Boost converter using axiall lead inductors??

Discussion in 'The Projects Forum' started by imbaine13, Jun 1, 2014.

  1. imbaine13

    Thread Starter Member

    Oct 6, 2013
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    Hello everyone,

    I'd like to find out if it is practical to build a boost converter using axial lead inductors. The input voltage would be 5 volts and the output 12 volts at 1 amp (switching freq is 500khz). The reason I ask is because, first, I'm totally new to using inductors (this is my first project involving one), and second, axial lead inductor's values are relatively easy to figure out (from their colour bands) as compared to winding one or taking unlabelled ones off of old PCBs.
    May I also know what factors determine the maximum current that can be passed through one without significant I^2R losses (from calculations, I came up with a peak current of 3.2 amps through the inductor). Is it the wire size, or are there other factors in play that I should consider. At what frequency would I have to start worrying about the skin effect. Also, at what flux density (B) do 'E' shaped ferrite inductor cores start to saturate.

    Appreciate all input.
     
    Last edited: Jun 1, 2014
  2. Papabravo

    Expert

    Feb 24, 2006
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    Axial lead inductors are generally not able to handle the power requirements. Your output power is 12V * 1A = 12 Watts. Assuming 80% efficiency, the required input power is 12 / .8 = 15 Watts. So 15 Watts at 5V is 3A input current. The inductor peak current will be twice that or 6A. Now you can see why torroidal inductors with heavy gauge enameled wire are used.
     
  3. crutschow

    Expert

    Mar 14, 2008
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    A purchased inductor will have a stated peak current rating which you should not exceed, otherwise the inductor will saturate.
     
  4. RichardO

    Well-Known Member

    May 4, 2013
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    Very true. And here is a true story...

    Recently I was demonstating a circuit that ran on a 9 volt battery. It ran for a few minutes and then quit working. I checked the battery voltage and I found that the battery was dead. In fact, it was warm to the touch!

    The next day I found the problem. I had miscalculated the frequency of the step-up voltage converter. The too low frequency had saturated the inductor.

    Always being the curious type, I decided to measure how much current the circuit was drawing before I fixed it. I hooked up my bench supply to supply power. Since the circuit was only expected to draw about 20 mA, I set my DVM to the 200 mA range. On power up, the meter read something for an instant and then read 0. [Insert 4-letter word here].

    I had blown the fuse in my meter. After replacing the fuse and using the 10 Amp range on the meter, I found the the current was over 600 mA. The 1/4" diameter by 1/2' long inductor was only rated for about 400 mA saturation current so it was way, way beyond its saturation limit at the peak current of the circuit.
     
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  5. Papabravo

    Expert

    Feb 24, 2006
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    Everyone should memorize the Power Out = 0.8 * Power In from my example above as the very first calculation for any switchmode power supply design.
     
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  6. #12

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    Nov 30, 2010
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  7. imbaine13

    Thread Starter Member

    Oct 6, 2013
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    Thanks for the replies everyone. That link was helpful, unfortunately, there's plenty of staff on there that I don't quite understand. I have salvaged a few E type core transofrmers and inductors from discarded power inverters and CTR monitors. They all look like they are in working condition and I'm planning to rewind some of them to get the required inductor characterists. (Use if an airgap to raise the saturating current where necessary). Might anyone know a link that would guide me through this?
    Thanks.
     
  8. #12

    Expert

    Nov 30, 2010
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    Same for me!
    That site goes into magnetics way farther than anything I have seen before. I probably couldn't learn half it in a week. Sorry you didn't find what you were looking for, but mark the page. It looks like a good reference source that will be handy some day.
     
  9. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Agreed!

    I would add that for a 5v -> 12v boost circuit the formula is closer to;
    Power Out = 0.7 * Power In
    :)
     
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  10. crutschow

    Expert

    Mar 14, 2008
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    What "formula" gives an efficiency of 70% for that particular boost ratio? It would seem that you can design boost switching regulators with significantly better efficiency then that.
     
  11. #12

    Expert

    Nov 30, 2010
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    The Texas Instruments Simple Switcher website quotes a lot of results over 80%, but I haven't built any of their stuff and tested it.
     
  12. Papabravo

    Expert

    Feb 24, 2006
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    The 80% rule is a typical number when considering the feasibility of an SMPS proposal. Some designs will do better and some will do worse. Doing this calculation weeds out a number of proposals that are simply not worth the effort for other reasons. In particular, designs where a boost converter has enormous peak current demands in the inductor. There are still people out there who believe you can get something for nothing.
     
  13. ronv

    AAC Fanatic!

    Nov 12, 2008
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    There is a lot of "stuff" to worry about with boost regulators.

    1- They can go screwy if you design for a fixed load but they need to run at a lower load. So tell us about you minimum load or put it in the calculator to size the inductor.

    2- Such a high frequency is problematic if you are doing a home brew. The FET and diode need to be very fast and the layout of the board very good.

    3- Unless you plan on a "soft start" the inductor may saturate on start up while it is charging the output cap.

    But having said that. Here is a handy calculator:
    http://www.daycounter.com/Calculators/Switching-Converter-Calculator2.phtml

    Keep in mind the peak inductor current shown in the calculator is on top of the steady state current.
     
  14. SgtWookie

    Expert

    Jul 17, 2007
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    You're far better off to plan for a worst-case scenario, as then you are more likely to avoid problems like inductor saturation, excessive power dissipation in a MOSFET, etc.

    That way if you plan carefully enough, if your efficiency is better than expected, you will have a pleasant surprise. Melt-downs usually aren't very enjoyable.

    Toroidal inductors are very commonly used for that power range because there is almost no RFI/EMI emitted by such an inductor. Axial inductors are at the other end of that spectrum.
     
  15. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    It's in the spirit of a ball park figure like Papbravo's post.

    For a buck made by a beginner I would say 80-90%, and a boost with 5v input say 70-80% range.

    And like Papabravo I aimed for the low end of the range. 5v to 12v at 1A output is quite a bit of peak current (3.5A avg Iin, so a few amps peak!) and peak current in a boost circuit is one area beginners tend to make mistakes with. :)
     
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