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20-60A adjustable ~200vdc current mode buck design

Discussion in 'Electronic Design' started by John Barrett, Feb 19, 2007.

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  1. John Barrett

    John Barrett Guest

    I'm looking to build a power supply for a krypton arc lamp, and need a
    design that can deliver an adjustable 20-60 amps into a LOW resistance load
    (around 5 ohms). I'm a programmer for a living and this is my first power
    electronics project (previous electronics projects have mostly been
    microcontroller based stuff)

    I've done quite a bit of reading, but am running into a brick wall scaling
    up some of the various designs that I've found online. I'm working with
    LTC's SWCad3 to test concepts, but I'm having problems with input currents
    spiking up way too high since my target output is so close to my current
    source (AC 220v 60A single phase rectified to 320vdc), and the 120hz line is
    just too slow to keep the input capacitor bank charged under heavy load
    without some serious high current spikes to recharge.

    Any suggestions to smooth out the current draw from the line would be MUCH
    appreciated !!
  2. John,
    such a power supply is rather tricky. The power
    level is rather high, and the nonlinear negative
    dynamic impedance load doen't make it simpler.
    You'll have to use a switching approach.
    Not recommended.

  3. Guest

    Must agree here.
    At this power level , I'd recommend you get a professional power
    supply designer
    to tackle this.
    DNA , you around :0)
    If you still decide to tackle this yourself , I recommend some eye
    protetion of some sort
    and prepair yourself for lots of bangs and smoke :0(
  4. Guest

    The old-fashioned

    ----------- ~ + ------ L ------------ +
    bridge ===
    ----------- ~ - --------------------- -

    'L' smoothes the charge peaks, needs an air gap, and will
    basically be a very heavy big piece of iron with copper.

    The output of this circuit is somewhat lower then 220V DC,
    so more current.
  5. John Barrett

    John Barrett Guest

    Ohh I'm quite prepared for a bit of mess :) Even got a few spare arc tubes
    just in case :) And a welding helmet for eye protection :)

    As stated... I'm planning a current mode buck regulator, though it has been
    suggested that I'll need a boost circuit in front of it to smooth out the
    line input current surges (to push up the voltage so that charge is
    transfered into the caps more effciently ??) . I've already got a spice
    model created that is close (need to work on the control loops a little
    more, and a better soft start circuit., and beat these 120+ amp surges that
    I'm getting)

    "nonlinear negative dynamic impedance load" -- I'm not an EE so I could use
    a bit of explanation I'll probably need to understand that as I'd like to
    create a spice model of the arc tube for the later stages of testing.
    (resistance decreases as current increases ?? am I even close ??)

    (cummon guys !!! so far everyone who has responded, here and in alt.lasers
    [because this is for a KW output ND:YAG laser] has had nothing to hand out
    but warnings !! I'm committed, it will get built !! )
  6. bernado

    bernado Guest

    IPM - International Perforation Management
    high-tech engineering China-Germany-Thailand
    fax : 0049 - (0) 1212-5-375-17-531
    Email : -

    We are using AC/DC converters and special DC constanter with 300 V/DC
    30 up to 60 Amps for nano or micro perforation machines since more
    then 20 years.

    There are several companies very well equiped - have a look to or

    Such AC/DC STABLIZED power supplies are to purchase by around 4000 -
    6000 bucks by one year guarantee so that it makes not sense to invest
    several 10,000 bucks in developments for NEW primary AC switching and
    secondary DC regulated units.

    Let me know if you need more or specific details - I'm glad to help
    and support you.
  7. John Barrett

    John Barrett Guest

    The only detail that could possibly help me at those kinds of prices is "do
    you give free samples ??" :) :)

    I makes very little sense to invest that kind of money when I've got less
    than $500 into this laser so far, and I dont think its going to cost me more
    than about $1000 more to bring it online. I'm not being funded by a
    corporation with deep pockets :) In any case, for that kind of money, I
    could afford to pump this rod with 808nm laser diode bars and skip the
    massive current requirements that the krypton arcs are forcing me to deal
    with. ($6,000 times the 3 supplies I need pays for a LOT of laser diode bars

    And I seriously doubt its going to cost me 10K or better to to design my
    own -- even if I count my time invested -- I'm pretty close to something
    workable now with less than a week invested, and between the folks here and
    what I'm finding online, working out the rest of the issues shouldnt take
    all that long. And besides -- I get to learn something new in the process !!

    [Flame On]
    So save the commercial hype for someone that doesnt want to be bothered with
    the nitty gritty details !!
    [Flame Off]
  8. Guest

    dude, your insane trying to build a 6Kw. lamp supply from scratch,
    those lamps blow if ya sneeze.
    I know I just blew one trying to build a 2 kW version. I have one

    contact bruce @ contact @ second source lasers do com, he has two
    eski lamp supplies in stock. BTW the series 6 pins and two connector
    bodies you asked for shipped yesterday.

    Steve Roberts
  9. John  Larkin

    John Larkin Guest

    Three phase AC!

    But what you're doing is like deciding it would be cool to climb a
    mountain for the first time, and buying a ticket to Nepal.

  10. D from BC

    D from BC Guest

    It's a mountain all right..
    I'm very new in smps..
    From near scratch, my little 130Watt (90% eff) converter project has
    gone on for > 2 months.
    I had to read on:
    mosfet switching losses
    trace inductances
    fast power diodes
    feedback control theory
    control topologies
    smps simulation
    paralleling power devices
    reducing EMI
    ground planes
    capacitor losses
    magnetics design
    line filtering and ov protection

    But, I got it working. :)
    Just have to fix some ringing....
    smps: 1 quality of life: 0
    D from BC
  11. Exactly.

  12. That's a mere foothill, and a small one at that.
    But certainly keep on climbing. :)
  13. John Barrett

    John Barrett Guest

    Thanks for getting those out, Steve !!

    and OF COURSE I'm insane.. even more so than you think because I need 8KW !!
    (7mm ID bore needing 195-205 volts !!)

    However -- the design is progressing -- I've got a model up and running that
    (without current regulation) stabilizes at a very smooth 48 amps into the
    tube (presuming 5 ohms tube resistance) after startup... its got a bit more
    ripple voltage than I would like, and its spiking the AC input hard, but at
    least I got the soft start and main current path worked out

    having a real problem with the current sense -- trying to use the switch
    model to trigger when the ISense voltage passes a preset threshold, but I
    must have it modeled wrong some how because it is not switching when the
    tube current spikes. (I know -- its a kludge, but I'm too lazy to add in the
    full pwm driver until I got everything else close to working as intended)
    I'll prolly see what I can do about putting at least a comparator in and see
    if that helps
  14. At this high output current, I would probably thinking about
    a 2 phase parallel current mode output stage (or pair of
    stages. This is a fairly simple way to divide the current
    load equally between two parallel paths, while having
    something like 80% or 90% of the current ripple cancel. Toy
    can work with one of the halves, till you pretty much get it
    working at half current and then build a second one. The
    trick is to come up with the dual complementary triangle
    wave to drive the error amplifiers, so that the output
    pulses are always almost precisely 180 degrees out of phase.
    I have made fairly high current, low ripple current mode
    laser supplies this way, though the voltage was much lower,
    and the load better behaved.
    I think you may need a more continuous current sense, rather
    than a peak trip.
  15. John Barrett

    John Barrett Guest

    I know I need continuous feedback -- but I'm still trying to get a handle on
    the main current path before I add complexity -- just wanted something
    simple in there to provide SOME regulation so I'd have an idea if I was
    getting close.

    I have made a deciscion over the last 24 hours -- I'm going with a 2 phase
    boost front end for active PFC and an internal 400v DC bus. One phase will
    be a non-inverting boost for the positive halves of the 2 line inputs, the
    other will be inverting for the negative halves, eliminating all the
    problems that I'm having because my negative input to the buck is a half
    wave pulse instead of a solid ground.

    I'm gonna spend today modeling the non-inverting boost supply.. see if I can
    get that working -- nice thing is -- it doesnt have to be strongly
    regulated... -- the not-so-nice thing is I'm going to have to do the full
    PWM setup because crude regulation methods wont work to drive the boost

    hehehe at this point I would kill for a PWM controller spice model generic
    enough to be used for this :) Gonna have to search around and see whats out
    there !!
  16. Genome

    Genome Guest

    John was talking about interleaving the buck stage but interleaving the
    boost stage wouldn't hurt either.

    Here you go....

    It's supposed to model a UCC3817....

    Actually it's not really that super brilliant but you'll get the idea.

  17. John Barrett

    John Barrett Guest

    Thank YOU !! Very much appreciated !!

    I was getting a little put out with half a dozen people telling me

    Thanks again for what I came here for -- a little education on SMPS design
  18. Genome

    Genome Guest

    Oh..... OK, fiddle fiddle fiddle. Try one of these,

    That's two stages operated in parallel with the clock waveforms shifted. The
    idea is sort of that the current error amplifiers receive the same demand
    signal from a single or master voltage error amplifier and program the same
    current in their respective power stages. You have to figure out how to tie
    the available parts in the ICs together to do the job.

    One thing you are forced to do is use current transformers to recreate the
    inductor current. At high powers this would be done anyway but at low powers
    you would use a sense resistor in the return lead to the input rectifier.
    Unfotunately when you try to operate things in parallel you can't do that
    any more.... they just won't connect together proper like.

    I've added some source sense resistors to the 'mosfets' but not implemented
    a current limit with them. Problem is that the drain current transformer is
    operated at high duty cycles and might saturate. You lose the signal and
    things go bang. Resistors don't, lose signals, so it's a kind of 'oh shit!'

    The peak limit is simplified a bit. It really needs slope compensation but
    the IC does not make that easy to add, that's why those transistors were in
    the previous one. Here I've just thrown in some appropriate current sources.

    I've diddled it to do 1KW per section from a 220V line..... I suppose you
    will need 8 of them because you are some sort of cross dressing freak. There
    are ways of synchronising these ICs by shoving a pulse up their oscillator.
    I'm sure if you read some data sheets you'll get some ideas.

    In the old days a 4017 Johnson Counter clocked at N (stages) times the
    desired base frequency with the reset pin on the appropriate tap did the
    job. These day I suppose you would use a PIC.

    Really all you are getting is something to practice your sums on. You will
    probably find some of the values I've left in there are sub-optimal. You can
    try putting in 'real' components to get some guesses about power losses. I'd
    recommend you grab hold of some SiC diodes for the main boost diodes.

    Have Fun and try not to go too fruit pie.

  19. John Barrett

    John Barrett Guest

    I've already located inexpensive isolated current sensors with models for 50
    or 100 amps and 3000v isolation, so I can easily do high side current
    sensing -- just gotta do a spice model for it, which looking at your models
    has helped me figure out (a voltage source defined via a formula referencing
    the load current)

    I'm seriously looking at the PFC chip you mentioned, and some others (I've
    already tried to model out one using the LT1248 but its not working right

    You want fruit pie ?? here is fruit pie -- what would it take if I wanted a
    true earth ground on the output of this mess -- I've considered running
    seperate boosts on the poitive and negative outputs from the bridge, using
    the isolated current sensor to get EVERYTHING out of the ground path.. one
    of them inverting to get a positive voltage (add more in pairs if 2 wont get
    me enough current at reasonable ripple)

    is that fruit pie enough for you ?? or is there a better way ??
  20. Genome

    Genome Guest

    Ermmmmm...... Current sense transformers, something like

    Each PFC section needs to control its own inductor current otherwise they
    won't share the total load. It's tied up with the volt-second balance,
    inductors integrate the voltage placed accross them as the current through
    them. If there is a mismatch in the drive to the inductors then one of them
    will hog the current.

    The local current error amplifiers force them to share it but you need
    separate sense signals for each stage so one honking big current sensing
    thing on the input won't do. You also need a reasonable frequency response
    from the devices if you want to compensate the current loops correctly.

    Ahaaaa, LT1248, shows you how crap I am for not checking if they did one or
    if LTSpice had the model. I knew they did but there is something wrong with
    my head. I suppose I'm fixated on TI because I grew up with Unitrode. It's
    all much of a muchness and once you get happy with one you can see that a
    lot of the internals are the same in all the others.

    If you really really need to ground the output of your beast....... Uhm,
    right, your side of the pond has strange electricity with something like
    110V balanced either side of ground. My head hurts but I think you might
    only get away with it if you use half wave rectification which is going to
    be dirty and waste half of the capacity of your supply.

    Since you seem to enjoy pain you might as well go the whole banana and
    dangle a transformer isolated coverter of the end of your PFC stages.
    Something like a half or full bridge. That will make any (well most)
    concerns about earth disappear.

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