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Schottky diode always shorted 1

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JuandelaCruz11

JuandelaCruz11

Automotive
Feb 12, 2019
5
thread240-96155

Hi to All,

This is my first time on these forums, i just like to ask if what maybe the cause of Schottky diode being shorted, We are using this schottky diode for protection circuit on our Burn In Boards, This is where the IC's are being loaded prior testing to BI (Burn In). We are having failures on the diode being shorted even without the loaded IC, we already tried using higher voltage diodes but still has the same response. Its always shorted after an hour of bias and temperature. BTW the diode we are using are compatible on the voltage and temperature we are running. Its 220V at 125C.

 
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It would help if you show part of the schematics. Plus what component (type number) you are using.

In general, schottky diodes have a limited reverse voltage and a rather high reverse leakage current. To add insult to that, the reverse current increases with temperature.

The power developed under reverse condition is therefore not insignificant and that makes temperature increase. It can lead to thermal instability at normal temperatures and if you do actual burn-in at 125 C and with a high reverse voltage it is almost a given thing that they will fail. And fail shorted.

The reverse current temperature dependency can make the use of a schottky diode in the "normal" way (like clamping the base of a BJT to increase switch-off speed) problematic when temperature gets high. Like under-the-hood in sunny weather.

Schematics and component number. Plus actual reverse voltage can help us helping you.

Gunnar Englund
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Half full - Half empty? I don't mind. It's what in it that counts.
 
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See attached actual schematics of protection circuit of the burn in board. This is the part number of the diode TST20H300CW. The voltage for these supply is 220V and the current is 0.26A. The temperature is 125C and run for 24hrs.
 
 https://files.engineering.com/getfile.aspx?folder=dad8b5ca-8950-449d-bd74-ccda1dfcd354&file=schematic_of_protection_circuit.JPG
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Please note that the part is specified for short duration reverse pulse tests (30 milliseconds) and not for 86 400 000 milliseconds.
At 220 V and initial 200 micro-amps reverse current, the dissipation will be around 44 milliwatts. Not much, agreed, but enough to heat the junction somewhat more and then the reverse current increases rapidly causing even higher temperature and a final melt-down.
I see no reason why you need a shottky there. I think that you should get rid of it altogether. the zener and the capacitor is protection enough.

Gunnar Englund
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
 
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Why do you even need a Schottky diode; aren't they primarily useful for fast recovery in switching from ON to OFF, since they store minimal space charge? You can replace it with a conventional power diode to protect against reverse voltage conditions.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! faq731-376 forum1529 Entire Forum list
 
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We cannot change the design of the Board as this is customer owned Burn In Board, previously the part number of diode used was MBR20H150CTG which has 150V Peak Repetitive Reverse Voltage and in series to sustain the 220V, They upgraded it to TST20H300CW with Repetitive peak reverse voltage of 300V but still encountered shorted.
 
 https://files.engineering.com/getfile.aspx?folder=4cbf2feb-b0fa-4e80-a16f-486803dcb05b&file=schematic_of_old_protection_circuit.JPG
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Ok, but they obviously made a change when the previous part failed. The fact that their supposed upgrade is still failing should be pointed out to them.

You are grossly exceeding the operating limits of the datasheet. Assuming 125C junction temperature, the leakage current is specified to be 10 mA. At 220V, that means the device is dissipating 2.2W. Using the datasheet values of 48C/W junction to ambient, the junction to ambient rise is 105.6C, and with an ambient of 125C, the junction is actually at 230.6C, which means it's fried, PERIOD. If you want to get pedantic, the reverse current at 230C junction temperature is probably around 1A and the chip dissipated over 220W for a few nanoseconds.

Forget about the thermal runaway, you're just cooking the junction and allowing the metal side of the Schottky diode to directly diffuse into and alloy the junction into a chunk metal and a dead short. Did no one at the customer bother to do the math? If anything, the previous diodes probably had a better chance at surviving than the new diode.

A conventional silicon power diode would have about 10 uA leakage at the same 125C junction temperature, resulting in mW of power dissipation. AND something like has a maximum junction temperature of 175C, giving you even more margin.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! faq731-376 forum1529 Entire Forum list
 
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image_muengs.png


Text shall, of course, be "Starts here.."

It's soo obvious that you can't do this with any success.

Gunnar Englund
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
 
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Semiconductors REALLY, REALLY, hate being above 150C. While the Si eutectic temperature is quite high, the contact area for any junction is likely rife with crystal defects, which allow the aluminum to migrate into the defects and spike through to the low resistivity Si underneath the junction. The high reverse field simply aggravates such behavior.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! faq731-376 forum1529 Entire Forum list
 
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Really appreciate the help guys, I learned a lot on this experience. Thank you very much..
as of now we already highlighted it to the customer and they are looking into it.
 
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