Fast charging protocol controller and method thereof. For example, a fast charging protocol controller for a power supply charger includes: a first transistor voltage-drop detector configured to detect a first transistor voltage drop from a first drain of a first transistor to a first source of the first transistor, the first transistor being configured to provide an output current of the power supply charger and an output voltage of the power supply charger; a resistor voltage-drop detector configured to detect a resistor voltage drop from a first resistor terminal of a first resistor to a second resistor terminal of the first resistor, the first resistor being configured to receive a resistor current that is equal to the output current in magnitude; and a logic controller coupled to the first transistor voltage-drop detector and the resistor voltage-drop detector.