FELICITAS PCBSIM – RELIABILITY OF
SIMULATION RESULTS
INTRODUCTION
Spice circuit simulation has now been around for more than 40 years. To my experience a simulation saves usually
at least one re-spin and lots of time. Furthermore, a circuit fine tuned in simulation is very cost efficient and
stable. Despite this, many engineers do not use simulation on a regular basis. We often hear doubts about the
accuracy of simulation.
This article will cover aspects which influence the accuracy of simulation and will give practical guidelines about
what results can be trusted.
FELICITAS PCBSIM
The following examples have been done with Felicitas PCBSim. This is
a SPICE circuit simulator which is seamlessly integrated into CadSoft
EAGLE. It is activated directly from the schematic editor of EAGLE and
adds context menu entries and specific dialogs for the simulation
parameters of all components.
The schematic is transferred to PCBSim with a single button. All
simulation settings are stored within the schematic of EAGLE.
Nevertheless, the following results and conclusions are valid for most
other SPICE simulators.
Figure 1: Felicitas PCBSim Circuit Simulator
SPICE ALGORITHM
The first version of SPICE (Simulation Program with Integrated Circuit
Emphasis) was created by the Berkeley University of California back in
1973. The source code was later released to the public and is now the
basis for many different Simulators.
Spice uses Kirchhoff's circuit laws to create a set of equations which
represents the circuit. Kirchhoff's junction rule state that the sum of
currents must be zero for each node (in Figure 2it would be applied to
nodes NODE1, NODE2 and GND).
Kirchoff's voltage law says that the sum of voltages must be zero for
each closed loop (for Figure 2: voltages across V1, R1 and D1).
Figure 2: Forward Biased Diode
For each circuit this leads to n equations for n unknown voltages. If the circuit is linear, solving these equations
is straight forward. For non-linear circuits, Spice deploys an iterative process (Newton-Raphson approximation).
Simply put, node voltages are guessed and currents and calculated. If the sum of currents for a node is not zero,
the node voltage is adjusted. If it all sums up, the solution is found.
The simulation starts at time zero and iterates with small time steps forward. For each time step the solution of
the previous time step is taken as a first guess to the algorithm.
Voltages across capacitors are calculated by integrating the current flowing into them. Inductors are treated
similarly.
If Spice knows exactly how the current through D1 depends on the voltage across D1, the simulation results will
be absolutely exact. More generally, the simulation results will be exact if the simulation model for each device
is exact.
We will now cover the accuracy which can be expected from commonly used simulation models.
SIMULATION MODELS FOR PASSIVE COMPONENTS
Ideal passive components are of course known to SPICE and do not need a simulation model, just a value. Felicitas
PCBSim directly uses the component values from EAGLE and understands the usual electronic notation, like 3k3
or 4u7.
The parasitics are the interesting part for the simulation.
Felicitas PCBSim allows to directly add these to the component. Many
times it will not be necessary to do this. For example resistors and
ceramic capacitors will behave like ideal resistors and capacitors
respectively in most applications.
Electrolytic capacitors are a different story. They have fairly large serial
resistance and also leakage. Both effects can easily be covered by the
simulation and should be added to the simulation parameter of the
component (see Figure 3).
The parasitics of inductors can be added likewise, here the series
resistance is most important.
Figure 3: Parasitics of a Capacitor
Therefore, the vast majority of passive components can be accurately simulated without adding information on
parasitics. Only a few passive key components of each circuit usually need more attention. Then the simulation
will also cover effects like inrush current, LC-filter bandwidth, capacitor losses, ripple current and voltages, etc.
SIMULATION MODELS
FOR SEMICONDUCTORS
Figure 4: 1N4007 forward current at different
temperatures
Since Spice was originally designed to
simulate integrate circuits, it has very
precise simulation models for different
kinds of semiconductors. Diodes and transistors (including JFETs and MOSFETs) are usually simulated very exactly.
See figure 3 for an example of the forward dc characteristic of 1N4007, a silicon rectifier diode.
Usually the models reflect
- dc characteristic
- junction and gate capacitances, voltage dependent
- temperature dependence of forward voltages
- current gain, resistance between source and drain
This allows very precise simulation of these semiconductors including frequency response, non-linear effects
and temperature drift.
Other semiconductors (thyristors, triacs, zener diodes, dual-gate MOSFETs,...) are modeled as small sub-circuits.
They are very accurate as well, but sometimes do not reflect temperature dependency precisely. The following
section about integrated circuits apply to these semiconductors as well.
SIMULATION MODELS FOR INTEGRATED CIRCUITS
Integrated circuits can be modeled in two ways: the Spice simulation model can directly represent the internal
schematic of the integrated circuit. This approach leads to very exact models but they are also very large and
therefore slow to simulate. Furthermore the model reveals all the internal details of the integrated circuit and
so allows competitors to easily copy the circuit.
A second approach is the so called macro-model. These models describe integrated circuits as artificial building
blocks. The main amplifier of an operational amplifier could for example be modeled as a voltage - controlled
voltage source with high gain. This approach usually leads to faster simulations, but often do not reflect all
aspects of an integrated circuit (for example supply current characteristics or temperature dependency).
Still both approaches are valid and most of the available models yield very accurate simulation results.
If in doubt it is easy to verify a simulation model. A small circuit from the data sheet of the component can be
copied and simulated. Comparing the results with the data sheet already gives a high level of confidence in the
simulation model. The small signal step response of an AD8038 from Analog Devices has been verified in this way:
Figure 3:
AD8038: Comparison
Between Data Sheet
and Simulation
The result shows that the simulation model is very accurately reflecting the real behavior of the operational
amplifier.
INFLUENCE OF THE BOARD LAYOUT
In some cases the board layout has a strong influence on the performance of the circuit. This is especially true
for high-frequency applications. Even before the layout is done, the influence of parasitic capacitances can be
tested by just adding them to the simulated circuit. In this way it can be checked whether these parasitic
capacitances have noticeable influence and how to compensate for
them. As a practical guideline, this should be considered with frequencies
above 10MHz for sensitive amplifiers, above 100MHz otherwise.
Even short PCB tracks behave like transmission lines with very fast
analog or digital signals. There is a special SPICE component
“Transmission Line” to simulate this. Felicitas PCBSim's Transmission
Line Calculator allows you to comfortably derive the properties of the
transmission lines from the PCB track geometry. Delay times, reflections
and even losses can be analyzed prior to the actual board layout. A 5cm
track for example should be considered as a transmission line for
frequencies higher than 300MHz or slopes faster than 1ns.
Figure 6: Transmission Line Calculator
Checking the influence of coupling capacitors and transmission lines improves the accuracy of the simulation
results for sensitive high-frequency circuits substantially.
The result of all these tests is also a much more detailed specification on how to do the board layout which of
course increases the chance of a first-time-right prototype considerably.
WHAT IF A SIMULATION MODEL CAN'T BE FOUND?
Felicitas PCBSim already comes with a large number of simulation models. Most vendors do supply simulation
models for their components. Usually, the models are written for a certain simulator. Felicitas PCBSim has an
integrated Universal Spice Parser which is able to import simulation models targeted for a number of popular
simulators. In this way most of the available models can be used.
If there is no simulation model for a certain device available (or if it is encrypted), it is often possible to use
models for similar parts from other vendors. If none can be found, either the vendor or Felicitas Customized
Engineering will be able to create such a model on request.
PRACTICAL EXPERIENCE
I have done or helped with the simulation of many different designs, like
- Amplifiers (HF/NF, low power,
high power, precision)
- DC/DC Converters
- NF/HF Filters
- Charge pumps
- Oscillators
- Modulators
- Demodulators
In the vast majority of cases the simulation was spot on and reflected the performance of the prototype including
frequency response, power consumption and efficiency.
CONCLUSION
A simulation is as good as the simulation models it uses and in most cases very accurate models are available
today. The simulation then allows to test and optimize a design before the first prototype is built. It gives unique
insight into the design which would be very hard or even impossible to obtain with real hardware.
Usually, the first prototype will work right from the start. Simulation therefore saves weeks and months of valuable
design time.