DC Buck & Boost

Discussion in 'General Electronics Chat' started by bumba000, Oct 9, 2015.

  1. bumba000

    Thread Starter Member

    Oct 7, 2015
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    Hello All,
    So I have created so far a boost converter which is driven by my arduino's pwm. Without drawing up a schematic of what I've done as I'm sure you're all knowledgeable enough to understand what I must have done, I am trying to keep a steady output voltage when various loads are attached.

    I am using a single cell 3.7-4.2v 18650 type of battery, a mosfet two capacitors, a shottky diode and have tried a wide range of inductors from 0.06uH to 0.110uH. I can get a lot of volts out this thing but the voltage is always reduced to almost nothing when I attach a load.

    I am attempting to power a small heating element. These elements range in ohms from 0.02 up to 1.8. My ideal output voltage would be 5V and I would like 10 amps. I cant figure out whether I need to do some type of load isolation, need more boost or what.

    While this isn't my first electronics project I am still pretty new to all of this and would really appreciate some help.

    Thank You, John

    P.S. I say buck and boost because I want the power out to be adjustable from 3 to 5 volts. As previously stated - I am triggering the mosfet via arduino pwm..
     
  2. bumba000

    Thread Starter Member

    Oct 7, 2015
    72
    2
    Is there anything that I can do here to trigger a response to my question?
     
  3. Lestraveled

    Well-Known Member

    May 19, 2014
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    Yes, you can. You can post a schematic. Please keep in mind we are not mind readers. Give us something to work to other than wild ass guesses.
     
  4. bumba000

    Thread Starter Member

    Oct 7, 2015
    72
    2
    Thank you. I didn't realize that there are actually many ways that this could have been laid out. I wasn't really sure how to depict the arduino's attachment here, but the rest is accurate.

    [​IMG]
     
  5. Lestraveled

    Well-Known Member

    May 19, 2014
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    Excellent. What frequency is your PWM running at? What is the part # of your schottky diode??
     
  6. bumba000

    Thread Starter Member

    Oct 7, 2015
    72
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    my arduino is actually a pro trinket 3.3v and it runs as 8MHz. So, as I understand it the PWM is running at 100KHz.

    I was using diode DO204AL but have pulled a much larger one out of an old power supply thinking that might help. it did not.
    The inductor is 33uH and I've have tried 8uH and 110uH. The capacitors are electrolytic . C1 is 16v 10uF and C2 was the same but am now using 35V 220uF.

    The mosfet that I'm using now is STP27N3LH5 I was using RLB8721
     
    Last edited: Oct 9, 2015
  7. Lestraveled

    Well-Known Member

    May 19, 2014
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    Your load, 5V @ 10 amps = 50 watts. A single 18650 battery is rated at 2.9 AmpHour, which will power your load for 3.5 seconds.
     
  8. bumba000

    Thread Starter Member

    Oct 7, 2015
    72
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    All I can say in response is that devices exist on the market that do exactly what I'm trying to do. They use a single 18650 battery and put out 5V plus at 50W plus. I own two of them but would like to build my own.

    Here's a pic of one that I tore into. This as most can also charge the battery ( I have that worked out ) and also has an OLED on the other side of this board ( I also have this worked out ).[​IMG]
     
  9. Lestraveled

    Well-Known Member

    May 19, 2014
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    Oops, sorry, I miss-calculated. One 18650 will power your load (50 W) for 3.5 minutes.
     
  10. bumba000

    Thread Starter Member

    Oct 7, 2015
    72
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    more or less that's roughly how much I get out of one of these things. They only run for 10 seconds or less at a time and need charged several times per day. Also, I've tried connecting the load directly to the battery and it barely gets warm. One of these devices is silent during and after operation. Another whines and clicks during and after operation.
     
  11. Lestraveled

    Well-Known Member

    May 19, 2014
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    OK, what do you want to do?? Your purchased device probably does a good job. What do you want to do different??
     
  12. bumba000

    Thread Starter Member

    Oct 7, 2015
    72
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    I want to do the same thing I just want to make my own. I didn't think it would be as difficult as this has proven to be. After all the time and cash that I have involved at this point now I really want to make it work. As I previously stated - this little circuit I've built does ramp up the voltage but as soon as I attach the load which is essentially a direct short at 0.2ohms the voltage drops to about 1 or 2 volts and the load ( small heating element) barely gets warm.
     
  13. Lestraveled

    Well-Known Member

    May 19, 2014
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    I doubt you can build it cheaper than what you can buy it.
     
  14. Lestraveled

    Well-Known Member

    May 19, 2014
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    Talk about what you really want to do with more detail. Perhaps there is a different way altogether to do it. (Something you haven't thought of.) There are a lot of very smart people here. Open up....
     
  15. tindel

    Active Member

    Sep 16, 2012
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    1. Your FET selection is lousy. You need a low voltage turn-on FET... preferably around 2.0V with > 30A rating, preferably. Actually a npn may be better in this application.
    2. You didn't specify your inductor (other than inductance) so it's hard to know if the inductor you have is suitable for the task. Preferably it will be low DC resistance and a good ferrite core creating a resonance about 5x greater than your switching frequency.
    3. How did you calculate your PWM is at 100kHz? What is the duty cycle of your PWM? Is the duty cycle fixed or is it variable? Is the frequency fixed or variable?
    4. Aluminum Electrolytic capacitors are awful for high frequency SMPS's. Get aluminum polymer caps - far superior. If you're budget can't afford that - get a several tantalums and put them in parallel.
    5. DO204AL is a package type - not a diode type. What diode are you using? Just because it's large does not mean it's better.

    Give us all the information you can. Anything you can think of. Designing Power Supplies is not easy. We're willing to help but you must give more information rather than string us along with little bits and pieces of information.
     
  16. bumba000

    Thread Starter Member

    Oct 7, 2015
    72
    2
    Ok, my inductor is this: https://www.digikey.com/product-detail/en/12LRS333C/811-2085-ND/2092061
    the other inductors that are smaller I pulled out of an old lcd monitor and another from some pci card. The larger inductor that I have is one that I made from a ferrite ring and some 30 awg mag wire.

    I do believe I've misspoken regarding the capacitors. These are what I have https://www.digikey.com/product-detail/en/UPW1C100MDD/493-1767-ND/589508 and
    https://www.digikey.com/product-detail/en/UVR1H0R1MDD/493-1095-ND/588836

    As for the PWM frequency - I didn't calculate anything. I figured this number based on information I've found regarding my pro trinket and other 3.3v arduino information.

    My diode is this: https://www.digikey.com/product-detail/en/SB130-E3/54/SB130-E3/54GICT-ND/4498220
    The larger one looks exactly the same just larger.

    I have these parts from digikey as I started out by building the circuit in figure 1 here: http://datasheets.maximintegrated.com/en/ds/MAX1522-MAX1524.pdf using the MAX1523. I guess I didn't realize then that that would give me 12V out. It does, I built it correctly and it actually worked ( the second time ). The first time I didn't realize that schematics don't always acculturate represent an IC's pinout and I smoked the little guy. Lucky I bought 4 of everything on that schematic. I realize now that I wanted to build figure 2 instead. Either way it's not going to do what I'm looking for.

    So what I've done since then is a bunch of tutorial watching and a bunch of question asking on SMPS, bucks and boosts.

    My goal is to build a circuit that will take the 3.7 to 4.2 volts from an 18650 battery and change it to 3 to 5 volts at up to 10 amps. This needs to be adjustable by input from my Pro Trinket (https://learn.adafruit.com/introducing-pro-trinket) / Arduino. I really don't know about duty cycle other than my trinket using analogWrite can be set from 0 to 255. When triggering the mosfet like this I understand that analogWrite(10) will hit the mosfet with roughly 3v in short bursts while analogWrite(100) would be longer bursts and 255 would be always on. The mosfet doesn't like 255. Right now I'm finding success and happiness from 10 to 60.

    The load that I'm trying to drive is a small heating element. It's just a coiled piece of wire and they come in ( or can be hand made ) in a wide array of resistances. Typically from 0.02ohm up to 1.5ohms.

    So I have written about 600 lines of code into my trinket that allows the PWM setting to be changed by pressing button a or b and then activates the PWM to the mosfet by pressing and holding button C.

    Thank You All, John
     
  17. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    The inductor in the link is rated at 2.1A, so will saturate and be unsuitable for your 10A load.
     
  18. bumba000

    Thread Starter Member

    Oct 7, 2015
    72
    2
    well I've been at this since about 10am again. re-reading some of your feedback from yesterday; all of my mosfets are NPN.

    I've been watching tutorials on inductors, resistors, capacitors and more off an on most of the day while working on my project. So the best that I've come up with so far has been to set up a second PWM pin on my arduino which triggers a second mosfet. This way they are taking turns charging the inductor. I've had the best results doing this. Also, I'm doing this using the inductor that I removed from one of these store bought devices. It only measures 8uH but is larger and heavier than the those that I purchased. I haven't found a way to measure the saturation voltage of an inductor using a multimeter or an lcr meter but will keep looking.

    If any of you happen to have a better solution to meet my end goal I sure would appreciate hearing it.
     
  19. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    You won't find one. Read up about saturation current ;).
     
  20. tindel

    Active Member

    Sep 16, 2012
    568
    193
    Your inductor is inadequate, as already pointed out. Get something like this... https://www.digikey.com/product-detail/en/HC2LP-2R2-R/513-1161-1-ND/754659. The inductor should be somewhere between 2.2uH and 100uH. v(t) = L * di/dt. Selection of the inductor value is largely based on this equation. You will have a linear change in current for a step voltage change.

    You didn't misspeak on your caps. Those are aluminum electrolytic. And they are shit for this application. You want aluminum polymer for this application. https://www.digikey.com/product-search/en?pv13=67&pv13=79&pv13=91&pv13=102&pv13=128&pv13=137&pv13=159&pv13=168&pv13=183&pv13=198&pv13=211&pv13=229&pv13=65&FV=fff40002,fff80532,fc000b,fc0013,fc016f,fc0173,fc0177,fc01ac,fc01c1,fc01c7,fc01e9,fc01ea,fc01eb,fc01ee,fc01f2,fc026b&mnonly=0&newproducts=0&ColumnSort=0&page=1&stock=1&quantity=1&ptm=0&fid=0&pageSize=25. Something in the 100u to 1000uF is probably appropriate for the input and output cap - but again the actual value you use will largely depend on your operating frequency!

    Your diode selection was good because it was fast, but it needs more current carrying capacity. Minimum of about 15A. Here's a 20 Amp diode that should do the trick. https://www.digikey.com/product-detail/en/VB20100S-E3/8W/VB20100S-E3/8WGICT-ND/1543677.

    I would use this mosfet: http://www.digikey.com/product-detail/en/IRLR6225TRPBF/IRLR6225TRPBFCT-ND/2538170. This one is way over spec'd but will work great.

    Remember - you're trying to get 50W out. So at a minimum battery voltage of 3.7V you will be drawing an AVERAGE of 13.5A (Current = Power / Voltage) out of your battery. And that's assuming 100% efficiency. it will probably be closer to 70% efficient at these loads. So parts should be rated for at least 20A, preferably more.

    All of this really hinges on your PWM frequency though. You need to know this. Usually your uC will have a clock frequency that is divided down to provide your PWM frequency. Then a capture-compare register sets your duty cycle. There are usually a few different modes that your PWM can work in as well. You need to research this and find out the answers to these questions or you will probably not be successful in your design. The way a SMPS usually works is to have a constant PWM frequency with a variable duty cycle. It doesn't sound like your arduino is setup to do this - which is probably part of your problem. You need feedback to regulate the voltage properly! This means you need to setup an analog input to your arduino to read the voltage, and increase or decrease the duty cycle if your voltage is low or high, respectively.

    I will help you select the rest of your parts - but I MUST know the PWM frequency. So look at your code and figure out what your operating frequency is. I will admit that I know nothing about arduino so I will be no help on that front - so either you will have to figure it out yourself or someone else will have to help you.

    You have discovered that building a power supply is no easy task. Every power engineer I know worth their weight in fecal matter is making six figures.
     
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