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12V / 6A Power supply. Help with Buck and Boost converters + OR'ing circuit

Discussion in 'Power Electronics' started by esm., Jun 7, 2012.

  1. esm.

    esm.

    26
    0
    Feb 19, 2012
    Hello all.
    Maybe someone here can give me suggestion of ICs and make considerations about this design.
    The picture below states how this power supply should work.


    [​IMG]


    Details:

    1) IC 1 probably will be a LT3650-8.4 from Linear Technology. It supports up to 32V of input voltage (40V peak). Suggestions of other ICs for this purpose?

    2) The power supply INPUT has only two states: (A) Floating (cut-off) and (B) 17 to 30VDC present at the input.

    3) The OR'ing circuit at the OUTPUT, as the picture shows, doesn't mean it will be done with diodes. It can be done with MOSFETs, relays, or any kind of logic control like, for example: When the INPUT is connected to a 17 to 30VDC PSU, a logic LOW is present in an ENABLE pin of the BOOST converter, so that this remains in idle state (Output in high impedance). When there is no power supply connected at the INPUT, the same ENABLE pin goes logic HIGH, so the BOOST converter goes to active mode and presents 12V at its output.

    4) The intention of using batteries is, case the INPUT be floating, maintain the circuit connected to the 12V OUTPUT working for at least 10 minutes (If more, better), through the BOOST converter

    5) Which BUCK and BOOST controllers could I use for this design (IC 2 and IC 3)?

    6) How can I do the Or'ing circuit between Buck and Boost controllers? Some suggestions?

    7) There are no restrictions if heatsinks are required. No restrictions with the final physical dimensions of the PCB. Just need to work.

    8) No problem if external MOSFETs are required, both on Buck and Boost converters

    9) Final cost of this PSU is not so important.

    10) Preference to SMD packages in all the circuit, if possible.

    11) The 12V OUTPUT can be designed to deliver more than 6A. No problem about that.

    12) No need of insulation (Flyback / forward topology, for example). The topology can be modified to Flyback / Forward also

    Maybe a Power supply like this already exist in the market (a final product I mean), meeting this specifications, or even better than, ready to use. I did some research about but didn't found any models.

    For the time, all I found is a Buck converter from Linear Technology, based on a LTC1624. Example of a 12V x 10A output with Vin range of 13 to 28V using this converter:

    [​IMG]


    Somebody help me ?
    Thanks in advance.
     

    Attached Files:

  2. BobK

    BobK

    7,601
    1,643
    Jan 5, 2010
    If you are expecting to get 6A at 12V from two 4AHour 3.7V lithium batteries, do you realize that they will need to put out over 10A and last about 24 minutes? Also, what is the maximum discharge rate for the batteries? I expect you are exceeding it.

    Bob
     
  3. esm.

    esm.

    26
    0
    Feb 19, 2012
    Hi. Thanks for you observation.
    I noticed the high current consumption from the battery now.
    This was just my initial idea
    Yes the output can vary 400mV, this is OK.

    There will be 4 devices connected to this 12V output, each device as a load of 1.5A, aproximately.

    Maybe I will need to limit the OUTPUT of this power supply, to supply power just to one device or maybe to two of these devices when running the Boost converter. So the current consumption would be 50% to 75% lower when running with the battery.

    I am trying to find a solution for this case yet, its not easy.

    I don't even know which Boost and Buck converters to use in such case
     
  4. esm.

    esm.

    26
    0
    Feb 19, 2012
    I would like to ask the users from the forum if they already used such kind of controllers, a particular buck or boost IC, similar to the ones I would need to meet the requirements of my circuit, and if the results were satisfatory.
     
  5. gorgon

    gorgon

    603
    23
    Jun 6, 2011
    From what I have read about Li-ion charging, I think the charger will not function with the load of a boost regulator messing up the charge cycle.
    The best would be to turn off the boost switcher until the main voltage pass a threshold, and then start the switcher.

    TOK ;)
     
  6. esm.

    esm.

    26
    0
    Feb 19, 2012
    Hi.
    thanks for the help.

    I can do what you said easily. When the input voltage is above 10V (example), the boost controller is placed in a off state. I can use a comparator to do that (maybe with some hysteresis also), or a simple circuit based on zeners and transistors to detect a threshold.

    But to do that, the boost IC must have an enable/shutdown control pin.

    First, I would like to make a list of boost and buck controllers that meet the circuit needs, then study the datasheets and choose one.

    It is always good to hear from someone who tested an IC in practice and how were the results. I am trying to get some recommendations.
     
    Last edited: Jun 9, 2012
  7. esm.

    esm.

    26
    0
    Feb 19, 2012
    I was rethinking about this design.
    The output current will be reduced to 4A at the maximum load, not 6A as the initial idea
    I think now I can have a single solution based on a buck-boost topology, right?
    My only experience with a buck boost IC was the TPS63000 from Texas, but it doesnt fit the requirements of this design.
    Some other recomendations of buck boost ICs now?

    Best regards
     
    Last edited: Jun 15, 2012
  8. khankll

    khankll

    110
    0
    Feb 6, 2011
    ..........there is no 18650 with capacity above 3000mAH.. those by panasonic whpo call themselves 3400mah are discharged well below normal voltage of 3.7 volts..
     
  9. gorgon

    gorgon

    603
    23
    Jun 6, 2011
    You should look for a SEPIC switcher in the case where you need to have both a buck and a boost regulation.

    TOK ;)
     
  10. esm.

    esm.

    26
    0
    Feb 19, 2012
    Did you had experience with some SEPIC / BUCK-BOOST switchers compatible with this design? The output current of this design will be reduced to 4,5A MAX, I think.
     
  11. gorgon

    gorgon

    603
    23
    Jun 6, 2011
    I've used the Webench here http://www.ti.com/ww/en/simple_switcher_dc_dc_converters/index.html

    One chip that came up is LM3481.

    I've never done a SEPIC design here, but several other designs.
    I tried to use your parameters for a design, tuned to max efficiency, and got a complete design in a .pdf file. I'll try to post it and see how it comes out.


    TOK ;)
     

    Attached Files:

  12. esm.

    esm.

    26
    0
    Feb 19, 2012
    Hi.
    Thanks for the help, i am checking this now...

    Have you ever seen a single Li-ion charger IC, capable of charging 4 Li-Ion cells connected in series? In a single chip?

    For this design, maybe will be better to stack 4 Li-ion cell in series, because the output power will cause I high currente to flow from the batteries and the circuit is running from them.
     
    Last edited: Jun 18, 2012
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