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LM5041EVAL: Parallel resistor for Differential mode inductor

Nesh Johny
Expert 1310 points
Part Number: LM5041EVAL

Hi Sir,

Keeping a (10E) resistor parallel to the diffrential mode inductor L1 (10uH) is mandatory or what is the practical advantage of the resistor in the circuit.

  • 0
    TI__Mastermind 25570 points

    Hello Johny

    I don't have access to the designer of this EVM but the resistor is almost certainly there in order to damp the resonance of the input LC filter (L1, C1,2,3,4).

    A power supply presents a constant power load to its source and its input filter can oscillate under certain conditions - this behaviour is explained at http://www.ti.com/lit/an/snva538/snva538.pdf and https://training.ti.com/power-tips-damping-input-filter among others.

    This input filter has a resonance at about 17kHz with a characteristic impedance of about 1 Ohm (jWL) at resonance so optimum damping would use a 2 Ohm resistor. The resistor appears in parallel with the inductor so it limits the ability of the circuit to filter out noise currents and I suspect that the designer used 10 Ohms because it was a good tradeoff between providing enough damping while keeping reasonable filter performance.

    I hope this helps - please let us know if you need any more information.

    Regards

    Colin