250mA voltage clamp

[Tony Williams]

~In article <[email protected]>,


Try also.

http://ece.colorado.edu/~book/book/book/chapter7/ch7_7.htm#7_7_4

Oh buggerit... try

http://ece.colorado.edu/~bart/book/book/chapter7/ch7_7.htm#7_7_4

 

[Walter Harley]

Oh buggerit... try

http://ece.colorado.edu/~bart/book/book/chapter7/ch7_7.htm#7_7_4

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
7.7.1, 7.7,3, and 7.7.6 is Vt, the thermal voltage (kT/q)?

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

Thanks,
-walter

 

[Tony Williams]

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

Looks like it.

..... Am I right that the subscript of the denominator term in
equations 7.7.1, 7.7,3, and 7.7.6 is Vt, the thermal voltage
(kT/q)?

Yes. As noted in Appendix 2.

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

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

 

[Winfield Hill]

Tony Williams wrote...

Looks like it.


Yes. As noted in Appendix 2.


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

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.

 

[Ken Smith]

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:

0.5 PMOS
----+-----\/\/----+-----------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.