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Message |
Guest
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 Posted:
Wed Nov 16, 2005 12:06 am Post subject:
MOSFET question: Drive voltage vs. Vgs(th) |
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Can anyone tell me the difference between the drive voltage and the
gate-to-source threshold voltage (Vgs,th) of a MOSFET, if any?
I was under the impression that these refer to the same voltage - the
voltage that is needed to turn the MOSFET 'on'.
However, if you look at the following datasheet for a Fairchild
MOSFET...
http://www.fairchildsemi.com/ds/FD%2FFDG311N.pdf
you'll see that the title says "2.5V Specified", which I understand
refers to the drive voltage. However, under the "On Characteristics,"
you'll see Vgs(th) min. = 0.4V and Vgs(th) max. = 1.5V. Why isn't
Vgs(th) max. = 2.5V, as the title suggests? Is there a difference
between Vgs(th) and the "specified voltage"?
Thanks!
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Christian S.
Guest
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| Posted:
Wed Nov 16, 2005 12:15 am Post subject:
Re: MOSFET question: Drive voltage vs. Vgs(th) |
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On 15 Nov 2005 10:06:42 -0800, accolade@gmail.com wrote:
| Quote: | Can anyone tell me the difference between the drive voltage and the
gate-to-source threshold voltage (Vgs,th) of a MOSFET, if any?
I was under the impression that these refer to the same voltage - the
voltage that is needed to turn the MOSFET 'on'.
However, if you look at the following datasheet for a Fairchild
MOSFET...
http://www.fairchildsemi.com/ds/FD%2FFDG311N.pdf
you'll see that the title says "2.5V Specified", which I understand
refers to the drive voltage. However, under the "On Characteristics,"
you'll see Vgs(th) min. = 0.4V and Vgs(th) max. = 1.5V. Why isn't
Vgs(th) max. = 2.5V, as the title suggests? Is there a difference
between Vgs(th) and the "specified voltage"?
Thanks!
|
The threshold voltage is a statistical value. It is typical 0.9V at
least 0.4V and at most 1.5V. So if you buy one most likely the
threshold will be around 0.9V.
Assuming a simple device behaviour ("square law") the threshold
voltage is the value at which the MOSFET starts to turn on. The
current you can "drive" is proportional to the square of the
difference between gate-source and threshold voltage ("overdrive
voltage").
Chris |
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CWatters
Guest
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| Posted:
Wed Nov 16, 2005 1:33 am Post subject:
Re: MOSFET question: Drive voltage vs. Vgs(th) |
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<accolade@gmail.com> wrote in message
news:1132078002.207502.207930@g49g2000cwa.googlegroups.com...
| Quote: | you'll see that the title says "2.5V Specified", which I understand
refers to the drive voltage. However, under the "On Characteristics,"
you'll see Vgs(th) min. = 0.4V and Vgs(th) max. = 1.5V. Why isn't
Vgs(th) max. = 2.5V,
|
FETs are imperfect devices. Sure they turn on at Vgs but the higher the
ACTUAL gate drive voltage the lower the on-resistance. "2.5V Specified"
probably means that the on resistance and other parameters were
measured/specified with the gate at 2.5V.
Many FETs are designed for particular applications. For example you will
sometimes see FETs called "logic level" devices. This means that Vgs is low
enough for them to be driven by 3V or 5V CMOS logic families.
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Guest
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| Posted:
Mon Nov 28, 2005 9:35 am Post subject:
Re: MOSFET question: Drive voltage vs. Vgs(th) |
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accolade@gmail.com wrote:
| Quote: | Can anyone tell me the difference between the drive voltage and the
gate-to-source threshold voltage (Vgs,th) of a MOSFET, if any?
I was under the impression that these refer to the same voltage - the
voltage that is needed to turn the MOSFET 'on'.
However, if you look at the following datasheet for a Fairchild
MOSFET...
http://www.fairchildsemi.com/ds/FD%2FFDG311N.pdf
you'll see that the title says "2.5V Specified", which I understand
refers to the drive voltage. However, under the "On Characteristics,"
you'll see Vgs(th) min. = 0.4V and Vgs(th) max. = 1.5V. Why isn't
Vgs(th) max. = 2.5V, as the title suggests? Is there a difference
between Vgs(th) and the "specified voltage"?
See how they define threshhold voltage. They state Vds=Vgs, meaning |
the gate is tied to the drain. Then to get .00025 amps Ids with the
mosfet configured this way you have to apply about .9 volts, more or
less, but it could actually be anything between .4 and 1.5. Make of it
what you will. For a lot of applications it's irrelevant.
Now look at Figure 1. That information is a lot more likely to be
useful. |
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