PROTECTION SYSTEM

Technical field

This invention relates to a protection system and in particular a system for protection against overcurrents designed to protect capacitors in the automotive sector.

Background art

Electronic control devices are extremely well developed in the automotive industry which require as input electrical quantities which are particularly filtered and which generate, in turn, oscillations, even in the order of tens of kHz, which can cause disturbances in the power supply network to which they are connected.

A reference example for this specification are the electric fans with electronic control comprising a direct current motor, or even a brushless motor, to which reference is made below, for driving a corresponding fan for cooling radiating bodies.

The brushless motors comprise, inside them, an electronic card, or control electronics, which supervises and controls the operation of the electric fan, and are powered in direct current by the battery of the vehicle.

The electronic card comprises a power driver, for example an inverter, suitably controlled, for supplying the windings of the motor, and a filter system for filtering any oscillations of the supply voltage coming from the line.

As mentioned, the need is felt in the reference sector for also protecting, in an increasingly stringent manner, the power supply line from voltage ripple generated, for example, by the driver.

In order to satisfy the electromagnetic compatibility requirements, filters are used which comprise electrolytic capacitors and which are inserted between the motor and the line itself in order to send any parasitic signals to earth.

These capacitors are not therefore protected by the electronics of the motor and are subject, being located at the input of the motor, to all the transients coming from the motor power supply line.

The electrolytic capacitors are delicate components from a point of view of control of any overcurrents or violent transients but, due to the need for high capacity values for the filtering, they cannot be replaced with other components.

In the presence, therefore, of the above-mentioned occasional events the capacitors can be damaged or even explode and the electromagnetic compatibility requirements remain unsatisfied.

Disclosure of the invention

In this context, the main purpose of this invention is to propose a protection system, designed in particular for the protection of capacitors, which overcomes the above-mentioned drawbacks.

A first aim of this invention is to provide a protection system which preserves the integrity of the filtering capacitors in the presence of occasional events such as overcurrents.

A second aim of this invention is to provide a protection system while maintains the reliability and efficiency of filtering capacitors even following potentially harmful events for the capacitors.

A third aim of this invention is also provide a protection system which is simple and relatively inexpensive.

A fourth aim of this invention is to provide a filter system which is the more reliable than the prior art solutions.

A fifth aim of this description is to provide an electrical machine which is reliable and satisfactory in terms of electromagnetic compatibility.

These aims are fully achieved by a protection system for a filtering system and/or by a filtering system and/or by an electrical machine having the features set out in claims 1 , 10, 1 1 and/or by the combination of one or more of the claims appended to this application.

In accordance with the first above-mentioned aim of this invention, this description relates to a protection system for an electrolytic capacitor.

The system comprises a controllable switch inserted between the electrolytic capacitor to be protected and the reference potential, generally the common potential or zero, in such a way that, following an opening the switch, the capacitor is no longer subjected to the dangerous transient event.

Advantageously, according to one aspect of this invention, the protection system is configured to close the switch at the end of the transient, restoring operation of the electrolytic capacitor.

In one embodiment, the protection system comprises a current divider which can be inserted between the electrolytic capacitor and the reference potential.

The current divider comprises a first branch, wherein there is the controllable switch, and a second resistive branch.

The controllable switch is, for example, a MOSFET, which has a relative resistor R1 of a few ιτιΩ, whilst the resistive branch comprises a resistor R2 which is much greater than R1 , for example of about 100 Ω.

Advantageously, as will be described in more detail below, the resistor R2 means that a voltage is always measurable at the terminals of the divider even when the MOSFET is open.

Moreover, since the resistor R2 is much greater than the equivalent series resistance R of the electrolytic capacitor, it bears the majority of the voltage when the MOSFET is open.

The protection system comprises a comparator stage for controlling the controllable switch to open and close the switch in such a way as to protect or restore the electrolytic capacitor. In one embodiment, the comparator stage has a first input for a reference threshold T1 , obtained, for example, by a voltage divider, and a second input for a signal S1 to be compared with the threshold T1 .

The comparator stage has an output G in communication with the controllable switch for changing between the respective closed configuration and the respective open configuration.

In the preferred case wherein the controllable switch is a MOSFET, the output of the comparator stage is in communication with the gate of the transistor.

The signal S1 to compare with the threshold T1 comes from an amplifier stage with gain which also forms part of the protection system.

According to one embodiment, the amplifier stage has an output in communication with the second input of the comparator stage for transmitting to the second input of the comparator stage the signal S1 . The amplifier stage has a first input in communication with the output, that is to say, the output is back-driven on the first input of the amplifier stage. The amplifier stage comprises a second input which is connected in parallel to the current divider to receive as an input the voltage at the terminals of the controllable switch.

According to one aspect of this invention, the amplifier stage has as input signal the voltage at the terminals of the current divider, that is, the voltage at the terminals of the MOSFET when it is closed and the voltage at the terminals of the current R2 when the MOSFET is open if R2 is much greater than the intrinsic resistor R1 of the MOSFET.

The amplifier stage comprises a circuit for controlling its gain.

The control circuit is controlled by the comparator stage for varying the gain of the amplifier stage between a first value A1 and a second value A2 with the first value A1 greater than the second value A2.

In one embodiment, the control circuit is configured in such a way that the gain of the amplifier stage has the first value A1 when the MOSFET in the divider is closed and has the second value A2 when the MOSFET is open. In this way, the comparator stage receives as input a signal S1 which is always an expression of the transient on the electrolytic capacitor even when the MOSFET is open.

In this way, the system keeps monitored the transient which has led to the opening of the MOSFET.

In one embodiment, the first gain value A1 is substantially equal, in terms of order of magnitude, to four times the second gain value A2.

In one embodiment, the second gain value A2 is substantially unitary. According to one aspect of this invention, the amplifier stage comprises an operational amplifier mounted in a non-inverting fashion.

The amplifier comprises the output, the first input and the second input of the amplifier stage.

The non-inverting assembly is such that the operational amplifier has the first gain value A1 obtained with a resistor R3 and a resistor R4 mounted as shown.

The resistors R3 and R4 define the first gain A1 as A1 =1 +R4/R3. In a preferred embodiment, the resistor R4 is three times the resistor R3, that is, R4 = 3 R3 in such a way that the gain of the amplifier is approximately 4.

So that the second gain A2 is unitary or substantially unitary, the amplifier stage comprises a second controllable switch, for example a MOSFET, inserted between the third resistor R3 and the reference potential.

The MOSFET has the respective gate in communication with the output G of the comparator stage and is controlled by it, like the MOSFET substantially in series with the electrolytic capacitor.

The second MOSFET can be controlled between a closed configuration, where the resistor R3 is closed on the reference potential, and an open configuration.

The closed configuration corresponds to the first gain value A1 of the amplifier stage whilst the open configuration corresponds to the second gain value A2 of the amplifier stage. The gain value A2 is substantially unitary in consideration of the high impedance of the inverting input such that the operational amplifier is, in practice, a voltage tracker that, is to say, in particular, of the voltage at the terminals of R2.

When the voltage falls below the above-mentioned threshold T1 the comparator stage closes both the MOSFETs and the protection system returns to the initial configuration with the electrolytic capacitor operating in its filter function.

In one embodiment, the protection system according to this invention comprises a voltage stabilising stage between the current divider and the second input of the amplifier stage.

In accordance with a second aspect of this invention, this invention relates to a filter system comprising an electrolytic capacitor and a protection system in accordance with the first aspect of this invention.

In accordance with a third aspect, this invention relates to an electrical machine comprising a rotor, a stator, a casing for containing the stator and an electronic control card for supervising and controlling the operation of the electrical machine wherein the electrical machine comprises a filter system according to the third aspect of the invention.

In one embodiment, the electronic card comprises the filter system.

In one embodiment, the electronic card with the filter system is inserted inside the casing.

The protection system prevents damage to the electrolytic filtering capacitor in the event of transients coming from the network and restores it at the end of the transients. For these reasons, the protection systems also with the electrolytic capacitor may be closed inside the motor, not requiring external intervention and avoiding the risk of an explosion of the capacitor.

In one embodiment, the electrical machine in accordance with the third aspect of this invention comprises a cap for closing the casing to form a sealed enclosure and the filter system is inserted inside the sealed enclosure.

According to one aspect of this invention, the filtering system uses the filter inductance normally present in the motor control card for filtering the power supply at the motor input.

The electrolytic capacitor is preferably connected to the supply line between the inductance and a power supply source of the electrical machine.

In one embodiment, the filter system is installed in a box outside the motor, along the relative power supply line.

Brief description of drawings

Further features and advantages of this invention are more apparent in the non-limiting description of a preferred but non-exclusive embodiment of a protection system, as illustrated in the accompanying drawings, in which: - Figure 1 illustrates a block diagram of a first embodiment of an application of a protection system in accordance with this invention;

Figure 2 illustrates a block diagram of a second embodiment of an application of a protection system in accordance with this invention;

Figure 3 illustrates a circuit diagram partly in blocks of a detail of the applications of the preceding drawings.

Detailed description of preferred embodiments of the invention

With reference to Figure 1 , the numeral 100 denotes an electrical machine in accordance with one aspect of this description.

The machine 100 is of a substantially known type and is described and illustrated only insofar as necessary for understanding this invention.

The machine 100 comprises, in short, a rotor, a stator, a casing for containing of the stator and an electronic control card 101 for supervising and controlling the operation of the machine 100. In a preferred embodiment of interest for this invention, the machine 100 is a brushless motor with permanent magnet rotor, to which reference is made hereinafter without limiting the scope of the invention.

The machine 100 comprises a cap for closing the casing to form a closed enclosure 102, preferably in a sealed fashion.

As illustrated, the card 101 is inserted inside the enclosure 102 and closed it.

The electronic card comprises a power driver 103, substantially of known type and not described, and a stage 104 for powering it, configured for filtering the electrical quantities at the input to the driver 103 and coming from a power supply line 105.

The substantially known stage 104 comprises, very briefly, a capacitive element 106, comprising at least one filtering capacitor, which is inserted in parallel to the power supply of the driver 103, and an inductive element 107 inserted on the positive supply line.

According to one aspect of this invention, the machine 100 comprises a further filter stage or system, schematically represented as a block 108, interposed between the stage 104 and the line 105 to filter any disturbances (EMI Electromagnetic Interference), generated by the operation of the motor 100, which go towards the power supply line 105, in such a way as to satisfy the electromagnetic compatibility requirements often requested.

The filter system 108, which will be described in more detail below, comprises an electrolytic capacitor, schematically illustrated with a block 109.

The filter system 108 also comprises a system for protecting the capacitor 109, associated with it and labelled with numeral 1 .

The system 1 protects the capacitor 109 from any transients coming from the line 105 supplying the motor 100 in such a way as to preserve its integrity and the functionality.

In one embodiment, the card 101 comprises the filter system 108. With reference to Figure 2, in the embodiment illustrated, the EMI filtering stage 108 is positioned outside the motor 100, between the power supply line 105 and the motor 100.

In this case, a supply line at +5 volts for the protection system 1 is picked up by the motor 100, for example by the relative driver 103.

The system 108 is, for example, inserted in its container 1 1 1 which may be installed in the proximity of the motor 100.

In automotive applications, for example, if the motor 100 is a motor for driving a fan for cooling radiating bodies, the container 1 1 1 with the filtering system may be positioned in a compartment of the vehicle in the proximity of the motor 100.

Figure 3 illustrates in more detail one embodiment, more specifically, of the protection system 1 of the electrolytic capacitor 109 filtering EMI.

For practicality, the potential denoted by the minus sign in Figures 1 and 2 is indicated as the reference potential or common potential or zero in Figure 3.

Moreover, the electrolytic capacitor 109 has been schematically illustrated with an equivalent network comprising a capacitance C, a resistance R and an inductance L.

The system 1 comprises a current divider 2 inserted between the electrolytic capacitor 109 and the reference potential.

The divider 2 comprises a first branch comprising a controllable switch 3, for example a MOSFET type electronic switch to which explicit reference will be made without thereby limiting the scope of the invention, having relative resistor R1 .

The divider 2 comprises a second branch comprising a resistor whose value R2 is much greater than R1 .

If, for example the resistor R1 of the MOSFET 3 is in the order of ιτιΩ, the resistor R2 has, for example, a value equal to 100Ω.

It should be noted that in this way the resistor R1 is also much higher than the equivalent resistance R of the capacitor 109. The first branch can be opened by the switch 3 which can be controlled from its gate.

The protection system 1 comprises a comparator stage 4, or simply a comparator, for controlling the switch 3.

The comparator stage comprises, for example, an operational amplifier OP1 and has a first non-inverting input 5 for a reference threshold T1 , a second inverting input 6 for a voltage signal S1 to be compared with the threshold T1 and an output G in communication with the gate of the switch 3 for controlling the switch between a closed configuration and an open configuration.

The threshold T1 of the comparator 4 corresponds, as described below, to the maximum threshold current permitted on the MOSFET 3, that is, the maximum acceptable current on the capacitor 109.

As illustrated, the stage 4 is supplied at a predetermined voltage, for example +5 Volts, which in the preferred case in the example, may be picked up from the card 101 of the motor 100.

The threshold T1 is defined by a voltage divider using the resistors RT1 and RT2 having, for example, values, respectively, of 1 .55 kQ and 47 kQ. The comparator stage 4 is configured for changing the switch 3 from the closed configuration to the open configuration when the value of the signal S1 is greater than the threshold T1 and changing the switch 3 from the open configuration to the closed configuration when the value of the signal S1 becomes less than the threshold T1 .

The protection system 1 comprises an amplifier stage 7 with gain to provide to the comparator stage 4 the signal S1 .

In one embodiment, the stage 7 comprises an operational amplifier OP2 comprising an output OP2_out, a first inverting input 8 and a second non- inverting input 9 which correspond to the inputs and the output of the stage 7.

As illustrated, the operational amplifier OP2 is mounted in a non-inverting fashion and has an gain having value A1 defined by a resistor R3 and a resistor R4.

The value A1 of the gain of the operational OP2 in the non-inverting assembly is given by A1 = 1 +R4/R3.

In one embodiment, the resistor R4 has value equal to three times the value of the resistor R3, that is, R4 = 3 R3, for example R3 = 10kQ and R4 = 30kQ.

As illustrated, the input 8 is in communication with the output OP2_out, the input 9 is in parallel with the divider 2 in such a way that operational OP2 receives as input the voltage at the terminals of the controllable switch 3 and at the terminals of the resistor R2.

It should be noted that the features of the operational amplifiers are considered to be known in this invention, including, in particular, a very high input resistance which it is assumed ideally to have an infinite value, and a very small output resistance, which, similarly, is assumed ideally to be zero, and the approximations due to them, also in the description of the connections with reference to the circuit of Figure 3.

The output OP2_out is in communication with the second input 6 of the comparator stage 4 for transmitting to the input 6 the signal S1 .

The amplifier stage 7 comprises a circuit 10 for controlling the gain controlled by the comparator stage 4 for varying the gain of the stage 7 between the value A1 , corresponding to the above-mentioned non- inverting assembly with the resistors R3 and R4, and a second value A2 with the first value A1 greater than the second value A2.

In one embodiment, the first gain value A1 is substantially equal to four times the second gain value A2.

The second gain value A2 is substantially unitary since, as will become clearer as this description continues, the stage 7 becomes a voltage tracker.

As illustrated in Figure 3, the amplifier stage 7 comprises a second controllable switch 1 1 , for example a MOSFET type electronic switch, inserted between the resistor R3 and the reference potential. The output G of the comparator stage is in communication with the switch 1 1 for controlling it between a closed configuration, which corresponds to the first gain value A1 of the amplifier stage, the resistor R3 being connected to the reference potential, and an open configuration, which corresponds to the second gain value A2 of the amplifier stage.

In this configuration, as mentioned above, due to the impedance of the input of the operational OP2 which is substantially infinite, the value A2 of the gain is equal to 1 .

The comparator stage 4 is configured for changing the switch 1 1 from the closed configuration to the open configuration when the value of the signal S1 becomes greater than the threshold T1 and changing the switch 1 1 from the open configuration to the closed configuration when the value of the signal S1 becomes less than the threshold T1 .

In the embodiment illustrated for example, the protection system comprises a voltage stabilising stage 12 between the current divider 2 and the input 9 of the amplifier stage 7.

With reference to the diagram of Figure 3, it should be noted, for a complete description, that the protection system 1 comprises, in the example illustrated, an RC filter at the input to the comparator stage.

The protection system 1 comprises, in the example illustrated, a resistor R5 between the output G of the comparator 4 and the gates of the MOSFETs 3 and 1 1 .

In use, in a normal operating condition, the capacitor 109 filters the electromagnetic disturbances generated by the motor 100, as it is operational between the power supply and the common potential.

Under normal conditions, the capacitor 109 sees in effect, between itself and the common potential, the MOSFET 3, which, given its known resistance, which is very small, appears as a short-circuit.

The MOSFET 3 is kept closed by the comparator 4 since the input signal S1 remains less than the threshold T1 and the output G is high.

The signal S1 , in particular, is generated by the stage 7 which amplifies, with gain of value A1 in the case in the example, the voltage at the terminals of the MOSFET 3.

The voltage at the terminals of the MOSFET 3 is proportional to the current on the MOSFET, that is to say, it is proportional to the current on the capacitor 109, so the signal S1 is significant of the current on the capacitor 109.

The stage 7 has a gain of value A1 since the MOSFET 1 1 is also closed as it is always controlled by the output G of the comparator stage 4 which is high.

Given the very low value of current circulating in the MOSFET 3 under normal operating conditions, the voltage is to be amplified in the stage 7 so that it can be compared, in the stage 4, with the predetermined threshold T1 .

The threshold T1 of the comparator 4 is a function, as mentioned, of the maximum threshold current permitted on the MOSFET 3, that is, the maximum acceptable current on the capacitor 109.

Whilst the current on the MOSFET 3 and, therefore, on the capacitor 109 exceeds a maximum permissible value, as can be interpreted from the comparison between S1 and T1 , the output G remains high and the MOSFETs 3 and 1 1 remain closed.

In the case of a violent transient, coming in general from the supply line 105, which is able to raise the current on the capacitor 109, and, therefore, on the MOSFET 3, above a value corresponding to the maximum permitted voltage, the output G becomes low and the MOSFETs 3 and 1 1 switch to the open configuration.

Basically, as soon as the signal S1 , which is proportional to the voltage at the terminals of the MOSFET 3 and amplified in stage 7, exceeds the threshold T1 , the output G lowers and the MOSFETs 3 and 1 1 open.

In this configuration, with the MOSFETs 3 and 1 1 open, the capacitor 109 is in series, towards the reference potential, the resistor R2 which, being must greater than the equivalent series resistance R of the capacitor 109 bears the majority of the voltage applied to the capacitor 109 protecting it from the transient.

The amplifier stage 7 sees, in this configuration, the voltage, which is relatively high, at the terminals of R2 and significant of the trend of the transient which has triggered the opening of the MOSFETs 3 and 1 1 .

Since the MOSFET 1 1 is open, the stage 7 passes to the gain A2, which is substantially unitary, as the resistor R4 is negligible with respect to the input impedance of the operational OP2.

The signal S1 is thus proportional to the voltage at the terminals R2 which is in turn proportional to the current on the capacitor 109.

During this step, therefore, the comparator 4 continues to compare a signal S1 proportional to the current on the capacitor 109, even if it is no longer amplified, with the threshold T1 .

S1 follows the trend of the transient which has triggered the opening of the MOSFETs 3 and 1 1 and when it falls below the threshold T1 , the output G of the comparator 4 returns high, closing again the MOSFETs 3 and 1 1 and restoring the normal operating conditions.

In other words, the filtering system 108 returns to the normal operating conditions only when the output G of the comparator 4 returns high and the gate of the MOSFETs 3 and 1 1 change, in practice, to +5V, closing the respective switches.

The protection system described above makes it possible to protect any filtering electrolytic capacitors, used for reasons of electromagnetic compatibility and inserted upstream of the electrical machine, from transients coming from the power supply line. During the transient the capacitor is protected by a very large resistance which bears the majority of the voltage applied but once the transient is finished the system returns to the normal operating conditions thanks to the closing of the electronic switches provided