Re: Diode vs TVS

George Herold wrote:
On Sep 15, 8:27 pm, "David L. Jones" <altz...@xxxxxxxxx> wrote:
Phil Allison wrote:
"David L. Jones"

Vladimir Vassilevsky wrote:

I tested the basic reverse polarity protection circuit:
12V battery, 100A fuse, diode.

If the TVS (1.5SMC24AT3) is used, it gets destroyed into the short
circuit. The fuse is blown, of course; but the device is damaged.
This happens at the very first attempt.

However if the trivial 1N5404 is used, it blows the fuse and
survives the test. Tried it several times.

Not surprising if you look at the specs.
Same basic 200A surge rating (1/2 sine 8.3ms) for both parts, but
the TVS is in a 165degC/W package and the 1N5404 is in a 15degC/W
package.

** False & irrelevant data and false comparison.

It's not irrelevant. It shows that the dissipation ability of the
devices
are not equal. That would be of some relevance when it comes to
overloading
the thing, and hints that it's not apples vs apples here.

Both packs have poor deg C/W figures UNTIL mounted on a PCB.

Then the diode becomes 53 deg C/W - not 15.

http://www.onsemi.com/pub_link/Collateral/1N5400-D.PDF

I got my 15deg J-A figure from Diodes
Inc:http://www.diodes.com/datasheets/ds28007.pdf
They do not specify the conditions, but you'd probably assume it's a
mounted
figure.

Vishay show 20degC/W for a board mounted 0.375" lead length, less
than half
the figure on-semi quote. So it looks
like this varies a far bit between
brands.http://www.vishay.com/doc?88516

In either case it's still a LOT lower (20-50deg) than the really
wimpy 165deg J-A figure of the TVS. Doesn't that hint at a little
something?

However: 8.3mS is NOT enough time for the mounting method to
come into play

Correct. It likely comes down to how beefy the internal silicon and
bonding
is etc, and the J-A
figure would be a fair bet to be representative of that, at least in
the
absense of any other firm data. If you have a better insight into
this then
please share it with us Phil.

so the 200 amp surge figure rules.

And both devices have the same 200A surge figure, yet one dies and
the other
doesn't.
IMO it's fairly obvious that the diode is likely to be beefier
internally
than the TVS, so would be more likely to survive an (unspecified in
this
case) out of spec overload better.

Real problem is, the OP's test is NOT defined -

ie what is the actual fusing time and what is the peak current ?

May well be more than 200amps.

Obviously it's above the 200A/8.3ms figure (or equivalent),
otherwise it
wouldn't go splat.

Dave.

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" > > However: 8.3mS is NOT enough time for the mounting method to
> > come into play
>
> Correct. It likely comes down to how beefy the internal silicon...
<snip> "


Yeah, every hunk of stuff, has a thermal time contant. How long does
it take for heat made inside to get out. I believe a friend of mine
told me that the time constant goes as the square of the linear
dimension. (double all the dimensions and the TC goes up by four.)
But don't take my word for it you can easily work it out for
yourself. (Time constant = heat capacity X thermal resistance). 8ms
strikes me as a small piece of stuff.. but I'm not sure how big.

Would be rather interesting to decapsulate the two devices and see the
physical difference.
Also the difference between different brand 1N5404's
Likely to be a few factors at play here though...

Dave.

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