TVS breakdown voltages for higher currents

Littlefuse.

-- Many thanks,

Don Lancaster voice phone: (928)428-4073 Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552 rss:

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Unfortunately they don't :-(

Epcos has V/I characteristics of their TVS products over decades of current:

They seem to only have the usual. For example, for the SR6K35M474X it's

Joerg a écrit :

I doubt you'll find anything like that. Plus their tempco will give you even more hard time.

Your mention of 20mA seems to indicate that your working impedance isn't too low, which would help... I had that sort of problem once and I resorted to 2 inductor-TVS stages (had to withstand 8/20 surge levels) followed by a resistor/zener clamp. My impedances where high (supplying CMOS logic)

In you case maybe you can do that, or a TVS clamp followed by a zener driven voltage limiter...

I find this tool useful for converting data sheet graphs into numbers...

The I sometimes run that data thru a curve fitter such as...

To get an equation I can use in behavioral modeling ;-) ...Jim Thompson

I was able to get that for a zener once. But it was a smaller manufacturer and they are often more flexible. They actually gave me their QC acceptance criteria sheets and they tested at many more current values than the one listed in the datasheet.

Actually it is unknown and can be very low. What I mean is that if the TVS is guaranteed to run at no more than, say, 20mA at 80V we'd be fine. Even 50mA would be ok. This is because those spikes will be short and seldom.

Probably that is the case for a TVS with 70V standoff voltage or possibly even for lower ones. But I need hard data for this design.

I sure would have liked to provide an electronic clamp using a big old FET but the problem is we did not have the required board space.

I was able to get that for a zener once. But it was a smaller manufacturer and they are often more flexible. They actually gave me their QC acceptance criteria sheets and they tested at many more current values than the one listed in the datasheet.

Actually it is unknown and can be very low. What I mean is that if the TVS is guaranteed to run at no more than, say, 20mA at 80V we'd be fine. Even 50mA would be ok. This is because those spikes will be short and seldom.

Probably that is the case for a TVS with 70V standoff voltage or possibly even for lower ones. But I need hard data for this design.

I sure would have liked to provide an electronic clamp using a big old FET but the problem is we did not have the required board space.

Yabbut ... I need guaranteed values here. Not just graphs or typical values.

The real world has no guaranteed values. Real engineers can work with a range. TINSTAAFL ;-) ...Jim Thompson

current:

He who works on mission-critical electronics shall be cautious about that. You can't step in front of an NTSB inspector and say "But the graph showed that this should normally not have happened!".

TINSTAAFL ;-)

Not "is", it's "ain't", man :-)

current:

graph.

Some of us are couth, some are not ;-) ...Jim Thompson

Many of these flea-sized TVS have really weak average power handling capabi= lity, usually just 10s mW, so those 100msec burst times of yours have a goo= d 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.

Clamping is the brute-force way to protect your gear. You're volunteering to absorb all the energy your enemy can throw at you. A more zen-like strategy is to step aside, namely limit the input current before you try to clamp it.

"When you can take this transient from my hand, Grasshopper, it will be time for you to leave."

Cheers

Phil Hobbs

Indeed! Clear back in the mid-60's I was turning off alternator regulators so that the (bipolar) devices went into BVcex mode and could ride out the transients without damaging dissipation. ...Jim Thompson

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.

Zen wasn't possible, I had about the square-footage of a fly to clamp at two places, no chance to have opening devices because they'd have to be quite major.

Anyhow, there will never be much energy above 80V and all I want is something that is guaranteed not to conduct for than a few tens of milliamps at 80V. The normal voltage range is less than 32V and the 80V peaks will be very short and a rare occurrence. I am fairly sure a 70V TVS can do that but a bench test isn't good enough here.

No they can't. The Vishay ds says repetitive power handling of 300W at 0.01% duty which comes to 30mW.