Test procedure and test system for testing Efuses

Abstract

The invention relates to a test method for checking the operating state of an e-fuse (1) that can be switched between at least one conducting and one non-conducting state and via which an electrical load (3) is electrically connected to a supply voltage source (2), wherein the electrical load (3) tolerates a supply interruption for a predetermined maximum interruption duration. The switchable e-fuse (1) is switched for a test time, T S , which is at most equal to the maximum interruption duration of the electrical load (3), is switched from the conductive to the non-conductive state. The switchable fuse (1) is switched during the test time, T SThe residual current flowing through the capacitor is detected as an actual residual current value by means of a test circuit (120) and compared with at least one desired residual current value. If the actual residual current value is below the at least one desired residual current value, the switchable fuse (1) is assigned a first operating state, and otherwise a further operating state. The invention further relates to a test system (100) for carrying out the test method.

Description

The invention relates to a method for testing switchable electronic fuses according to the preamble of claim 1.Electronic fuses, referred to below as "electronic fuses") are semiconductor circuits for limiting or interrupting a current consumed by an electrical load. Analogously to classic fuses, Eruses are destroyed if, in the event of an overcurrent (for example in the event of a short circuit occurring in the electrical load), a current which lies above a permissible rated current value flows through them for a predetermined time. An effect correctly destroyed in an overcurrent case is typically permanently (irreversibly) non-conductive, but resettable effects are also known, which can be reset back into the initial state after an overcurrent case.Switchable outputs can be electronically switched between a conducting and a non-conducting state until an overcurrent situation occurs. For example, a switchable fuse can be designed as a field effect transistor, for example as an n-channel metal oxide semiconductor field effect transistor, which can be switched between a conductive and a non-conductive state via a voltage applied to the gate electrode.As a result of errors during production or during operation or as a result of degradation of outputs caused by aging, it is possible that, in the event of an overcurrent, the electrical load to be protected is not reliably separated from the supply voltage. This impairs the operational reliability of the electrical load. For example, if such an electrical load is a safety-critical electrical subsystem of a vehicle, the operational safety of the vehicle may be impaired. Therefore, it is necessary to check the working state of such effuses regularly.For this purpose, the fuse can be tested in an operating mode with a suspended electrical load, for example in a parking state with the safety-critical electrical subsystem switched off. However, this method is not applicable to checking the operating state of an efuse in operation and is therefore not suitable for continuous monitoring.Furthermore, a redundant second fuse can be arranged in parallel with a first fuse between a supply voltage source and an electrical load to be protected. Both outputs can then be tested alternately separated from the electrical load and insulated without impairing the supply of the electrical load. However, this redundancy leads to increased manufacturing and operating costs, for example due to an increased space requirement on printed circuit boards or chips and / or to an increased power consumption, because additional expenses have to be provided.Document US 2001 / 0034070 A1 describes an apparatus and a method for testing fuses in a memory device. A voltage is applied across a control resistor, which voltage drops over time. The voltage dropped across the control resistor is compared with a reference voltage value by means of a comparator. The time period after which the comparator changes state is determined. Fuses are tested by comparison with this determined time duration by applying a decaying voltage across the fuse to be tested and determining the time duration until the comparator switches. In the case of a resistance value of the fuse according to the specification, the comparator switches its state within the time duration determined on the basis of the control resistor. By adapting the reference voltage of the comparator, the time duration within which the comparator is to switch for a specified resistance value of the fuse can be reduced and the test time can thus be shortened. However, this method is limited to testing non-switchable electrical connections and, in particular, to determining a state of sufficient conductivity (i.e., a sufficiently low resistance).There is therefore a need for an improved test method with which the operating state of a switchable fuse can be detected, wherein a distinction is to be made in particular between a safe operating state in which the electrical load is reliably disconnected from the supply voltage in the event of an overcurrent and an unsafe operating state in which the electrical load is not reliably disconnected from the supply voltage in the event of an overcurrent. Furthermore, it may also be necessary to detect operating states in which the electrical load is indeed disconnected in the event of an overcurrent, wherein, however, certain parameters (for example the switch-off time or the remaining residual conductivity along the fuse) do not correspond to the specified setpoint values.In a first aspect, the object of the invention is to specify an improved method for testing a switchable fuse. This object is achieved according to the invention by a method having the features of claim 1.The invention is further based on the object of specifying an improved test system for testing a switchable fuse. This object is achieved according to the invention by a test system having the features of claim 8.Advantageous embodiments of the invention are the subject matter of the dependent claims.The first aspect of the invention relates to a method for testing a switchable fuse.In the case of a switchable fuse which can be switched between at least one conductive state and one non-conductive state and is electrically connected to a supply voltage source via the at least one electrical load, at least a first, functional working state and a second, non-functional working state can be distinguished.In the first operating state, the at least one electrical load is reliably disconnected from the supply voltage source if the current drawn by the electrical load via the fuse is above a predetermined rated current intensity for a predetermined period of time. In the second operating state, a disconnection of the electrical load is not ensured or is not reliably ensured under these conditions.The at least one electrical load is configured to tolerate a supply interruption (i.e., a disconnection from the supply voltage source) for a predetermined maximum interruption duration.The interruption maximum duration is the maximum time period during which operation of the electrical load can be maintained taking into account all tolerances without the supply voltage source being supplied, for example by means of internal buffer memories in the electrical load.In a test method for checking the operating state of a switchable fuse, the switchable fuse is switched from a conductive state to a non-conductive state for a test time, wherein the test time is at most equal to the shortest maximum interruption duration of all electrical loads supplied by the switchable fuse.During the test time, the residual current flowing through the switchable fuse in the non-conductive state is detected as an actual residual current value and compared with at least one desired residual current value. A first (functional) operating state is assigned to the switchable fuse if the detected actual residual current value is below the at least one desired residual current value. If at least one setpoint residual current value is less than or equal to the detected actual residual current value, then a further operating state is assigned to the switchable fuse.The method makes it possible to test a switchable effect with regard to its functional reliability without impairing the ongoing operation of an electrical load supplied thereby and without using a redundant structure of switchable effects. This allows a particularly cost-saving, energy-saving and close-mesh functional checking of switchable uses.In one embodiment of the method, the actual residual current value is detected by detecting an actual value of a discharge time duration of the discharge of a capacitor fed in parallel with the electrical load from the supply voltage source via a resistor. The detected actual value of the discharge time duration is compared with at least one setpoint value for the discharge time duration, which is determined for a switchable fuse in a respective operating state.For example, a first setpoint value for the discharge time duration can be determined in a fully functional working state of a switchable fuse and a second setpoint value for the discharge time duration can be determined in a still functional, but already degraded working state. The actual value of the discharge time duration of a capacitor connected to the supply voltage source via a switchable fuse to be tested is then detected and compared with the first and second setpoint values. If the actual value of the detected discharge time duration is below the first setpoint value, the "functional" operating state is assigned to the switchable efuse to be tested. If the actual value of the detected discharge time duration is below the second setpoint value, but not below the first setpoint value, the switchable fuse under test is assigned the operating state "functionally limited". If the actual value of the detected discharge time duration is not below the second setpoint value, the "non-functional" operating state is assigned to the switchable fuse to be tested.In a development of this embodiment of the method, a measurement voltage across the capacitor is detected and compared with a reference voltage, for example by means of a comparator. The reference voltage can be provided, for example, by an adjustable reference voltage source.A clock signal having a predetermined clock period can be counted during a period in which the measurement voltage is above the reference voltage and the fuse has been switched to the non-conductive state. The actual value of the discharge time duration can be determined from the product of the count and the period duration of the clock signal. For example, if the reference voltage is selected to be sufficiently low (close to the voltage value of the measurement voltage for the fully discharged capacitor), the product of the count with the period duration can be formed directly. To shorten the measurement duration, however, it is also possible to choose a higher reference voltage and to extrapolate the (correspondingly lower) count determined therefrom, which would result for a reference voltage which would be close to the measurement voltage across the almost completely discharged capacitor.The counter required for a time measurement according to this further development of the method, like a precise clock generator, can be implemented very easily and cost-effectively or is already present. In addition, the counting operation can be very easily restricted to the time range in which the capacitor is discharged via the resistor. Thus, counter circuits or counter modules typically have a "capture" input which must be occupied by a positive (logically true) level and / or a rising (or falling) edge in order for the counting process to be triggered or the counter state to be detected. For example, this level and / or this edge for such a "capture" input can be obtained from the output voltage of a comparator, with which the measurement voltage is compared with the reference voltage. A logical combination (for example with a signal voltage which triggers the switching of the switchable fuse from the conductive state to the non-conductive state) can also be very easily implemented, for example via a transistor-transistor logic (TTL).It can thus be determined in a particularly simple manner on the basis of the discharge time period whether the fuse as intended disconnects the connection to the electrical load when it is switched from the conductive state to the non-conductive state. At an actual value of the discharge time period which lies above a predetermined corridor around an associated setpoint value of the discharge time period (corresponding to an insufficiently rapid discharge of the capacitor), an excessively high leakage current of the switchable fuse in the non-conductive state or a short circuit across the switchable fuse can be detected. In this case, the operational reliability of the electrical load is endangered in that it is not separated from the supply voltage in the event of a fault (exceeding the rated current).If the actual value of the discharge time duration is below this predetermined corridor, it is possible to detect an excessively low charge of the capacitor, which can be caused, for example, by the switchable fuse not switching completely on when it is actuated in the conductive state (that is to say that the resistance along the fuse is too high).In a development of the method, a current temperature value and / or a current value of the supply voltage and / or a deviation of the capacitance value of the capacitor and / or a deviation of the resistance value of the resistor from the respective nominal value is or are determined and used for correcting the actual value of the discharge time duration.For example, the actual capacitance value of the capacitor can be smaller than its nominal capacitance value and a shorter actual value of the discharge time duration can thereby be effected. In order to prevent a switchable fuse from being incorrectly positively classified as faulty, a corrected (correspondingly reduced) setpoint value of the discharge time duration is determined on the basis of the actual (corrected) capacitance value of the capacitor. Such a correction can be obtained particularly easily in a sufficient approximation by scaling the setpoint value of the discharge time duration (corresponding to the ratio of the setpoint value to the actual value of the capacitance value). Of course, (computationally equivalent) the actual value of the discharge time duration can also be scaled correspondingly inversely.In the same way, a correction of the actual resistance value is possible in the event of a deviation from the nominal value of the resistance value.The influences of the temperature of the switchable fuse and / or further components and of fluctuations in the supply voltage can also be taken into account by arithmetic correction of the setpoint value of the discharge time duration (or, computationally equivalently, of the actual value of the discharge time duration). It may be possible for such fluctuations or deviations to be incorporated nonlinearly in the computational correction. In this case, it will prove to be advantageous to acquire correction tables, for example a correction table for taking into account the operating temperature of the fuse, in which a correction term is acquired in each case for a series of temperature values, which correction term is to be applied additively, multiplicatively or according to another functional relationship to the acquired actual value of the discharge time period when the respective temperature has been acquired. In an analogous manner, a correction table can also be applied for different values of the supply voltage.Such correction tables can be stored in a non-volatile memory and called up by a test evaluation unit after the actual value of the discharge time duration has been detected as described. In such a nonvolatile memory, correction values for taking into account the deviations of the actual value of the resistance value and / or of the actual value of the capacitance value can also be stored, which are acquired once (with respect to a switchable effect in each case) in a calibration step.This further development of the embodiment enables the use of components of little precision (resistors, capacitors) and the execution of the method even in very different situations (at different, greatly varying supply voltages and temperatures). It thus improves the robustness of the method and enables a simpler and more cost-effective implementation.In an exemplary embodiment of the method, the test time (shut-down time) during which the switchable fuse is placed in the non-conductive state is limited to 100 microseconds. Since electrical loads, in particular those which are used in the automobile, have buffer memories (for example internal capacitances) which are sufficiently large to bridge such a short interruption without restriction, it is possible with this embodiment of the method to test Effects in a very large number of operating situations (with a wide range of electrical loads).In one embodiment of the method, detected actual residual current values (for example actual residual current values determined from actual values of the discharge time duration) are stored continuously. From this, a dynamically adapted setpoint residual current value is ascertained continuously in such a way that a further working state (i.e. a working state deviating from the working state "functionally") is assigned to the switchable fuse if a currently detected actual residual current value deviates statistically strikingly from currently detected actual residual current values. In this way, gradual processes (for example aging-related drifting) which do not impair the functionality of the switchable fuse can be distinguished from potentially error-prone sudden changes. This makes it possible to detect defective switchable effuses with particularly high specificity and sensitivity.In one embodiment of the method, the switchable efuse is arranged in a vehicle for supplying an electrical load. Such electrical loads (for example control units, sensor systems for detecting a vehicle environment or components of a drive train) are particularly relevant to safety and reliability. Their disconnection in the event of a fault (e.g. when a rated current value is exceeded) is therefore particularly important.In one embodiment of the method, a first operating state is assigned "functionally" to the switchable fuse if the actual residual current value is below a first setpoint residual current value. A second operating state "functionally limited" is assigned when the actual remaining current value lies between the first and a second desired remaining current value, which is higher than the first desired remaining current value. A third operating state "non-functional" is assigned if the actual residual current value is above the second desired residual current value.Analogously, if the actual residual current value is determined on the basis of the actual value of the discharge time duration, a first (comparatively smaller) setpoint value and a second (comparatively larger) setpoint value of the discharge time duration can also be used as the basis for the assignment of the first to third operating states.This embodiment has the advantage that already degraded, provisionally yet functional switchable effects (for example with an increased leakage current, which is still within the provided tolerance range, in the blocked, non-conductive state) are detected in good time, so that they can be replaced in scheduled maintenance work.According to a further aspect, the invention relates to a test system which is set up to carry out the method described above. According to the invention, the test system comprises a test circuit, a test control unit and a test evaluation unit.The test circuit is configured to detect the actual residual current value which flows through the switchable fuse when the latter is switched to the non-conductive state.The test control unit is configured to switch the switchable fuse from the conductive state to the non-conductive state for a predetermined test time.The test evaluation unit is configured to compare the actual residual current value detected by the test circuit with at least one target residual current value.The advantages of the test system correspond to those of the method according to the first aspect of the invention that can be carried out therewith.In one embodiment, the test circuit comprises an RC element having a resistor and a capacitor connected in parallel therewith, as well as a comparator and a timer.The RG element can be connected to a supply voltage source via the switchable fuse. The comparator is configured to compare a measurement voltage which is present across the RC element with a reference voltage which is provided, for example, by a preferably adjustable reference voltage source.The timer is connected to the test control unit and the comparator and is configured to record the time that elapses from the time when the switchable fuse is switched from the conductive state to the non-conductive state by the test control unit to the time when the comparator determines that the measurement voltage across the RC element reaches or falls below the reference voltage.With such a test circuit, an actual value of the discharge time duration of the capacitor can be detected via the resistor, from which the actual residual current value can be determined by the blocked switchable fuse (switched to the non-conductive state). Alternatively, instead of comparing the actual residual current value with a target residual current value, the detected actual value of the discharge time period can also be compared with a target value of the discharge time period corresponding to the target residual current value.This embodiment thus enables a particularly simple and accurate check of the actual residual current value and thus a check of the functionality of the switchable fuse.In one embodiment, the test evaluation unit comprises an analog-to-digital converter (ADC) which is configured to detect the supply voltage and / or which, in cooperation with a temperature sensor, is configured to detect the temperature of the switchable fuse. Alternatively or additionally, the test evaluation unit according to this embodiment comprises a nonvolatile memory for storing at least one calibration value and / or at least one detected residual current value behind.This embodiment has the advantage that fluctuations in the operating temperature of the fuse or the supply voltage and production tolerances of the electronic components, for example of the resistor and of the capacitor in the RC element, can be taken into account in the testing of the fuse. This allows a particularly specific and sensitive test of the fuse with at the same time a particularly simple and cost-effective construction of the test system. Further advantages correspond to those of the corresponding embodiment of the method described above according to the first aspect of the invention.Exemplary embodiments of the invention are explained in more detail below with reference to drawings.The following are shown: FIG. 1 schematically shows a test system for a switchable fuse, FIG. 2 schematically shows the time profile of a test voltage during a test phase, and FIG. 3 schematically shows a flow chart for the test evaluation of a measurement at a switchable fuse.Corresponding parts are provided with the same reference numerals in all figures.FIG. 1 shows, purely by way of example and schematically, a test system 100 for checking the operating state of a switchable fuse 1, which is arranged between a supply voltage source 2 and an electrical load 3.In the present exemplary embodiment, the switchable fuse 1 is designed as an n-channel metal oxide semiconductor field effect transistor (NMOS FET), but other forms of a semiconductor switching element are also possible in the same way. In the present case, the source electrode 1.S of the NMOS FET is connected to the supply voltage source 2. The drain electrode 1.D is connected to the electrical load 3 via a diode 4. The direction of flow of the diode 4 points from the switchable fuse 1 to the electrical load 3.The supply voltage source 2 can be designed, for example, as a vehicle battery of a vehicle, not shown in detail, which emits a supply voltage U V of typically between 8 volts and 17 volts. Alternatively or additionally, the supply voltage source 2 can be part of an electrical on-board power supply of the vehicle. The electrical load 3 can be designed, for example, as a control unit, active sensor, actuator or as another, electrically operated subsystem of the vehicle.The electrical load 3 must be supplied continuously with the supply voltage U V but can bridge interruptions of the supply voltage U V for a certain period of time (for example, by means of internal capacitors or other buffer memories). Typically, supply interruptions can be bypassed up to an interruption maximum duration of approximately 100 microseconds.The test system 100 comprises a microcontroller 110 with external circuitry which is arranged (typically together with the switchable fuse 1) on a printed circuit board, which is not shown in any more detail. On the microcontroller 110 are arranged a timer 111, a non-volatile memory 112 and a multi-channel analog-to-digital converter (ADC) 113.Furthermore, microcontroller 110 has an execution unit for executing program instructions, which is not shown in greater detail in FIG. 1. Program code executable by this execution unit comprises an effect test software 114 and an application software 115, which can be stored in a common or in separate program memories of the microcontroller 110. The program memory / memories of the microcontroller 110 is / are not shown in more detail in FIG. 1. For example, a non-volatile memory 112 can be designed as flash memory and can be configured for storing both program instructions and for non-volatile storage of data.The multi-channel ADC 113 samples the value of the supply voltage U V on a first channel and the value of a peripheral temperature sensor 101 on a second channel. The temperature sensor 101 detects the temperature of the printed circuit board on which the fuse 1 and the external circuitry of the microcontroller 110 are arranged.The timer 111 can be designed, for example, as a resettable counter which, upon a counter start, counts an internal clock of the microcontroller 110.Furthermore, the microcontroller 110, controlled by the program instructions of the fuse test software 114, provides a signal voltage U S which is fed to the gate electrode 1.G of the switchable fuse 1. The switchable fuse 1 can be switched with the signal voltage U S between a conductive state (for example at U S≈5 volts) and a non-conductive state (for example at U S≈0 volts).Between the drain electrode 1.D of the switchable fuse 1 and the ground there is arranged an RC element 102, 103, which is formed by a resistor 102 and a capacitor 103 connected in parallel thereto and which has a time constant τ, which results from the product of the resistance value R and the capacitance of the capacitor C (τ=R·C).The measurement voltage U RC across the RC element 102, 103 is detected by a comparator 104 and compared with a reference voltage U R which is provided by a reference voltage source 105. The comparator output voltage U K at the output of the comparator 104 assumes a high level (which corresponds to a positive value, for example U K≈3 volts) if the measurement voltage U RC is greater than or equal to the reference voltage U R. The comparator output voltage U K at the output of the comparator 104 assumes a low level (which corresponds to a comparatively smaller value, for example U K≈0 volts) if the measurement voltage U RC is smaller than the reference voltage U R.In the exemplary embodiment according to FIG. 1, test system 100 includes a test circuit 120, a test control unit 121, and a test evaluation unit 122.The test circuit 120 is formed in the present case by the timer 111, the RC element 102, 103 having the resistor 102 and the capacitor 103, the comparator 104 and the reference voltage source 105.The test control unit 121 is formed in the present case by the microcontroller 110 with the effect test software 114. The test evaluation unit 122 is formed in the present case by the microcontroller 110 with the ADC 113 and the nonvolatile memory 112 and the temperature sensor 101. However, it is also possible for the test control unit 121 and the test evaluation unit 122 to be realized independently of one another.The mode of operation of the test system 100 is explained in more detail below with reference to FIG. 2, which schematically shows the profile of the signal voltage U S, the measurement voltage U RC and the comparator output voltage U K.At a first time t 0 the efuse test software 114 lowers the signal voltage U S (for example from U S ≈5 volts to U S ≈0 volts) in such a way that the switchable efuse 1 is switched from the conductive state to the non-conductive state by the NMOS FET being turned off. The diode 4 prevents internal capacitances or buffer memories of the electrical load 3 from continuing to feed the capacitor 103 of the RC element 102, 103.In synchronism with lowering of the signal voltage U S the fuse test software 114 starts the timer 111.The capacitor 103 then discharges via the resistor 102, so that the measurement voltage U RC drops from the value of the supply voltage U V in the manner of a decaying exponential function until it falls below the value of the reference voltage U R at a second point in time t 1.When the reference voltage U R is undershot, the comparator output voltage U K changes from the high level (which corresponds to the logic value one or "true") to the low level (which corresponds to the logic value zero or "false").As a result, at the second time t 1 the timer 111 is stopped (for example, by the comparator output voltage U K being applied to a capture input of a counter of the timer 111).The value of the timer 111 (count at the second time t 1) thus represents the discharge time duration T during which the capacitor 103 has been discharged via the resistor 102 to the value of the reference voltage U R.At a third time t S the efuse test software 114 raises the signal voltage U S again (for example from U S, ≈0 volts to U S ≈5 volts) in such a way that the efuse 1 is switched from the non-conductive state to the conductive state by switching the NMOS FET to the on state. Thus, starting from the third time t S the electrical load 3 is again supplied by the supply voltage source 2.The distance between the first time t 0 and the third time t S is selected such that the shut-down time duration T S, during which the electrical load 3 is not supplied, is sufficiently large for a discharge of the capacitor 103 to the reference voltage U R of the comparator 104, but is considerably shorter than the maximum interruption duration during which the electrical load 3 can remain unremoved without functioning properly any longer. This prevents the operation of the electric load 3 from being impaired.Synchronously with raising the signal voltage U S at the third time t S the fuse test software 114 resets the timer 111 back to zero.Following the second time t 1 the fuse test software 114 evaluates the value of the discharge time duration T supplied by the timer 111. For example, the value of the discharge time duration T measured in this way can be compared with a setpoint value T soll which results from the aforementioned relationship for the discharge process:In various operating environments, in particular in a vehicle, the supply voltage U V can vary considerably. In addition, due to manufacturing tolerances or aging processes, the resistance value R and / or the capacitance value C may deviate from a respective nominal value. Furthermore, the switching behavior of a switchable fuse 1 and / or the discharge behavior of an RC element 102, 103 may also depend on the temperature.It can therefore prove to be advantageous, when evaluating the switchable fuse 1, to take into account not only the measured discharge time duration T, but also at least correction values for the precise determination of the resistance value R and / or the capacitance value C and in particular the current value (detected 0 at the first time t) of the supply voltage U V.In this case, from the aforementioned relationship to changed values of the supply voltage U V, of the resistor 102 and / or of the capacitance of the capacitor 103, a corrected setpoint value T' can be determined for the comparison with the discharge time duration T:In an analogous manner, temperature-related variations can also be taken into account, for example by recording characteristic curves for the switching behavior of the switchable fuse 1.Correction values, for example a correction value ΔR=R'-R for the resistor 102 and / or a correction value ΔC=C-C' for the capacitance of the capacitor 103, can advantageously be detected in a calibration step and stored on the nonvolatile memory 112.In one embodiment, the values of the discharge time duration T respectively recorded can also be stored continuously, at certain time intervals or for a certain period lying behind on the nonvolatile memory 112. By comparing a currently detected value with previous values, a gradual degradation of the switchable efuse 1 can be determined particularly early by means of the efuse test software 114, in particular already when the functionality is temporarily still ensured.For example, a working state of the switchable efuse 1 can be determined as a value of the list {"functional", "functional but degraded", "non-functional"} and transferred from the efuse test software 114 to the application software 115.The application software 115 can trigger an error response based on this value, for example deactivate certain functions of a vehicle controlled by the relevant electrical load 3 or output a warning message.The operation of the schematic circuit shown in FIG. 1 is explained in more detail on the basis of the schematic voltage time characteristics shown in FIG. 2. In the upper part, FIG. 2 shows the time profile of the signal voltage U S, in the middle part the time profile of the measurement voltage U RC and in the lower part the time profile of the comparator output voltage U K along the time axis t.At the time t=0 (in which the supply voltage U V is switched on or applied), the signal voltage U S is switched to a high value (typically of between 10 volts and 15 volts) which is sufficient to drive the switchable fuse 1 designed as an NMOS FET. Accordingly, the measurement voltage U RC present across the RC element 102, 103 will approximately assume the value of the supply voltage U V (only decreases by the comparatively slight voltage drop along the source-drain path, which is typically 100 millivolt to 1 volt).The comparator 104 compares the measurement voltage U RC with the reference voltage U R, The reference voltage U R is set to a value slightly above zero. The reference voltage U R can be determined, for example, such that for a leakage current which is just considered to be permissible in the case of a blocked switchable fuse 1, the voltage drop resulting therefrom across the RC element 102, 103 is calculated. If this voltage drop is not exceeded, the switchable fuse 1 can be considered to be functional. Typically, for this purpose, the reference voltage U R will be about 10 to 20 percent of the value of the supply voltage U V.When the switchable fuse 1 is operatively switched through (in the conductive state), the measurement voltage U RC is thus considerably greater than the reference voltage U R. A high positive voltage value of the comparator output voltage U K( typically 2 volts to 5 volts), is thus present at the output of the comparator 104, depending on the circuit technology and the supply of the comparator 104.At a first time t 0 the efuse test software 114 running on the microcontroller 110 triggers a short-term blocking of the switchable efuse 1 in order to test its functionality. For this purpose, up to a third time t S( i.e. for a shutdown time period T S= t S- t 0) the signal voltage U S is set to a low level, which corresponds to a value of 0 volt or close to 0 volt.In the case of a functional switchable fuse 1, the source-drain current is thereby completely or almost completely interrupted. The capacitor 103 then discharges via the resistor 102, wherein the voltage across the capacitor 103 (which is equal to the measurement voltage U RC ) drops, determined according to the decaying exponential function already described at the beginning, by the time constant τ=R·Cof the RC element 102, 103. After a time period designated as discharge time period T, at a second time t 1= t 0+ T, the value of reference voltage U R is undershot. Synchronously or with negligible delay, the output of the comparator 104 switches from the high to the low level of the comparator output voltage U K.The shut-down time duration T S is dimensioned to be sufficiently large in order to be able to reliably observe the decay of the measurement voltage U RC (in the case of a functional switchable fuse 1). It will prove to be advantageous in this case to provide a multiple of the discharge time T as the shut-down time period T S within which the measurement voltage U RC drops from the supply voltage U V to the reference voltage U R when the operable (in particular sufficiently blocking) switchable fuse 1 is operable.At the same time, the shut-down time duration T S is dimensioned to be so small that the electrical load 3 tolerates a failure of the supply voltage U V during this time duration without impairment. Such a time interval, during which the electrical load 3 can be operated reliably without impairment (i.e. taking into account possible tolerances of both the electrical load 3 and the supply voltage U V and further tolerances) even in the absence of the supply voltage U V is referred to here and in the following as the maximum interruption duration.For example, based on a known interrupt maximum duration, the shut-down duration T S may be selected as a fraction (e.g., half or one third) of the interrupt maximum duration. The RC element 102, 103 and the reference voltage U R are then dimensioned such that, in the case of a maximum permissible leakage current through a blocked switchable fuse 1 and in the case of a maximum permissible supply voltage U V the capacitor 103 is discharged via the resistor 102 during a discharge time period T to the value of the reference voltage U R which is a fraction (for example one fifth or one tenth) of the shut-down time period T S.To determine the discharge time duration T, a capture signal can be generated, for example by means of a further comparator, not shown in more detail in FIG. 1, or by conversion to a transistor-transistor logic (TTL), which capture signal assumes a high level and / or a rising edge when the signal voltage U S assumes a low level and at the same time the comparator output voltage U K still remains at a high level and which otherwise assumes a low level and / or a falling edge. By means of such a capture signal, a counting process of a known clock signal can be triggered at the timer 111, namely at a high level and / or a rising edge, or stopped, namely at a low level and / or a falling edge. In this case, the count of the timer 111, multiplied by the period of the known clock signal, corresponds exactly to the discharge time period T. Of course, the counting process can also be stopped when the count of the timer 111 exceeds a maximum count. In addition, the application software 115 can send a start and / or stop signal to the timer 111 in order to trigger and / or end the counting process.Alternatively, the comparator output voltage U K can also be fed directly to a negated capture input of the timer 111 in such a way that the timer 111 counts a clock signal during the time interval between the second time t 1 and the third time t S during which the comparator output voltage U K is at the low level. In this embodiment, the discharge time period T is the difference between the accurately known shut-down time period T S (triggered by the Efuse test software 114) and the measured time periodFIG. 3 shows schematically and purely by way of example a flow chart for explaining the evaluation carried out by the efuse test software 114. The flow starts at a starting point A and ends at an end point E.Starting from the starting point A, in a first step S 1, the current value of the supply voltage U V and the current temperature value are read in, which is detected by the temperature sensor 101 (typically arranged close to the switchable fuse 1). Both values are sampled and discretized by ADC 113.In a second step S2, at the second time t 1 the signal voltage U S is lowered to the low level and the switchable fuse 1 is thereby blocked (set to the non-conductive state). At the same time, the timer 111 is started in the second step S2.In a subsequent first decision step E 1, it is checked whether the time duration, during which the switchable fuse 1 is blocked, detected by the timer 111 has already reached or exceeded the predetermined shutdown time duration T S. In other words, it is checked whether the current time t is before the third time t S t<t 0+ T S= t S.If the shut-down time duration T S has not yet been reached, the method is paused over a predetermined waiting time duration in the subsequent third step S 3 and the first decision step E 1 is then carried out again. The predetermined waiting time period is dimensioned sufficiently small that the reaching of the predetermined shut-down time period T S can be detected with high accuracy. The predetermined waiting time duration is preferably one clock period of a clock signal supplied to the timer 111 and counted by the latter.If it is determined in the first decision step E 1 that the predetermined shutdown time duration T S has been reached, the switchable fuse 1 is switched back to pass (into a conductive state) in a subsequent fourth step S 4.Subsequently, in a fifth step S 5, the discharge time period T is read out, for example as a count of a counter multiplied by the clock period, which counts the clock signal in the time period between the first time t 0 and the second time t 1, in which the comparator output voltage U K is at a high level, but the signal voltage U S is at a low level.In a subsequent sixth step S 6, the read-out discharge time duration T is adjusted by taking into account calibration data relating to the resistance value R and the capacitance value C of the RC element 102, 103 and / or the current value (i.e. detected in the first step S 1) of the supply voltage U V and / or the temperature detected by the temperature sensor 101.For example, the deviation ΔR from a nominal resistance value R and the deviation ΔC from a nominal capacitance value C can be detected during a calibration step (not shown in more detail in FIG. 3 ) and stored in the nonvolatile memory 112. In addition, characteristic curves which map the time switching behavior of the NMOS FET which forms the fuse 1 and / or characteristic curves which map the source-drain leakage current of the switchable fuse 1 in each case as a function of the temperature can be stored in the nonvolatile memory 112. These data are read out in the sixth step S 6 and used for adapting the read-out discharge time duration T in such a way that a corrected value of the discharge time duration T is determined, which describes the behavior of the switchable fuse 1 independently of production tolerances, independently of temperature and independently of the supply voltage U V.In a subsequent second decision step E 2, the discharge time duration T corrected in this way is compared with a first threshold value. If the discharge time duration T is below this first threshold value, the switchable fuse 1 is considered to be operable without restrictions. In a first output step A 1, a first output code is then output "functionally" (which can of course also be encoded as a bit pattern or literal).If the discharge time duration T is not below the first threshold value, then a check is made in a third decision step E 3 following the second decision step E 2 as to whether the discharge time duration T is below a second threshold value, which is selected to be greater than the first threshold value.If the discharge time duration T is below this second threshold value, the switchable fuse 1 is considered to be functionally limited. This can mean, for example, that the switchable fuse 1 is to be regarded as functional only for a certain remaining lifetime with a certain probability and should be replaced before this remaining lifetime expires. In a second output step A 2, a second output code different from the first output code is then output with "restricted functionality" (which can of course also be encoded as a bit pattern or literal).If the discharge time duration T is not below the first threshold value, then a check is made in a third decision step E 3 following the second decision step E 2 as to whether the discharge time duration T is below a second threshold value, which is selected to be greater than the first threshold value.If the discharge time duration T is not below this second threshold value, the switchable fuse 1 is considered to be inoperative. In a third output step A 3, a third output code that is different from the first and second output codes is then output "non-functional" (which can of course also be encoded as a bit pattern or literal).The threshold values used in the second and third decision steps E 2, E 3 can be determined in particular by statistical investigations. For example, values for the discharge time duration T can be continuously recorded via a sufficiently large sample of switchable Euses 1. For a subset of failed switchable expenses 1 from this sample, a threshold value with respect to the discharge time period T can then be determined retrospective, with which a sufficiently large proportion of these failures would have been detected in good time without provoking an unacceptably high number of false positive detections (i.e., using this threshold value, switchable expenses 1) marked as next to fail, but in fact still functional.The term "output of an output code" is to be understood in particular as the transmission of such an output code to the application software 115. For example, if the efuse test software 114 has determined the output code to be "functionally capable to a limited extent", the application software 115 can output a warning and / or an indication until the switchable efuse 1 is to be replaced as much as possible. In the event that the efuse test software 114 has determined the output code "non-functional", the application software 115 can shut down a safety-critical subsystem which is supplied via the switchable efuse 1.Following the first, second and third output steps A 1, A 2, A 3, respectively, the value of the discharge time duration T is stored in the nonvolatile memory 112 in a seventh step S 7. End point E of the method is then reached.For the sake of simplifying the illustration, it has been assumed in the preceding that steps S 5, S 6 and S 7, decision steps E 2 and E 3, and output steps A 1, A 2 and A 3 are carried out temporally after the fourth step S 4, in which the switchable fuse 1 is switched back to continuity (into a conductive state). In principle, it is also conceivable that the aforementioned steps are already carried out from the earliest time of the availability of the discharge time duration T, namely from the second time t 1. This would have the technical advantage that the switching on of the fuse again could be made dependent on the result of the evaluation of the discharge time duration T. For example, for Efuses considered inoperative, it would no longer be possible to activate them at all.List of reference characters1 Switchable Effect 1.D Drain electrode 1.G Gate electrode 1.S Source electrode 2 Supply voltage source 3 Electrical load 4 Diode 100 Test system 101 Temperature sensor 102 Resistor, RC element 103 Capacitor, RC element 104 Comparator 105 Reference voltage source 110 Microcontroller 111 Timer 112 Nonvolatile memory 113 Analog-to-digital converter (ADC) 114 Effect test software 115 Application software 120 Test circuit 121 Test control unit 122 Test evaluation unit A Starting point A 1 to A 3 First to third output step E End point E 1 to E 3 First to third decision steps S 1 to S 7 First to seventh step t Time axis t 0, t 1, t S first to third points in time T discharge time period T S shut-down time period, test time U K comparator output voltage U V supply voltage U R reference voltage U RC measurement voltage U S signal voltageReferences included in the specificationThis list of documents cited by the applicant has been produced in an automated manner and is only included for the better information of the reader. The list is not part of the German patent application or utility model application. The DPMA does not take any adhesion for any faults or omissions.Patent Literature citedUS 2001 / 0034070 A1

[0007]

Claims

Test method for checking the operating state of an fuse (1) which can be switched over between at least one conductive and a non-conductive state and by means of which an electrical load (3) is electrically connected to a supply voltage source (2), wherein the electrical load (3) tolerates a supply interruption for a predetermined interruption maximum duration, characterized in that - the switchable fuse (1) is switched over from the conductive state to the non-conductive state for a test time, T S, which is at most equal to the interruption maximum duration of the electrical load (3), - the residual current flowing through the switchable fuse (1) during the test time, T S, is detected as an actual residual current value by means of a test circuit (120) and - is compared with at least one desired residual current value, wherein a first operating state is assigned, if the actual residual current value is below the at least one target residual current value and if otherwise a further working state is assigned.Test method according to Claim 1, characterized in that the actual return current value is detected by an actual value of a discharge time duration, T, of the discharge of a capacitor (103) supplied by the switchable fuse (1) in parallel with the electrical load (3) via a resistor (102), and is compared with at least one setpoint value of the discharge time duration, T, which setpoint value is determined for the switchable fuse (1) for a respective operating state and is assigned to a setpoint residual current value.Test method according to Claim 2, characterized in that a measurement voltage, U RC, is detected across the capacitor (103) and compared with a reference voltage, U R, and a clock signal is counted off during a time period during which the measurement voltage, U RC, lies above the reference voltage, U R, wherein the actual value of the discharge time period, T, is determined from the product of a count and a period duration of the clock signal.Test method according to one of Claims 2 to 3, characterized in that a current temperature value and / or a current value of a supply voltage, U V, and / or a deviation of the capacitance value of the capacitor (103) and / or of the resistance value of the resistor (102) from a nominal value in each case is / are determined and is / are used for correcting the actual value of the discharge time duration, T.Test method according to one of the preceding claims, characterized in that the setpoint residual current value is adapted as a function of actual residual current values detected behind in such a way that a further working state is assigned to the switchable fuse (1) if a currently detected actual residual current value statistically deviates from actual residual current values detected behind in a striking manner.Test method according to one of the preceding claims, characterized in that the switchable fuse (1) for supplying the electrical load (3) is arranged in a vehicle.Test method according to one of the preceding claims, characterized in that a first working state is assigned "functionally" if the actual residual current value is below a first desired residual current value, a second working state is assigned "functionally limitedly" if the actual residual current value lies between the first and a higher second desired residual current value, and a third working state is assigned "non-functionally" if the actual residual current value lies above the second desired residual current value.Test system (100) for carrying out a method according to one of the preceding claims, comprising a test circuit (120), a test control unit (121) and a test evaluation unit (122), characterized in that - the test circuit (120) is configured to detect the actual residual current value which flows through the switchable fuse (1) in the non-conductive state, - the test control unit (121) is configured to switch the switchable fuse (1) between the conductive and the non-conductive state for the predetermined test time, T S, and - the test evaluation unit (122) is configured to compare the actual residual current value detected by the test circuit (120) with at least one setpoint residual current value.Test system (100) according to Claim 8, characterized in that the test circuit (120) comprises an RC element (102, 103) comprising a resistor (102) and a capacitor (103) connected in parallel thereto, a comparator (104) and a timer (111), - wherein the RC element (102, 103) can be connected to a supply voltage source (2) via the switchable fuse (1), - wherein the comparator (104) is configured to compare the measurement voltage, U RC, present across the RC element (102, 103) with a reference voltage, U R, and - wherein the timer (111) is started by the test control unit (121) with the switching of the switchable fuse (1) from the conductive to the non-conductive state and is stopped by the comparator (104), when the measurement voltage, U RC, reaches or falls below the reference voltage, U R.Test system (100) according to Claim 7 or 8, characterized in that the test evaluation unit (122) comprises - an analog-to-digital converter (113), o which is configured to record an actual value of the supply voltage, U V, and / or o which, by means of a temperature sensor (101), is configured to record the temperature of the switchable fuse (1), - and / or a nonvolatile memory (112) for storing at least one calibration value and / or at least one actual residual current value.

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