Tool/software:
Hi Jiemo zhu,
Thanks for the inputs. I am going through single switch topology approach https://www.ti.com/lit/an/sluab06/sluab06.pdf?ts=1743574254094&ref_url=https%253A%252F%252Fwww.google.com%252F from page number 5 
Test step 1 w.r.t S1, S2 from figure 2-1 is understood.
In the 2nd step , I understood SW1,SW2 Means S1 & S2, am I right? and what about SW3, In figure 2-1 , couldn't see this. If you have any calculations on these formulas can you share it.