The relay serves no IRL purpose and is only there to prevent EC from finding an incorrect steady state solution via DC analysis and starting with that. It serves the same role as starting switches in other circuits with this issue, but in the case of this circuit that doesn’t work either as it says “Cannot find solution” after a couple seconds if you don’t throw the switch quick enough, so a relay does the job nicely by starting open and instantly closing.
This circuit was inspired by the LED driver I found in a real lightbulb (see http://everycircuit.com/circuit/5060621189775360 ) but upon close examination of the data sheet for the BP9916E chip I found that it switches on when the inductor stops conducting and switches off when the inductor reaches the current limit so the LED would flash very rapidly. Now, this isn’t really an issue given how fast the circuit operates, but I still liked the idea of having a steady constant current instead of a rapidly flashing imitation of constant current, so I made this circuit. It does operate at a fairly high frequency and probably has some pretty big switching losses as a result (especially if you have some more realistic MOSFET capacitances) but that can be fixed with a bigger inductor, although that has it’s own issues.
