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LMR51430: Web Bench recommended Inductor has confusing large current rating

Paul Bryson
Prodigy 10 points
Part Number: LMR51430

I used the Web Bench Power Designer to design a buck converter, Vin-20V-30V; Vout=12V @ 0.5A.  The inductors recommended have current ratings above 9.5A (one as high as 18A!).  This seems excessive and would add unnecessary cost and size to my design.  The datasheet only has one equation regarding inductor ripple current.  Web Bench lists 300mA as the inductor ripple current.   I do not see the reason for such a large current rating. Can someone explain it to me?  What is the actual current rating required for the inductor?

  • 0
    TI__Guru 50120 points

    Hello,

    The inductor saturation current rating should generally be higher than the peak current limit of the device. 

    That design recommendation sounds like it is selecting an overdesigned inductor for this application.

    Generally for most designs you would only need to pick an inductor with a 5-A saturation current  given that current limit is around 4.76A typical.

    Regards,

    Jimmy

  • 0 in reply to Jimmy Hua
    Prodigy 10 points

    a. So to check my understanding, the current limit you referenced would be a short duration instantaneous current?  And therefore concerns the saturation current more than the dc current rating of the inductor?

    b. What conditions would cause the circuit to reach these peak currents?

    c. So would the dc current rating for the inductor be based on the max output current plus the rms ripple current? 

  • +1 in reply to Paul Bryson
    TI__Guru 50120 points

    a. I Depending on the fault condition (overload or short circuit) the duration will be different. If it is an overload condition (resistance on Vout is low but not enough to be 0Ohm shorting VOUT to GND), it will be cycle by cycle current limit. If the Vout to GND is sustained, then it will enter hiccup mode which is a short duration. In both cases, the peak inductor current is railing at Isc.

    b. See explanation above

    c. When looking at an inductor, I would make sure that it the saturation current is at Isc. The saturation current in this instance is the current where the inductor derates by 10%. In the case of the LMR51430EVM, the inductor used is the 7447714068. 

    Looking at the inductor datasheet for a 6.8uH inductor to derate by 10% (eg. 6.8uH to ~6uH), the current is around 6.7A which is above the datasheet typical of 4.76uH, making the EVM inductor a suitable component. 

    Regards,

    Jimmy

  • 0 in reply to Jimmy Hua
    Prodigy 10 points

    So if I understand you, the current limit will only be reached during a fault in the load?  So if I have a well controlled load the inductor current rating could be much less?