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DC/DC Power Supply Help

Discussion in 'Power Electronics' started by Mozencrath, Dec 15, 2011.

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  1. Mozencrath

    Mozencrath

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    0
    Dec 15, 2011
    Hi all, I am in need of a DC-DC Power supply schematic that outputs between 12.7 and 14.5VDC and an input that could be anywhere from 12VDC up to 36VDC. The crutch is this-I need it to put out at around 80 amps. Realistically, the supply will be running 80% of the time under 20amps, but there are occassions where it will need to draw upwards of 60amps continuous.

    I found this schematic (attached), but it looks like it is only about 50% efficient assuming a 24v in and 12v out. Am I correct on this or should it be higher?

    Does anyone know of any simple circuits for a supply that will work for me, or know of one that I can simply modify to get the power output needed? Any thoughts would be appreciated. To give you an idea of my experience level, I was an EE major for 2 years but never completed my schooling.

    I don't need anything fancy, and really am trying to keep it as simple as possible and get 80% efficiency or better. Is this possible with BJT transistors, or will I need to use a switching supply or MOSFET design?

    Thanks !

    this is a link to the schematic I referenced: http://www.zen22142.zen.co.uk/Circuits/Power/3730.htm
     

    Attached Files:

  2. davelectronic

    davelectronic

    1,087
    12
    Dec 13, 2010
    Hi Mozencrath.
    Welcome to the forum.
    your link schematic is a large power supply, not a DC to DC converter, what your asking is possible to build, but would be an advanced project, given first the selection of voltages you want, and it would be a fairly large unit, given the current you want, retail units are few and far between with that current rating.

    Your schematic is an AC to DC power supply, using multiple transistors to up the current.
    So what you want is to take a DC source and make that a high current DC source, in there is a problem, i dont think you will get much more power from your DC source than it can deliver, so whats the high current driving, if you dont mind me asking ?.

    Ive seen your schematic before on the web site its on, some good circuits on there.
    Any way i think you need to clarify what you want to accomplish. :)
     
  3. jackorocko

    jackorocko

    1,284
    1
    Apr 4, 2010
    Dave is correct, if you have 12V input and want out 14.5V at 80A, then your input will need to deliver more then [email protected]% efficiency. You can not create power out of nothing, you can only convert quantities of power. ex. want more voltage, then you will need to decrease your current. want more current then you will need to decrease your voltage.

    I would try to keep your input V greater then your output V and do a search for dc-dc buck converter. Right now I think you are a little confused on exactly what you need. Hopefully we can clear some of that up if you provide us with some details.
     
  4. Mozencrath

    Mozencrath

    13
    0
    Dec 15, 2011
    Yes the circuit technically is an AC to DC power supply, but I simply used a 24V battery pack instead of an AC transformer. I will be a bit more clear for you as well. As far as input, I do not need it to accept 12VDC to 36VDC, I mean I can input 12, 24 or 36VDC since my power source is 12V batteries....

    What I am powering is actually my Prius' 12V system. The system is currently powered by an internal DC/DC Converter within the prius that converts the main 1.3kw battery (220V nom) into 13.3 - 14.1 VDC. It has a rated output of 100A, however the system will only ever draw 80 amps max on rare occassions. Typically the load is about 15-30 amps.

    I mean, I was even thinking of trying to find a simple circuit that can take my 24V battery setup and just output ~13-14V. Can I do something like this without transistors? Just an LM317 to regulate the voltage, and a capacitor to smooth it out?

    Your advice is GREATLY appreciated!
     
  5. jackorocko

    jackorocko

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    1
    Apr 4, 2010
    What do you have against the design that you need to change it? Essentially your gonna be creating the exact same thing. Why do you wish to reinvent the wheel?

    Yes, but the LM317 is only designed for at it's max 1A, you will however get less then that. Also, VR's are very inefficient the further the input V is away form the regulated voltage.
     
    Last edited: Dec 15, 2011
  6. duke37

    duke37

    5,364
    772
    Jan 9, 2011
    Dropping 24V to 12V using a resistive circuit will give an efficiency of 50%. To get a higher efficiency, you will need to store the energy in an inductor and send it to the output. Pulse width modulation will be needed to do this and will enable a constant output voltage with varying load and input voltage.

    Fets are better for switching circuits since they can have a very low resistance when turned fully on.
     
  7. Mozencrath

    Mozencrath

    13
    0
    Dec 15, 2011
    Do you have an example of a high current DC to DC that would utilize FET's?

    I have nothing against the schematic I posted, but I can not deal with the hassle of trying to cool hundreds of watts of power dissipation from the outboard transistors. And, with only 50% efficiency, I would need almost double the battery capacity to run the system.
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    This should tell you more about what you need.

    Such devices are already so common that it would probably be easier to buy rather than build one.

    Your question may be better asked at an electric vehicle forum as there are many DIY types out there who would have needed something similar (although typically from a higher input voltage).

    Here is one manufacturere of these devices.
     
  9. Mozencrath

    Mozencrath

    13
    0
    Dec 15, 2011

    Thanks for the links, but @ powerstream I would be looking at spending almost $1,000. My only option is to make my own. I work at an electronics supplier and we stock over 750,000 different parts obsolete and new, so my cost on parts would be essentially zero.
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    OK, you have 2 problems.

    Firstly (and I didn't notice this before) your minimum input voltage is below your output voltage, and the maximum is above it. This means that you need something that is neither a buck or a boost regulator. One solution to this is a boost followed by a buck regulator (and this doubles the complexity).

    The second is the power output. At these levels the design is very critical.

    For these reasons, the design would be somewhat complex.

    I am trying to think of a *simple* way to do it, and my best suggestion is to find a way to combine the output of several lower current devices. To do that is tricky, and the best I can suggest is that each regulator would need to be current limited and possibly isolated from the others by a diode.
     
  11. electro_pa

    electro_pa

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    Aug 14, 2011
    Prius 12v supply??

    Hi Mozencrath,
    You say:
    I wonder what you really are doing? Can you explain.
    I am a NHW11 Prius owner. I have the six 'Service Manuals', and I understand how the car operates in detail.
    Sounds like you wish to eliminate the 12v battery and sub a power converter running off the 'Traction battery'.
    Am I right?
    Where do the 12,24,36volt come in? That's a puzzle to me.
    I think you are really confused about the Hybrid Drive System. The traction battery of mine is about 272volts.
    The inverter for 12volts is a rather light affair. It charges the 12volt SLA 'Accessory battery'. Nothing like 100amps!
    I've seen the 12volt battery deliver 25 amps, while it pumps up the hydraulic brake accumulater, just after switch-on, then drops to less than 2 amps without lamps, wipers & steering loads.
    It doesn't charge the 12volt battery while the Hybrid System is not started.
    If there is no 12volts available at switch-on, nothing happens! All the system computers operate from 12volts.
    What is wrong with the Toyota system that you want to change?
    Clive.
     
  12. circuitronics

    circuitronics

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    Dec 18, 2011
  13. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Hahahahaha Linear regulator. Oh circuitronics you crack me up.
     
  14. jackorocko

    jackorocko

    1,284
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    Apr 4, 2010
    Seems the OP has no intention of explaining what he is really up to. I asked the same thing.
     
  15. Mozencrath

    Mozencrath

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    0
    Dec 15, 2011
    Sorry everyone I was not getting notifications on this thread. The NWH11 Prius is much different than the Gen II prius. I do not want to tap into the traction battery. I want to power the entire 12v system via an external power source i.e. batteries in either 24v or 36v configuration. Yes, the battery supplies 25amps at startuo and will show 2 amps once the car is started, but that 2 amps is not outgoing, it is 2 amps charging the battery. If you were to put your clamp meter on the converter output, you would see that the converter is supplying anywhere from 15 - 80 amps of 14v DC.

    The problem with the Toyota system that I want to change, is the fact that currently the cars computers and accesories are powered via the traction battery. The Prius' DC/DC converter converts the 200v traction battery into 14v. During typical driving in the Gen II, the traction battery is providing a constant 600 watts. This is essentially half of the capacity of the traction battery being wasted every hour. My intention is to disconnect the converter, and power the 12v system using an external power supply which is the build in question here.

    I hope this explains everything,
     
  16. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    So let's assume for a moment that your 600W is correct.

    And lets assume that this 600W could be provided by other external batteries.

    Where would the energy come from to charge them?

    Or do you want your car's range to be limited by the charge on a set of batteries that you can't recharge until you get home?

    (Are you intending to use deep cycle batteries?)

    The 600W must be almost insignificant alongside the power required from the batteries to actually drive the wheels. And like a regular car, the engine powers what is effectively a large alternator to recharge the battery anyway.

    The charging efficiency of the battery pack in the Prius is way better than a lead acid battery. And when are you ever going to be running an hour from just batteries?

    I'm pretty sure the designers of the Prius thought this one out.
     
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