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
Simulation circuit in SystemVision Cloud
Simulation result in LTspice
Simulation result in SystemVision Cloud /p>
Internal temperature of PTC
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 o C. 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.
The graph in Figure 5 clearly illustrates an idiosyncratic property of the VHDL-AMS language as a multi-disciplinary technology, not only dealing with analog electronics but also thermal design, where the internal temperature reached by components can be directly accessible. In a general way, the designs can also be digital, mechanical, magnetic, hydraulic, or of radiative nature.
We are now going to test the PTC VHDL models in a simplified (although realistic) inrush current limitation application, and we will adopt a known basic SMPS circuit (Figure 6, also visible at SystemVision Cloud), where we rectify and smooth a three-phase AC source voltage via diodes and a big capacitor (3 mF or more), producing a constant current applied to a load. The PTC network must limit the inrush current during the capacitance charge. Once the voltage around the capacitor has reached a defined voltage, the two switches close. The PTC network is thus disconnected and left for cooling, while the current is brought to the load.
Simulation in SystemVision Cloud of SMPS circuit working in normal conditions
For example, in this simulation, it is quite possible to visualize what will happen when the capacitor is short-circuited by bringing the value of the parallel resistance to the capacitor r13 (normally 100 MΩ:) to 0 Ω. The results are shown on Figure 7. In each of the parallel arms of the PTC network, one of the PTCs (the highest value u66 in the upper branch) will heat up above the switch temperature. All the voltage is then reported on the switching elements. The PTC network resistance increases globally and brings back the surge mains current to zero within less than 0.5 s, protecting the whole circuit.
Simulation in SystemVision Cloud of SMSP circuit with capacitor in short circuit
Many other parameters of the circuit can be tuned, i.e. the ambient temperature, PTC element tolerances, and capacitance value. In theory, this creates the possibility of calibrating the PTC network prior to experimentation and verifying if the protection is valid in the whole range of ambient temperatures.
In conclusion, we can say that every new contributing brick added to the wall of this never-ending task of component simulation is always welcome and SystemVision Cloud, VHDL-AMS programming facility, is easily one of them. The way multi-disciplinary models are built can even be a source of inspiration for SPICE. But as it is customary to end an article with a cliffhanger, that will be for Part II.
SystemVision Cloud is registered trademark of Mentor, a Siemens Business. All rights reserved.
LTspice is a registered trademark of Analog Devices Inc. All rights reserved.
Bibliography
- François Pêcheux, Christophe Lallement, Alain Vachoux. title VHDL-AMS and Verilog-AMS as alternative hardware description languages for efficient modeling of multidiscipline systems., IEEE Transactions on Computer-Aided Design of Integrated Circuits and Systems, IEEE, 2005, 24 (2), pp.204-225.
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