LM2678, 12v, 4amp load, exploded tantalum! - Page 1
Forum section: electronics / beginners

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: November 15, 2015, 08:05:26 am
content: So I've been building up a circuit on a home etched PCB, basically because the PSU here didn't go to plan:https://www.eevblog.com/forum/projects/pico-atx-psu-schematic-what-ic-is-used/msg727110/#msg727110(the -12volt is negative of the input voltage, and it all became too hard)anyway, new PCB, new design, plan C....I've assembled the -12volt section, all good, done the +12volt, and it failed with any load, schematic looks like this:Google told me I needed the 100nf nearer the IC, so I tacked on a second, hence the two C12's in the diagram.I've been testing it by connecting the load, then turning my bench PSU on.Tested it into a 2amp dummy load, and it seemed happy enough, when I tried 4amp, my bench PSU would hit its 3amp current limit, and drop to 11volts... and something on the LM2678 board buzzed.... I assumed the inductor...SO I ran my bench PSUs two channels in parallel, plugged in the power supply under test, and a tantalum promptly exploded, specifically, C11.. I guess it wasn't the inductor buzzingIs this circuit just extremely inefficient for some reason, that the 19volt, 3amp input, can't supply 12volt, 4amp output?(scratch that, any less than 85% efficiency would overload the PSU)But why did the tant explode? should I try again with the tant removed? could it just have been a bad part?Thanks!

author: Simon
author_rank: Unknown
timestamp: November 15, 2015, 08:21:30 am
content: maybe because of the lower voltage and higher input currents the tantalum was overworked and it's ESR x current peaks made too much heat. just a long shot.

author: Wirehead
author_rank: Regular Contributor
timestamp: November 15, 2015, 08:22:48 am
content: What's the voltage rating of the tantalum caps?

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: November 15, 2015, 08:26:36 am
content: Quote from: Simon on November 15, 2015, 08:21:30 amIt didn't explode till I gave it more current, and it instantly exploded... and it must have only been a 4amp peak draw for a fraction of a second. But its quite possible...Quote from: Wirehead on November 15, 2015, 08:22:48 amMeant to be 25volt... but I may have bought them from China

author: Ian.M
author_rank: Super Contributor
timestamp: November 15, 2015, 08:33:18 am
content: You are pushing your luck. Its normal to de-rate tants on power rails by at least 50%, and that's with good quality ones from manufacturers that care about their reputation.

author: Simon
author_rank: Unknown
timestamp: November 15, 2015, 08:58:21 am
content: just use a few good old aluminium electrolytics

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: November 15, 2015, 08:58:35 am
content: I powered it u with no load again, and another capacitor on the input blew its lid, hit the roof (the capacitor, and me) and burnt holes in the paper on the desk...I'm going to redesign this board with electrolyticsI probably only need 12volts at 2amps, however, the design was done as 5amps using the TI design tool...I do need 5volts at 5amps though, so might need to use some more real-estate on the board for those capacitors.Should I try removing the input capacitors and just sticking some electrolytic(s) on the input?

author: Ian.M
author_rank: Super Contributor
timestamp: November 15, 2015, 09:03:13 am
content: I'd replace the input ones with ceramics and bodge aluminium electrolytics accross them to maintain the bulk capacitance value.

author: DrGeoff
author_rank: Frequent Contributor
timestamp: November 15, 2015, 09:17:26 am
content: Tantalums love to explode, or catch fire...They tend to go short circuit, particularly if a reverse voltage appears across them, even for a short while. It could have been that the act of switching off the input supply caused the collapsing magnetic field in the inductor to present a reverse voltage spike across the tantalum.

author: wraper
author_rank: Unknown
timestamp: November 15, 2015, 09:22:09 am
content: Capacitors on the picture are 100% counterfeit epcos. Moreover Kemet bought epcos tantallum business 10 years ago.

author: Zero999
author_rank: Super Contributor
timestamp: November 15, 2015, 10:19:14 am
content: Quote from: wraper on November 15, 2015, 09:22:09 amI agree. Before going to the trouble of redesigning the board, try replacing them with tantalums from a reputable supplier and see how it goes.

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: November 15, 2015, 10:30:02 am
content: I stuck a 470uF, 50volt electrolytic on the board.Tested at 2amps just fine... with a 4amp dummy load (and 3amp input current limit, at 19volts), the LM2678 didn't power up, and something on the board buzzed... I'm too scared to try it with more input current in case something bigger explodes!

author: wraper
author_rank: Unknown
timestamp: November 15, 2015, 12:20:35 pm
content: Quote from: AmmoJammo on November 15, 2015, 10:30:02 amIf you bought it from china, very likely LM2678 is counterfeit too. At least on Cheap Chinese DC/Dc converter boards such ICs usually are fake.

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: November 15, 2015, 08:03:21 pm
content: The current PCB isn't using the turn on pin of the regulators, as I planned to switch the DC input to the board, but I don't think this is a great idea, and another reason to redesign the PCB...Thanks for the input guys!

author: Simon
author_rank: Unknown
timestamp: November 15, 2015, 08:58:46 pm
content: If you are worried about spikes and transients getting back out of the input use a TVS diode, it will prevent over volt spikes and will short out any rever polarity stuff.Yea chinese sduppliers are a bugger, L298's are £3 each, and I'm supposed to beleive I can buy genuine ST ones from china for 5 for a couple of quid, give me a break. I experimented and bought 2 for a quid and I'm sure that ST branding on it is dodgy and a poor copy.

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: November 27, 2015, 09:15:17 pm
content: Alrighty, I've redesigned the board to use evectrolytics, and acquired some Panasonic parts from work, cos, free!What should I be looking for in a TVS diode? cos, I have no idea where to start with that :pThanks!

author: Simon
author_rank: Unknown
timestamp: November 27, 2015, 09:17:51 pm
content: make sure your reverse stand off voltage is somewhat over the supply voltage. The other extreme is the max clamp voltage. You have the voltage the TVS is rated for and it's power, usually more powerful TVS diodes have a lower max clamp voltage.

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: December 05, 2015, 07:30:05 am
content: I've come back to this now, might etch the PCB later this evening when the weather cools down a bit.But the question now is, are my Chinese ebay special inductors up to the task?I smashed one open, because that's just how I roll, and they have two 0.4mm wires in parallel.I wired one across my bench power supply, and measured 0.13-0.14volt drop at 6amps, suggesting a DC resistance of 0.022-0.023ohm.The Texas Instruments suggests I want an inductor with 0.019ohm dc resistance for 5amp output, so I think I'll simply apply the "she'll be right" principal

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: December 06, 2015, 12:47:53 am
content: Hopefully now with less explosion and fire(5volt section isn't fully populated yet, as apparently futurlec can't read, nor issue refunds)I have to redo the heatsink as well, because the mounting holes aren't right...

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: December 07, 2015, 10:46:27 am
content: This is going inside an Amiga 600.I actually ran it off the bodged together power supply at the bottom of the pic for a few hours, the regulators and/or inductors got hot enough that you could smell them from outside the case, and there's a module that does +-12volts, with a resistor on the positive rail, as I was only using the negative rail... There's capacitors on the bottom, fake Rubycon most likelyThe final design above still isn't the final, one regulator isn't soldered on, as its the wrong part, and I plan to run bigger inductors all round, so I'll finish this board, make sure it works, then build another (I etched two already)

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: December 08, 2015, 10:00:49 am
content: How can I measure the switching frequency of these regulators?I stuck my scope on the switch pin and twiddled the settings till it read about 250kHz (meant to be 260) but with the same settings I can't get a reading anywhere near this for the other regulator... probably fakes.

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: December 08, 2015, 10:21:32 am
content: Ok, it seem that the 5volt regulators, with more than 16.5volts input, the output stops regulating, and just increases as the input voltage increases. (up to 7volts with 19volt input)This is with no load, but could this be an issue with my circuit (there's not much to it!) or are these just fake regulators (which I assume anyway, genuine ones are on their way)Schematic is this, with four 220uf capacitors on the input, for the two positive, and one negative regulator.On the scope, with the input voltage set to 19volts, the switch pin appears to no longer be switching...

author: dadler
author_rank: Unknown
timestamp: December 08, 2015, 05:49:05 pm
content: See input voltage vs load graph...

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: December 13, 2015, 01:08:53 am
content: I can't see anything listed in the datasheet about minimum load, and the TI simulator suggests this configuration should work with zero load as well.The simulator also gives the same 15uH inductor for upto 40 volts input, with the 5amp maximum output.

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: December 13, 2015, 03:34:41 am
content: I tried a 1k resistor across the 5volt rail, now it regulates.I increased this up to 3.9k, and it still regulated.I assume this isn't an issue due to no load on the output, but something else?

author: Tomorokoshi
author_rank: Super Contributor
timestamp: December 13, 2015, 06:00:09 am
content: Quote from: AmmoJammo on December 07, 2015, 10:46:27 amBoing!Now things are getting interesting!

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: December 13, 2015, 08:29:31 am
content: Quote from: Tomorokoshi on December 13, 2015, 06:00:09 amACA620, and an internal slot load CD drive.It also now has a heatsink on the accelerator, and an A604 memory expansion, with an Indivision ECS for VGA output

author: free_electron
author_rank: Super Contributor
timestamp: December 15, 2015, 02:56:41 pm
content: why would anyone deliberately use suicidal components ...

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: December 17, 2015, 10:45:30 am
content: I guess seeing no one seems to have an answer to the non regulating 5volt regulator, I'll just chuck a load on each of the outputs and see what happens

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: December 19, 2015, 12:15:31 am
content: With 16.5 volts into the 5 volt regulator, with no load, it stops regulating (11.5volts over the output)The 12volt regulator does the same, more than 11.5volts above the output voltage and it stops regulating...

author: langwadt
author_rank: Super Contributor
timestamp: December 19, 2015, 01:09:18 am
content: Quote from: AmmoJammo on December 19, 2015, 12:15:31 ammaybe is simply that the switch is too slow to make the duty cycle short enough for very low loads

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: December 19, 2015, 01:17:12 am
content: A 1.5ma load on the 5volt rail, and it still regulated upto 20 volts input?

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: December 19, 2015, 03:19:18 am
content: Quote from: AmmoJammo on December 19, 2015, 01:17:12 amA 22k resistor on the 5volt rail, and once again, it still regulated with 20volts input...

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: December 24, 2015, 02:48:19 am
content: ACA620, with heatsink, overclocked to 25Mhz.Internal slot load CD Rom drive.A604 and Indivision ECS.Internal DC-DC PSU (will happily do 5volts at 5amps, 12volts at 2amps)4gb compact flash card.Still has the original floppy drive too!