Control RGB LEDS (common cathode) with a Mosfet or a transistor

why i cant use a P mosfet directly? or a transystor

Because for high sided switching you turn on the FET or transistor with a low signal and turn it off with a high signal. If the voltage you want to switch is higher than the signal you have to switch it with then you need a driver first to make the signal of the same level as the voltage you want to switch.

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