Double Buck converter in series

Discussion in 'The Projects Forum' started by jeffpkamp, Jul 31, 2013.

  1. jeffpkamp

    Thread Starter New Member

    Jul 31, 2013
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    I am trying to run a petier cooler off of a 12V battery. I am using buck converters to lower the voltage. One problem is the cooler draws up to 6A, but the buck converter is only rated for 3A.

    My initial plan was to put two buck converters at 6V each (connected in parallel to the same battery) in series for a total of 12V and 3A through each converter. Then I could adjust either of the converters to adjust to total voltage.

    However, I realized that these are circuits, not batteries, and after drawing it out, it seems like hooking the - terminal of BUCK1 to the + terminal of BUCK2 should cause an electrical short... Am I wrong about this? Is there a better way to do this?

    I would appreciate any suggestions on using multiple buck converters to regulate the voltage on my cooler, with under 3A per buck. Thanks!
     
  2. donpetru

    Active Member

    Nov 14, 2008
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    Search for higher power buck converter; do not use two in parallel because it is not reliable and, in some cases, not worth it.
     
  3. cornishlad

    Member

    Jul 31, 2013
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    Ther is quite a lot of confusion in your question ! But here are some observations.

    first..as donpetru says..not a good idea to parallel connect them to get increase current.
    Almost all the buck converters you may be looking at (China..Ebay ?) do not have isolated outputs, so, as you have said, you CANNOT connect them in series.
     
  4. jeffpkamp

    Thread Starter New Member

    Jul 31, 2013
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    I am here to learn too, so humor my speculation for a moment, I have a torroidal transformer with 1:1 windings.

    If I hooked that up to one of the converters to isolate it, could I then use it in series? (see drawing, Voltmeter on the right is my device, ignore the stats for the inductors).

    I already have the buck converters, and the high power ones I saw are 50-100x more expensive, so it'd be nice if I could get this to work.
     
  5. cornishlad

    Member

    Jul 31, 2013
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    It's the parallel / serial that is confusing. Can you confirm that you want to get 6v at 6amps from a 12v battery and you have 2 x bucks 12v to 6v@3amp ? Or am I failing to see something....
    As for you last question you must know that you cannot use a transformer in a DC circuit.
     
  6. jeffpkamp

    Thread Starter New Member

    Jul 31, 2013
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    Sorry, I realized after I drew the circit that if I wanted to couple it that way I would have to do so at the primary induction coil, which would work since the switching of the converter would make it "AC", but it would not work at the position I had it drawn (I was drawing this conclusion from a diagram of flybuck converter, which is isolated.)

    As for the confusion... the Vin (+ and - ) for the two buck converters is in parallel. The Vout (+ and -) were what I wanted to put in series.

    I want to get a maximum of 12 V at 6A from this set up, in a way I can efficently modulate the voltage (Ie, I can turn down one of the buck converters to make it 6-12V, hence why they need to be in series).
     
  7. BobTPH

    Active Member

    Jun 5, 2013
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    Putting the two in series doubles the voltage and leaves that current unchanged, so that is not what you want.

    Putting them in parallel doubles the current while leaving the voltage unchanged. This is what you want.

    You can put them in parallel, but you will need a resistor in series with each one first so that they current will be shared equally.

    Bob
     
  8. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
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    What about a PWM driver instead.
     
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  9. jeffpkamp

    Thread Starter New Member

    Jul 31, 2013
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    Alright, Here is the jist of my original question. Can I treat buck converters as being synonomous with a battery? My assumption is no, since they share a common ground (these are connected in parallel to the same battery for Vin).

    Bob, correct me if I am wrong, but my current is only limited by the resistance in my circuit (I=V/R). The battery I have this hooked up to can put out over 100 amps no problem, and the buck converters don't have much resistance.

    The limiting resistance in my circuit is the Cooler I am going to hook this up to, which draws 5 amps at 12V.

    My buck converters can only safely conduct 3A, so I am trying to split the current between the two of them, by making them each do half the work (cooler draws 12V, 5A; each converter pushes 6V 2.5A when in series for a total of 12V, 5A).

    Bill I may just have and get a PWM, I have one, but something is wrong with it. I can measure voltage on the out terminals, but I have no current. I would assume if the PWM chip was at full off or even mostly off, I wouldn't detect the full voltage difference at the Vout terminals. Not really sure what is going on there, but I can get another one for 5 dollars...
     
  10. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Your post #1 says you want to connect the inputs in parallel, but the outputs in series. That cannot be done on buck converters, you must have SMPS converters with ISOLATED outputs to do that.
     
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  11. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
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  12. wayneh

    Expert

    Sep 9, 2010
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    A simple PWM setup would be better than horsing around with converters, IMHO. Or ditch the battery and use a 5V computer PSU if you want reduced current.
     
  13. jeffpkamp

    Thread Starter New Member

    Jul 31, 2013
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    Thanks for confirming my suspicions and for all the input everyone. I think I'll take spend the $5 and get an 8A PWM:)
     
  14. MrChips

    Moderator

    Oct 2, 2009
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    So you want to control the power delivered to a TEC ( thermo-electric cooler).
    Scrap the buck converters and simply use PWM as Bill suggests.

    Edit: sorry I was still on first page.
     
  15. cornishlad

    Member

    Jul 31, 2013
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    Bob has told you that even if you could put two converters in series you double the voltage but leave the current available unchanged. Two converters in series = 12v @ 3 amps max. peltier device needs 72 watts - you have 36 watts available. Give up !
    The current is not just limited by the battery but by the power rating of the bucks - which may be an over optimistic figure anyway if they have come from Ebay/foreign land.
    I don't know anything about Peltier coolers so I ask if they are something other than a pure resistive load ? Try you PWM again on a 12v lamp bulb. If yours is dud get another. It's the best and most efficient way to go if the Peltier allows.

    sorry..I too missed some previous posts.
     
    Last edited: Aug 1, 2013
  16. BobTPH

    Active Member

    Jun 5, 2013
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    Now I am totally confused. You have a 12V battery, your Peltier device draws 5A at 12V, but you want to convert down to 6V, put two in series getting 12V and connect this to your device. This makes no sense. It would have the same effect as connecting directly to the battery with no buck converters in between, except for the fact that they could only handle 3A and would probably burn out.

    If you intend to run the Peltier device at 12V and you have a 12V battery, just connect the battery to the device.

    Bob
     
  17. cornishlad

    Member

    Jul 31, 2013
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    I think he wanted to "modulate" (his word) one of bucks with its output pot. to get 12v variable down to 6v. And I assumed he was worried about efficiency hence the buck converters.
     
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