Design question about multiple +12V rail ATX PSU - Page 1
Forum section: electronics / projects

author: bte
author_rank: Regular Contributor
timestamp: July 25, 2025, 11:10:25 pm
content: This question is prompted by my late OCZ PSU (OCZ500SXS) going kaputt in a certain way. Using a PSU tester, I found out that the +12V2 rail (second +12V rail) is not working properly (on the device display, value for +12V2 switched between 11.4V and the "low/non-existent" indicator along with a beep). Quite luckily I was able to find a (hopefully close enough) schematic for this PSU (attached to this post). Since the PSU is advertised as having two +12V rails, my expectation was that each +12V rail would have its own winding and rectification elements etc, but it turns out that is not the case. The +12V2 rail is connected to the same point as the +12V1 rail. Unique to each rail is the inductor and capacitor (multiple capacitors in parallel on the PCB).I checked the ATX standard. Since this supply is from 2009 or thereabouts, I used an old version of the spec (Feb 2008). Regarding multiple +12V rails, it only has this stipulation "The +12V1 DC and 12V2 DC should have separate short circuit and over current protection". I also checked a later version of the spec (April 2013) and it doesn't have any additional requirements regarding multiple +12V lanes.How does this design ensure that this stipulation is upheld? What could have prompted the makers of this supply to advertise it having two +12V lanes with 18A capability each rather than having a single, 36A capable +12V rail (obviously after making the minor adjustments)? Also, in this design, I am wondering how the heck the +12V1 rail can be OK while +12V2 is not.In case anyone is interested, the PSU in question:http://hardwaresecrets.com/ocz-stealthxstream-500-w-power-supply-review/The same review, as originally posted in 2009: (more readable, with pictures showing properly):https://web.archive.org/web/20150626222253/http://hardwaresecrets.com/ocz-stealthxstream-500-w-power-supply-review/

author: tom66
author_rank: Super Contributor
timestamp: July 25, 2025, 11:23:32 pm
content: Multiple rails on an ATX PSU is mostly a misnomer. The requirement of the ATX specification was that no one rail could deliver more than 240VA. PSU manufacturers worked around this by creating multiple "rails" that were effectively in parallel and rating each to max 240VA (in your case, 216VA per rail). In theory the power supply should have independent current sensing per "rail", and trip the supply off if any one rail exceeds the rating. In practice many supplies omit this function. If there is a fault making one rail go bad, most likely it is soldering or a current shunt broken somehow. The current shunt might be used to implement rail sensing if they actually bothered with that.

author: bte
author_rank: Regular Contributor
timestamp: July 26, 2025, 11:41:00 pm
content: Quote from: tom66 on July 25, 2025, 11:23:32 pmThank you for the explanation. I looked into that 240VA a bit more. From what I can find, it is related to the concept of "hazardous energy" and is not limited to ATX supplies. In fact, I couldn't find any design stipulations about "240VA" in the ATX spec at all. There was only one version that mentioned about it in one chapter and that version only had it referenced as to how to test such power supplies (and as far as I can find, that chapter was removed from the spec in the next revision and the removed text was made into its own testing document).For completeness (and as a reminder to myself) here is what I found:This one is a bit historical and it talks about the origins of the "energy hazard" concept:https://incompliancemag.com/energy-hazard-a-history-and-analysis/This one, from 2015, talks about the "hazardous electrical energy" as defined in IEC/UL 69050:https://www.eeweb.com/hazardous-energy/This one says that 240VA limit is removed in the new standard "IEC 62368-1":https://www.tuv.com/landingpage/en/iec-62368-1/meta-navigation/faq/To quote from that page (item 7):QuoteRegarding the OCZ power supply, I will check the PCB more carefully with respect to current shunts. I know it uses PS223 chip as the monitoring solution. So far I only found one bad electrolytic capacitor but it was in the standby voltage circuit so most likely not related to my issue at all. I will update if I ever figure something out.Oh, BTW, I had mentioned that each +12V rail had its own inductor (L21 and L22 in the schematic). I took a picture of those "inductors" and attached to this post. Do these really work as inductors or did the designers just do a bypass? If these are just bypasses, why wouldn't they just bridge it out?

author: SteveThackery
author_rank: Super Contributor
timestamp: July 27, 2025, 01:01:54 am
content: Quote from: tom66 on July 25, 2025, 11:23:32 pmCan you steer me to a source for this information (that independent current sensing per rail is not provided)? I ask because this sounds like a fire safety risk. It implies that a short-circuited rail will not be current limited to the 240VA specified, but rather to the current limit of thecombinedrails, which might well exceed the current rating of the individual rail wiring.Or is this mitigated in some way?

author: Vovk_Z
author_rank: Super Contributor
timestamp: July 27, 2025, 04:44:38 am
content: L21 and L22 on the photo are rather shunt resistors but not inductors.

author: tom66
author_rank: Super Contributor
timestamp: July 27, 2025, 08:14:35 am
content: I don't know why the ATX people decided on 240VA (or if it was just copied directly from the '950 standard), but it was there. See page 20.https://web.archive.org/web/20080920191034/http://www.formfactors.org/developer/specs/ATX12V_PSDG_2_2_public_br2.pdfQuoteMost supplies didn't implement independent current sensing because they seemingly misunderstood the standard. They assumed that it applied to theratingof any one rail, not the actual current drawn from the rail ... or, they were cheap and this creative misunderstanding saved them money. Never mind the fact that most supplies used 18 or 20AWG, so one individual wire couldn't take the full 20 amps anyway.Either way, if the supply rails are not independently limited to 20 amps (iow exceeding 20 amps kills that rail or the whole supply) then technically they are not in compliance with the standard. How much this matters in practice is debateable. As has been seen from the Nvidia 12VHPWR debacle, there's still a risk of combustible components in a PC burning under fault conditions even with the best designed power supplies, because contact wear can be uneven and connectors can fail when effectively overloaded.In ATXv2.3 the requirement was removed.

author: aeg
author_rank: Regular Contributor
timestamp: July 27, 2025, 11:05:47 pm
content: The supervisor chip on schematic page 2 has separate current sense inputs for L21 and L22, so 12V1 and 12V2 have separate OCP.

author: bte
author_rank: Regular Contributor
timestamp: August 08, 2025, 10:47:54 pm
content: Thanks a lot to everyone who chimed in and provided help. I am sorry for my late reply, life got in the way. I don't like threads hanging (i.e. the OP disappears without any feedback) so here's the closure, for lack of a better word.Quote from: Vovk_Z on July 27, 2025, 04:44:38 amThank you for the correction. Interestingly, the L32 and L52 (corresponding elements in the 3.3V and 5V circuits) are proper inductors (see attached picture). I am assuming they have a controlled/known DC resistance to work as shunt resistors for their respective rails.Quote from: aeg on July 27, 2025, 11:05:47 pmThank you for the insight. I had checked the PS223 datasheet and seen the sample circuit that implements separate OCP for two 12V rails. What I missed in the PSU schematic was the letters "A", "B", "C" etc denoting the nodes that eventually connects to the PS223 (coupled with the fact that the schematic having the "L" markings in place of shunt resistors threw me off). Now I know how the circuit (and the OCP function) works.