Estimating the maximum current for very low pulse widths for diodes - Page 1
Forum section: electronics / projects

author: daqq
author_rank: Super Contributor
timestamp: April 26, 2017, 06:54:43 am
content: Hi guys,For an application I need to have a diode pass around 1.5 kAmps during a time of 2us, with a low maximum repetition rate (500Hz) - a duty cycle of 1 to 1000.For certain other reasons it would be great if the diode had a specific package as well as a higher voltage drop on it and be a higher voltage type. The best match I found was this:http://ixapps.ixys.com/Datasheet/DSEP2x25-12C.pdfThe max forward surge current (250A) is listed for a 10ms pulse, which does not necessarily mean that the numbers apply to a 2us pulse.How much current could one pass through a diode like this for the specified conditions?Thanks,David

author: tggzzz
author_rank: Super Contributor
timestamp: April 26, 2017, 07:32:51 am
content: Quote from: daqq on April 26, 2017, 06:54:43 amYes, those numbers do apply to a 2us pulse.That's serious energy. Good luck with the parasitic inductance/capacitance of the surrounding circuit, and how it affects peak voltage/current..QuoteYou will have to test that. Make sure you have good personal protection, so that you don't get injured by shrapnel from disintegrating components.Before delivering any product built with components you are using out-of-spec, consult your insurance company to see if you are indemnified, and your lawyer to understand the consequences of not being indemnified.

author: capt bullshot
author_rank: Super Contributor
timestamp: April 26, 2017, 08:42:46 am
content: There's some hint in the datasheet: Fig.7, Transient thermal resistance junction to case.Considering the diagram starting at 1ms, do the calculations for the junction temperature rise in a safe range (e.g. from 50° to 150°), which results in some kind of single pulse energy that can be absorbed. Do some pessimistic extrapolation on the VF over IF diagram, calculate your expected pulse energy - if it is within the range, give it a try.The outcome heavily depends on the actual diode construction, these high speed diodes are in general way less robust than some line frequency rectifiers. Diodes like this one are used for example in PFC stages, there you'd be required to have additional measures to protect them from the charging capacitor inrush currents. Line frequency rectifiers are not suitable for your pulses, they may not even turn on in 2µs.Have you tried the free wheeling diodes in the IGBT modules? These usually match the performance of the IGBT and should catch that pulse - well that differs between module manufacturers, often the diode dice are smaller than the IGBT dice inside the modules.Edit: Inside an IGBT module, see the difference between transistors and diodes - can you spot the small spot of non-fully escaped magic smoke?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 26, 2017, 12:52:57 pm
content: Once had a snubber diode application. Very brief transient, 100ns range, but with some 200A+ behind it. Duty cycle very low, <1%. Tried a 6A diode: failed. Didn't get hot. Ran for at least a few cycles. But not forever.Hmm.Tried a 12A diode. Ran for a few minutes, then same story. Hmm.Tried a 30A diode. Ran just fine.What's going on? Best guess, electromigration. It's probably not a thermal failure mode: even with the transient applied voltage being ~50V (yes, a real +50V across the diode junction, in the forward direction -- forward recovery is very much a real thing), the peak power isn't extraordinary, and at those currents, internal resistance ensures even current sharing -- this is well outside the range of thermal hot spots.What's in an I_fsm rating?Several things: thermal limits, bond wire ampacity, die ratings, electromigration -- whatever the worst of each of these contributes.For a typical 8.3 or 10ms "half sine" pulse, the limit is mostly thermal, and the thermal time constant lies between the die itself (~100s us) and the heatsink pad (~10s ms). It does not involve much from the package, leads or external heatsinking (~seconds).Where the limit is thermal, one expects the peak current to scale roughly as sqrt(t). Or, more precisely, as the applied waveform, combined with the V(I) curve, combined with the transient thermal impedance curve.For a short duration pulse (usually around 1-10us square), the limit is largely device physics, and the maximum peak current levels off.For extremely short pulses, capacitance dominates, I suppose; you might not get anything from the junction, anyway (in the 10ns range, a power PN junction diode will barely begin to conduct, even at fairly high applied voltages). Lead inductance limits dI/dt in this range, anyway.For 1.5kA, I would expect needing, well, about a half dozen beefy diodes in parallel -- or a module or puck type diode.Otherwise, you might try to reduce the peak current. (Ask me how I solved my snubber problem...)Tim

author: Someone
author_rank: Super Contributor
timestamp: April 27, 2017, 12:10:22 am
content: Quote from: tggzzz on April 26, 2017, 07:32:51 amThe peak current is highly related to the waveform being applied, its entirely possible to put kA through a diode such as that repetitively without a loss of lifespan.Quote from: T3sl4co1l on April 26, 2017, 12:52:57 pmSingle diodes are entirely possible in this space, but they need to be purchased through specialty suppliers (who test and remark other diodes for you) or qualified in house.Quote from: daqq on April 26, 2017, 06:54:43 amYou will need to work with your manufacturer to get a qualification from them (expensive), or test yourself and then negotiate a supply contract where they wont change the fab or design on you.