Please assist with the circuitry to make these paralleled Bucks share current? - Page 1
Forum section: electronics / general

author: ocset
author_rank: Super Contributor
timestamp: January 26, 2020, 12:31:30 pm
content: HiThe attached (LTspice sim and pdf schem) is two paralleled Buck converters. Please assist us in making out the circuitry required to make these Bucks share current equally between them.The circuitry shown inside dotted lines is what we have so far. However, we need the sharing to be done over the entire load range of zero to 8A.Clearly what we have here will not do the job over the load range. So we need circuitry to indicate the difference between the load currents, and then accordingly adjust the individual reference voltages into the error amplifiers of each Buck…to make them share current. As you know, the circuitry should reduce the reference voltage of the highest current Buck, so as to make it peg back its current and Vice Versa for the other Buck….to make them share equally.(I wasnt going to post, but then i considered that the answer may be useful to others too.)

author: ahbushnell
author_rank: Frequent Contributor
timestamp: January 26, 2020, 04:08:26 pm
content: Quote from: treez on January 26, 2020, 12:31:30 pmUse current mode control. Look it up.Basically you monitor the current in the inductors on each power supply. There are chips that can do this. by setting the current in the inductor it will share.https://www.ti.com/seclit/ml/slup340/slup340.pdfThis is a TI ap note. I bet they have other ap notes on this subject.Good luck

author: ocset
author_rank: Super Contributor
timestamp: January 26, 2020, 06:26:46 pm
content: Thanks, The thing is, we are considering a way of making totally separate power supplies to share with each other, into a single voltage regulated output…I apologise because the reason we are looking into this isn’t specifically because of the Buck example that I gave here in the top post…its because we wish to do a 12.5Kw, 800V power supply, from a 400V PFC’d output. (input is mains 240vac)....(it would have been too much detail to put that forward in the first place in the top post)...The best way we can think of is to have four sets of 3.15kW converters each comprising an interleaved 3.15Kw Boost PFC, feeding a 3.15kw Dual LLC converter with isolated stacked outputs (2*400v for 800v)……..then we will make the four 3.15kw, 800v outputs go into one 800v output…by use of a sharing technique between the four of them...this is why we are interested in "sharing" techniques.(Basically, we need a 800V, 12.5Kw power supply from a 240vac mains input.)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 26, 2020, 10:40:29 pm
content: Right. Well, industrial modules that are designed to work together in this way, provide master-slave connections, controlling currents so they match.Also have the option of integrating modules more closely, with phase shift, to get phase interleave. This saves on filter caps.Tim

author: IanB
author_rank: Super Contributor
timestamp: January 26, 2020, 11:01:02 pm
content: Also, curiosity compels one to wonder about which setting has a 240 V 13 kW single phase supply as a starting point? In a domestic dwelling this could be available, but unusual, since few things need so much power. In a commercial setting there would typically be a 415 V three phase supply available, which seems more reasonable for such a power demand. (UK mains is invariably provided on the low voltage side from a 415 V/240 V wye connected transformer output. Commercial premises get all three phases.)

author: MagicSmoker
author_rank: Super Contributor
timestamp: January 26, 2020, 11:02:03 pm
content: Quote from: treez on January 26, 2020, 06:26:46 pmCurrent mode control will still allow separate power supplies paralleled together externally to share current equally without requiring any special provisions or communication between power supplies.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 27, 2020, 12:08:14 am
content: Quote from: MagicSmoker on January 26, 2020, 11:02:03 pmIf you don't mind that one saturates to 100% before the next pulls in, and so on -- yes.Mind that it requires power supplies that are rated for continuous operation in current limiting. Which rules out supplies that don't have current limiting (or an accurate limit -- peak current mode flybacks usually aren't great here, and also usually drop to hiccup mode at lower output voltages). Hopefully they also have peak efficiency near 100% load, so that overall efficiency isn't worse.Or the supplies are matched very closely in voltage (manual trimming could be done), and which have a fraction of the output current included in the feedback (error amp), so that the output is resistive rather than constant-voltage. Poorer overall regulation, but more efficient than the old hack of paralleling linear regulators with series resistors from each one.Tim

author: David Hess
author_rank: Super Contributor
timestamp: January 27, 2020, 09:38:51 am
content: It is probably not suitable for your application but with current mode regulators, driving the compensation pin of one regulator with the voltage from the other compensation pin and disabling feedback will force one regulator to track the current of the other. Or maybe just tie the compensation pins together? Linear Technology has some application notes which show this.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: January 27, 2020, 03:26:11 pm
content: Quote from: treez on January 26, 2020, 06:26:46 pmYou can use droop regulation to share. Each power supply lets the voltage drop slightly as the current increases. This looks like there is a current sharing resistor in series but its actually the control system making the power supply behave that way.EDIT:https://www.us.tdk-lambda.com/media/292152/tdk-lambda-blog-072915.pdf

author: fcb
author_rank: Super Contributor
timestamp: January 27, 2020, 04:00:57 pm
content: You want to stack 400V stages to create your 800V and then use two of these stacks in parallel? 2x2?The control could get really interesting as you have to balance the voltage and current in each of the converters (or risk one half of a stack sitting at 300V and the other at 500V). Probably develop this whole thing as a two-phase SMPSU; treating each of the stacks as a single converter (basically parallel upper and lower switching FET/IGBTS drive circuits). You might want some circuitry to check that the caps of each stack don't exceed the Vmax of the capacitor as the thing ages.

author: ocset
author_rank: Super Contributor
timestamp: January 28, 2020, 06:20:54 am
content: Quoteyes you're right, i think just doing it with 4 stacked , 3.15kW, 200Vout LLC converters could be the best way...no paralleling.

author: XFDDesign
author_rank: Frequent Contributor
timestamp: January 30, 2020, 05:02:56 am
content: Ehhh a lot of overcomplication going on here, and usually tree's threads are hilarious on his own part.I make supplies which have variable input voltages and combine to a single output, shared current. It comes down to how you design the blocks. Parallel current sources can work together all day long. Abstract the buck converters as transconductance amplifiers. An error voltage gives a proportionate output current. Move from cycle-by-cycle current mode control and its artificial ramp to average current control, and the whole affair gets easier. With each single converter, have a very small current-sense node and let that be the feedback signal into the compensator. This converter now only cares about maintaining its own current output. Parallel N of these blocks, and you get N times the output current (assuming the source can supply them) for the same input signal. Suppose one converter gave 2A/V in closed loop. Set Vin = 2V, and it will attempt to spit out 4A. Tie 4 converters in parallel, and that 2V will give 16A into a short. These paralleled converters can be modeled as a much larger single gm stage. Now, you just need to design another feedback loop to compensate voltage regulation against the load.The actual art, frustration, and engineering is in the design of those feedback loops and the nuances that go into getting the total system stability. I'll be damned if I share the results of those hours of work here though. XD

author: ocset
author_rank: Super Contributor
timestamp: February 02, 2020, 12:25:45 am
content: Thanks, the attached shows something similar…..a single “master” converter closes its loop on vout. Then many other parallel “slave” converters monitor the current of the master and close their loops on duplicating the master’s current.The slaves should have a crossover frequency at least 3 x that of the master, to stop the feedback loops from fighting each other.

author: ocset
author_rank: Super Contributor
timestamp: February 12, 2020, 08:11:16 am
content: QuoteQuoteThanks, on this theme, we could have eight identical paralleled 1.5Kw two transistor forwards……..and put a current limiter on the output of each one…….so that those that hogged hit their current limit, and then the others start coming in……..”droop” sharing as you kindly describe.So we want 50V at 240A in total…….so for each two transistor forward we would add a 30A current limiter to its output and then the whole eight of them would share with no other circuitry required…and their feedback loops would not fight each other....what do you say?

author: ejeffrey
author_rank: Super Contributor
timestamp: February 12, 2020, 05:33:54 pm
content: That's not really what he meant by droop sharing. Add a small current sense resistor to each output with a high gain current sense amplifier that biases the feedback node. Then you get a power supply with a larger output resistance but with much less wasted power.

author: fcb
author_rank: Super Contributor
timestamp: February 12, 2020, 06:15:44 pm
content: Quote from: treez on February 12, 2020, 08:11:16 amI thought you wanted 12.5KW at 800V, not 50V??

author: ocset
author_rank: Super Contributor
timestamp: February 12, 2020, 08:45:05 pm
content: QuoteThanks, sorry, its been changed.........we now have an 820VDC source, and we need a 50V, 240A output........the 820VDC source is at the other end of a 9.5 ohm cable (go and return)

author: uer166
author_rank: Super Contributor
timestamp: February 13, 2020, 01:14:10 am
content: Is this for that tethered drone thingy?

author: ocset
author_rank: Super Contributor
timestamp: February 13, 2020, 07:36:40 am
content: Hi, Yes it is indeed...."motorised kite".