Current sharing:  suckling from 3 power rails. - Page 1
Forum section: electronics / projects

author: frogblender
author_rank: Regular Contributor
timestamp: March 26, 2021, 01:29:52 am
content: HotRod pcie cards (ie. graphics cards) have, in addition to the PCIe connector, a 2x4 power connector, and a 2x3 power connector, allowing max of 300W combined:pcie edge connector: 75W (12v@5.5A, plus 3.3@3A)2x4 connector: 150W (12v@12.5A)2x3 connector: 75W (12v@6.5A)TOTAL = 300WMy pcie card has a multiphase buck, delivering 300watts (800mV at an unholy (musk-like) number of amps).I wish to feed the buck from the three 12v rails. But I need to draw off the rails such that the 5.5A, 6.5A, and 12.5A maximums of the three are never violated.Option1: Feed most of the phases from the 2x4 conn, some of the phases from the 2x3 conn, and the remaining from the edge connector. Example: for a 10phase controller: 5 phases are fed from the 2x4 (150w), 3 from the 2x3 (75W) conn, and 2 from the pcie slot. The problem though is the ratios aren't quite right; I can't pull the full 300W (my 10-phase example here only pulls 250W before the 2x3 is maxed out).Option2: there exists LTC4370, a ludicrously expensive part ($8 in quantity?) that uses 2 fets to combine 2 rails at 50/50 (until one of the rails sags below a threshold, then it'll lopside).... and 50/50 doesn't help - I need finer control.Option 3:So... Anyone have any ideas how to suckle amps from three rails in any arbitrary ratio?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 26, 2021, 03:23:01 am
content: Is this in the first person because of a project you're actively working on, or is the phrasing intended to put you into the perspective of one of the designers of these boards for emphasis..? It seems an odd way to phrase it, as the first case will have design history to draw from so should already know the answer (and I don't know what other applications would not be very specifically powering a CPU or GPU and RAM? And if so why omit it?), and the second case can just be asked straightforwardly...Anyway, I don't happen to be in any of those groups, so all I can do is speculate what Iwoulddo.Note that the individual phases need to be current regulated. Otherwise, how can they share current in any meaningful way? -- they can't all be fixed PWM. Normally, those currents are all programmed the same, under control of the voltage regulator error amp. Well, with ten channels, there is some freedom for some to throttle up and down around that mean, and others to pick up the slack. The cost is increased ripple, due to the pulses not perfectly overlapping in that case. Preference should be given to symmetric phases: for example your 2 PCIe slot phases would be 180° apart, and the three 2x3 phases would be 120°; both would be interleaved with the remaining to give a reasonable even total. This may simply require additional capacitors (to handle the ripple current or maintain ripple voltage within limits), or may not matter at all (the two limits are already well met thanks to high performing parts?).I can also suggest looking at what the first grouphasdone. Look up the controller chips on the board, read their appnotes. Read manufacturer appnotes for what they recommend, as far as sourcing all that current. (Where do you think the card mfgs got their designs from?) Important layout details there too.Tim

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: March 26, 2021, 08:00:04 am
content: Hi,Let me share a few techniques.LTC4370This circuit shares in a 1:1 ratio because the current sense resistor are the same value. The LTC4370 tries to regulate the voltage between OUT1 and OUT2 to zero. If the two sense resistors are not equal in value the current will share in that ratio. If you use 1mand 2mthe current will share in a 2:1 ratio.This configuration is limited to two inputs and introduces power loss.Poly-Phase Buck - Current Mode ControllersThe LTC3855 is just used as 2-phase example. In this example the current share equally because the current control signals, Ith pins, are tied together and the two phases have the same, 1m, current sense resistors. If you change the current sense resistors (or manipulate the Ith signals) you can change the sharing ratio.Jay_Diddy_B

author: Berni
author_rank: Super Contributor
timestamp: March 26, 2021, 10:05:25 am
content: The solution is simpler than that...you simply don't do it.Graphics cards don't actually have logic to balance the load among the various power inputs. They typically don't bother to take much power from the PCIe slot, running mostly the basic housekeeping stuff from there. All of the high power rails are made from 12V.The ATX power supply always has these PCIe 12V power connectors on the same rail (Some old designs have 12V EPS CPU power rails but they stopped doing that too). So it doesn't actually matter exactly how much power you take from each plug as long as you don't make such a large imbalance that a wire overheats. To do that you can simply connect it together via shunt resistors so that the card can detect that power is indeed coming in from both ports. The reason for the 8pin and 6pin PCIe power connectors are to determine the power supplies capability. The ATX PSU is designed to handle a max load of that many watts on all of these connectors. A weaker ATX PSU will have less 8pin/6pin connectors so that you are not physically able to plug in too power hungry loads. At the same time graphics cards have the appropriate number of 6pin/8pin connectors for how much power they consume. As a result the end user runs out of connectors if the PSU is too weak to run whatever they are trying to plug in.This is also seen in the pin out of the 8 pin connector. The weaker 6pin connector has 3x GND and 3x +12V while the higher power 8pin connector has 6x GND and 3x +12V. So there are not actually providing more wires to carry the extra current. The reason for extra grounds is actually that on a 6 pin connector one of the grounds is actually optionaly used as a detection pin, not carrying current. To combat this the 8 pin introduces 1 extra ground pin to balance it out providing 3 ground to return the 3 power inputs, the last extra pin is used as a 8pin detect.So sharing doesn't matter. It's all about not overloading the PSU, not current sharing.

author: mariush
author_rank: Super Contributor
timestamp: March 26, 2021, 12:44:52 pm
content: Like the poster above me says, don't bother.Just put 3 pci-e 8 pin connectors on the card, and don't take much if anything from the slot. Maybe just enough to convert some 12v to 5v or whatever logic you have on the card.Majority of power supplies don't have any logic or measurement to see if you actually pull more than 150 watts through a 8 pin connector, or more than 75w through a 6 pin connector, but there often is some overcurrent limits (like for example let's say 400-450w on cable with 2 pci-e 8pin connectors)I'd use 3 connectors so that at least the current is split across two cables, each with 2 connectors. Typically, modular power supplies will have 4-6 pairs of 12v+gnd going to the 2 8pin connectors on the cable , so you have 300 watts split between 4-6 pairs.With more than 2 connectors, you're tricking end users into having to use 2 cables, so basically 2 x 4-6 pairs of wires, ending up with 8-12 wires to your card.The 3rd connector can be optional ... you'd probably join all 3 connectors to some power plane, maybe through some resettable fuses (make them trip at 175-200w per connector or something like that)Quote from: frogblender on March 26, 2021, 01:29:52 am

author: magic
author_rank: Super Contributor
timestamp: March 26, 2021, 01:02:19 pm
content: Pretty sure I have taken apart a few ATX supplies and they looked like they have a current sense resistor and individual OC protection for each of the "fake" 12V rails. I think it's even legally mandated in some parts of the world.That being said, just see what GPUs do and do the same.