see;
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Spank me with your keyboards!
Harry
- posted
10 years ago
SOAR OF N-CH FET
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- Vote on answer
- posted
10 years ago
For protection?
It may be helpful to put some sort of lowpass in there both to prevent transients from messing you up, and to allow for brief excursions beyond the SOAR during switching.
You know what device you're contemplating the use of, so you know (or can look it up in the data sheet) what sort of transient excursions outside of the static SOAR can be tolerated.
-- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
- Vote on answer
- posted
10 years ago
Hey, I make my dwgs on grid paper, so the lines are sort of straight. Sort of. Unless it's a whiteboard. Does anybody make a blue-grid whiteboard?
Did I mention this one?
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It's a multiplier of sorts. One could put a little resistance in series with the upper diode to adjust for the fact that some fets deviate from the constant-allowed-power curve at higher voltages.
The capacitor models the time aspect of the SOAR curves. The shutdown threshold should include a temperature component.
Your circuit is the moral equivalent of a foldback current limiter. That's OK, but you could maybe push the fet harder, safer with a more intelligent computer.
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-- John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
- Vote on answer
- posted
10 years ago
PG
My circuit is not just a foldback current limiter because of the positive temperature coeffi of Ron! SOAR is all about junction temperature and the R on is indicative of such. This circuit controls to a fix die temp then shut s down. The FET cannot be pushed any harder! No logs are needed to control smokeless operation.
Regards and I really like your dwgs (now I have said it), Harry
- Vote on answer
- posted
10 years ago
temperature coeffi of Ron! SOAR is all about junction temperature and the Ron is indicative of such. This circuit controls to a fix die temp then shuts down. The FET cannot be pushed any harder!
OK, I guess your fet is ON or OFF. My latest apps use the fet in linear mode, so I can't see Ron (whoever he is.) That's why I want a real Vd*Id multiplier.
I do real CAD schematics, or at least The Brat does, when we go to production. But I like to draw.
-- John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
- Vote on answer
- posted
10 years ago
y
.JPG
e temperature coeffi of Ron! SOAR is all about junction temperature and the Ron is indicative of such. This circuit controls to a fix die temp then sh uts down. The FET cannot be pushed any harder!
Did I say that mine ignores heat sinks, ambient temperature and all other secondary effects to get max FET operation. Just look in the data sheet and select the Ron at the die temp you want to stop at then set the amp gain. In the real system OFFN is latched and reset by a MCU.
Cheers, Harry
- Vote on answer
- posted
10 years ago
temperature coeffi of Ron! SOAR is all about junction temperature and the Ron is indicative of such. This circuit controls to a fix die temp then shuts down. The FET cannot be pushed any harder!
secondary effects to get max FET operation. Just look in the data sheet and select the Ron at the die temp you want to stop at then set the amp gain. In the real system OFFN is latched and reset by a MCU.
OK, OK! It's cute! You're measuring the ratio of Rds_on to Rs.
There are probably some timing subtleties, but the idea is good.
Mosfets have a second-breakdown sort of destruct mechanism that's from local heating or something, not a chip-wide thermal thing, but that happens at higher voltages, not your situation.
-- John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
- Vote on answer
- posted
10 years ago
"John Larkin"
** This recent app note from Semelab might interest you.
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It covers lateral MOSFETS for use in linear ( audio frequency) amplifiers. The possibility if using class G and H output stages to reduce dissipation with high supply voltage stages is mentioned.
The author "Robin" created the note in January 2011 - can't say if he has much hands on experience with MOSFET amplifiers but he has surely spoken with those that do.
Here is a list of the devices available:
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The 250V, 16A, 250W dual chip devices in TO3 would be most interesting.
... Phil
- Vote on answer
- posted
10 years ago
Ok you asked for it...
It oscillates!
Tim
-- Deep Friar: a very philosophical monk. Website:
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- Vote on answer
- posted
10 years ago
Ixys makes some brutal mosfets that are designed for linear operation, huge SOAR at higher voltages.
IXTH110N10L2 is rated 600 watts dissipation and 300 amps peak. 3500 watts for 1 millisecond, 22 kilowatts for 25 usec.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
- Vote on answer
- posted
10 years ago
"John Larkin" "Phil Allison"
** Well, this IS following a train of though right off a cliff ..... ** But it is not a lateral MOSFET and has none of the advantages.Try reading the links - you might just learn something.
If you ever get your fat head out of your arse.
.... Phil
- Vote on answer
- posted
10 years ago
Those wussy laterals are over an order of magnitude weaker than what I need. I'm driving lasers, not loudspeakers. Audio is boring.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
- Vote on answer
- posted
10 years ago
Not if is followed by a latch and you only get one cycle until the MCU res ets the latch. Larkin's switch is in a linear mode so Rds has no meaning. Mine is for a s olid state circuit breaker, where the surge current is 10X the holding curr ent to prevent false shut downs when starting. The over current loop operat es
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- posted
10 years ago
resets the latch.
a solid state circuit breaker, where the surge current is 10X the holding current to prevent false shut downs when starting. The over current loop op erates
an ir2127 would give you a gate driver, latch off, error out, undervoltage lockout and blanking so it doesn't trigger on turn on
it is meant for current limiting but I think with a single opamp could do the Rds trick
-Lasse
- Vote on answer
- posted
10 years ago
Ok. What I meant was, you didn't draw the latch :)
Tim
-- Deep Friar: a very philosophical monk. Website: http://seventransistorlabs.com