static drain-source on resistance

[kell]

On the datasheet for the STP80PF55
http://www.st.com/stonline/books/pdf/docs/8177.pdf
I can't make head or tail of figure 6. What is Vgs? It can't be 10
volts, the numbers all wrong for that.

 

[Robert Wilson]

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.

 

[Spehro Pefhany]

On the datasheet for the STP80PF55
http://www.st.com/stonline/books/pdf/docs/8177.pdf
I can't make head or tail of figure 6. What is Vgs? It can't be 10
volts, the numbers all wrong for that.

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

 

[Anthony Fremont]

On the datasheet for the STP80PF55
http://www.st.com/stonline/books/pdf/docs/8177.pdf
I can't make head or tail of figure 6. What is Vgs? It can't be 10
volts, the numbers all wrong for that.

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". ;-)

 

[Spehro Pefhany]

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

 

[Anthony Fremont]

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.

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

 

[John Larkin]

On the datasheet for the STP80PF55
http://www.st.com/stonline/books/pdf/docs/8177.pdf
I can't make head or tail of figure 6. What is Vgs? It can't be 10
volts, the numbers all wrong for that.

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

John

 

[Spehro Pefhany]

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

 

[D from BC]

On the datasheet for the STP80PF55
http://www.st.com/stonline/books/pdf/docs/8177.pdf
I can't make head or tail of figure 6. What is Vgs? It can't be 10
volts, the numbers all wrong for that.

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

 

[kell]

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

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

 

[Fred Bloggs]

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

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...

 

[Fred Bloggs]

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.

 

[John Larkin]

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.

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

 

[Spehro Pefhany]

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

 

[John Larkin]

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.

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