Testing 1N4007 with a megger

[taydin]

I have connected a 1N4007 to the megger in reverse polarity and did a ramp test to 2.5 kV. Attached is the voltage/current graph (Current is microamps).

As can be seen, at about 1500V, the first degradation of the junction occurs. Right around 1800V, another punch through occurs. Soon after that, at 1850V, the megger declares breakdown because the current hits 3 mA, the megger's limit.

After this test, I have connected the same diode again to the megger and started the ramp again. The megger declared breakdown right away, at 100V which is the ramp start voltage.

No surprises so far. But here is what surprised me: I have measured the diode after the breakdown using two multimeters, in the "diode check" range, and the diode is measured to be in working condition with BOTH multimeters. I have also measured the diode in the ohms range, and it had a forward resistance of 38 KΩ (a good 1N4007 has a 40 KΩ forward resistance) and a reverse resistance beyond the multimeter limit.

So, the diode has basically failed in state where it MEASURES to be working, but fails when under voltage. And indeed, I applied 30V to it in reverse polarity, and it drew about 1 mA.

This means, when we measure high voltage diodes, just a multimeter measurement won't be conclusive. The diode needs to be tested with rated voltage applied to it 

[taydin]

I did repeat the test with two more 1N4007's, and the graph was quite similar. One breakdown at around 1500V, and another breakdown around 1800V. And same measurement result with the multimeter.

[Gyro]

You're enjoying playing with your new megger aren't you! 

[taydin]

You're enjoying playing with your new megger aren't you! 

Yes, was quite educational But it needs to go back at the end of this week. If you guys can think of another test, let  me know! 

[thm_w]

Do you happen to have any 1N4005/1N4006 or anything lower rated?
My guess is 1N4001/2/3/4 and 5/6/7 are all the same, since the specs are equivalent.

[MagicSmoker]

I have connected a 1N4007 to the megger in reverse polarity and did a ramp test to 2.5 kV. Attached is the voltage/current graph (Current is microamps).

As can be seen, at about 1500V, the first degradation of the junction occurs. Right around 1800V, another punch through occurs. Soon after that, at 1850V, the megger declares breakdown because the current hits 3 mA, the megger's limit.
...

You can test avalanche breakdown safely *if* you limit current to maybe 100uA, or you limit the time so that the total avalanche energy is less than what the datasheet says is okay.

[taydin]

Those are good suggestions, guys! I don't have 1N4001 but I can get some from a local shop here.

For the avalanche, I can place series resistor during the test to limit the current. The megger's current limit is fixed at 3 mA.

[sourcecharge]

Wow, that is very educational.

I have about 500 of those 1n4007 diodes, and I have never tested a diode with rated voltage to check if it was bad.

What if you put 2 or 3 or 10 in series or even in parallel?

Do you think you would find thesame dimm test problem?

[taydin]

What if you put 2 or 3 or 10 in series or even in parallel?

Do you think you would find thesame dimm test problem?

Putting the diodes in parallel would just divide the fixed 3 mA current. But the diodes would still degrade. If you look at the voltage current graph, even after 1200V, there is a substantial increase in current. So even at that voltage, the diode probably won't be the same after the voltage is removed.

The diodes in series, you would add the breakdown voltage of each diode. The voltage drop across each diode won't be the same because they will have different insulation resistances. The one to reach break down first will just be in standby because current is limited by the other diode. Then that other diode will reach break down and then both will get damaged.

[taydin]

And another thing to keep in mind: This type of failure would only occur in the real world where the voltage source is rather low energy. Maybe getting continuously hammered by ESD, or maybe the diode is clamping a small inductor (small relay coil?) and gets continuously hammered with back EMF ...

But many times in the real world, the voltage source is high energy, so the diode will probably fail short circuit, and if the current continues, it will melt away and become open.

[mzzj]

Failed components like these are always real pleasure to try to find. Oldschool CRT monitors often had components that would measure ok with DMM but leak too much in real circuit.

[T3sl4co1l]

Hm, wonder if the avalanche withstanding capability varies with manufacturer.

I would expect a good diode to just simmer in avalanche at say 1200V or thereabouts.  Rather nasty, the results seen here!

Tim

[mzzj]

Hm, wonder if the avalanche withstanding capability varies with manufacturer.

I would expect a good diode to just simmer in avalanche at say 1200V or thereabouts.  Rather nasty, the results seen here!

Tim

Or you could interpret the results that leakage current increases  at 1350v and diode enters avalanche knee at around 1.8kV

It is indeed "simmering" with the 3mA current limit on the megger and 1.8kV applied = 5.4W !

If OP still has time to play with the megger it would be interesting to know what happens if you test couple of diodes with the current limited to 0.5mA or so. (less than 1 Watt power at avalanche)

[MagicSmoker]

...
It is indeed "simmering" with the 3mA current limit on the megger and 1.8kV applied = 5.4W !
...

Bingo! That's why you need to really limit the current (or the time spent) in avalanche for a runty little 1A diode which has a nominal Pd rating of maybe 1W... (and that's if you keep the legs extra long and don't place it next to anything else that gets hot).

[taydin]

Ok, attached are the results with 10MOhm in series of the diode, ramping to 2.5 kV, reverse polarized. The Y axis is again microamps.

I have also added the pdf output that has each individual voltage measurement.

[taydin]

After the above  test, I did another ramp, this time 5 kV. But the test stopped at 4.8 kV, because either the resistor broke down or the diode. Not sure which, will find out

[taydin]

I measured the resistor, it is still 10 MOhms. And just to make sure the diode is OK, I ramped it up to like 800V and it didn't breakdown. So I suppse the resistor arced internally, but the megger shut off the voltage before the diode would get killed. The resistor is 1/4W metalfilm.

[taydin]

So can we conclude that limiting the current prevents the junction from sustaining permanent damage under avalanche?

[taydin]

Nope, we can't ...

I picked 3 new 1N4007 from the bin and measured their insulation resistance at 1 kV. They are 65 GOhm, 87 GOhm, and 102 GOhm.

Then I measured the diode that just underwent the ramp test. It has an insulation resistance of 260 MOhms ... So it has permanently degraded.

[tooki]

I did repeat the test with two more 1N4007's, and the graph was quite similar. One breakdown at around 1500V, and another breakdown around 1800V. And same measurement result with the multimeter.

So... they all failed well beyond their maximum ratings. And this is a surprise why?

Look also at the derating graph in the attached datasheet. As temperature rises, reverse current rises dramatically.

Do you happen to have any 1N4005/1N4006 or anything lower rated?
My guess is 1N4001/2/3/4 and 5/6/7 are all the same, since the specs are equivalent.

Well, they can be the same, or not, depending on manufacturing method. E.g. the Vishay ones in the attached datasheet could very well just be binned dies of the same design, since they list identical specs other than breakdown voltage, while another brand's might not be. See also:
https://www.eevblog.com/forum/chat/why-1n400x/
https://electronics.stackexchange.com/questions/84471/what-is-the-difference-between-1n4001-and-1n4007-other-than-their-maximum-revers

[iMo]

It is a common practice manufacturers produce, for example, 1n4007 chips only, marking them 1n4001-1n4007. The same happens with transistors, memories, mcus, etc., etc.
They also buy chips from other vendors and package them under their brand, when in need.
The most expensive item in semiconductor business (except r&d) is the production of the sets of masks for photo-litography, the production of a silicon die costs almost nothing. You could save say 90% costs making the 1n4007 only.

[taydin]

So... they all failed well beyond their maximum ratings. And this is a surprise why?

The surprise is that the diode MEASURED to be working fine with a multimeter after it broke down around 1850V. I think I made that clear in my first post.

[jaromir]

Well, they can be the same, or not, depending on manufacturing method. E.g. the Vishay ones in the attached datasheet could very well just be binned dies of the same design, since they list identical specs other than breakdown voltage, while another brand's might not be.

This is indeed true. Let me share a bit of insider info.

More than decade ago, I worked for major semiconductor manufacturer other than Vishay. For simple rectifying diodes, they manufactured probably three dozens of diode types from one type of silicon. Silicon wafer was doped and gold was sputtered from both sides, making huge "mother diode" (with reverse voltage higher than 1kV) with 30cm diameter PN junction. This basic material was cut into smaller pieces, with variety of perhaps 5 sizes and shapes. For 10A diodes the pieces were larger and hexagonal in shape, 3A diodes were smaller sqares, 1A diodes were the same, with smaller lead cross-section - the limiting factor wasn't current density in silicon or leads, but heat dissipation. When buying larger diode, you are paying for copper too, not only silicon.
All diodes were manufactured as the "highest grade" of the family. For example, all 1N400x were manufactured as 1N4007, the "mother diode" was doped to have reverse breakdown voltage well above 1kV and vast majority of the diodes from manufacturing line had indeed breakdown above 1kV. There were no "mother diodes" for lower breakdown voltage. Lower voltage bins were filled at final diode testing by diodes which failed 1kV test. If those bins weren't full enough (the manufacturing yield was quite good), it was filled from 1N4007 bin. As an example, 1N4006 datasheet states breakdown voltage higher than 800V and 1N4007 (Vbr>1000V) fulfills this criterion. Diodes with higher than 1kV Vbr rating were made as stacks of 1kV chips in one package.

Vast majority of customers were aware of this and ordered 1N4007 exclusively. Reason for having lower grades diodes comes from past, when lower manufacturing yield made binning more reasonable. Nowadays it isn't as attractive and all grades are sold for the same price.

[floobydust]

There was a time 1N4005-1N4007 and 1N5405-1N5408 used to be spec'd with higher Vf, lower junction capacitance, longer reverse recovery time, higher thermal resistance - it depended on the manufacturer and their datasheet. The doping or construction was different.

It is in antique databooks the difference but now pretty much the same. It might be that sorting/binning the parts costs more than simply making the 1,000V part, or semi processes have improved.
Almost twice junction capacitance 1N4005-1N4007 Diodes Inc. aww its obsolete now.
Vishay S1A note 800V, 1,000V part differences

So they aren't all the same but who really designs that tight nowadays.
It was discussed here https://electronics.stackexchange.com/questions/84471/what-is-the-difference-between-1n4001-and-1n4007-other-than-their-maximum-revers

[tooki]

So... they all failed well beyond their maximum ratings. And this is a surprise why?

The surprise is that the diode MEASURED to be working fine with a multimeter after it broke down around 1850V. I think I made that clear in my first post.

I dunno, IMHO if a part has been so grossly abused, it’s suspect, period. And my years of working as a computer tech taught me that parts often semi-fail, leaving them in marginal states where they sorta work.