Part 1: From Desktop LTspice to VHDL-AMS in the Cloud
When a design engineer evaluates a new electronic simulation tool, they should keep as many of the following points in mind as possible:
- Make better descriptive or even predictive models
- Prepare practical experiments more efficiently
- Always obtain a solution within a reasonable timeframe
- Reduce costs as much as possible
The most well-known and widely used analog circuit simulation tool at this time is still SPICE.
In the numerous articles already published on the “Planet Analog” blog (You can do a search for Alain Stas in the upper right corner of Planet Analog Home Page) and dedicated to Vishay non-linear passive components, we’ve shown beyond any doubt that, for Vishay non-linear resistors, LTspice computations accurately reproduce the experimental results observed in the components and their applications. LTspice is extremely fast and allows for the easy import of SPICE models from sources other than Analog Devices. This is one of the most important qualities of an electronic simulator utility.
As we modelled NTC thermistors1 and PTC thermistors2, where many electrical characteristics were temperature-dependent, there were very important thermal aspects to consider (like self-heating or delays in heat transfer), but it was always possible to handle this by applying some tricks. For example, it’s possible to represent the thermal time constant of a thermistor with an electrical RC circuit3. The object temperature itself becomes a voltage and self-heating was simulated with analog behavior sources just the same4.
But what if the designer would like to combine the electrical effects on one side with the thermal effects on the other? Then they should turn to SABER RD, which is quite a popular software, or SLPS (a combined platform between PSpice and MATLAB Simulink).
If we keep costs in mind, one of the possible alternatives to SPICE modelling for multi-disciplinary simulation is offered by the IEEE standard VHDL-AMS (very high speed IC hardware description language for analog and mixed signals). Such a facility is offered by SystemVision Cloud, a cloud-based simulation tool from Mentor that is free for regular users.
A particularly interesting example of a complete system modeled with the VHDL-AMS language (incidentally also with VERILOG-AMS) is the case of an airbag system, starting with the capacity sensor, going through the signal handling, and ending up with the chemical reaction5.
Coming back to thermistors — as the methodologies for building component models in SPICE and in VHDL-AMS can be somewhat different, and also because of the different program options — it was important to verify the agreement of the results obtained in the same kind of simulations using both SPICE and VHDL-AMS software. For example, in the case of the simulation circuit in Figure 1, where PTC switching is performed at different ambient temperatures. As there is no parameter sweep possibility in SystemVision Cloud, we duplicate the PTC devices four times and give each one an ambient temperature, which is not easily possible in LTspice. This leads us to the simulation results of Figure 2. We see from the graphs of Figure 3 and Figure 4 that the results are similar. Repetition of this kind of simulation and quantitative analysis shows that the small differences in current amplitude are linked to the deployment of the Monte Carlo tolerance in LTspice, which is not supported in SystemVision Cloud at this time.
Simulation circuit in LTspice
The simulation in SystemVision Cloud allows us to visualize the temperature variation of the PTC components for each simulation case in Figure 5, where we see a temperature rising starting from the ambient temperature up to 160 oC. This internal temperature visualization hasn’t been implemented in LTspice, partly because it would have affected the simulation speed, and also because it would have needed to add an awkward virtual output pin "read out only" on the PTC symbol.