Dumb Q: Why few shottky bridges?

Very few applications. In general they are incorporated into IC's where they would be useful.

Also, a low voltage 200mA device can serve most small signal requirements. More powerfull apps would use discretes. The concept of prepackaged bridges is nothing more than encapsulating 4 diodes in a little box, but most power schottkys are not used in std. bridges. Far more likely to find them used in pairs and YES, there are a lot of schottky pairs available! So perhaps 2 of them?

i.e. Diotec has a 1A series CS10D to CS 50D. They are made, but the market is small. They are for low voltages only, because Schottky is only up to 100V and then there are some Si-carbite for higher voltages too. The reason is that you do not need them for mains transformer rectifiers, standard recovery is sufficient there. And in SMPS you have almost always a dual winding(fewer turns needed) and use only two diodes (higher efficiency). Also they are too expensive, compared with the 1N400X series. They also have a very high reverse current, 1mA at room temp and 10mA at

100deg. (1N5817). This can lead to thermal runaway and self destruction.

No they have only common cathode ones, you cannot make a bridge out of those.

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ciao Ban
Apricale, Italy

------------- George -

AT least they are repairable with available parts. While I can understand replacing capacitors, especially electrolytic, in equipment that is 30 years old -- when it happens to new computer equipment - people get upset !

Many good motherboards (and computers) dumped because of this issue. I have rescued a few -- but there are tens of thousands of them out there !

gb

Your question is answered already, but I thought I'd add: if the bridge recitifiers are not dissipating heat as you believe they should; why don't you add a small heatsink to the package?

IIRC, there is heat-transfer epoxy available. Just glue a the sink to the top of the package. It couldn't hurt.

jak

Hello George,

It might make sense to do a "cobbling up" to dissipate more of the heat into the air instead of the board. Obviously they haven't done their homework in the due diligence after the design. A thermal test should have been completed before releasing that scope design.

It's easy to buy powerful single Schottky diodes. If you want to keep it neat I'd look at regular silicon bridge rectifiers and let them ride a bit higher than normal, assuming this is just the 60Hz rectification and there are no stability issues. Take some that are larger than the previous ones for more "air contact".

Question: What's the exact type of the scopes and where did you find them? Could be a nice bargain if these have chronic problems and die prematurely.

Once I bought a nice shortwave transceiver for a song. The manufacturer had botched the design of the driver stage and nobody knew how to fix it. I guess it'll be the same for classic photography freaks since some of the otherwise really nice Minolta cameras have poor electrolytics in them that should now be failing by the bucket load. That causes the shutter to quit for good. Easy fix, but the repair shops will likely charge so much that people rather give the things away. Some might not even know how to fix these.

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Regards, Joerg

http://www.analogconsultants.com

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.

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Hello Sofie,

Huh? Schottkys drop 200mV less than silicon rectifiers. That's about

25-30% less dissipation. Nothing to sneeze at I'd say.

--
Regards, Joerg

http://www.analogconsultants.com

They also leak in the reverse mode, and cant stand much voltage, and might be expensive.

Hello Sjouke,

Leakage isn't so bad anymore these days. I have never seen it crimp the efficiency much in my designs. No high voltage, of course. But yes, they do cost a lot which is why I only use Schottky if I really have to.

--
Regards, Joerg

http://www.analogconsultants.com

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!

--
ciao Ban
Apricale, Italy

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.

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.

Even so it's an iffy thing.

If the scopes have been well-treated and kept in a lab, and were not left on 24/7, they can often be resuscitated with new capacitors, new diodes, and a general cleaning.

But many have been bouncing in a car trunk for 20 years, or run 24/7 in a salt-water factory, or slid off from on top a coke machine, or shipped loose on a pallet, or worse.

You can lessen the overall risk by buying a bunch of them, then using the hopeless ones for knobs, CRT's and other misc unobtainable parts to fix the better ones.

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.

Get the Philips datasheet. With 0.13W and 100K/W (standard mounting) this will result in +13K, more than doubling the the leakage, etc. Read a datasheet first pls.

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ciao Ban
Apricale, Italy

This begs the question why anyone would use them according to your logic. Yet they do so. I think you must have made some wrong assumption somewhere ! Think about thermal equilibrium.

Graham

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.

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ciao Ban
Apricale, Italy

I did, apparently you didnt look too closely.

on the Fairchild datasheet, the spec is 1ma at room temp, 10mA at 100C.

At a more typical 80K/W, and with the leakage going down with voltage, and the voltage being half a sine wave, applied only half the time, that only raises the temp a very very few degrees, much less than the

13 you suggest.

Still worth doing the runaway calculation though!. The "Onsemi" datasheet for this diode goes into this with charts graphs, equations and tables.

Look a little closer. The 10mA is at 100C, at room temp it's only 1mA.

And it's about half that for the newer diodes, which are only 38 cents for a 100V diode, so runaway isnt an issue.

And it will be even less in a typical rectifier circuit, where the peak voltage only occurs for part of a cycle, every other half cycle.

Still one should glance at the runaway diagrams, "onsemi" has particularly nice ones.

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So don't buy the inferior Philips parts?

12-volt dc supply using a bridge circuit with capacitive filter

such that IDC = 0.4 A (IF(AV) = 0.5 A), I(FM)/I(AV) = 10, Input

Voltage = 10 V(rms), RJA = 80°C/W.

Step 1. Find VR(equiv). Read F = 0.65 from Table 1,? VR(equiv) = (1.41)(10)(0.65) = 9.2 V.

Step 2. Find TR from Figure 2. Read TR = 109°C@ VR = 9.2 V and RJA = 80°C/W.

Step 3. Find PF(AV) from Figure 4. **Read PF(AV) = 0.5 W@I(FM)I(AV)= 10 and IF(AV) = 0.5 A.

Step 4. Find TA(max) from equation (3). TA(max) = 109 - (80) (0.5) = 69°C.

**Values given are for the 1N5818. Power is slightly lower for the1N5817 because of its lower forward voltage, and higher for the 1N5819 I copied this from the ON semi datasheet. You can see the equivalent reverse voltage is *only 9.2V* and thermal runaway occurs at 69 degrees. And this is a 10Vrms only secondary. Do it for 15V and see what comes out.

In reality they didn't consider transformer losses and mains overvoltage, which would give a lot less margin. That is probably why so many of those TEK power supplys failed.

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ciao Ban
Apricale, Italy