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Help with current amplification

Discussion in 'General Electronics Discussion' started by J_M, Jun 9, 2011.

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  1. J_M

    J_M

    4
    0
    Jun 9, 2011
    I am hoping I can get assistance with the following modification to an existing circuit:


    * Existing circuit: Output ~5VDC - 12VDC, 10 watts.

    I need to increase the current capability ~500 watts.

    I understand it might be simple to add a secondary power supply with a mosfet but I don't have the knowledge to design the modification. Would this be as simple as connecting a mosfet to the existing output? Would I require any type of isolation?

    I would greatly appreciate a simple circuit diagram to help me get my mind around how this type of amplifier would work so I can attempt some basic testing.

    Thanks.

    P.S. If I have left out any important info please advise.
     
  2. davenn

    davenn Moderator

    13,883
    1,966
    Sep 5, 2009
    500W at 5 - 12VDC ?

    you are going to need a very hefty PSU for that one built for the job.
    you cant produce extra power from something that cant supply it, ie. you cant modify the existing one.

    500W / 12V = 41.6 Amps

    one 13.8V 40 amp PSU I have at home for powering my transceiver radio cost ~ AU$300 from a supplier called Jaycar. My transceiver at full output pulls ~ 30A so the PSU still has some headroom... ie. its not running at peak output.
    in reality if you need 12V @ 500W (42A) contineously, then the PSU is going to have to be rated even higher than that. So that it also is not being driven too hard.
    Prob something ~ 45A minimum peak load would be advisable.

    its gonna cost you a few $$$ not something you can make easily, and a switch mode PSU would be the most efficient and lightest to move around.

    Dave
     
  3. J_M

    J_M

    4
    0
    Jun 9, 2011
    Agreed, however most likely I won't require the full 500 watts but I do want to allow for it as I need continuous duty.

    At this time I will be testing with an old tranformer/rectifier/capacitor bank from a very old industrial battery backup system that can handle the load. I will most likely end up using an SP series Meanwell, I am expecting to pay about $200 for a 500w/12V unit.

    My problem is not with the secondary power supply but with how to use the existing controller to control the higher current.
     
  4. davenn

    davenn Moderator

    13,883
    1,966
    Sep 5, 2009
    forget about the existing unit. you really just need a complete replacement supply.
    Why would you have the requirement for the existing unit to control a bigger supply ?

    explain you use and need a bit clearer so we can get an idea as to what you are trying to achieve :)

    Dave
     
  5. J_M

    J_M

    4
    0
    Jun 9, 2011
    I think you may be misunderstanding the problem. The real issue is finding a way to use the existing controller (which varies output voltage from 5v-12v) to provide a higher current while still maintaining the variable output voltage.
     
  6. daddles

    daddles

    443
    3
    Jun 10, 2011
    I think you've given the idea of your problem, but you haven't stated requirements clearly enough -- engineers need clear specs to design things. And, by the way, power isn't the same as current.

    So, you might say something like: I need a circuit that will take a 5-12 volt signal in and will present an Xi ohm load to this 5-12 volt supply. This should then drive an output load of Xo ohms with Y amps of current, limiting the total power to 500 W. Or somesuch... Include a specification of how close you need this behavior to mimic the input and how quickly. Even better, give something like a transfer function or a simple mathematical relationship that maps the input voltage into the output behavior you want. You also should give an idea of how much money you're willing to spend, as that tends to drive what solutions can be used.
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,499
    2,839
    Jan 21, 2010
    I think you misunderstand what you're asking.

    At a minimum, here are the things you would need to do:

    1) replace the existing mosfets (or parallel more) -- assuming the switching elements are mosfets) -- with ones rated for about 200A (the existing ones are probably rated for about 4A).

    2) (1) wull cause the power dissipation to increase substantially, so a 50 times larger heatsink (or forced air cooling) will likely be required.

    3) (1) will increase the capacitance and therefore current drive required, so the gate drivers will need to be beefed up considerably.

    4) The increased current will requite that any current sensing (used for circuit protection) will need to be reduced to 2% of its current value.

    5) Consequent to (4) you will have to beef up the wires.traces required to carry this current. Also any connectors will need to be changed.

    6) The inductors used will need to be replaced for ones capable of 50x the current (you're looking at 200A at least I would say) and these will be more than 50 times the size of the existing ones.

    7) Diodes used in the supply for rectification or flyback (depending on the topology) will need to be replaced with ones capable of the higher current. This may require additional heatsinking (remember 50 times the current). Since the voltage drop increases with current, the power dissipated may exceed 50 times the amount dissipated previously.

    8) Due to additional losses caused by (7) you may need to reconsider the input current and inductor specs to allow for the additional power loss.

    9) Capacitors used to reduce ripple will now be subject to 50 times the ripple current and will need to be hugely increased to allow for this. Generally speaking, the same capacitance will be required, but with a 50 fold reduction in ESR. Simply speaking, 50 times larger.

    10) to reduce some of the "50 times" effects, you could increase the frequency. However this may require that the controller be changed, and you are then going to need to re-spec the mosfets, the inductor, any diodes, and the filter capacitors.

    How much would be left of the original? Not much I expect.
     
  8. J_M

    J_M

    4
    0
    Jun 9, 2011
    I see, for all intents and purposes it would likely be more efficient to build a higher spec controller from the ground up. It sounds like I might be better served sourcing a new controller circuit with PWM and using a solid state relay. Cost will probably be higher but I am outside my area of expertise with regards to 'beefing up' the existing circuit.

    Thanks for your input.
     
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