logic level mosfet resistance

[John O'Flaherty]

For a logic level mosfet with RDSon specified at 0.09 ohms at about
10a, with VGS at 5V, will it act as a .09 ohm resistor even for low
currents, like 10 to 100mA, giving a drop of 1 to 10 mV, or is there a
voltage burden? The data sheets that have graphs don't show this area
very well.

 

[colin]

For a logic level mosfet with RDSon specified at 0.09 ohms at about
10a, with VGS at 5V, will it act as a .09 ohm resistor even for low
currents, like 10 to 100mA, giving a drop of 1 to 10 mV, or is there a
voltage burden? The data sheets that have graphs don't show this area
very well.

yes, the data sheets usualy show the V/I curve is equal to rdson below some
current and it does go all the way down to zero.

Colin =^.^=

 

[Rich Grise]

For a logic level mosfet with RDSon specified at 0.09 ohms at about
10a, with VGS at 5V, will it act as a .09 ohm resistor even for low
currents, like 10 to 100mA, giving a drop of 1 to 10 mV, or is there a
voltage burden? The data sheets that have graphs don't show this area
very well.

I just wonder why you're using a 10A MOSFET at 10 to 100 mA.

Thanks,
Rich

 

[Andrew Holme]

For a logic level mosfet with RDSon specified at 0.09 ohms at about
10a, with VGS at 5V, will it act as a .09 ohm resistor even for low
currents, like 10 to 100mA, giving a drop of 1 to 10 mV, or is there a
voltage burden? The data sheets that have graphs don't show this area
very well.

No. Rds is non-linear.

See http://freespace.virgin.net/ljmayes.mal/comp/vcr.htm

This link is from an article about using a JFET as a voltage-variable
resistor in an audio compressor; however, what it says about Rds linearity
could be applied to MOSFETs.

 

[Gary Reichlinger]

For a logic level mosfet with RDSon specified at 0.09 ohms at about
10a, with VGS at 5V, will it act as a .09 ohm resistor even for low
currents, like 10 to 100mA, giving a drop of 1 to 10 mV, or is there a
voltage burden? The data sheets that have graphs don't show this area
very well.

I would expect it to act like a resistor at currents below
maximum and a fixed VGS. However, if the answer was important to your
circuit, I would get a voltmeter and a few selected resistors and
actually test it. Just put a resistor in series with the mosfet to
limit the current to your desired level and measure the voltage across
the mosfet.

 

[John O'Flaherty]

I just wonder why you're using a 10A MOSFET at 10 to 100 mA.

I want the low RDSon because I'd like to measure a single ground
referenced voltage and be able to calculate the resistance of a load
and the power delivered to it. With this arrangement...

+5V Regulated
+
|
|
.-.
| | Rload (variable)
| |
'-'
|
|------------
| measure V
.-.
| |
| |10 ohm
'-'
|
|
||-+
Duty Cycle ||<-
-------||-+
Control |
|
===
GND
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

the load current is the measured voltage divided by 10 ohms, and the
load voltage is 5V-the measured voltage. The measurements will be taken
on the fly.

If there's a low power unit that still has low RDSon, that would work
too. Any suggestions?

 

[John O'Flaherty]

yes, the data sheets usualy show the V/I curve is equal to rdson below some
current and it does go all the way down to zero.

Thanks, Colin.

 

[Tim Wescott]

I want the low RDSon because I'd like to measure a single ground
referenced voltage and be able to calculate the resistance of a load
and the power delivered to it. With this arrangement...

+5V Regulated
+
|
|
.-.
| | Rload (variable)
| |
'-'
|
|------------
| measure V
.-.
| |
| |10 ohm
'-'
|
|
||-+
Duty Cycle ||<-
-------||-+
Control |
|
===
GND
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

the load current is the measured voltage divided by 10 ohms, and the
load voltage is 5V-the measured voltage. The measurements will be taken
on the fly.

If there's a low power unit that still has low RDSon, that would work
too. Any suggestions?

The following will be insensitive to RDSon. Instead of worrying about
it you can just get any old logic-level FET. If you want to read the
voltage at the load you'll have to tap that separately, of course.

+5V Regulated
+
|
|
.-.
| | Rload (variable)
| |
'-'
|
|
||-+
Duty Cycle ||<-
-------||-+
Control |
o------------
| measure V
.-.
| |
| |10 ohm
'-'
|
|
===
GND
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)


--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

 

[Joerg]

Hello John,

I want the low RDSon because I'd like to measure a single ground
referenced voltage and be able to calculate the resistance of a load
and the power delivered to it. With this arrangement...

+5V Regulated
+
|
|
.-.
| | Rload (variable)
| |
'-'
|
|------------
| measure V
.-.
| |
| |10 ohm
'-'
|
|
||-+
Duty Cycle ||<-
-------||-+
Control |
|
===
GND
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

the load current is the measured voltage divided by 10 ohms, and the
load voltage is 5V-the measured voltage. The measurements will be taken
on the fly.

Can't you use a mux and also measure the voltage at the drain? Then a
subtraction would yield the voltage across the 10ohm resistor.

 

[John O'Flaherty]

The following will be insensitive to RDSon. Instead of worrying about
it you can just get any old logic-level FET. If you want to read the
voltage at the load you'll have to tap that separately, of course.

+5V Regulated
+
|
|
.-.
| | Rload (variable)
| |
'-'
|
|
||-+
Duty Cycle ||<-
-------||-+
Control |
o------------
| measure V
.-.
| |
| |10 ohm
'-'
|
|
===
GND
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

I did want to get by with a single measurement to get all the load
information- I,V, Power. Plus, with the FET ground referenced, I'll be
better off by about a volt or so on the drive voltage (for the higher
end of the current range). I found the IRF3711 for $1.11 that has 5.9
mOhm (typical) RDSon at 4.5V gate drive, so that will only be 0.059% of
the 10 ohms.

 

[John O'Flaherty]

Hello John,


Can't you use a mux and also measure the voltage at the drain? Then a
subtraction would yield the voltage across the 10ohm resistor.

Well, if everything goes according to plan, I'll be able to use a
single measurement, saving the trouble of multiplexing and the time to
make the second measurement. It should work ok if I can rely on <10
mOhm resistance even at lower current levels.

 

[Rich Grise]

Well, if everything goes according to plan, I'll be able to use a
single measurement, saving the trouble of multiplexing and the time to
make the second measurement. It should work ok if I can rely on <10
mOhm resistance even at lower current levels.

If you're saturating the thing, why not put the current sense resistor
in the source lead and get a single-ended measurement?

Good Luck!
Rich

 

[colin]

Well, if everything goes according to plan, I'll be able to use a
single measurement, saving the trouble of multiplexing and the time to
make the second measurement. It should work ok if I can rely on <10
mOhm resistance even at lower current levels.

You can get current sense fets if all you want to do is measure current -
they have a fourth terminal wich is conected to a current sense resistor
through wich a small part of the current flows.

However the on resistance of a normal FET rises with voltage/current,
therefore it is lowest at zero volts, some SMPS controller ICs actualy use
the VDS to monitor the current.

How accurate do you want to be ?

Colin =^.^=

 

[John O'Flaherty]

If you're saturating the thing, why not put the current sense resistor
in the source lead and get a single-ended measurement?

My hope was to get V, I, R and P of the load with a single measurement.
I should be able to do that within the needed precision with the
circuit I had, and have 1 volt or so of extra drive to VGS, as long as
the RDS is in the area of 10mOhms, and if it remains negligible over a
range of currents from, say, 5mA to 100mA. The RDS could even be higher
and be calibrated out, unless it's non-linear and/or temperature
sensitive. I may not even need to worry about those factors if I start
wiith the lowest RDS possible. I found a fet with <10mOhm for a little
over $1, so I'm happy with the way I'm going; I just wasn't sure if the
simple resistance remained valid at low current levels. The consensus
seems to be that it does.

 

[John O'Flaherty]

You can get current sense fets if all you want to do is measure current -
they have a fourth terminal wich is conected to a current sense resistor
through wich a small part of the current flows.

However the on resistance of a normal FET rises with voltage/current,
therefore it is lowest at zero volts, some SMPS controller ICs actualy use
the VDS to monitor the current.

How accurate do you want to be ?

Not just current . I want load current, voltage, power and
resistance with a single voltage measurement, since my circuit will be
lazy as sin.
I'll be digitizing with 10 bits, so anything better than 0.1% would be
a waste. With a power fet running a maximum of 100mA, with 10mV RDS and
at about 500Hz, endogenous heating won't be a problem! I hadn't heard
about current-sensing FETS; thanks for mentioning them.