2kw dc/dc converter

Discussion in 'General Electronics Chat' started by Huby, Jul 5, 2012.

  1. Huby

    Thread Starter New Member

    Jul 5, 2012
    24
    1
    Hi.
    We are building one relatively strong dc/dc converter.

    This are the parameters:
    Input voltage:72V
    Output: 400V
    Power: 2kw
    Transformer: toroid
    --------------------
    I dont know what we are doing wrong but this is the situation.
    When the circuit is in idle mode all is working fine, but when i set the voltage up, block is activated and circuit stops to work. I have to press reset button to activate it. One more thing, the circuit is working only if i press the reset button. Could the LM393 be the problem ?
    I changed the capacitors but without success.

    Schematics is on the link.
    I hope that someone can help me out with this one.
    http://oi49.tinypic.com/111pcsi.jpg

    Huby
     
  2. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
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    I don't know who came up with that schematic, but you need to hire a power supply designer. There is no way that could get anywhere near 2kW. The two transistors specified are 8A devices. That whole design is non feasible: do a simple power calculation: output of 400V/2kW is about 5A. On the 72V input side of the transformer, the current will be about 35 - 40A. You need that much switching horsepower on the input side and a transformer which can handle it.
     
    Last edited: Jul 5, 2012
  3. takao21203

    Distinguished Member

    Apr 28, 2012
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    You need transistors with at least double Amps specification.

    If you can archieve 80% eff. your heat developement will be 400W, quite a challenge. The AUX supply using a shunt transistor is not very appreciate.
     
  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,974
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    where is the rest of the circuit??
     
  5. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
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    The power transformer and output stage is not shown. I assumed the two switch transistors go to the transformer primary labeled "OUT 1", "OUT 2", "OUT 3"..

    But I agree, most of the circuit that would actually represent a 2kW converter is not there.... I actually designed a couple of them and that one surely won't get there.:D

    There's a whole lot more involved.......
     
  6. praondevou

    AAC Fanatic!

    Jul 9, 2011
    2,936
    488
    Mmh, a 2kW push-pull? Seems not to be the appropriate topology to me. And why is Out2 connected to GND, maybe it's not a push-pull after all?

    Is the 555 supposed to be a pulse-by-pulse current limiting? Is it good enough to prevent flux-walking?

    As far as I remember THR to 12V resets the outputof the 555, therefore with IC7C IC9A's output is enabled (after power reset delay) and then also the pulses to the 2 gate drivers. Maybe S1 is a NC?
     
  7. Huby

    Thread Starter New Member

    Jul 5, 2012
    24
    1
    Now i know that am on the right forum !
    Thank you for your reply's.

    I will describe the whole project and i really hope that, with your help i will finish this converter. The orginal idea is to create 8/16kw converter but i was thinking to start with some basic idea and then to expand this to some bigger design. If you are willing to help we can start immediately with the real version.

    Transformer should run on 200/400 Hz. This one is quite big and he can easy work on this frequency and he can easy give that much power. We tested on 800Hz and it worked without problem.

    Our first design was giving better results and we could slow down the protection to 500ns but when we connected the transformer over current was activated and we stuck again in the same problem. The capacitor didn't discharge....
    So we ware thinking, ok let's make new board with LEM then we can avoid some startup problems. It didn't change much. The idle mode is still working perfect until we connect the transformer.

    This was our first design:
    http://oi48.tinypic.com/2py11l0.jpg

    Because we are spinning in circle with this design i would accept any idea on changing the elements / project plan. If we have to start over from new, ok i will but i have to have some insurance that the new design will work.
    Someone wrote "where is the rest of the circuit", well the presented one was the whole circuit.

    Please help me out with this and let's try to make some basic concept.

    Huby
     
    Last edited: Jul 6, 2012
  8. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    How big IS this transformer? I remember a hard-switched 2.5kW DC/DC converter with an E65-core connected to an H-bridge, this was switching at 30kHz...

    I imagine a 2kW transformer switched at 200 to 800Hz to be much bigger than this.

    So why this low frequency?
     
  9. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
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    I don't think you have the basic fundamentals to do this. As I said before: you are trying to get 2kW of load power (actually, you now say that you want 8/16 kW of power) from a circuit topology that can not possibly work. let me repeat, it can NOT POSSIBLY WORK to generate those power levels.

    As I said before: do a basic power calculation. You initial design spec is to use 72V input. To get 2kW, that would mean at least 30A of current (perfect efficiency) in reality at least 40A peaks through the switch transistors. To get 8kW, the input side current levels would be about 160A.

    The design you illustrate in the schematic is not feasible. It can not work, it can not be made to work. If you don't see why, I respectfully re suggest that you need to get a power supply designer. Anyone with such experience would know in two seconds the topology you are pursuing is not feasible.

    I actually designed several 2kW power supplies back in my career. The input power was derived from 220VAC creating a 300V DC source and followed by a full bridge (H bridge) converter.

    We did build one 10kW supply which strapped four 2.5kW converters in parallel all operating in constant current mode (using a current mode controller). To be honest, that one never worked right. The control loop could not be stabilized over the full operating voltage range.
     
  10. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
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    Sounds similar to what I used. I think that transformer was approximately the size of a softball. I know it fit into a 5" rack chassis, but could not go into the 3.5" chassis. Standard ferrite core.

    Just bring the fork lift.....
     
  11. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
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    I have to ask:

    Was this design "borrowed" from a different lower power product with the belief it could be made to work at 2 kW?

    It looks like something that might work at a few hundred Watts.... maybe.
     
  12. takao21203

    Distinguished Member

    Apr 28, 2012
    3,577
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    Not so heavy...500VA toroid = 4kg.

    When I built some switching circuits, and tested them with 12v 50W bulbs, two bulbs in parallel would instantly fry the MOSFET.

    For a converter that is meant to be for 5 Amps but has capability for 12 Amps permanently, I use a 32 Amps power PNP.

    For instance to deal with large inrush currents.

    For 2kW you need a transistor bank- not just to deal with the generated heat. The inrush currents will be much higher than 30 Amps! So if you use inappreciate cable and then startup a large motor, this can and likely will cause a fire risk.

    To deal with this further more information is required:

    -The complete setup
    -400V AC or DC? One phase or more?
    -Where does the schematic come from? What is modification track record?
    -What is the reason not to use IC but to build it with discrete components?
    -Why it was not tested at lower levels for correct functionality? And if, what was the point where it stopped working correctly?
    -List also if possible complete testing track record.
     
  13. takao21203

    Distinguished Member

    Apr 28, 2012
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    See my questions, if they can become answered, it is much easier to help this problem further.
     
  14. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Even if that's true, it means 2kW is 36 pounds assuming no losses, about 45 pounds in real application.

    I'm not lifting it.

    It's still ridiculous in my opinion when there are much easier ways to do it. This thread is a waste.
     
  15. Huby

    Thread Starter New Member

    Jul 5, 2012
    24
    1
    Thank you for your wish to help !

    Transformer details:
    - toroid core - M4 metal
    - dimensions: 384/200/40mm
    - induction B = 1,5T
    - voltage 72V/400V
    - frequency 200/400/800Hz
    - current 125A/20A
    - power 9.000VA
    --------------------------------
    This transformer was used for some other things and we bought it because that was the cheapest solution at the time. Nevertheless i have place so the dimensions are no problem.

    To answer your questions
    - the idea was "borowed" from 800W DC/DC converter so we was thinking that we can adjust the parts to suit our needs.

    -400V AC or DC? One phase or more?
    ** DC, one phase

    -Where does the schematic come from? What is modification track record?
    ** we build the schematics on our own with some lower power designs and from
    examining others dc/dc converters.

    -What is the reason not to use IC but to build it with discrete components?
    ** there was no particilar reason. The idea was to build some cheaper solution that should give good results.

    -Why it was not tested at lower levels for correct functionality?
    And if, what was the point where it stopped working correctly?
    ** this is the part where we are now and it stoped to work when we connect the transformer.

    -List also if possible complete testing track record.
    ** i have to collect this data, but i can pass it tomorrow.

    --------------
    I have no problem to change the design but i need some starting point and we want to make it on our own and of course with your help.

    Huby
     
    Last edited: Jul 6, 2012
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