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Quick TVS diode question. Vclamp < Vbreakdown

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T3sl4co1l:

--- Quote from: kellogs on September 03, 2022, 10:09:15 am ---Wow, I need to revisit college notes...

>> then we can take its slope of ~ -0.443

How did you derive this number out of that logarithmic graph in fig.1 ?

--- End quote ---

Take the rise and run.  Take the log of each, then take the ratio of logs.  That is:

\[ a = \frac{ \ln \frac{y_2}{y_1} }{ \ln \frac{x_2}{x_1} } \]

for pairs of points on the curve (x1, y1), (x2, y2) such that \$y \propto x^a\$.  I noticed that, for 1 decade (10 times) of y, there was about a 180 times span in x -- you don't need absolute points, you can read the ratios off the axes directly.

Then, say, knowing that you need to extrapolate 10 times further than the plot shows, go \$10^a\$ times further along the other axis.



--- Quote --->>The load dump peak power is Vc * (Us - Vc) / Ri or 4.8kW,

 Of course (2)! Also, reading the note on load dump impulse in my attached picture a few posts back:


--- Quote ---a
 If not otherwise agreed, use the higher voltage level with the higher value for internal resistance, or use the lower voltage level with
he lower value for internal resistance.

--- End quote ---

Does that mean that under worst circumstances, for the diode in question, P_peak yields about 29 * 50 / 0.5 =~ 3 kW ?

--- End quote ---

That or 29 * 72 / 4, which is lower, so yeah.  Or inbetween values, which I guess would be at customer request (if someone (OEM, say?) is looking for a particular combination).



--- Quote --- >>probably two 30kW in parallel

Would that not require perfectly matched V_clamp ?

--- End quote ---

Not so much: Vclamp is moderately resistive, and has a positive tempco.  They might not share evenly at first, but once the die heats up, both will be bearing the load.

Which is why you need extra capacity: they won't track perfectly, so you lose some difference in capacity.  The result is strictly more capacity than a single, but how much more does depend.

I wouldn't have a problem using several in parallel at say... 30% derating?  Or less in greater numbers.



--- Quote ---These 15 kW diodes are over budget anyway, so, if MOVs are that bad... reed switches! Yes I would actually use a comparator to trigger the reed relay(s). Many of them are specc'ed at 1ms max engage time, which would be great I think. Most of the automotive impulses are ~ uS long, so the reed switch would not even engage them - a smaller TVS diode may be employed for them. And when the big one hits, then TVS takes out a bit of their energy and then 1ms later reed switch relays the rest of it into an 1M resistor or such. But... their carry / switching current ratings are quite lousy. What are they going to do, weld into place ?

--- End quote ---

Engage, sure, what about disengage time?  What's a 1M going to do?  (Did you mean 1m?)  You don't have many ms left before it has to open into the full 100A surge.

Welded?  Melted?  Absolutely.  The contacts are quite small.

If only we had some kind of technology, that could switch faster... ???

Tim

kellogs:
Got it, thank you! Actually t_d will be 400/2 = 200 ms, not 100 ms. That takes the diode ability further down to 1.45 kW, so 3 of these in parallel is probably what I need. Too expensive...

Reed switches will be cheaper if I can make them equally share current. Is the 1M resistor set up like below not suited for absorbing the load dump energy ?

T3sl4co1l:
What energy, it's a meg?  It doesn't need to go anywhere, you can just open-circuit it...

You know that 100V 5A MOSFETs are cheaper than reed relays..?

Tim

kellogs:
Right, of course...

Hmm, there are also (shorter) pulses defined for +150V and -220V... And what about their driving BJTs, these will need some sort of protection as well.

T3sl4co1l:
What about BJTs?  Like for general purpose load switching (lamp/solenoid/etc. driver)?

A small TVS or MOV handily absorbs the higher voltage, shorter, and importantly: higher impedance, pulses.  With the inductance trick if you like. :)  A typical MOSFET load dump switch won't respond fast enough for those, passing such pulses easily to a TVS.

Tim

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