Triggering Mosfet with Vgs (2*Vds) > Vds????

[Franco]

Hi guys, I was wondering if there is any inconvenient of triggering
Mosfets (N-Channel BTW) with Vgs twice as big as the Vds???

Cheers...

Franco.

 

[Winfield]

Hi guys, I was wondering if there is any inconvenient of triggering
Mosfets (N-Channel BTW) with Vgs twice as big as the Vds???

Cheers...

Franco.

Not sure what you mean by "any inconvenient," but I can
tell you it's not an uncommon situation, and involves no
unusual difficulties if a supply voltage higher than Vds
is available to power the gate driver.

 

[Franco]

Not sure what you mean by "any inconvenient," but I can
tell you it's not an uncommon situation, and involves no
unusual difficulties if a supply voltage higher than Vds
is available to power the gate driver.

I meant that I could not find any written material supporting
that way of using the Mosfets. Anyways, if that approach is usual
there should not be any inconvenient.

Do you have any example when to use that configuration?

Greetings...

Franco.

 

[Spehro Pefhany]

Hi guys, I was wondering if there is any inconvenient of triggering
Mosfets (N-Channel BTW) with Vgs twice as big as the Vds???

Cheers...

Franco.

It might be very inconvenient if Vds was 600VDC.

Best regards,
Spehro Pefhany

 

[[email protected]]

I meant that I could not find any written material supporting
that way of using the Mosfets. Anyways, if that approach is usual
there should not be any inconvenient.

Do you have any example when to use that configuration?

You'd want a device without protection diodes on the gate. The SD214
comes to mind.

http://www.ortodoxism.ro/datasheets/linearsystems/SD214.pdf

I've no idea where you could buy one these days - Farnell doesn't seem
to stock them any more. Joerg will know.

 

[Winfield]

It might be very inconvenient if Vds was 600VDC.

Thanks, I needed that!

 

[Winfield]

I meant that I could not find any written material supporting
that way of using the Mosfets. Anyways, if that approach is
usual there should not be any inconvenient.

Do you have any example when to use that configuration?

If you study your written materials, app notes, etc.,
you'll see the issues of Vgs to turn on the MOSFET, and Vds
are independent; there's no relationship between the two.
The most common situation where Vds is much lower than Vgs
occurs when a MOSFET is switched on and Vds is nearly zero.
And of course we expect to use a reasonably-high Vgs voltage,
such as 12V, etc., to insure that the MOSFET is very well ON
in that case. Later, when Vgs = 0 and the MOSFET is OFF,
the drain voltage is free to rise, e.g. up to the Vd supply
voltage, etc, but consider, why should this be very high at
all? For example, the MOSFET might be acting as a rectifier
in a high-current, low-voltage SMPS, delivering 1.8 volts at
60A to a microprocessor. High Vgs voltages are sill required
to turn on the MOSFETs.

 

[Jim Thompson]

Not sure what you mean by "any inconvenient," but I can
tell you it's not an uncommon situation, and involves no
unusual difficulties if a supply voltage higher than Vds
is available to power the gate driver.

Student, wrapped up in equations rather than thinking things through.

...Jim Thompson

 

[Franco]

"America: Land of the Free, Because of the Brave"...

Patriotic American, wrapped up in fights rather than thinking things
through.

 

[Franco]

If you study your written materials, app notes, etc.,
you'll see the issues of Vgs to turn on the MOSFET, and Vds
are independent; there's no relationship between the two.
The most common situation where Vds is much lower than Vgs
occurs when a MOSFET is switched on and Vds is nearly zero.
And of course we expect to use a reasonably-high Vgs voltage,
such as 12V, etc., to insure that the MOSFET is very well ON
in that case. Later, when Vgs = 0 and the MOSFET is OFF,
the drain voltage is free to rise, e.g. up to the Vd supply
voltage, etc, but consider, why should this be very high at
all? For example, the MOSFET might be acting as a rectifier
in a high-current, low-voltage SMPS, delivering 1.8 volts at
60A to a microprocessor. High Vgs voltages are sill required
to turn on the MOSFETs.

It confuses me that there are some curves indicating that
Vds > Vgs - Vth. But, as you mention, the channels (N or P) should
be created independent of the voltage between the terminals.

The mosfets have are commanded by a higher Vgs because I am using
a Brushless Driver motor chip that internally regulates up the
voltages
in the gates for driving the upper mosfets in three half mosfet
bridges.

On the other hand, the voltage being sent to the motor may drop
under the level of that Vgs generated by a bootstrap cap.

Thanks for the answer!

Franco.

 

[Richard the Dreaded Libertarian]

"America: Land of the Free, Because of the Brave"...

Patriotic American, wrapped up in fights rather than thinking things
through.

You must be new around here. Jim's been our resident brownshirt for
a number of years now.

Cheers!
Rich

 

[Jamie]

"America: Land of the Free, Because of the Brave"...

Patriotic American, wrapped up in fights rather than thinking things
through.

Now that's no way of making friends.

 

[Jim Thompson]

"America: Land of the Free, Because of the Brave"...

Patriotic American, wrapped up in fights rather than thinking things
through.

An exhibition of one of the many reasons I kill-filter on "@gmail.com"
.... sort of a free first-cut IQ meter ;-)

...Jim Thompson

 

[Franco]

Ok... so, that's why he can treat others as he wants.

 

[Michael A. Terrell]

Student, wrapped up in equations rather than thinking things through.

Suggestions for new newsgroups:



Any other suggestions?

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Michael A. Terrell]

"America: Land of the Free, Because of the Brave"...

Patriotic American, wrapped up in fights rather than thinking things
through.

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--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Martin Griffith]

"America: Land of the Free, Because of the Brave"...

Patriotic American, wrapped up in fights rather than thinking things
through.

The US population are pretty easily manipulated by corporate lies, but
eventually they sort themselves out.

Win once spake:

"Americans generally do the right thing, after first exhausting all
the available alternatives"

It's a pity the US don't look to the French for methods of telling the
Gubberment that they suck

http://news.bbc.co.uk/2/hi/europe/7093373.stm

But the RIAA/Exxon brigade will not permit stuff like this to happen
in the US


Martin

 

[Martin Griffith]

Suggestions for new newsgroups:



Any other suggestions?

Martin

 

[Joerg]

It confuses me that there are some curves indicating that
Vds > Vgs - Vth. But, as you mention, the channels (N or P) should
be created independent of the voltage between the terminals.

The mosfets have are commanded by a higher Vgs because I am using
a Brushless Driver motor chip that internally regulates up the
voltages
in the gates for driving the upper mosfets in three half mosfet
bridges.

On the other hand, the voltage being sent to the motor may drop
under the level of that Vgs generated by a bootstrap cap.

Thanks for the answer!

To illustrate Win's point with a real-life example: Take a single fuel
cell. It generates about 650mV and an incredible current may be
available. However, you cannot really use 650mV directly for anything
electronic. So, a few big FETs are arranged, typically in two banks. One
bank pulls down one primary, then turns off and now the other pulls the
2nd primary down. Vds is never higher than 650mV.

Those FETs still need 10V or so of Vgs to switch reliably. Mostly those
kind of converters generate higher voltages at their outputs but that
won't be there without first switching the FETs a few times. Therefore,
a little helper oscillator is used that generates 10V or more at low
power. This voltage is used to drive an oscillator, the regulator loop
and so on. The oscillator feeds buffers and those drive the FET gates
hard from 0V to 10V and back.

So here you have it: Vds is between 0V and 0.65V all the time while Vgs
is between 0V and 10V.

 

[Michael A. Terrell]

Martin

Those are all covered by eurosheep.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida