Timeline for Modeling MOSFET Pole and Transconductance for use in Larger Overall Control System
Current License: CC BY-SA 4.0
7 events
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| Mar 10, 2021 at 13:20 | comment | added | bobflux | This scheme makes the FET capacitance the dominant pole. But then, to drive it with current you need a transconductance amp. If you don't need DC accuracy, a single bipolar transistor will do the job very well as a transconductance amp. But if you need DC accuracy, it's more complicated. So it depends what the goal of the whole circuit is. | |
| Mar 10, 2021 at 13:19 | comment | added | bobflux | Here's another thing. The opamp has a pole so it needs a compensation cap which is set for the worst case (low Vds). But at high Vds the FET performs much better than worst case, so a lot of open loop gain is wasted through the compensation cap instead of being used to improve the performance of the circuit. If the FET gate is driven not with voltage, but with current instead, the variable capacitance (depending on Vds) results in variable gain, and the lower capacitance/higher gain at higher Vds can be used instead of wasted. | |
| Mar 10, 2021 at 13:15 | comment | added | a concerned citizen |
You could try to use the ready to find documentation about small signal MOSFETs, but those parameters will involve calculations that depend on the dynamical usage of the circuit: you have a feedback that settles at a certain operating point (assuming static operation). In the end, this method might be easier since it's a one click away (plus cursors or .meas statements). Otherwise, here's a quick hack for a MOSFET (not VDMOS, what you have there), that shows an equivalent circuit that might be used with a bit of tweaking.
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| Mar 10, 2021 at 13:04 | comment | added | banjoeschmoe | Got it. So seems like my MOSFET "block" will be a slightly more complicated mess. I suppose I'll just find my gm from my operating point, and do some small-signal modeling to get appropriate equations. I'll edit my post with my solution when I get around to it. | |
| Mar 10, 2021 at 9:02 | history | edited | bobflux | CC BY-SA 4.0 |
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| Mar 10, 2021 at 2:19 | comment | added | banjoeschmoe | I should have clarified, I understand about the stability concerns here. I know the OA has its dominant pole, and the MOSFET introduces another pole. What I really want to know is how I can treat the MOSFET as its own "block" in the block diagram shown in my OP. I assume it has the form gm*{something}/(1+sTau), but I'm not sure how to identify the missing terms. | |
| Mar 9, 2021 at 22:35 | history | answered | bobflux | CC BY-SA 4.0 |
