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TPS54291: inductor selection

leinho
Expert 1230 points
Other Parts Discussed in Thread: TPS54291, SWITCHERPRO, LM2678

Hi, 

How are you?

We are selecting the right discrete components to build a dc dc converter with a tps54291 chip...

We have some questions about the inductor selection:

Why did they choose that inductor in the design example on page 21? The irms is 1.51 A and the peak current is 1.76 A but they chose an inductor with a saturation current of 3.7 A...

Why? Isn't that too much?

Is there any restriction about the resistance of this inductor? They chose one with 10 mohm but they don't mention anything about it....


Thanks!


  • 0
    TI__Guru**** 191445 points

    There are a couple of things to consider when choosing inductors.  Of course you need to ensure that the ratings meet or exceed your rms and peak current requirements.  You may want to choose higher ratings if you want the inductor to not saturate or overheat during fault conditions.  Also, for the same inductor value, higher current rating means lower DCR and better efficiency.  I usually will pick the highest current rating inductor that is not physically too big for my design to get the best efficiency.  Sometime 2x or 3x the peak and rms currents for my design.

  • 0 in reply to JohnTucker
    Expert 1230 points

    Thanks John,

    That explain me a lot of things...

    I have a last question:

    Why in the design example there is a 10 ohm and a capacitor of 470 pF in the SW pin?

    There is nothing told about it.

    How did he pick it?

  • 0 in reply to leinho
    TI__Guru**** 191445 points

    That is a snubber circuit.  It is used to reduce ringing on the switching node.  The fast rise times can cause over-shoot and ringing, and the snubber can reduce some of that at the cost of  small loss of efficiency.  If you contact me off line, I can provide some general documentation on RC snubber component selection.

  • 0 in reply to JohnTucker
    Intellectual 930 points

    Hello John.

    If I can add my contribution to the discussion, usually small inductances are wired on iron powder cores.

    They usually have a specified inductance value far from the saturation current, this means that at the saturation current you could have 40-50% lower inductance value than expected.

    It depends from your design and from you application, but if you want to guarantee a CCM condition in you complete range it could be better to select bigger inductances than expected...

    The "problem" could be avoided using gapped ferrite cores, but it is usually more expensive and it is justified only when you are thinking toward bigger power...

    Have a nice day.

    Paolo

  • 0 in reply to Paolo Reggio
    Expert 1230 points

    Thanks Paolo!

  • 0 in reply to JohnTucker
    Expert 1230 points

    Hello John,

    Thanks for your response.

    I didn't remember about snubber circuits... I will look about it deeper later.

    I have two other questions in order to finish my pcb:

    1) I am calculating the total power dissipation in order to know how much will it dissipate.

    Related to the design example, where did he get the mosfet capacitance in order to calculate switching losses?

    2) I am thinking of dissipate along the ground plane of my circuit... How do i get the minimum area according to the Watt i need to dissipate?

    Thanks!

    Regards!

  • 0 in reply to leinho
    TI__Guru**** 191445 points

    Not sure where those numbers came from.  Most likely he got them from the IC designer.  You could also use switcherpro software to calculate the losses.

    The usual manner is to use the GND plane along with some additional ground area to dissipate the heat.  We have a calculator for that, but the TPS54291 is not yet supported.

    You could use the layout recommendation in the datasheet for an estimate of the copper area.

  • 0 in reply to JohnTucker
    Expert 1230 points

    I guess i have seen that calculator but my tps54291 and my two lm2678 ( 5 V fixed, and 11 V adjustable, Vin between 14 and 17 V) aren't supported...

    Do you have any advise about the power dissipation calculation and layout pattern of the lm2678... Nothing appears in its datasheet...