Buck and Boost converter

Discussion in 'The Projects Forum' started by Humanityrulz, Jul 2, 2010.

  1. Humanityrulz

    Thread Starter Active Member

    Jul 21, 2009
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    Hi,
    First I gone through previous posts realted Buck boost converter, but I haven't found what I need.
    Q) Can I use MC33063A as buck and boost at same time?
    (manufacturer given either buck or boost ckts. Excel sheet attached in ZIP format.)
    Vin=12V,
    Vout= 5V(min) to 15V(max)(by adjusting the R1(6.7k to 2k ohms) value in datasheet).
    My load current max 500mA(1A for safe side).

    Thankyou in Advance.
     
  2. sceadwian

    New Member

    Jun 1, 2009
    499
    37
    Not sure about that specific controller, but the keyword to search for with boost + buck converters is sepic. This allows continuous operation from bellow at or above input/output voltage.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    Humanityrulz likes this.
  4. Humanityrulz

    Thread Starter Active Member

    Jul 21, 2009
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    Thank you for your replies.

    Hi SgtWookie,
    Thnaks for doc..It really help full, I calculate most values for my design but I am stuck about Cout value( pag-19), by considering ESR value.
    In doc.. design example Cout calulated value is 15.7uF, Cout value they considered 330uF(!!! Don't know why?) for ESR =0.12ohms, I tried to match these values with ESR equation given in page 19, I am got negative vlaues of ESR or Cout... I didn't get it...

    By reading the document I undertstand Cout is on of the important component in operation,even Rsc and Ct are also critical components as I believe.
    Can I use any PNP transistor and diodes(schottkey) for my design(similar Vsat and Vfd1,Vfd2)?
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Now that I think of it, I'm sorry that I gave you that link to the ONSemi's application note.
    Their datasheet for the MC33063/MC34063 has some rather annoying errors in it; for example in figure 17 on page 7, more than one of the formulas uses the variable being defined on both sides of the equation (regarding Ton and Toff). Somebody pulled a fast one. I found that out perhaps 6 months ago when helping another member with a boost circuit.

    It would not surprise me to find similar foul-ups in that applications note. If you used their same formulas and came up with a negative ESR for what they calculated, then obviously something is wrong. However, did you work it all out by hand and write everything down? Go back and double-check your work.

    Have a look at Texas Instruments' application note for the same IC:
    http://focus.ti.com/lit/an/slva252b/slva252b.pdf

    Something else to consider is ONSemi's Excel spreadsheet design tool for the NCP3063. This IC is very similar to the MC33063/MC34063, except it switches at a higher frequency.
    The Excel spreadsheet tool is on this page:
    http://www.onsemi.com/PowerSolutions/supportDoc.do?type=tools&rpn=NCP3063

    Unfortunately, it does not have a buck-boost configuration in the tool; only buck, boost and inverting.
     
  6. Humanityrulz

    Thread Starter Active Member

    Jul 21, 2009
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    To be honest, application really clarified most of my questions expect few errors.
    Except ESR formula all seems good, ESR formula might be right but their considered values are giving -ve, something wrong. They didn't given much info abt that any way.
    Yeah, I work it all out by hand and write and I cross checked with application note values.
    I will check again, for my application As I said I am working on variable output voltage min 5V to 15V.

    So I calculated values for both 5v and 15V. Rsc, Ct, R1,R2,L values are different for 5and 15V.

    By using pot I can manage the R1 value to satisfy both output voltages 5 and 15V(R2 constant) but Rsc, Ct are critical components(very low values) How can choose these values for output voltage because Vout depends Rsc ,Ct and L.
    If I choose Rsc, Ct values for output 5V or 15V, those are giving too much noise at 15V output(noise around 5v) How can I select these vaues .
    Suggest me in this...

    I uploaded .rar(similar excel sheet) file in my first post. any way thankyou for those links...

    Any suggestion... will help.....:)
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    You can add a filter stage to the output. Anywhere from 2.7uH to 10uH in series, with a 100uF to 200uF cap to ground will eliminate much of the ripple. It negatively impacts response time, of course.
     
  8. tom66

    Senior Member

    May 9, 2009
    2,613
    213
    I found a very nice online calculator which does a lot of the work for you:

    http://www.nomad.ee/micros/mc34063a/index.shtml

    You'll want to overspecify the Cout capacitor however due to imperfections with real world capacitors. Also the inductor should be a bit bigger than Lmin. What you can do is use the buck-boost (inverting) converter. This produces a negative voltage though. It shouldn't be a problem if the device doesn't need to interface to any other parts - just put ground as the positive wire and -5V (or whatever) as your ground (for your device.)

    The MC34063A should work from 3V to 40V. When using the calculator, specify the minimum input voltage you need as the input voltage, otherwise the switch current might be too small or the inductor underspecified to generate the voltage.
     
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  9. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
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    In post #6, Did you say:
    ?

    Sorry... ;)

    As for bucking and boosting, Using ICs are very efficient, but not always required. There is some good information relating to SEPIC circuits that I have attached.

    Also, you may want to spend and hour watching this video on TIs website. It is Power Supplies 101 and it has some really good info on all types of regulators
    http://e2e.ti.com/videos/m/power_management/180921.aspx
     
  10. zxsa

    Member

    Jun 11, 2010
    31
    2
    For a product that I was working on, I had to find a buck-and-boost regulator as well. You can use a SEPIC circuit, but I found a regulator which, at least in my opinion, is more efficient. Have a look at TI's TPS55065.
     
  11. Humanityrulz

    Thread Starter Active Member

    Jul 21, 2009
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    It is good regulator for 5V out and variable input voltage 1.5V to 40V, but I am looking for adjustble output voltage Vout(min)=5V to Vout(max)=15V by input 11.7V to14.5V.
    This IC doing job for 5V output, I think corect me if I am wrong.
     
  12. zxsa

    Member

    Jun 11, 2010
    31
    2
    Sorry, I missed the part where you said you wanted an adjustable output.

    In that case, it might be easier having a two-stage regulator. Stage 1 is a boost-regulator generating 16V and Stage 2 is a variable buck regulator allowing you to select the output in the range 5V-15V.

    How are you wanting to control the output voltage? By pot (mechanical input) or maybe by software in a microcontroller?
     
  13. Humanityrulz

    Thread Starter Active Member

    Jul 21, 2009
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    Why I can't adjust output voltage using pot(R1 )?
    R1 and R2 ratio changes the comperator set range and controls the gate if it less than nominal voltage.
    clarify me.. in this please...

    I will watch .. Thank you for link..
     
  14. Humanityrulz

    Thread Starter Active Member

    Jul 21, 2009
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    Ok.. Thanks for idea..have you tried this way.. two stage.. if possible..give some info..please..
    .... I am using mechanical pot.... for my application..
     
  15. zxsa

    Member

    Jun 11, 2010
    31
    2
    I haven't done a two-stage like this before. But, with a two stage circuit like what I suggested, you could find a smaller, more efficient circuit for each stage. Maybe your total circuit will be bigger and a little more expensive, but I'd think it will be easier to get it running stable.

    Where possible, I would try to find regulator ICs using a higher switching frequency (like 2MHz). Higher frequency means your inductor becomes smaller.

    For the pot: what you should do is to use a normal resistor and a pot in series. When your pot is at the zero-position, the normal resistor sets the one boundary. This way, you can use a smaller resistance value pot and have better (more accurate) control over the feedback voltage.
     
  16. Humanityrulz

    Thread Starter Active Member

    Jul 21, 2009
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    I am trying to reduce the components as much as possible.. for stable output , I don't worry abt that ofcourse.
    Higher switching frequency.. might work but timing capacitor and Rsc values for this IC will lower..for 50KHz Rsc=0.22ohms Ct=524pf that means for 2MHz it's lessthan 0.22ohms and 524pf which bit worry.. I guess.
    If you got any IC which works at high freq, I will try ...anyway..
    Yeah.... I am doing that.. Thanks for details...
     
  17. hdinhhien

    New Member

    Dec 28, 2010
    1
    0

    I want output current of MC33063 greater than 500mA ?
    we can get more transistor but I do not know the value of the resistor to open transistor ?
    can you help for me.
    thankyou!
     
  18. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    Have you looked at any of the solutions from National or Maxim yet?
     
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