Abstract:
A one way conductor includes a MOSFET and a driving device. The MOSFET has a source and a drain respectively serving a positive end P and a negative end N of the one way conductor. The driving device including a BJT differential amplifier detects a voltage difference between the source and the drain of the MOSFET. When the voltage of the positive end P is higher than the voltage of the negative end N, the driving device outputs a driving voltage to a gate of the MOSFET to turn on the MOSFET. If the voltage of the positive end P is lower than the voltage of the negative end N, the driving device cannot output the driving voltage for turning on the MOSFET, and the one way conductor is turned off at this time. Consequently, the one way conductor of the invention has the one way conductive property.

Description:
This application claims the benefit of Taiwan application Serial No. 93113765, filed May 14, 2004, the subject matter of which is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates in general to a one way conductor, and more particularly to a one way conductor having a low forward voltage. 
   2. Description of the Related Art 
   A diode is one of the indispensable components among various electronic components required by the electronic circuit. However, the diode always has a drawback that the forward voltage (VF) of the diode is not 0V but is about 0.6V. For example, a Schottky diode has a forward voltage of about 0.4V, which may be obtained by changing the semiconductor manufacturing process. Although the Schottky diode can satisfy the requirements in designing most of the circuits, it still cannot satisfy the design of the special circuit, which requires the one way conductive property of the diode and must have a relatively low forward voltage. Thus, there is a need to develop a one way conductor having a very low forward voltage so as to reduce the power loss of the circuit and enhance the efficiency of power usage. 
     FIG. 5  shows a conventional power supply circuit  500  using a diode. The battery BT 1  and the battery BT 2  provide the power for the load RL, which is, for example, a notebook computer. When the potential of the battery BT 1  is higher than that of the battery BT 2 , the diode D 1  is turned on and the diode D 2  is turned off because the diode D 1  is forward biased and the diode D 2  is backward biased. The load RL can access the power supply voltage from the battery BT 1  with the higher potential. On the contrary, if the potential of the battery BT 1  is lower than that of the battery BT 2 , the load RL can access the power supply voltage from the battery BT 2  with the higher potential. Because the diodes D 1  are D 2  are typical diodes, the voltage of the load RL is substantially 0.45V lower than the power supply voltage of the battery BT 1  or BT 2 . 
   SUMMARY OF THE INVENTION 
   It is therefore an object of the invention to provide a one way conductor using a MOSFET and a BJT differential amplifier such that the one way conductor has the one way conductive property with a relatively low forward voltage. 
   The invention achieves the above-identified object by providing a one way conductor, which includes a first transistor and a driving circuit. The first transistor has a source, a drain and a gate. The driving circuit is coupled to the first transistor and includes a second transistor, a third transistor, a first impedance, a second impedance and a third impedance. The second transistor has a second emitter, a second base and a second collector. The third transistor has a third emitter, a third base and a third collector. The third emitter is coupled to the source, the third collector is coupled to the gate, the second base is coupled to the third base, and the second base is coupled to the second collector. The first impedance has a first end coupled to the drain and a second end coupled to the second emitter. The second impedance has a first end coupled to the second collector and a second end coupled to a constant voltage. The third impedance has a first end coupled to the third collector and a second end coupled to the constant voltage. 
   Other objects, features, and advantages of the invention will become apparent from the following detailed description of the preferred but non-limiting embodiments. The following description is made with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a one way conductor according to a first embodiment of the invention. 
       FIG. 2  shows a Pspice simulation result. 
       FIG. 3  shows other Pspice simulation results. 
       FIG. 4  shows a one way conductor according to a second embodiment of the invention. 
       FIG. 5  shows a conventional power supply circuit using a diode. 
       FIG. 6  shows a power supply circuit using the one way conductor of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows a one way conductor  100  according to a first embodiment of the invention. The one way conductor  100  utilizes a MOSFET Q 1  and a driving device  102  such that the one way conductor  100  has the one way conductive property with a relatively low forward voltage. The one way conductor  100  includes a MOSFET Q 1 , a PNP bipolar junction transistor (BJT) transistor Q 2 , a PNP BJT transistor Q 3  and resistors R 1 , R 2 , R 3 , R 4  and R 5 . The MOSFET Q 1  is a P channel transistor (i.e., PMOS) having a source S and a drain D respectively serving as an negative end N and a positive end P of the one way conductor  100 . The transistors Q 2  and Q 3  constitute a BJT differential amplifier. The base B 2  of the transistor Q 2  is coupled to the collector C 2  of the transistor Q 2  through the resistor R 4 . The base B 3  of the transistor Q 3  is coupled to the collector C 2  of the transistor Q 2  through the resistor R 5 . The collector C 2  of the transistor Q 2  and the collector C 3  of the transistor Q 3  are respectively grounded through the resistors R 2  and R 3 . The collector C 3  of the transistor Q 3  is coupled to the gate G of the PMOS transistor Q 1 , and the emitter E 3  of the transistor  03  is coupled to the source S of the PMOS transistor Q 1 . A first end of the resistor R 1  is coupled to the drain D of the transistor Q 1 , and a second end of the resistor R 1  is coupled to the emitter E 2  of the transistor Q 2 . The resistances of the resistors R 2  and R 3  are substantially the same, and the resistance of the resistor R 2  is far greater than that of the resistor R 1 . Preferably, the resistance of the resistor R 2  is several hundred times that of the resistance of the resistor R 1 . 
   The operation principle of the one way conductor  100  will be described detailedly in the following. When the one way conductor  100  is forward biased, the voltage of the positive end P is higher than that of the negative end N, and a static current IE 2  flows through the transistor Q 2 . After the static current IE 2  flows through the resistor R 1 , a cross-over voltage VR 1  between two ends of the resistor R 1  is generated. The cross-over voltage VR 1  is preferably several tens of millivolts (mV). When the voltage of the P terminal rises such that the voltage difference between the P terminal and the N terminal is higher than VR 1 , the voltage of the emitter E 2  of the transistor Q 2  rises with the rise of the voltage of the P terminal. Because the resistance of the resistor R 2  is very large, the static current IE 2  of the transistor Q 2  is almost kept constant although the voltage of the P terminal rises. Thus, the VEB 2  (the cross-over voltage between the emitter E 2  and the base B 2 ) of the transistor Q 2  is almost kept constant, too. Because the voltage of the emitter E 2  of the transistor Q 2  rises with the rise of the voltage of the P terminal, the voltage of the base B 2  of the transistor Q 2  also rises with the rise of the voltage of the emitter E 2  of the transistor Q 2 . Consequently, the voltage of the base B 3  of the transistor Q 3  also rises with the rise of the voltage of the base B 2  of the transistor Q 2 . However, because the voltage of the negative end N of the one way conductor  100  is kept constant, the voltage of the emitter E 3  of the transistor Q 3  is also kept constant. Thus, the VEB 3  (the cross-over voltage between the emitter E 3  and the base B 3 ) of the transistor Q 3  decreases. Consequently, the current IC 3  flowing through the collector C 3  of the transistor Q 3  decreases such that the cross-over voltage VR 3  of the resistor R 3  decreases. Accordingly, the voltage of the collector C 3  of the transistor Q 3  decreases. When the voltage of the collector C 3  of the transistor Q 3  decreases such that the VSG (the cross-over voltage between the source S and the gate G) of the transistor Q 1  is greater than an absolute value of a threshold voltage Vth of the transistor Q 1 , the transistor Q 1  is turned on and the forward current ID flows from the positive end P to the negative end N. 
   On the contrary, when the voltage of the negative end N is higher than that of the positive end P, the voltage of the emitter E 3  of the transistor Q 3  is higher than that of the emitter E 2  of the transistor Q 2 , and the transistor Q 3  is turned on accordingly. The collector C 3  of the transistor Q 3  has a high voltage such that the transistor Q 1  is completely turned off. Thus, the one way conductor  100  is backward biased and the one way conductor  100  is turned off. When the voltage difference between the negative end N and the positive end P is higher than the VEB 2  of the transistor Q 2 , a backward current flows from the negative end N to the source B 3  of the transistor Q 3  through the emitter E 3  of the transistor Q 3 , then flows to the base B 2  of the transistor Q 2  through the resistors R 5  and R 4 , then flows through the emitter E 2  of the transistor Q 2  and the resistor R 1 , and finally flows to the positive end P. The existing resistors R 4  and R 5  can reduce the backward current. If each of the transistor Q 2  and the transistor Q 3  has the VEB voltage higher than the highest voltage of the negative end N, the resistors R 4  and R 5  can be omitted in the one way conductor  100  of this embodiment, and the bases B 2  and B 3  of the transistors Q 2  and Q 3  may be directly and electrically connected to each other. 
   The first embodiment of the invention will be further described with reference to various resistances in conjunction with PSpice simulation results.  FIG. 2  shows a Pspice simulation result of the relation of the voltage difference VPN between the positive end P and the negative end N of the one way conductor  100  and the forward current ID, and  FIG. 3  shows Pspice simulation results of the relation of the VPN and the voltage VQ 2 C (curve  302 ) of the collector C 2  of the transistor Q 2  and a voltage Vg (curve  304 ) of the gate G of the transistor Q 1  when the voltage of the negative end N of the one way conductor  100  is a constant of 10V, the resistor R 1  is 1.5K Ohms, the resistors R 4  and R 5  are 100K Ohms, and the resistors R 2  and R 3  are 1M Ohms. As shown in  FIG. 2 , the horizontal axis represents the voltage difference VPN between the positive end P and the negative end N, and the vertical axis represents the forward current ID. In  FIG. 2 , when VPN equals 30 mV, the transistor Q 1  is turned on. After the transistor Q 1  is turned on, the value of the voltage Vg and the slope of the forward bias VPN V.S. the forward current ID are determined by the common emitter current gain β of Q 3 . As shown in  FIG. 3 , when the voltage Vg equals the voltage VQ 2 C of the collector C of the transistor Q 2 , Vg and VQ 2 C substantially equal the cross-over voltage VR 1  of the resistor R 1 , and VR 1  is substantially equal to (VP−VBE 2 )×(R 1 /(R 1 +R 2 ))=(10−0.6)×(1.5 k/(1.5 k+1000 k))=14 mV, wherein VP is the voltage of the positive end P. The voltage Vg of the gate G of the transistor Q 1  decreases with the increase of the VPN. 
   In this case, when the forward current ID is smaller than 0.2 amperes, the transistor Q 1  is not completely turned on. The impedance of the transistor Q 1  increases with the decrease of the forward current ID. When the forward current ID is zero, the transistor Q 1  is completely turned off. The actual curve of the forward current ID of  FIG. 2  is determined according to the property of the transistor Q 1 . 
   Each of the transistors Q 2  and Q 3  is preferably a twin transistor such that the transistors Q 2  and Q 3  have similar but not completely the same properties and parameters. In order to disable the one way conductor  100  from generating a backward current, the forward voltage of the one way conductor  100  when the conductor  100  is turned on has to be greater than the offset voltage of the transistors Q 2  and Q 3 . The value of the forward voltage can be determined according to the resistance value of the resistor R 1  and the threshold voltage of the gate G of the transistor Q 1 . The larger the resistance value of the resistor R 1  is, the larger the forward voltage is. The smaller the resistance value of the resistor R 1  is, the smaller the forward voltage is. Thus, it is possible to change the forward voltage of the one way conductor  100  when the conductor  100  is turned on by adjusting the resistance value of the resistor R 1 . 
     FIG. 4  shows a one way conductor  400  according to a second embodiment of the invention. The one way conductor  400  includes an N channel MOSFET Q 1  (NMOS) and a driving device  402 . The transistor Q 1  has a source S and a drain D respectively serving as a positive end P and a negative end N of the one way conductor. The configuration and principle of the one way conductor  400  are similar to those of the one way conductor  100  of  FIG. 1 . The one way conductor  400  may be obtained by replacing the P channel transistor Q 1  of the one way conductor  100  with an N channel transistor and replacing the PNP transistors Q 2  and Q 3  with NPN transistors. 
     FIG. 6  shows a power supply circuit  600  using the one way conductor  100  of the invention. When the one way conductor  100  of the first embodiment of the invention is applied to the power supply circuit  600 , the voltage of the load RL is only several tens of mV lower than the power supply voltage of the battery BT 1  or BT 2 , and the voltage drop of the several tens of mV is far smaller than that in the power supply circuit  500  using the typical diode, as shown in  FIG. 5 . 
   The one way conductors according to the embodiments of the invention have the following advantages.
     1. The forward voltage is very low.   2. The backward leakage current is lower than that of the Schottky diode.   3. The forward conductive and backward cut-off operations are very precise without the generation of the large backward current.   4. When the one way conductor is converted from the forward bias into the backward bias, the forward saturation region of the MOSFET is converted into the working region and then into the backward cut-off region in a gradually manner. So, when the voltage difference between the positive end P and the negative end N of the one way conductor approaches zero volts, the one way conductor of the invention is free from the unstable oscillating phenomenon.   5. The efficiency can be enhanced when the one way conductor of the invention is used in a rectification circuit.   

   While the invention has been described by way of examples and in terms of preferred embodiments, it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures.