Is it possible to modify this voltage booster to regulate input voltage instead of output voltage?

Discussion in 'The Projects Forum' started by rudyauction8, Oct 14, 2014.

  1. rudyauction8

    Thread Starter Member

    Jan 27, 2012
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    I'm working on a project involving 10 volt solar panels charging 12 volt batteries. I'm looking at a voltage booster on ebay, that regulates its output voltage. The problem is that solar cells are current limited which means that if the booster wants more current than the solar panels produce the voltage will drop until the booster can't boost the voltage any more. I'm wondering whether I can modify this booster to regulate the input voltage to the panel's most efficient voltage and boost all of the available power unregulated through a diode into a battery.

    My plan is to cut the feedback trace and solder in my own feedback loop coming from the input voltage.

    Below are links to the booster and datasheet.

    http://www.ebay.com/itm/310777154161

    http://www.ti.com/lit/ds/symlink/lm1577.pdf
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    No you can't. You should wire the solar panels in series to boost the voltage and parallel to increase the current. Then regulate down to the charging voltage.
     
  3. rudyauction8

    Thread Starter Member

    Jan 27, 2012
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    I know I 'should' put the panels in series but I won't. I have only one panel and don't want to get a second. All cells in the panel are already in series.

    So beyond the 'should' and 'shouldn't', why would this not be possible - please give facts not opinions.
     
  4. tcmtech

    Well-Known Member

    Nov 4, 2013
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    You need a MMPT (maximum power point tracker) to do what you are asking for.

    I dont think that circuit board and IC you are referencing to could be modified to ever work that way.
     
  5. rudyauction8

    Thread Starter Member

    Jan 27, 2012
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    From what I've heard those require higher input voltage from the panel than the battery's fully charged voltage, sometimes exceeding 150 volts.

    I'll make a circuit diagram showing my intended modification and post it later.
     
  6. Papabravo

    Expert

    Feb 24, 2006
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    The fact is that a solar panel is a current device and not a voltage device. The voltage from a solar cell cannot be regulated arbitrarily. The cell supplies an amount of current based on the amount of illumination. The voltage depends on how much current the load draws from the cell. It is what it is and can't be anything else. It is possible to design a DC-DC converter that takes an input voltage that is higher or lower than the output voltage, but that is not exactly what you described. Is it?
     
  7. rudyauction8

    Thread Starter Member

    Jan 27, 2012
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    Basically I know that if I leave the booster as it is, when the battery's voltage is low it'll try to draw more current than the panel creates therefor causing the panel's voltage to drop to near 0 until the booster stops accepting the power. However if I modify the booster to regulate based on input voltage instead of output voltage the booster will only take what the panel provides (about 10 volts under load), and the battery will serve to regulate output voltage by taking any and all current available from the booster.

    Since the panel's current is nearly the same whether it's at 1 volt or 10 volts, the most power will be available when the load is just enough to drop the voltage to 10 volts, so using a booster that regulates the input voltage to 10 volts (or runs at max power if the panel can provide enough power) is what I'm trying to do. The output will be unregulated, but fed into a large battery through a diode which will stabilize the output to prevent damage to the booster caused by the high open circuit output voltage created by the booster.
     
  8. rudyauction8

    Thread Starter Member

    Jan 27, 2012
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    2
    I'm using an unregulated booster (555 and inductor) but I have to set the duty cycle different for different lighting conditions. When properly tuned it works great in full sunlight but I'd like it to regulate itself and adjust for different lighting. Modding this booster is the cheapest and easiest way I can think of.
     
  9. Papabravo

    Expert

    Feb 24, 2006
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    Let us know if it works, or lets out the magic smoke.
     
  10. rudyauction8

    Thread Starter Member

    Jan 27, 2012
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    That's what I'm not sure about. I'm worried that if the feedback pin is fed from the input instead of the output the chip might malfunction and possibly burn out. I'll order a handful on the 16th and give it a shot.
     
  11. Lestraveled

    Well-Known Member

    May 19, 2014
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    Rudyauction8
    What you describe is exactly what an MPPT does. It does not matter whether you convert a high panel voltage down to charge your batteries, or convert up, it still is the same function. I am attaching a white paper about doing what you are interested in.
     
  12. Lestraveled

    Well-Known Member

    May 19, 2014
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    The BQ2031 is a smart battery charger. The MPPT feedback is used to modify the current output in the "bulk" mode. You would use the same approach to modify the voltage output of your LM2577 based on the panel voltage. This should be an improvement. Although, if you could modify your LM2577 to use current feedback, instead of voltage feedback, you would get much better performance.
     
  13. Lestraveled

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    May 19, 2014
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    Here is an example of a Simple Switcher buck converter that I designed to operate as a voltage supply or as a current supply. (SW in the up position = constant current, SW in the down position = constant voltage). The feedback is the same as your LM2577.

    [​IMG]
     
  14. rudyauction8

    Thread Starter Member

    Jan 27, 2012
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    Looks like I have lots of reading to do. Thanks for all of the info, I'll do a bit of research and give it a try. I may also mess around with a microcontroller and see if I can get it to adjust the regulation based on the amount of light reaching the panel.

    Side note, maybe I could use a photoresistor in the feedback loop to lower the input voltage when less light is hitting the panels to increase efficiency during shade and morning/evening hours. I'd get an mppt charger but I don't have the cash and designing my own would be a good experience for me.
     
  15. Lestraveled

    Well-Known Member

    May 19, 2014
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    Wait a few more hours and I will have a design for you.
     
  16. rudyauction8

    Thread Starter Member

    Jan 27, 2012
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    THANK YOU! I'll check back tomorrow morning.
     
  17. Lestraveled

    Well-Known Member

    May 19, 2014
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    OK here it is. The basic concept is based on the BQ2031 white paper I posted earlier. I adapted it to the LM2577 converter you are using.

    [​IMG]
    - U1 has to be a rail to rail op-amp. I indicated an LMC6482 but you can use any well behaved slow r2r op-amp.
    - You get to select the components that make the LM2577 work.
    - R11 and D2 set the maximum voltage out. Vmax = 1.23V + Vzener. So, the 1N4743a is a 13 Volt zener and will give you a Vmax of about 14.2 volts. Good for a lead acid battery
    - R13 sets the nominal current output. Iout = 1.23/R13.
    - R4 sets the max power point. The sensed panel voltage is scaled to 50% by R1 and R2. So, your MP point is scaled at 50% also. Look at it as a pivot point. Panel voltage above this point will tell the regulator to output more current. Panel voltage below this point will tell the regulator to output less current.
    - R8 sets how much the output current will change as the panel voltage changes. R9 will effect this also. If you can't get what you want by adjusting R8, you can lower R9 for more effect and increase R9 for less.

    The components that are to the left of C2 (plus R9) make up the simple MPPT controller. All that circuit does is produce a inverted and scaled voltage based on the panel voltage. The components to the right of C2 is your simple switcher but it uses feedback from a current sensing resistor, so it will output a constant current. The reference for the LM2577 is 1.23 volts. If R 13 was a 1.23 ohm resistor the switcher would output a constant 1 amp. The MPPT control adds or subtracts to that feedback voltage. (Pin 2 of the LM2577 is the summing point for the current feedback and the MPPT control.) As the panel voltage drops, the MPPT controller produces a higher voltage. This voltage is added to the current feedback and will cause the LM2577 to output less current. Use the BQ2031 white paper as reference.

    OK, there you go. It took me two beers and a cup of coffee to design this up. Feel free to ask questions, I will assist. I want you to keep us updated on this project. I especially want you to show Papabravo and tcmtech what a positive attitude can achieve. :D
     
    Last edited: Oct 15, 2014
  18. Papabravo

    Expert

    Feb 24, 2006
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    I think the insight required to translate poorly stated requirement(s) has more to do with the result than a positive attitude. I said:

    "It is possible to design a DC-DC converter that takes an input voltage that is higher or lower than the output voltage, but that is not exactly what you described. Is it?"

    and the OP never bothered to respond to the question. You however took the suggestion and gave him what he needed, instead of what he wanted. That's fine and you are to be commended, but the positive attitude works both ways. I'm just sayin'
     
  19. Lestraveled

    Well-Known Member

    May 19, 2014
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    I think the OP described what he was looking for very well. He provided a data sheet and a link to the board he wanted to modify.
     
  20. Papabravo

    Expert

    Feb 24, 2006
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    I respect your opinion.
     
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