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Old 25th April 2003, 06:26 AM   #11
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Default 50 ways

Sajti is absolutely right. I would add that if you increase the # of devices, you can increase the source resistors. For say 25 devices you can use 2 Ohms and still have an equivalent source resistor of 80 milliOhms or something like that. But because each sees 2 Ohms, matching is unnecessary.

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Old 25th April 2003, 06:34 AM   #12
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Default Re: 50 Output Stage Transistors?

Quote:
Originally posted by thylantyr
Nelson Pass has inspired me when he said there
is no limit on how many output stage mosfet
transistors you can parallel provided that you
can drive them
Yes, exactly

Driving many pairs is hard because the gate-drain capacitance will get huge eventually which means highly capacitive load for the poor driver stage.

Why don't you start in the other end? Max stort term continous current, min practical load => minumum amount of pairs.

Bear in mind that wiring becomes very important if you have many pairs.

...but it's cool to have many transistors, impresses your mates
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Old 25th April 2003, 08:05 AM   #13
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Cool information.

Taking notes.

Thanks.

I'm laying out the proven 20 output
transistor design right now.. I will
probably make different flavors
of PCB to accomodate the madness..

When layout is done, I will make
the crazy version and order some
pcb's so I can do some smoke tests
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Old 25th April 2003, 09:27 AM   #14
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Default Re: Re: 50 Output Stage Transistors?

Quote:
Originally posted by peranders
Bear in mind that wiring becomes very important if you have many pairs.
Take a leaf from the book of aero engine designers of yesteryear. Lay the fets out radially in two circles, all the upper fets in one circle and all the lower ones in another circle. Sort of like two paddle wheels. Have all the leads point toward the centre. This will give you the lowest resistance, lowest inductance and shortest lead length possible, with no fet out on the end of a conductor but rather equal conductor length to all.
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Old 25th April 2003, 10:20 AM   #15
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another factor to consider is the positive coefficient for Rds vs. temperature. If one mosfet is taking on increasing load, its temperature will go up, causing its Rds to go up as well, which in turn reduces the load on that mosfet.

I have used one pair of 540/9540 without the source resistors and no apparent ill effect.
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Old 25th April 2003, 01:04 PM   #16
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Quote:
Originally posted by millwood
another factor to consider is the positive coefficient for Rds vs. temperature. If one mosfet is taking on increasing load, its temperature will go up, causing its Rds to go up as well, which in turn reduces the load on that mosfet.
That effect is really only most apparent when the fets are fully switched on, like when used in a switching power supply etc. For linear use, the gate threshold has a *negative* tempco of typically -5.5 mV / deg C which has a much greater effect than Rds variation. So for a fixed bias voltage and linear usage, the hottest fet is going to turn on further than the rest.

P.S. Hexfet type mosfets at least. e.g. IRFxxx types. Not sure about lateral fets.
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Old 25th April 2003, 03:40 PM   #17
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The temp co thing is only useful for lateral types, the old 2SK135/2SJ50 and similar ones. Modern Fet's has this effect at 4-6 A, Hitachi at 100-200 mA. Quite a differnce

The tempco can be as much as -10 mV /deg for some types.
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Old 25th April 2003, 04:06 PM   #18
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Default Re: Re: Re: 50 Output Stage Transistors?

Quote:
Originally posted by Circlotron

Take a leaf from the book of aero engine designers of yesteryear. Lay the fets out radially in two circles, all the upper fets in one circle and all the lower ones in another circle. Sort of like two paddle wheels. Have all the leads point toward the centre. This will give you the lowest resistance, lowest inductance and shortest lead length possible, with no fet out on the end of a conductor but rather equal conductor length to all.



14" PCB.

Cool.
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Old 25th April 2003, 05:37 PM   #19
JoeBob is offline JoeBob  Canada
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A big circle like that would work well for hooking up the FETs, but what about heat dissipation? You might want to think about that, I'm guessing you'll be using huge heatsinks, since you've got that much power, so you might want to spread out the heat sources a little more. Might think of a larger circle, or maybe several smaller circles spaced out. Otherwise you might have to end up using a larger heatsink to combat the inefficiency of how you mounted your fets.
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Old 26th April 2003, 01:46 AM   #20
haldor is offline haldor  United States
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Quote:
Originally posted by JoeBob
A big circle like that would work well for hooking up the FETs, but what about heat dissipation? You might want to think about that, I'm guessing you'll be using huge heatsinks, since you've got that much power, so you might want to spread out the heat sources a little more. Might think of a larger circle, or maybe several smaller circles spaced out. Otherwise you might have to end up using a larger heatsink to combat the inefficiency of how you mounted your fets.

Why not put a small individual heat sink on each FET and use a motor to spin the whole PCB? Use ring shaped tracks and brushes to get power into and signals in and out of the board. I can see it now, this could be like one of those crazy lightning machines from an old Frankenstein movie! "It's Alive... ALIVE!!"

Better go take my meds...

Phil
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