Electrical Engineering

It uses MC34063 as switching controller. I wanted to adapt this circuit, and I was busy trying to pick R1 value. The datasheet has following formulas: Image alt text

The issue I have is with calculating ton/toff, since Vsat is defined as saturation voltage of switch. The switch is IRF740, and since it is a MOSFET, it does not have a defined saturation voltage, but Rds. Therefore, the saturation voltage is dependent on drain-source current. This is where I get confused, and uncertain regarding how to approach this. How do I find Ipeak that is required for selecting R1 using MOSFET as a switch? It seems like datasheet was written with the assumption that an NPN transistor would be used as a switch.

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How to define Vsat for MOSFET as a switch?

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