[Question] No.1 S6 Win 7 USB Driver

[AMV]

Hello
Since a few weeks I am in possession of a S6. I've got usb drivers installed and am able to get complete access to internal and external phone memory.
My problem now is when phone is off and trying to flash with sp flash tool phone wont be recognized no matter what i do. I pluged it in with battery and it's unknown device i always get an error in flashtool. I also pluged it in without battery it's also unknown device and always connects and disconnects in a interval of 2-5 seconds. When phone is back on everything works fine again when connecting to pc. My question is: Is there any specific driver i need. I used one from pdanet but maybe it's not really compatible with phone. Now i had flashtool half way through and then another error. Now i am stuck on mediatek bootlogo and phone wont start. I need proper driver for mt6589 preloader for this phone so i can flash new rom. I've spent lot of time searching but without results.
Maybe i can find help here.

Thanks for any help in advance