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UCC28722 Transformer Bobbin and Core?

Sharmaine Quia
Prodigy 80 points

Hello. This is my first time to develop and make a power supply

The specs is already given in the we-bench, but I don't know how to construct it?

What bobbin to use? What core to use? How many turns in the primary?secondary?auxiliary?

I really need help and support from the experts. Please help me deal with this matter.

Thank you for your help and support.

  • 0
    Prodigy 185 points
    Hello,

    I have recently design a HF transformer for flyback topology, I have followed switchng power supply design, from abhram pressman.
    Steps can be highlighted as follows for transformer design in a nut-shell.
    1) Calculate the required input power based on the load requirements and take say efficiency as 0.7.
    2)Calculate the area product based on the power requirement.
    3)Your calculated area product should be less than the selected core. Thus you can accomodate your power supply requirements within that core.(say EE20,30 etc.)
    4)Take certain assumption i.e. max. duty cycle (0.45-0.4)current density say 300A/cm^2,flux density say 0.2T for ferrite cores, and that can be verified from data sheet of the core.
    5)You should calculate the no. of primary turns based on your output voltage + consider diode drop also and your input voltage and duty cycle
    6)Calculate the required primary inductance and air gap to fulfil it.
    6)Secondary turns and further turns of other secondaries can be calculate considering primary turns.
    7)Need to cross-check and verify in the end if flux density is not exceeded, and n1*a1(turns ratio*area of strand of all wires dont exceed the window area,etc.)
    8) You can select corresponding bobbin for your core as in the data sheet of the core and also based on your number of secondaries if it accomodates or you need any flying winding as all bobbin sometimes may not support if you have more than say 3 to 4 isolated secondaries.

    Transformer design is tedious, but once I made my own detailed excel sheet, with proper understanding it feels comfortable. and also need to learn new things. As magnetics is the crux of the power supply design.
    Apart from these follows EMI/EMC,device selection,snubbers etc. But i am really interested in power supply design and looking towards getting more and more indepth.
    All the best.

    Regards,
    Mukesh
  • 0 in reply to mukesh chaudhary
    Prodigy 80 points

    Thanks for the reply :)

    1. Do you have a softcopy of the book? Where can I download it?

    2. If Area Product is calculated already, How will I know which core should be use? For example, I calculated 0.04cm2 as the Area Product, then I will use RM8, How will I know if RM8 is the right core for me to use?

    Please help me regarding this matter. I'm quite confuse regarding the choice of core and bobbin.

  • 0 in reply to Sharmaine Quia
    Prodigy 185 points
    Hello,

    I have the hard copy of the book here. I dont know where to get soft copy. If i will get it will forward you.
    Each core has specific area product i.e. core area x window area(AexWa), your calculated area product should be less than this, in order for that core to accomodate your power requirement. say 0.04cm^4 is the area product you calculated then you select core is area product more than this and thus it will be sure that.

    In your case RM8's area product is Ae=63mm^2 and We= 30.9mm^2 and thus area product comes out to be 0.189 cm^4, which is greater than 0.04cm^4,so you can comfortable go with this core.

    Regards,
    Mukesh
  • 0 in reply to mukesh chaudhary
    Prodigy 80 points
    Hello,

    May I know sir what's the formula of the area product on that book?
    I've searched in the internet but there are so many formula.
    I'm confuse what formula to be used.
    Another concern sir, I'm only seeing Ae in the datasheet.
    Is there a formula also for We? or it also can be seen in the datasheet?
  • 0 in reply to Sharmaine Quia
    Prodigy 185 points
    An is the window area as given in the data sheet and nomenclature of the data sheet.
    Formula for the are product in Ap=(2*E*10^(4)/ku*Bm*J) (cm^4) in some books its Ap=(2*E*10^4/Bm.Ku.J)^1.14.
    where, E is the energy of the core, 0.5L*I^2, and ku is the window utilization factor Bm the flux density and J the curent density
    It is basically derived from transformer equation. Just see that this Ap is sufficient for your wattage of the core.

    Regards,
    Mukesh