In this blog, we will discuss how to perform RF and thermal analyses using HFSS and Icepak. The thermal analysis will be performed using the electronic desktop Icepak. HFSS, 3D Layout, or Q3D can be used to generate power loss (dissipated) data for Icepak.
For the simulation of RF electromagnetic fields, HFSS is an extremely powerful and accurate tool. However, HFSS is limited in its ability to solve for low frequencies. Q3D is used for low frequency applications. This will be discussed in another blog. DC loss thermal analysis is solved using SIwave or 3D Layout.
There are certain applications where a device needs to transmit a large amount of power through a metallic waveguide (e.g. Earth stations to geostationary satellites). To build a proper cooling system around the waveguide, the designer needs to know the waveguide's temperature.
| 3D Layout | Q3D | HFSS | SIwave | |
| Loss | DC Loss | AC Loss | RF Loss | DC Loss |
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Applications
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Multiple PCBs (Data Centers / Servers / )
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Power lines/ Bus bars Frequencies < 1 MHz or Small size (Dies)
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High power waveguides (Earth stations / Satellites / Radars) or High power amplifiers (Wireless Transmitters / Optical Modulators) or Biomedical Implants or Microwaves |
Single PCB
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| Icepack limitation | Full 3D | Full 3D | Full 3D | No access |
Start by building your model, or import a one. HFSS supports the import of STEP and SAT files in the standard MCAD format
Our model today is a power divider. The waveguide is WR975, which operates between 0.75 and 1.2GHz.
We are injecting 20000 Watts into one port, and would like to know how much power is dissipated inside, and what the waveguide's temperature is.
Start the HFSS setup. Initially, assign materials to all waveguides, say copper.
After that, assign ports as follows
The next step is to add a solution setup:
There is no need to sweep. Due to the fact that thermal analysis is always performed at a single frequency point.
The final step before starting the solution is to right-click on the model name and select to set the temperature, enable feedback for thermal analysis, and set the current temperature to 20cel. Now the model is ready.
Press OK, and start solving.
Display the S-parameters by right-clicking Results. You will notice that only -50dB reaches one branch. There is a large amount of reflection, around -5.5dB.
In order to perform a thermal analysis, right-click on the name of the model and select Create Target Design. An Icepak model is created as a result.
Expand the Thermal section of the setup:
The model has two openings, opening 1 is simply an air flow opening.
The air is coming from Opening2. The air velocity is specified here.
When solving for temperature and flow, you will need these two Openings. You do not need them if you solve for the temperature alone. Take note of the air region surrounding the waveguide. Thermal analysis requires this information.
At this step, you can add any other component you want, like a fan, a heat spreader, other PCBs, enclosure, ...etc
Double-click the setup to verify it
Select the discrete ordinates for the radiation. For the solver settings, change the air velocity to 0.1 to match the openings:
Start solving.
Check the loss numbers in the profile:
The energy loss in each section can be seen here. You notice that the dissipation is so small. In addition, the loss is negligible on the waveguide with no fields, i_Waveguide_5_2_2. The majority of the loss occurs in the main waveguide (input side). The junction dissipates 191W, while the other two branches dissipate 54W each.
After selecting all the waveguides, right-click and select Plot Fields, Temperature, then Temperature.
Plot on surface,
The results are as follows:
There is not a great deal of power dissipation, which is why the temperature did not rise too high. The size of the waveguide absorbs and distribute all the heat. On the branch where no wave is traveling, the temperature remains at 20 degrees Celsius.
In order to perform thermal analysis for high power RF applications, the user must follow this procedure. In the event that there are a very high temperature, due to a lossy design, the user may activate the two-way analysis. This can be accomplished by activating the two-way analysis in Thermal. Select Add 2-Way Coupling from the right click menu of the thermal model setup.
Press OK to set the number of iterations to 2,
In the HFSS model, right-click on the copper material and select properties to open the material panel. Click on the Edit material button:
In the View/Edit modifier for, select the Thermal Modifier option:
In other words, you are allowing the copper conductivity to change. The thermal formula for conductivity should be edited as follows:
The formula can be modified here in accordance with the supplier's instructions. In accordance with the default formula, the conductivity remains nominal if the temperature is between -25 and 1000 degrees Celsius.
Once you have clicked OK, click OK again.
The setup for a two-coupling is now complete. Thermal analysis results are sent to HFSS, HFSS performs the analysis, and Icepak receives the results. The process is repeated two times. In our case, the temperature is still around 20 degrees Celsius. As a result, the conductivity will not change.
