LT8357 Sepic - not working at low voltage input

Question

Hi, I have designed a SEPIC converter, based on LT8357. I used the LT Power CAD and verified the design with excel sheet based on the datasheet. 
 The design spec is 9V-36V input, 15V output, 3A. The problem is: When input voltage in <10.8V, output voltage drops to 5V. 
 My load is ~2A at 15V.

Any idea? 
 
 Ohad.

MarkusHP · Accepted Answer

Hello, 
 1. You have used a 40V FET -> in a SEPIC the Switch does see the sum of VIN and VOUT, hence 51V at 36V Vin -> You need a 60V FET. And the diode has to be a 60V type as well. 
 2 With my approach LTpowerCAD suggests a Sense resistor of 4.7mOhm. You have chosen 8mOhm, hence you limit the possible output power at lower VIN.)

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LT8357 Sepic - not working at low voltage input