SM3040 Repair/Teardown and a few questions on SMPS's - Page 1
Forum section: electronics / repair

author: grumpydoc
author_rank: Super Contributor
timestamp: March 28, 2015, 05:35:18 pm
content: So, back on the bench trying to fix ebay junkThis time a Digimess SM3040 power supply. Something of a beast of a PSU supplying up to 30V/40A, it's a switcher (well, a 1.2kW linear would be a bit of a handful), without PFC as far as I can see.I forgot to take a photo of the front but here's a shot of the closely related SM5020 I found on the web.It's the well known Mastech chassis again - there must be literally hundreds of different makes and models based on these designs.The fault was "blows mains fuse", actually that wasn't quite true as it took out the breaker when I tried it without touching the fuse at all - not surprisingly I found that two of the main switching IRG4PC50UD's are dead short and I was equally unsurprised to find this soft start thermistor hasliterallyblown its lid.Something a bit odd is going on around the leg of this IGBT as well - not sure whether it has got hot enough to melt the solder or whether this is left over from manufacture.Overall construction is reasonable, primary side components are on one half, then heatsink and magnetics in the middle (where they can be cooled by the fan)However there aren't any air gaps on the PCB to help isolate the high voltage stuff, probably not that much of a problem given that the main board isn't especially crowded and the overall design provides fairly good separation. I haven't checked that adequate creepage has been allowed everywhere though and I'm a little suspicious about the various daughter boards, eg the aux power supply board has the hight voltage DC on part of it, no slots on the PCB and then feeds low voltage DC into the control circuits.The high current paths are all heavily re-enforced so it should stand up to the max load of 40A with no problems - except, curiously, for the positive output (see photo below). Although this is connected by an 8 AWG cable (good for 75A or so) it isn't soldered all that well to the board, nor is this trace endowed with extra copper. The heavy 8AWG cables run to the terminals on the back, then lighter 12AWG cables run to the front binding posts. This really is another weakness of the design - whilst it's clear that you are supposed to connect high current loads to the rear binding posts there nothing limiting current from the front post as far as I can see - and more than enough capacity to melt the banana socketsThe meter current shunt looks the part thoughFinally the schematic - this is for the SM5020 but is basically the same except for a few component substitutions and, in practice, a few of the components on the board differ from the schematic as well (the diagram is a bit large - click on the "thumbnail" below for the full Monty, all the photos above are linked to full resolution versions as well).I confess to having only a tenuous grasp of SMPS functions so I do have a couple of questions for the experts. The design is based on a TL494 PWM - I've seen quite a few similar to this - with an auxiliary supply built around a TOP223. After the basic SMPS there appears to be a linear regulator stage built around a 75NF75 FET. So far so good.One thing I don't understand is the function of the thyristor SCR001, especially as it is paralleled by quite a low value resistor. In fact this is an SG210 inrush suppressor in the unit I have although the schematic has 27ohms for the 5020 and 20 ohms for the 3040 (at least I think it's 20ohms, it says 10ohms x 2 but doesn't say whether series or parallel). This is supposed to have a cold resistance of 7ohms and a resistance at rated current of 0.2ohms.Another thing I can't figure out is the regulation - I assume the SMPS is supposed to track a little above the output voltage then the final regulation is a linear regulator (the 75NF75 and regulator built around U401) but I can't see how the voltage feedback is done - there's a path back to the TL494 via R302 but I don't see how any tracking is achieved.I've not got this up and running yet as I'm waiting for replacement parts - I've opted for IRG4PC40UD's as they are 1/3 the price of the 50 and I can't see that a 55A part is actually called for. In any case the schematic says IRF460LC which is only rated at 21A. Similarly I'm assuming that the thyristor, should it need to be replaced does not need to be a 55A part either - does that sound reasonable?

author: grumpydoc
author_rank: Super Contributor
timestamp: April 07, 2015, 08:10:18 pm
content: Hmmm, that did not go wellHaving checked out the rest of the primary components - bridge, SCR, main caps etc I replaced the 4 IGBTs, unsurprisingly when powered up it all went pop.I wasn't especially surprised because when I replaced them I noticed that the two shorted ones (Q202B and Q203B) had already been replaced before I got the PSU suggesting someone else had had a go at fixing it and given up at that point - there must be another fault.I'm not ready to give up on the PSU yet but am not keen to blow up too many IGBT's as they aren't exactly cheap. I did use the 40A version but even so it has a pulse rating of 160A so it isn't going to die without a fight!Does anyone have any suggestions as to where to go next?

author: SeanB
author_rank: Super Contributor
timestamp: April 07, 2015, 08:42:02 pm
content: Check C203, preferably by either using high voltage along with capacitance checking, or by changing them.If only one half of the bridge is going bang with both transistors shorted and cooked, check the snubber resistors and capacitors, along with the base resistors. Take the transformer out, and check the winding insulation is still fine. Check the wiring connecting the 2 transformers, it could have a broken wire or loose crimp.Testing use a light bulb between the power caps and the switching devices, that way a fault will just make the light glow.The thyristor across the resistor is there to reduce power dissipation BTW, it just shorts out the resistor when the aux power supply is powered up, it is triggered by the aux switcher turning on and running after the caps are charged up and the TOP223 starts up after it does it's power on restart.

author: grumpydoc
author_rank: Super Contributor
timestamp: April 07, 2015, 09:23:46 pm
content: Thanks for the pointers.Quote from: SeanB on April 07, 2015, 08:42:02 pmIn the "3040" variant C203 is a wire link, not a cap.QuoteI presume you mean R207A/B, R209A/B, C206A/B and C207A/B - they seem fine although, now I look again, one of the 27ohm resistors is blackened - the one by (I think) Q203B . Not sure whether it has been overheating itself or whether it was the IGBT adjacent to it. It still measures 27 ohms though. All the caps measure a fraction over 2.2nF (generally about 2.4 but the tolerance on those is going to be quite loose).Quotethey all measure 15 ohms, so they look OK as well.QuoteI think that's a good idea. I hope that the transformer hasn't failed. I'll check the secondary bridge isn't shorted as well.QuoteWhat sort of wattage - 100W OK?I nearly did use the light bulb trick but decided to go for the alternative of feeding the PSU from an underpowered isolating transformer instead (assuming it would just complain badly but survive the few seconds that it took me to turn the power off). Turns out it wasn't underpowered enoughQuoteOK, I was sort of thinking that might be it but, as I said, the plain resistor has been replaced by an NTC inrush suppressor in the PSU I have (in fact a couple of photos I found on the net show the same NTC in this position). Given that the "hot" resistance of the NTC is pretty low it seemed slightly redundant to have a thyristor across it.

author: SeanB
author_rank: Super Contributor
timestamp: April 08, 2015, 05:56:09 pm
content: Thyristor across the NTC is to ensure that it stays cold after the supply is turned on, so that it is high resistance if it is turned off for a second or two then back on. Hot NTC means a blown fuse then.100W bulb will work, and those caps really should be there, otherwise the transformer will saturate with uneven duty cycle from the controller. Place them in in place of those links.

author: grumpydoc
author_rank: Super Contributor
timestamp: April 08, 2015, 08:53:57 pm
content: Quote from: SeanB on April 08, 2015, 05:56:09 pmExplained like that it almost makes sense - except I then start to wonder why bother with an NTC?QuoteI wonder why they are omitted in the 3040 design - in the 50V/20A or 100V/10A variant they are there.But 225J? I assume 225 = 22x105pF which is 2.2uF, but what does the "J" signify?EDIT: Ah, OK it seems to mean 5% tolerance AFAICS

author: tautech
author_rank: Super Contributor
timestamp: April 08, 2015, 08:58:31 pm
content: Yep, a letter after the value is tolerance. and 225 is 2.2uF.

author: ass20
author_rank: Regular Contributor
timestamp: September 27, 2024, 09:55:48 am
content: Hiany body have photo of HY013 board with TL084 ?