MC34063 Boost Voltage Dropping

Discussion in 'General Electronics Chat' started by sonar_abhi, Mar 19, 2016.

  1. sonar_abhi

    Thread Starter New Member

    Mar 19, 2016
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    Hello All,

    I have built a voltage boost circuit using MC34063 to boost from 3.2V to 5V with 350mA output current.

    The problem is I am getting 5V output when measured witha DMM but when I connect even a small load like a microcontroller, the output voltage drops to around 1.5 or 2 V.

    I have used the standard schematic of voltage boost mentioned in the datasheet. The values are as follows:

    R:180 ohms
    Rsc: 0.2 ohms
    Ct: 22pF
    L: 22uH
    Cin: 10uF
    Cout: 10uF
    R3/R4 for boosting: 3.3K and 10Kohms

    The inductor is a small one which resembles a 1/2 watt resistor.

    Please help 1458392105186-124527122.jpg
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Not all 22uH inductors are the same. Another important specification on an inductor is saturation current. You will not get 350mA of saturation current out of that little inductor. You need something with a core that won't saturate.
     
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  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    I agree with Gopher that the problem is the inductor. Also, since your peak collector current is less tha 1.5 A (and actually less than 1 A), you do not need the external transistor. The circuit for this is on the datadheet.

    ak
     
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  4. Picbuster

    Member

    Dec 2, 2013
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    Hi, your schematic is hard to read but If D1 = 1n5913(zener) you should use a schottky diode.
     
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  5. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    The datasheet calls for the Zener. The chip can also handle 1.5 A so there is no reason for the more complicated circuit the OP is using with the TIP41 - his 350 mA could be supplied directly by the chip.

    Also, the datasheet clearly has 2 bars on each inductor showing a core (of some ferromagnetic material) is needed for higher saturation current.

    Look at the circuit photographs on page 9 and 10 of this document for the type/size of the inductors used.

    http://www.onsemi.com/pub_link/Collateral/MC34063A-D.PDF

    The one labelled Coilcraft on page 10 and those with "221" on page 9.

    If you don't have one handy, you could try about 5 close wraps of insulated wire (~22 to 26 gauge) around a good sized nail (standard 3" nail, aka 10 penny).
     
    Last edited: Mar 19, 2016
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  6. sonar_abhi

    Thread Starter New Member

    Mar 19, 2016
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    Ok grt. Thanks for the response guys.
    So inductor seems to be the problem. Unfortunately I dont have access to a better inductor right now.
    Can I parallel two or three inductors to improve the current handling capacity? Will that work?
     
  7. Papabravo

    Expert

    Feb 24, 2006
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    Only if the result gives you the right inductance and you can eliminate coupling effects of closely space inductors. It is best not to take shortcuts with SMPS circuits.

    Since this is a boost converter it is worth mentioning that the inductor current may be twice the output current.
     
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  8. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    What us the saturation current of those inductors you want to parallel?
     
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  9. sonar_abhi

    Thread Starter New Member

    Mar 19, 2016
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    Hi. Unfortunately the current rating is not mentioned on the inductor. But the package resembles a 1/2 W resistor (snap was included in the forst post). If anybody can point out the saturation current of that device it would be helpful.

    Thanks
     
  10. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    I don't think it will work - even if you had a few dozen.

    Try the nail trick i mentioned in bottom of post 5. You may notice some heating of the nail because of Eddie currents but it is only an experiment. It may need more than 5 turns. Maybe 10 or so - lots of variables in inductors and the circuit can withstand lots of variables too. Finally, try the more simple circuit (first one in the datasheet because no transistor should be needed at the low currents you are using).
     
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  11. sonar_abhi

    Thread Starter New Member

    Mar 19, 2016
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    Ok. I will try that but I need a proper solution for production also so will try to get a proper inductor also. What should be the current rating that I should go for? 22uF/1A or 22uF/2A? Also what is the difference between an axial inductor and an inductor choke?
     
  12. Picbuster

    Member

    Dec 2, 2013
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    Hum they use a 1n5219 correct? www.onsemi.com/pub/Collateral/1N5817-D.PDF this sheet is for 17/18/19 and is a schottkey
    However correct me when wrong.
    picbuster
     
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  13. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    @sonar_abhi

    See post above. You need a different diode than what you have listed in your schematic. A Zener diode that you used is not correct. And Thank @Picbuster for a very good detailed look at your schematic.
     
  14. sonar_abhi

    Thread Starter New Member

    Mar 19, 2016
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    Yes...my bad. Actually I didnt get 1N5817 model in Proteus for simulation hence used what was available in Proteus.
    In the actual circuit, I have used a 1N5817. Sorry, I forgot to mention that in the post.

    @GopherT, what inductor should I use? 22uH/1A or more? 1A signifies the saturation current or the peak current?
     
  15. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    1 A should work for your 350mA peak demand.
     
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  16. sonar_abhi

    Thread Starter New Member

    Mar 19, 2016
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    Thanks man...you've been very kind to help. Actually everybody on this forum, I thank you all. I will get the inductor tomorrow and post the results

    Regards
     
  17. Willen

    Member

    Nov 13, 2015
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    Break any damaged CFL fluorescent light or something which has an big inductor in ferrite. Or wind 20 turns of 24SWG enamel coated copper wire on any ferrite core.
     
  18. sonar_abhi

    Thread Starter New Member

    Mar 19, 2016
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    Hello Guys,

    I made several changes in the circuit and tested in for the following changes:
    1] Inductor: 22uH/2A
    Shorted the pins 1 and 8 (to use it in a darlington configuration)
    Timing capacitor: 390pF
    Output capacitor: 100uF/16V
    Removed the external transistor and checked it with transistor also

    2] Inductor: 42uF/2A
    Shorted the pins 1 and 8 (to use it in a darlington configuration)
    Timing capacitor: 470pF
    Output capacitor: 1uF/16V
    Removed the external transistor and checked it with transistor also

    Still the same problem persists. The output voltage at no load is 5V proper but under load, it drops to 3.2V.

    The reason I persisted with MC34063 was its low cost (it costs Rs. 10/piece in India compared to Rs. 60/ piece for Micrel MIC2288) and that there are people who have posted that they have used the MC34063 chip with the same Vin and Vout at 300/350mA. But I guess for me, there is no other option than to use the Micrel chip.

    Thank you all for your help.
     
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