Abstract:
Rectifier circuits which are usable, instead of diodes, for rectifying alternating voltages, and which, like diodes, form two-terminal networks having a cathode terminal and an anode terminal. The power loss of these rectifier circuits is clearly less that the power loss of silicon p-n diodes. These rectifier circuits also include voltage clamping functions.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a rectifier circuit. 
     BACKGROUND INFORMATION 
     For the generation of direct voltages from AC voltages, at the present time mostly rectifier bridges are used which are made up of an interconnection of diodes. 
     The conversion of an AC voltage to a DC voltage takes place in a motor vehicle, for instance, in which an AC voltage is generated by a generator which is converted by a post-connected rectifier bridge to a DC voltage. 
     An example for such a voltage supply of a motor vehicle is shown in  FIG. 1 . This includes a generator device GEN which has an excitation winding G and radially connected phase windings U, V and W. The phase voltages provided at phase windings U, V and W are supplied to a rectifier bridge RF, which provides the desired DC voltage at its output. Rectifier bridge RF includes three bridge branches. In each of these bridge branches two silicon p-n diodes D are provided. 
     Conditioned physically, during a forward operation of a silicon p-n diode in rectifier applications, diode forward voltages of ca. 800 mV to ca. 2 V come about. These diode forward voltages, at meaningful dimensioning, are usually not able to be lowered below ca. 1 V. Therefore, especially in the case of the rectification of lower alternating voltages, power losses are created at the rectifier diodes. In generators used for the voltage supply of passenger vehicles, the alternating voltages that are to be rectified usually amount to about 17 V peak-to-peak between the terminals of two phase windings. Since the current flows via two rectifier diodes, because of the rectification, a voltage reduction is created averaging about 2 V. In this example, the power loss in the rectifier at corresponding load amounts to about 20% of the power output. The power loss converted in the rectifier has to be eliminated in the form of heat by costly cooling elements. In addition, the power losses directly affect the fuel consumption of the respective vehicle. 
     SUMMARY OF THE INVENTION 
     By contrast, a rectifier circuit having the features described herein has the advantage that the power loss is reduced, and for this reason, the expenditure for cooling may also be reduced. 
     This is achieved essentially in that, in a rectifier bridge, instead of silicon p-n diodes, rectifier circuits according to the exemplary embodiments and/or exemplary methods of the present invention are used, each silicon p-n diode of the rectifier bridge being able to be replaced by such a rectifier circuit. Circuit engineering changes of the overall system are not necessary. The rectifier circuits according to the exemplary embodiments and/or exemplary methods of the present invention require no separate power supply and also no separate signal inputs. 
     The forward voltages of silicon p-n diodes in rectifier operation can usually not be lowered below ca. 1.1 V. By using rectifier circuits according to the present invention, instead of silicon p-n diodes, the forward voltages are able to be lowered to ca. 25 mV. This makes it possible clearly to reduce the power losses of rectifiers and the expenditure for their cooling. 
     Further advantageous characteristics of the exemplary embodiments and/or exemplary methods of the present invention are yielded by the following explanation of exemplary embodiments with reference to  FIGS. 2-7 . 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an example for a voltage supply of a motor vehicle. 
         FIG. 2  shows a rectifier circuit according to a first exemplary embodiment of the present invention. 
         FIG. 3  shows a rectifier circuit according to a second exemplary embodiment of the present invention. 
         FIG. 4  shows a rectifier circuit according to a third exemplary embodiment of the present invention. 
         FIG. 5  shows a rectifier circuit according to a fourth exemplary embodiment of the present invention. 
         FIG. 6  shows in exemplary fashion the current-voltage characteristics line of a silicon p-n diode and the current-voltage characteristics line of a rectifier circuit according to the present invention. 
         FIG. 7  depicts the implementation of a rectifier circuit according to the present invention in the form of an electronic component. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2  shows a rectifier circuit according to a first exemplary embodiment of the present invention. 
     The rectifier circuit shown in  FIG. 2  may be used, for example, in a rectifier bridge instead of a silicon p-n diode. It has a cathode terminal K 1  and an anode terminal A 1 , the same as a silicon p-n diode. MOS transistor T 1  and inverse diode D 6  are connected in parallel, and from a technological point of view, in this circuit, together they form a microelectronic component. 
     The rectifier circuit shown in  FIG. 2  has a symmetrically designed differential amplifier, which is formed by transistors T 2  and T 3  and resistors R 1 , R 2  and R 3 . A first input of this differential amplifier is connected via a diode D 1  to cathode terminal K 1  and the drain terminal of MOS transistor T 1 . A second input of this differential amplifier is connected via a diode D 2  to anode terminal A 1 . This differential amplifier amplifies the potential difference present between cathode terminal K 1  and anode terminal A 1  of the rectifier circuit. Because of the symmetrical construction of the differential amplifier, temperature differences and ageing effects act only slightly on the properties of the differential amplifier. 
     The output signal of the differential amplifier is available at the collector of transistor T 3 , and is passed on via a resistor R 4  to the input of a power amplifying stage. This power amplifying stage is made up of transistors T 4  and T 5 , whose bases are connected together. Zener diode  5  acts as a protective element for transistor T 1  and protects its gate from overvoltages. 
     In the case of the rectification of an alternating voltage, an alternating voltage of frequency f is present between cathode terminal K 1  and anode terminal A 1 . At a positive potential at cathode terminal K 1 , MOS transistor T 1  with its integrated inverse diode D 6  is in blocking operation and capacitor C 1  is able to charge via diode D 3  and resistor R 5 . The voltage present at capacitor C 1  is used for supplying the additional components of the rectifier circuit. 
     If, on the other hand, the electrical potential at cathode terminal K 1  is more negative than the electrical potential at anode terminal A 1  of the rectifier circuit, then the gate-to-source voltage of MOS transistor T 1  is positive and greater than its threshold voltage. At these conditions, MOS transistor T 1  is conductive, a current flow having this current direction causing only a slight voltage drop. 
     If the electrical potential at cathode terminal K 1  of the rectifier circuit is again more positive than the electrical potential at anode terminal A 1  of the rectifier circuit, then the gate-to-source voltage of MOS transistor T 1  is less than its threshold voltage. Under these conditions MOS transistor T 1  blocks. For this reason, the current flow through MOS transistor T 1  is only very small. 
     If the electrical potential at cathode terminal K 1  of the rectifier circuit is more positive than the electrical potential at anode terminal A 1  of the rectifier circuit and if this potential difference exceeds a value set by Zener diode D 4 , the input potential of the power amplifying stage consisting of transistors T 4  and T 5  is raised. This also increases the gate-to-source voltage of MOS transistor T 1  and a current flow comes about between the drain and the source of MOS transistor T 1 . At the conditions given, this current flow limits the electrical potential difference between cathode terminal K 1  and anode terminal A 1  of the rectifier circuit to a predetermined value. This feature of the limiting of the potential difference represents voltage clamping and constitutes a load dump protection in special cases. 
       FIG. 3  shows a rectifier circuit according to a second exemplary embodiment of the present invention. The design and the functionality of the rectifier circuit shown in  FIG. 3  agree to a great extent with the design and functionality of the rectifier circuit shown in  FIG. 2 . The rectifier circuit shown in  FIG. 3  differs from the rectifier circuit shown in  FIG. 2  only in that the bases of the two transistors T 9  and T 10 , which form the power amplifying stage, are not connected to the cathode of diode D 9  via a Zener diode and a resistor. Accordingly, the exemplary embodiment shown in  FIG. 3  does not have the feature of limiting the potential difference between cathode terminal K 2  and anode terminal A 2  of the rectifier circuit, that is, the feature of voltage clamping. 
       FIG. 4  shows a rectifier circuit according to a third exemplary embodiment of the present invention. The design and the functionality of the rectifier circuit shown in  FIG. 4  agree to a great extent with the design and functionality of the rectifier circuit shown in  FIG. 2 . The rectifier circuit shown in  FIG. 4  differs from the rectifier circuit shown in  FIG. 2  in that the functional features of voltage clamping and power amplification are not provided. The control of the control input and of the gate terminal of MOS transistor T 11  takes place directly from the output of the differential amplifier, which in the exemplary embodiment shown in  FIG. 4  is formed by transistors T 12  and T 13  and resistors R 10 , R 11  and R 12 . 
     In this exemplary embodiment, by omitting the power amplifying stage, conditioned upon the dimensioning of the additional components of the rectifier circuit, the power consumption of the circuit is able to increase. Furthermore, the maximum frequency f of the voltage that is to be rectified is also able to be reduced, since the charging and discharging of the gate of MOS transistor T 11  takes place more slowly at these conditions. 
       FIG. 5  shows a rectifier circuit according to a fourth exemplary embodiment of the present invention. The design and the functionality of the rectifier circuit shown in  FIG. 5  agree to a great extent with the design and functionality of the rectifier circuit shown in  FIG. 2 . The rectifier circuit shown in  FIG. 5  differs from the one shown in  FIG. 2  in that the first input of differential amplifier T 15 , T 16 , R 13 , R 14 , R 15  is not connected via a diode, but directly to cathode terminal K 4  of the rectifier circuit and to the drain terminal of MOS transistor T 14 , and moreover, in that the second input of this differential amplifier is not connected via a diode, but directly to anode terminal A 4  of the rectifier circuit. In this exemplary embodiment we assume that the base-to-emitter inverse blocking capability of transistor T 15  of the differential amplifier is always greater than the maximum voltages present there during the operation of the rectifier circuit. 
       FIG. 6  shows in exemplary fashion the current-voltage characteristics line of a silicon p-n diode and the current-voltage characteristics line of a rectifier circuit according to the present invention. It is clear from  FIG. 6  that forward voltage UARF of a rectifier circuit according to the present invention is relatively small compared to forward voltage UPND of a silicon p-n diode. 
       FIG. 7  depicts the implementation of a rectifier circuit according to the present invention in the form of an electronic component. Rectifier circuits according to the present invention may be composed of discrete components or of specially developed components. Such a low-loss electronic component is seen in  FIG. 7 , which is made up of a MOS transistor MOS, a capacitor C, a mounting rack B and an integrated circuit IC. Integrated circuit IC includes all electronic components of the rectifier circuit except the MOS transistor and the capacitor. The electronic component according to  FIG. 7  is interconnectable in the same way as a silicon p-n diode. In this context, anode terminal A of the electronic component corresponds to the anode terminal of a silicon p-n diode, and cathode terminal K of the electronic component corresponds to the cathode terminal of a silicon p-n diode.