mosfet gate threshold voltage

http://www.powerdesigners.com/InfoWeb/design_center/articles/MOSFETs/mosfets.shtm
says:

4.5 Gate Threshold Voltage, VT , {VTH , VGS(th) }
VT is the minimum gate voltage at which the transistor will turn
ON.

Does this mean, 100% full ON? Or that the gate is Starting to turn On
(1% ON, for example)?

Do mosfets prefer to be fully on or fully off, or can they be run at
some point in the middle, continuously?

If I wanted a MOSFET to run at, say, 50% of full ON, would a MOSFET
cheerfully comply, or would it blow up?

Thanks,

Michael

 

[Eeyore]

mrdarrett@gmail.com wrote:

http://www.powerdesigners.com/InfoWeb/design_center/articles/MOSFETs/mosfets.shtm
says:

4.5 Gate Threshold Voltage, VT , {VTH , VGS(th) }
VT is the minimum gate voltage at which the transistor will turn
ON.

Does this mean, 100% full ON? Or that the gate is Starting to turn On
(1% ON, for example)?

Threshold means starting to turn on..

Do mosfets prefer to be fully on or fully off, or can they be run at
some point in the middle, continuously?

Whichever you like subject to chip power dissipation and die temperature limitations.
Google for Motorola's AN1040 which goes into some detail about heatsinking.

If I wanted a MOSFET to run at, say, 50% of full ON,

Define 50% full on !

would a MOSFET cheerfully comply, or would it blow up?

It all depends. Cheerfully either.

As ever the devil is in the detail which you will have to learn I'm afraid.

Graham

 

[Phil Allison]

<mrdarrett@gmail.com>

http://www.powerdesigners.com/InfoWeb/design_center/articles/MOSFETs/mosfets.shtm
says:

4.5 Gate Threshold Voltage, VT , {VTH , VGS(th) }
VT is the minimum gate voltage at which the transistor will turn
ON.

Does this mean, 100% full ON? Or that the gate is Starting to turn On
(1% ON, for example)?

** Go look up the word " threshold " in a dictionary - fool.

Do mosfets prefer to be fully on or fully off, or can they be run at
some point in the middle, continuously?

** Why do you keep posting questions you already know the answer to ?

Troll.

If I wanted a MOSFET to run at, say, 50% of full ON, would a MOSFET
cheerfully comply, or would it blow up?

** For you - it would rather blow up than be mauled by an idiot.



...... Phil

 

[default]

On Fri, 24 Oct 2008 16:18:14 -0700 (PDT), mrdarrett@gmail.com wrote:

http://www.powerdesigners.com/InfoWeb/design_center/articles/MOSFETs/mosfets.shtm
says:

4.5 Gate Threshold Voltage, VT , {VTH , VGS(th) }
VT is the minimum gate voltage at which the transistor will turn
ON.

Does this mean, 100% full ON? Or that the gate is Starting to turn On
(1% ON, for example)?

that means the minimum gate voltage - subject to temperature - that it
begins to conduct. See a typical data sheet and you'd get the idea -
check the charts.

Do mosfets prefer to be fully on or fully off, or can they be run at
some point in the middle, continuously?

They can be used in the linear region (partially on) like audio amps
or (on-off) saturated switches. Preferences? They don't say. I
doubt they care all that much.

They find a lot of use in switches because, generally speaking, they
waste less power in many applications. Rds (resistance drain to
source) on high current devices is frequently in the milliohm region,
when switched fully on. - and you can use them in parallel to further
reduce the "on" resistance.

If I wanted a MOSFET to run at, say, 50% of full ON, would a MOSFET
cheerfully comply, or would it blow up?

It would comply providing you remove the heat generated. Think in
terms of a variable resistor and the amount of power dissipated in the
mosfet - then provide a way to remove the heat if you need to.

Thanks,

Michael

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[langwadt@fonz.dk]

On 25 Okt., 01:18, mrdarr...@gmail.com wrote:

http://www.powerdesigners.com/InfoWeb/design_center/articles/MOSFETs/...
says:

4.5 Gate Threshold Voltage, VT , {VTH , VGS(th) }
VT is the minimum gate voltage at which the transistor will turn
ON.

Does this mean, 100% full ON? Or that the gate is Starting to turn On
(1% ON, for example)?

Starting to turn on, the datesheet will say what current, usually
something
like a few hundred uA

Do mosfets prefer to be fully on or fully off, or can they be run at
some point in the middle, continuously?

as long as you can get rid of the heat generated it doesn't matter

If I wanted a MOSFET to run at, say, 50% of full ON, would a MOSFET
cheerfully comply, or would it blow up?

Thanks,

Michael

what do you mean by 50% on?

as long as you stay within the limits of the datasheet things should
be ok

-Lasse

 

[pimpom]

<mrdarrett@gmail.com> wrote in message
news:98b4d381-0db0-4908-a8c5-a15613d61815@o4g2000pra.googlegroups.com...

http://www.powerdesigners.com/InfoWeb/design_center/articles/MOSFETs/mosfets.shtm
says:

4.5 Gate Threshold Voltage, VT , {VTH , VGS(th) }
VT is the minimum gate voltage at which the transistor will
turn
ON.

Does this mean, 100% full ON? Or that the gate is Starting to
turn On
(1% ON, for example)?

Here you have to define what ON and 100% ON mean. The gate
threshold voltage is the gate voltage that will just start to
make an appreciable current flow through the MOSFET. Increasing
the gate voltage will cause a progressively larger current to
flow between source and drain until the current reaches a level
limited by the load.

Example: Vth = 4V, P.S. = 10V, Rload = 1 ohm.
As you increase the gate voltage above 4V, the drain current will
increase, but that current will cause a voltage drop in the load
resistor until Id = 10A. When that level is reached, the whole
10V is dropped across the load resistor and drain current can no
longer increase even if you keep on increasing the gate voltage.
That's when it is said to be fully (or 100%) ON.

You can set the current at any value between 0 and 10A by
controlling the gate voltage.

However, if the load resistance is high, a gate voltage of 4V or
very slightly above it may be anough to turn the transistor fully
on. If the load resistor is, say, 1000 ohms, the maximum current
possible is 10V/1000ohms = 10 mA. And that level of current may
already be reached at or slightly above Vth.

Do mosfets prefer to be fully on or fully off, or can they be
run at
some point in the middle, continuously?

If I wanted a MOSFET to run at, say, 50% of full ON, would a
MOSFET
cheerfully comply, or would it blow up?

In the above example with a 1-ohm load, when the current is
somewhere between 0 and 10A, that part of the supply voltage that
is not dropped across the load resistor exists across the
transistor. Now the transistor is absorbing both voltage and
current. Voltage*current = Power. And that power heats the
transistor.

Suppose you set the gate voltage so that Id = 4A. That will cause
a drop of 4V across the load resistor, leaving 6V across the
transistor. 6V*4A = 24Watts. Now you have 24W of power heating
the transistor, and that will quickly burn it out unless you
provide adequate cooling.

 

On Oct 25, 7:50 am, "langw...@fonz.dk" <langw...@fonz.dk> wrote:

On 25 Okt., 01:18, mrdarr...@gmail.com wrote:

http://www.powerdesigners.com/InfoWeb/design_center/articles/MOSFETs/...
says:

4.5 Gate Threshold Voltage, VT , {VTH , VGS(th) }
VT is the minimum gate voltage at which the transistor will turn
ON.

Does this mean, 100% full ON? Or that the gate is Starting to turn On
(1% ON, for example)?

Starting to turn on, the datesheet will say what current, usually
something
like a few hundred uA

Thanks - you've been most helpful

Do mosfets prefer to be fully on or fully off, or can they be run at
some point in the middle, continuously?

as long as you can get rid of the heat generated it doesn't matter

Thanks!

If I wanted a MOSFET to run at, say, 50% of full ON, would a MOSFET
cheerfully comply, or would it blow up?

Thanks,

Michael

what do you mean by 50% on?

1/2 ID_max

as long as you stay within the limits of the datasheet things should
be ok

-Lasse

Thank you very much for the helpful replies.

Michael