How to convert 12V/80A to 36V/10A?

Discussion in 'The Projects Forum' started by espguitarist, Jul 19, 2012.

  1. espguitarist

    Thread Starter New Member

    Jul 19, 2012
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    Hey guys,

    I'm working on a board that will control some components that dissipate 210W at 36V. The power source will be a generator that puts out 12V at 80A.

    I'm a mechanical engineering student, but I haven't done any circuits yet so I am in way over my head on this. I'm not really even sure if I've worded my question correctly, so hopefully I won't be flamed too badly. ;)

    I've got most of the board and components figured out, but I can't seem to figure out what kind of components I should use to get the voltage and current that I want.

    Cost and size are factors here. I know I could go and buy some kind of big transformer or amplifier that will do what I want, but I don't need something that complex or expensive.

    Thanks!
     
  2. takao21203

    Distinguished Member

    Apr 28, 2012
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    The attached circuit can easily be changed for boost converter configuration.

    For 10A you need more than one MOSFET.

    Or large power NPN transistors eventually.

    If you need storage coils, I have coils that are good for upto 6 Amps each, and can be used 2x parallel for double current.

    Having 12V input, the TL494 supply is more simple- directly power it from the 12V input.

    I made modifications to the schematic but don't have a printed schem.
    However they are trivial.

    Difference between buck/boost maybe you can research for yourself.

    The top part of the circuit is unneccessary, only to blink LED.

    So it can be built with much less components than shown.

    If you don't have much experience then I guess aquiring the storage coil (or coils) could be a problem, as well the cooling.

    I use a $9 large VGA Cooler here for buck mode SMPS. The power transistor is mounted upside down like DPAK, and two TO220 components are added as well for padding (the cooler is large). It is mounted using two holes, and hold in place with spring tension elements.

    For 10A or more, it is actually worth to consider other circuit solutions than based on storage coil, for instance transformer based. 10A would be just right on the margin in terms of Watts. Depends how much efficiency you need but you write about a larger generator.
     
  3. espguitarist

    Thread Starter New Member

    Jul 19, 2012
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    Okay, a boost convertor is what I need to design. It's a lot easier to Google things when you know what they're called. :)

    I could use the TL494, but I was planning to use an ATtiny85 as my microcontroller. Could I go to an IC with more inputs and outputs (ie ATMEGA328), and just run the boost convertor control through my IC along with everything else? Or would it be better to keep the systems separate?

    Also, the components (very large solenoids) that need the 36V are going to be activated on/off by a MOSFET and a signal from the microcontroller.

    Seeing that the boost converter needs a MOSFET (or a few) to work, and is run by PWM, could I eliminate the MOSFET that runs the solenoid and just use the activation of the boost converter via PWM to turn my solenoid on and off?

    I'm not sure what I need to know about storage coils (inductors?). I did find a website that has some information about building a boost converter, and it has a calculator that tells me the specs I need for the components involved. Let me know if it looks accurate, here's the link.

    http://www.ladyada.net/library/diyboostcalc.html

    I haven't even considered cooling yet, I was hoping that everything would be okay just running without cooling, but as I've gotten deeper into this project I've realized that I'm playing with a considerable amount of power that will generate a lot of heat.

    I should probably create a schematic so that this all makes more sense, but I don't have anything drawn up at the moment. Let me know if I'm not being clear enough.

    Thanks for your help!
     
  4. takao21203

    Distinguished Member

    Apr 28, 2012
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    Since the feedback will need high frequency, it's better to use a seperate controller. Yes it is possible to do SMPS using a microcontroller.

    yes this is easily possible even for TL494.

    such calculators may have limited use when you don't have experience with the related components. 10A is a power class where you can't just buy a read-made one and will have a guarantee it will work well.

    Coils have to have the right properties:

    Diameter in terms of flux carry capability.
    Wire diameter for current capability.
    Magnetic material in terms of frequency and intended application.

    It's hard to imagine to do a 210W SMPS without any kind of cooling.
    Maybe build a smaller one firsthand for let say 1A or 2A, and even for these, you will see considerable heat developement.

    If you don't have experience with power electronics, SMPS are not that easy for 200W class.

    I have many years experience with circuits, so when I built related circuits, I did know how to proceed. Was a lot of work tough.

    The circuit I have shown here is not difficult in terms of components, or construction. But modification well makes sense.

    Buck or boost configuration is not much different, for storage coil + buck often a PNP is used, and for boost converter, a NPN.

    What you need to understand is how these power transistors (or MOSFETs) are worked in the circuit.

    If you need any of these coils, I have many of these here, they are actually quite suitable for these kind of circuits.

    Building a HF transformer if you did not build circuits before, well upto you, myself I would not want to deal with the calculations involved.
     
  5. takao21203

    Distinguished Member

    Apr 28, 2012
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    from the linked website:

    For many small projects, its cheaper and easier to DIY a boost converter than to buy a specialty chip

    This is wrong. The TL494 costs about 50 cents.
    And it does not need special components.

    However, frequency and dead-time (thus duty-cycle) can be adjusted.

    For solenoids, you don't even need filtering or large output capacitor. They can live with quite a large ripple current.

    How about the solenoids? Do they need 10A each? Or do you operate more than one?

    You could build a smaller TL494 based circuit, maybe for 3A to 5A, and replicate as many times as needed.

    TL494 is cheap!
     
  6. espguitarist

    Thread Starter New Member

    Jul 19, 2012
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    There will be two solenoids, however they won't ever be operating at the same time, and they shouldn't ever be activated for more than 10 seconds.

    They don't need 10A, they need 36V and they are capable of dissipating a max of 210W, so the max amperage would be 5.8A (theoretically...I don't know if that's actually how this works :rolleyes:).
     
  7. takao21203

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    Apr 28, 2012
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    then build a 5 Amps circuit having a VGA cooler (only a smaller one),
    it can be overloaded with 6 or 7 Amps for a while (some 10 secs).

    If you have two solenoids, use two circuits.

    It is possible to build it for $25 or something the like. Depends where you get the components.

    I would exploit the fact that the solenoids are never worked permanently, so you can overload the SMPS a little.

    microcontroller can't drive large power transistor directly, so you need a driver stage anyway.
     
  8. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    +1 A forward converter commonly called "push pull" is the best design for running off 12V input. We built a bunch of them for portable launchers and other devices that ran off car batteries in the field.
     
  9. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    A boost converter for over 200W is a tough design, not the best choice IMHO. There is a reason you don't see commercial boost converters for that wattage.
     
  10. takao21203

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    Apr 28, 2012
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    I have one boost converter with 150W specification, however I heavily doubt it can reach that level. Using one single IRF n-CH MOSFET.

    12V->35V.

    I have actually modified it for higher voltage. At 82V the MOSFET said goodbye...then I repaired it. It still works. One ATX PSU was ruined.

    UC3843 I think, not so easy to work with as TL494.

    And based on single toroid inductor only.
     
  11. espguitarist

    Thread Starter New Member

    Jul 19, 2012
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    Okay...so a boost converter probably won't work for 200W and stay relatively cool.

    What about using large capacitors? I really only need the 36V for the initial surge to make sure that the solenoids have plenty of force at full stroke. Once they are fully closed, they don't need more than 12V to stay there.

    Is there some way that I could fire them with a capacitor at 36V and then have 12V flow to it as long as I need it to be activated?

    Or, alternatively, is there a capacitor big enough to provide 36V for 5-10 seconds?

    Charging time would have to be taken into consideration, it is possible that they will need to be fired in rapid succession.
     
  12. takao21203

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    Apr 28, 2012
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    http://www.ebay.com/itm/DC-DC-10V-3...-For-Module-Mobile-Notebook-Car-/221004399645

    this module can output 35V.

    Using a large capacitor is an interesting approach.

    But somehow you would have to turn off the DC/DC module, and then quickly turn on the solenoid. Maybe 10000 uF will already be enough, given the fact you would also supply 12v holding voltage via a diode.

    I would experiment! There is certainly potential to archieve a solution NOT using a full 10A boost converter.

    But you need to understand how this dc/dc booster works. Then for instance you could turn it on/off by manipulating it's feedback path. You would only have to supply a relatively small voltage artificially at the right point.

    To turn on the solenoid from the dc/dc output (+ larger capacitor), a MOSFET could be used. N-channel MOSFET needs connection to ground, and the load from Vcc towards the MOSFET.

    Do you follow this explanation, or do I need to draw a schematic?

    I don't know the current levels however that are needed for the solenoid.
     
  13. espguitarist

    Thread Starter New Member

    Jul 19, 2012
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    Oh, that thing is much smaller than I was expecting. That doesn't look too complicated at all...you mentioned that solenoids can live with large ripple currents, so the booster wouldn't necessarily need filtering or larger capacitors. Could the booster in your link be simplified and potentially built cheaper for my application? Could I use two per solenoid so that I have 300W available? Or does it not work that way?

    So you're saying that I should use the booster to charge the capacitor that will fire the solenoids? I suggested the capacitors as an alternative to the booster, I thought I could just charge them using the 12V source and get them to output 36V. Or is that not how they work?

    I tend to blow things up when I experiment. :p I'm going for a slightly more educated approach for this project.


    An N-type MOSFET is how I was planning to utilize the capacitors, so I am glad that's how you say it should be done. I do understand that.

    I don't think the solenoids will ever need more than 6A. They are rated for 210W at 36V and a resistance of 6.66 ohms. Using P=IV and V=IR, these numbers give an amperage range of 5.4-5.8A. If this is incorrect, please let me know, I am just applying what I have learned in my physics classes. I haven't gotten to any circuits classes yet, so my understanding is very basic.

    I should mention, it is possible for us to get solenoids that are lower resistance and require less voltage, but they would draw more amperage. The lowest voltage one available is 23V and 2.70 ohms, still 210W. Would one of these be easier to work with?
     
    Last edited: Jul 20, 2012
  14. takao21203

    Distinguished Member

    Apr 28, 2012
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    I doubt this converter can reach full 150W. At least it would need a larger cooler. I have one of these here, but I rather used it to produce higher voltages (I modified it).

    Yes the output is DC, so if you adjust them for exactly the same voltage, it is possible to use two in parallel. But I don't think it is professional. You would have to try if it works out. I don't say it does not work for your particular application. I would rather say, it may work out very well.

    The circuit is very simple, based on UC3843. There is no schematic, and this IC is not so easy to work with. It seems to be optimized in terms of storage coil. They are not always so easy to find/to buy.

    yes this seems reasonable, as they only need 36V to turn on. Do you understand to use a diode to supply 12v holding voltage?

    But, you would need to turn off the DC/DC, somehow, or it will continue to supply 36V. This is not difficult, you only need to supply a fake feedback voltage (there is a small adjustable resistor on the PCB).

    No you can't easily charge capacitors from 12V to 36V.

    I don't know, I would need to see a picture of the solenoid. Have you ever experimented with them? Or is the said 12V generator all you have, but you need 36V somehow?

    You seem to know already the holding voltage is just 12V. The power consumption at this voltage also will be lower.

    210W solenoids really must be hudge! :D
     
  15. espguitarist

    Thread Starter New Member

    Jul 19, 2012
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    I don't mind building my own converter to handle more power, we're already building a circuit board for this thing anyway. I'd like to keep it professional, also, so if I need to integrate two of those into a single board of my own design, that's what I'll do.

    Is the UC3843 the ideal IC to use for this? Or could I just as easily use a different IC, such as the TL494?

    I am pretty sure I understand using a diode for holding voltage. It should be as simple as running it in parallel with the capacitor and using the diode to make sure that current can only flow towards the solenoid, and not back to the 12V power supply. Right?

    I need to shut off the DC/DC when the capacitors are fully charged? Or when they are discharging?

    So I guess the advantages of this would be that the DC/DC won't be on all the time, reducing heat, and I won't have to supply a constant 36V when I want the solenoids activated. It seems like a reasonable approach to me.

    I've got one solenoid that I've been using for testing. We're working on getting some more, but we're sorta waiting until we have a bigger list of things to order from Newark.

    The 12V generator is just a standard generator used in what my project is for. What I'm working on will be added onto another system, so 12V is what we have to work with. I just need 36V to power the solenoids, that's it.

    To run 100% duty cycle on these solenoids, you can only give them 11.5V, and they have a holding force of 120.1N. We won't be anywhere near 100% duty cycle, so we could run 36V constantly, but there's simply no need. 120.1N is more than enough holding force. The problem is that at 10mm stroke and 12V, the starting force is only about 3N, and that's at 20°C. Our ambient temps will be closer to 60-70°C, so I am afraid that the starting forces of the solenoid won't be nearly enough.

    At 36V, the starting force is 40N, which should be plenty.

    After 8ms at 36V, the solenoid will be at about 1.5mm from closed, and at that point 12V will produce ~72N of force. So, as long as the capacitors can provide 36V for 8ms, that design should work perfectly.

    I'll send you an email with a link to the solenoid we're using.
     
  16. takao21203

    Distinguished Member

    Apr 28, 2012
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    These are two ICs that are often used for SMPS.
    The UC3843 does not have a schematic in the datasheet.
    The one's I have found so far are very specialized.

    I'd say TL494 is easier to work with. I saw one schematic where it is used for 80Amps converter.

    bountyhunter explained that for >1 Amps., sometimes a push/pull converter is prefered. But it is possible to build a converter using a storage coil for upto 10 Amps. The main problem is to find a suitable toroid or core. If you use the right one, only one circuit is needed, but might be easier/cheaper to use two smaller one's.

    yes.

    You need to shut off the dc/dc, when you switch on the solenoid, or it will continue to boost voltage.

    And rely on the capacitor to switch on the solenoid. 10000uF to me looks like a starting point for this, might need more capacity, may work with less.

    As the dc/dc won't work with 100% duty, you could simply charge 10000uF without current limiting, After that, the dc/dc won't consume power.

    If you use a very large capacitor, then the MOSFET needs to have capability for the surge currents. There will also be a limit given by the storage coil.

    I'd also recommend maybe you can ask on the TI forum, they do make various DC/DC chips.

    Eventually you don't even need a large capacitor, you could simply overload the dc/dc for a brief moment, then turn it off, as 12V is supplied as holding voltage.
     
  17. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Anything is possible if you want to use a big enough hammer, my point is that using a boost above 100W powered from a low voltage source is just a terrible choice for a starting point. I always try to do things the easy way.

    A boost is a "store-dump-repeat" transfer of energy topology, a forward converter delivers energy directly across the transformer so is much more efficient. The push-pull gives the maximum utilization of input voltage and maximizes efficiency when running off a low input voltage.

    http://www.national.com/AU/design/courses/214/214_push-pull_power_converter_topologies.pdf
     
    takao21203 likes this.
  18. espguitarist

    Thread Starter New Member

    Jul 19, 2012
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    How would I determine what toroid I need to use? They don't look overly complicated to make, so I could do that if I could just find the core I needed.

    Also, if I'm using a capacitor to provide the initial 36V to the solenoid, could I use a lower amperage DC/DC to charge the capacitor? That way I wouldn't need to build a 10A unit, I could just use one of the ones you linked on ebay.


    Okay, it shouldn't be too hard to make sure the DC/DC goes off when the solenoid comes on.

    I don't really want to overload the DC/DC, I don't want to have to worry about stressing the components and having potential premature failure issues.

    I like the capacitor idea, I think it will work nicely, and if I can use a lower amperage DC/DC, then it will allow me to keep the heat down.

    So just to run through everything, to make sure I have understood everything up to this point...

    Start with a 12V source, send that to the DC/DC and get 36V. Send 36V to the capacitor to charge it, then run the capacitor to a MOSFET that is activated by my IC output. Run 12V source in parallel with the capacitor with a diode to prevent reverse current. When it is time for the solenoid to be fired, it receives 36V from the capacitor, until the capacitor is discharged, at which point it receives 12V from the source to remain closed until the IC ends its firing sequence.

    Does that sounds right?
     
  19. takao21203

    Distinguished Member

    Apr 28, 2012
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    The linked document is very useful.

    Using a HF transformer for 200W, and you need:

    1. Do the maths and calculate it's dimensions
    2. Or don't calculate it, and over-dimensionate
    3. Do a mix of both, calculate a little + try different cores

    I have so far only built single inductor based converters.
    Recently I made a small flyback transformer.
    I did not calculate anything, from the material,
    to the number of turns or wire diameter. For 240V -> 3v (but adjustable upto 20v).

    It worked straight away! Literally I just added one full layer fine wire, and about 15 turns thick wire for the secondary.

    For 200W, it is unavoidable to calculate it, at least somehow.
    Or to experiment with many different cores.

    I must also say I have taken apart a lot of stuff when I was younger, so there is some experience for dimensioning of components.

    The magnetic cores I bought from Radionics.
     
  20. espguitarist

    Thread Starter New Member

    Jul 19, 2012
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    How does a boost converter compare to a push-pull converter in terms of cost? What about overall temperature during operation? Looking at the schematic you posted, it looks like it would have four coils?

    I guess I've determined that I only need 210W for 8ms at a time (10% duty cycle on solenoid), after that I only need 21W (100% duty cycle on solenoid).

    If a capacitor can provide the wattage/current that I need for the first 8ms, then does that mean I could use a lower wattage converter (of either type)?

    I would assume that a lower wattage converter will take longer to charge the capacitor all the way, but that might be okay depending on how the timing works out. That's something I'll have to determine once we have a working prototype.
     
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