PMP8913: Current and power limit

200W is about 8X the power level of PMP8913 (28W), so component losses will be much higher. I suggest that you start by downloading Power Stage Designer on the TI website:

http://www.ti.com/tool/POWERSTAGE-DESIGNER

This tool will allow you to analyze the effects of inductance value, fsw, input voltage, load current, etc on the voltage and current waveforms.

Depending on your input voltage range, the FET current may be quite high, which could make the losses high. Three factors affect the FETs losses; switching current, switching voltage, and fsw. If all three are high, then the losses in the FET will be large. You will likely require a TO-220/TO-247 package and a heatsink. I suggest a lower switching frequency of 100KHz or 200KHz. Since the FET will switch 100V, you need a very fast switching (low total gate charge) part.

The rectifier and the FET both see currents of Iout/(1-D) plus 1/2 of the delta I (dI = V/L * dt). So for 20Vin, D ~ 0.8, so 2A/(1-0.8) = 10A (plus 1/2 of dI). This is significant. So your rectifier losses will be in the 1.5W-2W range.

The inductor may also be large. If you drop your fsw to 200KHz, your inductor could be in the range of ~22uH/15A. Make sure core losses are considered also. High core losses effectively reduce the dc current levels (to keep the temp rise to 40C max).

Total losses would be approx 10.5W if the converter were 95% efficient, so you will certainly need to consider thermal management.

Hope this helps,

regards,

John Betten