KiCad Simulation – Node sequence

I’ve just started using KiCad, and I’m really impressed. However, setting up a simulation can be awkward, because you need to match up the pin numbers from your schematic (which defines what is connected to what) with the node numbers that you ngspice model has.

Most people encounter this first with transistors. eg. a typical BJT has the following pin assignments:

  1. Emitter
  2. Base
  3. Collector
This is fine – KiCad is mainly for creating PCBs, so the most important thing is that the pins on the schematic match up to the pins on the footprint on the PCB.

However, our simulation models have nodes, and each node will correspond to a pin on the schematic and PCB, so we need to make sure that each pin is connected to the right node.

From the datasheet you’ll be able to get the pin assignments – e.g. pin 2 is the Base. Where do you find the ngspice node assignments? It depends on the type of spice model you’re using.

  • Built-in .MODEL. These are used for standard components, so the behaviour of the component can be defined with just a few parameters – e.g. for a basic simulation all you need to know about a resistor is the resistance. There are 17 types of model.
  • Custom .SUBCKT (Sub-circuit). These can be used for components that don’t fit into the standard categories, or for a complete add-on board.

Which one is it? In KiCad, look at the spice model editor:

The first highlighted text shows that this is a .MODEL

.MODEL

To get the information you need, you’ll need to look at the ngspice manual.

This includes the list of 17 standard models (reproduced here):

CodeModel Type
RSemiconductor resistor model
CSemiconductor capacitor model
LInductor model
SWVoltage controlled switch
CSWCurrent controlled switch
URCUniform distributed RC model
LTRALossy transmission line model
DDiode model
NPNNPN BJT model
PNPPNP BJT model
NJFN-channel JFET model
PJFP-channel JFET model
NMOSN-channel MOSFET model
PMOSP-channel MOSFET model
NMFN-channel MESFET model
PMFP-channel MESFET model
VDMOSPower MOS model

Go back to KiCad Spice Model Editor

The second highlighted text shows that this is an NPN BJT.

So, now look at the section of the ngspice manual for BJTs:

This tells us that ngspice expects the nodes to be listed in the order collector, base, emitter.

We’ll use this later.

.SUBCKT

Sub-circuits can be easy to figure out, or they can be hard, depending on whether the creator of the model has made sensible choices about the node names.

Look at the KiCad Spice Model Editor – this example is from a manufacturer spice model of an N-channel MOSFET, that has chosen to use .SUBCKT instead of .MODEL:

The stuff immediately after the .SUBCKT is what we need – the first thing is the type of device, the next 3 items are the nodes, helpfully lablled as drain, gate and source in that order.

Alternate Node Sequence

Now we’re ready to fill in the Alternate Node Sequence box in KiCad:

In this you write a list of the pin numbers, separated by spaces, in the correct node sequence.

e.g. for our BJT example we know that the correct node sequence is

collector, base, emmitter

Recall our pin assigments:

  1. Emitter
  2. Base
  3. Collector

So I’ll rewrite the node sequence with the pin number in brackets:

collector (3), base(2), emmitter(1)

Now all we need are the pin numbers, so strip out everything else:

3 2 1

And put that in the Alternate Node Sequence box:

Hit OK and we’re done!

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