dc dc boost 12v to 18v, 4A [need help]

Discussion in 'The Projects Forum' started by mdagli1, Dec 4, 2012.

  1. mdagli1

    Thread Starter New Member

    Dec 4, 2012
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    Circuit: DC DC boost converter
    Chip: MC34063A
    Requirements: 12V in, 18V 4A out, 70-80% efficiency
    Goal: Because of the space, heat and EMI limitations, Simple, low cost, low profile, as few components as possible, conventional components will be used.
    Use: A PSU from a PC will be used to power a small but powerful amplifier circuit that I'm designing.

    Hi there all
    Just starting to get involved into electronic engineering again after 2 years.
    I have an 8 pin step-up inverting switching regulator but need some help calculating what values the components are suppose to be. I'm primarily work visually/pragmatically rather than numerically so any images/simulators would be useful.
    I've primarily copied this from a variety of sources but need assistance to verify the relationship between curtain parts.

    R1, R2: Current limiters for input voltage? don't think I need them.
    R3, R4: no idea...
    R5, R6: Feedback loop to control output voltage. (find out as I go along with some pots)
    C2: timing capacitor, no idea how to determine or adjust the frequency of this.
    D2: Is this really needed?

    Attached image:
    [​IMG]

    Edit: linked it to larger version
     
    Last edited: Dec 4, 2012
  2. crutschow

    Expert

    Mar 14, 2008
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    Have you looked at this?

    D2 doesn't seem to be needed and would, in fact, seem to keep the circuit from working. Do you know why it is in the circuit?

    Edit: If you want an easy design then use a device available from Linear Technology or TI who offer free design aids and simulators for their switching regulator circuits.
     
  3. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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  4. mdagli1

    Thread Starter New Member

    Dec 4, 2012
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    Thanks guys for the input, very useful.

    Just need to balance the frequency with the capacitor and inductor now.

    Only a few questions left before I put this all together (still waiting on the inductor to come in the mail).

    R4: Seems to be required for the switch in the documentation for the chip. What does this do and why would I need it?
    D1: Would using 4 of them into a full bridge rectifier improve efficiency and reduce ripple?
     
    Last edited: Dec 4, 2012
  5. crutschow

    Expert

    Mar 14, 2008
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    R4 is to discharge the large gate capacitance of the MOSFET and help turn it off more quickly.

    D1 can not be made into a bridge rectifier. Even if you could, it wouldn't help efficiency or reduce ripple.
     
  6. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    For a 4A Load, you probably want to look at a different regulator.

    The Peak current from the MC34063 is 1.5A for the internal switching transistors, meaning the output current will be a good deal less than that for the boost you are looking for.

    One candidate is the LM2587 Simple Switcher Flyback boost converter which has a rated 5A output. They are not at the low cost end, due to internal switches, but only a bit more complicated to set up.

    Another option would be to modify your existing switching power supply to output 18V, That option only for those with experience in the area, which includes not being averse to electrocution. This method, however, is not available for discussion on this forum for many reasons, safety being the major issue.
     
  7. mdagli1

    Thread Starter New Member

    Dec 4, 2012
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    So I need a completely different switcher chip? OK
    I got this chip because I was ordering a bunch off stuff all together and that was the only one available and didn't want to waste postage.

    An old work colleague has advised me to use a programme called Proteus. How does it compare to spice?

    Would be useful having something like this as I'm starting to waste money on parts I may or may not need...
     
  8. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    LTSpice is a good simulator and free. Proteus makes some things easier, though it isn't free, and many of those who help here use LTSpice, so exchanging schematics isn't as simple.

    Before ordering parts, you'll want to look at the datasheet to ensure what you are ordering matches what you are looking for. The times when something is overlooked is how a parts inventory for future projects gets built up. :)
     
  9. mdagli1

    Thread Starter New Member

    Dec 4, 2012
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  10. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    One would be sufficient for the small jump from 12V to 18V. Though putting them in parallel as suggested is an option for increasing output current, for double the cost.

    There are many design assistants around, such as This One for TI

    Here is a download and install package for the Simple Switcher Series I suggested.
     
  11. mdagli1

    Thread Starter New Member

    Dec 4, 2012
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    Chip: LM3478
    Still struggling a bit to get this thing to work.
    [​IMG]

    After running several simulations on the TI site, this seemed to be perfect.
    Got the whole thing running on my workbench here but is not maintaining the 18V under 33 Ohm load.

    I don't really understand why when the simulation seems to dictate otherwise.

    edit: OK, this is ******* me off now. Just noticed the chip is rated at 1A output, then why the hell did TI workbench recommend it? The simulation shows a steady state of 5A, what a load of bull. Man, do I feel misled by the stupid small sample size.
     
    Last edited: Dec 24, 2012
  12. mdagli1

    Thread Starter New Member

    Dec 4, 2012
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    Yup, I should really be going after monolithic chips as they generally rated at high continuous currents.
    As you suggest LM2587 and the LT1370 is what I'm after but in parallel. They seem quite costly so going to need to think about it carefully.
     
  13. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Can we make that a sticky?

    If not, can I forward it to my ex boss?:p
     
  14. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    If you had asked an old power supply designer, he would have warned that a 75W boost is not trivial. It's going to be pulling high peak input curretnts (over 10A) and it's tough to do that with monolithic devices.

    A better approach might be a two transistor push-pull using a SG3525 IC. Just change the transformer turns ratio to get a different output voltage.


    http://electronics-diy.com/electronic_schematic.php?id=883
     
    Last edited: Dec 24, 2012
  15. mdagli1

    Thread Starter New Member

    Dec 4, 2012
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    Thanks bountyhunter, very interesting and the most complete schematic I've seen so far. Completely missed the significance of the push pull stage.

    Will look into further.
     
  16. mdagli1

    Thread Starter New Member

    Dec 4, 2012
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    Well, has been very refreshing reading up on transistors. Can't believe on how much I've actually forgotten...

    Just tried the push-pull transistor set up next to the switch pin and getting interesting results.

    without: 20secs to charge 1F cap up to 17.8V
    with: 16sec to charge 1F cap up to 18.2V
     
  17. mdagli1

    Thread Starter New Member

    Dec 4, 2012
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    Well, managed to get hold of a pair of LM2587 and got it all working in parallel using a pot to adjust the regulated output to 18V. Right up to the point until I put on a 33 ohm load... (which worked fine on my STA540)

    My bench supply went well over 5A and should of only drawn 1A. Disconnecting the load had no effect and was stuck like that. Suspicious that they had blown up I then tested them both of them separately.
    1 is now allowing over 2A to go straight to ground with no voltage output.
    The other outputs 18V then drops down to 12V and draws over 0.5A with no load.

    I really don't understand WTF happened here... Those were rather costly to get hold of and don't particularly want to do it again.
     
  18. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    They don't work in parallel.
     
  19. takao21203

    Distinguished Member

    Apr 28, 2012
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    You can use it for more than 1.5A with external transistor.

    Not difficult for low voltage.

    The coil has to be a good one for high currents. If it is bad, you can see it will heat up. It is OK if it heats up a little, slowly, but any time it heats up fast, it is not suitable, bad efficiency.
     
  20. mdagli1

    Thread Starter New Member

    Dec 4, 2012
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