Abstract:
A control device for a power output stage is proposed, which includes a power output stage control which generates a triggering signal as a function of at least one control signal, for triggering at least one power output stage, which switches an electric load, preferably an inductive load having a recovery diode, a recording device being provided which records a measure for an electrical magnitude, which is applied to electric load. A feedback system is also provided, which supplies the power output stage control with at least one feedback signal as a function of the measure for the electrical magnitude, so as to influence the trigger signal.

Description:
FIELD OF THE INVENTION 
   The present invention is based on a device for controlling a power output stage. 
   BACKGROUND INFORMATION 
   A power output stage circuit having PWM (pulse-width modulation) operation and permanently closed operation is described in European Patent Application No. 899 860. A series-connected control circuit controls the pulse-width modulation generator and the power output stage into a permanently open state, a PWM operation having pulse-width modulated pulses and a permanently closed state, as a function of an externally predefined setpoint value, an actual value of the power output stage, and a reference voltage derived from the supply voltage. An increased power loss and a high EMV (electromagnetic compatibility) in the extreme range toward full load output may be avoided by the feature that it goes over prematurely from PWM operation to permanently closed operation as a function of a predefined setpoint value and of the supply voltage, and through a hysteresis of these values it returns again to PWM operation. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide a power output stage control which further increases the EMV compatibility and decreases the power loss. 
   The present invention relates to a device for controlling a power output stage which may generate a trigger signal for at least one power output stage, as a function of at least one control signal, in order to trigger the power output stage, in a switching operation, which may switch an electric load having an inductive component, which may have a recovery diode, for example. The device may further include a detection unit which may acquire a measure for the electrical magnitude applied to the load. It may be distinguished by having a feedback system provided, which may supply at least one feedback signal to the power output stage control, as a function of the measure for the electrical magnitude, for influencing the trigger signal. Because of this, both line-conducted and radiated interferences, particularly with pulse-width modulated power controllers for inductive loads in motor vehicles, may be further reduced, since the feedback may permit an optimized triggering of the power semiconductor. Interferences may be caused by high-frequency, steep-skirted switching on and off, in particular, by the time delays during transition from conducting operation to blocking operation, or vice versa, in a recovery diode connected in parallel to an electromotor. This time delay of the recovery diode at the transition from conducting operation to blocking operation, and vice versa, may be detected with the aid of a voltage dip or a voltage increase and may be used for a failure-optimized triggering of the power output stage. At the times of the transition from the conducting to the nonconducting state of the recovery diode, the steepness of curve of the drain voltage of the power output stage may be reduced in a controlled manner. This may be achieved by the feedback system, which may send a feedback signal on to the power output control, so that the response time of the power output stage may become slower during the transition time. At times when the switching action of the diode is finished, the power output stage may be triggered so as to reduce the overall power loss at maximal response time, i.e. at maximal steepness of curve. 
   In an example embodiment, a feedback activating device may be provided which controls the passing on of the feedback signal. The feedback activating device may pass on the feedback signal for the reduction of the steepness of curve only when the switching action of the diode is not yet terminated. Thereby interferences may be minimized. If, however, the switching action of the diode is finished, the feedback activating device may detect this with the aid of the decline of the voltage increase or the voltage dip, and may deactivate the passing on of the feedback signal. Thereby the switching behavior of the power output stage may be influenced in a controlled manner, in order, on the one hand, to minimize interferences during the switching action of the diode, and on the other hand to accelerate the switching behavior of the power output stage at the end of the switching action of the diode for the reduction of power loss. 
   In a further example embodiment, the feedback activating device may pass on the feedback signal when the measure for the electrical magnitude exceeds or falls below a boundary value. For example, the beginning of the switching action of the diode may become noticeable in the form of a voltage increase or a voltage decrease, which may be detected by boundary value comparison. 
   In a further example embodiment, it may be provided to store the electrical magnitude in a storage element, such as, for example, in an RC (resistance-capacitance) element, temporarily as a reference value. If the electrical magnitude exceeds or falls below the temporarily stored reference value, it may be concluded that there is a voltage peak, and the feedback is activated. The voltage peak may be used as feedback signal, if necessary, after signal adjustments via resistors. This analogous switching implementation may ensure an interference-minimized and loss-minimized triggering of the power output stage. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a circuit configuration for controlling a power output stage. 
       FIG. 2   a  shows a time-dependent voltage curves of the drain voltage of the output stage, of the diode voltage at the recovery diode, as well as the resulting total voltage at a closed process. 
       FIG. 2   b  shows the corresponding time-dependent voltage curves at open processes. 
       FIG. 2   c  shows the time-dependent curve of the drain voltage at feedback according to the present invention. 
   

   DETAILED DESCRIPTION 
   An inductive load  10  is provided as the electrical load, such as an electromotor. A recovery diode  12  is connected in parallel to this inductive load  10 . Inductive load  10  is connected via power output stage  14 . For the detection of the current flowing through power output stage  14 , a measuring resistor  20  connected to ground is provided, whose potential is picked off and supplied to a PWM control  18 . A setpoint value is also supplied to this PWM control  18 . From the setpoint value and the actual value, PWM control  18  generates a control signal which is used by a power output stage control  16  as an input variable. Power output control  16  includes a push-pull stage  22 , made up of two transistors whose respective bases are connected to each other in an electrically conducting manner. Both control signal  19  via a series resistor and a feedback signal  21  are supplied to the base of push-pull stage  22  in an electrically conducting manner. Push-pull stage  22  is fed by positive supply voltage UB and connected to ground on the other end. The emitter of one transistor of push-pull stage  22  is connected via a series resistor to the gate terminal of power output stage  14  for passing on a trigger signal  15 . The two transistors of push-pull stage  22  are interconnected as emitter follower. 
   Supply voltage UB, which may be picked off at positive pole  56 , reaches the parallel connection of inductive load  10  and recovery diode  12  via a first choke  50 . Between positive pole  56  and a negative pole  58  there is a first capacitor  48 . The minus potential is taken by second choke  52  to ground. Also, a second capacitor  54  is provided which couples the positive supply potential of inductive load  10  to ground. First and second chokes  50 ,  52  and first and second capacitors  48 ,  54  are used for interference elimination. 
   A feedback system  24  is controlled by the positive supply voltage as input value. The positive supply potential is connected to the base of a second transistor  30 , whose collector is connected via a second resistor  32 , on one hand to ground, and on the other hand to the base of a first transistor  26 . The collector of first transistor  26  is connected to ground via a first resistor  28 , the emitter of first transistor  26  is brought together with the output signal of a sixth resistor  46  in an electrically conductive manner, and thereafter reaches the base of push-pull stage  22  as feedback signal  21 . Feedback system  24  includes an RC element  35 , which is formed from a fourth resistor  36  and a third capacitor  38 , connected to ground, and is fed by the positive supply potential. The common potential of fourth resistor  36  and third capacitor  38  reaches the emitter of second transistor  30  via a third transistor  34 , and on the other side the common potential is connected to the base of a third transistor  44  as well as via a fourth capacitor  40  to the collector of third transistor  44  in an electrically conducting manner. The positive supply potential of inductive load  10  also reaches the emitter of third transistor  44  via fifth resistor  42 . The collector output of third transistor  44  is connected via sixth resistor  46  to the output potential of the emitter of first transistor  26 , in an electrically conducting manner, thus creating feedback signal  21 . 
   The functionality and operating mode of the circuit shown in  FIG. 1  will now be described in greater detail with the aid of  FIGS. 2   a  through  2   c . The signal curves shown in  FIGS. 2   a  and  2   b  show the drain voltage UDS, diode voltage UDI as well as the total voltage Uges resulting from these at switching on ( FIG. 2   a ) and switching off ( FIG. 2   b ) of power output stage  14 . If one looks at the time-dependent voltage pattern of total voltage Uges, a voltage dip and a voltage peak may be determined. These voltage dips and voltage peaks are related to the switching behavior of recovery diode  12 . At the appearance of the (change-over) switching actions, recovery diode  12  must first make free space of the barrier-layer capacitance in order to change its switching state. There is almost immediately a short-circuit if power output stage  14  is already turned on, but recovery diode  12  is not yet completely in blocking operation. The result is a voltage dip of total voltage Uges. This voltage dip has a negative effect on line-bound as well as radiated interferences. Now, according to the present invention, the detected voltage dip or voltage increase is used to influence the trigger signal for power output stage  14  in a controlled manner. This influence is clearly shown in  FIG. 2   c . During the change-over switching action of recovery diode  12 , power output stage  14  is triggered with a lesser steepness, i.e. the switching action is deliberately slowed down. This slowing down of the switching behavior of power output stage  14  shows up in the second range  62  as well as in the framework of the fifth range  65  (when switching off power output stage  14 ). Once the change-over switching action of recovery diode  12  is ended, which may be recognized because of the ending of the voltage dip or the voltage increase of total voltage Uges, power output stage control  16  generates a trigger signal  15  in such a manner that the gate voltage UDS of power output stage  14  changes with maximum steepness (third range  63 ), until ground potential has almost been reached. The corresponding happens also for the switching-off phase of power output stage  14 , as shown by fourth to sixth ranges  64  to  66 . 
   The signal curve shown in  FIG. 2   c  may be achieved, for instance, by the circuit shown in FIG.  1 . On account of feedback system  24 , the voltage peaks (voltage dip in the switch-on phase, voltage increase in the switch-off phase) which are created by the time-delayed switching behavior of recovery diode  12 , have, at the point in time of their creation, a negative feedback on power output stage control  16  in the form of negative feedback signal  21 . Thereby the switching behavior of power output stage  14  is slowed down only during the times up to achieving complete blocking operation or conducting operation of recovery diode  12 , on account of which the resulting EMV interferences are greatly reduced. Power output stage control  16  is configured so that, without feedback, the maximum switching speed of power output stage  14  is attained, which becomes noticeable from the very steep skirts of the first, third, fourth and sixth ranges  61 ,  63 ,  64 ,  66 . 
   In RC element  35 , which is made up of fourth resistor  36  and third capacitor  38 , almost the normal level of total voltage Uges is stored, that is, when no voltage dip or voltage increase occurs. Now PWM control  18  creates a control signal  19  in such a manner that power output stage  14  is to be switched on. Power output stage control  16  creates a trigger signal  15  which generates a drain voltage UDS with maximum switching speed (first range  61 ). During this timespan the feedback is not yet active, and as yet no feedback signal  21  is being generated by the feedback system. For up to now, total voltage Uges did not yet exceed the reference voltage stored in RC element  35 , so that none of the three transistors  26 ,  30 ,  44  turns on. 
   Subsequently, because of the change-over switching action of recovery diode  12 , a voltage dip of total voltage Uges may come about during the switching on action of power output stage  14 . If the voltage dip of total voltage Uges falls below the reference value stored in RC element  35  by a certain amount, the voltage relationships have the effect of biasing into conduction first and second transistor  26 ,  30 . The voltage dip of total voltage Uges reaches the base of push-pull stage  22  as feedback signal  21 , via first and second transistor  26 ,  30 . Control signal  19 , also supplied to the base of push-pull stage  22 , is reduced by feedback signal  21  corresponding to the voltage dip (negative feedback). Thereby the switching speed of power output stage  14  may be reduced. The drain voltage UDS of power output stage  14  drops off at a lower slope in second range  62  than in first and third ranges  61 ,  63 , in which no voltage dips of total voltage Uges were detected. 
   In the meantime, recovery diode  12  may be completely reversed. This is shown by the decline of the voltage dip. The total voltage Uges now approximately attains again the value stored temporarily in RC element  35 . First and second transistor  26 ,  30  are no longer turned on, so that no feedback signal  21  reaches the base of push-pull stage  22 . Power output stage control  16  now generates a trigger signal  15  with maximum switching speed, which may become noticeable in the form of the maximum steepness of curve in third range  63 . 
   A control signal  19  from PWM control  18  now reaches power output stage control  16 , which may effect switching off power output stage  14 . In fourth range  64 , power output stage  14  is first activated with maximum switching speed. Because of the change-over switching actions in recovery diode  12 , a voltage increase of total voltage Uges develops, as in  FIG. 2   b . In RC element  35  the voltage level of total voltage Uges is before the voltage increase is stored. If the voltage peak exceeds the reference voltage stored in the RC element by a certain amount, third transistor  44  is turned on. Now the positive voltage peak reaches the base of push-pull stage  22  as feedback signal  21 , via third transistor  44 . Thereby a trigger signal  15  is generated in such a manner that the switching speed is reduced. This goes along with a less large voltage change of drain voltage UDS in the fifth range  65 . If the change-over switching action of recovery diode  12  is ended, the voltage increase of total voltage Uges is reduced. If the voltage increase again falls below a certain boundary value, third transistor  44  is no longer turned on. Feedback signal  21  no longer reaches the base of push-pull stage  22 . Power output stage  14  is once again activated with maximum switching speed (sixth range  66 ). 
   The two chokes  50 ,  52 , as well as the two capacitors  48 ,  54  may be used to improve the EMV behavior. Besides that, the two chokes  50 ,  52  may ensure that there will indeed quickly be a detectable voltage dip or voltage increase in total voltage Uges. This voltage dip or this voltage increase may thereby be more easily detected, and after an appropriate signal adaptation via resistors  25 ,  32 ,  34 ,  42 ,  46 , is also used correspondingly as feedback signal  21 . 
   The circuit as in  FIG. 1  described above may accordingly also be used if power output stage  14  is connected to the positive pole, and inductive load  10  together with recovery diode  12  are connected to the negative pole. The levels have to be adjusted accordingly. 
   This circuit may be applied for triggering an actuating drive in a motor vehicle, for example, for blower control or for flap adjustment. However, it is not limited to these.