Spice is a powerful tool for circuit analysis. But it often surprises users with
. holes in its abilities and strange definitions
. lack of industry-wide standards
. the need for the user to model the physics of their circuit, not just draw the schematic

I hope this blog will educate users and promote discussion in these areas.

Inaccurate power MOSFET models

The MOSFETs we use are almost always 3 terminal power MOSFETs, not 4 terminal integrated circuit MOSFETs. The few exceptions are small signal MOSFETs for things like charge amplifiers and RF front ends.

Spice does not have a built-in device model for a power MOSFET, so it requires a subcircuit to accurately describe one.

Available power MOSFET models range from trash to sophisticated. There are ancient models out there that use just a Spice .model line. These lack even the body diode that is an essential part of a power MOSFET. Without it, many switching power supply circuits simply don't work.

Based on pioneering papers of 25 years ago, an accurate power MOSFET subcircuit requires a minimum of one diode plus two MOSFETs. Additional diodes may be used to model the power MOSFET’s voltage dependent capacitances. 

An accurate alternative to model trench-gate type power MOSFETs uses one diode plus one MOSFET that uses Spice's BSIM3 MOSFET model. A BSIM3 MOSFET .model line can be identified by the parameter LEVEL=7 or LEVEL=8.

Check the subcircuit’s text to see what you have!

A given vendor’s models tend to be consistent, good or bad, except where they acquired another company with models of a different quality.

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