[email protected] wrote:
[email protected] wrote:
[email protected] wrote:
Hello Folks, Does anyone know a manufacturer that
provides a wee bit more info in datasheets for
their TVS? The issue: We have SMAJ78CA in a board
because long cables, filters and such can cause
nasty ringing. The board must tolerate several
100msec bursts of 80V amplitude in a row, but only
on rare occasions and then such a burst is a
one-time event. Long story short, these things
won't clamp well before 100V is reached in the
ringing and then some stuff with 100V abs max
ratings can go phhhhut.
Many of these flea-sized TVS have really weak average
power handling capability, usually just 10s mW, so
those 100msec burst times of yours have a good chance
of blowing the SMAJ78CA. You better start with the
repetitive rate power handling of the part before you
waste a bunch of time looking for V/I limit
guarantees.
SMA isn't exactly flea-sized. They can take a watt or
two for a few hundred msec. That's all I am asking them
to do. [...]
No they can't. The Vishay ds says repetitive power
handling of 300W at 0.01% duty which comes to 30mW.
Of course not continuously. That's why I wrote "for a few
hundred msec". As mentioned before, the duty cycle will be
extremely low, and it'll be less than 0.01%. I am not going
to sink 300W pulse into them but maybe 1-2W pulses. --
Regards, Joerg
http://www.analogconsultants.com/
Okay , from Fig 5.
http://www.mouser.com/catalog/specsheets/smaj50.pdf the
transient thermal impedance is less than 10oC/W (~7) for the
100ms-1000ms pulses, so it's a non-issue. But anything
approaching continuous and Rja goes towards 120oC/W in that
flea package.
That is exactly what I meant. It is a non-issue _if_ you know
how much it will conduct at 80V, worst case maximum. And I
don't, hence my inquiry and my post here. -- Regards, Joerg
http://www.analogconsultants.com/
You could combine the precision of a zener with the peak energy
handling of the TVS to get your results- the zener sets the peak
firing voltage, TVS is low voltage not critical except that its
threshold for significant conduction exceeds steady state worst
case maximum DC on line with sufficient margin so that SCR shuts
off by holding current deprivation. Please view in a fixed-width
font such as Courier. . . . . . .
----------+-----------+------------ . |
| . | | . |
| . ___/ ___/ . // \
// \ . --- ZD --- TVS . |
| . | | . |
| . | ----- .
+---------/ \ SCR . | ----- .
| | . [390] | .
| | . | | .
| | . ----------+-----------+-------------- . .
That is a pretty good idea. Well, if I had the space for a big
enough
SCR. But there is another problem: The SCR will not let go until
the TVS
has largely come out of conduction and the decay of the 80V surges
is
going to be very slow. That would cook out the TVS.
So essentially I have to leave everything under 80V untouched or
sink
only a very small current whenever the circuit is below 80V. Just
to
avoid overheating any of the protective devices.
The main issue is the fast onset of the 80V surge which creates
ringing.
It's only this ringing I have to cap. So potentially I cold do
something
with a high-pass but ... no space.
--
Regards, Joerg
http://www.analogconsultants.com/