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Registered Member #1563
Joined: Wed Jun 25 2008, 03:55AM
Location: Wimer Oregon, Wewt for sticks!
Posts: 30
Right now with my current coil, I have a maximum of 400A at 800V. With the diode anti-parallel to the coil's rating being an average current of 50A, I have been fine (figuring peak current handling being 10x that of average). However I want to vary coil dimensions and size, and the and soon I run into currents of 4kA or more. Any suggestions on what I should do at that point, or is a damped coil my only *affordable* solution?
banned on 5/26/2009 Registered Member #1877
Joined: Mon Dec 22 2008, 02:03AM
Location:
Posts: 147
400a is not much for a diode to handle, espescially in a pulse. you can sample diodes from stmicro that can handle 1000a pulses. and the little rule of thumb of 10x the average current rating does not necessarily apply to all semiconductors, i got some SCR's from vishay, 50 amps average current, but can take a huge 1490 amp pulse in 8.3ms, and 1430 amps in 10 ms. so as for what you should do, you should buy a few diodes, and parallel them to handle the full pulse.
Registered Member #1563
Joined: Wed Jun 25 2008, 03:55AM
Location: Wimer Oregon, Wewt for sticks!
Posts: 30
I don't know if my figures are off, but say I wanted to toss in a 90 turn coil of specs: Inner Diameter: 8mm outer Diameter: 21mm length: 51mm inductance: 24uH Resistance: 34mOhm
It would be nice to test with such a thing... but with 800v and 1.1 mF, it has a negative current of 3kA. I wouldn't trust that many paralleled diodes...
banned on 5/26/2009 Registered Member #1877
Joined: Mon Dec 22 2008, 02:03AM
Location:
Posts: 147
BlakFyre wrote ...
It would be nice to test with such a thing... but with 800v and 1.1 mF, it has a negative current of 3kA. I wouldn't trust that many paralleled diodes...
why wouldent you? i have a 5 kj bank and use 18 parralell diodes that still work. they are 400amp pulse each. and even i think its overrated.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
Paralleling silicon semiconductor junctions is more complicated than it seems, thermal runaway is time dependant.
At low currents and long pulses, the advise is don't. With the junction "on" voltage dominating the voltage drop, one device will indeed heat and hog all of the current.
At high currents with short pulses, the intrinsic resistivity of the diodes dominates the forward drop, which may be volts. Each diode looks more like a resistor, and, while they're heating, they share nicely.
This effect can be seen in the SOA curves for bipolar devices, which show a decreasing power handling as the pulse lengthens, it's a measure of how long the many parallel junctions that they're made of will remain sharing reasonably well under heating conditions.
As a precaution, take the I2R implied by a single device's pulse rating as well as the peak I, and derate both by a feelgood factor (3?) for your calculated pulse length and magnitude, for the degradation of sharing that will occur. A good precaution to start with would be to have them all from the same batch.
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Do I need a super diode?
