to slow down and stabilize over time. Increasing time steps reduces computational time without significantly reducing accuracy while modeling the temperature dynamics in the component.
  4. Locate the Boundary Conditions section.
  5. In the Type dropdown list, select Convection.
  6. Click on Group1coil.
  7. In the External temperature text field, type 20 deg C.
  8. In the Heat transfer coefficient text field, type 5.0. Generally, the Heat transfer coefficient for natural convection is around 5.0 W/m2K. It can also be calculated using the Heat transfer coefficient calculator link or empirical equations.
  9. Repeat for other boundaries.
  10. Enter the Total Group Power for each component in the Heat Sources section as per the table below. Heat sources distribute the heat uniformly among the components.
    Heat Sources Group1core Group1coil Group2coil Group1bobbin
    Total Group Power (W) 1.5 2.0 2.0 0.0
    Thermal settings showing initial steady state temperature and boundary conditions for various components.

    Initial steady-state temperature setting and configuring Boundary Conditions in the Heat tab.

Mesh

  1. From the Model Builder, click on the Mesh button.
  2. In the Settings section, from the Mesh Refinement dropdown list, select Coarse.
  3. Click the Evaluate Mesh button.
  4. Click the Compute Mesh button.
    Depicting meshed geometry and showcasing how to modify element size.

    Generated mesh for the simulation.

Solve

  1. From the Model Builder, click the Solve tab.
  2. Click the Simulation Setup button followed by the Run Simulation button.
    Solver settings to run the simulation for the inductor.

    Solver settings to run the simulation.

Results and Discussion

  1. From the Model Builder, click on the Results tab.
  2. Click on the Summary tab to see the losses set by the user and the computed hotspot temperature.
  3. Further, the user can switch between the Results tabs to visualize the processed results.
    Hotspot and losses are displayed in Results Summary tab.

    The Results Summary tab displays the calculated hotspot temperature and the user-defined average loss values.

    Since the losses are predefined by the user in the Heat Source section, it must be verified that the losses shown in the Summary tab match those already specified in the Heat Source. In this case, the Total Group Power under Heat Source is set to 2 W for each winding and 1.5 W for the core. The Results Summary shows almost the same values in this case.

    The losses in the two coils are uniformly distributed, as they are considered to be heat sources.

    Nearly equal losses in both windings of the inductor.

    The losses are observed to be evenly distributed throughout the core and across the two windings, with almost equal losses in the two coils. This even distribution is because the losses are predetermined in the Heat Source section and assumed to be uniformly spread within the component.

    Temperature distribution in the inductor showing heating in coil, core and parts of bobbin in proximity to them.

    Temperature distribution on the inductor surface at time = 300s.

    The Temperature tab presents the transient heating profile of the inductor over time steps, up to 300 seconds. The figure below illustrates the temperature distribution at Time = 300 seconds, highlighting a temperature rise within the core and windings. The hotspot temperature reaches 53.5°C, remaining well below the inductor’s critical temperature threshold.

  4. Click the Open in ParaView button to open simulation results in post-processing software. It allows manipulation with results, changing color schemes, making slices, and doing any other manipulations with results.

    Detailed analysis and visualization of losses and temperature in ParaView.

    Temperature and loss distribution on the inductor surface at Time = 300 using ParaView for detailed analysis.

    ParaView is utilized for detailed visualization and analysis of the temperature and loss distribution, as illustrated in the figure above. The temperature rise is evident in the coil, core, and sections of the bobbin in proximity to these components.

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