Opinion about fissured diode packages (w/ pictures) 5

[xj25]

Hi,

We have had some faults (last year like 3 in total 60 equipment) due to a diode failing in open circuit. It is on top of a DC control circuit just after the MCB, and has the function of avoiding reverse supply to upstream circuits.

The diode is 3A RMS and after checking we have seen some of them fissured, but not failed yet (see pictures). The diode is sized for the current but we can not discard some incorrect maneuver in maintenance or transient that could be causing an overcurrent.

As an example, we have a similar diode in other older equipment but sized to 10A (with exactly same control circuit downstream) that has not given problems in 8 years.

Before changing the remaining to the 10A one, I would like to check some of your opinions about the possible causes of package fissuring. My previous idea was that the most common fault mode for a diode was shortcircuit but it seems not occuring here.

Regards!

 

[itsmoked]

Classic fast thermal events. Rupture and almost always open-circuited. You are probably seeing excessive current events - as you suspect. They must be whoppers too since most of these diodes have large pulse ratings.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[IRstuff]

Your statements are scary. 3A RMS is only one possible rating to be considered. Peak current as alluded to would be another. So, you say "diode is sized" but did you size for RMS or for peak? The fact that the 10A ones haven't failed speaks volumes. I would hazard a guess that someone was looking to save money and incorrectly assumed that the 10A diode was over-specified for a 3A RMS current, and neglected to check for peak current.

TTFN
faq731-376

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Of course I can. I can do anything. I can do absolutely anything. I'm an expert!

 

[Comcokid]

Silicon diodes fail in short circuit. To fail them in open circuit there needs to be a pulse big enough to blow them apart.

The diode appears to be a Semikron SK 1/16 which is a 1600V with Ifrms of 3 Amps (absolute maximum continuous) but a Ifav (more of a normal operating current) of 1.45 Amps (85 C with 10 mm lead length on each side of the body).

Just like Police Chief Martin Brody said in Jaws "I think we're gonna need a bigger boat", I think you need a bigger diode.

 

[xj25]

I traced back the defined component and it says one of these:
SK3/16
P600S
DA6 1600

The first I checked and it is for 1,8A avg, that should be enough for the regular load.

Anyway the fact only a small number of them have failed and, as presently known, always in open circuit (we will check because maybe some could be shorted and not detected) brings me to upgrade to a bigger model.

Thanks for the discussion!