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east electronics 12th May 2007 08:11 AM

mosfet amp protection diodes
 
many design have in the gates of mosfet diodes for gate overdrive protection

any body knows if this technic can be also applyied to designs that have quasi output configuration ????

thanks sakis east_electronics@yahoo.gr

fotios 12th May 2007 07:15 PM

Try this. But be carefull because i don't have check it in practice. Also as you know, i don't like Mosfets. This method used by John Linsley Hood in his well known 150W Mosfet amplifier project.

http://www.eal.gr/LEGEND%20SAKIS.jpg

fotios 12th May 2007 08:01 PM

This scheme of protection zeners becames from the project of JLH slightly modified during 1985 in the hellenic magazine "«◊Ō”" (Sound & Image) from Theodoros Spinoulas if i remember well. This amplifier was the first and the last Mosfet amplifier that i have build in my life. For this protection scheme i am in posetion to confirm that it works fine.

http://www.eal.gr/LEGEND%20SAKIS_1.jpg

jcx 12th May 2007 08:48 PM

mostfets gates without internal gate protection diodes are as likely to fail from excess reverse V as they are from forward V - so the opposing diode in series with the protecting Zener in the above schematics leave the gate oxide at risk

Nelson Pass 13th May 2007 12:40 AM

In both schematics, you can get by with only the single zener
diode on each output set - it will not be forward biased in linear
operation and will protect against large negative Vgs as well as
positive.

Also in both schematics, it is wise to put the zener diodes on
the outside of the gate resistors, otherwise you will occasionally
encounter parasitic oscillation.

:cool:

fotios 13th May 2007 06:54 AM

Quote:

Originally posted by Nelson Pass
In both schematics, you can get by with only the single zener
diode on each output set - it will not be forward biased in linear
operation and will protect against large negative Vgs as well as
positive.

Also in both schematics, it is wise to put the zener diodes on
the outside of the gate resistors, otherwise you will occasionally
encounter parasitic oscillation.

:cool:

As i said before, i don't have experience about mosfets. Simply i try to help Sakis. According to your suggestions i changed the schematic to this below. It is correct now?

http://www.eal.gr/LEGEND%20SAKIS_2.jpg

Leolabs 14th May 2007 02:54 AM

Some LATFETs already incoporate protection diodes inside them.

Workhorse 14th May 2007 03:05 AM

Quote:

Originally posted by Nelson Pass
otherwise you will occasionally
encounter parasitic oscillation.

I haven't encountered yet any oscillation from placing the zeners directly to gate-source terminals

Nelson Pass 14th May 2007 04:05 AM

Quote:

Originally posted by fotios
It is correct now?
If you drop D1 it will swing more positive output.


Quote:

Originally posted by Workhorse
I haven't encountered yet any oscillation from placing the zeners directly to gate-source terminals
Not typical, but I have seen it.

:cool:

fotios 14th May 2007 06:18 AM

Letís start from the beginning because I believe that has become confusion. Below it is the original design of BORAíS LEGEND but redrawn so that it is comprehensible. Because the initial drawing that I had received from sakis was inconceivable.
Maybe mr. Jagodic can confirm that there is not any mistake in the values of components and in the connections of this drawn?

http://www.eal.gr/LEGEND%20BORA.jpg


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