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LM25037-Q1 Push-Pull topology

tateo yamashiro
Guru 10890 points
Other Parts Discussed in Thread: LM25037, UCC28250

Hi,

My customer has question about LM25037.
Could you tell me about questoin?

[Question]
 My customer considers to use LM25037 under the following condition.

  Topology: Push-Pull
  Vin: 6V to 16V
  Vout: +/-20V
  Iout=12.5A
  fsw=100kHz

 - How can we calculate minimum input capacitor requirement and Cin ripple current?

 - When primary MOSFET is turn-on, it occurs the spike current on MOSFET.
   So R-C filter is needed on CS pin.
   Why does it occur the spike current?

Best Regards,
tateo

  • 0
    TI__Expert 7395 points
    Yamashiro-San,

    Thanks for sending along. I will contact the appropriate engineer to answer.

    Regards
    John
  • 0
    TI__Intellectual 1550 points
    Yamashiro-San,

    To answer your questions:

    1. If you look at the primary-side current of push-pull converter (its shape looks the same as in full bridge as shown in Fig. 44 of UCC28250 datasheet), the peak-to-peak ripple current of Cin is basically equal to the peak value of the inductor current transposed to the primary side. You will need to use the min Vin (i.e., max duty cycle and max inductor current ripple) to get the max current ripple of Cin.

    To get the minimum required input capacitance, you will need to derive the input capacitor ripple by integrating the current of Cin over Ton (i.e., on-time of one primary switch) and then divide the integration result by Cin. This is because the min Vcin occurs at the beginning of the switching period while the max Vcin occurs at t=Ton.

    2. The spike current is caused by the reverse recovery of the diode on the secondary. LM25037 implements leading-edge blanking which alleviates the requirement of the external RC filter. This in turn improves the current sense response.

    Let me know if you have further questions.

    Regard,
    Wangxin