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LM51561H: LM51561H-Q1

karthik j
Prodigy 20 points
Part Number: LM51561H
Other Parts Discussed in Thread: LM5156

Hi,

   I am trying to create SEPIC regulator configuration using LM51561HQPWPRQ1 IC. My input voltage range is 9V to 16V and required output is 9V/300mA.

   I have attached 2 simulation circuit with this and only difference between 2 circuit is that MOSFET p/n is different. Intention of simulation is to find the current flowing through inductor and observe voltage across the MOSFET drain and source terminal to choose the appropriate part. 

  When i try simulation with 16V input, output is same for both circuit and no anomaly is observed. But when i try simulation with 9V input, output differs for both circuit. I am seeing significant increase in voltage across drain to source mosfet terminal and higher current flowing through inductor. Like to know why there is change in output significantly as it will affect the selection of the MOSFET and inductor. Also like to know whether increased value observed is a simulation model issue and i can ignore the same.

LM5156_TINA_2.TSC

LM5156_TINA_1.TSC

  • 0
    TI__Guru 59445 points

    Hello karthik,

    Please give us some time to understand your simulations and check out what is going on there.

    Best regards,
    Brigitte

  • 0 in reply to Brigitte
    Prodigy 20 points

    Hi Brigitte,

        Any update on the query?

  • 0 in reply to karthik j
    TI__Guru 59445 points

    Hello karthik,

    We observed the difference in your two simulation files, but up to now, we do not understand where this is coming from. I expect that the difference is a simulation issue, not a real circuit issue.

    Best regards,
    Brigitte

  • 0 in reply to Brigitte
    TI__Mastermind 45361 points

    Hi Karthik,

    what you observe in the simulation will most properly also be visible in the real hardware.
    Due to the very high gate resistors you have selected (100Ohm) the transition time from on to off and also off to on this very high.

    This will lead to a high loss during this transition.

    With the large transition time and the high Rds,on of the IRFR420 (3Ohm) the SEPIC will shift energy between the inductors and the coupling capacitor.

    A similar behavior can be observed with a SEPIC which uses and coupled inductor with a very high coupling factor.

    When you reduce the gate resistor to a few Ohms (e.g. 3Ohm) you will see that this issue can be solved.

    But still leave the question of the IRFR420 is good to be used in this application - it looks very over dimensioned from the voltage rating which on the other hand lead to a very high Rds,on.

    Best regards,

     Stefan

  • 0 in reply to Stefan Schauer
    Prodigy 20 points

    Hi Stefan, 

      I tried simulation with Zero gate resistance but the issue still exist. So can i consider that by choosing MOSFET with low Rds ON resistance i can fix this issue?

  • +1 in reply to karthik j
    TI__Guru 59445 points

    Hello karthik,

    Yes, lower rDS(on) should solve the issue.

    Best regards,
    Brigitte