toroidal core for high current
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From: Jenkins on 19 Jul 2010 22:37 On Jul 19, 9:32 pm, "Tim Williams" <tmoran...(a)charter.net> wrote: > "Jenkins" <phreon...(a)gmail.com> wrote in messagenews:3e5cb4e2-a242-461e-8bbc-7fd0fe300363(a)w12g2000yqj.googlegroups.com... > > Just a thought but I wonder if I could parallel several smaller buck > > converters, with or without isolation such as the flyback or fullwave, > > which would allow me to switch them in as I need more current. > > You can. In fact, this is what most multiphase controllers do automatically, with the bonus of phase shift between outputs, which reduces average current ripple. > > If you're making one from scratch, remember to throttle them in parallel, so none are hogging current. > > Tim > That maybe the way to go then. Maybe not as cost effective because of the added parts but probably a bit easier to do if I can get the control issues figured out. Although I don't know if there is a huge difference between doing the 50A and 100A versions. BTW, those should be amps instead of watts ;)
From: Grant on 19 Jul 2010 23:52 On Mon, 19 Jul 2010 19:23:43 -0700 (PDT), Jenkins <phreon111(a)gmail.com> wrote: >On Jul 19, 3:23 pm, Tim Wescott <t...(a)seemywebsite.com> wrote: >> On 07/19/2010 12:01 PM, Jenkins wrote: >> >> >> >> > On Jul 19, 1:12 pm, Tim Wescott<t...(a)seemywebsite.com> wrote: >> >> On 07/19/2010 10:22 AM, Jenkins wrote: >> >> >>> Guys, it should be obvious it was 50khz and not 50Mhz, I made a >> >>> mistake when I was entering the data to paste it in. >> >> >>> Volts In 250 V >> >>> Volts Out 25 V >> >>> Load Current 100 A >> >>> Freq. 50 KHz >> >>> Vripple 0.25 V >> >>> Duty Cycle 10 % >> >>> Ipp Inductor 1 A >> >>> Ipk Inductor 100.5 A >> >>> Irms 99.500418759588 A >> >>> L 450 uH >> >>> C 800 uF >> >> >> And look! The inductor value has changed! >> >> > duh! Did I say they wouldn't? Just curious... have you ever made a >> > mistake in your life? I made a mistake entering the data... get over >> > it. It's not the end of the world. >> >> I wasn't criticizing; forgive me if it came across that way. >> >> And I do make mistakes -- sometimes I think I've screwed up when I >> haven't :-). > >hehe, ok. np. > >> >> >> Regardless, going from 250V to 25V means that the output stage will only >> >> be on for 10% of the time, which is putting severe demands on an already >> >> heavily used inductor. Why don't you want to use a transformer? >> >> > I thought the whole point of smps was reduce the transformer size? >> >> When I and others say "why don't you use a transformer", we mean "why >> don't you use a switching regulator topology with a transformer". If >> you use a transformer with a turns ratio sufficiently less than 10:1 to >> insure 5V out when the input is at it's lowest ebb then you will >> minimize the amount of energy that must be stored in the magnetics, and >> hence their size. >> > >Ok, I don't have a huge problem with it but it seems that it may add >significantly to the cost. I'd prefer the isolation provided by a >transformer but I'd also like to reduce costs. A simple buck converter >topology would work fine except I may have issues with discontinuous >mode which I would like to get around. In fact I need a somewhat >variable supply of ~1A to ~100A. > >I'm not sure I could implement an efficient topology such as Full- >Bridge Push Pull converter at those currents and voltages. The main >issue I'm having is determining the transformer and inductors to use. >The site you gave me shows some cores that should work but I'm not >sure how well as they still seem awful small. I would expect a >transformer to be at least the size of my fist and not one of those >ones one typically sees in PC SMPS's. Of course I may be wrong if the >size is somewhat proportional to the switching frequency. > >Just a thought but I wonder if I could parallel several smaller buck >converters, with or without isolation such as the flyback or fullwave, >which would allow me to switch them in as I need more current. I could >have one 50W, 25W, 10W and 1W. This would require current sensing but >shouldn't be an issue as it doesn't have to be accurate. I guess the >problem here would be current sharing? Possibly not, if you let the smaller ones go into constant current mode at their max output, providing you can setup the voltage feedback to allow bigger ones to shutdown when not required? Some multiphase controllers shutdown some phases when not required to meet output current, needed for example with those modern CPUs go mA to many tens of Amps in microsecs, depending on spot demand. Grant.
From: Tim Wescott on 20 Jul 2010 00:39 On 07/19/2010 07:23 PM, Jenkins wrote: > On Jul 19, 3:23 pm, Tim Wescott<t...(a)seemywebsite.com> wrote: >> On 07/19/2010 12:01 PM, Jenkins wrote: >> >> >> >>> On Jul 19, 1:12 pm, Tim Wescott<t...(a)seemywebsite.com> wrote: >>>> On 07/19/2010 10:22 AM, Jenkins wrote: >> >>>>> Guys, it should be obvious it was 50khz and not 50Mhz, I made a >>>>> mistake when I was entering the data to paste it in. >> >>>>> Volts In 250 V >>>>> Volts Out 25 V >>>>> Load Current 100 A >>>>> Freq. 50 KHz >>>>> Vripple 0.25 V >>>>> Duty Cycle 10 % >>>>> Ipp Inductor 1 A >>>>> Ipk Inductor 100.5 A >>>>> Irms 99.500418759588 A >>>>> L 450 uH >>>>> C 800 uF >> >>>> And look! The inductor value has changed! >> >>> duh! Did I say they wouldn't? Just curious... have you ever made a >>> mistake in your life? I made a mistake entering the data... get over >>> it. It's not the end of the world. >> >> I wasn't criticizing; forgive me if it came across that way. >> >> And I do make mistakes -- sometimes I think I've screwed up when I >> haven't :-). > > hehe, ok. np. > >> >>>> Regardless, going from 250V to 25V means that the output stage will only >>>> be on for 10% of the time, which is putting severe demands on an already >>>> heavily used inductor. Why don't you want to use a transformer? >> >>> I thought the whole point of smps was reduce the transformer size? >> >> When I and others say "why don't you use a transformer", we mean "why >> don't you use a switching regulator topology with a transformer". If >> you use a transformer with a turns ratio sufficiently less than 10:1 to >> insure 5V out when the input is at it's lowest ebb then you will >> minimize the amount of energy that must be stored in the magnetics, and >> hence their size. >> > > Ok, I don't have a huge problem with it but it seems that it may add > significantly to the cost. I'd prefer the isolation provided by a > transformer but I'd also like to reduce costs. A simple buck converter > topology would work fine except I may have issues with discontinuous > mode which I would like to get around. In fact I need a somewhat > variable supply of ~1A to ~100A. You're talking about a 500W supply -- that's not going to be cheap unless you can build them 10000 at a time like the PC manufacturers do. > I'm not sure I could implement an efficient topology such as Full- > Bridge Push Pull converter at those currents and voltages. The main > issue I'm having is determining the transformer and inductors to use. > The site you gave me shows some cores that should work but I'm not > sure how well as they still seem awful small. I would expect a > transformer to be at least the size of my fist and not one of those > ones one typically sees in PC SMPS's. Of course I may be wrong if the > size is somewhat proportional to the switching frequency. The last switcher project that I worked on was a 300W unit that had two transformers, each a cube about 1.25 inches on a side. Yes, transformer size goes down as switching frequencies go up, for the same reasons that inductor sizes do. > Just a thought but I wonder if I could parallel several smaller buck > converters, with or without isolation such as the flyback or fullwave, > which would allow me to switch them in as I need more current. I could > have one 50W, 25W, 10W and 1W. This would require current sensing but > shouldn't be an issue as it doesn't have to be accurate. I guess the > problem here would be current sharing? You could, but you'd have all the same issues with each buck converter as you would with one big one. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html
From: Robert Baer on 20 Jul 2010 01:51 Jenkins wrote: > How physically large does a toroidal core need to be to handle a 100A > smps buck or flyback configuration? > > I used http://www.daycounter.com/Calculators/Switching-Converter-Calculator2.phtml > to calculate the inductance, > > Volts In 250 V > Volts Out 25 V > Load Current 100 A > Freq. 50000 KHz > Vripple 0.25 V > Duty Cycle 10 % > Ipp Inductor 5 A > Ipk Inductor 102.5 A > Irms 97.510683175401 A > L 0.09 uH > C 0.8 uF > > which seems awful small? For a toroid, L = u*(rN)^2/D, and I played > around with different sizes and giving me relatively large > inductance's(much larger than 0.1uH). > > steel core, r = 1cm, D = 5cm, N = 20, L = 700uH. > > It seems that the inductance is pretty low for large currents, > > Load Current 1 A > Vripple 0.25 V > Duty Cycle 10 % > Ipp Inductor 0.001 A > Ipk Inductor 1.0005 A > Irms 0.99950004168751 A > L 450 uH > C 0.008 uF > > So it should be relatively easy to get an inductor for doing 100A's > for a buck converter? Even an air core would work? > > If the calculations above are correct what are the real issued > involved for high current inductors used in smps? Core saturation[how > to calculate?]? Heat dissipation[Need thick wires => physically large > inductor]? High frequency issues? > > Can one buy off the shelf inductors that can handle 100A+ or would > they be difficult to obtain the core material? > > > > Core area is the key; toroid, cup core, e-core, whatever will give the cross-section needed. But. I think you will find toroids cost a lot more.
From: Paul Keinanen on 20 Jul 2010 03:01
On Mon, 19 Jul 2010 19:23:43 -0700 (PDT), Jenkins <phreon111(a)gmail.com> wrote: >Just a thought but I wonder if I could parallel several smaller buck >converters, with or without isolation such as the flyback or fullwave, >which would allow me to switch them in as I need more current. I could >have one 50W, 25W, 10W and 1W. This would require current sensing but >shouldn't be an issue as it doesn't have to be accurate. I guess the >problem here would be current sharing? If you are going to use multiple converters, why not look more closely on some multiphase system, running the separate converters from a single clock with different phase shifts. |