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Old 30th October 2008, 11:54 PM   #11
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Quote:
Originally posted by Mooly
Hi Nico,
"Nothing negative to report" only the tempco. And we like that . And they are tough, I once years ago accidently shorted the speaker output to one of the rails on an earlier amp. Flash/bang, but no harm done.
Just interested really in what I can do with the verticals, can I match the sonics of,
My MOSFET amplifier designed for music.

The verticals are just so cheap now, not that that's a reason in itself of course.
I find the lateral fets very interesting devices, I'd use them but too bad they are expensive. I'm not that surprised the transistors survived the rail-to-output short though.

I deliberately tried to blow a lateral FET up last year:
Lateral MOSFET torture...

50V 10A power supply over D-S and hit it with 9V on the gate on a too small heatsink... And it survived! Did it ten times and it still works!

Expect for the transconductance decreasing a lot as it heats up there seems to be another pretty non-obvious mechanism protecting it. Seems like the internal protection zeners start conducting like a thyristor when the temperature gets high enough. This clamps gate voltage to ~1V provided gate current drive is limited and at this gate voltage and temperature the transistors don't let much current through. It was 0.2A with the transistor I tested on the heatsink I had.

This might explain why they are so hard to blow up as many have experienced.

To take advantage of this one has to make sure:
* External gate zeners limit the gate voltage so the current can't go high enough to blow the internal drain wire or damage the transistor in other ways. Probably more of a problem with the metal cans as the plastic ones have the wire embedded in epoxy, increasing it's overload capacity.

* Gate current is limited. The gate resistors and/or VAS should limit the gate current to a not-to-high value if the transistor starts drawing current into the gate. With a standard topology current limited VAS is needed anyway as the external zeners should be protected.

If the VAS is not current limited the VAS transistor is probably the first component to blow with a shorted output. This will then take out resistors and stuff, hopefully the output stage gate resistors will fuse without outputs blowing but the amp is broken nevertheless.
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Old 31st October 2008, 02:07 AM   #12
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well.. i did mean there is a bias pot, but the jumpers would allow quick change from a 1.0-2.0V range to 2-4V, 3-8V ranges in order to quickly get "in the ballpark" for different output device combinations. using a simple 8 pin (2x4) header and a jumper block. this would be for prototyping only. i wouldn't want this feature available to consumers. too much temptation for the curious mind.....

i've seen it before.... "hmm...what does this do????....... SNAP!!!! SSSSZZZZTTBOOOOMMMM....."



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Old 31st October 2008, 03:08 AM   #13
gain is offline gain  United States
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Mooly, good to see you. as long as the amp sounded good i would have no problem using L-MOSFET in the OPS. i (very slightly) prefer the sound of bipolars to mosfets actually, but thats just my purely subjective opinion.


Quote:
Originally posted by unclejed613
i've seen it before.... "hmm...what does this do????....... SNAP!!!! SSSSZZZZTTBOOOOMMMM....."

ROFL! Yeah i've seen this a few times too and been guilty of it myself as well from time to time i must admit. ahhh, cats and curiosity.
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Old 31st October 2008, 07:49 AM   #14
Mooly is offline Mooly  United Kingdom
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Bob wrote,
"A Zobel network on the gate" interesting thought, I hadn't even considered that.
I haven't run into any stability problems so far with the verticals in a CFP arrangement, I use a 100mhz 'scope and all seems well.
180 ohms on the gates, I am amazed how docile they seem.
Are they going to bite when I least expect it ?
Quiescent current was another worry. Folks seem to say "more is better" when it comes to VFets. Again I found the non linearity using the CFP stage dissappeared at around 100ma, but a Vgs multiplier in contact with the outputs seems a must.

Zobel on the gate eh !!

Thanks
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Old 31st October 2008, 08:04 AM   #15
h_a is offline h_a  Europe
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Quote:
Again I found the non linearity using the CFP stage dissappeared
That's also more or less what D. Self found! Pretty well performing for the bad mosfets

Have fun, Hannes
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Old 31st October 2008, 10:57 AM   #16
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unlike tubes, gates should never draw any current. unless they're shorted to the channel or they have internal zener protection. with RF there's also the possibility of RF current into the gate capacitance.

would the zobels go to the source or ground? probably makes sense to go to the source since that would shunt any RF to the source.

i have a book here with a test circuit for testing various "drop in" output stages. its basically a generic class B amp with (socketed?) "DUT" spots where you connect BJT's, darlingtons, Sziklai pairs, or MOSFETs. that's why the idea of the jumper configurable bias.

VFETs actually have a positive tempco only up to a point (up to about 100mA or so, sometimes as high as 1 amp) after that it turns negative like L-MOSFETs. that explains why they behave better with higher bias current.
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Old 31st October 2008, 11:21 AM   #17
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Quote:
Originally posted by megajocke
[snip]50V 10A power supply over D-S and hit it with 9V on the gate on a too small heatsink... And it survived! Did it ten times and it still works![snip]

Of course it survived! You turned if fully on, what is the Rdson, about 0.01 ohms? At 10 Amps (if your supply was holding up, which it probably wasn't) that's about 1Watts disspation.....

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Old 31st October 2008, 11:22 AM   #18
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"unlike tubes, gates should never draw any current. unless they're shorted to the channel or they have internal zener protection. with RF there's also the possibility of RF current into the gate capacitance."

The laterals have internal zener protection. They seem to latch up at lower voltage at high temperature protecting the transistor.
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Old 31st October 2008, 11:47 AM   #19
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by unclejed613
VFETs actually have a positive tempco only up to a point (up to about 100mA or so, sometimes as high as 1 amp) after that it turns negative like L-MOSFETs. that explains why they behave better with higher bias current.
I've read and re-read this and I'm confused.
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Old 31st October 2008, 11:48 AM   #20
h_a is offline h_a  Europe
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Quote:
VFETs actually have a positive tempco only up to a point (up to about 100mA
I think you mixed the two up. The laterals have zero tempco at about 100mA, verticals are usually at several amps.

Have fun, Hannes
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