We are using STM32l412 microcontroller in one of our project but every time if we need to flash the firmware we need to power the target board USB is not powering the controller while programming

[Tesla DeLorean]

Ok, and what's the question here? That the L4 needs to be suitable powered? Your own board, or some more standard one?

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[S.Ma]

if using stlink its vdd is not a supply for the application, for example its regulator may overheat if your board requires over usb current budget such as 2G modems. stlink and application target supply should be independent.