Abstract:
A rectifier assembly including a fusible cutout is configured as a polarity reversal protection device. The fusible cutout includes a semiconductor chip having a positive temperature coefficient so that when it overheats, it becomes unsoldered and interrupts the current flow.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a rectifier assembly. 
     BACKGROUND INFORMATION 
     A rectifier assembly is referred to in German Published Patent Application No. 199 08 697, although no fuse measures to protect against polarity reversal are discussed therein. 
     SUMMARY OF THE INVENTION 
     The exemplary rectifier assembly according to the present invention, i.e., the fusible cutout for use in a rectifier assembly, provides that the circuit may be interrupted at an early point in time, in particular in the case of polarity reversal, and thus prevent a fire caused by overheating of the rectifier elements. The fusible cutout provided for this purpose has the close manufacturing tolerances required in the range of high currents (100 A), so there is no risk of tripping the fuse mechanism during normal operation of the generator. 
     Exemplary embodiments of the present invention are illustrated in the drawings and explained in the following description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a rectifier assembly. 
         FIG. 2  shows the semiconductor chip of a fusible cutout. 
         FIG. 3  shows a cooling plate. 
         FIG. 4  shows the contacting of a fusible cutout. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a rectifier assembly for a three-phase generator in a motor vehicle. The input includes three phase terminals u, v and w and is connected in a known manner to six semiconductor rectifier elements  1  through  6 , in particular semiconductor diodes. The anodes of diodes  4 ,  5  and  6  are connected to a ground terminal. The cathodes of diodes  1 ,  2  and  3  are also connected to one another. They are connected to output B+ of the rectifier assembly across a fusible cutout  10 . 
     The three-phase current of a three-phase generator whose three-phase terminals are connected to input u, v, w is rectified by the bridge rectifier formed by diodes  1  through  6 . Output B+ is connectable to the positive pole of the vehicle battery, the negative pole is connected to ground, so the rectified current of the three-phase generator is able to charge the vehicle battery. The battery is prevented from discharging by the diodes of the rectifier. However if the poles of the battery are connected in reverse (e.g., in the case of a starting aid), the battery may discharge across the three diode segments  1 ,  4  or  2 ,  5  or  3 ,  6 , which are thus connected in the direction of flow. Depending on the size of the battery and its charge, discharge currents of up to 900 A may result. Fusible cutout  10  is provided to prevent such discharge currents, which could last for more than 60 seconds, until the circuit would be interrupted due to destruction of the rectifier diodes or melting of feeder wires. This prevents hazardous situations in which high temperatures could occur in the rectifier components and cause ignition of plastic insulation, some of which could burn with a naked flame and trigger an automotive fire. The fusible cutout here is a silicon-based fuse, i.e., the same material as that of the rectifier elements. 
       FIG. 2  illustrates semiconductor chip  100  of fusible cutout  10 . This semiconductor chip includes an n-doped middle zone  110 , where the doping corresponds to the basic doping of the semiconductor wafer used in its production. Edge areas  120  and  130  of the chip are highly n-doped and are contactable via metallic coatings  140  and/or  150  applied to their outside surfaces. 
     The chip is soldered into a casing, which is also used for the diodes of the rectifier. This procedure is exactly the same as that known for rectifier diodes as referred to in German Published Patent Application No. 195 49 202, for example. The chip here is secured on a press-fit socket by using solder, and the head wire is soldered to the other side of the chip. Perpendicular to the direction of current flow, the chip includes a cross-sectional area approximately the same size as the corresponding cross-sectional area of a rectifier diode. This achieves the result that the thermal resistivity between the chip and the casing bottom is equal to the corresponding thermal resistivity of the rectifier diodes. 
     In another exemplary embodiment, highly doped zones  120  and  130  may be omitted and fusible cutout chip  100  may be made completely of an n-doped region  110 . 
     The fusible cutout chip is made of a silicon wafer doped homogeneously with phosphorus, for example, having a thickness between 130 and 200 micrometers. The dopant concentration amounts to 10 18  cm −3 , the specific resistance at 25 degrees Celsius is approximately 23 mΩcm. It is provided with a metallic coating on both sides to produce the exemplary embodiment without any additional doping steps and is divided into individual square chips having an edge length of approximately 4.5 millimeters. This yields a cross-sectional area of approximately 20 square millimeters, which is suitable for generator currents of more than 120 A. Coating with metal may be performed, for example, by sputtering with chromium, followed by nickel-vanadium and finally silver. The chips are divided by using a diamond saw. If the resistance of the fusible cutout chip is to be reduced, the configuration illustrated in  FIG. 2  is expedient. In this case, before applying the metallic coating, additional doping of the wafer that has already been doped and a diffusion step are performed to produce highly n-doped regions  120  and  130 . 
     Although the forward voltage of a diode has a negative temperature coefficient, the temperature coefficient of the homogeneously doped fusible cutout chip is positive. The resistance of the fusible cutout chip may therefore be adjusted so that in normal operation of the rectifier, the resistance and thus the electric power loss are comparable to the electric power loss of a rectifier diode. In the case of polarity reversal, the resistance of the fuse increases drastically due to the positive temperature coefficient. The increase in temperature unsolders the fusible cutout chip out of the casing and the circuit is broken after only a short battery discharge time. The resistance of the fuse is adjustable with a close tolerance, and the electric power loss is adapted to the power loss of the rectifier diodes through the doping and geometry. The cooling is also comparable due to the use of the same casing. Thus, in the event of a reversal of battery poles, the fuse will always become unsoldered before the rectifier diodes. 
       FIG. 3  shows a top view of a cooling plate  20  including recesses  30 ,  40 ,  50 ,  60  and  70 . Recesses  20 ,  30 ,  40 ,  50   60 , and  70  have the same diameter. 
     To produce the rectifier assembly, rectifier diodes  1 ,  2  and  3 , which are manufactured in the form of press-fit diodes, are pressed in a known manner into recesses  30  through  50 . In the same manner, the casing of the fusible cutout chip, which is identical to the casings of the rectifier chips, is also pressed into recess  60 . Recess  70  is used for mounting a contact for the fusible cutout chip. This contact is described below in the explanation of FIG.  4 . 
       FIG. 4  shows a detail of cooling plate  20  including a press-fit fusible cutout  61  which is pressed into recess  60 , i.e., a press-fit casing, such as that also used for the rectifier diodes in which, however, a fusible cutout chip has been mounted instead of a diode chip. Fuse  61  is arranged with its press-fit socket  63  in recess  60  in the cooling plate. A contact screw  71  which is electrically insulated from the cooling plate by insulation  72  is introduced into recess  70 . A contact wire  74  clamped by a contact nut  73  is electrically connected to head wire  62  of fusible cutout  61  via a soldered joint  75 . 
     Contact screw  71  forms output B+ of the rectifier assembly. Current flows over cooling plate  20 , which forms the electrical connection of the cathodes of diodes  1 ,  2  and  3 , over press-fit socket  63  into the fuse and from there over head wire  62 , contact wire  74  and contact screw  71  into the vehicle electrical system and/or the battery of the motor vehicle. The thermal resistivity between the bottom of the individual casing of the diodes (not shown here) and fuse  61  is identical. Diodes and the fusible cutout have the same cooling conditions. The total thermal resistivity between the chip (i.e., the diode chip or fusible cutout chip) and the cooling air is approx. 2 K/W at a rotational speed of 3000 rpm of the three-phase generator. In the case of a fusible cutout chip without highly doped regions  120 ,  130  of an n-doped silicon wafer having a specific resistance of 23 mΩcm at a temperature of 25 degrees Celsius, a chip thickness of 200 micrometers and a chip area of 20.3 square millimeters, an equilibrium temperature of 215 degrees Celsius is established on the fusible cutout chip with the cooling plate arrangement described here, e.g., in normal operation at a generator current of 120 A and a cooling air temperature of 120 degrees Celsius. The voltage drop amounts to approximately 0.4 volt, and the electric power loss is approx. 48 watt. In comparison with that, the chip temperature of the rectifier diodes is 182 degrees Celsius, the total power loss of the three diodes  1 ,  2  and  3  is 108 watt. The electric power loss may be further reduced by using chips having a smaller thickness of approx. 130 micrometers, for example. The equilibrium temperature on the fusible cutout chip is then 177 degrees Celsius, the voltage drop is 0.24 volt and the power loss is 28 watt (i.e., approx. 25% of the uniform power loss of the three diodes  1 ,  2  and  3  of 108 watt, which is independent of chip thickness).