Some of us have a habit of trying to be experts at everything. The one thing I have had to learn time and again, whether in software or hardware, is that there is no need to constantly reinvent the wheel. When you need to get an idea of how your device operates, you don’t need to build a simulation program from scratch. Thankfully, there are a number of tools that make it easy to simulate the behavior of your circuit, and you don’t need to be an electronics simulation expert.
We can’t all be experts at everything. That’s not necessarily bad, it just lets you focus on being the best at what you do. When it comes to running simulations based on your PCB design, there are a number of powerful tools that have been developed and refined over time. SPICE is the most prominent of these tools, and this open-source platform gives you the power to analyze many important aspects of your device functionality.
Great PCB design software packages include SPICE simulation capabilities, but desktop-based packages can create a strain on your budget. If you need to validate your device without breaking the bank, you should consider using an online PCB design software platform. SPICE is powerful enough to give you a deep look into the functionality of your device while still being compact enough to be run in browser-based design software.
Salt or Pepper?
If you aren’t familiar with SPICE models, just know that it has nothing to do with salt and pepper. SPICE stands for Simulation Program with Integrated Circuit Emphasis. It is a generalized simulator for analog electronic circuits, with a focus on simulating integrated circuits. Based on how you combine common circuit elements, you can create an integrated circuit with multiple inputs and outputs.
The great thing about SPICE simulators is that they are open source, meaning anyone with some programming skills can download the source code and modify it. Different companies have built and sold their own SPICE spin-offs focused on different applications, including packages specifically for PCB design.
When you’re working with your design program, there is always the possibility that you need to use a component that isn’t in your component library. Maybe the part is very rare or it has been fabricated in house. Whatever the case may be, you need to tell your design software how your component actually functions. This is where SPICE models come in. After building a SPICE model for your component, you can run simulations on any PCB that includes that component.
Most simulators that are built to simulate your design will use SPICE models that are attached to the components on your board. The great thing about component SPICE models is that they are designed to interface with each other, allowing your simulation software to quickly build a larger model for your entire board. Once your device starts to take shape, you are ready to start running simulations.
Close-up of a red PCB
SPICE Component Models
If you’ve never designed a new component, the process is rather straightforward. New chips are designed by combining the basic analog electronic elements (resistors, capacitors, and inductors) into a single complete circuit. Once the circuit is designed, it needs to be packaged into a single unit: this unit is called an integrated circuit, or IC. Most people refer to ICs as computer chips, although they are not always used in computers.
Your components are like the critical organs in your device, and you need to know that the way they are connected together will create the device you need. This is why your components each have a SPICE model that defines their functionality. All of the outputs from SPICE models for each component feed into the inputs of the downstream components. This is how your design software creates a full simulation of your device.
The same idea applies to the internal circuit elements in each of your components. Your PCB design software can build a SPICE model for your device based solely on how you connect the various circuit elements that make up your component. Determining the voltage and current in your equivalent circuit simply requires working through Kirchoff’s Laws. The methods for solving these equations are well-known and yield highly accurate or exact solutions.
Nonlinear and Semiconductor Devices in SPICE Simulations
SPICE simulations can also be used with circuits and components that include nonlinear elements like diodes and transistors. Once you include these nonlinear circuit elements in your circuit, the equations defining your system are typically transcendental, and an iterative numerical technique must be used to solve the system.
Because these more complicated models with nonlinear circuit elements require numerical techniques, you will need to define convergence conditions for your simulator. These conditions specify a certain level of accuracy that you are seeking in your results. You will need to define these convergence conditions when you run your simulation.
Rather than building SPICE models for every component in your device, component manufacturers will define their own SPICE models for their components. These models are thoroughly validated by manufacturers before being released, and you can be assured of their reliability. PCB design software with a great component library will likely include the most current SPICE models for these components.
ICs on a blue PCB
SPICE Model Outputs
Once you are ready to run your SPICE model, you may be unsure what you are looking for or what type of simulation to perform, especially if you are not familiar with SPICE models. The exact simulation you need will depend on how your device is intended to function. You can run SPICE simulations on specific components, or you can run a SPICE simulation for your entire device.
To get a better idea of how your device should function, it’s best to look at specific examples. If you are building an analog device that is intended to produce a voltage or current change in a specific frequency range (for example, as is the case in a filter), then you want to use a simulation that outputs voltage and current as functions of frequency. This type of simulation models the output signal for AC input signals with specific frequencies.
In some cases, you need to look at the functionality of a digital system. The inputs to your device are set as a set of digital signals, and your SPICE simulation outputs a set of digital signals. This allows you to verify the truth table that describes the functionality of your device. A great SPICE simulator will also return the propagation delay for digital components. This is extremely important for ensuring your signals are synchronized in your high speed circuits.
In dynamic components and devices, you will want to look at how the output signal evolves over time for specific inputs. This is displayed on a graph of voltage (or current) versus time. This type of output lets you compare the input and output signal strengths, propagation delay, and signal integrity. You can also use this to verify your design by comparing your simulated output with the theoretical output.
Limitations of SPICE Simulations
SPICE simulations are excellent for getting a better idea of how your device responds to different digital and/or analog inputs. These simulations are also useful for validating your device and ensuring that it will function as designed. However, validation can only be performed up to a certain point.
SPICE simulations only consider components that are placed on the board, and these simulations do not consider the effects of parasitic capacitance and inductance. The geometry of your traces, copper pours, the substrate material, and component geometry all modify the parasitic impedance. Components have parasitic inductance and resistance from the component pins/leads and the parasitic capacitance due to the component packaging.
SPICE models that are generated from a PCB generally cannot include the effects of parasitic impedance. The effects of parasitic impedance can be modeled by adding capacitors and inductors at specific locations within the circuit model that describes your device. More advanced SPICE simulators can take this into account. These parasitic effects become extremely important in high speed and RF circuits.
If you’re looking for proper PCB design software with the easy-to-use user interface that you can rely on to get your designs from idea to manufacturing, then look at Altium Designer. The simulation and design capabilities within Altium Designer are stellar and encourage your design security throughout the end of the design.
If you ’re looking to know more about the simulation capabilities of Altium Designer, talk to an expert at Altium today.
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