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Spockie-Tech
Site Admin
Joined: 31 May 2004
Posts: 3160
Location: Melbourne, Australia
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Chris Baron from Robot Power would probably be the man to ask - You can find him on the OSMC mailing list at http://tech.groups.yahoo.com/group/osmc/
There are lots of other power fet gurus on that list as well (its where I learned most of my Fet knowledge) who might take you up on it - but Chris has more experience at designing things suitable for combat environments, most of the other gurus are more industrial control guys who might not take into account shock loading and regular overloads as much.
Last time I talked to Chris, he had just landed an OEM motor controller contract so was fairly busy and might not want to take on a new project, but hes usually willing to chip in with some ideas, and if you can convince him it would be a saleable product, he might be interested.
Instantanous power dissipation is the tricky bit to deal with when switching high current loads - much like Jolt has to deal with harsh shock loads because of its rigidity that causes unexpected areas to break - having a "stiff" power supply/batteries that can near-instantly deal out large chunks of watts makes things go boom..
if you can soften up the "hit" when the switch changes state at the source of the power (not at the switch which would cause fet heating) then you can get away with a lot less precision power drive cleverness required in the fet drive circuit - limiting the batteries ability to deliver hundreds of amps with a thin wire or other resitive element will makes the controllers job a lot easier - it might add a small amount to your spin up time but shouldnt reduce the final energy level reached majorly.
I have enough on my plate at the moment that I can only offer some general advice, but not a complete design service at this time sorry.
Another possibility might be the cafe electric guy who makes the "Zilla" controllers - http://cafeelectric.com/shop/index.php?main_page=index&cPath=1 - the most powerful consumer ESCs in the world that I know of - 300v at 2000amps - around 600Kw or 800Hp ! - I dont know if he is available for custom electronics design, but if anyone knows how to spin a high power SSR together, he should. _________________ Great minds discuss ideas. Average minds discuss events. Small minds discuss people
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Mon Jun 23, 2008 1:42 am |
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Spockie-Tech
Site Admin
Joined: 31 May 2004
Posts: 3160
Location: Melbourne, Australia
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P-Channel Power Fets do exist, but they are usually not available in as high a power rating as N-Channels for some reason, and are usually much more expensive for a given power as well.
Which is why most H-Bridge controllers do all the jumping through hoops necessary to drive N-Channel fets on the high side of the bridge as well as the low side rather than use P-Channel for the high side (which would be much simpler to drive, no Vs sensing and gate voltage charge pumps would be required for P-types on the high side)
Repeated high speed switching (like PWM) does mean you spend more time in the "transitional zone" (where both the current flow and voltage drop across the Fet are high which means high power dissipation and booms), than a single-switch event.
But, even a single poorly-driven switch event is enough to destroy a fet if the gate drive is weak and more than fractions of a microsecond are spent transisitioning from off to on, so you still need to be able to charge and discharge the Fet gate capacitance quickly, even although you're only switching occasionally.
As an analogy (maybe not a great one, but it will do for now), you cant make a gun barrel out of cardboard, even if it does only need to fire occasionally.. the high energy event still needs to be contained and directed for it to work even once.
Higher power fets tend to have higher gate capacitance and need a correspondingly higher current gate drive to switch rapidly. So make sure you check the Gate Capacitance spec for your 300amp fets and rate your drive circuit appropriately..
For example - The IBC's HIP chips can supply a momentary current of 2.5 amps to charge the 1405 Fets 5 Nanofarad gates in under a few nanoseconds.
Putting in higher power rated fets (or more fets in parallel) increases the capacitance to be charged and thus slows down the switching time (causing more heating) correspondingly so you dont get as much benefit as would first appear by just banking up fets or putting bigger ones in. The 8-Fet controllers (40nf total) are about the limit for the 2.5a Hip drive drives afaik. _________________ Great minds discuss ideas. Average minds discuss events. Small minds discuss people
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Mon Jun 23, 2008 11:05 pm |
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