Redundancy diodes getting really hot, can I backfeed a meanwell psu? - Page 1
Forum section: electronics / projects

author: ITman496
author_rank: Regular Contributor
timestamp: January 16, 2021, 11:04:06 pm
content: Hello!I have a redundant power supply circuit that I cloned from an Astron dc power supply to feed my 12v diesel heaters.They draw combined a total of 26 amps on startup but drop to around 8 when running.This is the circuit:It allows either the power supply to feed the load, plus charge the battery slowly (psu output set to SLA float voltage)or if psu fails due to power outage, it seamlessly clicks over to battery.The issue I'm having is that one of the diodes in my "100A" bridge rectifier with integrated heatsink is getting up to like 350f on startup at 26A!My theory is as follows:100A chinese part really means 50A.But thats 50A when the load is spread over two diodes (for rectifying). I'm using 1 diode, in DC mode, which isn't even A which is even worse since they only draw 30A at the peak of the waveform.So that means my "100A" 50A bridge rectifier is actually only able to handle something like 25A AC current per diode, and maybe only 15-20A DC current.Am I understanding?Anyway, my main quick patch solution is.. Can I just hook the power supply straight to the output and only have the diode on the battery, and in fact parallel the two diodes? Does a meanwell LRS-350-12 mind having battery voltage on it if the AC input is disconnected?

author: engineer_in_shorts
author_rank: Regular Contributor
timestamp: January 16, 2021, 11:08:32 pm
content: You need to dissipating approximately 15watts of heat. What heat sink are you using? Any pics?

author: ITman496
author_rank: Regular Contributor
timestamp: January 16, 2021, 11:21:52 pm
content: this is what I bought, I just have it bolted into the cabinet.It only draws the startup surge for about 3 minutes, then settles down to 8Ahttps://www.amazon.com/gp/product/B07LBQ4YL1Also to make it clear, the diode inside itself is getting hot, not the connections, I'll go get my thermal camera and download the picture later.

author: TimNJ
author_rank: Super Contributor
timestamp: January 16, 2021, 11:27:13 pm
content: Have you considered an ideal diode controller with MOSFETs? Maybe you’re trying to avoid the slight increase in complexity?https://www.analog.com/en/technical-articles/low-iq-ideal-diode-controller-with-reverse-input-protection.htmlYou can probably keep the het down to a minimum.

author: ITman496
author_rank: Regular Contributor
timestamp: January 16, 2021, 11:30:12 pm
content: Yeah! I was looking at those, but I am indeed just simply trying to keep costs and complexity down. Just simple stuff.. Just want it to work and keep working forever. I want to be able to trust it, unlike my other inventions,Right now I'm trying to see if I can either just have backfed power on the meanwell, or maybe some bigger diodes..

author: trobbins
author_rank: Frequent Contributor
timestamp: January 17, 2021, 12:12:06 am
content: ITman496 you appear to have forgotten about heatsinking and how important that is to the current rating of pretty much any semiconductor device. The side of a cabinet is likely a very poor heatsink. Perhaps start by looking through some heatsinking101 tutorials and then do some simple calculations on what heatsink C/W thermal rating you will need to use - if you are still unsure then post details of your setup (part numbers). A minute is an eternity for junction temperature rise.

author: ITman496
author_rank: Regular Contributor
timestamp: January 17, 2021, 12:14:05 am
content: @trobbins Well, I do think about heatsinking quite a lot, but I was hoping I'd rather not have to think about it, given that this module IS a heatsink.. It has no flat plate on it to couple to the cabinet at all, its just its own thing?A few posts up I posted an amazon link to the bridge I'm using. It does not have a clear shot in the product pictures, but the top of it is all fins, its not a flat surface. It looks like its designed to be on its own.

author: TimNJ
author_rank: Super Contributor
timestamp: January 17, 2021, 12:41:34 am
content: I wonder if the rating of the Amazon link is based on forced air fan cooling. But being an Amazon diode bridge, specification seems to be limited.

author: ITman496
author_rank: Regular Contributor
timestamp: January 17, 2021, 01:11:47 am
content: Agreed, which brings me back to my original question... Does anyone have any experience with voltage staying present (due to a battery) on the output of a SMPS like this?Does it care?I am thinking of redoing the wiring so that it uses two diodes of the bridge between the battery and output, and the PSU direct on output, so that it can still charge slowly, but if the psu shuts off, it will still have voltage being fed back into it via battery thru the diodes.This layout:

author: bdunham7
author_rank: Super Contributor
timestamp: January 17, 2021, 03:09:09 am
content: Is your PSU current-limited? If so, why not do it the other way--use the diodes for the power supply and let the battery always be connected to the load. If you are using float voltage, there's no need to limit the charge current to less than the PSU output.I don't know the answer for you specific PSU, but of those type generally, there's good chance they'll blow up if backfed power.

author: ITman496
author_rank: Regular Contributor
timestamp: January 17, 2021, 04:28:59 am
content: Its current limit is to 'hiccup' the output until the overcurrent condition is fixed, I believe.I am thinking of taking it back out and hooking my bench supply, limited to a few mA, to it and backfeeding to see what happens..

author: trobbins
author_rank: Frequent Contributor
timestamp: January 17, 2021, 05:28:09 am
content: That diode bridge has 5C/W Rj-c, and 8C/W Rc-a. That Rj-c is likely to be worse if just one diode in the bridge is continuously conducting. Vf is up to 1.1V at 17A, and would get higher at 25 to 32A, even if Vf fell with junction temp (the datasheet is very poor on any such info). If you were passing 32A continuously (which is your achilles heel), even for just a few secs, that implies a dissipation of circa 30-50W, and Tj would rise at least 150C, so you have cooked your goose.Diode current ratings catch so many people out - it must be the click-bait of the Ifav rating combined with no effort in pre-design.

author: trobbins
author_rank: Frequent Contributor
timestamp: January 17, 2021, 05:40:20 am
content: With respect to your system, the first or second schematic seems fine for function. Either schematic does not have current limit (apart from fuse) for the battery, so some types of load fault could cook its steering diode (to see how that can happen you need to understand the fuse spec). Given the 2nd schematic removes the need for one steering diode then that seems like a benefit.If your PS has a voltage setting at the battery float voltage or a little higher, and you insert a steering diode in with the 5 ohm, then you should have no possibility for back-feed in to the PS unless the PS fails internally.

author: bdunham7
author_rank: Super Contributor
timestamp: January 17, 2021, 06:11:23 am
content: I think thesimplest solution is to just use appropriate rectifiers. And this would probably be the most reliable as well. These are about 10X as good as what you have.https://www.mouser.com/ProductDetail/Vishay-Semiconductors/VS-50PF40W/?qs=u%252Bh7cTrMUz9H6cWMtf2V6w%3D%3DYou'll need a modest, insulated heat sink, but both rectifiers can share it.

author: ITman496
author_rank: Regular Contributor
timestamp: January 17, 2021, 08:42:06 am
content: Yikes. I have never worked with diodes like this before, I guess it is not as simple as I had hoped.Thank you so much for the guidance..I may do something like this, perhaps?The idea being that the voltage output has to be live from the 12v psu in order for the 12v relay to click on, which then allows the 120v relay coil to get power from AC. That then clicks on and connects the PSU to load.If the power goes out, the AC relay opens, cutting off the PSU immediately. I figure this is okay, as I've used power supplies to feed big chunky capacitor'd things before and they didn't seem to care that it held the voltage up for a little while anyway..After the caps in the psu drain, the 12v opens as well. So its a delayed until power output good connect when firing up, and immediate disconnect upon power loss. I suppose the only mode where this doesn't work okay is where the PSU itself fails but power is still on. But if it fails... well, it can eat as much 12v as it wants, its broken and I'm not going to bother trying to repair such a cheap supply. Blow the fuse for all I care.I am, however, going to hook the power supply to my current limited power supply tomorrow and backfeed it with the AC on and off to see what happens. If I can just sit there floating output voltage all day on it and nothing happens I'll probably just hook it up diode-less for the psu side of things, like my second schematic, and parallel the main diodes on battery, knowing that it might not be optimal but it will probably live, and move on with my life?Anyway, thank you guys very much for your feedback, I've achieved my main goal with these projects, learn something new, in this case, learn a WHOLE BUNCH new stuff about something I thought was cut and dry and dead simple, finding out that it really, really, is not simple.

author: ITman496
author_rank: Regular Contributor
timestamp: January 18, 2021, 04:12:12 pm
content: For the sake of closure:The diode bridge failed shorted and tested if my power supply can be back fed for me. It can!So now I have a circuit very much like my second schematic. Power supply direct to output, batteries behind a steering diode + charging resistor. Seems to work great. Will upgrade the battery steering diode ASAP but this will at least survive an emergency shutdown.