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LM5041: Transformer Design

Nesh Johny
Expert 1310 points
Part Number: LM5041

Dear Sir,

The transformer used in LM5041EVAL board is having 1.25mH primary inductance. (Np :8T, Ns: 1T)  

in LM5041 the input is 35V-70V then after buck stage it'll be around 22V then current fed push-pull stage, which is having transformer which has a secondary of 2.5V.

1. From the core datasheet the AL value should be around 24000 nH/T2. is it correct?

2. This much big AL value core is required or around 5000nH/T2 is fine . magnetizing current will be high or which loss I have to consider in this scenario.

3. What will be the minimum primary inductance required for a current fed push pull stage transformer. (any reference document)

Thanks & regards,

Nesh

  • 0
    TI__Mastermind 34465 points

    Hello Nesh,

    The A9786-AL is an 8T primary transformer (pdf attached) with a primary inductance of 1.25mH

    LM5041.pdf

    The AL = 1.25e-6/(8*8) = 19.52nH 

    This means the core is gapped by the manufacturer to give an AL equal to this value.

    Please check the method you used to calculate the AL.

    The value of the primary inductance is set by the operating voltage and the max on time of the transformer.

    For reasonable power loss in the core the max operating flux density (Bmax) is limited to 100mT

    This calculations required are detailed in the magnetics section in the following TI seminars:

    Please enter "magnetics" into the topic section.

    Regards

    John

  • 0 in reply to John Griffin1
    Expert 1310 points

    Dear Sir,

    Thanks for the reply,

    Please correct me in the calculation  of AL, if I'm wrong.

    I'm getting 19531nH  (instead of 19.52nH )

    Is my calculation wrong because of any unit conversion.

    Regards,

    Nesh

  • 0 in reply to Nesh Johny
    TI__Mastermind 34465 points
    Hi Nesh,
    Sorry you are correct. The AL is 19531nH/T2 or 19.5uH/T2
    This number is only relevant in terms of calculating the inductance associated with a number of turns.
    If you want to calculate the magnetizing current you need to know the the following:
    I_mag = V*Ton/L
    V is the voltage across the winding
    Ton is the time the voltage is applied to the winding
    L is the inductance value.
    You can see that the magnetising current will be smaller for larger magnetising inductance.
    Regards
    John
  • 0 in reply to John Griffin1
    Expert 1310 points
    Hi sir,

    Does the higher magnetizing current lead to any instability of the converter or higher loss (since primary current will become more).

    thanks
    Nesh
  • 0 in reply to Nesh Johny
    TI__Mastermind 34465 points
    Hi Nesh
    Yes. A high magnetising current will lead to high loss in the transformer primary.
    It is best to minimise the magnetising current as much as possible.
    Regards
    John
  • 0 in reply to John Griffin1
    Expert 1310 points
    Thank you sir