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Quick TVS diode question. Vclamp < Vbreakdown
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T3sl4co1l:
L what?  Show an equivalent circuit.

If you're thinking about the source: load dump isn't an RLC equivalent circuit -- well, maybe you could design a pulse generator that way, but the real deal is an equivalent due to a positive feedback effect in the alternator; there will be some ESL still (winding inductance) but it isn't dominant, and it's modeled as a waveform from an ideal Thevenin source.

Tim
kellogs:
Ugh, L is the inductor in between the two protection devices in https://itecnotes.com/electrical/electronic-two-gas-discharge-tube-in-parallel-a-fast-acting-gas-acting-tube-and-a-gas-discharge-tube/

L_min would be my calculated minimum value for it.

Thanks
T3sl4co1l:
Ah, that way.

That's only needed for fast-ish rise times, where the GDT breakdown delay matters.  You're well past that here.  Just pick a GDT low enough to work.

More important is the GDT needs a TVS in series with it, so it doesn't short out the supply after the load dump.  It's a discharge, it stays latched on until current falls below holding.

Which is a TVS that still needs to handle the full current of said dump, so, doesn't save much compared to a full size TVS just clamping the whole thing.  The GDT also needs a quite generous energy rating, as its voltage drop during breakdown is some 10-20V or whatever.  I haven't checked but I'm guessing they're neither recommended for, nor available for, load dump application.

...You're also spark-discharging any capacitors on the bus, which, whatever, it's going to be less energy than load dump itself, but still, it's more stuff going on, and that extra backwards current might do weird things to attached devices, who knows.

Tim
kellogs:
GDT in series with TVS would clamp the voltage way to high for automotive.

Back to the linked diagram, I would not use a GDT, but, as said, a MOV (more precisely the GMOV that @Tomorokoshi has mentioned - https://media.digikey.com/pdf/Data%20Sheets/Bourns%20PDFs/GMOV_Series_DS.pdf)


--- Quote from: T3sl4co1l on September 01, 2022, 05:33:08 pm ---Which is a TVS that still needs to handle the full current of said dump, so, doesn't save much compared to a full size TVS just clamping the whole thing.

--- End quote ---

Huh? Tim sir, can I possibly get a practically sized TVS diode that would actually handle that whole blast of energy by itself? Oh, and within budget too would be nice :D - say max $10 a pop.
T3sl4co1l:
Just use the MOV straight up?  MOVs are just sloppier TVSs...


--- Quote from: kellogs on September 01, 2022, 06:11:23 pm ---Huh? Tim sir, can I possibly get a practically sized TVS diode that would actually handle that whole blast of energy by itself? Oh, and within budget too would be nice :D - say max $10 a pop.

--- End quote ---

I'm sure the name brands get theirs for less; but then, I suppose your production quantity is somewhat less than 10 million/year, too, huh? ::)

:-DD

Tim
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