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UCC28951-Q1: SLUC222D MOS RMS current

Shyama Agrawal
Intellectual 860 points
Part Number: UCC28951-Q1

Hello,

In SLUC222D, for QA, B, C, D power loss calculation. The equation used is as shown in the image below:

It considers transformer primary current as MOS current for conduction loss (Ideally the RMS in MOS will be lesser than transformer primary) and has completely ignored turn OFF loss. Please explain if it is a way of simplifying the model or should we use a different equation to correctly size the MOSFET.

  • +1
    TI__Guru* 86286 points

    Hello,

    I reviewed the tool and it is using all of the transformer RMS current to estimate I^2*R losses of the FET.  This is an over estimate of power dissipation in the FET by a factor of 2. 

    Generally in a design you will select components for a power rating of 125 to 150% of the maximum power rating.  So  if you used the tool to select the FETs based on the tool calculations they will be rated for 200% of the power required.  You could use a lower power rated FET if desired.

    Regards,