UCC28740: About DESIGN CALCULATOR TOOL

Hi Kenji,

Thanks for your interest in TI solutions.

1. You'll find that the ideal primary-to-secondary turns ratio is dependent upon a voltage "Vbulkvalley." This is the minimum voltage expected across the bulk capacitors, accounting for ripple across these caps. As you reduce the value of these caps, there will be greater voltage ripple across these capacitors resulting in a lower Vbulkvalley, which will affect your primary-to-secondary turns ratio.
2. The MOSFET VDS rating also includes the forward drop of the secondary rectifying diode along with the output current multiplied by the DC resistance of the inductor reflected back. You'll also find that there is a 1.3x multiplier applied to arrive at the final VDS rating -- this is so that operation isn't near the maximum of what the MOSFET can handle.

Regards,

Michael Wu

Hi Kenji,

The equation is:

Vds,rated = (Vbulk_max + (Nps * (Vout_cv + Vf + (DCR_Lout * Iocc_target)))) * 1.3

In the above equation, the DC resistance of the output inductor is expressed in milliohms. If you have any questions about the variables represented in the equation, you can find them in the Excel spreadsheet either by searching the document or by using the drop-down menu to the left of the formula bar (MS Office 2010).

Regards,

Michael Wu