LT8708 How to estimate the NMOS power in Buck-boost mode

Hi team,

My client cares about heat around MOS when using the LT8708.

I want to estimate the power of 4 MOSs from M1 to M4.

As explained in the application information of the data sheet, the formula is either Buck or Boost.

What is the formula in Buck-Boost mode?

Assuming a forward direction of 12V to 12V.

Best regards,

T-yoshi

  • Power dissipation of the MOSFETs is difficult to determine accurately in buck-boost mode because the percentage of time in buck verses boost is varying and there is a hysteresis factor as well. You can simulate or measure to get typical values or use the table to get worst case values.

  • Hi Charlie-san,

    Thank you for your response.

    I understand that it is difficult to know the timing of switching accurately in buck-boost mode.

    First, let's proceed to confirm with the customer by simulation.

    However, I think it is important for customers to estimate with the worst conditions.

    Could you please explain how to use the table to get worst case values?

    In their usage, it basically boost and back-boost when the voltage approaches, but is it correct that the total power of the four FETs is larger in back boost?

    If boost mode consumes more power from the four FETs, I think it is sufficient to calculate the boost mode power from the table in the data sheet as the worst case.

     

    Best regards,

    T-yoshi

  • Hi T-yoshi-san,

    Buck-boost mode has the lowest efficiency as shown in the plot on page one of the datasheet and therefore the greatest total power dissipation. In buck-boost mode, some percentage of the time it is in boost mode and the remainder of the time it is in buck mode. Each FET must be able to handle the worst case situation for that particular FET (buck or boost and forward current or reverse current). The table makes it clear which mode is worst case for a particular FET.

  • Hi Charlie-san,

    Thank you for your support.

    I understand that buck-boost mode is worst case.

    LT8708 is not registered in LTPowerCAD, so LTspice is the only way to seek efficiency.

    I encourage customers to use LTspice.

    Best regards,

    T-yoshi