A power transistor chip and an application circuit thereof have a junction field effect transistor to act as a start-up circuit of an AC/DC voltage converter. The start-up circuit can be turned off after the PWM circuit of the AC/DC voltage converter operates normally to conserve the consumption of the power. Besides, the junction field effect transistor is built in the power transistor chip. Because the junction field effect transistor is fabricated with the same manufacturing process as the power transistor, it is capable of simplifying the entire process and lowering the production cost due to no additional mask and manufacturing process.