DC to DC Converter

Discussion in 'General Electronics Chat' started by R!f@@, Mar 19, 2011.

  1. R!f@@

    Thread Starter AAC Fanatic!

    Apr 2, 2009
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    What type of Converter can convert a 12VDC to 24VDC.
    Power out needs to be atleast 350W.

    I would atleast require an input I of 30Amps. Quite easy for a heave duty PC PSU.

    Any one done this type of work or have a schema I can refer to?

    Or Is it possible to take a part a PC PSU and rewind the former for proper output ?
     
    Last edited: Mar 19, 2011
  2. Papabravo

    Expert

    Feb 24, 2006
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    It is a type of switching regulator called a "boost regulator" The efficiency of this device will be in the range of 80-95% depending on your skill with understanding magnetics and proper layout among other things. Linear Technology, National, and others have many helpful application notes.

    So for an output power of 350 Watts Output, assuming 80% efficiency for starters you will need:
    Code ( (Unknown Language)):
    1.  
    2. 350 / 0.80 = 437.5 Watts Input Power
    3.  
    4. 437.5 / 12 = 36.45 Amps
    5.  
    6. Inductor Peak Current = 2 * 36.45 ≈ 73 Amps.
    7.  
    Based on the 700 circular mills per amp rule I'd say the wire for your inductor needs to be about a quarter inch in diameter (AWG 3). To find the core material and wind such an inductor will be a substantial challenge. Good Luck. You might want to get your feet wet with switching regulators by building something a bit more modest. I doubt you have the stones to pull it off with your first shot. BTW -- my first attempt at a boost regulator to drive a thermal printer was an unmitigated disaster.

    Last point. DC power supplies can't be "rewound". Only an AC transformer or an inductor can be "rewound". A PC PSU will put out the same regulated DC voltages over a wide range of AC inputs.

    Why not just design and build the power supply you need?
     
    Last edited: Mar 19, 2011
  3. R!f@@

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    Apr 2, 2009
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    I do have some power supplies that produce 24V from mains and I think they can handle quite a power, problem is I am having a hard trying to fix them as they were taken from telephone equipments and are faulty.

    I try to fix one and just does not seem to maintain an output. it's too difficult to desolder haevy duty components and is too packed to do a descent work on.

    Still I do have one more PSU of the same kind.
    My question is will it's former work for me.

    Rather than building a AC to DC one, I though a Low voltage dc to dc will be a bit of a less challenge. I have been getting a lot of resources from lotta guys so Now building a PCB will be easy, although it would be a challenge for being a first timer I thought of trying it.

    Like you said , I too thought the difficult part will be to get the TX right. But since I have plenty of real powerful SMPS transformers of various kind I thought I could try using one of them. They have multi strand copper windings. I have read that multi strand are better than single wire windings.

    What would I need to see that if those tx works. I presume I will need an Inductance meter as such.

    Can You gimme a list of things I should find and look for?
     
  4. Papabravo

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    Feb 24, 2006
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    A boost regulator does not require a transformer, it requires an inductor. IMHO it is a fools errand to try to reuse existing magnetics for purposes other than the ones they were designed for. It is quite difficult to obtain the information you need on core material and other things without some pretty highe end equipment.

    As far as transformers are concerned I don't see how they do anything for you since the outputs of power supplies do not depend on the transformers, but on the regulators. The regulators can be linear or switching and they will continue to work over some range of AC inputs.

    Maybe you could clarify what you are trying to accomplish.
     
  5. debe

    Well-Known Member

    Sep 21, 2010
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    Hi R!f@@, I have dismantled some SMPS transformers & rewound them for 12V small inverters. There is 2 ways to dismantle the transformers, the slow way is soak in paint stripper & dismantle. The quick way is in an old Microwave oven, I put the T/F in with a mug of water (i find the magnetron lasts a little longer this way) Heat T/F untill all glue holding core together is soft & dismantle. Oven in pic is on 2nd Magnetron I have a vast supply of good 2nd hand magnetrons from old microwave ovens. So be aware this is hard on a microwave magnetron but does work. The windings may sparkle a little sometimes.
     
  6. R!f@@

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    Apr 2, 2009
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    Hey debe, I like your way of working, your method never hit me on how to take apart a ferrite core Tx. Cool ..and thanks, I will definitely try that method.

    @papa.
    My aim is to use a PC PSU to power my CNC. I got this pretty nifty casing and was wondering that it would make a great casing for my CNC machine. I can put everything in it, u know the PSU and stepper drivers plus the PC.
     
  7. Papabravo

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    Feb 24, 2006
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    A PC PSU would not be my first choice to power motors, either steppers or servo motors. The main reason is that SMPS have a hard time dealing with load transients. When a phase is first energized in either a stepper of a servo (industrial, not hobby) there is a large demand for current until the back EMF comes up to oppose the in-rush current.

    Remember when I told you about my first boost regulator for the thermal printer. Without jumping through enormous hoops there was really no way to power that beastie from a 3.7 V Li-Ion battery. Every time we turned it on the SMPS went to it's knees and did not bother to stop there. If you want to run motors on a CNC, for heavens sake don't do it on the cheap from your junk box. Do it right and turn out all sorts of cool parts on the CNC.
     
  8. R!f@@

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    hehehe...so U suggest that I try it with a linear PSU
     
  9. tom66

    Senior Member

    May 9, 2009
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    I've modified a 12V ATX supply to put out 24V on the 12V rail before. If you have some old junk ones lying around you might want to try it. It's quite easy, you just have to find the feedback resistors, and solder a pot across them. Adjusting the pot will usually allow you to change the voltage. Be careful not to short it - as the current limits are usually spec'd for 12V, and you might need to remove an SCR for the overvoltage protection. And replacing the electrolytic caps wouldn't be a bad idea.
     
  10. R!f@@

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    Old ones...I have new powerful ones.
    Do you think by hacking the feedback I can get 24V at the rated power?
     
  11. Papabravo

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    Hacking an SMPS can double the voltage but will cut the current in half for about the same power output. The efficiency may not be what you expect and exceeding maximum ratings on other components like capacitors and diodes is a distinct possibility. But hey -- you're young and you probably think you're invincible. I certainly did when I was your age.
     
  12. tom66

    Senior Member

    May 9, 2009
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    Yep, probably. I managed to get mine to about 30V before it cut out. Not sure what caused that to happen, but a simple reset and it would work again. Wasn't really bothered about going higher so I left it there. Note the fan often runs off the 12V rail, so it will increase in speed with a higher voltage. You might want to leave it at a higher speed though to improve airflow.

    I would recommend replacing the electrolytics with higher rated ones - 50V minimum, to improve lifespan. The Schottky diodes on the output (as part of the flyback converter circuitry) may be a concern. Worst case, a cap pops or a diode blows and the supply dies. It's very unlikely to blow the main fet even under an overload condition. And the supply is isolated.
     
  13. R!f@@

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    Ok let's I have PSU that can handle say 16Amps at V1, V2 & V3. Total I can draw 48Amps Con.
    Now I know about capacitor rating and diode ratings, I can replace those with proper ones.
    By Hacking into the feed back, you are saying that I can double the Voltage. I know I can increase it but doubling is something I did not expect.

    The Power will be constant, so at 24V current will be at 24Amps. Not bad. May be this is possible.
    One question though, Like papa said, will it handle the 4 Nema23 300oz steppers driven by an allegro TB6560 Driver chip. These steppers draw 6Amps max. Will the start sequence shut down a PC PSU, or is this open for speculation.

    Do I have to destroy a PSU to find out. I mean putting a PSU back in order after all the trouble is not easy. This I know. But if it works then this will make my casing a whole lot lighter. I do have a 24VAC 600VA Transformer. The weight is too much for me.
     
  14. tom66

    Senior Member

    May 9, 2009
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    Yes, it's more than possible. Here's a video I made: http://www.youtube.com/watch?v=mK_rIs7uFlo

    I wouldn't rely on the maximum output current. Keep within 80% of that rating. Most supplies are over-rated on this specification.

    Unfortunately the modification is usually permanent, although if it's a pot I suppose you could desolder it at a later date. I'd *highly* recommend you remove the ATX connector so someone doesn't plug it in and kill their motherboard or two SSDs.
     
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