Abstract:
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.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is related to a voltage regulator circuit and particularly to a power transistor chip with a function field effect transistor (JEFT) built therein and an AC/DC voltage converter using the power transistor chip. 
     2. Brief Description of the Related Art 
     Due to the semiconductor technology being developed progressively, the digital products such as the computer and the peripherals thereof are capable of being upgraded continuously. The fast change of the manufacturing process for the semiconductor results in a variety of demands for the power source of the integrated circuit (IC) employed in the computer and the peripherals thereof. Hence, various combinations of voltage regulators using such as the boost converter and the buck converter to meet the need of different power sources of the integrated circuit become one of the most important factors to offer versatile digital products. The AC/DC voltage converter is widely taken as the primary stage circuit of the voltage regulator circuit because it is capable of converting the AC power input to the needed steady direct power output. 
     Referring to  FIG. 1 , a circuit diagram of the conventional AC/DC voltage converter is illustrated. The AC/DC voltage converter  10  includes a bridge type rectifying circuit  11 , a power transistor chip  12 , a pulse width modulation (PWM) circuit  13 , a start-up circuit  14 , a transformer circuit  15 , a filtering and feedback circuit  16  and a working power circuit  17 . The pulse width modulation circuit  13  produces the modulated PWM signal to control and output the direct power output V o  according to the magnitude of the feedback voltage of the direct power output V o . However, the pulse width modulation circuit  13  usually is driven by a low voltage direct power and there is no direct power available for operating the pulse width modulation circuit  13  at the time of the AC/DC voltage converter  10  initiating the work thereof. Therefore, it is necessary to use the start-up circuit  14  and the working power circuit  17  to supply the successive power needed by the pulse width modulation circuit  13 . 
     When the AC/DC voltage converter  10  initiates the work thereof, the output terminal of the bridge type rectifying circuit  11  outputs a rippling direct power to the pulse width modulation circuit  13  via the resistance in the start-up circuit  14  for operating the pulse width modulation circuit  13 . Then, the pulse width modulation circuit  13  performs the normal job thereof afterward according to the magnitude of the feedback voltage of the output direct power V o  and produces the modulated PWM signal such that the time duration of ON and OFF of the power transistor can be controlled for outputting a steady direct power V o . The working power circuit  17 , which is connected to the transformer circuit  15 , supplies more steady working power for the pulse width modulation circuit  13  performs the job thereof much steadily after the AC/DC voltage converter  10  is started up and outputs the steady direct power V o . 
     Although the preceding way allows the AC/DC voltage converter to work normally, the start-up circuit  14  keeps in a state of supplying the power to the pulse width modulation circuit  13  unnecessarily. In order to improve the deficiency, the depletion metal oxide semiconductor field effect transistors (Depletion MOSFET)  221 ,  331  are employed instead to output a start-up signal st via the pulse width modulation circuit  23  or  33  to turn off the operation of the depletion metal oxide semiconductor field effect transistors  221 ,  331  respectively for conserving the power consumption. 
     The difference between  FIG. 2  and  FIG. 3  is in that the depletion metal oxide semiconductor field effect transistors  221 ,  331  are integrated in the power transistor chip  22  and the pulse width modulation circuit  33  respectively for starting up the AC/DC voltage converter. However, the manufacturing process of the chip becomes more complicated due to the additional process of the channel (N channel or P channel) of the depletion metal oxide semiconductor field effect transistors  221 ,  331 . 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide a power transistor chip and an AC/DC voltage converter using the power transistor chip, which has a junction field effect transistor built in the power transistor chip to act as a start-up circuit such that not only the power consumption is conserved but also no additional mask and process are involved for simplifying the manufacturing process and lowering the production cost. 
     In order to achieve the preceding object, the power transistor chip with a built-in junction field effect transistor according to the present invention is adaptable to the AC/DC voltage converter and comprises a first pin, a second pin, a third pin, a fourth pin, a fifth pin, a power transistor and a junction field effect transistor; wherein, the power transistor acts as a power switch of the AC/DC voltage converter and has a first source/drain, a second source/drain and a power transistor gate with the first source/drain coupling with the first pin, the second source/drain coupling with the second pin and the power transistor gate coupling with the third pin; and the junction field effect transistor acts as the start-up circuit of the AC/DC voltage converter and has a third source/drain, a fourth source/drain and a junction field effect transistor gate with the third source/drain coupling with said first pin, the fourth source/drain coupling with the fourth pin and the junction field effect transistor gate coupling with the fifth pin. 
     Further, the AC/DC voltage converter according to the present invention is capable of converting the AC power input to the steady DC power output and further comprises a bridge type rectifying circuit, a transformer circuit, a pulse width modulation circuit, a filtering and feedback circuit and a working power circuit in addition to the preceding power transistor chip with the built-in junction field effect transistor. 
     Wherein, the bridge type rectifying circuit has a power input terminal, which receives the AC power input, and a rectifying output terminal; the transformer circuit has a primary coil, a secondary coil and an auxiliary coil with the primary coil having an terminal coupling with the rectifying output terminal and another terminal coupling with the first pin of the power transistor chip, i.e., coupling with the first source/drain of the power transistor and the third source/drain of the junction field effect transistor. 
     The pulse width modulation circuit has a start-up power control terminal, a working power terminal, a pulse width modulation signal output terminal and a current sensing terminal with the start-up power control terminal coupling with the fifth pin to control ON and OFF of the junction field effect transistor, the working power terminal coupling with the fourth pin to receive a start-up power from the junction field effect transistor, the pulse width modulation signal output terminal coupling with the third pin to output a pulse width modulation signal according to the magnitude of a feedback voltage for modulating the DC power and the current sensing terminal coupling with the second pin to sense a current passing through the power transistor. 
     The working power circuit couples with the auxiliary coil and the working power terminal of the pulse width modulation circuit to supply a power needed by the pulse width modulation circuit at the time of the start-up power output by the junction field effect transistor being turned off by the pulse width modulation circuit; and the filtering and feedback circuit couple with the secondary coil to filter and output the steady DC power and supply a feedback voltage needed by the pulse width modulation circuit. 
     In short, a power transistor chip and an AC/DC voltage converter using the power transistor chip according to the present invention has a junction field effect transistor built in the power transistor chip to act as a start-up circuit of the 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 fabricated with the power transistor chip with the identical manufacturing process such that it is capable of simplifying the process and lowering the production cost due to no additional mask and manufacturing process. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The detail structure, the applied principle, the function and the effectiveness of the present invention can be more fully understood with reference to the following description and accompanying drawings, in which: 
         FIG. 1  is a circuit diagram illustrating the conventional AC/DC voltage converter; 
         FIG. 2  is a circuit diagram illustrating the conventional AC/DC voltage converter employing the depletion metal oxide semiconductor field effect transistor (Depletion MOSFET) as a start-up circuit; 
         FIG. 3  is a circuit diagram illustrating another conventional AC/DC voltage converter similar to  FIG. 2 ; 
         FIG. 4  is a circuit diagram illustrating an AC/DC voltage converter of a preferred embodiment according to the present invention; 
         FIG. 5  is a diagram illustrating the junction field effect transistor of the power transistor chip according to the present invention in a state of ON; and 
         FIG. 6  is a diagram illustrating the junction field effect transistor of the power transistor chip according to the present invention in a state of OFF. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 4 , an AC/DC voltage converter of the preferred embodiment according to the present invention is illustrated. The AC/DC voltage converter  40  is capable of converting the AC input power V in  coming from the power terminal  411  to a steady direct output voltage V o . it can be seen in  FIG. 4  that the AC/DC voltage converter  40  includes a bridge type rectifying circuit  41 , a power transistor chip  42 , a pulse width modulation circuit  43 , a transformer circuit  45 , a filtering and feed back circuit  46  and a working power circuit  47 . 
     The power transistor chip  42  shown in  FIG. 4  has a first pin  421 , a second pin  422 , a third pin  423 , a fourth pin  244 , a fifth pin  425 , a power transistor  426  and a junction field effect transistor  427 . The junction field effect transistor  427 , which acts as the start-up circuit of the AC/DC voltage converter  40 , is built in the power transistor chip  42  to simplify the manufacturing process of the chip arranged in the AC/DC voltage converter  40 . Although the power transistor  426  and the junction field effect transistor  427  are exemplified with N-type metal oxide semiconductor field effect transistor and N-type junction field effect transistor and the resistance  428  connected to the gate of the power transistor  426  is built in the power transistor chip  42 , persons skill in the art realize different type transistors such as the P-type metal oxide semiconductor field effect transistor or P-type junction field effect transistor can be adopted as well. Further, the built-in resistance  428  can be optionally integrated in a chip with the pulse width modulation circuit  43  instead of being included in the power transistor chip  42 . 
     Wherein, the power transistor  426 , which is a power switch of the AC/DC voltage converter  40 , has the first source/drain coupling with the first pin  421 , the second source/drain coupling with the second pin  422  and the gate coupling with the third pin  423 . The junction field effect transistor  427  has the third source/drain D coupling with the first pin  421 , the fourth source/drain S coupling with the fourth pin  424  and the gate G coupling with the fifth pin  425 . 
     When the AC/DC voltage converter  40  is initiated to work, the bridge type rectifying circuit  41  receives the AC power input V in  from the power source terminal  411  and rectifying the full wave of the AC power input V in  such that a rippling direct power is output to the first pin  421  via a primary coil of the transformer circuit  451 , which has an terminal coupling with the rectifying output terminal  412  of the rectifying circuit  41  and another terminal thereof coupling with the first pin  421  of the power transistor chip  42 . 
     In order to supply the power to operate the pulse width modulation circuit  43 , the working power terminal  432  of the pulse width modulation circuit  43  couples with the fourth pin  424  of the power transistor chip  42 , i.e., the fourth source/drain S of the junction field effect transistor  427 . Meanwhile, the start-up signal st output by the pulse width modulation circuit  43  is a low reference level signal and the built-in transistor  436  is still in a state of OFF such that the working power terminal  432  and the start-up power control terminal  431  of the pulse width modulation circuit  43  have the same potential as the VCC. 
     Besides, the start-up power control terminal  431  couples with the fifth pin  425  of the power transistor chip  42 , i.e., the gate G of the junction field effect transistor  427 . The working power terminal  432  couples with the fourth pin  424  of the power transistor chip  42 , i.e. the fourth source/drain S of the junction field effect transistor  427 . Hence, the fourth source/drain S of the junction field effect transistor  427  and the gate G of the junction field effect transistor  427  have the same potential as the VCC and are in a state of ON (See  FIG. 5 ) such that the rippling direct power output by the bridge type rectifying circuit  41  can be transmitted to the fourth source/drain S of the junction field effect transistor  427 . 
     Meanwhile, the pulse width modulation circuit  43  obtains the power for initiating the operation and starting up the work thereof. In addition, the pulse width modulation circuit  43  produces the modulated PWM signal in accordance with the magnitude of the feedback voltage of the direct power V o  output by the AC/DC voltage converter  40  and the current of the power transistor chip  42 , which is sensed from the current sensing terminal  434 . Then, the modulated PWM signal is sent to the PWM signal output terminal  433  and transmitted to the gate of the power transistor of the power transistor chip  42  via the third pin  423  and the resistance  428  to control the time duration of ON and OFF of the power transistor  426  such that the AC/DC voltage converter  40  is capable of outputting a steady direct power V o . 
     When the pulse width modulation circuit  43  initiates to work, the secondary coil  452  is induced a voltage, and then the voltage is filtered by the filtering and feedback circuit  46 , which couples with the secondary coil  452 , to output the steady direct power V c . The reference feedback voltage needed by the pulse width modulation circuit  43  is obtained by means of the isolated detection of a light emitting diode  461  and an optical transistor  435  in the filtering and feedback circuit  46 . 
     In addition, an auxiliary coil  453  of the transformer circuit  45  produces an induced voltage simultaneously and the induced voltage is filtered and transmitted to the working power terminal  432  by the working power circuit  47 , which couples with the auxiliary coil  453 . Under this circumstance, the steady working power supplied by the working power circuit  47  has been capable of meeting the need of much steadiness of the pulse width modulation such that the pulse width modulation circuit  43  raises the start-up signal st to a high reference level to activate the built-in transistor  436  for lowering the potential of the start-up control terminal  431  to the GND ground potential. In this way, it creates a reverse bias voltage between the fourth source/drain S and the gate G of the junction field effect transistor  427  to cause pinch-off as shown in  FIG. 6  such that the start-up power output by the junction field effect transistor  427  is turned off after the steady working power supplied by the working power circuit  47  is capable of constantly offering the power required by the pulse width modulation circuit  43 . In this way, the power consumption of the AC/DC voltage converter  40  is conserved significantly. 
     Referring to  FIGS. 5 and 6 , the power transistor chip  42  according to the preferred embodiment of the present invention provides the built-in junction field effect transistor  427  and the junction field effect transistor  427  has the similar doped structure as the power transistor  426  such that the identical manufacturing process is capable of being performed for fabricating both the power transistor  426  and the junction field effect transistor  427  with no additional mask and process. Therefore, the whole process is simplified and the production cost is lowered advantageously. 
     While the invention has been described with referencing to a preferred embodiment thereof, it is to be understood that modifications or variations may be easily made without departing from the spirit of this invention, which is defined by the appended claims.