Patent Publication Number: US-2004051398-A1

Title: Device for rapid short-circuit protection in a power semiconductor

Description:
BACKGROUND INFORMATION  
       [0001] The present invention is directed to a device for rapid short-circuit protection in a power semiconductor according to the preamble of the independent claim. A sense high-side switch having important protective functions integrated into it is described in the article “Sense-Highside-Schalter übernimmt Sicherungsfunktionen [Sense High-Side Switch Assumes Safety Functions],” by A. Blessing, A. Graf, P. Sommer in the journal  Components,  5-6/97, pages 32 to 35 (BTS 640S2). An overheat cutoff and a current-limiting function, which are continuously active, are provided. A signal proportional to a load current may be picked up at a sense output of the power semiconductor. This sense voltage is analyzed by an A/D converter of a microcontroller and processed further, e.g., for fusing purposes.  
       [0002] However, in this power semiconductor there is no possibility for altering the internal current-limiting function, i.e., the value of the cutoff current from the outside. Depending on the use of the power semiconductor, maximum peak currents of consumers connected to it may vary greatly. In most cases, the current limit is set very high by the manufacturer of the power electronic unit to ensure protection of the power semiconductor itself. Since a power semiconductor that is accurately adapted for each application with regard to continuous current and/or maximum peak current is not available for each application, it is often necessary to use oversized power semiconductors. This in turn results in, for example, an unnecessarily high current flowing over the plugs, the circuit boards, i.e., the printed conductors, the power semiconductor, the cable and short-circuit sink, e.g., in the case of a short circuit until detection and initiation of countermeasures. In order not to have to dimension components that might be subject to a short circuit for the short-circuit current of the power semiconductor, it is desirable to have a short-circuit shutdown of the power semiconductor, the value of which is designed to be rapidly applicable. The applicable short-circuit shutdown is especially important in conjunction with a dual-voltage vehicle electric system (12 V/42 V) to make it possible to control a short circuit between the two voltage levels.  
       [0003] Such a multivoltage vehicle electric system is described, for example, in German Patent Application 199 448 33, which has been published subsequently. Short-circuit protection means are provided between the two voltage levels of the multivoltage vehicle electric system to largely reduce a short circuit and/or prevent the effects of a short circuit between the two voltages and/or protect or shut down endangered consumers in the event of a short circuit. Analysis of a possible overcurrent is controlled by a program in a microcontroller.  
       [0004] The object of the present invention is to provide a device which will increase the security with respect to short circuits. This is to be accomplished in an inexpensive manner.  
       [0005] This object is achieved through the features of the independent claim.  
       ADVANTAGES OF THE INVENTION  
       [0006] The device according to the present invention for rapid short-circuit protection in a power semiconductor includes at least one power semiconductor via which a load current may be applied to at least one electric load. Current detection means are provided, providing a measure of the load current applied to the electric load. A semiconductor protective circuit triggers the power semiconductor into protective operation in the event of an imminent impairment of the power semiconductor. According to the present invention, in addition to the semiconductor protective circuit, at least one additional electric component is provided to compare the load current or a measure of the load current with a limiting value, where monitoring means trigger the power semiconductor into protective operation with regard to the electric load when the monitored value exceeds the maximum limiting value or drops below the minimum limiting value. The additional load current monitoring is implemented according to the present invention by a hardware circuit. In comparison with a software-based analysis, there are advantages with regard to the speed with which a possible overload is detected. Therefore, countermeasures may be initiated rapidly to reliably protect the electric load. The value of the short-circuit shutdown of the power semiconductor is preferably adjustable by the user. The user is thus given an opportunity to use the power semiconductor for triggering any loads by selecting a suitable dimension for the limiting value.  
       [0007] In an expedient refinement, a locking circuit is provided to suppress activation of the power semiconductor when the monitored value drops below the limiting value in the meantime. The locking circuit increases protection against permanent damage to the power semiconductor and/or the electric load because the on and off operations in particular constitute a special risk for the power semiconductor and the electric load. It is possible to inquire as to the status of the lock for further processing. The power semiconductor is enabled to resume normal operation only by a specific unlocking signal. This targeted control increases the ability of a user to influence the protective function of the power semiconductor.  
       [0008] Additional expedient refinements are derived from additional dependent claims and from the description. 
     
    
    
     DRAWING  
     [0009] Exemplary embodiments of the present invention are illustrated in the drawing and explained in greater detail in the following description.  
     [0010]FIGS. 1 and 2 show typical embodiments of the power semiconductor;  
     [0011]FIG. 3 shows an additional protective function implemented in the power semiconductor;  
     [0012]FIG. 4 shows a protective function implemented outside the semiconductor; and  
     [0013]FIG. 5 shows a typical dual-voltage vehicle electric system in which the power semiconductors are preferably used. 
    
    
     DESCRIPTION OF THE EXEMPLARY EMBODIMENTS  
     [0014] An integrated power semiconductor  10  has at least one load output  12  by which an electric load  24  may be supplied with a load current IL, which flows toward ground  26 . A switching means  20  is provided for activating power semiconductor  10 ; when the switching means is closed, a control input  16  of power semiconductor  10  is at a logical reference potential  22 . Power semiconductor  10  has a current balancing output  14  at which a current proportional to load current IL flows across a shunt  18  to logical reference potential  22 . Voltage drop VIS induced across shunt  18  is analyzed.  
     [0015] In the exemplary embodiment according to FIG. 2, the individual components of power semiconductor  10  are shown in greater detail. Various protective and analyzing functions are provided, such as a voltage source  30 , a surge suppressor  32 , a current-limiting device  34 , a gate protection  36 , the actual power switch  38 , a voltage sensor  40 , a charge pump  42 , a protective circuit for inductive loads  44 , a current detection unit  46 , an electrostatic discharge protection  48 , a logic circuit  50  and a temperature sensor  52 . Otherwise, the external components correspond to those in FIG. 1.  
     [0016] The exemplary embodiment according to FIG. 3 is used for an internal protective circuit acting directly on current-limiting device  34  of power semiconductor  10 . To do so, current balancing output  14  is connected to control input  16  across shunt  18  for the case when switching means  20  is closed and thus power semiconductor  10  has been activated. Voltage drop VIS across shunt  18  is compared with a reference voltage  60  by a comparator  62 . The output signal of comparator  62  is sent to current-limiting device  34 .  
     [0017] In the exemplary embodiment according to FIG. 4, voltage drop VIS across shunt  18  is smoothed by a filter  70  composed of an RC element, for example. The smoothed output voltage is sent to a transistor stage  72  or as an alternative to an inverting input of a comparator  74 . If smoothed voltage VIS exceeds a certain limiting value VCC, then the output signal of transistor stage  72  as well as that of comparator stage  74  become logical 0. These output signals are sent to a first AND gate  76  whose output signal is sent as an input signal to a second AND gate  78 . The output signal of first AND gate  76  goes to the second input of first AND gate  76  via a seal-in resistor  80 . An unlocking signal  84  may also go via a diode to the second input of first AND gate  76 . In addition, the condition of the locking circuit and/or the seal-in circuit may also be queried via two resistors via the pin through which unlocking signal  84  of the locking circuit may also be sent. Regular triggering  82  of power semiconductor  10  (and thus of load  24 ) is sent to the second input of second AND gate  78 . When activation is requested in normal operation, switching means  86  is triggered, so that control input  16  of power semiconductor  10  is at a logical reference potential  22  so that load current IL is applied to electric load  24  (not shown in FIG. 4).  
     [0018]FIG. 5 illustrates the essential components of a dual-voltage vehicle electric system of a motor vehicle. Specifically, generator G, e.g., a claw-field three-phase generator driven by the vehicle engine is shown. Generator G supplies an output voltage U0 of 42 V, for example, which is used directly to charge battery B 1  with a rated voltage of 36 V. The line resistance between generator G and battery B 1  is symbolized by resistors R 1  and R 2 . The consumers, which are to be supplied with voltage U0, are connected to generator G by a signal/power distributor V 1 . In particular, three consumers R 6 , R 7  and R 8  are shown here as examples of electric load  24 , connectable to generator G via power semiconductors H 1 , H 2  and H 3 . These power semiconductors H 1 , H 2  and H 3  have inverse diodes D 1 , D 2  and D 3  and internal resistors R 3 , R 4  and R 5  determined by the design.  
     [0019] A second battery B 2  is charged by generator G via a d.c.-d.c. converter W 1 . The d.c.-d.c. converter W 1  converts voltage U0=42 V into a voltage U1=14 V which is suitable for charging battery B 2  having a rated voltage of 12 V. Voltage U1 is supplied from voltage converter W 1  to battery B 2  via switch S 1  and the line having line resistance R 9 . Resistance R 9  also includes the internal resistance of battery B 2 .  
     [0020] Battery B 2  is used to supply consumers which require a lower voltage, e.g., 12 V or 14 V. The connection is accomplished via signal power distributor V 2 . These consumers are labeled as R13, R14 and R15, and they may be switched on via power semiconductors H 4 , H 5  and H 6  having inverse diodes D 4 , D 5  and D 6 , respectively. The line resistances between consumers R13, R14 and R15 are labeled as R10, R11 and R12.  
     [0021] The consumers that are to be supplied with 12 V or 14 V power via SLV2 also include, if so decided, a Zener diode Z 1  and another diode D 7 , which together form a surge suppressor. Zener diode Z 1  and additional diode D 7  are mentioned only as examples of possible voltage-limiting means. It is also possible to use other limiter circuits.  
     [0022] The consumers for one voltage level or the other are selected depending on the voltage requirements for optimum operation of these consumers. For example, the starter may be connected to either the 12 V battery or the 36 V battery. When using power semiconductors on the 14 V side, the switch having the short-circuited 14 V load becomes conducting through the inverse diode of the respective power semiconductor which is always present and thus connects all the 14 V consumers to 42 V, so that the consumers which are not designed for this voltage level are endangered. FIG. 5 shows such a short circuit. A resistor RK which is between resistors R 8  and R 13  on the voltage side represents a short circuit, the effects of which are to be ameliorated according to the present invention. The following discussion will explain how the effects of a short circuit, symbolized by resistance R 16 , may be limited.  
     [0023] In the exemplary embodiments according to FIGS. 1 and 2, shunt  18  converts output current IS of current balancing output  14  into a voltage signal VIS which is proportional to load current IL, usually being directly proportional. Shunt  18  is dimensioned so that the current range of interest for the given application, between a value of zero and the peak current, which is converted to a conventional voltage range for an A/D converter, e.g., 0 to 5 V. As soon as voltage drop VIS across shunt  18  is greater than 5 V, this is outside the desired current range. This usually signals a fault case such as a short circuit in the overall system. In this case, power semiconductor  10  should be triggered into protective operation. Protective operation is understood to be, for example, operation with a current-limiting function or complete shutdown of power semiconductor  10 .  
     [0024] As a rule, power semiconductor  10  does not have any possibility of picking up logical reference potential  22  to detect the voltage drop across shunt  18  based on this logical reference potential  22 . To overcome this problem, it is proposed that voltage drop VIS across shunt  18  be measured relative to the potential picked up at control input  16 . In the case of triggering of power semiconductor  10 , switching means  20  is closed and thus control input  16  is at logical reference potential  22 . Control input  16  is suitable for this application, however, because monitoring is of interest only in the activated state of power semiconductor  10 .  
     [0025] According to FIG. 3, comparator  62  as an electronic component compares voltage drop VIS across shunt  18  with reference voltage  60 . For the reasons explained above, this voltage is 5.5 V, for example, to reliably detect when the working range of load current IL is exceeded. If voltage drop VIS across shunt  18  exceeds reference voltage  60 , the output signal of comparator  62  activates current-limiting function  34  already integrated into power semiconductor  10 . The current-limiting circuit either causes a direct shutdown of power semiconductor  10 ,  38  or it regulates the voltage drop across shunt  18  to a maximum of 5.5 V. This would result in limiting load current IL to a value proportional to reference voltage  60 . The user may adapt reference voltage  60  to the particular application case, i.e., to electric load  24  to be triggered as desired. This supplementary circuit is relatively small in comparison with the circuit of power semiconductor  10 , which is already present, and thus it increases the additional cost only slightly. However, the functionality of power semiconductor  10  is greatly increased without having to perform intervention measures in the semiconductor circuit itself. Thus, the user&#39;s previous wiring may remain the same.  
     [0026] In the exemplary embodiment according to FIG. 4, external monitoring means are provided, simultaneously producing a rapid shutdown of power semiconductor  10 . For implementation of the protective function, contrary to the exemplary embodiment illustrated in FIG. 3, this embodiment no longer relies on internal current-limiting function  34  of power semiconductor  10 . Instead, power semiconductor  10  is shut down via control input  16 , as explained below. In normal operation, load current IL is within the allowed range. Therefore, the output signal of first AND gate  76  has the logic  1  state, so that triggering  82  reaches the control input of switching means  86  unhindered. If triggering  82  signals a requested activation of load  24 , then switching means  86  sets control input  16  at logical reference potential  22 . Therefore, load current IL is applied to load  24 . The signal which is proportional to load current IL is available at current balancing output  14 . Voltage drop VIS across shunt  18  is smoothed by (optional) RC element  70 . The output signal smoothed in this way is sent either to transistor stage  72  or to comparator stage  74  to perform monitoring whether a definable limiting value has been exceeded. In the exemplary embodiment, the limiting value is selected as the VCC signal, so it is approx. 5 V. If smoothed voltage drop VIS across shunt  18  exceeds reference voltage VCC of 5 V, then either transistor stage  72  or comparator stage  74  will output an output signal of logical zero. This output signal of logical zero is sent to first AND gate  76 , whose output signal also assumes the value logical zero. Since the output signal of first AND gate  76  is also used as the input signal of second AND gate  78 , the output signal of second AND gate  78  changes its logical state to logical zero. Thus switching means  86  is no longer triggered so that control input  16  is no longer at logical reference potential  22 . Therefore, power semiconductor  10  is switched off. Current flow IL through load  24  is suppressed. A locking circuit is provided to suppress immediate renewed activation of power semiconductor  10 . To do so, the output signal of first AND gate  76  goes across seal-in resistor  80  to the second input of first AND gate  76 . Thus, the logical zero signal is also at the second input of AND gate  76  once the monitoring function is activated, so the output signals of the two AND gates  76 ,  78  remain constantly at logical zero. The condition of the locking circuit may be queried by signal  84 . This may be used for additional analysis purposes. A locking signal having the logical zero state signals that the protective function has been activated. To be able to return power semiconductor  10  to operation, the user must apply a signal  84  having the logical  1  state to the second input of first AND gate  76 . In the normal case, load current IL will not have exceeded limiting value VCC, so first AND gate  76  is acted upon by two logical  1  signals, so that its output also assumes the logical  1  value. Triggering signal  82  is thus switched through to the output of second AND gate  78  to allow activation of switching means  86  as desired with the corresponding action upon control input  16 . Semiconductor  10  may thus be triggered again, so that load current IL may flow through electric load  24 .  
     [0027] Power semiconductors  10  may be used as power semiconductors H 1 -H 6  in the manner already described in the exemplary embodiment according to FIG. 5. In the event of a short circuit between different voltage levels U1 and U0 of the multivoltage network in particular, the electronic components contribute toward early detection of an unacceptable load current IL and initiation of countermeasures.