static drain-source on resistance

Well it's reasonably obvious what it is, it's the voltage required between the gate and the source to produce the specified current in the draon source connection. Have you not used one before? It's an honest question, asked with all due respect.

Sincerely,

Robert.

kell wrote:

I suggest that Vgs is probably 10V, but that graph is so screwed up you're probably better off to ask ST for a correct one. At least the Y axis is wrong in title, and probably in values by 10:1.

Best regards, Spehro Pefhany

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It's a pretty strange looking graph but, if I understand, the horizontal axis represents Vgs's from 2.5V thru ~7V. The vertical axis represents the amount of drain current (Id) required to create an arbitrarily chosen 5V drop (Vds). Since there is a well defined curved line, I suppose that proves that Rdson is fairly static for a given Vgs. I don't know why they felt the need to make a puzzle out of it. This looks allot like the work of "The Center of Confusion". ;-)

One hint that something is seriously wrong is that a graph called "Static drain-source on resistance" does not have ohms as the units for the dependent variable. A second hint is that if you interpret the Y axis as ohms, it's about 10:1 too high to match the typical Rds(on) figure of 16m ohms. A third hint is that Vgs is not stated. Garbage.

Best regards, Spehro Pefhany

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Oops, I'm sorry. I was looking at the graph above it.

Bad caption? Looks like the substrate diode curve to me.

John

They duplicated that curve from elsewhere. The identical graph (with the identical ID number) shows up in Figure 11, with the correct caption.

BTW, have you done any work with Philips MOSFETs? I'm looking for a small (preferably smaller than DPAK) low-voltage (eg. 30V) MOSFET with a gargantuan SOA for handling one-shot exponential-decay pulses.

Best regards, Spehro Pefhany

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I'm no pro but will take a shot at answering this...

Look at figure 3 to understand figure 6. I'll guess the Vgs for figure 6 about 4Volts.

Vgs of 10V on figure 3 is probably just to show the max test range. It's confusing. D from BC

There you go, that explains it. Thanks John & Spehro.

Mmmmmmmmmmmmmmm..nah. The graph is okay. You might note that in both active and saturation region the Gfs is a product of mu.sub.p x Cox (per unit area) x (W/L) x (VGS-VTH). RDS,ON is 1/Gfs in active region. Computing the Gfs from Fig 6 at the various VGS in saturation region, allows for accurate extrapolation to RDS,ON in active region for a given VGS...

I should correct that to RDS,ON= 1/(dId/dVds) in active region, which is related to 1/(dId/dVGS)=1/Gfs in saturation region, the same product factors. I guess you could call RDS,ON= 1/Gds in active region. Gds=dId/dVds there.

No, I don't think I've used any Philips fets. Do you need to absorb a lot of energy? Maybe use a series resistor and a tiny, low Rds-on fet?

John

Yes, I've been looking at doing that (it's even possible to get SMT WW resistors) but it might not be acceptable for other reasons. The total energy is not huge, perhaps 1J.

Best regards, Spehro Pefhany

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You could probably dump a joule into a SOT-23 fet, certainly into a power-pad SO-8 or one of the 6-lead power-pad SOT-23's.

How about a pcb trace to soak up some of the energy? If you had a tiny

20 mohm fet, and put 20 mohm of pcb trace in series, that cuts the fet stress by 2:1. That would be about 0.25" of 6 mil, 1 oz trace.

Get some SOT-23 fets and some caps and dump joules until the fets fry. Fun!

John