EuroCase ATX PSUs

[Rainbow]

These PSUs use the following circuit as aux. power (at least I've seen it in 200W, 250W and 350W versions - the PCB is the same):


The problem is C12. When it fails, the output voltages of this aux power supply goes too high, destroying the TL494 (usually not high enough to destroy the 7805 for +5VSB). Some of these PSUs have crappy cap there.
The 200W one (model 200XA) and 350W (model 300XX) had JunFu and HEC craps. One of them was 2.7uF, the other one only 1.25uF.
In the 200W one, C12 and TL494 (some of the pins shorted to ground) were bad and also one of the small transistors that drive main switching transistors through a transformer. It worked fine after replacing them.
The 350W one was damaged more - transistor Q16 (replaced with KSE13007), diodes D5 a D6, zener diode ZD2, blown fuse and of course TL494. Also two 10K resistors were a bit discolored but OK.
The 250W one had Jamicon - this one was still working fine, the C12 was 12.5uF (after about 5 years of use - the oldest of these three PSUs).
I've used 33uF/63V@105 Jamicons as replacement.
BTW. Don't be confused by the polarity of C12 - it's OK in the schematic. It filters negative pulses.

[PeteS in CA]

Re: EuroCase ATX PSUs

Looks like it's self-oscillating, fixed duty cycle. The voltage to the TL494 is not regulated - no feedback loop - so if something changes the duty cycle, e.g. C12, that voltage will change. Are the main inverter switch(es) transistors or MOSFETs? The TL494 is very mature technology, designed in the late 70s; the uA7805 is even more mature. Mature technology, used right, can be both inexpensive and reliable.

[Rainbow]

Re: EuroCase ATX PSUs

The main switching transistors are NPN transistors - the main circuit with TL494 (or KA7500) seems to be similiar in almost all PSUs (both AT and ATX - maybe except for the high-end ones).
But these aux power supplies are different among manufacturers - and many of them are crappy (e.g. http://www.pavouk.org/hw/en_mxr-250p.html). Better PSUs use regulation through optocoupler (seen in DTK PTP-2007).

[PeteS in CA]

Re: EuroCase ATX PSUs

The KA7500 is Samsung's (IIRC) of the SG3524, which went into production in 1975. The self-oscillating standby regulators are probably limited, by the 7805, to .5A to 1.5A. Higher current circuits tend to use better regulator topologies.

[PeteS in CA]

Re: EuroCase ATX PSUs

Those two general-purpose ceramic disc capacitors at CY1 and CY2 scare me. Good P/Ss use special types that have been approved by various countries' safety agencies.

[Rainbow]

Re: EuroCase ATX PSUs

One of these PSUs died today in my machine @work. Results: motherboard Epox EP-3PTA dead (shorted super i/o), graphics card Gainward GF2MX dead (voltage regulator shorted), HDD maxtor D740X dead (nothing seems to be shorted - but does not work), DVD-ROM Toshiba dead (servo controller chip shorted). Only CPU Celeron Tualatin 1.3GHz, memory JetRam 256MB, FDD (Teac) and network card Realtek (why?!?!) survived.

[willawake]

Re: EuroCase ATX PSUs

wah! thats too bad. are they gonna pay for a new pc?

[Rainbow]

Re: EuroCase ATX PSUs

I'm the network admin and hw "specialist" (all-in-one - it's a small company) so I'll rebuild the machine from some components that are lying around there. I've fixed that PSU (THE cap was bad, of course - it fried TL494 and one small transistor) and also another one of the same type - nothing was fried, THE cap was still about 10uF (instead of 22uF) so I replaced it (of course), only jammed fan and two caps bulged as a result of overheating.
I'm new there but looks like I'm going to open all computers to find these PSUs and replace THE offending caps before they fail.
Hope the servers don't have these PSUs (or even the Skyhawk/Deer crap )

[willawake]

Re: EuroCase ATX PSUs

hehe it sounds like my job. i hope they have more money than my company.

if you need a rack cabinet check out http://www.triton.cz they are a great company

[japlytic]

Re: EuroCase ATX PSUs

Since the polarity of the voltage measured between C12 and GND (0V) may change, I recommend using a non-polarized electrolytic capacitor.

[Rainbow]

Re: EuroCase ATX PSUs

Another 200XA died today - the machine was running and after a while it was not. I knew what to do - immediately disconnect and replace PSU. Luckily, no hardware was damaged. Only the HEC crap-cap (1.87uF...), KA7500B and one C945 transistor was bad in the PSU. I put Partsnic cap inside (as I ran out of Jamicons), hope that they're good.

[linuxguru]

Re: EuroCase ATX PSUs

This was a good find - this topology is used in a wide variety of ATX SMPSes, and the critical cap in all cases is C12. It regulates the on-time of Q16 (and hence the duty cycle & voltage of the flyback converter) in conjunction with D5, ZD2, R82, C15 and P2 flyback winding. R82/C15 provide the base drive during the time Q16 is 'on'. If the potential on C12 is low, ZD2 will not conduct and this facilitates the startup of the oscillator - during this time, it runs as a free-running self-oscillating converter. When the potential on C12 reaches approximately -6.2v, ZD2 starts conducting on each cycle, limiting the base drive to Q16 (most variants of this circuit have a small resistor Re in series with the emitter of Q15, sometimes using that to drive an NPN pull-down transistor that removes the base drive to Q15 when the potential on Re reaches Vbe, thus providing hysteresis).

As can be seen, the potential on C12 cannot exceed the sum of Vd5 (forward drop on D5) + Vz (zener breakdown voltage) + Vebo (emitter-base breakdown voltage of Q15). This is ~0.7 + 6.2 + 7 V, or ~14V. Therefore, a 25V rating for C12 is sufficient in this topology (50V is overkill). The value is not critical - it only changes the time-constant of auxiliary regulation loop and hence the transient response of the auxiliary supply (Vsb is separately regulated by a 7805, and is not affected).

I'm replacing this cap (pre-emptively where possible) with 100uF/35V or 100uF/50V United Chemicon LXA long-life, and this is working well. You may want to try 47uF/50V or 22uF/50V if you're more cautious and do not want to alter the transient behaviour of the circuit.

Some variants of this circuit have large cap(s) on the secondary at the cathode(s) of D7, D8 - these often bulge/leak and must be replaced with high-quality low-ESR high-ripple current 105c cap(s).

[trodas]

Re: EuroCase ATX PSUs

I did replaced the "C12" in question with Panny FC 47uF 50V and it seems to working great We see in time, I was using a fanless approach there, so we might see failure in some years that come
https://www.badcaps.net/forum/showthread.php?t=2198

[yanz]

Re: EuroCase ATX PSUs

amazing you can still get FC to 47uf i use nichicon HE 47uF 25v or ruby YXF 47uF 63v

so that i see now, 100uf is okay too.

[trodas]

Re: EuroCase ATX PSUs

Well, it was recommended to me use the 50V ones, just to get safe, so I did and digi-key have FC 47uF 50V ones with this number: P10321-ND
d6.5 x 12mm suxxka

And I'm not sure anymore with the capacity overshooting too much - I mean if it is safe - read - if it produce nice clean voltage at output in all cases