KDMPower MIPC MI-X8775CD [PCB WF-C rev:E]

[PeteS in CA]

Re: KDMPower MIPC MI-X8775CD [PCB WF-C rev:E]

BJTs from the 1300x family are one of those parts just like LM358, LM317, 555 timer, and etc. - i.e. parts that aren't really the best in what they do, but very simple, cheap, and universal, so too good to go away / get replaced by something new.

When Motorola introduced their Switchmode series BJTs - 2N654x in TO-3 (actually, TO-204) and MJE1300x in TO-220 - they were the first BJTs optimized for use in switchmode power supplies. So they were some of the best at what they did ...... in the late 1970s. Similarly, LM358, et al, were top-notch parts, when National Semi first introduced them, and now they're still too good at what they do to obsolete them. I don't know whether On Semi still makes the MJE1300x series anymore.

I suspect you are correct about their persistence in very low-end PSUs. They are part of a suite of inexpensive, well known parts - TL494-clone PWMs, 13007/13009 BJTs, very mature ferrites (inexpensive and available from lots of sources), Micrometals -26 powdered iron cores, last-for-a-few-years junky sort-of-low-impedance output capacitors.

And that's all I've found to test bad so far - 6 parts total (or 8, if you count the two 13009 BJTs.) Hopefully the SDC2921 didn't take a hit, because the base of Q12 (and Q11) are directly connected to it. With Q12 blown, though, that is a possibility. Of course, it could be that Q12 blew first, causing the primary BJTs to run only "one-sided" - i.e. either only Q15 or Q16 switching, causing the PWM to up the pulse until Q15/Q16 blew. But more likely, since both 13009 BJTs were blown... I think they blew first, which then sent a huge pulse of voltage on the secondary of the driver transformer, taking out Q12 and the rest of the parts on BJT drive circuit on the primary. Whatever, I guess we will see when I replace all the bad parts.

My guess would be that one one of the 13009s shorted, took out the other, and then the resulting big current pulse blew back through the proportional drive transformer, T2, into the Q11/Q12 circuit.

BTW, your SDC2921 is a PWM-supervisor combo IC. The PWM circuit looks similar to the old SG3524 type voltage-mode PWM. The supervisor provides PG and the output voltages window detector can shut down the PWM in case of a UV or OV fault. https://pdf1.alldatasheet.com/datash...C/SDC2921.html

[momaka]

KDMPower MIPC MI-X8775CD [PCB WF-C rev:E] - the autopsy results

OK, since I do think I am going to end up fixing this PSU... first thing that needs taking care of is the blown components on the primary.

Before removing anything, though, I decided to draw a rough schematic of the BJT drive circuit. Well, I did that... and perhaps I got a little carried away.


I just wish I didn't use a scrap piece of paper, as I though this would only be a rough schematic that I could later throw away. Now I don't feel like throwing away this garbage paper (at least until I re-draw / copy it.)

Basically, the above partial schematic of the PSU contains the BJT drive circuit on the primary, the BJT driver circuit on the secondary, and even the OPP circuit traced out (since it was connected to the BJT drive circuit on the secondary - might as well do that too, right. ) With that said, please don't assume it to be 100% accurate. After scanning it and just as I was ready to upload it here, I already found a mistake (which I corrected in Photoshop.) If something doesn't add up, please feel free to point it out.

Anyways...
So what does the autopsy reveal? Well, I wrote that on the schematic paper above, but here it is in text format too:
- R50 and R52, which were the 1-Ohm resistors, are completely OPEN.
- R51 measures 1.4x KOhms... but from what I can see, the first band on one side of its color code is Red. So no idea what this is, but the resistance likely doesn't match. Thus, this resistor is bad for sure.
- R53 measured 0.875 KOhms. However, upon cleaning some of the soot on it with my finger, it started reading 0.93-0.95 KOhms. I can't read its color bands, but it's positioned exactly the same way as R51, just for Q15 instead of Q16, so it likely should be the same value as R51. So R53 is bad too.
- Either D14 or D12 is bad (I can't tell which one had the bad part, as I removed all parts first, then measured them aftewards and lost track which one is which.) I suspect it was D14, since R53 seems to have more damage.
- And finally, one of the C945 driver transistors on the secondary, Q12, has a low resistance reading (about 100 Ohms) on BE junction... so that one is bad too.

And that's all I've found to test bad so far - 6 parts total (or 8, if you count the two 13009 BJTs.) Hopefully the SDC2921 didn't take a hit, because the base of Q12 (and Q11) are directly connected to it. With Q12 blown, though, that is a possibility. Of course, it could be that Q12 blew first, causing the primary BJTs to run only "one-sided" - i.e. either only Q15 or Q16 switching, causing the PWM to up the pulse until Q15/Q16 blew. But more likely, since both 13009 BJTs were blown... I think they blew first, which then sent a huge pulse of voltage on the secondary of the driver transformer, taking out Q12 and the rest of the parts on BJT drive circuit on the primary. Whatever, I guess we will see when I replace all the bad parts.

For the primary caps, we get a pair of CapXon LP, 200V, 820 uF… in 22x42 mm size. IMO, that is suspiciously small for that capacity. I shall find out later.

And their real capacity is....
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*drumroll*
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625 uF only! (well, at least one of them is - I didn't remove both.)
I checked several times too, just to make sure it wasn't a mistake on my meter. It wasn't. The meter measured that one "820 uF" CapXon LP every time at 625-628 uF. That's not even 680 uF worth of capacity! More like halfway between 560 uF and 680 uF.

ESR-wise, that cap showed up as 80 mOhms, which I suppose is OK. Vloss was 0.5% or less.

That said, even if these caps are 625-ish uF, that's still plenty of capacity... for a 300-350 Watt PSU. But I guess the primary couldn't even do that much.

Well, that's all I have for now.

[momaka]

Re: KDMPower MIPC MI-X8775CD [PCB WF-C rev:E]

But can you explain one thing for me, you say this is a half-bridge design but that it uses 13009 BJT transistors for the half-bridge.
The design examples I found when I Google show the half-bridge topology using two MOSFET switches.
Is it just a misnomer for ATX power supplies or what is up with that?

Well, you CAN build a half-bridge PSU with MOSFETs too... or probably just about any switching device, if designed accordingly. That goes for other topologies too.

Here is, for example, one H-bridge PSU posted recently that appears to use MOSFETs:
https://www.badcaps.net/forum/showth...91#post1003591

As to why cheap PSUs continue to use BJTs for half-bridge designs... that I'm not 100% sure about. I suspect it has to do with the fact that the design is well-understood and easy to copy (after all, half-bridge schematics have been widely-available on the internet for a long time now.) I also suspect it has to do with the fact that H-bridge PSUs can be built with readily-available dirt-cheap parts. BJTs from the 1300x family are one of those parts just like LM358, LM317, 555 timer, and etc. - i.e. parts that aren't really the best in what they do, but very simple, cheap, and universal, so too good to go away / get replaced by something new.

As a result of using these cheaper BJTs, the switching frequency is also typically lower... which works out fine with the rest of the PSU too, because then you don't need very low ESR caps or output toroid rated for higher frequency. On that note...

The output toroid core is also the lower-tier Micrometals-52 and hence is going to be a little more loss-y.

That should be Micrometals-26 in post #2.
Micrometals-52 is the better core (slightly higher frequency and permeability specs) - typically colored green with blue on one side.

They didn't even put good quality glue on the back of the label!

Actually, it might be the paint on the case that's the problem. It has a very fine eggshell finish, so probably most labels won't stick to it properly.

On that note, I didn't see a warranty sticker anywhere. Not that I was expecting this PSU to have one anyways.

Now, seriously, I am impressed. This psu is so much better than those Hantol/Powertech/t&p that I posted some months ago.

That is true.

When you put it that way, it does seem like a PSU worth fixing up. And indeed I think I will.

Looks like it really is time to break apart that Raidmax RX-380K crapjob I liked above and use its parts in better PSUs. The only reason I've been saving it is because it uses the most simplistic H-bridge design with a KA7500 controller and discrete transistors for the protections. A long while back, The_Unique posted a tutorial/writeup how to roughly convert such PSUs to adjustable ones and it's been on my mind to try that for a long time now (but just among many other projects, so that's why I never got to it.)

These are the psus I really like to play with. I would add input filter, check primary caps, try to fix the psu problem and replace all secondary capacitors with 2200uF caps. And 5vsb always gets 2x1000uF japanese low esr caps to be safe.

Well, as I've come to find out, getting an input filter for these PSUs is the hardest part - at least the EMI/RFI chokes, that is. Sure I have some scrap TV boards that do have proper EMI/RFI filters... but a lot of times, those don't fit (usually too big.) And getting new filter parts (at least reputable ones) is not exactly cheap (some places ask as much for a filter as this PSU costs. )

I could get a bunch of these, though, and wind my own chokes since I already have the wire:
https://www.ebay.com/itm/5X-22mm-x-1.../152506497515?

The other thing is... when it comes to recapping, I'm always low on cap stock by the time I get to my "lesser" PSUs. Main problem is (well, IDK if it's a problem, really ) I've acquired too many good Delta and HiPro PSUs, and those take precedent when it comes to getting recapped. So if by any chance I get any leftover caps, only then the lower-tier PSUs may get recapped.

And then play with the fan controller.

I guess I forgot to mention, but this PSU already has one. Not sure how well it works, though. Probably worth looking into, as you noted... once the PSU is fixed.

Nooo!

Having enough headroom on the secondary side actually helps a lot with the efficiency and the total power the psu can handle. It also helps tuning the fan to be slower and quieter.

I know.
But it won't help too much in this case. The single not-so-thick wire on the main PS output toroid for the 12V rail will very likely start pushing the limits around the 15 Amp mark. At 20 Amps (if the core can even handle that), I suspect it will get very hot. So there's no point in keeping too much headroom on any of the rectifiers if the output toroid wires (nor the core itself) can handle that kind of load.

The other thing that throws me off is how the primary blew up even with (supposedly) 13009 BJTs. If those are truly 13009 and not cheapo knock-offs, then I expect at least a few components on the secondary (like the toroid inductor) would have seen some heating from the large load the PSU was tasked with. But that doesn't seem to have happened, so I think there is also a possibility there may be something not quite right with the design. The OPP should have also acted before the primary BJTs blew (and it didn't). So for these reasons, I don't see why the secondary should be so over-built, when it really is the primary that's going to be my limit again. I actually rather the secondary side rectifiers go first, as that usually triggers the short-circuit protection and the PSU usually just shuts down. Usually.

[goodpsusearch]

Re: KDMPower MIPC MI-X8775CD [PCB WF-C rev:E]

What's funny is that label keeps peeling off by itself. While I was taking the case shots above, I had to re-apply pressure on it at least several times to stop it from rolling onto itself and falling off. It's as if it knew it was telling me lies and so was shamefully trying to cover itself, fall off, and hide somewhere.
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They didn't even put good quality glue on the back of the label!



Now, seriously, I am impressed. This psu is so much better than those Hantol/Powertech/t&p that I posted some months ago.

It even has proper 35 transformer, primaries that I hope they would at least be 560uF real C and secondary side with proper output filtering on the main voltages. I mean it even has coils!

These are the psus I really like to play with. I would add input filter, check primary caps, try to fix the psu problem and replace all secondary capacitors with 2200uF caps. And 5vsb always gets 2x1000uF japanese low esr caps to be safe.

And then play with the fan controller.

On that note, if I do end up fixing this, I will probably remove one of the 20 Amp rectifiers from the 12V rail to use in another PSU. A single 20 Amp rectifier on the 12V rail should still be more than enough. But anyways, those are all just "rough ideas" for this PSU.

Nooo!

Having enough headroom on the secondary side actually helps a lot with the efficiency and the total power the psu can handle. It also helps tuning the fan to be slower and quieter.

[Per Hansson]

Re: KDMPower MIPC MI-X8775CD [PCB WF-C rev:E]

Ahh yes of course I forgot, the fuse is for a 875w PSU with half-bridge design of around 70% efficiency so of course that explains the thick wire inside it

But can you explain one thing for me, you say this is a half-bridge design but that it uses 13009 BJT transistors for the half-bridge.
The design examples I found when I Google show the half-bridge topology using two MOSFET switches.
Is it just a misnomer for ATX power supplies or what is up with that?

[momaka]

Re: KDMPower MIPC MI-X8775CD [PCB WF-C rev:E]

I saw it in this photo, do I get the cookie?

And we have a winner!
(Then again, it appears you were also the only participant in that game. )

Judging by this better view of the fuse I'm not sure if I would rate it as a fuse or more a wire link?

Why not both?

Well, it is a legit fuse, after all. But it's one of those small, short ones. Yes, it's rated for 250V, but it's a glass fuse and not heatshrinked. So I imagine there is a chance that it could arc-over in the case of a hard short-circuit. Then again, if the BJTs blew open before it did... lol!

Also, another thing I noticed today (though I forgot to take a picture): on the secondary side next to the 12V wires, there is silkscreen "12V1" and "12V2" that points with arrows how the supposedly "dual" 12V rails should be connected. But looking underneath on the PCB, there are no separate traces for two 12V rails. It's all coming from one copper trace. So this PSU was never even capable of having dual rails to begin with by design.

On that note, if I do end up fixing this, I will probably remove one of the 20 Amp rectifiers from the 12V rail to use in another PSU. A single 20 Amp rectifier on the 12V rail should still be more than enough. But anyways, those are all just "rough ideas" for this PSU. First I'm going to try finish fixing up my better PSUs. Finished recapping two HiPro's this week and have an old Delta, FSP, and Bestec lining up in the queue. Maybe even a cool Topower/Zumax I posted a while back.

[Per Hansson]

Re: KDMPower MIPC MI-X8775CD [PCB WF-C rev:E]

Interestingly enough, the fuse (a free e-cookie for anyone that can find it ) was intact. I suppose this part (and perhaps others) blew up to save the fuse, as usual.

I saw it in this photo, do I get the cookie?
Judging by this better view of the fuse I'm not sure if I would rate it as a fuse or more a wire link?

[momaka]

KDMPower MIPC MI-X8775CD [PCB WF-C rev:E] - part 2

Of course, who cares about safety, so long as this PSU can do 775 Watts, right?
Well, can it? Let’s look at the secondary side.

We get one STPS2045CT rectifier for 3.3V and same for 5V rail. Meanwhile, the 12V rail gets 2x MBR20100CT in parallel. So the label is a complete LIE in terms of the current capabilities on each rail (but I think we expected that already. ) On the other hand, this is probably one of the few cheapo PSUs that I’ve seen with much better rectifier configuration on the 12V rail than the 5V rail. But that also means the 5V and 12V rectifiers are not tied together in any way, so I suspect this PSU probably does not have good cross-regulation.

The windings for the 5V rail on the main output toroid inductor sign a similar tune. In the past, most half-bridge PSUs would typically get 2x thick wires for the 5V rail and a single wire for the 12V rail. In this unit, both the 5V and 12V rail appear to have the same wire thickness on the output toroid… and it’s not very thick, mind you. :\ IMO, it won’t be able carry even 24-25 Amps… let alone the current of “dual” 24-Amp 12V rails. The output toroid core is also the lower-tier Micrometals-52 and hence is going to be a little more loss-y. Thus, I expect this PSU probably had mediocre efficiency. And with all of that said and shown above, I don’t think this PSU is realistically capable of more than 250 Watts continuous DC due to those output toroids. Maybe it can peak at 300 Watts, but maybe not. I wouldn’t suggest anyone who has one of these to push it that far. And even if the output toroid was bigger, we also have to account for the output ripple, which is where the output electrolytic caps come into play.

On that front, I think it’s clear this unit won’t do too well there either. Each rail has only 2x 1000 uF caps (though at least the cap spots are for 10 mm diameter.) There are also PI coils for each rail - yes those tiny ones. The -12V rail gets none, though. It only has a single 470 uF cap. Meanwhile, the 5VSB has a 16V, 1000 uF cap and a 10V, 470 uF cap. All output caps are by HANGCON… or is it KANGCON? You tell me.

OK, maybe that picture didn’t come out too well. But either way, those output caps probably won’t allow for more than 150 Watts total before the ripple goes out of spec. As a side note, we can see on the above picture the caps are dated early 2019 (3rd week, I presume?) So this PSU is, at most, maybe almost 2 years old. Judging by the lack of dust, though, I don’t think it has seen much (if any?) use. In fact, it still has that “new China smell” to it. I suspect this may even be on of those cases where the user just built a new system with this PSU, powered it On, and the thing blew up almost instantly or in very short order.

Another interesting thing I find is that the SDC2921 PWM IC datasheet says the IC can be configured to provide OPP. Tracing my unit, I spotted an arrow mark on the PCB silkscreen pointing to three resistors labeled “OPP” (and the resistors are installed.) Now, whether or not it was done properly, that’s another question. Perhaps it was, and maybe the primary blew because those 13009 BJTs were dodgy. Or maybe the Base drive circuit was not built with good enough parts. Or maybe the OPP is incorrectly set to sky-high levels.

Whichever the case, now I have me another PSU to repair… or should I?
Part of me does NOT want to, because I have one nice Enermax PSU that’s awaiting an ATX wire harness. On the other hand, these cheapo H-bridge PSUs are usually super easy to repair. In fact, if there is one good thing about this PSU, it’s how easy it is to find components and work on. A lot of functions are even labeled on the PCB with text. Probably a great PSU for anyone that wants to learn how to repair/troubleshoot electronics.

I guess I’ll sleep on it for a while and then decide what I shall do with it. Knowing me, I most likely will end up fixing it with parts from “lesser” PSUs (I’m looking at you Raidmax RX-380K!) But we will see. Don’t stay tuned for this one - it will likely go in the back of the queue on my list of projects.

[momaka]

Remember that cheap $15 PSU that dmill89 posted a while back here?
https://www.badcaps.net/forum/showthread.php?t=72229
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So I was randomly browsing on eBay (as always) and saw a listing for supposedly the same “775 Watt” PSU with starting bid at $1. Description stated “for part or repair”, with the owner/seller mentioning the PSU had blown/shorted. Surely I could spend a few $$ more and get the same PSU as dmill89 right from Amazon brand new and (probably) working… but where is the fun in that?!

So I got this one instead for $13 shipped to my door ($0.99 for the PSU and the rest was shipping. ) If dmill’s PSU was able to handle 350 Watts without croaking, it really made me wonder what was wrong with this one. I got the unit nicely packed, and it honestly looked pretty much unused with no dust anywhere:


On the outside, it appears nearly the same as dmill89’s PSU: standard black finish, non-modular unit with a 120 mm fan and voltage selector switch (i.e. no AFPC.) Only difference was really that mine appeared to have a see-through fan with LEDs.

And here is a bottom-shot:

I think one way to tell if you’re dealing with a cheap PSU is when you see those stamped PCB stand-offs on the case. On the positive side, at least there are vents on the back of the PSU to (hopefully) avoid hot spots.

Here are also all of the connectors this PSU has:

Aside from the standard motherboard stuff (20+4 pin ATX, 4+4 pin 12V CPU power), this unit also has 2x 6+2 pin PCI-E 12V power, 6x SATA, 3x Molex, and 1x floppy/Berg. The output wires are a mix of 18 and 20 AWG, with the ATX connector, 4+4 pin 12V CPU, and first 6+2 pin PCI-E connector being 18 AWG. The 2nd 6+2 pin PCI-E is simply piggy-backed from the 1st one, but with 20 AWG wire. The Molex and SATA connectors are also all 20 AWG. I do like how they arranged the drive connectors though, with 2x SATA followed by a single Molex on each string. And all wires are UL listed, at least.

OK, let’s have a look at the label finally.

It appears like this is not the same PSU model as dmill’s. Whereas his was an MI-08775V2 and rated for 875 Watts, this one is a MI-X8775CD and *only* rated for 775 Watts… like we are going to believe that one, too. What’s funny is that label keeps peeling off by itself. While I was taking the case shots above, I had to re-apply pressure on it at least several times to stop it from rolling onto itself and falling off. It’s as if it knew it was telling me lies and so was shamefully trying to cover itself, fall off, and hide somewhere.

On a different note, I’m surprised the shipping box didn’t get blown away by the wind when the unit was delivered to my house. There’s not much weight to this PSU. Probably a good half of it is from the output cables. On the other hand, all those output cables scare me - imagine someone trying to plug in a beefy video card with two 8-pin PCI-E power connectors! Technically speaking, since there are supposedly two 12V rails rated at 24 Amps each and no mention of the total max combined current, one could assume this unit is capable of 48 Amps (or 576 Watts) on the 12V rail. But maybe that is possible on this unit, after all. What do I know if I haven’t opened the unit yet? I suppose that part comes next.

But first, would it B OK if I posted a picture of the fan? (Oh look at me trying to be funny now. )

This one is a rated for 0.12 Amps - just a tad lower than the one in dmill’s PSU. When I hooked it on 12V directly, it does move quite a bit of air, though. And the blue LEDs are very bright and pretty. (Perhaps that also explains the lower rated power of the PSU - those LEDs need a lot of juice, so they had to lower the rating on the unit. )

Anyways, here is the unit inside.


Right off the bat we can see… well, there isn’t a whole lot to see, really. Initially, I thought this might be a 300-350 Watt capable unit. Sure the input filtering is missing, but I expect that to a degree in any cheap PSU. What really got me, though… before I had the unit opened and was looking through the vents, I was honestly very excited, because I could see two TO-3P transistors on the primary heatsink and a PDIP-16 chip on the secondary. This got me thinking the PSU must use the same double-forward design as the 875W unit, but perhaps also with added OCP (due to that PDIP-16 IC.) Then after opening the unit, I looked at the parts on the primary heatsink and noted the “13009” text on what I thought was one of the MOSFETs. Surely this quickly extinguished my excitement.
- That ain’t no MOSFET! That’s a BJT.
And only then I saw the obvious - that there are 3 transformers in the middle. The realization hit me like a rock on the head… this is yet just another half-bridge PSU. Looking up info on the SDC2921 PWM controller confirms this 100%.

Ah well, at least the thing has a bridge rectifier and a properly-sized (tall) 35 mm main transformer. The heatsinks are also not bad. Certainly we can’t expect 775 Watts from that. Heck, does a half-bridge design over 500 Watts even exist as far as ATX PSUs go? If yes, I don’t want to know how much one of those weights or what kind of BJTs it has on the primary. It certainly won’t be anything like you see in the pictures above. But anyways, let’s continue forward.

I think I see what blew up in there.

Interestingly enough, the fuse (a free e-cookie for anyone that can find it ) was intact. I suppose this part (and perhaps others) blew up to save the fuse, as usual. 5VSB should still be good, though. It uses a 3313D IC (whatever that is, I can’t seem to located a datasheet) and has the typical 50V, 47 uF “startup” cap.

Time for an autopsy on the primary, so out goes the heatsink.

We have not one but two charred small “components”. Initially, I thought these were diodes, but the PCB silkscreen suggests they are resistors. I get quite different readings for the two, despite both being connected the same way. Next to these, you may also see two small 1-Ohm resistors (one in front of Q15, and the other to the right of Q16 Base) - these are blown open. One even appears to have a hole / burn mark on it. The reverse-polarity diodes might be OK, though. Will check if/when I decide to dig deeper into it.

Next, here’s our beloved MOSFETs…. Oops sorry, I meant BJTs.

They look OK, but they are NOT OK. The left one is completely blown open on all pins. The right one has a short between Base and Collector. The Emitter has decided it won’t talk to these two in any way - completely open in any direction. Given the no-name manufacturer (or at least I can’t seem to find it), no telling if these are knock-offs or real 13009. I suspect it may be the former. In any case, the MIGHTY fuse defeated them.

Speaking of which, I think this picture just barely shows the fuse… among other parts.

For the primary caps, we get a pair of CapXon LP, 200V, 820 uF… in 22x42 mm size. IMO, that is suspiciously small for that capacity. I shall find out later. Also interesting to note there is a 2nd spot for a thermistor (which is bridge by a jumper, since there is one already) as well as an empty spot for a connector for PPFC coil. As for the blue cap on the left of the CapXon’s: that should be a Y2-class cap between ground (earth) and negative (-) primary-side bus… but surely enough, it ain’t Y2-rated and rather just a standard 2.2 nF, 2 kV ceramic cap. In terms of EMI/RFI, this PSU certainly won’t pass… and hence why we don’t see any FCC or UL marks on the label.

On the other hand, the PCB doesn't seem so bad, at least on the solder side:

The soldering is very good, overall. Spacing between primary and secondary might be a bit questionable in a few places... but overall, nothing too serious, it seems.

continued in part 2 below due to 10k char limit...