Buck Converter, Problems Problems and Failures!

Discussion in 'The Projects Forum' started by Mustafatarhan49, Oct 18, 2012.

  1. Mustafatarhan49

    Thread Starter Member

    Aug 16, 2012
    56
    0
    I have been working on a DC DC buck converter. I have faced with a lot of problem. I really need the your help. I will post the problems that I have up to now. So may be you can comment it.

    - I need help for high side gate driving circuit, I searched about bootstrap capacitor but I did not understand at all.
    - I need help for the feedback loop- Any help how to do that?
    -Component selections=> I have found many website and read many article but each time I got different values.

    To inform that I used parameters such Vin:50V Vout:5volt Iout =30mA fswitch =100~150kHz
    Any schematic or other things will be perfect!
     
    Last edited: Oct 18, 2012
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,979
    744
    Last edited: Oct 18, 2012
  3. Mustafatarhan49

    Thread Starter Member

    Aug 16, 2012
    56
    0
  4. Mustafatarhan49

    Thread Starter Member

    Aug 16, 2012
    56
    0
    Any help from this forum???
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Do you have a schematic drawn yet?

    We need to have something to work with.

    You have left many items not addressed.

    What will be your input voltage range?

    What is the desired output voltage and current?

    Here is a Design and Application Guide for MOSFET Drivers by Laslo Balogh:
    http://www.ti.com/lit/ml/slup169/slup169.pdf
    You will need Adobe Reader, Foxit Reader or another PDF viewer installed to see it.

    Discussion on High-side non-isolated drivers begins on page 18. Bootstrap technique discussion starts on page 22. However, I strongly suggest that you read the entire document; as if you missed the information in the beginning (or don't know it yet), you will be very, very lost and confused.
     
  6. Mustafatarhan49

    Thread Starter Member

    Aug 16, 2012
    56
    0
    Input voltage range is in between 14.4Volt and 50V and My desired output voltage is 6 volt and Max current is 50mA.


    please check the .asc file in the attachment which I've built this circuit today with my shallow knowladge. I used Frequency generator and I've applied below parameters to the circuit.

    Vin:10V
    Fswitch:50kHz
    D=%10
    Cout : 100nF (instead of 22uF in the simulation file)

    According to that circuit,I saw that I could switch on and off my mosfet on the breadboard. But i could not see the desired output voltage. I applied 10V to the input and I was getting 17V and I've got many ripples in the output.
    I mean my buck converter worked as a boost converter :D I am really inexperienced ...

    (I've just made this error amplifier and comparator which I am not sure if it really works, I have not tried it in the breadboard)



    Thanks a lot for the datasheet! I will start reading now!
     
    Last edited: Oct 19, 2012
  7. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    IMHO, trying to build a switcher from discrete components is ridiculous if you are actually building a project when there are cheap, integrated devices available. It will end up costing more, taking up more space, and be less reliable.

    If it is a homework assignment, that's different.

    Since I used to design power supplies, what jumps out is your lack of specs to design from:

    Does it need to be short circuit protected?

    Thermal protection?

    Tolerance on the input voltage and output voltage?

    Size allowed?

    All SMD or through hole components?

    In electronics you have to define what it is you are seeking to do before you can figure out how to do it.

    BTW: what first came to my mind seeing this:

    Vin:50V Vout:5volt Iout =30mA

    is what a waste of engineering design effort it is to build a switcher when a simple linear could do it with a total power dissipation of less than 1.5W. Usually, the complexity and cost of switchers is only justified when the power savings are significant or it is battery powered and power MUST be conserved.
     
    Last edited: Oct 19, 2012
  8. Mustafatarhan49

    Thread Starter Member

    Aug 16, 2012
    56
    0
    Actually, This is not homework.Basically, I am searching for cheap and efficient power supplies for this range.
    I am tottally not against to use cheap IC if I can find it. But,the meaining of cheap component or Ic depend on what you do.
    In my case, there is already running cheap Linear regulator for this conversion which is combination of two step, one decreasing voltage with zener to 14.4 and for the second part L.Regulator to get stable output but the thing that we want to develop is, to develop a switched supply which can increase efficiency and reduce cost (I don't expect the cost to be less than LReg but I want to see something comparable).

    I checked with the suppliers and I found really cheap inductor (150uH for a few euro cents) That's why I decided to try this. If you have any other advice, I am willing to look in to that too.

    I will try to answer all of these above questions.
    -It is not mandotary to have short circuit protection, It might be an optional property.
    -Thermal protection is somehow important. We need to consider for the high temperature too.
    -Input voltage can go high up to 60V Output voltage can go up to 5.6V
    - Smd design will be good for the design.
     
  9. crutschow

    Expert

    Mar 14, 2008
    13,002
    3,229
    What amount of ripple voltage can you tolerate?

    What is the load?
     
  10. Mustafatarhan49

    Thread Starter Member

    Aug 16, 2012
    56
    0
    I can tolarete 100mV. My output is 5V. I will connect to load to a uC or to a sensor. I will also try to read some ADC value from this output.
     
  11. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    Gotcha.

    You want a discrete component switcher that costs less than an IC switcher and costs comparable to a linear.... but you did not specify the total cost you have to meet.

    Makes me nostalgic for my old job.:D

    I used to get requests like that every day.

    good luck
     
  12. Mustafatarhan49

    Thread Starter Member

    Aug 16, 2012
    56
    0
    LOL, :D My price target is in the range of 0.60€-1€ range. I have found really cheap inductor arround 0.04€ and now focusing on the rest of the parts :)
     
  13. Mustafatarhan49

    Thread Starter Member

    Aug 16, 2012
    56
    0
    I have tried to make buck operation here, but I got boosted result :D
    Any idea why I get my result like this in the attachment ?
     
    Last edited: Oct 22, 2012
  14. crutschow

    Expert

    Mar 14, 2008
    13,002
    3,229
    Try a larger Cout, like 100uF or more.
     
  15. Mustafatarhan49

    Thread Starter Member

    Aug 16, 2012
    56
    0
    I just made a quick simulation with Ltspice.
    Output ripples are avoided here after replacing with the 100uF.
    But my problem is output voltage. It is too high. What should I do to decrease it to the desired value (Vout =Vin*D =1V)
     
  16. crutschow

    Expert

    Mar 14, 2008
    13,002
    3,229
    You have the P-MOSFET source and drain reversed (normal operation is for the current direction to be from source to drain). The way you have it the intrinsic source-drain diode is forward biased and the transistor cannot shut off the voltage. Notice that the drain voltage is going from 10V (full on) to about 9.4V when the diode is conducting.

    Also notice that the turn-off time of the MOSFET is very slow (top trace) due to the high gate capacitance of the MOSFET. You need to reduce the value of R1.
     
    Last edited: Oct 23, 2012
  17. Mustafatarhan49

    Thread Starter Member

    Aug 16, 2012
    56
    0
    I changed the circuit like what you advise. I reduce the Rsg to 200ohm and increased the Cout to the 100uF. Now I see that my mosfet have enough time to turn on and off. But look, Even though I am applying 0.1Duty cycle I can not get expected output voltage... I can decrease the duty cycle but I know it is not possible in practical application. please see the attached file.
     
  18. RamaD

    Active Member

    Dec 4, 2009
    254
    33
    Your load is 1K, means the current is too low, below this buck converter's minimum. For your 10% duty cycle, if the expected output is about 1V, your minimum load should be about 300mA, say about 3 Ohms load.
    You can check this out by changing your load to 3 Ohms, and the output voltage should actually fall below 1V, because the above calc's are approx. without taking into account the diode & MOSFET drops.
     
  19. Mustafatarhan49

    Thread Starter Member

    Aug 16, 2012
    56
    0

    3ohm mean is not too small ? A wire can have 3ohm resistance too.
     
  20. RamaD

    Active Member

    Dec 4, 2009
    254
    33
    3 Ohms is a lot of resistance for a wire. For example, 25 SWG wire with a cross sectional area of 0.2 sq mm will have a resistance of 80mOhm for 1 metre length.

    Since we are expecting 1V output, 3 Ohms load will draw ~333mA for it and will satisfy the minimum load requirement. If the current is more, then the resistance is even less!

    I was wondering if you could simulate to check the output voltage.
     
Loading...