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CurrentSense Comparators for electronic load

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hli:

--- Quote from: Jwillis on February 02, 2020, 04:06:27 am ---Because no matter how well you pick your components perfect balancing would not be possible since  a tiny difference in threshold voltage between MOSFETs results in a substantial current mismatch . Other factors also effect the balancing like temperature and how well matched the other components are.

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This is only true when you don't provide a feedback loop. If your basic operating principle is 'provide voltage X to the FET gate to achieve current Y', then yes, the current depends heavily on factors such as the gate threshhold voltage, temperature, and the phase of the moon (or whatever might incluence the FET too).
But this is not how the usual constant current circuit work. It uses the current shunt to create a feedback, and to keep the current at the same level regardless of the FETs parameters. There are just two factors (apart from the current set-point) which influence the actual current: the exact value of the shunt resistor, and the offset voltage of the driver OpAmp. Both can be correct for statically, when needed.
When you goal is to keep all FETs at the same temperature, you might need different feedback. Note that a FET with a slightly higher Rdson will reach a slightly higher temperature at the same current (because it just dissipates a little bit more power). If you need to, this can also be corrected for statically.
I suggest you just simulate the classical circuit, with lets says 3 FETs, in LTSpice - just use three different FETs (different Vgsth) and see how it behaves.

From what I understand from your (very small) explanations you want to achieve a similar kind of feedback ('I can control the gain'), but need just more components (controlling the driver gain introduced an error, and the comparators itself have an offset voltage). The usual problem with that is that each additional component in the feedback loop introduces a phase lag, so the loop becomes more unstable (and tends to oscillate). I suggest you too simulate yout circuit, and vary the component parameters a little bit to see which ones introduce errors.


--- Quote from: Jwillis on February 02, 2020, 04:06:27 am ---And I simply don't understand why trying to make something work a little better causes such a controversy.

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For one, no-one here understands how your cicruit is supposed to work. It would help to explain its operating principle in more detail (how does the feedback loop from the output current back to the FETs gate work).
Second, most people here know that your other premise "the classic curcuit does not work very well because it does not balance properly" is wrong - it works very well, and can be calibrated to arbitrary precisio if needed. They also that the latter is usually just pointless - the correct function of your system should not depend on achieving such precise balancing. (If you depend on balancing each FET with one degree C to each other, what happens if your room temperature rises by 2°C? What happens when you airflow gets disturbed by accumulated dust and one FETs just gets cooled a little bit less than the others? You need to have some error budget for such things, and it is much bigger than jst 1% imbalance between the FETs.
Yes, it might be an interesting exercise to see how good the balancing can be, but it is just a problem of accounting for slightly different shunt resistor values (and the offset voltage can be eleminated by using precision OpAmps).

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