Most of today’s wireless applications call for a reduction in the physical size and an increase in the functionality of RF and microwave components, forcing designers to adopt a host of new materials and structures. These densely packed circuits present a number of engineering challenges from increased electromagnetic coupling to increased dissipated power. Integration and miniaturization exacerbate the interdependency between electrical, thermal and mechanical behavior, calling for engineering tools that not only simulate each domain accurately but capture the coupling between them. ePhysics version 2.0 from Ansoft does just that by dynamically coupling thermal and stress analysis to HFSS for electromagnetic-based multi-physics analysis. The combination of ePhysics with HFSS allows engineers to investigate quantities such as temperature, stress and electrical performance for an arbitrary 3D structure without the need to build and test a physical prototype.

Coupled Analyses

ePhysics v2 provides powerful multi-disciplinary analysis within an electromagnetic-based design flow. A unique dynamic link mechanism supports automated data exchange between dedicated solvers. Through this coupling between the ePhysics and HFSS solvers, a device’s behavior can be fully investigated. With these coupled simulators, engineers can account for the mechanical and thermal consequences of the electromagnetic fields that contribute significantly to a design’s overall performance. There is no limit to the number of projects and analyses that can be coupled in a “daisy chain” fashion to reproduce the desired application. In such linked designs, variables can be exchanged between any two adjacent designs and used wherever needed along the daisy chained designs. For example, a multi-domain capability is exercised as temperature distributions resulting from HFSS derived electromagnetic fields that are then channeled into the ePhysics electrostatic solver to evaluate the induced mechanical stress and resulting deformation, as shown in Figure 1. Note that in this application the magnetostatic solver in Ansoft’s Maxwell product provides the distribution of the nonlinear biasing DC field in the ferrite component such that HFSS can accurately calculate the corresponding HF fields and losses in the ferrite.

In this multi-domain analysis, the thermal analysis provides the temperature distribution source used in the stress target design for the computation of the resulting deformation and stress. The thermal capability in ePhysics provides nonlinear steady-state and transient thermal analyses, including all heat transfer mechanisms: conduction, convection and radiation (which can be coupled to Ansoft electromagnetic solvers), and stress solvers. The power loss with volumetric or surface density distribution information obtained by the 3D electromagnetic solver HFSS serves as a highly accurate heat source for thermal analysis. This allows engineers to obtain a complete thermal profile of a device including the overall temperature distribution and location of hot and cold spots in a steady-state thermal analysis or for any instant in time if the thermal analysis is transient. In addition to using thermal sources based on HFSS derived losses, independent thermal sources may also be applied to the same or different objects in ePhysics. For example, this is very useful to model additional heat sources that are not of electromagnetic nature.

The linear stress-analysis engine in ePhysics is especially useful for analyzing deformation and stress due to electromagnetic force density distribution, thermal deformation, structural stress due to temperature distribution and more. Power losses computed by HFSS for microwave and RF applications are used to determine the corresponding temperature distribution; subsequently, the temperature distribution at selected time steps is used to calculate the corresponding deformation and structural stress in a microwave/RF component.

Design Automation

To reduce engineering set-up time and enhance usability, the new user interface for ePhysics v2 is based on the same Ansoft desktop that is common to HFSS, Q3D Extractor and Ansoft Designer, implementing numerous automation features specific to an electromagnetic-centric design flow. For instance, parameterization of model geometries and materials is easily defined for all model design objects and design optimization is available through Optimetrics. To assist the engineer in defining complex multi-domain analyses, multiple links may be used in the same solution setup. In the example of a thermal transient design, apart from selecting the power loss distribution calculated in the “source design,” there are two additional links that can be created and used in the solution process. The first is a mesh link to indicate the source of mesh (based on a design with identical geometry) and a link to a thermal static design that allows the use of an existing static solution as the distributed initial condition (temperature distribution in the model at Time = 0).

Further automation supports design and/or project variables being used in coupled projects. For instance, a variable that has been defined in the source design can be controlled in the target design. This capability allows a user to perform parametric sweeping of an HFSS object directly from within an ePhysics simulation. Thus, the whole range of simulations in HFSS and ePhysics are performed such that data flow between coupled solvers occurs automatically, without stopping the global solution process. The results of the parameter sweep are available in the post processor for viewing without any additional intervention from the user. Additionally, coupled designs utilize data caching technology to eliminate the need for re-simulating previously solved structures.

The link between simulators includes various mechanisms that automate simulator-to-simulator coupling for multi-domain analysis. Key automation features include:

• import a starting mesh from the coupled simulator of any eligible design (designs must share a common geometry)

• import the initial temperature distribution from a static thermal solution (if thermal transient solution is used)

• usage of adaptive mesh refinement (insures the mapping of the applicable fields and automatically monitors between different meshes in the coupled designs)

• automatic mapping of parameters between coupled designs

• power loss density from HFSS is mapped automatically to any ePhysics coupled thermal designs

New Analysis Capabilities

The latest release of ePhysics supports a new solution sequence from HFSS “transient” to ePhysics thermal transient solver. This new capability is demonstrated in an example of a high power ferrite circulator wherein the power loss distribution corresponding to the power loss peak calculated by HFSS is channeled to ePhysics for simulation of the transient temperature.

The ePhysics coupling supports HFSS arbitrary excitation sequencing such that an arbitrary sequence of power pulses can be efficiently generated to create the desired ePhysics input as a function of time, based on a single HFSS solution. The user definable parameter setup allows the thermal solver to calculate global quantities such as object-wise average temperature, hot spot and cold spot temperatures and their respective locations. Also, field distributions and other calculations can be performed using the post processing. Additional multi-domain analysis can be used to simulate the stress and deformation at user-specified moments and the corresponding temperature distributions, as shown in Figure 2.

The features available in the thermal transient solution sequence are exhibited in the following formulation

In this formulation, Q_v represents the power loss distribution, k is the thermal conductivity tensor, ρ is the mass density and c is the specific heat. The thermal diffusion equation is solved using the initial temperature distribution throughout the model and the boundary conditions specified by the user.

Advanced convective and radiative boundary conditions can be defined by the user to account for free and forced convection and thermal radiation effects.

Forced convection is often used in real life applications to cool down the high temperatures present in devices operating under high power steady-state or transient (pulsed) conditions. A structure with forced cooling can be simulated in ePhysics via the dedicated forced convection and radiation boundary conditions. Forced convection is reflected in a model featuring user-specified convection channels that allow the flow of a variety of cooling fluids at user specified velocity, such as the case of the liquid cooled ferrite circulator with cooling channels located in the walls of the waveguide structure, as shown in Figure 3. The setup of the forced convection on the walls of the channels allows the user to choose among a few available fluids or specify newly defined ones.

Other analysis capabilities introduced in ePhysics v2 include the ability to handle nonlinear thermal properties (conductivity vs. temperature), a new anisotropic stress solution and the optional distributed analysis that provides distributed computation of parametric sweeps for multi-physics applications resulting in a drastic acceleration of overall simulation time for large parametric studies and design optimization.

Conclusion

Growing concern over optimal design, reliability and shortened design cycles demands simulation technology that provides the utmost insight into the electrical, thermal and mechanical behavior of complex microwave/RF components. Linking dedicated simulators provides an unprecedented look into the true nature of the hardware being developed by engineers today. Simulation technology and automation are critical factors in establishing a multi-domain analysis capability. Such capability is especially important to designers working with high density and high power applications where thermal and stress considerations are no longer insignificant contributors to overall device performance.

Ansoft Corp.,
Pittsburgh, PA
(412) 261-3200,
www.ansoft.com.
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