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Webench design for LM2678 does not simulate

Kishen Someshwar
Intellectual 1625 points
Other Parts Discussed in Thread: LM2678

Hello Team,


I have a customer who is interested in using the LM2678 for their upcoming designs. They are using the webench simulation tool to study the device. For their input parameters Vmin = 8V Vmax = 40V, Vout = 12V and Iout = 5A, there was no simulation generated. I tried with the same parameters and got the same result. However when I input 20-22V, Vout= 12V Iout = 5A, I do get a simulation. Is there an issue with webench for this device. How can the customer evaluate this device for the specs they need.


Thank you for you time and attention towards this issue.


Sincerely,


Kishen

  • 0
    TI__Expert 6365 points

    Hi Kishen,

    The LM2678 is BUCK device and it's only configured in WEBENCH as a BUCK. For the design to be created, the VinMin has to be greater than Vout.

    Hope this answers your question.

  • 0 in reply to Howard Chen-WEBENCH Design Center
    TI__Intellectual 1625 points

    Hi Howard,

    Thank you for your attention towards this post. I took your advice and I tried simulating from 15 V to 40 V , Vout 12V , 5A  and still could not get a simualtion. Basically my customers main concern is this:

    " What I am try to understand is the size of the inductor needed to get 12V at 5 A, and the effect of lower inductance due the saturating current going through it. I.e if the current at load increases that causes the inductance value to drop how does the output voltage behave (increase in ripple ?). I can model this with Vin on 21V, but will that simulation be accurate at Vin of 24V ? Because of space restrictions I cannot put a bigger size inductor that can handle 5A without saturation.

    If we cannot resolve this, I will design this with the LTM8054 where no external inductor is needed."

    I appreciate your support in this.

    Sincerely,

    Kishen

  • 0 in reply to Kishen Someshwar
    TI__Mastermind 41310 points

    Kishen,

    There is a lot of information that is missing here. Does your customer already have a design that they are using?

    It appears that they already have an inductor chosen for the design. It also appears that the VIN needs to be 24V, with a Vout of 12V and output current of 5A. This design can be created in WEBENCH® successfully. You can easily run a few steady state simulations with the design and vary the inductance to see the effect on the output voltage, but I am not sure that is what your customer is looking for.

    It actually sounds like your customer wants to see the effect of the inductor saturating and the impact that has on the output voltage. I am sure that the basic spice models used for inductors would not capture this effect. The saturation of the inductor will cause the inductance to decrease, which in turn will cause the peak current to increase. The ripple current will also increase. If held that way for a long time, the heat and winding losses will become extensive and could cause damage to other components and the inductor.