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Ultra fast rectifier
2

thread240-386510

Ultra fast rectifier

Ultra fast rectifier

(OP)
Hi all.

Is it possible for something to cause a ultra fast semiconductor rectifier diode to a temporarily act as a short and allow reverse current flow? Temperature, Voltage transient, etc.?

I have some diodes that were claimed to be allowing reverse voltage for an extended period of time (1-5 seconds minimum). Short of sending the diodes out for an optical analysis, I do not see anything wrong. I have little to zero knowledge of the application or environment this happened in.

I understand this is a broad question. I am just looking for a place to start.

Thank you All.

RE: Ultra fast rectifier

The rectifiers maybe bad. Can be checked with a common ohm meter. Also be aware that any semiconductor has leakage currents. The larger the semiconductor, the large the leakage currents. These leakage currents in a high impedance circuit can result in reverse or "let through" voltages.

RE: Ultra fast rectifier

(OP)

Thank you.

Checking it with a MM shows good, both as an ohm meter and using the diode check setting. It is also acting as it should in the circuit. What ever happened, (if something happened) was temporary. So I am asking more on a theoretical level is there something that could cause reverse current flow (full short), cause no damage to the diode, and revert back to acting as it should?

Low impedance circuit, relatively small semiconductor, roughly 5mA leakage Max. Wouldn't cause what was claimed to have seen.

RE: Ultra fast rectifier

Schottky diode have considerable leakage when they get hot. You didn't specify the diode.

RE: Ultra fast rectifier

That part can leak up to 1/2mA in reverse.

It like all others will Zener breakdown and flow backwards if the reverse voltage gets high enough.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Ultra fast rectifier

(OP)

For it to act how it was described to me. It would have to be leaking at least 500mA for an extended period of time, and then act normal again. As I said I do not see a problem with it, so I am asking on a theoretical level, is this something that is possible? If it is operating in the breakdown region, especially for an extended period of time, its my understanding that the diode will be damaged and not recover.

RE: Ultra fast rectifier

If the claim is as written above, the reply should be "How much have you been smoking today?"...

Dan - Owner
http://www.Hi-TecDesigns.com

RE: Ultra fast rectifier

2
I wouldn't say "extended period" bur the Reverse Recovery Current (google) can extend for microseconds in some large diodes. Nanoseconds is more common. And, yes, the diode conducts heavily until the carriers (aka electrons and holes) have withdrawn from the junction and created a depletion layer. This is one of the mechanisms that produce HF interference in many power circuits. See also snubber.

The Germans have a wonderful word for it: "Trägerstaueffekt"

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Ultra fast rectifier

(OP)

MacGyverS2000
Haha I would love to tell them that... but I also would like them to keep buying the product.

Skogsgurra
Thank you

Like I said I see nothing wrong with the diode, nor do I have an understanding on how this could happen. The product, as returned, was completely functional. I just wanted to throw the question out there to some Electrical Engineers in case I was missing something. It is more probable than not that the customer misdiagnosed the issue.



RE: Ultra fast rectifier

You need to find out how the rectifier is turned off and what the forward current was. Was the forward current at its maximum rated value, or close to it? This would result in a maximum of stored charge in the junction. If the diode is not reverse biased sufficiently, then the stored charge may take an inordinate amount of time.

TTFN
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RE: Ultra fast rectifier

(OP)
The idea being that; the depletion region is completely charged, then when turned off, if there is only a partial reverse bias, electrons are still flowing for sometime?

Forward current draws roughly 700mA MAX, typically 350mA where the rectifier is rated for 3A.
The reverse bias is 24 VDC.

RE: Ultra fast rectifier

is it really a diode or a TVS? whats the part number?
if the voltage is high enough the TVS will conduct for as long as the voltage is high.
5 seconds is NOT reverse recovery

RE: Ultra fast rectifier

It's known to check Diode with an ohm meter but I'm curious if anyone has used a Megger for testing leakage? The diode would not be in it's forward biased state at the time but it would be subjected to a higher voltage than just an ohm meter so would a Megger test give a better indication of the Reverse leakage potential?

Chuck

RE: Ultra fast rectifier

see: https://portalvhds963slh4m3fqg2.blob.core.windows....

The current is still no more than 1mA, which is typically where most diode reverse breakdown specs occur, so there is no reason or need for a higher voltage. Moreover, a 30kV potential, particularly applied as a pulse is tantamount to an ESD zap, especially with the ability to dump milliamp level currents. Such conditions can damage some diodes.

TTFN
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RE: Ultra fast rectifier

Ok sort of like getting hit by a bolt of lightning, Thanks for you input IRstuff.

Chuck

RE: Ultra fast rectifier

That is a very impressive Megger, it has many functions my Fluke does not,including dielectric absorption/discharge test. Understandably they are very proud of it. The lowest price I could find was $2500.00.... Yeow! I'm too small time for all those features I guess.

Chuck

RE: Ultra fast rectifier

The max Vrrm for the diodes specified is 200V and may be as low as 50 volts. So even a 500V megger would put these diodes into breakdown. However the diodes can be checked for damage using an ohm meter that has an open circuit voltage of greater than 1 volt. The then units in question conduct in one direct and not the other, than more than likely they have not suffered any damage. This is a step up from visual inspection.

RE: Ultra fast rectifier

"more than likely they have not suffered any damage"

That depends on the diode. Some devices are damaged immediately, but the consequences are not revealed for some number of power cycles. TTL emitter diodes are like that; they can get damaged, but it takes some time before they completely go bad, as each iteration of junction current further drives that aluminum spike into the junction, until it's eventually shorted out for good, or bad...

TTFN
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RE: Ultra fast rectifier

Possibly a strong Radio Frequency transmitter was operating near the circuit, this could cause conduction in the diode, especially if the circuitry happened to resonate at the frequency.

RE: Ultra fast rectifier

ES3D is a 200 Vrev 3 Amp 20nS fast recovery diode.
there is no such thing in this diode that with 24V you can get 5 seconds of reverse current.
what happens after 5 seconds? The data you are giving us is faulty or the diode is broken and should be replaced with another of the same part number.
I have used these diodes in several switching power supplies. if you have 5 seconds of reverse current in a rectifier circuit the transformer will burn up because you are making a short across the winding.

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