Hi everyone. I want to pull-down the buck converter enable pin whenever the node voltage reach 15V. the buck regulator acts as a startup and it needs to be switched off, when power from Tx. aux. winding will flow. suggest some design idea without comparator.

 

That description seems like an application for a zener diode plus an NPN transistor to do the pull down. It will need a pull down for the transistor base to compensate for the zener leakage, plus a series resistor to limit base current from the node. AND, probably a collector resistor to limit the current as it pulls down the enable pin. BUT no comparator!!

 

What buck regulator are you using?
What is the regulator input voltage?

Need to know the En pin exact characteristics.

 

Buck regulator: LMR38015FDRRR
Regulator input voltage: 15V to 77V (abnormal); 28VDC(normal)
EN pin threshold: 0.95V

 

The buck regulator will stop automatically due to its feedback connection if the load is supplied from another source.

 

Finally able to implement. Thanks for the support.

 

Since they are in series, you only need one resistor for the Zener and Q1's base,

 

The whole thing is pointless. If the voltage on the output of the buck regulator is 15V, and the buck regulator is set to regulate to 12V then it WILL ALREADY be shut down.