Dumb Q: Why few shottky bridges?

[John Popelish]

Did you download that datasheet? I was astonished myself about this. Look at
figure11, maybe my interpretation is wrong, but it seems to indicate that
fact. And look how bad the Rth is and with a 125deg. max junction temp.you
come to a usable dissipation of less than a watt for 55° ambient.

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.

 

[ian field]

Did you download that datasheet? I was astonished myself about this. Look
at figure11, maybe my interpretation is wrong, but it seems to indicate
that fact. And look how bad the Rth is and with a 125deg. max junction
temp. you come to a usable dissipation of less than a watt for 55°
ambient.

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!

 

[Joerg]

Hello George,

You can sometimes find pallet-loads of these when a company upgrades
(downgrades) to more modern scopes. Check out the usual electronic
liquidator sites. I tend to go for the 465/475/485 variety. I'd stay
away from the 22xx series or the 455's, they're of much lower quality
than the other ones. Also avoid the 78xx and 79xx series-- they're
excellent scopes, but have fragile and unobtainable IC's in them.

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

Just don't pay more than $40 each for the good-looking ones, or $20 for
the hopeless ones, m and you can come out ahead, if you consider fixing
scopes as "fun" and not drudgery.

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

 

[Joerg]

Hello George,

se Mr. Know-All has designed in such bombs as bridge


Funny, but the specs say 10mA max leakage at 100 C. With 13 volts,
that's only 0.13 watts. Unlikely to cause thermal runaway. Well, then
again it might, if you have the diode clipped onto those little
Radio-Shack clip-leads with the number 28 wires.

When hooked up to actual terminals, or soldered onto reasonable-size PC
board traces, these shouldnt run away.

BTW shottky's are not all that expensive anymore, Fairchild lists the
1N5819 at 9 cents in quantity.

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

 

[Joerg]

Hello Ban,

It's good that you are so blissfully unaware of the shortcomings of
Schottkys, because Mr. Know-All has designed in such bombs as bridge
replacements.
Look at Tmax. operating, K/W j-a, capacity and especially reverse voltage
vs. power dissipation. The 1N5819 cannot even have 13V reverse in 0.5 duty
cycle and will produce a thermal runaway. And a 0.5 duty cycle is what
happens in a bridge rectifier. And this diode is rated for 50V!

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.

 

[Joerg]

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.

 

[Joerg]

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).

 

[Fred Bartoli]

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.

Onsemi's Schottkys are MBRS, not MURS.

 

[Ban]

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.

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.

 

[Ancient_Hacker]

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......

 

[Ban]

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.

 

[Ban]

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.

 

[Ban]

snip irrelevant

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).

 

[Joerg]

Hello Ban,

Maybe you didn't hear anything because the company had chosen a new
consultant?

They still use me. A lot.

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.

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

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.

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.

 

[Chris Jones]

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).

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

 

[Joerg]

Hello Chris,

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

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.

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.

 

[Ban]

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

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

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?

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°.

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

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

 

[Joerg]

Hello Ban,

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.

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.

2. Have a look at Vf. With 0.79V at 1A not really a dissipation saver, but
still 27% less than a 1N4002.

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
300mA I'd look for another one.

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°.

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.

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

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.