Electronic – Are LEDs better than we think

Conventional wisdom about LEDs says their maximum reverse voltage \$V_{R(max)}\$ is quite limited, usually in the 5V-8V range.

So, for experimentation purposes, I wanted to bring an LED into controlled breakdown, using my current-limited power supply.

Of course I expected the actual breakdown voltage to be somewhat higher than the reported guaranteed \$V_{R(max)}\$, but I could never have expected the outcome I found. I tried with different kinds of "el cheapo" no-brand Chinese indicator LEDs (3mm and 5mm, red, green, blue, yellow and white) and I couldn't bring them in the breakdown region, even at 32V (where my power supply reached its max)!

Therefore I wanted to double check my assumptions and I systematically browsed
many datasheets (about 40) of current devices (standard 3mm and 5mm LEDs, for both indicator and lighting applications) from different manufacturers (e.g. Vishay, Nichia, Kingbright, Fairchild, Cree). Almost all of them reported a \$V_{R(max)}=5V\$, with some Vishay devices rated at 6V.

I was extremely puzzled. OK, manufacturers tend to be conservative, but a >25V margin seemed a bit too high. After all, guaranteeing a \$V_{R(max)}=25V\$ (or something like that) could make LEDs good candidates for some useful applications or allow circuit simplifications (e.g., no need to protect LEDs from low-voltage reverse spikes). Anyway, that would be another bullet in the list the marketing people could boast about!

Of course my test was limited to a dozen LEDs of unknown manufacturer, but I suppose they can't be better than those from reputable sources. Or did I experience a sort of reversed Murphy's law, where I found the only box of LEDs on the planet with such a feature?!?

Question(s): Is my finding something that is known in the industry? Why do they keep specifying LEDs with so low a \$V_{R(max)}\$ when the actual devices seems to be much better? Do I miss something?

EDIT

(to clarify some points that possibly prompted comments/answers that didn't actually give me the explanations I'd like to get)

Things I already know

• Stresses beyond absolute max ratings reported in the datasheet might damage the device, and usually will damage it if the stresses are well beyond those limits.

• When you exceed those max ratings you cannot demand anything from the manufacturer. You are on your own in unknown territory. You can neither sue him nor complain.

• No sane designer would use a part in his design outside the specs given in the datasheet. Good designers will make sure the part will stay well below the stated max ratings. As I stated at the beginning, I was experimenting, purposely entering unknown land to verify my expectations and my knowledge about reverse breakdown.

My assumptions (possibly wrong; and if they are wrong I'd like to know why)

• The main limiting factor for any diode max reverse voltage rating is its breakdown voltage. In other words, you can safely reverse bias a diode as strongly as you want until breakdown (either Zener or avalanche) kicks in.

• Breakdown is not destructive in itself. The sudden increase in reverse current causes an enormous increase in dissipated power, especially at high reverse voltages, therefore the PN junction will be destroyed, unless you limit the current somehow.

• LEDs breakdown mechanism is not different from that of other PN junction diodes, like regular silicon rectifiers or Zeners.

• Since LEDs are not designed (as opposed to Zeners) to work in breakdown, the BD voltage is not a well specified parameter, so the manufacturing spread could be quite big. Therefore the manufacturers choose a suitable safety margin and declare that as the max reverse voltage.

• Although some safety margin is needed, it can't be huge. IIRC, BD voltage depends on doping levels and the geometry of the metallurgical junction and those parameters also influence the diode characteristics when forward biased. If the "useful specs" of the LED have to be reasonably consistent, so the doping and the geometry must be; hence also BD voltage values can't be too wildly spread out.

What puzzled me and made me think there are more issues beyond protecting an LED from entering breakdown

• So big a difference between rated max reverse voltage and actual BD voltage (at least +400%) should mean something and should have a rationale behind it. Given the assumptions above, I can't believe the same model of LED can have a BD voltage spread that big, i.e. I can't believe the same process (even across different batches) can yield one part that enters breakdown at, say, 10V and another one that enters it at 30V (I stand to be corrected).