Help with LTSpice Boost Convertor Sim

Discussion in 'General Electronics Chat' started by spinnaker, Dec 30, 2010.

  1. spinnaker

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    I put together this boost converter in Tina.

    [​IMG]

    I want to boost 3VDC to 5VDC. It seems to work OK in Tina. Maybe a bit high at around 5.8 VDC out.

    Tina is really nice and really easy to use but the problem is that the demo version does not save. So I tried to do the same thing in LT Spice. I am just starting to learn LT Spice so I am sure it is just how I have my simulation set up.

    Can anyone take a look and tell me what I am doing wrong?

    [​IMG]
     
  2. hgmjr

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    Jan 28, 2005
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    You should be using an NPN transistor, not a PNP transistor in this configuration.

    hgmjr
     
  3. spinnaker

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    I have seen a couple of simple boost converter circuits and they both used N channel MOSFETs. I guess I just assumed that pnp is the same as an N channel. I still have lots to learn (relearn).

    I changed it to a NPN and it still does not work in LT Spice.

    Works fine in Yenka and Tina.
     
  4. Ron H

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    Try running this simulation.
     
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  5. spinnaker

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    Thanks Ron, that is exactly what I needed.

    It made me get just a little more comfortable with Spice.
     
  6. SgtWookie

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    Similar idea, but a regulating boost supply made from two transistors and a transformer. Regulation is pretty decent up to 100mA.
     
  7. spinnaker

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    Thanks Sarge I will take a look. In my (rather hgmjr's) circuit, I was getting a 7V spike when it first started, so I put a .1 mfd across that 10ufd. It seemed to take care of the spike.

    My supply is going to be two 1.5 batteries. Should I still have regulation?
     
  8. SgtWookie

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    I think you meant Ron_H's circuit, also similar to what you started off with?

    Realize that 0.1mfd = 100uF. the "mfd" designation is actually somewhat antiquated.

    I showed a 2.8v supply so that you could see how it performs with partially discharged batteries.

    Try it with various input voltages, and see what you get. Don't go above around 4.8v though, or you won't get regulation anymore.
     
  9. spinnaker

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    Double oops yes Ron and I meant a .1 ufd (micro farad :)).

    I do want something simple. It is just to power an LCD so maybe if battery is low the PIC can turn on an LED or something.
     
  10. SgtWookie

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    How much current does your display require?
     
  11. spinnaker

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    The display itself only uses 3ma max. But the datasheet on the back light is a little confusing to me. It says < 100ma but I know I currently have one working on another project that uses far less than that. With a current limiter on the BL and still providing plenty of light, the whole project uses only about 12ma.
     
  12. SgtWookie

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    Do you actually want to use the backlight?

    If so, figure out how much current it needs to provide a readable display, and what the Vf is (measure voltage from A to K.)
     
  13. spinnaker

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    Have to. I had another project where I tried not using the BL. The display is not readable with the BL off.

    The plan is to shut off the BL and LCD when not in use. That is what I did with the other project.


    The forward V of the BL is (according to the datashheet) is 3.2V. I will verify a bit later by measuring it myself. If that is what you want me to do.
     
  14. SgtWookie

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    Actually, I want to know what current you need to make it readable.
     
  15. spinnaker

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    In the one project I have a 220 ohm resistor and it is still very readable with a 5V supply.

    I've got to get things hooked back up again to measure anything. That project is in the middle of being put into a project case so things are disassembled. What I will probably do is just breadboard something. That way I can experiment with different current limiters.
     
  16. SgtWookie

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    OK, so (5V-3.2V)/220 = 1.8/220 = 8.18mA.

    Sound about right? Better measure it to make sure though.
     
  17. spinnaker

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    I can but I have measured previous and from memory that sounds just about right.

    I'm going to breadboard something up to measure again and be sure.

    Breadboarding won't go to waste. I have a bunch of extra PICS and some small scraps of PCBs. An LCD tester / terminal display (using the PICs UART) sounds like a nice little project.
     
  18. SgtWookie

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    Ever think about using a PIC output pin and a PWM channel?

    Have a look at Roman Black's page:
    http://www.romanblack.com/smps/pic-smps.htm
    His is a buck; you'd need a boost, which would require a transistor or transistors. You could probably cut the parts count quite a bit from what I posted.
     
  19. spinnaker

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    Yeah has a boost on the other part of his site but it contains it's own PWM. If I can figure out how to break that apart and it still works.

    Anything wrong with the one I started and Ron got to work? Maybe not ideal but pretty simple for a first try?
     
  20. SgtWookie

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    No, it's an OK starter place.

    L1 could be much smaller; perhaps 330uH.
    C1 should be larger.
    D1 should be a model, like a 1N4148 or 1N914 fast switching diode.
    Q1 should be something like a 2N2222, 2N4401, 2N3904.
    R1 should be 200 to 250 Ohms to more closely emulate your load. You could replace it with a current sink of anywhere from 10mA to 20mA.
    Try to get your switching frequency somewhere between 18kHz and 50kHz.
     
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