DC to DC Convertor 22.2v Lipo to 12v 1a

Discussion in 'The Projects Forum' started by rockier, Jun 19, 2010.

  1. rockier

    Thread Starter New Member

    Jun 18, 2010
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    I am trying to do a project to make a DC to DC convertor to converte a Lipo that is 22.2v at 2100mah to output 12v at 1.5a.

    I have some NTE966 regulators I wanted to use.

    From the datasheet I am thinking I need something like this.
    Datasheet can be found here http://www.nteinc.com/specs/900to999/pdf/nte966.pdf

    I need a 0.33uF or larger tantalum or mylar capacitor on the input side and I am not sure how to get 10w resister.

    Can someone give me some giudance here?
     
  2. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,699
    905
    It is a little confusing whether you want a voltage regulator or current regulator.

    For voltage regulation, you will be dissipating 15.3 W in the device (i.e., voltage drop X current). Look at the thermal specifications for that device. The junction to case resistance is 5°C per W. Thus, if the case were at 30°C, the junction would be at 76°C. The max. junction temp is 150°C. So far so good, right? HOWEVER, look at the junction to ambient resistance is 65°C, which means that without a heat sink, the junction temp would quickly go to toward 995°C (65 X 15.3). It will either thermally shut down or pop long before that occurs.

    You will need a good heat sink to allow such a large power dissipation. Do you have the space for that? Your efficiency for the voltage conversion will be quite low. Can you use a different lipo configuration (e.g., 4S). Note: 22.2 V is the nominal voltage. When fully charged (e.g, 4.2 V/cell), the the voltage will be about 25V.

    For current regulator, will your powered device handle 25 ? 10W resistors are available from Ohmite and other manufacturers at most parts dealers, such as DigiKey and Mouser. You will also need to consider the heat dissipation.

    Have you considered a switching regulator?

    John
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You really need to look at building or buying a switching power supply.

    A linear regulator will waste about half the power you have available, like John said.

    Look at the attached schematic and simulation. It's a DC-DC "buck" converter that's relatively simple, yet 94% efficient.

    L2 and C6 could be removed to make it even more simple, but at a cost of more noise on the output.

    The parts used in the simulation are very specific.

    No, you can't use the NTE parts to build it with.
     
  4. rockier

    Thread Starter New Member

    Jun 18, 2010
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    0
    Thanks everyone.

    I will look at the one SgtWookie said. I just had the NTE parts allready and was think I could use them but I have no problem changing. I am learning more every day and this fourm is great. It has a lot of information.

    I normally get my parts from Mouser and will look for the parts I do not have allready now.

    Agian you guys are great. Thanks for the help.

    Is there anything I need to now to build DC-DC "buck" converter.
     
  5. rockier

    Thread Starter New Member

    Jun 18, 2010
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    0
    Last edited: Jun 20, 2010
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    I'm attaching LT1171 12v buck supply.asc to this post.

    You will need to have Linear Technology's LTSpice installed in order to view it.

    LTSpice is a free download. Google is your friend.

    Once you have the simulation loaded, click View => Bill of Materials => Show on Schematic.
    Then View => Zoom to Fit

    [eta]
    Noticed that I didn't have a part number in one of the caps. Updated; here is the text BOM:
    Code ( (Unknown Language)):
    1. --- Bill of Materials ---
    2.  
    3. Ref.    Mfg.    Part No.    Description
    4. C1  KEMET   C0603C105K8PAC  capacitor, 1µF, 10V
    5. C2  --  --  capacitor, 100nF
    6. C3  TDK C575OX7RIH475M  capacitor, 4.7µF, 50V
    7. C4  KEMET   T495X107K016AS  capacitor, 100µF, 16V
    8. C5  Nichicon    UPL1J181MRH6    capacitor, 180µF, 63V
    9. C6  Nichicon    UPL1V221MPH6    capacitor, 220µF, 35V
    10. D1  --  MBRS360 diode
    11. D2  --  MBRS360 diode
    12. D3  --  1N4148  diode
    13. L1  Coiltronics CTX50-5-52  inductor, 50µH, 5A pk
    14. L2  Wurth Elektronik    7447745039 WE-PD2 5820  inductor, 3.9µH, 2.6A pk
    15. R1  --  --  resistor, 10.4K
    16. R2  --  --  resistor, 1.2K
    17. R3  --  --  resistor, 470
    18. U1  Linear Technology   LT1171  integrated circuit
    For the 100nF cap, just use a metal poly or ceramic.

    [eta]
    Instead of trying to find a 10.4k resistor for R1, use a 10k fixed and a 500 Ohm or 1k pot in series.
     
    Last edited: Jun 20, 2010
  7. rockier

    Thread Starter New Member

    Jun 18, 2010
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    0
    Thanks a lot.
    Have the LTSpice allready loaded.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Keep in mind that you should not try to "breadboard" this circuit.

    The internal oscillator of the LT1171 regulator runs at a pretty high frequency. You should make a proper circuit board. Keep the traces as short and as wide as you can.
     
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