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Original question:

LM5155: tr, tf

LM5155: CISS/ CRSS/ VDS, OFF

Kuramochi Tadahiko
Guru 28715 points
Part Number: LM5155

Hi,

The question is the same as the last question of the related thread.

I heard that Rise/Fall time is calculated by the following equation according to the following document and the calculator uses the gate charges of the MOSFET as these a common values in the datasheet and the gate drive voltage of the MOSFET is know. .

https://www.ti.com/lit/ml/slua618a/slua618a.pdf

Could you tell me the actual values?

CISS:

CRSS

VDS, OFF

I would like to verify the calculation.

Best Regards,

Kuramochi

  • 0
    TI__Guru* 78240 points

    Hi Kuramochi,

    In design we usually refer to total gate charge Qg to estimate the switching transient, and T_on= Qg/Ig_source, and T_off= Qg/Ig_sink.   The referred article is a more sophisticated calculation, and I will have to forward your thread to the our FET group. 

    Thanks,

    Youhao 

  • 0 in reply to Youhao Xi
    Guru 28715 points

    Youhao-san,

    Thank you for your support.

    What my customer would like to know is how tr and tf are calculated on the above calculator.

    Best Regards,

    Kuramochi

  • 0 in reply to Kuramochi Tadahiko
    TI__Mastermind 38190 points

    Hi Tadahiko-san,

    Thanks for inquiry. I did not work on the calculator referenced in your earlier post. As you may have seen, there are many methods to estimate MOSFET switching times. At the link below, you will find an Excel-based non-synchronous boost FET power loss estimation tool. In the equations tab, there are detailed equations showing how the turn on and turn off times and associated switching losses are calculated. I believe this is similar but more detailed calculation. Please note, the switching times specified in the FET datasheet are very dependent on the driver circuit, PCB layout and MOSFET parameters including gate charge and internal gate resistance. Please refer to the blog in the second link below.