Abstract:
A switch-off threshold of an electronic component for protecting from a temperature-specific overload is determined by comparing a parameter directly dependent on the temperature of the electrical component with a parameter indirectly dependent on the temperature of the electrical component. In this way, standard components can be used, power transistors in particular, which are monitored by an external temperature sensor.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention relates to a device and a method for protecting an electronic component, in particular a field effect transistor from temperature-related destruction and/or overload.  
         [0003]     Published, European patent application EP 0 323 813 A1 discloses a device for protecting an integrated power circuit from over temperatures. The device contains two sensors integrated in the circuit. In this case, the first sensor supplies an output value dependent on the temperature of the integrated circuit and the second sensor supplies a second output value dependent on the current through the component, in this case a power transistor.  
         [0004]     With the known circuit configuration, the current flow through the transistor is set as a function of the output value of the two sensors. In this case, with an increased temperature, the threshold for a reduction of the collector emitter current is reduced in the integrated circuit.  
         [0005]     In order to be able to provide a reliable power amplifier, it must be protected from overload damage. In addition, the temperature sensors are integrated into the power amplifier, as mentioned above. One measure for the performance of the protective circuit is the thermal transition between the electronic component and the thermal sensor element. With a bad thermal connection and a fast heating process, which can occur with large currents for example, extremely large temperature differences can exist between the electronic component and the temperature sensor. Therefore, the actual temperature of the electronic component can be considerably higher than that of the sensor element. The result of this is that the electronic component can be damaged or destroyed, because an evaluation circuit linked to the sensor element has not yet determined an over temperature.  
         [0006]     With field effect transistors (FET) and metal oxide field effect transistors (MOSFET) the temperature of the barrier of the semiconductor device may amount to no more than 175° C. With bipolar transistors, the maximum admissible barrier temperature is somewhat higher, 200° C. in fact. With the maximum admissible barrier temperature, the residual currents of the semiconductor components can be 100 times greater than at 25° C. The failure probability of the semiconductor device increases with an increasing barrier temperature. Thus, the semiconductor device should be reliably protected from over temperatures. The drain-source current of a transistor is referred to here as the residual current.  
         [0007]     In order to achieve reliable protection, the temperature sensors are mainly integrated into the electronic component, disposed directly thereon or disposed as close as possible to the region of the component (in this case the barrier of a power transistor) reacting to the increased temperature. This configuration is nevertheless disadvantageous in that instead of standard components, only components with integrated temperature sensors can be used.  
       SUMMARY OF THE INVENTION  
       [0008]     It is accordingly an object of the invention to provide a method and a device for protecting an electronic component that overcomes the above-mentioned disadvantages of the prior art methods and devices of this general type, which protects the electronic component as much as possible from a temperature-specific destruction, as a function of the thermal transition between a temperature sensor and the electronic component.  
         [0009]     With the foregoing and other objects in view there is provided, in accordance with the invention, a method for protecting an electrical component from a temperature-specific malfunction. The method includes determining a first measurement value being directly dependent on a temperature of the electronic component, determining a second measurement value being indirectly dependent on the temperature of the electronic component, and comparing the first measurement value with the second measurement value. The electronic component is switched on or off, if a result of a comparison exceeds or undershoots a predetermined threshold.  
         [0010]     In this case, a first measurement value is compared to a second measurement value. If the result of the comparison exceeds or falls short of a predetermined threshold value, the electrical component is switched off or on. In this way, the first measurement value is directly dependent on the temperature of the electrical component and the second measurement value is indirectly dependent on the temperature of the electrical component.  
         [0011]     Furthermore, the electrical component providing the second measurement value that is indirectly dependent on the temperature of the component is not integrated in the electrical component to be examined. This allows standard components that do not have integrated protective circuits to be used. The configuration of a circuit of this type results among other things in a considerable cost-savings.  
         [0012]     In a preferred embodiment of the invention, the first and the second average value are compared by subtraction.  
         [0013]     In this way in a particularly simple manner, exceeding or not reaching the predetermined threshold value can be established by a change of sign in the result of the subtraction.  
         [0014]     In a further preferred exemplary embodiment, the method and/or the device has a further switch-off threshold that depends exclusively on one of the two measurement values.  
         [0015]     Furthermore, a second measurement value can be compared to the first measurement values of a number of electrical components.  
         [0016]     This produces particular advantages in terms of the construction space required and the number of electronic components required since an additional external temperature sensor can be used for a number of electronic components.  
         [0017]     Other features which are considered as characteristic for the invention are set forth in the appended claims.  
         [0018]     Although the invention is illustrated and described herein as embodied in a method and a device for protecting an electronic component, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.  
         [0019]     The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]      FIG. 1  is a circuit diagram of a first exemplary embodiment of a circuit configuration according to the invention;  
         [0021]      FIG. 2  is a circuit diagram of a second exemplary embodiment of the circuit configuration according to the invention; and  
         [0022]      FIG. 3  is a graph of a first and a second measurement value over time. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]     Referring now to the figures of the drawing in detail and first, particularly, to  FIG. 1  thereof, there is shown an electronic component B, which serves to switch an electronic load L on or off. A series circuit containing the electronic component B and the load L is disposed between a first potential VCC and a second potential GND of an operating voltage source. A link point between the component B and the load L has been indicated here by reference character P 1 .  
         [0024]     The electronic component B is shown here schematically with a resistor R DS,on  and a circuit ST, as an equivalent circuit diagram. The resistor R DS,on  and the switch ST are connected in series.  
         [0025]     Component B can be a circuit breaker, a FET, MOSFET or IGBT for example. This can also be a part of a half bridge configuration to control a load L, such as a gearbox in a motor vehicle for example.  
         [0026]     The switch ST is switched on or off by a voltage U G . The voltage U G  is made available by a control circuit GC (Gate Control), as a function of two input parameters. The control voltage U G  is generated by the control circuit GC as a function of an on/of f switching signal U C  and the output voltage U ST  of a comparator K. The output voltage U ST  of the comparator K supplies the control circuit GC, with a switch-on or switch-off signal for the circuit S T , as a function of the temperature of the component. The control signal U ST  can be linked to the switch-on/switch-off signal U C  for example, by an AND gate. The circuit S T  is then only switched on if both voltages U ST  and U C  have a level such that should result in the circuit being turned on, according to the predefined threshold values.  
         [0027]     In order to ensure a prompt switch-off of the electrical component B with a temperature T that is no longer tolerable, temperature sensors are disposed outside the component, in this case a PTC resistor R PTC . The comparator K determines on the one hand the electrical current U PTC  dropping over the PTC resistor R PTC , and on the other hand an electrical current U RDS,on  dropping over the resistor R DS,on . The resistor R DS,on  represents a temperature-dependent parameter of the component B, in this case the resistance between drain and source of the field effect transistor. This is recorded using a differential amplifier A, and fed to the comparator K on its inverted input (−). The load current I L  is assumed here either as constant or determined via a measurement resistor, thereby taking account of a possible change in the load current I L  during the evaluation of the measurement result.  
         [0028]     On the one hand, the PTC resistor is linked to a third potential V SV  via a resistor R 1 , in this case 5V, and on the other hand to a second potential GND of the supply voltage source. The voltage U PTC  dropping via the PTC resistor is supplied to the comparator K at its noninverting input (+). The control voltage U St  is determined here by a comparison, in this case a subtraction of two voltages U RDS,on  and U PTC . The first measurement value, the voltage U RDS,on  is thus directly dependent on the temperature of the electrical component B. As the resistor R PTC  is disposed in a thermal coupling to the component B, the second measurement value, the voltage U PTC  is indirectly dependent on the temperature of the electrical component B. The quality of the thermal coupling between the component B and the PTC resistor depends on its spacial arrangement in relation to the component B.  
         [0029]     The circuit configuration is useful in that, as a result of the indirect or direct coupling, the two temperature-dependent voltages U RDS,on  and U PTC  drift apart at the source of the temperature change, due to the different thermal time instants of the coupling, in other words, the rate of increase of the two voltages U RDS,on  und U PTC  are different.  
         [0030]     In the present case, the switch-off threshold is selected such that the electrical component B is switched off, once the voltage U ST  at the output of the comparator K is approximately equal to 0. In this case, the voltage U RDS,on  would be equal to the voltage U PTC  dropping via the PTC resistor.  
         [0031]     In addition to subtracting two of the voltages U RDS,on  and U PTC  dependent on the temperature of the electrical components B, other operations are also possible, such as an addition, a multiplication, or also a division of these two input parameters U RDS,on  und U PTC  for example. The switch-off threshold, here U ST  approximately equal to 0 volts, is selected such that the temperature of the electrical component B for the switch-off threshold exceeds a predetermined temperature of 120° C. for example.  
         [0032]     As a result of comparing or linking the directly temperature-dependent voltage U RDS,on  and the indirectly temperature-dependent voltage U PTC , the excess current switch-off threshold is achieved and the power amplifier is switched off in an overload case, by rapidly heating the component B even with lower temperatures. The excess current switch-off threshold is dependent on the current difference between the voltage U RDS,on  and the voltage U PTC . In this way, the loading of the electrical component B is reduced and thus guards against the failure of the component.  
         [0033]      FIG. 2  shows a further exemplary embodiment of a device for protecting an electronic component. In this diagram, functionally identical components are given the same reference characters as in  FIG. 1 .  
         [0034]     The electrical component B is shown again here, in this case an integrated circuit containing an N channel MOSFET T and a control circuit GC. Diode D 1  is disposed parallel to the drain source stretch of transistor T. The diode D 1  is a substrate diode present in any event on a MOSFET.  
         [0035]     The MOSFET T is electrically connected with its drain connection D to the first potential VCC of the supply voltage source and with its source connection S to a node P 1 . The node P 1  is electrically connected to the second potential GND of the supply voltage source via the load L, as shown in  FIG. 1 . A PTC resistor R PTC  is also disposed here spacially separated from the integrated electronic component B. The electronic component B and the PTC resistor R PTC  are combined here into one component assembly BG. This combination is intended to clarify the spacial proximity of the component B and the PTC resistor R PTC .  
         [0036]     On the one hand, the PTC resistor R PTC  is connected to a resistor R 1  and on the other hand to the second potential GND of the voltage supply. The second connection of the resistor R 1  is connected to a third potential V 5V . The resistors R PTC  and R 1  form a voltage divider, the voltage U PTC  fed to the comparator K being varied as a function of the value of the PTC resistor R PTC .  
         [0037]     The control circuit GC also has an input for a switch-on/switch-off signal U C  and the output voltage U ST  of the comparator.  
         [0038]     In a similar manner to the exemplary embodiment according to  FIG. 1 , the voltage U RDS,on  dropping over the drain source route of the transistor T is determined by a difference amplifier A and fed to the inverting input (−) of the comparator K. On the one hand, the voltage dropping over the PTC resistor R PTC  is fed to the noninverting (+) input of the comparator K, on the other hand, it can be fed to a non-illustrated further circuit configuration by a node P 2 . By way of example, this can contain a microcontroller with an analog-digital converter. If a predetermined threshold value U PTC,max  is exceeded, the electronic component B can also only be switched off as a function of the voltage U PTC  dropping over the PTC resistor. This prevents an absolute maximum temperature, T max =130° C. being exceeded for example.  
         [0039]     The resistor U RDS,on  has a positive temperature coefficient. This fluctuates in a region between 0.7% K =1 ≦α≦1.8% K −1 . The positive temperature coefficicent a increases the loss performance of the MOSFET T operating as a circuit and can, in extreme cases, result in a malfunction of the transistor T.  
         [0040]      FIG. 3  shows the graph of the first and the second measurement value, voltages U RDS,on  and U PTC  as a function of time. As illustrated in this diagram, the voltage U RDS,on  has a larger increase than the voltage U PTC . This results in U PTC &lt;U RDS,on  for t=0, in that the two curves intersect each other after approximately nine seconds. In both exemplary embodiments according to  FIGS. 1 and 2 , this intersection point results in an output voltage U ST of  0 volts at the comparator K. In this case, the electronic component B would be switched-off.  
         [0041]     The first measurement value U RDS,on  is directly dependent on the temperature of the component B, in other words, the measurement value is directly tapped at the location of the temperature change. The second measurement value U PTC  changes as a direct consequence of the thermal coupling by the heat derived from the component B. The second measurement value U PTC  is measured physically distanced from the component B and is determined based on the measurement value of the temperature of the component B.  
         [0042]     This application claims the priority, under 35 U.S.C. § 119, of German patent application No. 10 2004 020 274.5, filed Apr. 26, 2004; the entire disclosure of the prior application is herewith incorporated by reference.