Powering USB to serial CH340 converter - does it need to be powered from USB port?
It is not necessary to power the USB to serial converter device from the USB cable. There are plenty of "self powered" USB devices that are USB compliant. 

I have made multiple products using a chip like the FTDI FT232RL and powered it from the local power rail of the MCU (in my case with that chip it was a local +5V). 

You may still want to monitor the 5V coming from the USB connector so that you can tell if it is plugged into the upstream port. A simple connection of the USB 5V to a parallel pair of components (0.1uF capacitor to ground and a 10K resistor to ground) can then feed into the input of a 74LV14 to provide a USB VBUS detect.