Can diodes die due to capacitive transient loads? (eg plug in wall power supply) - Page 1
Forum section: electronics / projects

author: incf
author_rank: Frequent Contributor
timestamp: April 16, 2025, 02:44:52 pm
content: Suppose a product has a "beefy" >100W 24V DC wall power supply with a barrel jack connector. (shared between different boards in a power bus/chassis)And both the power supply and the product has a significant quantity (>1000uF) of low ESR decoupling capacitance.And the product uses something like a single S34 schottky diode  without current limiting resistor to supply it's power (capacitor inrush flows through diode)In this situation, could the diode be damaged by the inrush currents generated by plugging and unplugging while the power supply is live?How do you know "how close to the edge" you are?https://www.mouser.com/datasheet/2/389/stps340-1851525.pdf

author: moffy
author_rank: Super Contributor
timestamp: April 16, 2025, 03:08:04 pm
content: Diodes are very rugged devices able to sustain large surges, in this case 75A/105A for 10ms, for a plug pack that would be hard to sustain. I would be more concerned about inductive voltage spikes if the supply is disconnected under load.

author: incf
author_rank: Frequent Contributor
timestamp: April 16, 2025, 04:05:56 pm
content: I feel like the inrush current could be substantially higher amplitude but shorter duration than the rated ~100A (8.3ms sine wave). But I don't know for sure.

author: TimFox
author_rank: Super Contributor
timestamp: April 16, 2025, 04:13:02 pm
content: Power diodes usually have two different maximum current ratings:IF (avg)is the average forward current, which raises the temperature of the device.IFSMis the peak forward current, specified under various specific conditions (e.g., 8.3 ms for 60 Hz rectification).The latter can be 30 to 50 times the former.Destruction due to exceeding the maximum reverse voltage is probably faster: this is usually specified only for repetitive waveforms (as in normal rectification).

author: bdunham7
author_rank: Super Contributor
timestamp: April 16, 2025, 06:13:07 pm
content: Quote from: incf on April 16, 2025, 04:05:56 pmYou'd have to test that and if you end up in uncharted waters beyond the datasheet spec, you'd want to characterize your diode to see what short pulse surge currents it actually takes to blow it up.My guess is that your surge currents won't be that high and also that the diode can take µs current surges well beyond the 8.3ms rating. The inductance of your barrel jack lead wires alone will probably smear out your pulse enough to keep currents inline.https://www.vishay.com/docs/84120/84120.pdf

author: Siwastaja
author_rank: Super Contributor
timestamp: April 16, 2025, 06:23:51 pm
content: Yes, diodes do die due to inrush current into DC bus filter capacitors. And yes, diode datasheets generally suck and convey too little information to properly analyze the situation. I believe most designs are realized "by luck": choosing sensible combination of diode rating for the output current, and amount of filter capacitance again for the output current, tends to almost always work out, giving that warm fuzzy feeling that diodes are indestructible.

author: incf
author_rank: Frequent Contributor
timestamp: April 16, 2025, 06:53:01 pm
content: and there is no sensible way to extrapolate from datasheet values? (without elaborate testing or access to unpublished data)I'd guess the bond wires are one failure mode, and localized heating is another.I don't know how to guess the capacity of bond wires of unknown size.But maybe it is possible to speculate/extrapolate about thermal effects during very short transients? Filling a 1000uF capacitor to 24V through 30mOhm and 70nH of contact resistance results in about 700 amps peak that tails off for a few dozen microseconds.edit: I didn't see the link to the Vishay paper. That looks interesting and like it might address part of this.

author: bdunham7
author_rank: Super Contributor
timestamp: April 16, 2025, 07:32:05 pm
content: Quote from: incf on April 16, 2025, 06:53:01 pmIDK how much you can extrapolate unless the datasheet specifies a fusing energy and even then I'd think it was iffy. But I'd prefer some simple testing over hypothesizing and extrapolating. And then applying the bane of the thrifty engineers existence--a safety factor.Are your other numbers there anywhere near realistic? The total resistance is going to be the sums of the ESRs of the capacitors plus the wire plus "everything else". The capacitors have some self-inductance and at those speeds the effective ESR will likely be quite a bit higher. A 1mm diameter wire 50cm long has over 600nH and 10mR resistance (and there are two wires...). But if you want a universal solution, add a small resistor.

author: bdunham7
author_rank: Super Contributor
timestamp: April 16, 2025, 07:42:14 pm
content: Quote from: Siwastaja on April 16, 2025, 06:23:51 pmI prefer the art of "impedance protection".