Increasing output current of smps with multiple darlington transistors

Discussion in 'Power Electronics' started by uraniumhexoflorite, Feb 18, 2017.

  1. uraniumhexoflorite

    Thread Starter Member

    Oct 23, 2016
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    I've been working on a buck converter using an MC34063, but the output current is far lower than I need. I want to be able to pull at least 700 ma. I was able to get 70 ma without an external transistor. Yesterday, I set up an external transistor and increased the current to 150 ma. The external transistor I used is a TIP116. I was wondering if having another TIP116 would help to increase the current output more. Basically making a darlington transistor using darlington transistors. I apologize if this question seems a bit ridiculous, but I have very little experience with amplifiers. I have op amps aswell, but I'm not sure if they are what I should use. Thanks. :)
     
  2. RichardO

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    May 4, 2013
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    We need your exact circuit diagram to give an accurate answer. What is your switching frequency and what is the specification for the inductor that you are actually using?

    Just guessing from what you said, I suspect that your inductor is not rated for high enough current and is saturating. The MC34063 by itself appears to be able to supply enough current without a buffer transistor.

    Adding a Darlington transistor is not likely to help much. In fact it may make your problems worse because it drops more voltage when on and has a lower switching speed. At these low currents a non-Darlington transistor might help some.
     
  3. uraniumhexoflorite

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    Oct 23, 2016
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    How do I tell if the inductor is saturaring?
     
  4. RichardO

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    May 4, 2013
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    You need an oscilloscope. You monitor the current through the current sense resistor (Rsc in the examples in the data sheet).

    This is best done by using two scope probes with the scope set for differential mode. Put one probe on each end of Rsc.
    The waveform should be a linear ramp. If the waveform starts out as a ramp and then suddenly increases then the inductor is saturating.

    This measurement is hard enough to do that it is usually easier to get a data sheet for your inductor or buy an inductor that has the correct rating. The data sheet has the equations to calculate the peak inductor current.
     
  5. uraniumhexoflorite

    Thread Starter Member

    Oct 23, 2016
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    It might not be the inductor. I'm using one I salvaged from a broken cigarette lighter adapter charger. The circuit in the adapter charger put out approximately 700 ma.
     
  6. ian field

    AAC Fanatic!

    Oct 27, 2012
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    Darlington transistors barely saturate, so they increase on time losses. There's not much you can do to increase carrier purging from the B/E region, so turn off time can be slower which increases transition losses. Its best to favour any schemes in the appnote that don't involve Darlingtons.
     
  7. uraniumhexoflorite

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    Oct 23, 2016
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    would a FET or a BJT be better?
     
  8. ian field

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    A MOSFET is generally better, but AFAICR; there are no examples in the appnote.

    You have to remember that the MOSFET is voltage driven instead of current driven. Most power MOSFETs need at least 6V gate drive to turn them fully on and minimise on time losses. Logic level types are OK with only a few volts gate drive. The main problem you could run into is charging and discharging the gate capacitance quickly enough to minimise transition losses - but its no worse than purging charge carriers from a bipolar B/E region.
     
  9. uraniumhexoflorite

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    Oct 23, 2016
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    So I should use a BJT instead of the TIP116? The MC34063 is switching at 4 MHz.
     
  10. uraniumhexoflorite

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    Oct 23, 2016
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    I did some more experimenting. I accidentally set up the MC34063 in an external NPN configuration, but the TIP116 still works better than a 1011 BJT or a TIP110.
     
  11. uraniumhexoflorite

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    Oct 23, 2016
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    I did some more experimenting. I accidentally set up the MC34063 in an external NPN configuration, but the TIP116 still works better than a 1011 BJT or a TIP110.
     
  12. ian field

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    4MHz is more than the ft of most power transistors for that kind of application - you may have to search the selection guides for a MOSFET that's fast enough.
     
  13. uraniumhexoflorite

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    Oct 23, 2016
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    I'll try it at a lower frequency.
     
  14. ian field

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    You probably need to stay under 500kHz - the 34063 data sheet will tell you what its capable of. Even at that frequency you'll have growing challenges with some of the components.
     
  15. RichardO

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    May 4, 2013
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    I would run the oscillator at more like 50KHz as per the example in the attached data sheet. Make sure that your inductor works at that frequency. You need to at least measure its inductance to have a chance of using it.

    I also attached a couple of application notes that you should find useful.
     
  16. RichardO

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    Both of these observations say that you have something fundamentally wrong.

    I suggest that you select the most basic circuit and get it working -- no external transistors. Maybe at the lower frequency things will work a lot better.
     
  17. uraniumhexoflorite

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    Oct 23, 2016
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    I had it working in the basic step down configuration, but it couldn't supply enough current. All I'm trying to get it to do now is supply more current.
     
  18. uraniumhexoflorite

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    Oct 23, 2016
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    I can lower the frequency substantially. I'll try it with some larger timing capacitors.
     
  19. ian field

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    50kHz is probably near the bottom end of where you need to start thinking along the lines of multilayer ceramic chip filter capacitors.

    You certainly won't get away with cheap electrolytics, smaller MLCC caps in parallel would be useful to reduce temperature rise.
     
  20. RichardO

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    I believe that the most basic circuit will deliver a current similar to what you want. As an example, see Figure 10 in the data sheet I posted earlier.
     
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