Laptop Power Supply from 12V car battery

Discussion in 'General Electronics Chat' started by hazim, Jul 2, 2010.

  1. hazim

    Thread Starter Active Member

    Jan 3, 2008
    419
    13
    Hi all

    I want to build the circuit below to use it as a power supply for my laptop from 12V car battery power source. The circuit is said to handle 65W (19VDC 3.4A). My laptop power adaptor output is rated 19VDC 4.74A (90W). So I want to increase the output power from the circuit. Actually I'm new to smps circuits, and so I don't know whether the inductor should be changed to increase the output power (current) or not. D2 is MBR1645 (16A 45V), it's not available and I'll use MBR2045 (20A) instead. By the way, why they use much higher current semiconductors than the actual current in the circuit? to reduce heating??). Using 6800uF for C8 my solve the problem or what to do?
    If the input voltage varies from approx. 11V to approx. 14V, how much will this affect the output voltage?

    Any help is appreciated.

    Regards,
    Hazim

    [​IMG]
     
    Last edited: Jul 2, 2010
  2. Ante

    New Member

    Dec 25, 2009
    10
    1
    Even if your original power adaptor produces 90W its not necessarily what the demand is.
    I would find out first how much power the computer really consumes, perhaps (and very likely) the 3.4A may suffice.
     
  3. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    I agree. It will have a higher demand during battery charging cycles, but the power supply is probably over-rated to keep it cooler.
     
  4. Ante

    New Member

    Dec 25, 2009
    10
    1
    Or maybe you could go for another circuit with higher output current like this one:

    [​IMG]
     
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  5. hazim

    Thread Starter Active Member

    Jan 3, 2008
    419
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    Maybe you are right. I don't know the real power consumption of the laptop, it depends.. It's Toshiba Satellite A300 and the consumption may be so high sometimes (high performance, watching dvd, high CPU usage...etc).
    I'm thinking to elimenate the protection part of the circuit and put a 5A fuse instead, in case there is over current/short circuit. What do you think?
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    Have a look at the attached, it's pretty simple and the efficiency is around 92% when you have at least 13.3v available for the input. Your vehicle's typical system voltage will be around 13.8 after the engine has been running for awhile.

    If you download LTSpice (Google those two words) you can download the .ASC file into the Program Files\LTSpice\SwitcherCad directory and run the simulation yourself.

    Note that if you run it on low voltage (engine off, battery partially discharged) the regulator IC will start dissipating a lot of power, > 6W. This means that it will require a heat sink.
     
  7. Ante

    New Member

    Dec 25, 2009
    10
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    That’s a good choice, its even simpler with a lower part count. However as with all switchers a certain awareness is required for layout and wiring to get maximum reliability and efficiency.
     
  8. hazim

    Thread Starter Active Member

    Jan 3, 2008
    419
    13
    Thank you SgtWookie's. I have almost all the components of the circuit I provided. Building the circuit will not cost me more than 3$ :) while the circuit you posted uses LT1170 which is hard to find here, and it's expensive too. It's obvious better in many things from mine. But anyway I may build yours, I'll postpone this project a while and I may change my opinion. Can you provide me with information how to make the inductors?
    Isn't this one easier :)
    [​IMG]
     
  9. t06afre

    AAC Fanatic!

    May 11, 2009
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    Just so you know about it. Modern Laptop computers require to a sense a factory power supply in order to charge the battery. So with your setup you will most probably NOT be able charge the battery. But I think the computer will allow running without charging the battery
     
    Last edited: Jul 2, 2010
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  10. SgtWookie

    Expert

    Jul 17, 2007
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    Hazim,
    You have posted a circuit designed for a lower amount of current, that I posted in another thread.

    If you tried to use that one, you would have much higher ripple voltage than shown in the simulation, where the load is only 2.7A.

    Also, the current rating for D1 would be exceeded.

    The circuit that I posted in this thread has a filter stage on the output, comprised of a 10uH inductor and a 220uF low-ESR capacitor. This greatly reduces the output ripple; if you look at that simulation you will see that the ripple is only about 5mV. Without the filter, you might see ripple of over a volt, which would not be acceptable.

    The inductors and capacitors used in the simulation are very specific. Download LTSpice and install it, download the simulation file I attached in my first reply, and open it using LTSpice. Click on View, then Bill of Materials.

    Look up the data for the capacitors and the inductors. You need to match or exceed the specifications to obtain similar efficiency.

    Ante's point is well-taken. The wiring in the simulation is perfect; zero resistance, zero inductance, no signal coupling, etc. It will be challenging to design a really good board. Short, very wide traces are the way to go. The smaller the board can be made, the better. Your board will have heavy peak currents in the 10A range.

    Since you will be attempting to make your own inductors, the board design will have to wait.

    I do not know what kind of inductors or raw materials that you can obtain where you are, nor do I know what kind of test equipment you have. A dual-trace oscilloscope with >=20MHz bandwidth is a minimum requirement for building these kinds of projects.

    You should look at Ronald Dekker's page:
    http://www.dos4ever.com/flyback/flyback.html

    There is an "inductor test bench" about halfway down the page. You can build it very inexpensively. It will be a big help in measuring your home-made inductors. If you don't have a 40106, you could substitute a 555 timer circuit that is set up for wide PWM.

    That whole page is well worth reading through several times.
     
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  11. hazim

    Thread Starter Active Member

    Jan 3, 2008
    419
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    Yes you posted it in an old thread. I have LTSpice already, I downloaded the file and I'll check it. The page of Ronald Dekker is important, I saved it on my PC. As I said before, I'm new to switch-mode power supplies and I'm interested in such pages/books... I'll upload a useful pdf I have about switching regulators in a separate thread.
    For the circuit board I have, I got the circuit from source referenced it to "electronic for you" magazine, the PCB is provided.
    You can download the complete article from here:
    http://www.4shared.com/file/HFxJztgY/Laptop_power_converter.html
     
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