A charging circuit including a transformer, a storage element, a switch element, a first resistor, and a current detection unit is provided. The transformer includes a primary coil and a secondary coil. The storage element is coupled to the secondary coil. The switch element is coupled to the primary coil. The first resistor is coupled to the primary coil. The current detection unit detects current flowing through the first resistor. When the current reaches a set current, the current detection unit sends a full signal to de-activate the switch unit.