Electro Thermal Simulation
The increasing complexity of nowadays wireless RF devices increases the demand for accurate and efficient simulations of large and complex RF designs. Identifying and predicting potential issues early in the design process saves resources, time, and money. Heat can degrade the performance and the reliability of electronic devices, thus thermal analysis plays a crucial role to determine weather the device will perform as intended to be in real operating conditions. Electromagnetic losses including the dielectric and conductor losses are one of the main heat sources and thus an accurate calculation of these losses is essential to predict the actual performance. Using ANSYS HFSS, the EM losses can be accurately calculated, which can then be imported into ANSYS thermal analysis and simulated using ANSYS Icepak CFD solver to help engineers solve the most complex thermal challenges and to predict how their designs will perform with temperature changes.
Overview
In this blog we will be using the ANSYS Electronics Desktop (AEDT) to run a two-way coupled electro-thermal simulation of high power RF coaxial filter. ANSYS HFSS will be used to perform the electromagnetic simulation to calculate the RF dielectric and conductor losses. The RF losses will then be coupled to ANSYS Icepak CFD solver to perform the thermal analysis including conduction, convection, and radiation modes of heat transfer and calculate the temperature profile. The temperature profile will then be fed back into HFSS to modify the temperature dependent material properties and recalculate the EM losses. The EM losses can be retransferred into Icepak for a number of iterations to improve the accuracy of the thermal analysis.
Workflow
The work flow of this demo consists of the following steps:
- setting up the HFSS simulation to enable temperature feedback by defining temperature dependent materials and setting the objects temperature.
- Simulating the electromagnetic losses through all solids using ANSYS HFSS
- transfer the geometry with material assignments to Icepak with the calculated EM losses
- setup the Icepak simulation, including boundary conditions, mesh setup, and solver setup
- setup 2-way Icepak simulation
- simulate the 2-way coupling analysis until results converge
- postprocess the results
HFSS Setup
In this demo we will simulate the below RF Coaxial filter and calculate the RF conduction losses due to the finite conductivity of the materials used and the dielectric losses.
To enable the 2-way coupling analysis between HFSS and Icepak, the material properties in HFSS need to be modified to include a thermal modifier. To add the thermal modifier;
- Open the material properties,
- Click on View/Edit Materials..,
- Check the box to include the Thermal Modifier.
The thermal modifier in this example will be applied to the Aluminum Bulk conductivity and the Teflon material relative permittivity and loss tangent.
The second modification to the HFSS design to enable the Icepak temperature feedback is to set the object temperatures. To set the object temperature;
- Right click on the HFSS design in the project manager
- Select Set Object Temperature...
- Check the box to Include Temperature Dependence
- Check the box to Enable Feedback
- Enter the temperature value
- Click Ok
After setting up the HFSS design, we can start the HFSS simulation. The below figure shows the S parameters plot of the filter response.
Using the filed calculator we can calculate the surface loss density on the finite conductive surfaces and the volume loss density in the lossy dielectric volumes as seen below. The input power can be modified in the Edit Sources window. In this demo the input power is set to 1W.
Icepak Setup
To create the Icepak design using the 2025R1 release,
1) right click on the HFSS design, and select Create Target Design...
2) Under the General Tab, select Target Design Type to be Icepak
3) Select the Analysis Setup
4) Under the Icepak Tab, Select Forced Convection
5) Select the Flow Speed to be 1 m/s
6) Select the Flow Direction to be +Z
This workflow will create an Icepak design with the geometry, boundary conditions, and analysis setup defined. It will also automatically link the EM losses to the right geometry.
The solution type for the Icepak analysis is Steady State with Temperature and Flow.
The defined boundary conditions are as seen below, an Inlet Velocity and Outlet Pressure.
To setup the 2-way coupling analysis between Icepak and HFSS;
- Right click on the Icepak analysis setup and select Add 2-Way Coupling
- Select the number of Coupling Iterations
- Check the box to continue Icepak iterations during Coupling
- Enter the max Icepak iterations per coupling
Before starting the analysis, the mesh can be viewed. Notice the thermal simulation mesh is different than the HFSS generated mesh.
- In the mesh viewer, check the “Show” option and make sure that “Cut plane” is selected.
- In the “Define plane drop down, choose “X plane through center”
- Turn the model so that you are looking at other planes
The analysis setup for this model is as seen below. Temperature and Flow problem types with a Flow Regime of Turbulent. The radiation model is Discrete Ordinates and gravity is included. Both are necessary for natural convection. To increase the simulation accuracy, the number of iterations can be increased.
Under the “Solver Settings” tab, the user is setting initial values for the simulation equations
- Allows steady state models to converge faster
- 1 m_per_sec is set opposite to gravity (positive z direction)
- This panel, however, can be ignored if you want
- Press the “Advanced Options” to review recommended settings
After the simulation is completed. We can review the solution profile and observe the residual plot for the convergence criteria setup in the solution.
We can confirm that the EM losses calculated by HFSS has been mapped correctly by looking at the total mapped EM loss in the simulation profile. As seen, the total EM loss is the same as the summation of the HFSS calculated surface and volume loss densities. We can also see that 2 coupling iterations have been performed.
The HFSS simulation profile should also include the temperature feedback from Icepak, as seen below.
Below is the temperature profile and the velocity plot of the coaxial filter simulation.
A complete demonstration is provided in the video link below: