The future of thermal design – earlier electrothermal analysis
During a panel discussion at the recent SEMI-THERM Thermal Technologies Workshop about the ‘Future of Thermal Design’ one of the speakers imagined a future where (paraphrased) “Electrical spice files and thermal spice files could be used together for early concurrent electrothermal system design”. My interpretation of this statement was that now, and going forward, electrical and thermal systems must be designed as one, and that continuing to work in electrical and thermal silos will begin to carry more and more inefficiencies, errors, re-work and risk.
Electrical-thermal design: What are Boundary Condition Independent Reduced Order Models?
At the time of this conference, an electrical-thermal-spice workflow was simply not practical in a general sense. While thermal spice files do indeed exist, unlike electrical spice files, they can only be created for use in a single, fixed, thermal environment. It’s important to note that until only recently, no method had been identified to create a thermal spice file with a known accuracy at any operating power cycle, or even to support multiple heat sources in a device. The introduction of the thermal netlist export from Simcenter Flotherm 2020.1 (described here) this summer solved the accuracy and multi-heat source problem, but the single thermal environment limitation remained.
This has all changed now. The recent release of Simcenter Flotherm 2020.2 can export FANTASTIC1 Boundary Condition Independent Reduced Order Models (BCI-ROMs) in the IEEE Standard 1076.1 VHDL-AMS format, making it compatible with many 1D and functional schematic circuit simulation tools. The BCI part of the name is critically important. It means the model will retain accuracy in any thermal environment. This is the accurate, portable, version of the future ‘thermal spice file’ described above, albeit with a different name.
So what’s now possible? A thermal BCI-ROM in VHDL-AMS can directly connect to electrical circuit elements. The thermal circuit shown below will communicate temperature to the electrical circuit element, and the electrical circuit will send power back. This is not actually a ‘back and forth’ co-simulation approach. Thermal and electrical circuits are solved concurrently as they are described in the same language.
Thermal Model Supply Chain and connected BCI-ROMs
The model supply chain is well established for electrical design. It’s common today to request and receive behavioral models of devices (in spice or VHDL-AMS formats) as part of the functional electrical design process. These electrical models are portable, i.e., they will interact properly with any other component in the functional schematic and can be consumed by many simulation tools. The same is now true for thermal models, thanks to the BCI nature of these thermal BCI-ROMs and their implementation in VHDL-AMS, an IEEE standard format. A thermal BCI-ROM may be placed in any thermal situation/environment and respond accurately, and used by any simulation tool that supports VHDL-AMS.
A semiconductor vendor for example, can now support their customers by supplying both electrical spice files and thermal BCI-ROMs. The customer can create a thermal BCI-ROM of the PCB, connect them together with other thermal BCI-ROMs and electrical models to quickly assemble a functional schematic representation of their product that captures all electrothermal behavior.
The availability of an accurate electrothermal functional schematic early in the design process brings together thermal and electrical design to enable concurrent design at the system level. The electrical design benefits from an understanding from the beginning of how the electrical functionality and performance of the product will be impacted by temperature and temperature differences. The thermal designer benefits by gaining access to accurate power dissipation values far earlier in the design process, early visibility on what components are likely to need remedial thermal action, and the ability to design power control strategies from the onset.
Smartphone Power Control, Thermal Throttling – Digital Electronics Example
Let’s look at an example how this works with Simcenter Flotherm, electronics cooling software, generating BCI-ROMs from 3D thermal models and using them in SystemVision Cloud, circuit simulation software. You can try out the electrothermal circuit simulation example in a link below, but first a tour of what is included.
The example model is an electrothermal design of a generic Smartphone, with a power throttling algorithm defined. This is the same application that my colleague Mike Donnelly wrote about earlier this year, but this time the thermal circuit is built with connected thermal BCI-ROMs instead of a single thermal netlist.
Processor 3D thermal model and BCI-ROM
Let’s focus first on the processor. In Simcenter Flotherm, the processor model looks like this:
There are three named heat source zones on the die: CPU, GPU, and Connectivity. There are two defined boundaries from which heat can exit the model: Top and Bottom. When this Flotherm model is converted into a FANTASTIC thermal BCI-ROM and imported into SystemVision Cloud it looks like this.
Each heat source has a thermal connection to carry power and temperature information, as does each defined boundary face. Note that each boundary face requires a thermal resistance (Rth) to be specified. More info below.
Incorporating multiple thermal BCI-ROMs into the electrical circuit model
The BCI-ROM is then connected to the electrical circuit, establishing the power-temperature connection, and also connected to the rest of the thermal circuit. Each connection for a thermal boundary face requires a user input for thermal resistance between the BCI-ROM and the next thermal object in the circuit. This is how the Boundary Conditions for the BCI-ROM come into play. In this example, the Bottom face of the processor is connected directly to the appropriate location on the PCB BCI-ROM with Rth_Bottom = 0.05 K/W (assuming a small thermal resistance between the bottom of a component and the board is usually reasonable). The top of the processor also has a boundary face Rth. This generally could represent a connection to a heat sink, airflow, etc, but in this case it represents a direct connection to the phone casing via a thermal interface material (Rth_Top = 0.2 K/W) between the processor lid and the casing.
In addition to the processor, two memory chips are included as thermal BCI-ROMs, and finally the ‘rest’ of the Smartphone (containing the PCB, battery, display, temperature sensor, and phone casing) is included as another BCI-ROM. Note that the thermal BCI-ROM boundary faces for the components are connected directly to corresponding PCB BCI-ROM heat sources. This specifies that heat exiting the components will enter the ‘PCB and Case’ BCI-ROM at the correct physical location, and then accurately move within that BCI-ROM before arriving at the ambient reference temperature.
The electrical side of the circuit contains three items of note:
- ‘Digital Power’ VHDL-AMS. The digital power model is an abstract representation of the power dissipation behavior of any clocked digital IC. The instantaneous power dissipation is a function of the applied voltage, the clock frequency, the operating state, and the temperature. The user can specify the nominal values for these parameters, and the model dynamically adjusts the power dissipation as they change during transient simulation.
- Operating State Definition. For this example, the system is Idle for 100 seconds, then in Gaming Mode for 1500 seconds.
- Dynamic Thermal Management (DTM) Controller. The sampled-data algorithm is attempting to keep the sensor temperature (on the phone casing, just above the processor) between 40 °C and 45 °C. It does this by adjusting the global clock rate in a simple manner:
- Sensor Temperature is too hot? Halve the clock frequency
- Sensor Temperature is too cold? Double the clock frequency
Transient electrothermal circuit simulation power – aware control
The DTM controller responds as shown below. The increased power dissipation from ‘gaming mode’ at the maximum clock frequency causes the sensor temperature to exceed 45 °C at ~750 seconds activating thermal throttling control response. the DTM controller halves the frequency in response, dropping the power dissipation and the sensor temperature, and of course dropping the performance of the gaming software at the same time.
Explore for yourself using BCI-ROMs, generated by Simcenter Flotherm, in SystemVision Cloud
The SystemVision Cloud design is linked to below. There are a few parameters (look for the blue boxes) that are available to change directly.
- Note – Things to Try:
- Determine the impact on device performance of eliminating the heat path from the top of the processor to the phone casing by setting Rth_Top = 50 K/W
- Comment: this causes the Processor temperatures to be much higher obviously, but it also introduces a slower thermal response to processor power changes. In this situation, the DTM controller halves the clock frequency 3 times before the sensor temperature drops below 45 °C, creating a very uneven gaming experience for the user.
- Determine the impact of changing the nominal power dissipation for the CPU to 2W.
- Try different values for the DTM policy, max and min temperatures, sampling period
- Determine the impact on device performance of eliminating the heat path from the top of the processor to the phone casing by setting Rth_Top = 50 K/W
Launch SystemVision Cloud example: https://www.systemvision.com/node/388554
Also note, to access the full interactive design, click the ‘Edit in SystemVision Cloud’ button, make a copy of the design and explore!
Further reading including thermal design with reduced order models
Please view the Simcenter Flotherm 2020.2 release overview blog by my colleague John Parry to learn about new features including faster joule heating simulation.
For other information on BCI-ROM technology, please see the following:
Video: BCI-ROMs – Faster transient electronics thermal simulation
Thermal Netlists
Blog: Electrothermal circuit simulation enabled by VHDL-AMS thermal netlists
Video: [TECH TIP] Thermal netlist export in .sp format workflow for users
Supporting 1D fluid flow network analysis with Boundary condition independent reduced order models (BCI-ROMs in FMU format)
Video: How to use Simcenter Flotherm Reduced Order Models (BCI-ROM) in Simcenter Flomaster – a liquid cooling example
References:
- L Codecasa, V d’Alessandro, A Magnani, N Rinaldi, PJ Zampardi, “Fast novel thermal analysis simulation tool for integrated circuits (FANTASTIC)”, 20th International Workshop on Thermal Investigations of ICs and Systems (THERMINIC) article 6972507 United Kingdom, 2014.
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