Inductor for boost converter.

Discussion in 'General Electronics Chat' started by IcedFruits, Apr 15, 2014.

  1. IcedFruits

    Thread Starter Member

    Jan 15, 2014
    84
    2
    I am thinking about building a low voltage boost converter, but not sure to choose inductor. I think i have some <100uH, 900uH, 5-6mH inductors with me.

    Please give some ideas, and suggest some reading on how to select the same, with formulae reference if possible. :)
     
    Last edited: Apr 20, 2014
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,287
    1,252
  3. IcedFruits

    Thread Starter Member

    Jan 15, 2014
    84
    2
    looks interesting and detailed. going to read, thanks.
     
  4. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
  5. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,287
    1,252
    What voltage and currents are you after?
     
  6. IcedFruits

    Thread Starter Member

    Jan 15, 2014
    84
    2
    @Jony130 thanks

    fairly low, got a 5.5v 100ma solar panel from ebay, and trying to make some constant voltage between 3 - 3.3v from it. lets say 1v to be minimum for boosting and after 3.3 it will be buck.

    would be good if i could find low voltage booster ics, but cant find them in shops. guess, i will have externally power the boost driver circuitry, or at least kickstart with external power - a low power tss555, 2n7000 and some low power opamp.
     
    Last edited: Apr 17, 2014
  7. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,287
    1,252
    Real low voltage parts can be hard to find. Here is one that is kinda cute. I think you might be able to use the disable to switch it on and then clamp the output to 3.3 volts with a zener or voltage reference. This assumes your panel can't put out more than 100 ma. or so.
    http://www.ti.com/lit/ds/symlink/tps61097-33.pdf
     
  8. Moon968

    New Member

    Apr 14, 2014
    10
    0
    I have used this. This is show good result.
     
  9. Evil Lurker

    Member

    Aug 25, 2011
    117
    23
    Picking out the right inductor for a specific application requires a decent understanding of how they work or in the alternative some wicked voodoo magic ritual involving virgin sacrifice and a dart board with datasheets attached to it.

    The circuit you are actually wanting to build (or I should say need) is some itty bitty ridiculous high switching frequency thing (like 250-650kHz or more range) with a tiny little inductor. Reason you need to go this fast is to keep the ripple down on your caps (which will be ceramic SMD) and to keep the inductance value down on your inductor to reduce losses from copper wire resistance. Heck you may even want to go to with a boost-buck or SEPIC design with a coupled inductor. And yes let me emphasize when I say little I'm talking tiny... a driver in an SOIC-8 package or even smaller like a SOT-363 or some other nonsense package. The inductor you will need is probably a 7mm OD SMD package.

    Anyhoos I'm tired so I'm just going to hotlink some random pictures with no recommendation whatsoever for this company... [​IMG]

    See the three little ones in the right hand bottom corner on the other side of the 330uH and 47uh inductor? I'm guessing one of those is probably going to be about the right physical size. I'm also going to guess that the unshielded one is going to be around ~5.5mm OD with the others 7mm and ~10-11mm respectively.

    Your driver may look something like this...

    [​IMG]
     
  10. IcedFruits

    Thread Starter Member

    Jan 15, 2014
    84
    2
    thanks, it looks rather low profile but i can find it online, and they are selling it a bit overpriced. i will note it, in case i need one for future use. i was more like looking for something in the league of max757/756 of 3.3 or 5v outputs.

    @Evil Lurker
    is that so ? :confused: i was thinking of winding up some 20 gauge wires on ferrite rod, as i have some of those 20 & 26 gauge wires in my toolbox..... it should be ok, as long as i keep the wire length small between board and coil, right ?

    p.s thats a wacky looking spider u made, thanks for showing.

    ---------------
    now from the boost converter mathematics, i understood the following (i will take the o/p voltage as 5v for ease of calculation , and that i m not thinking about the buch part now ):
    (1)
    [​IMG]
    to keep the inductor peak current higher than 2x of normal current, i need to keep the inductor small.

    (2)
    [​IMG]
    for input voltage range from 1v to o/p of 5v i need 80% duty cycle pulse.

    (3)
    [​IMG]
    need some high frequency to match the small inductance of eq (1).

    ok, i am still a bit confused about the (a) load current part & (b) how to adjust the o/p voltage with varying i/p voltage part. 555 duty cycle wont go below 50%, unless i invert it. even if so, what is the way to vary R2 of a 555, a mosfet ?

    or should just put the clock input to "off" with a comparator ? i.e. introduce duty cycle to a steady clock ?
     
    Last edited: Apr 17, 2014
  11. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    You don't need a boost. The solar panel will output its full voltage long before it makes any significant amount of current.

    So you can think of it as a "5.5v solar panel" at all times, that will make 5.5v at a few mA when overcast and 5.5v at 100mA when in full sun.

    You need a 5v->3.3v buck IC. Not boost.
    :)
     
  12. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,287
    1,252
    Silly me.:rolleyes: RB is right. The voltage changes very little with light (see curve) it's all current. So you could just use a shunt regulator made from a zener like the 3.3 volt one shown in the table Here:
    http://www.mouser.com/ds/2/308/1N5913B-D-11179.pdf
    Cathode to +.
     
    • iv.jpg
      iv.jpg
      File size:
      51 KB
      Views:
      76
  13. IcedFruits

    Thread Starter Member

    Jan 15, 2014
    84
    2
    u didn't understand, when i i am home after a day's office work, solar panel doesn't give more than 2 volts under home fluorescent tube. :p

    thanks for the info on reaching full voltage, but i kind of thought of juicing it as much as i can.

    but, on the other hand , when i tested it last time, the voltage did increase from 3.5 (shade) to 5+ (full sunlight), so i thought it was 100ma that it can max support at any given voltage. am i wrong ?

    @ronv 3.3v zener is pretty bad, table in ur link shows it's rated against 113mA current.

    p.s. i need help with post 10
     
    Last edited: Apr 17, 2014
  14. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,287
    1,252
    Alas, your problem is you don't have enough light. It is not just voltage, but power you need to think about. So it is voltage X Current. If you look at the curves it is mainly the current that goes down with less light.

    I suspect the voltage you measured was with only the voltmeter attached. If you add a resistor to draw some current - even a little bit - it would drop to zero.

    No, 100ma only in full sun.:(


    You need to have a desired current to finish.
    Some form of feedback is needed to control the voltage by changing the duty cycle. I have built some simple bucks using a comparator and a voltage reference. It is kind of crude, but works for low current.
    Check out RB's site. I think he still has a simple boost that would be fun to build.
     
  15. IcedFruits

    Thread Starter Member

    Jan 15, 2014
    84
    2
    @ronv
    ok, i understand. but my goal is to build a low voltage boost, solar panel is just an excuse i bought from ebay. if i cant get power from sp, i will fetch it from one NiCd. [​IMG]
    (i will go out again to test the sp, with some loads, when i get to see the sun next time)

    i have checked the first site, need time to read the other sites. hopefully i will be able to come to some understanding by this week end.

    so, tell me about ur buck, u used 555 clock and changed its duty cycle as per the o/p voltage ?

    how did u do it / changed value of 555 resistances ?
    -----edit, just asking, u probably used the 555 pwm configuration with diode between 6 & 7 ?
     
    Last edited: Apr 17, 2014
  16. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,791
    1,103
    You can get~1%-99% with this config:
     
    IcedFruits likes this.
  17. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,287
    1,252
    I probably shouldn't do this. Both are not real good circuits, but maybe ok to learn from. ;)The first is a buck converter that just turns on and off to control the voltage. You can adjust R5 or R1 to change the voltage. You can download LTSpice IV free from here:http://www.linear.com/designtools/software/?gclid=COCbrre56r0CFU1cfgodZlwATw#LTspice Then you can probe the circuit. I used one like this in my golf cart to drop 48 to 5 volts for a battery gauge.
    The second is a constant current boost regulator for a bunch of LEDs in series. A guy on another forum built this for his ATV. The 555 is set up to run at about 125Khz and the feedback goes to the CV pin to change the duty cycle.
    The .asc file is a spice file if you decide to try it.
     
    IcedFruits likes this.
  18. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    Solar panels will give their full output voltage right down to 1/100 of their rated current or even much less.

    The solar panel voltage will only drop low when there is incredibly low light. Your internal room lighting with a fluorescent lamp might be a million times less light hitting the solar panel than in direct sunlight.

    You should never expect to get any useful power out of a solar panel when it is indoors. They don't tell you that in the marketing. :(
     
  19. IcedFruits

    Thread Starter Member

    Jan 15, 2014
    84
    2
    ^u're right ! i took the panel out in the ambient daylight, it was giving ~3.5v, and when i connected a 10ohm load, it dropped down to 0.7v with a load current of 2.1mA! expectations ....... :mad:

    @ronv thanks for sharing the circuit, what u achieved through the 2222 in boost circuit, i am planning to achieve with a pwm as alec said.

    also, please suggest some easily available part names of low offset and/or low voltage (1.5 - 2v) opamps .......
    :)
     
  20. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    Yep, even in ambient daylight your panel was only making 0.7v at 2.1mA, that is 1.47mW of power.

    If the panel is specced at 5.5v 100mA that is 550mW of power.

    So the difference between ambient daylight (ie shade) and full sunlight is 550 / 1.47 = 374 times less power out in shade compared to full sun.

    Of course you only get full sun for about 4 hours, where there are NO clouds.

    And that 550mW is only if the ebay advertising for the panel is true! Often their advertising is "Chinese Honesty". ;)

    Have you measured the panel output volts and amps in proper full sunlight at noon? (You can just put the ammeter shorted directly across the panel to measure the amps, no need for a resistor.)
     
Loading...