How to turn on a buck just in case the voltage is higher some value

I want my buck to become active just in case the voltage is higher than 8v. If it is lower than the current goes bypassing it directly to the load. What is the best way to make it real?

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Unnecessary. Just set the buck output to a voltage you need.

In case there is no voltage difference it becomes unstable and don’t provide enough current.

What source is so unstable and what voltage do your devices need?