Dumb Q: Why few shottky bridges?

I think this thermal sensitivity is exactly why you so seldom see Schottky power bridges. The same 4 devices, that work spread out over a square inch of board, go into thermal runaway if packed into a single package, unless heroic heat sinking is added.

Many Shottky-barrier diode data sheets I've seen carry an ESD warning, so I imagine mains transients would be a real concern if SB diodes were used on the secondary of a mains transformer!

Hello George,

Thanks. The 485 is a nice one, 350MHz AFAIR. I do remember that they got quite hot and this somehow never seemed right.

Not exactly fun but every time I have to repair lab gear I learn something.

Hello George,

Look at the MURS series. Diodes like the MURS120 can be had for well under that cost. Used it many times, no problems whatsoever.

Hello Ban,

I suggest that you take a look at more modern devices such as the ONSemi MURS series. Who'd use a 5819 these days?

Bombs? I haven't counted how many units have been produced to date. Probably six digit over about 10 years. No problems with the Schottkys.

Hello Ban,

Ignore my previous reply. The MURS isn't a Schottky. Can't remember the one I used back in the 1993 design, clients sometimes require handing over or destroying all docs when done.

Anyway, check out the ONSemi MBR16100. That would be a heavy duty example. No thermal runaway there I'd think.

Hello George,

Ignore previous reply, mixed something up. But ONSemi is still a good source. Take a look at their MBR series (for example MBR16100).

"Joerg" a écrit dans le message de news:WvCmg.27105$ snipped-for-privacy@newssvr14.news.prodigy.com...

Onsemi's Schottkys are MBRS, not MURS.

Maybe you didn't hear anything because the company had chosen a new consultant? :-) But you always answer with these acecdotical stories of your carriere, almost like JL. I tell you, I do not believe them, and I doubt you have ever used any Schottkys if you even dont know these simple facts you find in every datasheet. If you look at a modern device like the MBR20100 (Onsemi) you can see, that above 30deg. the losses by reverse current are higher than those by forward current at rated voltage and at 80° without heatsink the derating allows zero forward current.

Ban, Try opening up almost any PC power supply and lookup the parts numbers on the rectifier diodes, all of them. They're sometimes plain diodes, but often on the secondary side they're Schottky. Lesee here, picking two diodes I've salvaged, at random:

BYS28-45 - Schottky 20 amp dual diode

D83-004 Schottky 40 amp dual diode.

.... hmmm... about 600,000,000 PC power supplies made.... I've opened up about a dozen of them, and never seen a faulty diode. Lots of frozen fans, popped capacitors, blown fuses, never a blown schottky diode.

So hundreds of PC power supply designers are wrong, 600 million PC power supplies are defying your laws of physics, and you're right about schottky.s hmmmmm......

What do you argue? Where are these diodes? In 5V and 3,3V supplies. And why do you think they need TO247 cases. And how long does a cycle last there? I didn't say Schottky is bad, you misunderstood me. If a designer knows what he is doing and knows the shortcomings of the parts he can design a perfectly reliable supply. You were asking the admittedly dumb question about *bridge rectifiers*. Have you ever found one in those 6000000 computer PS? And I have answered it comprehensivly for anyone. Do the calcs for 3.85V and 5.55V squarewaves and you will see it's well under the point of no return, even at higher temperatures. I'm happy to have pointed out something new for you, and looking at some datasheets I also had to correct my understanding a bit, I admit. But maybe my discourse could explain why so many of those TEK supplys have failed. Just read again my posts from the beginning and tell me what I told you wrongly, I think it was consistent.

Well, the ON-Semi is only specified to 40V, even worse. BTW did you find out what was causing the limitation of your HP-scope to the positive range. I'm also still waiting for those screenshots you wanted to do with your TEK scope.

I want to explain to you why I replied to Joerg this way. He bashed the absolutely valid and intelligent answer of Sophie.

"A shottky bridge will not run any cooler than a regular bridge when it is rectifying 50 or 60 Hz (line or mains) power.... the schotky application is for high frequency switching power supplies and the like where the faster recovery time of the schotky is required."

So please do not interfere and rather learn to spell "Schottky" in the first place (not that others also should).

Hello Ban,

They still use me. A lot.

You don't have to believe me. I can live with that ;-)

At 30C IR is about 0.1-0.2mA. At 100C it's 1-2mA and I don't run my Schottky designs that high. Oh, but you don't believe I do Schottky designs. Anyhow, where's the problem?

BTW, if you want really low IR look at figure 3 in the MBR16100 data sheet.

Provided you are rectifying a low voltage at a high current, the Schottky diode will be be more efficient than the PN diode, both at high frequencies and also at low frequencies. The advantage of the Schottky will of course be greater at high frequencies. Whether or not the diodes are connected in a bridge does not alter this situation. As you pointed out, Schottky diodes are inferior for use at high voltages.

Chris

Hello Chris,

No bashing here, at least it wasn't meant to be.

Check out the MBR1000 from ONSemi. It's a 100V/1A Schottky. Reverse current under 40uA at 80V and a quite toasty 100C. Not bad, ain't it?

The trick is not to oversize the diode for the task.

More appropriate would be : Sorry, was out of my ignorance. :-)

You should also learn to read the datasheet in a right way. I refer to the MBR1100 here, since there is no MBR1000 on the site.

1. We use the max. spec., you cannot make comparisons with the typical specs.
2. Have a look at Vf. With 0.79V at 1A not really a dissipation saver, but still 27% less than a 1N4002.
3. The reverse current is specd *at 2% duty cycle* and 300us pulse width. Now make that 50% and 10ms and tell me the result pls. Use the 80K/W thermal mounting. BTW the max values are 0.5mA for 25° and 5.0mA for 100°.

Leave your "tricks" in the bag, cannot be more absurd.

Hello Ban,

Yes. But when the max specs have hardly any conditions listed (and are most likely for max reverse voltage) you need to contact the mfg to obtain more detailed data.

Often max ratings are similar to what you find on the pack sheets of medication. You've seen it: ".... severe liver damage or in extreme cases death", but when you are really sick you must take them.

A Schottky operated at its abs max is not a smart design IMHO. I do not do that. Realistically this diode is good for a few hundred mA, above

Sure, 5mA would not be a good result as you might burn a few hundred mW in the reverse phase. However, there is no mention under what conditions (voltage) this applies. Most likely max. These diodes draw far less. For a design in you'd need more data from the mfg. Sometimes they give you that, sometimes they don't (or don't have it). I had to do that for opto couplers, PIN diodes and other stuff and mostly that was no problem.

Don't know what's absurd about that. A diode that is too small will, as you pointed out, not save much in terms of Vf. Got to stay out of the "mushy" range. A diode that is way oversized will cause unnecessary losses because of reverse current.