Monday, November 21, 2016

Another attempt at simulation

I really haven't been in the mood to finish any of the PCB layouts, but I didn't want to let this project languish untouched. One of the items on my list has been to get valid simulation results so I can predict whether my choice of pull-up resistors is valid.

In the interim there's been an update to the published Spice model for the FDV301N MOSFET. One of the changes was to change the Cgs value from 78pf to 8.5pf, which makes much more sense to me. I re-ran the simulation from June 2012, and found the new model reduced the per-stage propagation delay from about 200ns to 105ns. This is an improvement, but still longer than the 70ns or so I'm seeing on the breadboard when the effects of the scope probe loading is factored out.

I'm still comparing the simulation results with the actual tracings to understand what needs tweaking to make the simulation more accurate. To make it easier to understand this rather complex model I drew the FDV301N model using the LTspice schematic capture:

The resulting netlist differs from the original model only in comments and minor formatting; otherwise it's identical right down to the net names. I no longer wonder why simulations using this model are so slow.

In the process I think I've discovered an error: the negative output of the EDB voltage source is connected to net 0 (ground), which makes no sense given that the positive output is connected via diode D to the FET's drain pin.  It seems to me that only things related to the temperature input (net 50) should be referenced to ground. I don't think this is affecting the test simulations, since all the Source pins (net 30) are connected to ground, but it would affect some of my real circuits.

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