Specing reverse protection diodes / Diode failure modes - Page 1
Forum section: electronics / projects

author: rickmoll
author_rank: Newbie
timestamp: July 10, 2016, 09:44:08 pm
content: I recently repaired a linear bench PSU, a Tektronix PWS2323 (32V, 3A). I had momentarily reverse connected a 6V lead-acid battery across the output. Fortunately the only damage was a blown 1N5408 (3A, 100V) reverse protection diode. The diode was shorted, but it did it's job and apparently prevented any other damage. There was space for a larger diode, so I substituted a 6A4 diode (6A, 400V).But it got me thinking about specing reverse protection diodes:1) Is it always better to use a reverse protection diode with a larger current rating, since it will handle a larger current before blowing?2) Do general purpose diodes all tend to fail in the same way? That is, do they always tend to short when they fail? I assume at some point they will turn into a fuse, and open circuit before the wire lead itself melts.3) Would a Schottky diode work just as well or better as a reverse protection diode? Does it have a similar failure mode?Rick

author: uncle_bob
author_rank: Unknown
timestamp: July 10, 2016, 11:15:30 pm
content: HiDiodes do not all fail the same way. If they fail open (in this case) you don't notice it happening. If they fail short, there is *some* amount of current that will open them back up again.I would put in the biggest current rating diode that will fit ....Bob

author: TheMG
author_rank: Frequent Contributor
timestamp: July 11, 2016, 05:35:11 am
content: Reverse polarity protection diodes wired in parallel are usually considered sacrificial. Consider that even a relatively small lead-acid battery can still put several tens of amps through the protection diode. You would need a very large diode to cover all possible scenarios.Indeed, most times when the current rating of a diode is grossly exceeded, they will fail short at first, then if the current is sufficient it may eventually go open.Usually you'd want a fuse in circuit such that the fuse will blow before the diode goes open circuit.

author: danadak
author_rank: Super Contributor
timestamp: July 11, 2016, 10:18:12 am
content: This might help -http://www.diotec-usa.com/Diff.htmlRegarding schottky, they turn on at a lower Vf than other silicon in the application, sopotentially can offer better protection. I did see this on wikihttps://en.wikipedia.org/wiki/Schottky_diode"In comparison with the power p–n diodes the Schottky diodes are less rugged. The junction is direct contact with the thermallysensitive metallization, a Schottky diode can therefore dissipate less power than an equivalent-size p-n counterpart with adeep-buried junction before failing (especially during reverse breakdown). The relative advantage of the lower forward voltageof Schottky diodes is diminished at higher forward currents, where the voltage drop is dominated by the series resistance."But I think if you look at some of the newer offerings their parasitic R is much better.Regards, Dana.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 11, 2016, 10:59:18 am
content: Quote from: rickmoll on July 10, 2016, 09:44:08 pmFor power supplies, probably.For signals, the junction leakage and capacitance will be a concern. This limits, for example, the amount of protection you can attach to USB lines.QuoteYes and yes! They may not be guaranteed to, and they may fuse at an unfortunately low current, though.TVS diodes usually carry the rating that they will fail in this manner.Generic parts may be wire-bonded, and those tiny wires will blow up at relatively low currents. Still, the part must survive at least whatever I_FSM is. If you're going to use it at higher levels, it may fail in an undesirable manner.QuoteYes, AFAIK.Beware that a schottky of the same current rating will have a smaller die, which probably means more series resistance. You don't notice it within ratings, because Vf is low in general. Again, study the surge ratings, and estimate the series resistance from what data is given.Also, note that schottky diodes have very high leakage, in some cases, fatally so! They basically come in two versions: lower Vf and high leakage (~100mA max!), and higher Vf and low leakage (~mA max). The difference in Vf is maybe 0.1V, so it's not a big deal. The difference in leakage is ahugedeal at high voltages: the diode can dissipate more due to reverse leakage than to forward current, and cause a runaway failure! Make sure they're selected appropriately.As for how much you really need, that depends on the source, and how it's fused.A lead-acid battery might offer hundreds of amperes short circuit, so you need a total stack of (parallel) diodes with RS significantly less than this (that is, say, 12V / 500A = 24mohm), or added series resistance at the load terminal to reduce the fault current.You also need enough diodes to bear the fusing I^2*t. For an automotive fuse, this might be 10s to 100s, and you'll need about a half dozen D2PAK size diodes to safely handle it (i.e., with a low chance of failure).Tim