How on earth does this power supply work? - Page 1
Forum section: electronics / projects

author: TheMG
author_rank: Frequent Contributor
timestamp: May 02, 2015, 06:48:32 pm
content: This is a Coutant/TDK Lambda MML400 series switching power supply configuration 24G (24VDC nominal 15A 400W max).What I simply can not quite understand is how the feedback from the secondary circuits get to the primary switching circuits.Typically, a switchmode power supply would use either a small transformer or opto-isolator to provide an electrically isolated feedback signal, necessary to obtain voltage regulation.However, this power supply has neither. The ONLY connection between the secondary circuits to the rest of the power supply is through the main transformer's secondary windings, represented by A, B, and C on the picture. B is the center tap of the transformer secondary.I can not find any service manuals or schematics for this power supply online.The only major power-handling components on the secondary side are the usual schottky diode and filter inductor and capacitors. So there's no further voltage regulation done on the secondary side.There are no surface mount components on the backside of the PCB on the secondary side. The only components on the other side of the board, that can't be seen in the picture, are the schottky diode, capacitors, and a few flameproof resistors. The LM393D ICs that can be seen, are dual differential comparators according to the datasheet. Without reverse-engineering the whole circuit, what sort of voodoo magic is going on here?Clearly, there has to be some form of feedback across the isolated gap between primary and secondary (otherwise there would be no way of obtaining precise voltage regulation), but how are they doing it? Any ideas?

author: TheMG
author_rank: Frequent Contributor
timestamp: May 02, 2015, 07:03:27 pm
content: Quote from: edavid on May 02, 2015, 06:54:21 pmUnfortunately I don't think that's what's going on here. The voltage on this power supply is adjustable from the secondary side (see the blue 10-turn pot in the picture).If the flyback primary sensing topology was used, voltage adjustment would have to be done on the primary side if I'm not mistaking. The power supply is also equipped with remote voltage sensing capability.PS: the main controller IC on the primary side appears to be a UC3844 which to me looks like a bog standard SMPS controller.

author: DanielS
author_rank: Frequent Contributor
timestamp: May 02, 2015, 07:16:12 pm
content: My guess: primary-side sensing for rough regulation, magnetic regulation on the secondary side for trimming.

author: wraper
author_rank: Unknown
timestamp: May 02, 2015, 07:21:19 pm
content: If it is UC3844 based, there should be optocoupler hidden somewhere.

author: wraper
author_rank: Unknown
timestamp: May 02, 2015, 07:28:15 pm
content: Post a photo of the PCB where UC3844 is located.

author: Andy Watson
author_rank: Super Contributor
timestamp: May 02, 2015, 07:32:38 pm
content: Quote from: TheMG on May 02, 2015, 06:48:32 pmThe regulation is probably performed on the secondary side - so that you can have one main power transformer supplying several secondary modules. Can you see the connections to the "filter inductor". I bet there's more than input and output. The regulation is done by controlling how near to saturation the filter inductor operates. Essentially the filer inductor is being used as a magnetic amplifier. The primary side being operated continuously at full bananas.

author: ajb
author_rank: Super Contributor
timestamp: May 02, 2015, 09:50:24 pm
content: Quote from: Andy Watson on May 02, 2015, 07:32:38 pmThat's either a hilarious autocorrect, or my new favorite bit of jargon. I'm okay with either one.

author: atferrari
author_rank: Frequent Contributor
timestamp: May 03, 2015, 02:16:10 am
content: Quote from: Andy Watson on May 02, 2015, 07:32:38 pm

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 03, 2015, 03:37:27 am
content: Quote from: Len on May 03, 2015, 02:25:21 amIs that Freelee the Banana Girl's car by any chance?Back on topic, I'm going to guess that they just have a postregulator.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 03, 2015, 03:50:35 am
content: Yeah, postreg is a possibility.Nearly impossible to say without a breakdown of all boards. That thing is packed. I count six, maybe eight boards, and that's just from this angle?Tim

author: Psi
author_rank: Super Contributor
timestamp: May 03, 2015, 04:41:31 am
content: It's been like 10 years since i opened my MML400 up, but from memory its a dual stage powersupply design.One big psu that generates unregulated high frequency AC from the rectified mains. And each module which takes this high frequency AC and generates its own output voltage.So if i'm remembering correctly the feedback will be internal to each module.With the high frequency AC available to each module they only need a tiny high freq transformer inside. (possibly a magnetic amp as others have said)I do remember being confused also when i took mine apart

author: Whales
author_rank: Super Contributor
timestamp: May 03, 2015, 05:00:10 am
content: Quote from: atferrari on May 03, 2015, 02:16:10 amFull bananas = very full or very highie the primary side is being operated at a constant high level, rather than it reducing when less power is needed. "It's being run at full bananas".

author: TheMG
author_rank: Frequent Contributor
timestamp: May 03, 2015, 10:43:40 pm
content: Quote from: Andy Watson on May 02, 2015, 07:32:38 pmWell, I scoped it (as well as I could with my crappy scope anyways) and can see no change in the secondary waveform while adjusting the output voltage. The waveform amplitude is also quite a bit higher that the expected output voltage.So there has to be something going on in the output module. The thing is, the ONLY power device is a single TO-247 device (unfortunately I can not see the part number as it is sandwiched between a board and the heatsink) labelled on the PCB as "D30".I suspect despite being labelled as "D30" it is probably not just one of those dual-diode things commonly found in switchmode supplies, but possibly a MOSFET being used for both regulation and rectification.This is made even more likely as I had another closer look at how the output module is connected to the main transformer's secondary. There is in fact just one winding without a center tap. The contacts I labelled "A" and "C" in the picture are in fact joined together under the top PCB.I wish I could tell what "D30" is, but it would take some serious disassembly to get to it.There's 3 inductors in there, all with only one winding.I've attached a quick sketch of what lies beneath the top board that you can see in the picture. Please excuse the crudity of the paint drawing.

author: DanielS
author_rank: Frequent Contributor
timestamp: May 03, 2015, 11:04:49 pm
content: Quote from: TheMG on May 03, 2015, 10:43:40 pmFrom your diagram, I have a hard time imagining that D30 being anything other than a dual schottky diode. A synchronous or otherwise controlled rectifier would require some form of control/timing circuit attached to its gate, which would usually be pin #1.

author: TheMG
author_rank: Frequent Contributor
timestamp: May 03, 2015, 11:39:29 pm
content: I really wish I could find something somewhere on how this works. It's definitely got my curiosity as it's the first I've seen designed this way.I have a few other configurable/multi-output power supplies from other manufacturers and while the output modules share a common PFC pre-regulator, each output module is a self-contained switching power supply operating from a common high voltage DC bus.Can't even find any mention of any patent numbers or anything anywhere.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 04, 2015, 12:05:18 am
content: Look up magnetic amplifier regulator - commonly used for auxilliary supplies on PSUs with multiple outputs.

author: TheMG
author_rank: Frequent Contributor
timestamp: May 04, 2015, 03:38:35 am
content: I think that's exactly it. Magnetic amplifier. I read up on it a bit and it does seem to match what's happening in this PSU.Now it makes perfect sense but since I had previously never heard of a magnetic amplifier I couldn't make any sense of what was going on in the circuit.Interesting concept for sure. I had heard of and seen power supplies using core saturation to provide voltage regulation - but those were big heavy 60Hz units. Never heard of a similar principle being used in switchmode power supplies until now.As Dave said "don't turn it on, take it apart!". Lots to be learned by doing so! I initially took it apart to check out the build quality (and I'll add that it's a top-notch power supply in that regard, can't fault it), that's when I noticed there was no apparent feedback path between primary and secondary.

author: DanielS
author_rank: Frequent Contributor
timestamp: May 04, 2015, 04:31:50 pm
content: Quote from: TheMG on May 04, 2015, 03:38:35 amIt has been known and documented for 20+ years but the extra cost and complexity have kept it from becoming common. With today's highly integrated PWMs and drivers, it is often cheaper and simpler to power auxiliary outputs from auxiliary DC-DC converters than magnetic regulators.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 04, 2015, 07:16:11 pm
content: Saturable reactors have been used for decades on the +3.3V rail in ATX supplies. Usually a peculiar plastic-cased inductor near the output side (what the heck is that?). This is also why the 3.3V rail has wonderful regulation (usually much better than spec), while 5 and 12 (which are derived from common windings) are more mediocre (near max spec).Tim

author: Andy Watson
author_rank: Super Contributor
timestamp: May 05, 2015, 12:12:13 am
content: I thought the banana scale was a widely recognised engineering frame of referenceI didn't realise it was so controversial!The attached pdf is the schematic of a Coutant 5V power unit that employs a magnetic regulator. The input from main power transformer is between the terminals F ans S. L1M is the saturable core inductor - aside from some filtering there is no other regulation between it and the output terminals. The action of the D2 will cause the core to saturate and pass maximum current. Regulation is achieved by driving the other winding on the core, L1R, to take the core away from saturation. That's the oddity with this configuration - toreducethe current through the core it has to be driven (via D3 and TR1).Apologies for the cruddy layout. This schematic has been recovered from a 20 year old Easytrax file - it originally printed on fan-fold paper. I can't remember why I reverse engineered it - must have had some big plans at the time!

author: TheMG
author_rank: Frequent Contributor
timestamp: May 05, 2015, 12:38:30 am
content: Quote from: T3sl4co1l on May 04, 2015, 07:16:11 pmCan't say I've ever seen that in any ATX power supply I've ever taken apart. Maybe this is the case in some higher end power supplies but most run-of-the-mill ATX supplies don't employ such extra regulation. Most of them simply use the 3.3V rail with a remote voltage sensing wire as the reference for the entire power supply. The 5V and 12V rails are usually just along for the ride, as evidenced by the fact that loading the 3.3V output of a typical ATX power supply will cause the remaining unloaded rails to climb in voltage.More modern ATX power supplies only produce 12V out of the main switcher, and the 5V and 3.3V rails have their own DC-DC converters powered from the main 12V output, therefore as each rail is individually regulated there is no need to have any load on either the 5V or 3.3V rails.

author: TheMG
author_rank: Frequent Contributor
timestamp: May 05, 2015, 12:51:50 am
content: Quote from: Andy Watson on May 05, 2015, 12:12:13 amSchematic appears to be not too far off.Odd configuration indeed, but seems to work quite well.I'm guessing the main objective by the manufacturer was to be able to use the exact same primary switcher and transformer for a number of different power supply configurations, and simply bolt in the necessary output modules according to customer's request. I'm also assuming that with the magnetic amplifier configuration, only a limited number of component changes are required for a wide variety of different voltages and currents.So it's all about minimizing manufacturing and design costs while producing custom power supply configurations to suit the customer's needs. Not having to custom-wind different transformers is probably a big money and time saver - ie, the same transformer is probably used in all 400W power supplies regardless of output configuration.

author: DanielS
author_rank: Frequent Contributor
timestamp: May 05, 2015, 01:25:57 am
content: Quote from: TheMG on May 05, 2015, 12:38:30 amMost affordable ATX power supplies (as in sub-$80 or so) are group-regulated affairs in either flyback or half-bridge configurations. The oldest ones regulate the 5V output while the not so old ones regulate based on the 3.3V sense line. In modern Haswell-ready models though, 12V is the new king.I do not remember seeing magnetic regulation in any of the PSUs I have opened either. The only reason I knew about them is because I skimmed a chapter about them in a switching supply design book many years ago.

author: TheMG
author_rank: Frequent Contributor
timestamp: May 05, 2015, 02:13:36 am
content: Quote from: DanielS on May 05, 2015, 01:25:57 amYeah. It's been a long time coming but were almost to the point of having PCs that run on all-12V (actually, HP has started doing this in most of their recent business PC lineup).With the ever decreasing cost of DC-DC converters it just makes sense to put the lower voltage regulation on the boards themselves instead of in the power supply.Most modern "high-end" ATX supplies use DC-DC converters to produce the 3.3V and 5V outputs. Eventually I think we'll get to a point where even this will be dropped and PC PSUs will be 12V-only.Personally, I can't wait. This sure will take some of the rat's nest out of computer cases!They also need to come up with a higher current standard connector. It's getting a little out of hand with some graphics cards having 12 or more pin 12V power connectors to supply the necessary current, each with pin having its own 18AWG wire. IMHO they should pick a new standard connector with a much greater current rating for the motherboard, graphics card, and any other high-current hardware devices.I have to say as an amateur radio operator I have a slight bias toward Anderson Powerpole connectors. Reliable, configurable, and the connector size is fairly small despite 30A per pin rating! Other nice thing is the gender-less design. I'd like to see those used in PCs too.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 05, 2015, 03:52:35 am
content: Weird, I've not taken apart one whichdidn'thave one!12V supplies make sense both from being more power-dense, and for being more consistent with mobile platforms -- laptops generally have a master 14-20V rail from which the rest are supplied (including battery management and ideal-diode circuits). Wouldn't mind 24V really, you get even more power through less wire.Tim

author: dom0
author_rank: Super Contributor
timestamp: May 05, 2015, 06:44:34 am
content: Quote from: TheMG on May 05, 2015, 02:13:36 amI think we can do better:Should be good for a couple hundred amps :-)

author: DanielS
author_rank: Frequent Contributor
timestamp: May 05, 2015, 12:15:41 pm
content: Quote from: TheMG on May 05, 2015, 02:13:36 amNot really: the high-current version of MiniFitJr can already handle 13A per pin, so the 6-pin connector could handle GPUs up to about 450W if the GPU measured wiring losses to detect the use of high-current connectors and heavier gauge wires, then set power limits accordingly.On the other hand, if Nvidia's last few generations are any indication, the trend is for GPUs to get considerably more power-efficient with a heavy focus on making more efficient use of their limited memory bandwidth. When the 14/16nm chips come out, I would not be surprised if even high-end single-GPU cards ended up well under 150W: the 980Ti in stock reference configuration uses only 180W and that's still a 28nm part. The days of high-end (single) GPUs being power hogs are coming to an end.

author: mariush
author_rank: Super Contributor
timestamp: May 05, 2015, 12:41:24 pm
content: Quote from: TheMG on May 05, 2015, 02:13:36 amThe USB 3 standard introduces 20v as a valid voltage for usb devices. A usb device can choose between 5v, 12v and 20v and by default everything defaults to 5v. AFAIK there's few controllers which actually support negociating for higher than 5v voltages and probably no motherboard that would negociate to 12v for usb 3.There are already some motherboards that have a standard 2.5 barrel jack that allows the motherboard and everything to be powered from an 18-19v laptop adapter (the mb has a bunch of sata power connectors on it and the board has dc-dc converters to produce 3.3v and 5v) but such motherboards still have the atx connectors as an option.I would love to see power supplies drop 3.3v and -12v and offer just 5v (because dropping 5v stand by would be too big of a change) and 12v (for compatibility with existing video cards and sata drives and fans) and 20v (for cpu and mb and future video cards).

author: amyk
author_rank: Super Contributor
timestamp: May 05, 2015, 02:27:26 pm
content: Quote from: DanielS on May 05, 2015, 01:25:57 amhttp://danyk.cz/s_atx_en.html29/44 of those ATX PSUs there use a magamp to regulate the 3.3V, the other 15 use either a discrete linear regulator or a separate coil on the transformer (more expensive).