Should I double the Schottkys ? - Page 1
Forum section: electronics / beginners

author: Kozmyk
author_rank: Regular Contributor
timestamp: July 25, 2011, 04:49:20 am
content: I'm attempting a repair on a WAECO DCDC10 12>24 converter/charger.My mate brought it around, if I can fix it, it's mine to use.Could be useful for topping up my field-charge batteries (for RC flying) and our GoGo mobility trike.It's a pretty solid piece of kit, it does have some quirks though.The fuses are ATO blade type soldered straight onto the PCB, no holders.I'll put holders in just in case. Easier than soldering aluminium to copper anyway.I've improved the way the big coil is fitted to the PCB.Even though WAECO are German, I think the boards are put together in China.Fault presented - 40A input and 15A output fuses blown, Schottky output rectifier s/c.The big choke coil must have got very hot, it nearly fell off the board, the silicone goop held it on, not the solder.FETs read OK ... well they're not shorted at least.I'm working blind, no circuits available that I can find and the chips have been helpfully tarred over.If the control circuits are damaged then I'd be lost pretty quickly. I've no idea what frequency the converter runs at or anything.Haven't fed it any volts yet but I'm hoping that it's just the diode that died, in fact I'm banking on it.It's a MBRF20100CT 20A 100V.There's a place on the board for another one in parallel.I was wondering if it might be a good idea to use that space and bung another one in there.It could make it just that little bit tougher.Can anyone who knows about the workings of this type of power supply think of a reason not to?Even better if anyone has direct knowledge of WAECO stuff.I was hoping that a quick trip to my local Maplins would yield a replacement diode but the highest working voltage they have is 60V so I guess I'll have to be patient and wait for mail order on the correct replacement.

author: Neilm
author_rank: Super Contributor
timestamp: July 25, 2011, 08:22:14 pm
content: You can't really parallel diodes - subtle differances in the construction will mean one will carry more current than the other. There are some diodes that have 2 devices on one substraight. These will be closely matched so they can be used in parallel.YoursNeil

author: Kozmyk
author_rank: Regular Contributor
timestamp: July 25, 2011, 11:05:44 pm
content: Thanks for that.I did wonder though that WAECO had the place ready for a parallel pair.They have a range of devices in this range and I expect the board gets used in another configuration.Or maybe they've tried two in parallel and thought better of it.On the other hand, given that for this unit, one diode pair will handle the max current then the fact that there may be a difference in the current being carried by two shouldn't really matter to the devices themselves.I had wondered whether putting two in parallel might affect the switching adversely.I've since had the word from a PSU designer that two might well work OK in parallel but to test the repair using just one first.Flying blind as I am, it all rests on whether it works with the heavy devices replaced or not.If the control circuitry is damaged I'll be stuck.Fingers crossed eh?See the vacant spot for another diode there on the right?

author: Kozmyk
author_rank: Regular Contributor
timestamp: July 29, 2011, 03:38:33 am
content: Fixed!Reseated the output coil, I don't think it was ever in right; the leg lengths were curiously uneven.Fitted fuse holders instead of soldering ATO fuses to the board.Tested repair with the one Schottky rectifier pair then fitted a second in parallel just to lighten the load.Soak testing now.I do like it when a repair is nice and simple.