Abstract:
An electric heater for a motor vehicle uses heat generated by power semiconductors as the heat source. The heat output by the power semiconductors is used directly for heating. The power semiconductors are regulated by circuit regulators to be able to adjust the heating power continuously. In addition, switching devices are provided which interrupt or shut down the respective branch circuits individually in the event of short circuits in the power semiconductors.

Description:
BACKGROUND INFORMATION  
         [0001]    The present invention relates to an electric heater for a motor vehicle, using the heat generated by power semiconductors as the heat source.  
           [0002]    Such a heater is known from German Patent No. 34 42 350 C2. With this known heater, the power semiconductor controls the electric drive motor. The power semiconductor is connected to a cooling body through which a liquid coolant flows, so the heat generated is transferred to the liquid coolant by heat exchange. The liquid coolant circulates in a closed line system having a pump and the actual heater installation.  
           [0003]    The efficiency of this known electric heater is not especially great, because the heat generated by the power semiconductor must be converted repeatedly. In addition, the heater installation has a complicated design, depends on the engine current present and thus cannot be regulated independently of the latter.  
         SUMMARY OF THE INVENTION  
         [0004]    The object of the present invention is to create an electric heater of the type defined in the preamble where the efficiency is greatly increased with a simple design and independent regulation of heating power is possible.  
           [0005]    This object is achieved according to a first embodiment of the present invention by connecting several branch circuits, each with one power semiconductor operated in high power loss operation, in parallel for generation of heat, or according to a second embodiment by connecting several branch circuits, each with two series-connected power semiconductors operated in high power loss operation, in parallel for generation of heat.  
           [0006]    In these embodiments, the current is converted directly into heat by the power semiconductors, which greatly increases efficiency. Another advantage of the new heater is that no additional control module is needed for the heater. Installation of the heater in the motor vehicle is also greatly simplified. In addition, the cabling complexity and manufacturing costs of the new electric heater are also reduced.  
           [0007]    No separate fuse protection for the heater in the vehicle electrical system is necessary. When starting operation of the heater, the high starting current surge can be prevented by a regulated smooth current rise. The new heating module can be cascaded in any desired fashion to increase the heating power and can also be integrated easily into a fan regulator.  
           [0008]    To protect the power semiconductors, one embodiment provides for a switching device that responds to overload to be connected in series with the power semiconductor in each branch circuit. In the event of a fault, the branch circuit affected can be shut down with this switching device without having to lose heater function as a whole. Heating power is reduced only by the ratio of defective branch circuits to total branch circuits.  
           [0009]    According to one embodiment, regulation of the heating power is easily made possible by the fact that the power output by the power semiconductors can be regulated individually by a common predetermined setpoint and by actual values derived from the power semiconductors, or by the fact that the powers output by the respective first power semiconductors of the branch circuits can be regulated individually by a common predetermined setpoint and by actual values derived at these power semiconductors, and the powers output by the respective second power semiconductors can be regulated individually by a fixed predetermined control voltage and by actual values derived at these power semiconductors.  
           [0010]    If the branch circuits are to supply power at the output to a low-resistance series resistor as a load impedance, then the heat generated by the series resistor can contribute to an increase in heating power. Each power semiconductor can supply power to an individual series resistor. All the power semiconductors may also supply power to a common series resistor, or groups of power semiconductors may each be connected to a group-individual series resistor.  
           [0011]    The switching devices for interrupting the branch circuits can be implemented in various ways. Thus, according to one embodiment, the switching devices may be designed as a printed conductor part of the branch circuits which burn out in the event of a fault at the elevated current occurring in the respective branch circuit. The same effect can also be achieved by looping the switching devices as shunts into the branch circuits, which burn out in the event of a fault at the elevated current occurring in the respective branch circuit, in which case the shunt can also be used to derive another control signal. Finally, the branch circuit can also be interrupted by using the connecting wires of the power semiconductors which burn out in the event of a fault at the elevated current occurring in the respective branch circuit.  
           [0012]    A controlled reduction or interruption in the current in a defective branch circuit occurs when measures are taken to ensure that in the event of a short circuit in one of the two power semiconductors connected in series in a branch circuit, an additional control signal can be derived from the defective branch circuit to reduce the power output by the respective second power semiconductor or switching it to a disconnect status. The control signal picked off at the shunt can be used as the control signal.  
           [0013]    The structural design of the new electric heater can be simplified by designing it as a heater module, with the power semiconductors mounted in thermal contact on a cooling body, with the heat transfer via the cooling body being improved.  
           [0014]    Simple temperature monitoring can be achieved with the electric heater by the fact that the power semiconductors and/or the cooling body are monitored by temperature sensors to detect whether a predetermined maximum temperature is exceeded, and by the fact that the output signals of the temperature sensor(s) reduce the power output by the respective power semiconductors or all the power semiconductors or switch them to a disconnect status. If the power semiconductors are monitored by individual temperature sensors, the expense of this is reduced by integrating the temperature sensors into the power semiconductors.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    The present invention will now be explained in greater detail on the basis of embodiments illustrated in the drawings, which show:  
         [0016]    [0016]FIG. 1: a circuit diagram of an electric heater with n parallel branch circuits, each containing one power semiconductor, and  
         [0017]    [0017]FIG. 2: a circuit diagram of an electric heater with n parallel branch circuits, each having two series-connected power semiconductors.  
     
    
     DETAILED DESCRIPTION  
       [0018]    As FIG. 1 shows, branch circuits Z 1 , Z 2 , . . . Zn are connected in parallel to one another at power supply voltage Ubatt, with each branch circuit Z 1 , Z 2 , . . . , Zn having a power semiconductor FET 1 , FET 2 , . . . , FETn. The connection to power supply voltage Ubatt is by way of switching devices FUSE 1 , FUSE 2 , . . . , FUSEn, which perform an individual interruption of branch circuit Z 1 , Z 2 , . . . , Zn in the event of a fault, e.g., a short circuit of the power semiconductor at which the multiple current occurs. A shunt which is not shown in detail may be added to the connection of power semiconductors FET 1 , FET 2 , . . . , FETn at the ground potential, where an individual actual value Ist 1 , Ist 2 , . . . Istn can be derived for the branch circuit Z 1 , Z 2 , . . . , Zn. In addition to the actual value picked off at the shunt, a setpoint Isoll is supplied to the gate terminal of power semiconductors FET 1 , FET 2 , FETn via a comparator or operational amplifier that serves as a circuit regulator to permit continuous regulation of the power in the respective power semiconductor. If the actual value exceeds setpoint Isoll, then the circuit regulator switches the power semiconductor into the disconnect status or reduces the power output. In addition, switching device FUSE 1 , FUSE 2 , . . . , FUSEn can completely interrupt branch circuit Z 1 , Z 2 , . . . , Zn in the event of a short circuit of respective power semiconductor FET 1 , FET 2 , . . . , FETn.  
         [0019]    Printed conductor segments of branch circuit Z 1 , Z 2 , . . . , Zn themselves can be used as switching devices FUSE 1 , FUSE 2 , FUSEn. Depending on the design of branch circuits Z 1 , Z 2 , Zn and the respective circuit regulator, the current may increase to a level 25 to 50 times higher in the event of a short circuit, so the printed conductor part burns out. The shunt can also be used as a switching device if it burns out with this current rise and interrupts branch circuit Z 1 , Z 2 , . . . , Zn. Even the connecting wires of power semiconductors FET 1 , FET 2 , . . . , FETn can be dimensioned to assume the function of switching devices FUSE 1 , FUSE 2 , . . . , FUSEn. The electric heater of this type may be designed as a heater module, mounted on a cooling body and integrated into a fan regulator; furthermore, the heater module itself need no longer be fused with respect to the vehicle&#39;s electrical system. However, it may be necessary to fuse the feeder lines to the heater module.  
         [0020]    In the embodiment according to FIG. 2, each branch circuit Z 1 , Z 2 , . . . , Zn has two series-connected power semiconductors FET 11  and FET 12 , FET 21  and FET 22 , . . . FETn 1  and FETn 2 , each controlled by its own circuit regulator. As in the embodiment according to FIG. 1, a switching device FUSE 1 , FUSE 2 , FUSEn and a shunt can be looped into branch circuits Z 1 , Z 2 , . . . , Zn. Switching devices FUSE 1 , FUSE 2 , . . . , FUSEn in turn can be designed in the variants described. Control signals characterizing actual value Ist 11 , Ist 21 , . . . Istn 1  of branch circuit Z 1 , Z 2 , . . . , Zn can be picked off at the shunts of the branch circuits and sent to the circuit regulators of the respective first power semiconductors FET 11 , FET 21 , . . . , FETn 1  to which can also be sent a setpoint Isoll to regulate the power in branch circuit Z 1 , Z 2 , . . . , Zn. The second power semiconductors FET 12 , FET 22 , . . . , FETn 2  are controlled by separate circuit regulators to which are sent a fixed predetermined control voltage ust and an actual value Ist 12 , Ist 22 , . . . , Istn 2 , which is derived from the voltage drop at the first upstream power semiconductor FET 11 , FET 21 , FETn 1 . In the event of a short circuit or defect in a power semiconductor such as FET 11  with this design of the circuit regulators, the respective second power semiconductor, such as FET 12 , in branch circuit Z 1  can be shut down or the power output by it can be reduced. However, the functioning of the remaining system is not affected, and the heating power is merely reduced by the ratio of defective branch circuits to total branch circuits.  
         [0021]    If both power semiconductors, e.g., FET 21  and FET 22 , are short-circuited, then the switching device, e.g., FUSE 2  as in the embodiment according to FIG. 1, goes into operation and interrupts the branch circuit, e.g., Z 2 , at the high current rise occurring.  
         [0022]    If only one common heat-dissipating, low-resistance series resistor is used as the load impedance for all branch circuits Z 1 , Z 2 , . . . , Zn to increase the heating power, then this resistor is looped into the common line leading to battery voltage Ubatt. This series resistor does not change the operation of the electric heater, it merely limits the current rise to a lower level in the event of a short circuit in a single power semiconductor (FIG. 1) or both power semiconductors (FIG. 2), but this lower level is still sufficient for a reliable response of switching device FUSE 1 , FUSE 2 , . . . , FUSEn. The heat generated by the series resistor is also used for heating, but it entails a power distribution which can be utilized at a predetermined maximum heating power to expand the temperature use range for the heater.  
         [0023]    Each power semiconductor or each pair of power semiconductors can also be connected to battery voltage Ubatt across an individual series resistor. Groups of branch circuits may also supply a series resistor. In any case, all the series resistors are involved in the production of heat.  
         [0024]    Temperature monitoring can easily be incorporated into the new heater. Thus, a temperature sensor may be provided for each power semiconductor and may also be integrated into the power semiconductor. If a predetermined maximum temperature is exceeded at the power semiconductor, the output signal of the temperature sensor then controls the respective power semiconductor so that its power output is reduced or it is completely shut down.  
         [0025]    It is also possible to provide just one temperature sensor for measuring the temperature of the cooling body, with all the power semiconductors of the electric heater being in thermal contact with it. If the temperature of the cooling body exceeds a predetermined maximum temperature, then all the power semiconductors are controlled with the output signal of the temperature sensor in such a way that their power output is reduced or they are completely shut down. Different values of the output signal of the temperature sensor can be used for this purpose, with the output signal initially triggering a power reduction at the first lower value and a complete shutdown at the second higher value of the output signal.