Patent Description:
Current sensors are used in a wide variety of applications, ranging from electrical vehicles (EVs), battery monitoring, EV charging stations to solar inverters, for example. In many applications, an overcurrent in an electrical system must be quickly and reliably detected. Therefore, many of semiconductor integrated circuit devices are conventionally provided with an overcurrent protection circuit as one of their abnormality protection circuits. Short circuit is a type of overcurrent. For example, an in-vehicle intelligent power device is provided with an overcurrent protection circuit, which restricts the amount of output current flowing through a power transistor not to exceed an overcurrent set value, for the purpose of preventing the device from breaking in a case of a short-circuit in a load connected to the power transistor. Circuit breakers, e.g. magnetic circuit breakers, fuses and overcurrent relays are commonly used to provide overcurrent protection mechanisms.

As an overcurrent event can potentially damage the system and even lead to a danger for a user, the system is usually arranged to perform a safety action when such an event occurs. For example, the electrical source may be disconnected from the load by opening a relay, or blowing a fuse. Therefore, an overcurrent event must not only be reliably detected by the current sensor, but it also needs to be reliably reported, or communicated to the system electronic control unit (ECU).

Prior art solutions are known wherein an overcurrent event is reported as a digital output, e.g. as a binary signal. If no overcurrent is detected (i.e. the measured current is lower than a predetermined threshold), the digital output is set to a logical low state and in the opposite case, if an overcurrent is detected (i.e. I > said threshold), the digital output is set to a logical high state (or vice-versa). Such a type of communication is not immune to electromagnetic interferences. A further drawback is that an electronic control unit cannot diagnose a failure in the overcurrent reporting unit, as it only receives an indication of whether or not overcurrent has occurred.

Application <CIT> is concerned with an overcurrent protection device. The device comprises a current unit that can determine in real-time a maximum allowed current on an instantaneous level of a voltage across a load to be powered. The device further comprises a power switch unit to provide power to the load and a PWM unit that generates a control signal to control the power switch unit.

In <CIT> a safety device is presented which is adapted to automatically interrupt the current supply to an electrical wiring system when an anomalous event occurs. Electrical parameters of the system are monitored with a set of electrical current and voltage measurement stages. Via a rule-based expert system and/or numerical classifier supervised by a training system an immediate disconnection of the wiring system can be carried out in case a dangerous situation is detected.

<CIT> relates to transmitting and receiving signals across an isolator in for example solid state lighting systems or DC/DC converter feedback regulation control systems. When a digital input signal is at a logic low level, a digital signal with a first modulation frequency is sent across an isolation barrier and when the input signal is at a logic high level, a digital signal with a second modulation frequency is sent.

In <CIT> a method and system for detecting a current sensor error are described, wherein a current is measured associated with a first phase of an electric motor using a first current sensor and a current associated with a second phase of the motor using a second current sensor. A target value is determined for the second phase current and from a comparison of the measured second phase current with the target value a current sensor error a current sensor error can be identified.

There is thus a need for a device wherein events like overcurrent can be detected and next reported in a more reliable way.

It is an object of embodiments of the present invention to provide for current supervisory device that allows for safe reporting on a detected event like e.g. the occurrence of an overcurrent.

The above objective is accomplished by the solution according to the present invention.

In a first aspect the invention relates to a current supervisory device as defined in independent claim <NUM>.

The proposed solution indeed allows for the use of a modulated signal to report the detection of an event, for example the occurrence or non-occurrence of an overcurrent. The processing circuit of the current supervisory device detects the event. The outcome indicates one of at least two states (typically occurrence or non-occurrence) and is used to generate an event signal. The processing circuit outputs an output signal to the output terminal. The modulation means provided in the processing circuit are configured to modulate a reporting signal at least in case one of the at least two different states is detected. For example, The current supervisory device may output a modulated signal as reporting signal as long as no fault (e.g. an overcurrent) is detected. When the processing circuit detects an overcurrent, the reporting signal may turn into a non-modulated signal. However, it may also be that in that case the another modulated signal is emitted as reporting signal, with at least one different modulation parameter.

In a preferred embodiment the modulation means is arranged to adapt a pulse width of pulses of said reporting signal. In other words, the duty cycle of the pulses is adaptable.

In a preferred embodiment the modulated signal is modulated in frequency. In yet further preferred embodiments the modulated signal is modulated both in frequency and in pulse width.

In preferred embodiments the modulated signal is a one-bit signal.

Advantageously the processing circuit comprises pulse shaping means for generating the reporting signal, said reporting signal being a pulsed signal.

In some embodiments the modulated signal has a frequency higher than the operational bandwidth or cycle of said current supervisory device, for example two times higher or ten times higher.

In advantageous embodiments the modulated signal is the modulated event signal. In other embodiments the generated event signal is e.g. used as a trigger signal for the modulation means to produce a modulated signal.

In some embodiments the processing circuit is arranged to detect the event by comparing the received signal with a predefined threshold. The processing circuit is then preferably arranged to detect the event by comparing the received signal with a plurality of predefined thresholds, whereby with each predefined threshold corresponds a different event signal and consequently to a different modulated signal.

In a preferred embodiment the event is the occurrence or non-occurrence of a fault. The fault may for example be an overcurrent.

According to the invention, the processing circuit is arranged to output for a further state of the at least two different states a further modulated signal having at least one modulation parameter different from the modulated signal.

In another embodiment the current sensing means comprises a shunt resistor for generating a voltage drop and/or a magnetic sensing element.

In one embodiment the current supervisory device further comprises voltage detection means for sensing a voltage and/or temperature detection means for sensing a temperature, and the modulation means of the processing circuit is arranged for modulating the output signal based on said event signal and on a signal indicative of the sensed voltage and/or the sensed temperature.

In some embodiments the current supervisory device comprising a further output terminal arranged for outputting the signal indicative of the sensed current.

In another aspect the invention relates to a system comprising a current supervisory device as previously described and an electronic device arranged to receive the modulated event signal. The electronic device may for example be an electronic control unit if a vehicle, a fuse or a circuit breaker.

In advantageous embodiments the system comprises filtering means to filter the modulated event signal before applying the modulated event signal to the electronic control unit.

In another embodiment the filtering means have a cut-off frequency lower than the modulated signal frequency.

In another embodiment the system comprises further comparing means to compare said modulated signal with a predetermined threshold level.

In some embodiments the current supervisory device comprises a low-pass filter having a cut-off frequency higher than the frequency of the modulated signal and is arranged to shape bandwidth limited pulses.

The present invention discloses a current supervisory device arranged to detect an event, e.g. the occurrence or non-occurrence of a fault like an overcurrent, and to report about the detection by means of a modulated output signal based on an event signal generated in accordance with said detecting.

A basic block scheme of a current supervisory device (<NUM>) according to embodiments of the present invention is shown in <FIG>. A sensor (<NUM>) is arranged to sense a current, e.g. a current in a conductor (<NUM>). The sensor output signal (<NUM>) is applied to a circuit (<NUM>) where the signal is converted into a signal (<NUM>) indicative of the sensed current, for example a digital signal comprising sampled current values. The sensed values of the current to be measured are output by the circuit (<NUM>) and fed to the processing circuit (<NUM>). The processing circuit determines (detects) based on the signal indicative of the sensed current whether or not a certain event, e.g. an overcurrent situation, has occurred. The occurrence an non-occurrence of the event can be seen as two different states. The detection may for example be established by a comparator comprised in the processing circuit that compares the received signal with a predetermined threshold value. For example, if the predetermined threshold is exceeded it is decided that an event has indeed taken place and if the signal stays under the threshold value, it is decided that the event did not occur. Obviously, also the opposite meaning can be given to the threshold level being exceeded or not. The skilled person will readily appreciate that various alternatives are available, for example making a comparison based on function of two or more sampled current values. A signal, herein referred to as event signal, is then generated in the processing circuit in accordance with the outcome of the detection, e.g. that an overcurrent situation was detected or that no overcurrent situation was detected. A reporting signal is output via an output terminal of the interface circuit. The processing circuit comprises modulation means to modulate, upon detection of at least one of the different states (e.g. occurrence or non-occurrence of an event), the reporting signal via one or more modulation parameters in accordance with the outcome of the detection. The reporting signal (<NUM>) is then applied to an output terminal (<NUM>) of an interface circuit of the current supervisory device. In some embodiments the modulated reporting signal is a modulated event signal. In other embodiments a signal based on the event signal is used to derive the modulated reporting signal. It may be in some embodiments that also another state gives rise to a reporting signal being a modulated signal. In other embodiments the reporting signal corresponding to that other state may not be modulated.

In some embodiments the interface circuit may also comprise a further output terminal for the signal indicative of the sensed current. The signal indicative of the sensed current may in some embodiments be a digital signal coming from an analog-to-digital converter (ADC) comprised for example in the circuit <NUM> or a digital signal supplied to the interface circuit by the processing circuit (<NUM>) comprising A/D conversion means. Alternatively, the analog signal, e.g. the signal output by an amplifier in circuit (<NUM>) can be applied to a terminal of the interface circuit. The interface then may for example have a terminal for an analog signal, i.e. an analog pin, and a terminal for a digital signal, i.e. a digital pin. However, also other options are available, as will be mentioned later in this description.

The sensor (<NUM>) is in advantageous embodiments a magnetic sensing element, e.g. a Hall sensor. Alternatively, the current sensing can be performed by means of a shunt resistor adapted to accurately produce an expected voltage for the supplied current. In case a shunt resistor is used, the current supervisory device preferably comprises a differential amplifier to sense a voltage drop across the resistor. In some embodiments the sensing means may comprise a first sensor used for current measurements and a second sensor for event detection as already mentioned. For example, the first sensor may be a Hall sensor and the second sensor a shunt resistor, or vice versa. In some embodiments the shunt resistor may be external to the device.

<FIG> illustrates a possible embodiment wherein the processing in the processing circuit (<NUM>) is performed in the digital domain. In <FIG> the circuit (<NUM>) that converts the sensor signal into a signal useful for further processing, comprises an amplifier to amplify the sensor signal and an analog-to-digital converter to obtain a digital input signal to the processing circuit. As the skilled person will readily recognize, the A/D conversion may in other embodiments be part of the processing circuit. The processing circuit (<NUM>) monitors, i.e. detects, based on the received digital signal whether an event has occurred or not. The processing may comprise a digital comparator to perform that task by comparing the received digital values with a predetermined threshold level. The threshold level may be stored in a memory in the processing circuit. Alternatively, the predetermined threshold level can be set from outside the current supervisory device and loaded into the device. For example, the threshold level may be based on (derived from) or equal to an external voltage applied to a dedicated terminal of the current supervisory device. A corresponding event signal is then generated, which is subsequently used for the reporting signal. For at least one of the states corresponding to the detection the modulation means provided in the processing circuit produce a modulated reporting signal. The reporting signal is then output via a terminal of the output interface (<NUM>). Again it may be that in some embodiments also another state yields a modulated reporting signal, but that modulated reporting signal then has at least one different modulation parameter. Optionally, in some embodiments the processing circuit also outputs a signal corresponding to the indication of the sensed current via the interface circuit as already mentioned. This, however, is not shown in <FIG>.

Many options are available to modulate the reporting signal based on the value of the generated event signal. Assuming that a modulation in frequency is applied, one can for example, in case no fault is detected, modulate the output signal at a frequency f1 and in case a fault was detected use a non-modulated signal, e.g. a DC signal with a given constant level. Another option could be to modulate the output signal in frequency (e.g. using frequency f1) in case no fault is detected and, in case a fault is observed, to modulate the output signal in frequency using another frequency (e.g. a frequency f2 greater than f1). The skilled person will recognize that the given options are merely examples and are in no way limiting. As an alternative, a pulse width modulation (PWM) can be employed. One possibility may be to use in case no fault is detected, as reporting signal a PWM modulated signal having frequency f1 and a duty cycle DC1, and in case a fault is detected a PWM modulated signal having the same frequency f1 but a duty cycle DC2 different from DC1. For example, DC1 may be lower than <NUM>%, or lower than <NUM>%, e.g. <NUM>% and DC2 may be higher than <NUM>%, or higher than <NUM>%, e.g. <NUM>%. As a further alternative the above options of applying modulation in frequency or in pulse width can be combined, so that the reporting signal is modulated both in frequency and pulse width.

In the embodiment shown in <FIG> a fully analog implementation is adopted. In circuit (<NUM>) the signal output by the sensor (<NUM>) is converted into a signal that can be used in the processing circuit. The main component of circuit (<NUM>) is an amplifier (<NUM>) to amplify the sensed signal (<NUM>) output by sensor (<NUM>). The signal output by circuit (<NUM>), e.g. the amplified signal is then fed to the processing circuit, which comprises an analog comparator that compares the amplified signal with a predefined threshold level and a corresponding event signal is applied to a frequency generator FG, where a frequency modulated reporting signal is produced and output based on the received event signal. The processing circuit in <FIG> comprises another, parallel path wherein the amplified signal output by circuit (<NUM>) is applied to an analog processing block. This block may optionally be connected to an output terminal of the interface circuit to output an indication of the sensed current. The analog processing block may for example perform filtering and/or clamping. It may for example also add an offset to the analog signal.

In some embodiments the current sensing means comprise more than one sensor. For example, the signal sensed by one sensor can be used, via circuit (<NUM>), which, for example, may have a second path comprising a dedicated amplifier in parallel to the path shown in <FIG>, and the processing circuit (possibly the same processing circuit (<NUM>) is shared among the two paths), to produce an output signal indicative of the sensed current. Another sensor may then provide a sensed signal which is subsequently used as set out above to observe and detect the occurrence or non-occurrence of an event and to produce a corresponding modulated output signal based on the generated event signal to report about the event.

In one embodiment all the processing blocks in the current supervisory device (<NUM>), i.e. all blocks shown in <FIG> except for the current sensing means, can be integrated in a single semiconductor package or a single integrated circuit. In other embodiments also the sensing means may be integrated.

In advantageous embodiments the reporting signal is a pulsed signal. The current supervisory device then comprises pulse shaping means. The pulse shaping means are preferably part of the processing circuit. In preferred embodiments the pulse shaping means is capable of generating bandwidth-limited pulses, e.g. pulses with smooth edges, and/or a limited rise time and a limited fall time. The bandwidth limited pulses may in some embodiments have a cut-off frequency which is higher than the modulated signal, for example five or ten times higher.

In order to monitor the possible occurrence of an event, in some embodiments the processing circuit checks at a given rate, e.g. the rate at which the signal indicative of the sensed current was sampled, whether the predetermined threshold value is exceeded or not. In some embodiments a pulse is generated when the threshold value is exceeded and no pulse is generated otherwise. In other embodiments it is just the other way round and a pulse is only generated if the threshold value is not exceeded. In yet further embodiments a pulse of a first type is generated if the threshold value is not exceeded and a pulse of a different, second type is generated if the threshold value is exceeded. The pulses may have different pulse characteristics. Advantageously, the pulse (i.e. the reporting signal) of second type is next modulated with at least one different modulation parameter than the pulse of first type. Hence, in the latter cases a modulated reporting signal is also generated when no event is detected. This eases checking whether the reporting circuit is still alive. It further makes the reporting less sensitive to EMC perturbations (e.g. from a magnetic interference or a power supply micro-cut).

In some embodiments the reporting signal forms a train of modulated pulses. In some embodiments the pulses may be rectangular pulses. In other embodiments the pulses may be bandwidth limited pulses obtained, for example, by filtering a square wave in order to remove higher harmonics and slow down the voltage transitions. The pulse shaping means can be implemented in an analog manner (e.g. via low pass filtering) or digitally, using a predetermined data sequence and a DAC, for example. In alternative embodiments the modulated output signal may be derived from a triangular signal or a sine or cosine signal. An example of a predetermined data sequence is given in <FIG>, where two bandwidth limited pulses are shown.

The modulation means in the processing circuit can modulate the reporting signal by setting one or more appropriate modulation parameters. In a preferred embodiment the modulation means is capable of adapting the pulse width of pulses of the reporting signal. In other words, the duty cycle of the pulses can be varied. Pulses indicating that an event has been detected then have a different width than pulses indicating no event was observed. For example, in such case a pulse with a greater pulse width and consuming more power is used than in case no event occurs. For example, the duty cycle can be lower than <NUM>%, or lower than <NUM>%, for example <NUM> % if no event was observed and higher than <NUM>%, or higher than <NUM>%, for example <NUM> % in case there is detection of an event (e.g. overcurrent detected, or a fault in some electrical subsystem). In other embodiments, the duty cycle is higher when no event was observed, and lower when an event is detected. As already explained, in some embodiments the reporting signal is not modulated when detecting one of the possible states, for example the state corresponding to the occurrence of an event being detected. In other embodiments the frequency is exploited as modulation parameter to indicate the detection of an event. The frequency of the reporting signal is then adapted, e.g. increased, when an event (a fault) is detected, e.g. when the threshold level is exceeded by the sampled current value.

In a digital implementation of the current supervisory device each operational cycle of the supervisory device can be seen as comprising three stages, namely the acquisition of a signal by the current sensing means, subsequently the processing as described above whereby a modulated output signal is produced based on the generated event signal and finally the outputting of the modulated output signal. The operational cycle corresponds to a certain time duration. The current supervisory device operates at a certain number of operational cycles per time unit. In an analog implementation of the current supervisory device the used signals occupy a certain operational bandwidth. In preferred embodiments of the current supervisory device according to the present invention the modulated reporting signal is higher in frequency than the operational bandwidth of the current supervisory device or than the number of operational cycles per second. In other words, the modulated output signal, i.e. the modulated reporting signal, supplies information on the monitored event at a faster rate than the rate at which the current sensing is performed. This higher rate offers the advantage that an alarm (in the form of the modulated output signal) can be reported much quicker in case e.g. an overcurrent situation is detected. In advantageous embodiments the modulated reporting signal has a frequency at least two times or five times or ten times higher than the operational speed (in cycles/s) applied in the supervisory device. For example, the operation speed of the current supervisory device may be <NUM> or <NUM>, and the modulated reporting signal frequency may be <NUM> or <NUM> or <NUM>.

An illustration of the operation of a current supervisory device according to the invention is provided in <FIG>. It shows a few operation cycles of the device. The example of an overcurrent situation is considered. As indicated an overcurrent (OC) situation occurs in the first cycle. The detection can only occur in the next cycle. At the end of this next cycle the event is reported. In the first timing diagram shown, this detection leads to an output signal with a different pulse width. Note that in this example the modulated reporting signal is the modulated event signal. Where the pulses had a duty cycle of <NUM> % before, a modulated signal is output with a duty cycle of <NUM>% when the overcurrent is detected. In the second timing diagram pulses are emitted at a higher frequency than the rate at which samples of the current values are received. The detection of the overcurrent leads to a change in pulse width of the modulated reporting signal that is output, while the frequency remains unchanged in this example. In other implementations, however, also the frequency may change. In certain embodiments the reporting signal is synchronised with the operation cycle (see for example the first timing diagram in <FIG>), whereas in other embodiments the reporting signal is output asynchronously (as illustrated in the second timing diagram).

<FIG> illustrates the reporting of a circuit failure. A case is considered whereby a modulated pulse is output as reporting signal as long as no circuit failure is detected. After the detection of a failure no modulated signal is emitted, but rather a signal with a constant level as shown in <FIG>. Alternatively, a signal with a ripple may be used, or for example a signal with a floating voltage. The reporting signal is output to e.g. an electronic control unit (ECU), which obviously must be capable of detecting the reporting signal. In the case depicted in <FIG> the ECU will detect the absence of a modulated pulse and so become aware that a failure has occurred. The second timing diagram illustrates an advantage of using a higher frequency for the reporting signal : the missing pulse is detected earlier and due to that reduced delay appropriate measures can be taken quicker.

Although in this description often the example is used of the occurrence or non-occurrence of an overcurrent, hence overcurrent monitoring, as an event, the skilled person will readily understand that there are many examples of other events to be monitored that can be considered. Other possible events may for example relate to monitoring the occurrence of a short circuit or of a malfunctioning electrical subsystem in a car, or functioning or malfunctioning of a car battery, or even the occurrence of a car crash.

In some embodiments a plurality of predefined thresholds can be provided. The threshold values can for example be stored in storage means of the processing circuit or loaded into the processing circuit from outside the current supervisory device. To each threshold then may correspond a different event signal and different modulated reporting signal. For example, different pulse widths and/or different frequencies can be used for the signals output as reporting signals. In some embodiments each threshold value is related to a different event, e.g. the occurrence (or non-occurrence) of a different fault like a current being above a certain threshold, an overcurrent, a malfunction in an electrical subsystem etc, as already mentioned before.

In other embodiments the plurality of predefined thresholds can be used to select an appropriate value from. The selection may for example be based on the state of a vehicle comprising the current supervisory device. The state may be e.g. the vehicle being parked or the vehicle driving or may be determined as a function of the vehicle speed. The selection may be performed inside the current supervisory device based on one or more identified parameters or, alternatively, may be performed from outside the current supervisory device, for example from the ECU.

Advantageously in some applications apart from the current a second physical quantity is monitored, like e.g. a temperature (e.g. the temperature of the current supervisory device itself) or a voltage, for example a battery voltage. In such embodiments the current supervisory device comprises voltage detection means and/or temperature detection means. The processing circuit then also receives a signal indicative of the one or more other sensed quantities. In accordance with the detected quantity a corresponding event signal is generated, which subsequently also serves as a further basis for modulating the output signal, together with the event signal that was generated from the signal indicative of the sensed current. In further embodiments more than one other physical quantity can be monitored apart from the current, e.g. both voltage and temperature. The current supervisory device may output a signal that differentiates between the different events. For example, the duty cycle may be <NUM>% in case of a safe state, <NUM>% in case an overtemperature was detected, <NUM>% in case of an overvoltage, and <NUM>% in case of a overcurrent.

In an aspect the invention relates to a system comprising a current supervisory device as described above and a conductor wherein the current to be sensed flows. The conductor may for example be a busbar for distributing power e.g. in a vehicle. Note that in that case all processing is still performed close to the busbar, more in particular in the processing circuit of the current supervisory device as explained above.

In an aspects the invention relates to a system comprising a current supervisory device as described above and an electronic device, like for example a fuse or a circuit breaker.

In a specific embodiment the electronic device is an electronic control unit (ECU) to which the modulated output signal is reported. To allow such reporting a communication channel towards the ECU is needed. When implemented in a vehicle, the system typically comprises such an ECU. In some embodiments this ECU may comprise a processing subcircuit that performs at least a part of the processing required in the derivation of the event signal. The processing subcircuit can be seen as a subcircuit of the processing circuit (<NUM>) discussed above and illustrated in <FIG>. The processing subcircuit may in some embodiments comprise a comparator (see also below).

In preferred embodiments the electronic device is provided with appropriate demodulation means for demodulating the modulated reporting signal received from the current supervisory device. The demodulation means may comprise duty cycle detection means and/or frequency detection means.

In one embodiment there is no filtering applied on the modulated reporting signal. The modulated reporting signal is then further processed by the demodulation means of the electronic device in a conventional way.

In some embodiments a low-pass filter is provided in the system in order to shape the pulses (i.e. reduce electromagnetic emissions). The pulses are preferably so shaped that high-frequency components at the entrance of the ECU are avoided as much as possible. The cut-off frequency of this low-pass filter is usually higher than the modulated signal frequency (e.g. <NUM> times higher or <NUM> times higher) such as to reduce higher harmonics of the signal. The low-pass filter can be implemented in the current supervisory device or in the ECU (e.g. in the above-mentioned processing subcircuit of the ECU) or may be performed on the communication channel between the current supervisory device and the ECU. The ECU is arranged to detect and demodulate the filtered modulated reporting signal. The ECU may for example be configured to measure pulse duration or signal frequency. This can be realized using conventional means known in the art, like for example timers and/or frequency-to-voltage converters and/or phase-locked loops (PLL) circuits. <FIG> provides an illustration. The modulated signal output by the current supervisory device is applied to a low-pass filter LPF1 and the filtered signal is next fed to the ECU. The filter LPF1 has a cut-off frequency higher than the frequency of the modulated signal. In an alternative embodiment the low-pass filtering is replaced by a pulse shaping by means of a digital-to-analog converter and a predetermined data sequence. This is another way to reduce or remove high-frequency content in the pulses. A predetermined data sequence corresponding to a bandwidth limited pulse (see e.g. <FIG>) can be stored in a memory in the current supervisory device, and the DAC can be configured such as to output the data sequence (for example, to convert digital data into a voltage sequence) and to produce the bandwidth limited pulse.

In other embodiments another low-pass filter is implemented that transforms the modulated output signal into an averaged signal that somewhat resembles a DC signal. What is meant, is a signal with typically a substantially reduced peak-to-peak amplitude, e.g. less than <NUM>%, or less than <NUM>%, or less than <NUM>% of the pulse amplitude. The level of the averaged signal is proportional to the duty cycle. For example, if a <NUM>% duty cycle is used for the pulses, then the level of the averaged signal is approximately <NUM>% VDD. The cut-off frequency of this low-pass filter is then lower than the modulated event signal frequency, e.g. <NUM> times lower, or <NUM> times lower. Again the filtering can be performed in the current supervisory device or in the ECU or on the communication channel in between. The ECU can detect a voltage level (almost DC), e.g. based on a comparator provided in the ECU. For example, there can be an analog comparator coupled to a digital input of the ECU. Alternatively, the modulated reporting signal can be connected to an analog input of the ECU to digitize the signal and then perform the comparison digitally. In some embodiments the analog comparator can be integrated in the ECU. Optionally the ECU can also detect the pulse. Due to the modulation and low-pass filtering a decoupling of the supervisory device gets and the ECU is achieved and the rejection of spurious external interferences is improved. This only has a negligible impact on the filtered signal. An illustration is given in <FIG>. The modulated reporting signal output by the current supervisory device is applied to a low-pass filter LPF2 to yield a DC-like averaged signal and this filtered signal is next fed to the ECU. The filter LPF2 now has a cut-off frequency lower than the frequency of the modulated signal.

In the embodiment shown in <FIG> the ECU comprises two inputs for receiving the modulated signal : one for the modulated signal as output by the current supervisory device and one for a low-pass filtered version of the modulated signal (a DC-like averaged signal). As also indicated in <FIG>, the averaged signal may optionally be compared via a comparator, which in some embodiments may be part of the ECU. The ECU may detect both the modulated signal in the time domain that was output by the current supervisory device and in the voltage domain at the output of LPF2, e.g. for redundancy, or only in the voltage domain at the output of LPF2. In other words, applying the modulated signal in the time domain is optional, as indicated by the dashed line in <FIG>. As already said, optionally a comparator can be provided to compare the signals obtained as output of the voltage domain and time domain detection.

In yet other embodiments both low-pass filters LPF1 and LPF2 as previously described are present. That means that both a low-pass filter for shaping the pulses and a low-pass filter to obtain an averaged, DC-like signal are present in the system. The various filters can be positioned in various ways, as already mentioned above. However, filter LPF1 should always precede filter LPF2. This is illustrated in <FIG>.

<FIG> represents a timing diagram for an embodiment of a system comprising a current supervisory device according to this invention. It shows a modulated signal (with a duty cycle of circa <NUM>%), a bandwidth limited pulse derived from the modulated signal (using a cut-off frequency fcutoff > sample frequency fs), and a filtered averaged signal (fcutoff<fs) and detection thresholds for the event and 'no event' condition. As can be seen from the figure, the swing, or peak-to-peak amplitude of the averaged signal is relatively low, e.g. <<NUM>% or <<NUM>% of the modulated signal amplitude. In an embodiment, a single threshold is sufficient (e.g. at VDD/<NUM>, with VDD the supply voltage). In general, threshold levels can be set taking into account the supply voltage and applied duty cycle. In another embodiment, two thresholds can be used. For example, if the filtered signal is between th1 and th3, a safe state can be detected. In a variant, a safe state can be detected if the filtered averaged signal is below th1, and an overcurrent is above th2. In yet another embodiment, four thresholds can be used to check the presence of the filtered averaged signal. A safe state can be detected if the observed signal amplitude sf<th1 and sf>th3, and an overcurrent can be detected if the signal amplitude sf>th2 and sf<th4, and a different failure can be detected if the signal is not within these bands (e.g. short circuit to VDD or ground GND, or broken wire if the voltage is floating between the bands).

Claim 1:
A current supervisory device (<NUM>) comprising :
- current sensing means (<NUM>) for sensing a current,
- a circuit (<NUM>) arranged to convert a signal (<NUM>) received from said current sensing means into a signal (<NUM>) indicative of the sensed current,
- a processing circuit (<NUM>) arranged for receiving said signal (<NUM>) indicative of the sensed current, for detecting an event based on said received signal (<NUM>), said detecting yielding an outcome corresponding to one of at least two different states, and for generating an event signal in accordance with said outcome, said processing circuit comprising modulation means for performing modulation based on said event signal,
- an output terminal (<NUM>) arranged for outputting a reporting signal (<NUM>) received from said processing circuit, whereby at least one of at least two different states gives rise to said reporting signal being a modulated signal,
characterised in that
said processing circuit is arranged to output for a further state of said at least two different states a further modulated signal having at least one modulation parameter different from said modulated signal.