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Power Mosfet Loss calculation CSD17551QA

Manjunath Inder
Prodigy 190 points

Hi there,

I'm struggling to find the the total power loss across the MOSFET  for the following application.

The DC electronic load which is used to test voltage regulators.The load draws pulsed current of set amplitude and frequency. My doubt is ,how we calculate the total loss across the MOSFET .The waveforms are shown

below.      

 What is the total  power loss across the MOSFET (conduction and switching) 

Will the MOSFET be able to withstand the power loss for the above condition. i.e Switched current (pulsating current of 10A(peak) 1KHz and 10% duty ratio).  voltage 3.3V max

Regards 

Manjunath I

  • 0
    TI__Mastermind 18200 points

    Manjunath, 

    The switching frequency is very low so we can probably just look at conduction losses. What is the voltage across the FET when it is conducting? 

    I would just use P = I * V * Duty Cycle. 

  • 0 in reply to Brett Barr1
    Prodigy 190 points

    Hi brett,

    The voltage across the MOSFET when it is conducting is 3.2V .So the Max power is (3.2 * 6 * 0.1) which is ~ 2W . Can I use 3.1W absolute power rating MOSFET.  

    P.S

    3.2V is the max voltage across the MOSFET when it is conducting and 6A is the peak current. 

    Regards 

     

  • 0 in reply to Manjunath Inder
    TI__Mastermind 18200 points

    Manjunath, 

    This does not appear correct. In your waveform, the voltage across the FET is only 3.2V when the FET is off.

    When the FET is on, it drops much lower (assuming you are fully enhancing the FET). What is the voltage across the FET during conduction? Or another way we could do this is to calculate the loss through I^2*R but then we would need to know what the gate drive voltage is while the FET is on to get an idea of the resistance. 

  • 0 in reply to Brett Barr1
    Prodigy 190 points

    Hi brett ,

    Sorry for the mistake .The voltage across the FET when it is conducting is around 2.3V .I'm not operating the MOSFET in ohmic region  its in the saturation region I suppose . 

    Regards Manjunath I  

  • 0 in reply to Manjunath Inder
    TI__Mastermind 18200 points

    Then your average power loss will be 2.3V * 10A * 10% = 2.3W. This is a decent amount of power for a QFN - for instance the most power we would recommend an SON5x6 when mounted on a 1x1" 2oz Cu board is right around 2.4W so whether the FET can handle this depends on your thermal environment.