250mA voltage clamp

In message , Tony Williams writes

Oh buggerit... try

Helpful resource - thanks! Interesting that it doesn't seem to mention the existence of JFETs anywhere.

If I understand correctly, equation 7.7.1 is missing an "n" in the denominator?

The font used for the italicized subscripts doesn't have quite enough pixels. Am I right that the subscript of the denominator term in equations

And in the last step of 7.7.5 I don't understand how he got from 2q to q.

Thanks, -walter

He set the surface potential, phi_S, to 2 phi_F, which should have made the last square-root term, q Es / phi_F, but it appears he mistakenly left the 2 in place. Note in equation 7.7.2, which he's deriving here, he has no 2, but he's kept that nice Na factor. :-) Na is the doping density, which is a huge number, like 10^17 /cm^3 (see page 247, 266, etc), to go with q = 1.6 x 10^-19.

BTW, Prof. Van Zeghbroeck's 2002 Acrobat .pdf version (page 283) fixes the missing n in 7.7.1, but it still has the other errors.

When evaluating a power MOSFET, we don't know all the terms in equation 7.7.2 (the math on page 271 has some typical values), but it's clear n must be greater than 1, as Kevin asserted.

The (Vg - Vth) term in 7.7.1 makes little sense to me, because we're below the threshold-voltage Vth anyway, so why bother with that? One can use Vg by itself, and adjust the proportionality term, Ix, in front of the exponent. In our case modeling power MOSFETs, it's all empirical anyway, fitting measured bench data.

Looks like it.

Yes. As noted in Appendix 2.

That bit is beyond me, especially as an Na seems to have disappeared as well.

I'm back from Xmas.

Two suggestions:

(1) Make the 1.6 Ohm a PTC thermistor/circuit breaker to drop the current to near zero if the short lasts for very long.

(2)

If the input side rises slowly enough we can do something like this:

----+-----\\/\\/----+-----------S D---------+---- ! ! G ! ! V Schottky ! ! ! --- ! ! ! ! ! ! ! [51R] ! ! ! ! ! ! ! +---------------[20K]---+ ! ! ! ! pnp B ! ------------E C-----------+--

The 20K and the HFE of the pnp make for a fairly sloppy current limit point but I think the advantage of the foldback effect could outweight it.