Power transistor chip with built-in junction field effect transistor and application circuit thereof

An AC/DC voltage converter, which is capable of converting an AC power to a steady DC power, comprising:

• a power transistor chip further comprising:
• a first pin;
• a second pin;
• a third pin;
• a fourth pin;
• a fifth pin;
• a power transistor acting as a power switch of said AC/DC voltage converter and having a first source/drain, a second source/drain and a power transistor gate, wherein said first source/drain couples with said first pin, said second source/drain couple with said second pin and said power transistor gate couple with said third pin;
• and
• a junction field effect transistor acting as a start-up circuit of said AC/DC voltage converter and having a third source/drain, a fourth source/drain and a junction field effect transistor gate, wherein said third source/drain couples with said first pin, said fourth source/drain couples with said fourth pin and said junction field effect transistor gate couples with said fifth pin;
• a bridge type rectifying circuit having a power input terminal, which receives the AC power, and a rectifying output terminal;
• a transformer circuit having a primary coil, a secondary coil and an auxiliary coil, wherein said primary coil has an terminal coupling with said rectifying output terminal and another terminal coupling with said first pin;
• a pulse width modulation circuit having a start-up power control terminal, a working power terminal, a pulse width modulation signal output terminal and a current sensing terminal, wherein said start-up power control terminal couples with fifth pin to control ON and OFF of said junction field effect transistor, said working power terminal couples with said fourth pin to receive a start-up power from said junction field effect transistor, said pulse width modulation signal output terminal couples with said third pin to output a pulse width modulation signal according to the magnitude of a feedback voltage for modulating said DC power and said current sensing terminal couples with said second pin to sense a current passing through said power transistor:
• a working power circuit coupling with said auxiliary coil and said working power terminal to supply a power needed by said pulse width modulation circuit at the time of said start-up power being off by said pulse width modulation circuit;
• and
• a filtering and feedback circuit coupling with said secondary coil to filter and output the DC power and supply a feedback voltage needed by said pulse width modulation circuit.