Buck converter and Boost converter

Discussion in 'General Electronics Chat' started by porky67, Nov 24, 2010.

  1. porky67

    Thread Starter New Member

    Nov 19, 2010
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    hi guys.. i have some question regarding Buck Converter and Boost Converter...
    Theoretically, I think I know the function of both converter..

    My question is that, I am encouraged to use Converter IC for both the converter.. But I am never exposed to this kind of IC before..
    Can you guys recommend some of the Converter IC and explain how the Converter IC work??
    In my impression, Converter IC includes inductor along? It's a package of the whole thing?
    Please enlighten me..

    Thank alot guys..
     
  2. timrobbins

    Active Member

    Aug 29, 2009
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    Suggest you search for buck and boost controllers from major IC controller manufacturers - they have good introductory comparisons and recommendations.
     
  3. Wendy

    Moderator

    Mar 24, 2008
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  4. SgtWookie

    Expert

    Jul 17, 2007
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    Some low-power out ICs have inductors inside the package, but they are not typical.

    Watch this video for starters:
    http://e2e.ti.com/videos/m/power_management/180921.aspx
    The 1st 20 minutes covers various linear regulators. At around 20:30, switching regulators are introduced. Even if you are somewhat familiar with the basics, you should watch the video, if only to practice the terminology.

    Linear Technology has a number of DC-DC converters that are quite easy to use. I've been experimenting with the LT1171's lately; they are 2.5A regulators that can output from 1.2v to 30v+. They can be used in boost, buck, and flyback/inverting topologies.

    Linear Technology also provides a very good (and free) Spice simulator known as LTSpice. Google: "LTSpice download" to obtain the latest version from their website. You do not have to register if you don't want to; however their occasional mailings are very interesting.

    There is a support group on Yahoo! Groups:
    http://tech.groups.yahoo.com/group/LTspice/?yguid=185176242
    Lots of models/symbols to download, and support in the forums.
     
    porky67 likes this.
  5. porky67

    Thread Starter New Member

    Nov 19, 2010
    18
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    @timrobbins
    i did search for the ICs in the web.. however, I don't know much about IC at all..
    that's the reason i tried to get suggestion here.. thank you btw... =)

    @Bill_Marsden
    thanks! a good start for my boost converter..

    @SgtWookie
    Wookie! Thank you for your detailed description!
    you have answered my questions!
    And included a software too! For now, I will try to find more info for myself.. =)
     
  6. porky67

    Thread Starter New Member

    Nov 19, 2010
    18
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    @SgtWookie,
    I have been looking around for the Boost Converter IC.. However, it is in vain.. I can't find much IC for boost converter. I have been reading about LT1171 as you mentioned about.
    However, I'm not sure about the LT1171 from the datasheet..
    My input for the boost converter is around 17-18V and my output would be 20V.. What is the range for the output current for LT1171?
    Hope to get your response soon! =)
    Thanks
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Look at the bottom of Page 1 of the datasheet.
    On the left is a boost converter using the LT1170, which is the 5A version of the regulator.
    The LT1171 is rated 2.5A.

    On the right side of the page is a plot showing power out for the LT1170 in various configurations. The LT1171 would be capable of 1/2 the output wattage.

    So roughly, a 1170 would be capable of around 65 Watts out with 17v to 18v in, so the LT1171 would output roughly 32.5 Watts.

    32.5Watts / 20v out boost = 1.625 Amperes.
    As the plot shows, this is approximate.
     
  8. porky67

    Thread Starter New Member

    Nov 19, 2010
    18
    0
    So, LT1170 is capable for higher output right?
    LT1170 itself is capable in handling 5A output?
    But is the 5A output is fixed?
    All I can is just manipulating the output voltage and getting 65W output for 17V/18V input?
    Because I would like to maintain the output power by changing the current and voltage.
    I mean i would like to lower the 5A current for higher voltage to maintain around 65W..
    I hope you understand what I mean..
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    Yes.

    No; the feedback input will regulate the output current down if necessary to the voltage you select.
    With 17-18v in, ~65W is your limit with the LT1170. See the plot.

    You can select a lower output voltage if you like. Power out is limited by the input voltage, the IC itself, and what's fed back to the FB input.
    You control that by selection of the feedback resistors. FB attempts to maintain the output so that FB measures ~1.24v.

    Read the datasheet.

    [eta]
    By the way, what are you using for the 17v to 18v input?
    If it's a pair of 9v "transistor" PP3 batteries wired in series, it's not going to work.
     
    Last edited: Dec 8, 2010
  10. porky67

    Thread Starter New Member

    Nov 19, 2010
    18
    0
    Thank for your detailed explaination!
    To be honest, I don't really know how to read the datasheet.
    I not sure which is which for the information I wanted.

    my input is actually a solar panel.. which might vary from 12V-17V and I am putting 18V at the highest limit for the solar panel output. and i need to boost them, and then using PWM to regulate them to suitable voltage to charge a battery.. which is 12V.
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    Well, it's kind of silly to boost the output to 20v and then waste power by draining it back down.

    You should have specified what you were doing in the first place.

    Are you charging just one sealed-lead-acid battery, or what?

    You need to give more details, or you'll waste a lot of time.
     
  12. porky67

    Thread Starter New Member

    Nov 19, 2010
    18
    0
    I'm sorry..
    Because at first, I am here asking for only the boost and buck converter..

    I am doing Maximum Power Point Tracking Solar Charge Controller.
    So I have to boost up the output of the solar panel and reduce it back to find the maximum power point to charge the battery(yeah, it is a sealed-lead-acid battery)..

    I have to maximize the power to charge the battery due to the sunlight intensity and temperature which will vary the output of the solar panel..
    And my MPPT controller is to control that part..
    I hope you get what I mean..
     
  13. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    So, is this MPPT controller one you bought from somewhere? Or are you trying to build one along with this boost converter, or buck-boost converter, whatever it turns out to be?

    I'm having to guess you already have a pre-made MPPT controller, otherwise how would you know that it requires 20v? Or is that just a number that you made up?

    12v sealed-lead-acid batteries need more than 12v to charge. At 12v, they're over 50% discharged. You need the charge rate (current flow) controlled as well as the voltage.

    Look up the datasheet for the battery you want to charge.
     
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