Buck Converter Design

Discussion in 'General Electronics Chat' started by hazim, Dec 31, 2011.

  1. hazim

    Thread Starter Active Member

    Jan 3, 2008
    419
    13
    Hi everybody.

    I haven't visited the forum from a long time, hope you are all fine.
    I want to design a step-down DC-DC converter with input voltage range 24V~32V and any output between 20V and 24V, say 22V assuming a dropout voltage around 1V. The output current is actually high, it is 5A at minimum, and may be I'll need it up to 10A.
    For this high current I think I'll need a buck controller IC and use external MOSFETs.
    I don't want to use expensive parts (ex. not more than 5$ for the IC). Also I'm limited with the availability of parts here. There are 10A Simple Switcher power modules that my fit my need, but these aren't available here.
    "The newest additions to the SIMPLE SWITCHER® power module family feature load currents up to 10A, frequency sync, current sharing, and pin-to-pin compatibility to deliver an entire portfolio of ease-of-use solutions."

    See this reference: http://www.ti.com/ww/en/simple_switcher_dc_dc_converters/index.html

    The efficiency isn't very important in this case, but I don't want to use linear regulation!

    What do you recommend?

    Thank you and Happy New Year!
     
    Last edited: Dec 31, 2011
  2. CraigHB

    Member

    Aug 12, 2011
    127
    15
    Did you look at this section? They probably offer something that would fit the bill. The plug-in modules are most simple to use if you can find one that will work for you. Dont' know if that meets your price and availablity, but that would be my first choice.

    Otherwise, if I had to build one myself and I wanted external switches, I'd go to this section to find a suitable controller.

    BTW, I like the TI parts a lot. Great documentation and support.
     
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  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Hello again Hazim,

    You started telling me awhile ago that you have these regualtor ICs on hand:
    KA7500B
    UC3842
    KA3525
    SG3524

    You also sent me to your local vendor's website (which had a TERRIBLE layout), and I started to catalog all of the transistors/MOSFETs that you could purchase via their website, because it was so difficult to figure out what was what. I actually went through 139 out of 189 pages, copying all of the part numbers - but I got distracted.

    Anyway, I am attaching the list I have compiled thus far; pages 1-100 and 150 thru 189.
     
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  4. hazim

    Thread Starter Active Member

    Jan 3, 2008
    419
    13
    Thank you. You made a great effort and spend a lot of time collecting the available transistors part numbers... so thank you again, This helps me in the future too.
    Right, I have those ICs, and we know the available transistors, where to start? we need any buck converter design using one of those ICs. I found this one in KA7500 datasheet:
    [​IMG]

    Actually I don't know how to determine the value of the coil and what transistor is good here, I prefer using a MOSFET and not a BJT.
    Here are the available coils: http://www.ekt2.com/ekt/prodList.asp?idCategory=675

    I want the converter to supply up to 8A or even more, and want it to function normally if the load was light too, like 1A.

    Regards,
    Hazim
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    The KA7500B is a pin-compatible alternative to the TL494.

    EKT doesn't have much of a supply in the way of inductors/transformers, and the only transformers they have, the only specifications they give are "BIG" or "VERY BIG", which is very useless!

    I think this topic is a continuation of this thread:
    http://forum.allaboutcircuits.com/showthread.php?t=60986
    unless I am mistaken?

    In that thread, you were saying you needed in the neighborhood of 2-3 Amperes; and now it's 8 Amperes. You should consider what you really need to get out of such a converter/regulator, and then add 20% as a safety margin. You don't want to be operating at 100% capacity of anything.

    You need to tell us what your load actually requires. If you over-state your requirements, your converter will wind up being more complicated and expensive than it could be.

    You are going to need inductors that are capable of over 2x your desired output current. Since you are now talking about 8A output, that means you will need at least 3 inductors rated at 8A in pararallel, since at this point, 8A is the largest capacity they seem to stock.

    Three inductors at 100uH in parallel works out to be 33.3uH.

    If this were strictly a buck converter, you could get by with just those three inductors. However, it needs to be a buck/boost or SEPIC - as you need at least 12v-13v output, and the input will drop at least that low. Unless you can add more windings (double the number of turns) to these 100uH inductors to make them into transformers, you will need to get 3 additional inductors the same size (total of six), and use capacitors between them to transfer the power.

    I have not worked out the details yet; I'm talking more in generalities than specifics.
     
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  6. hazim

    Thread Starter Active Member

    Jan 3, 2008
    419
    13
    There is no specific load to be used, actually the circuit will be used for internet wireless devices (access points...) and so, and there my be one, two, three, four, or five devices that to be connected to the circuit. I will make several circuits for several stations, each station contains some number of devices. I'll make sure how much exactly each device drains current at 24V to know what is the maximum current needed, and then get back to this thread. We may design say a 4A version, and an 8A one, but first let me know how much the loads consume exactly.
     
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