Abstract:
The invention relates to a MOSFET circuit having reduced output voltage oscillations, in which a smaller CoolMOS transistor (T 2 ) with a zener diode (Z 1 ) connected upstream of its gate is located in parallel with a larger CoolMOS transistor (T 1 ), so that, during a switch-off operation, after the larger transistor has been switched off, the smaller transistor (T 2 ) carries a tail current on account of the zener voltage still present, which tail current attenuates output oscillations of the voltage.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a MOSFET circuit having reduced output voltage oscillations during a switch-off operation during which the current flowing through the circuit falls to zero. 
     BACKGROUND 
     In SMPSs (SMPS=Switched Mode Power Semiconductor or switched mode power supply) with MOSFETs, the switch-off thereof gives rise to high-frequency output oscillations which bring about interference and thus adversely affect the interference spectrum. This applies in particular to SMPSs which use compensation components, namely so-called CoolMOS-FETs, as MOSFETs. 
     Compared with SMPSs with CoolMOS-FETs, corresponding circuits with IGBTs are distinguished by a lower-interference switching behavior in which significantly fewer high-frequency output oscillations arise during switch-off. 
       FIG. 4  shows a MOSFET T, which is located with its source-gate path and a load L in series between a voltage source +U and a reference-ground potential and is driven at its gate G. If this MOSFET T is switched off at an instant t 0 , then a current I through the MOSFET falls steeply, while an output voltage Uout rises suddenly and exhibits high-frequency output oscillations especially in the case of a CoolMOS-FET, as is illustrated in  FIG. 5 . 
     By contrast, during switch-off an IGBT used instead of the MOSFET T supplies a so-called tail current Itail, which delays the fall in the current I after switch-off at the instant t 0  and thus attenuates output oscillations of the output voltage Uout, so that the interference spectrum is adversely affected to a lesser extent in comparison with a CoolMOS-FET (cf.  FIG. 6 ). 
     It is an object of the present invention to provide a MOSFET circuit, in particular for a switched mode power supply, which is distinguished by reduced output voltage oscillations during a switch-off operation, which is also intended to hold true when CoolMOS-FETs are used for the MOSFET circuit. 
     SUMMARY 
     This object is achieved according to the invention by means of a MOSFET circuit having the following:
         a first MOS transistor having a first number of cells,   a second MOS transistor having a second number of cells, the second number being less than the first number and the second MOS transistor being provided with its source-drain path in parallel with the source-drain path of the first MOS transistor between a voltage source and refererence-ground potential, and   a constant voltage element between gate of the first MOS transistor and gate of the second MOS transistor.       

     A zener diode may advantageously be used for the constant voltage element. A first resistor may be provided in parallel with said zener diode, so that a parallel circuit formed by the zener diode and the first resistor is present. 
     A second resistor may be arranged in series with the parallel circuit formed by the constant voltage element or the zener diode and the first resistor. 
     The zener diode and the first resistor may advantageously be integrated with one another. This may be done for example by the zener diode and the first resistor being formed by a highly doped polycrystalline layer of the first conduction type and a polycrystalline layer of the second conduction type that is in contact with the latter. In this case, the polycrystalline layer of the second conduction type may be located on the polysilicon gate plane of the MOSFET circuit. 
     Polycrystalline silicon on the polysilicon gate plane may likewise be used for the second resistor. 
     The first and second MOS transistors may advantageously be integrated into a chip or semiconductor body. Silicon, silicon carbide, compound semiconductor or another suitable semiconductor material may be used for the semiconductor body. 
     The doping concentration in the highly doped polycrystalline layer forming the zener diode and the first resistor and also in the polycrystalline layer of the second conduction type should not be higher than 10 19  charge carriers cm −3  in order in any event to avoid a short circuit. 
     The MOSFET circuit according to the invention is realized in a particularly advantageous manner with CoolMOS-FETs, since the reduction of the output voltages has a particularly advantageous effect therein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in more detail below with reference to the drawings, in which: 
         FIG. 1  shows a schematic circuit diagram of the MOSFET circuit according to the invention, 
         FIG. 2  shows the profile of the current/voltage characteristic curve of a zener diode in the case of the MOSFET circuit according to the invention, 
         FIG. 3  shows a schematic sectional illustration through two transistor cells with a realization possibility for a zener diode, 
         FIG. 4  shows a schematic circuit diagram with a CoolMOS-FET, 
         FIG. 5  shows a diagram for elucidating the switch-off behavior in the case of a MOSFET, and 
         FIG. 6  shows a diagram for elucidating the switch-off behavior in the case of an IGBT. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 4 to 6  have already been explained in the introduction. 
     In the figures, the same reference symbols are in each case used for mutually corresponding structural parts. 
       FIG. 1  shows an exemplary embodiment of the MOSFET circuit according to the invention with a first, “larger” MOS transistor T 1  and a second, “smaller” MOS transistor T 2 . In this case, “larger” and “smaller” are to be understood such that the first MOS transistor T 1  has more cells than the second MOS transistor T 2 . In this case, the first MOS transistor T 1  may have more cells than the second MOS transistor T 2  by a factor of 10, for example. However, it is also possible to provide only the value 2 or less or a value of more than 10 for said factor. By way of example, the transistor T 1  may have 1000 cells. The transistor T 2  may then be provided with about 100 cells. 
     The two MOS transistors T 1  and T 2  are located with their source-drain paths in parallel with one another between a voltage source +U and a reference-ground potential or ground. 
     A load L may additionally be provided between the parallel circuit formed by the two transistors T 1  and T 2  and the voltage source +U. 
     A control terminal St is connected to gate of the first MOS transistor T 1  and, via a resistor R 1  and the parallel circuit formed by a zener diode Z 1  and a resistor R 2 , is connected to gate of the second MOS transistor T 2 . The resistor R 1  may have a very low resistance and, if appropriate, also be omitted. Only the parallel circuit formed by the zener diode Z 1  and the resistor R 2  is then located between the control terminal St and gate of the MOS transistor T 2 . 
     The MOS transistors T 1  and T 2  are n-channel MOS transistors, for example, which, in particular, are preferably embodied using compensation technology. Thus, CoolMOS transistors are preferably used here. 
     If the two transistors T 1  and T 2  are both in the on state, then a current I flows from the voltage source +U via the load L and the parallel circuits of the two transistors T 1  and T 2  to reference-ground potential. In this case, these two transistors T 1  and T 2  are switched on practically simultaneously by a corresponding signal being applied to the control terminal St. 
     If the two transistors T 1  and T 2  are then switched off at an instant t 0 , the gate voltage drop at the transistor T 2  is delayed by a certain time duration until after the gate voltage drop at the transistor T 1 , since the zener voltage of the zener diode Z 1  is still present momentarily at the gate of the transistor T 2 . This means that the switch-off of the transistor T 2  is delayed with respect to the switch-off of the transistor T 1 . A “tail current” thus continues to flow momentarily, so that a switch-off behavior corresponding to  FIG. 6  for an IGBT is present for the MOSFET circuit of  FIG. 1 . 
     Instead of the zener diode Z 1 , it is also possible to use a different constant voltage element provided that the latter has a characteristic curve as is illustrated for example in  FIG. 2  for the current I as a function of the voltage u across the zener diode Z 1 . 
     The two transistors T 1  and T 2  are expediently integrated in a semiconductor body or on a chip. It is then advantageous also to integrate the resistor R 1 , the resistor R 2  and the zener diode Z 1  in the same semiconductor body or chip. 
     One exemplary embodiment for this is shown in  FIG. 3 . It should be noted in this respect that the conduction types specified may in each case be reversed. Equally, instead of silicon, as has already been mentioned above, it is also possible to use another suitable semiconductor material. 
     Situated in a silicon body  1  with an n + -conducting layer  2  and an n-conducting layer  3  there are p-conducting body regions  4 ,  5 , in which respective n + -conducting source zones  6 ,  7  and p + -conducting body terminal regions  8 ,  9  are incorporated. Source metallizations  10 ,  11  made of aluminum, for example, are connected to the source zone  6  and, via the body terminal region  8 , to the body region  4  and, respectively, to the source zone  7  and, via the body terminal region  9 , to the body region  5 . 
     The metallizations  10 ,  11  are essentially situated on an insulating layer  12  made of silicon dioxide, for example, in which gate electrodes  13  made of n + -doped polycrystalline silicon are incorporated. One of said electrodes  13  may have a p + -doped polycrystalline region  14 , which thus forms a p + /n +  diode with zener-like behavior, that is to say the zener diode Z 1 . In this case, the pn junction between the region  14  and the region  13  constitutes the resistor R 2 . 
     The zener diode with resistor R 2  formed by the regions  13 ,  14  is provided with a metallization  15 . 
     The metallizations  10 ,  11  are introduced into the insulating layer  12  via first contact holes KL 1 , while the metallization  15  leads to the region  14  via second contact holes KL 2  in the insulating layer  12 . 
     It is also possible for p-doped compensation regions  16  to be embedded in the layer  3 , which may provide for charge compensation in the drift path of the transistors and be floating or nonfloating. 
       FIG. 3  shows two cells of a transistor T 1  or T 2 . Each of said transistors may have a multiplicity of such cells, zener diodes (cf. reference symbols  15 ,  14 ) only being incorporated in the cells of the transistor T 2 . 
     Since the MOS transistors T 1  and T 2  each have cells with an identical construction, for example corresponding to the diagram of  FIG. 3 , the MOS transistors T 1  and T 2  may readily be integrated in a single semiconductor body. 
     The resistor R 1  may readily be realized by polycrystalline silicon on the insulating layer  11 . 
     
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 List of reference symbols 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 T1, T2 
                 MOS transistor 
               
               
                   
                 R1, R2 
                 Resistor 
               
               
                   
                 Z1 
                 Zener diode 
               
               
                   
                 St 
                 Control terminal 
               
               
                   
                 U, u 
                 Voltage 
               
               
                   
                 I, i 
                 Current 
               
               
                   
                 L 
                 Load 
               
               
                   
                 Uout 
                 Output voltage 
               
               
                   
                 t0 
                 Instant 
               
               
                   
                 Itail 
                 Tail current 
               
               
                   
                  1 
                 Semiconductor body 
               
               
                   
                  2 
                 Semiconductor layer 
               
               
                   
                  3 
                 Semiconductor layer 
               
               
                   
                  4, 5 
                 Body region 
               
               
                   
                  6, 7 
                 Source zone 
               
               
                   
                  8, 9 
                 Body terminal region 
               
               
                   
                 10, 11 
                 Source metallization 
               
               
                   
                 12 
                 Insulating layer 
               
               
                   
                 13 
                 Gate electrode 
               
               
                   
                 14 
                 p + -conducting region 
               
               
                   
                 15 
                 Metallization 
               
               
                   
                 KL1, KL2 
                 Contact hole 
               
               
                   
                 G 
                 Gate 
               
               
                   
                 T 
                 Transistor