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12V buck converter with decent eff?

Discussion in 'Electronics & Electrics' started by RussellK, May 2, 2018.

  1. RussellK

    RussellK Member

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    The other day Bunnings had Ozito 18V 4AH batts for $39, so I grabbed a few with a view to using them for 12V LED lighting - they're a much handier form factor than the 12AH gel cells I've been using.

    I tried an Ebay LM2596 buck converter module that I had laying about and did a bit of bench testing.

    Dropping the battery's 23V to 12V with a 0.6A load gave 74% efficiency, rising to 78% with a 1.1A load. Neither figure seems particular stellar, and means I'm losing around a quarter of the pack's 72WH capacity.

    Anyone know of a better efficiency converter for this voltage range?
     
  2. pantner

    pantner Member

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  3. merlin13

    merlin13 Member

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    If the OP is using any 5 metre LED strips with 5050 LEDs on them then those those MP2307 regulators might struggle a tad.

    I've got a few 5050-sized Cool White ones (instead of the 'effing Greens that I ordered...) that eat well over 2 amps at 12 to 13-and-a-bit volts supply.


    OP - too lazy (and on too many meds atm) to out to the 'shop and set up a test myself, but any chance you've measured efficiency on that LM2596 at a supply voltage well below 23 DC?
     
  4. OP
    OP
    RussellK

    RussellK Member

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    Cool - so damn cheap I'll order a bunch of both and see how they perform.

    I did find these things: https://au.rs-online.com/web/p/non-isolated-dc-dc-converters/8111426/

    Which claim 96% efficiency for 24V in 12V out at 1A, but they're 22 clams. That said, for the application, that efficiency is worth the money, although 2A output would be more useful.
     
  5. OP
    OP
    RussellK

    RussellK Member

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    I'm powering 12V downlights, which are handy for the application, as they're easy to buy (Bunnings) and have useful colour temperatures and beam angles. Obviously there are some losses in the equation, including a diode bridge as the lights are designed to use AC, and whatever LED driver is inside them, but they work well for an exhibition/event stand, and the battery setup saves having to shell out the exorbitant fees that they charge for having a mains supply.

    As for testing, I used the Ozito 18V batt, which at this low load and freshly charged, measured about 23V. I could test the LM2596 at a lower input voltage, and maybe it'd be more efficient, but I'm looking for a solution for these Ozito batts.
     
  6. RobRoySyd

    RobRoySyd Member

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    Have you looked at the LM2596 spec sheet?
    Admittedly buck/boost converters aren't my strong point but I seem to recall there's optimal external component values for a given input/output voltage and current. This might be worth looking into before jumping to another chip. If you've got a variable power supply it could be revealing to plot the performance over input voltage.

    Relevant discussion here: https://electronics.stackexchange.c...ieve-the-max-efficiency-from-a-buck-converter
     
  7. OP
    OP
    RussellK

    RussellK Member

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    I did have a look at the datasheet, which sent me looking for Ebay modules with a fixed 12V output rather than the variable version, in the hope that the fixed one might have optimal components. That said, the datasheet seemed to suggest that with a 18-23V input and 12V output, the adjustable version should be able to do around 90% at 3A. A you say, I might have to spend a bit more time with module I have and see if I can get better performance out of it.
     
  8. merlin13

    merlin13 Member

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    Other trick might be to look at going the other way perhaps - stick two LEDs in series and fiddle with an appropriate converter up to a stable 24 volt output?

    Or if the peak voltage rating on the actual LEDs you've got there is kind, you could even go really nuts and simply chuck a PWM generator and simply average out the ON time to give "Something That Works". Done this before back when serious output LEDs were first released in the wild (late 80's) for a couple of specific lighting applications...

    This'll also be assuming the slightly unstable light spectrum over time as the battery volts drops doesn't affect your actual application of course.
     
  9. mtma

    mtma Member

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    The LM2596 is a design that was borne out of an era where P-MOSFETs were far more expensive for the same capabilities, and thus Natsemi's designers used a BJT series pass element and not only that but an emitter follower, which for various reasons injures the efficiency: particularly so at lower operating current levels. Some newer IC's do feature inherent advantages in this aspect.

    Secondly, the ebay "It's cheaper than what I could buy any one component for" LM2596 boards suffer from having component values selected so that the converter works across a very wide range of input voltages and outputs - this means that there is probably a specific set of component value selections that may overall be better for your specific usage case. Also the lowest loss inductor might not be in use, which in a buck changes overall efficiency considerably.

    Finally, the IC's mounted on the ebay boards are very probably counterfeit in some way or another. I have some of those boards with also the current limit comparator, and it is not a 150kHz LM2596 at all, despite the burned in markings on it. It switches at a leisurely 30kHz, which nowhere near specification for a fixed frequency regulator with no burst mode.


    ED-

    Merlin's suggestion of seriesing up the lamp is possible, however if one LED lamp blows you might find that they both end up blowing should they have their own internal current regulators. The fact that they maintain equal series voltage across their terminals is largely maintained by luck that their individual loadings happen to be similar.

    Alternatively, you may find - also by luck - that the lamps just run off 23VDC anyway, thanks to the magic of power electronics.
     
    Last edited: May 3, 2018
  10. fref99

    fref99 Member

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    Hi,

    If your willing to go DIY have a look at the RECOM R-78B-2.0 series (R-78B12-2.0). I've used them in a few projects and with a 16Volt input, 12Volt output at 1.5amps I'm seeing an efficiency of about 90%. They're not cheap but easy to wire up (LM78xx pin compatible) and have a sealed case.

    Regards
    FREF99
     
  11. OP
    OP
    RussellK

    RussellK Member

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    Thanks for the suggestions - I do want to end up with a 12V system, as it's a handy voltage for adding other things on.

    I've had a play with TI's WEBENCH tool, and as suggested, the Ebay LM2596 module's cap and inductor values are way off what's suggested for my voltages and load. I might see what low ESR caps and inductors I have laying about, and maybe try subbing in better values.

    That said, I do like the simplicity of that R-78B12-2.0 part :)
     

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