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TPS65950 with large inductors

Shawn Workman
Genius 15210 points
Other Parts Discussed in Thread: TPS65950

In reviewing the TPS65950 and I came across something that needs clarification. The external inductor of the switching power supplies is specified with a minimum inductance of 0.7uH, a nominal 1uH, and a maximum of 1.3uH.

 

What is the implication of making the inductor greater than 1.3uH?  What are the limitations? Is it a loss of efficiency, transient response or instability?  This application is subject to large magnetic fields in an MRI environment for a patient monitoring. There is an investigation into using a ferrite core inductors where the nominal inductance is 10uH but when the core is saturated due to a large external magnetic field it drops to 1uH.

 

In my experience, most switchers don’t mind operating with larger inductors once they are in continuous mode so my only thought is that this part is designed to run in discontinuous mode only or the compensation network is borderline with low phase margin. Can I use a larger value inductor if the change the capacitor to keep the corresponding LC filter the same?

 

Best regards,

Shawn 

  • 0
    Intellectual 290 points

    Sizing the inductor for the TPS65950 or any switching power supply is an exercise in design optimization.  The value range suggested in the datasheet is designed to get you "in the ballpark" when it comes to inductor selection.  Increasing the inductance has the benefit of reducing the peak inductor current at the expense of requiring a physically larger core.  Larger inductance also affects the phase margin in the feedback loop, and with an inductance big enough to sustain continuous current throughout the cycle the inductor becomes part of the feedback loop and regulator instability can result if not compensated for.  If the inductor was sized properly, increasing the inductance and core size brings only a very slight gain in efficiency before efficiency begins to drop off due to greater core losses.  The trick is finding that sweet spot where the efficiency is near maximum and peak currents are not so large that they pose filtering problems. This results in the smallest physical size possible for the power inductor that will deliver the given output power without heating up excessively.  That's the goal here, a grand compromise that yields the smallest possible inductor.  If you aren't designing portable systems however, maybe you don't care about this.

    Regards,

    -Jeff

  • 0 in reply to Jeffs_Fairway
    TI__Genius 15100 points

    Hi Shawn,

    It is difficult to guarantee the device performance with a large inductor. Theoretically, even if it is possible we cannot guarantee this because our lab tests are not done with these values. Whatever we recommend on the data sheet is tested in lab and TI can guarantee the device behavior.

     

    Regards,

    Gandhar.