Test circuit for a battery disconnect system, associated devices and method

Abstract

Test circuit for a battery disconnect system, associated devices and method. The present invention relates to a test circuit (30) for a disconnect system for a high-power electrical source (12), the disconnect system comprising a plurality of arms arranged in parallel, each arm having contactors placed back-to-back and separated by an intermediate point (P1, PI, PN), each contactor comprising a transistor, the test circuit (30) comprising: - for each intermediate point (P1, PI, PN), a respective current source (32_1, 32_I, 32_N) connected to the intermediate point (P1, PI, PN), each current source (32_1, 32_I, 32_N) being suitable for supplying a current, and - a reading unit (34) suitable for delivering a voltage representing the sum of the currents supplied by each current source (32_1, 32_I, 32_N). Figure for the abstract: Figure 2

Description

Title of the invention: Test circuit for a battery disconnect system, associated devices and method

[0001] The present invention relates to a test circuit for a high-power power source disconnection system, in particular a battery. The present invention also relates to a battery disconnection system and a battery comprising such a test circuit. The present invention also relates to a vehicle incorporating such a disconnection system. The present invention also relates to a corresponding test method.

[0002] Typically, a battery comprises one or more current storage cells, also called electrochemical generators, cells, or elements. A battery is an electricity-producing device in which chemical energy is converted into electrical energy. The chemical energy comes from electrochemically active compounds deposited on at least one face of electrodes arranged in the battery. The electrical energy is produced by electrochemical reactions during a discharge of the battery. The electrodes, arranged in a container, are electrically connected to current output terminals that ensure electrical continuity between the electrodes and an electrical load to which the battery is connected.

[0003] To increase the electrical power delivered, several sealed accumulators can be connected together to form a battery. Thus, a battery can be divided into modules, each module being composed of one or more accumulators connected together in series and / or in parallel. For example, a battery may comprise one or more parallel branches of accumulators connected in series and / or one or more parallel branches of modules connected in series.

[0004] A charging circuit is generally provided to which the battery can be connected to recharge the accumulators.

[0005] Furthermore, an electronic management system comprising measurement sensors and an electronic control circuit, more or less sophisticated depending on the application, can be associated with the battery. Such a system makes it possible, in particular, to organize and control the charging and discharging of the battery, in order to balance the charging and discharging of the different cells of the battery with respect to each other.

[0006] For high power applications, typically batteries with a capacity greater than 25-30 Ah and / or operating at currents greater than 100 A, a switching system is also used comprising a plurality of parallel branches, each branch having at least one silicon-type contactor.

[0007] Because the cut-off system provides isolation between the battery and its load, the proper functioning of each contactor must be checked.

[0008] To achieve this, it is known to perform such detection using specific tests. A parallel circuit is used to supply voltage upstream or downstream of the contactors, depending on the group being tested. A chaser then allows the opening or closing of each arm of each group to be checked one after the other to verify their proper operation. Such an assembly is complex to control and involves a large number of auxiliary components, thereby compromising the reliability of the complete system.

[0009] There is a need for a test circuit for a switching system to allow for simpler control of the proper functioning of the contactors.

[0010] To this end, the description describes a test circuit for a high-power electrical source disconnection system, the disconnection system comprising a plurality of arms arranged in parallel, each arm having contactors placed back-to-back and separated by an intermediate point, each contactor comprising a transistor, the test circuit comprising:

[0011] - for each intermediate point, a respective current source connected to the point in tertiary, each current source being capable of supplying a current, and

[0012] - a reading unit capable of delivering a voltage that is an image of the sum of the currents supplied by each power source.

[0013] According to particular embodiments, the test circuit has one or more of the following characteristics, taken individually or in all technically possible combinations:

[0014] - each transistor is an insulated-gate field-effect transistor.

[0015] - the reading unit includes a resistor.

[0016] - the reading unit includes an amplifier.

[0017] - the current source comprises a junction field-effect transistor.

[0018] - the contactors connected to an output of the high-power electrical source form a first set, the first set being controlled by a first control unit.

[0019] - the contactors connected to an input of the system to be powered forming a second together, the second set being controlled by a second control unit, the second control unit being separate from the first control unit.

[0020] The description also describes a high-power electrical source switching system comprising a plurality of arms arranged in parallel, each arm having contactors placed back-to-back and separated by an intermediate point, each contactor comprising a transistor, the switching system comprising a test circuit.

[0021] The description also proposes a high power electrical source equipped with a cut-off system, the high power electrical source preferably being a battery.

[0022] The description also relates to a vehicle comprising a high-power electrical source, the vehicle being chosen from the list consisting of an aircraft, a means of rail transport, a means of road transport, a means of maritime or river transport.

[0023] The description also describes a method for testing a high-power electrical source disconnection system, the disconnection system comprising a plurality of arms arranged in parallel, each arm having contactors placed back-to-back and separated by an intermediate point, each contactor comprising a transistor, the test method being implemented by a test circuit comprising:

[0024] - for each intermediate point, a respective current source connected to point in termedial, and

[0025] - a reading unit,

[0026] the process comprising a step of:

[0027] - supply of current by each current source, and

[0028] - output by the reading unit of a voltage representing the sum of the currents supplied by each power source.

[0029] In this description, the expression "specific to" means interchangeably "suitable for", "adapted to" or "configured for".

[0030] Some features and advantages of the invention will become apparent from the following description, given solely by way of non-limiting example, and made with reference to the accompanying drawings, in which:

[0031] - [Fig. 1] [Fig. 1] is a schematic representation of an installation comprising a battery, a cut-off system, and a system to be powered, and

[0032] - [Fig.2] [Fig.2] is an electrical diagram of a test circuit of the system cut of the [Fig.l].

[0033] An installation 10 is schematically represented in [Fig.1].

[0034] The installation 10 includes a battery 12 equipped with a cut-off system 14 and a power supply system 16.

[0035] In a manner known per se, a battery is generally an arrangement of a plurality of electrochemical elements but in the interest of simplifying the subject, a case with a single electrochemical element is described in what follows, knowing that the transposition to other arrangements is immediate.

[0036] The battery 12 comprises at least one electrochemical element and an electrochemical element management system.

[0037] As explained previously, an electrochemical element is an electricity-producing device in which chemical energy is converted into electrical energy.

[0038] The electrochemical element therefore delivers a current and a voltage between two terminals.

[0039] The management system is a system specific to managing the electrochemical element.

[0040] In the present case, battery 12 is a high-power battery, typically capable of supplying around ten kW

[0041] The system to be powered 16 here refers to any load requiring such power.

[0042] Depending on the case, the system to be supplied 16 may include one or more pieces of equipment.

[0043] According to a particular example, the system to be powered 16 is a vehicle, so the battery 12 is an on-board power supply.

[0044] According to one embodiment, the vehicle is chosen from the list consisting of an aircraft, a means of rail transport, a means of road transport, a means of maritime or river transport.

[0045] For the remainder, an active sign convention is chosen for battery 12.

[0046] Thus, the discharge current, that is to say a current flowing from the battery 12 to the system to be powered 16, is positive while the charge current, that is to say a current flowing from the system to be powered 16 to the battery 12, is negative.

[0047] As can be seen in [Fig.1], the switching system 14 is interposed between the battery 12 and the system to be powered 16.

[0048] This means that the switching system 14 is connected on one side to an output 12S of the battery 12 and on the other side to an input 16S of the system to be powered 16.

[0049] The cut-off system 14 is suitable for ensuring isolation between the battery 12 and the system to be powered 16 when it is desired that no current flows between the system to be powered 16 and the battery 12.

[0050] The switching system 14 is also suitable for allowing the flow of current between the battery 12 and the system to be powered 16.

[0051] The cutting system 14 comprises a plurality of arms 18 arranged in parallel.

[0052] In the example of [Fig. 1], the cutting system 14 comprises N arms, only the first arm 18_1, second arm 18_2, third arm 18_3 and Nth arm 18_N being represented.

[0053] The number N is an integer preferably greater than or equal to 10.

[0054] As regards the arrangement, each arm 18_1 to 18_N is connected on one side to the output 12S of the battery 12 and on the other side to the input 16S of the system to be powered 16.

[0055] Each arm 18_1 to 18_N has the same structure, i.e. two contactors 20_1 and 20_2 in series.

[0056] For the remainder of this text, the contactors connected to the battery 12 are referred to as first contactors 20_l, while the contactors connected to the system to be powered 16 are referred to as second contactors 20_2.

[0057] In the chosen convention, the first contactors 20_1 can be described as upper contactors as opposed to the second contactors 20_2 which can be described as lower contactors.

[0058] A contactor is a controlled electronic component used to open or close a connection between two elements.

[0059] According to the example described, each contactor 20_l or 20_2 is a semiconductor contactor.

[0060] A semiconductor contactor is more often designated by the abbreviation SSR, which refers to the corresponding English term "solid-state relay," generally translated as contactor or static relay.

[0061] Such a contactor is a circuit allowing a connection between two elements to be opened or closed without the use of a mechanical or electromechanical element.

[0062] In the example described, each contactor 20_l or 20_2 includes a transistor 22_1 or 22_2.

[0063] This does not exclude the presence of other components if necessary.

[0064] Here, each transistor 22_1 or 22_2 is an insulated gate field-effect transistor.

[0065] Such a transistor is more often called a MOSFET transistor.

[0066] The acronym MOSFET refers to the English name "metal-oxide-semiconductor field-effect transistor" which can be literally translated as "metal-oxide-semiconductor field-effect transistor".

[0067] In the following, each first transistor is denoted first transistor T1 and each second transistor is denoted second transistor T2.

[0068] The use of MOSFET transistors in this context makes it possible to obtain a low series resistance with low losses to allow the arms 18_1 to 18_N to dissipate the high power of the battery 12.

[0069] Thus, according to the case of [Fig.1], each transistor T1 or T2 comprises a drain, a source and a gate.

[0070] The drain, source and gate of the first transistor T1 are respectively noted T1D, T1S and T1G while the drain, source and gate of the second transistor T2 are respectively noted T2D, T2S and T2G.

[0071] The transistor T1 or T2 is used to modulate the current flowing between the drain and the source according to the control signal applied to the gate.

[0072] Depending on the nature of the majority carriers forming the current flowing between the drain and the source, the MOSFET transistor is classified as a P-type MOSFET or an N-type MOSFET.

[0073] In the case of [Fig.1], each transistor T1 or T2 is an N-type MOSFET.

[0074] Furthermore, by construction of a MOSFET transistor, each transistor T1 or T2 includes an intrinsic diode 24 arranged in antiparallel (anode connected to the source T1S or T2S of transistor Tl or T2 and cathode connected to the drain T1D or T2D of transistor Tl or T2).

[0075] Alternatively, each transistor T1 or T2 is of a different type than a MOSFET.

[0076] For example, each transistor T1 or T2 is a different field-effect transistor from a MOSFET.

[0077] Such a transistor is more often called a FET transistor, the acronym FET referring to the corresponding English term "field-effect transistor".

[0078] According to another example, each transistor T1 or T2 is an insulated-gate bipolar transistor.

[0079] Such a transistor is more commonly known as an IGBT transistor, the abbreviation IGBT corresponding to the corresponding English designation of "insulated-gate bipolar transistor". According to the example described, the contactors 20_1 and 20_2 are mounted back-to-back.

[0080] In the case mentioned, the head-to-tail arrangement implies that, for each arm 18, the drain T1D of the first transistor T1 is connected to the drain T2D of the second transistor T2.

[0081] As discussed more specifically with reference to the operation of the switching system 14, the head-to-tail arrangement ensures a good switching between the battery 12 and the system to be powered 16.

[0082] More specifically, this allows for a bidirectional cut (allowing upstream and downstream opening and closing).

[0083] An intermediate point PI, P2, P3, ... PN is also defined for each arm 18_1, 18_2, 18_3, ... 18_N, the intermediate point PI, P2, P3 or PN being a point located between the drain T1D of the first transistor T1 and the drain T2D of the second transistor T2.

[0084] The intermediate point PI, P2, P3 or PN constitutes here a floating node.

[0085] In the case of P-type MOSFET transistors, it is the sources of each transistor that are connected and the intermediate point PI, P2, P3 or PN is located between the two sources.

[0086] The switching system 14 further comprises two separate control units 26 and 28.

[0087] The first control unit 26 is suitable for controlling a first set El of contactors.

[0088] The first assembly El brings together the first contactors 20_l.

[0089] According to the example described, the first control unit 26 is a grid control unit.

[0090] More specifically, the first control unit 26 is suitable for controlling the grid T1G of each first contactor 20_l.

[0091] For this purpose, the first control unit 26 is connected to the grid T1G of each first contactor 20_l.

[0092] In operation, the first assembly El enables charging, i.e. the supply of a current from the system to be powered 16 to the battery 12.

[0093] The second control unit 28 is suitable for controlling a second set E2 of contactors.

[0094] The second set E2 brings together the second contactors 20_2.

[0095] According to the example described, the second control unit 28 is a grid control unit.

[0096] More specifically, the second control unit 28 is specific to controlling the T2G grid of each second contactor 20_2.

[0097] For this purpose, the second control unit 28 is connected to the T2G grid of each second contactor 20_2.

[0098] In operation, the second assembly E2 allows discharge, that is to say the supply of a current from the battery 12 to the system to be powered 16.

[0099] The switching system 14 includes a test circuit 30.

[0100] As will be detailed later, the test circuit 30 is specific to testing each branch 18_1 to 18_N.

[0101] The test circuit 30 is shown more specifically in [Fig.2].

[0102] The test circuit includes current sources 32_1 to 32_N and a unit of Reading 34.

[0103] Each current source 32_1 to 32_N is connected to a respective intermediate point PI toPN.

[0104] The test circuit 30 thus includes as many current sources 32_1 to 32_N as the switching system 14 has arms 18_1 to 18_N to be tested.

[0105] In this case, it is assumed that all arms 18_1 to 18_N of the switching system are to be tested.

[0106] However, for the sake of clarity, only three current sources are shown on [Fig.2], the first current source 32_1, the i-th current source 32_i and the N-th current source 32_N.

[0107] Each current source 32_1 to 32_N is suitable for supplying a current.

[0108] For certain applications, in order to reduce consumption, it may be in interesting that the current delivered by each current source 32_1 to 32_N is relatively low, typically on the order of milliamperes.

[0109] As can be seen in [Fig.2], each current source 32_1 to 32_N comprises a field-effect transistor with a junction 36 in series with a resistor 38.

[0110] Such a transistor 36 is more often called a JFET transistor 36, the abbreviation JFET referring to the corresponding English term "junction field effect" transistor.

[0111] The JFET transistor 36 has a drain 36D, a source 36S and a gate 36G while the resistor 38 has a first terminal 38B1 and a second terminal 38B2.

[0112] The first terminal 38B1 of the resistor 38 is connected to the drain 36D of the JFET transistor 36 while the second terminal 38B2 of the resistor 38 is connected to both the intermediate point PI, Pi or PN and to the gate 36G of the JFET transistor 36.

[0113] More generally, each current source 32_1 to 32_N is implemented by any electrical circuit enabling the function of current supply to be performed, such as a resistor powered by a voltage source, an integrated circuit or a source diode.

[0114] The reading unit 34 is suitable for delivering a voltage image of the sum of the currents supplied by each current source 32_1 to 32_N.

[0115] According to the example in [Fig.2], the reading unit 34 includes a resistance 40 called the biasing resistance 40.

[0116] The biasing resistor 40 has two terminals 40B1 and 40B2.

[0117] The first terminal 40B1 is connected to a power supply 41, and more specifically to the positive pole of the power supply 41, the negative pole being connected to ground.

[0118] The power supply 41 is used here to bias the biasing resistor 40 and therefore delivers a sufficiently high voltage between its two poles to allow such an effect. A voltage typically greater than 30 V is sufficient to achieve the biasing.

[0119] On the side of the second terminal 40B2, the biasing resistor 40 is connected in series with each current source 32_1 to 32_N, and more precisely with the source 36S of the corresponding JFET transistor 36.

[0120] The biasing resistance 40 is thus traversed by the sum of the currents delivered by each current source 32_1 to 32_N.

[0121] The biasing resistor 40 converts the current flowing through it into a voltage.

[0122] In this sense, thus positioned, the biasing resistor 40 serves as an adder.

[0123] The reading unit 34 also includes an amplifier 42.

[0124] According to the example described, the amplifier 42 is an operational amplifier 44 in a differential configuration.

[0125] The operational amplifier 44 has a first input 44E1, a second input 44E2 and an output 44S.

[0126] The amplifier 32 further comprises four resistors RI, R2, R3 and R4 as well as two capacitors Cl and C2.

[0127] The first resistor RI has a first terminal R1B1 connected to the first terminal R40B1 of the biasing resistor 40 and a second terminal R1B2 connected to a second terminal C1B2 of the first capacitor CL

[0128] The second resistor R2 has a first terminal R2B1 connected to the second input 44E2 of the operational amplifier 44 and a second terminal R2B2 connected to an output 44S of operational amplifier 44.

[0129] The third resistor R3 has a first terminal R3B1 connected to the second terminal R40B2 of the biasing resistor 40 and a second terminal R3B2 connected to the second input 44E2 of the operational amplifier 44 as well as to the first terminal R2B1 of the second resistor R2.

[0130] The fourth resistor R4 has a first terminal R4B1 connected to a first terminal C1B1 of the first capacitor Cl and a second terminal R4B2 connected to the first input 44E1 of the operational amplifier. The second terminal R4B2 of the fourth resistor R4 is also connected to the second terminal C1B2 of the first capacitor Cl and to the second terminal R1B2 of the first resistor RL

[0131] The first terminal C1B1 of the first capacitor Cl is connected to ground while the second terminal C1B2 of the first capacitor Cl is connected both to the second terminal R1B2 of the first resistor RI, to the second terminal R4B2 of the fourth resistor R4 and to the first input 44E1 of the operational amplifier.

[0132] The second capacitor C2 has a first terminal C2B1 connected to the first terminal R2B1 of the second resistor R2 and a second terminal C2B2 connected to the second terminal R2B2 of the second resistor R2.

[0133] Such an amplifier 42 makes it possible to obtain at the output a voltage such that:

[0134] y - (_^.y v S \ R3+R4 R3 P + R3

[0135] Where: • Vs denotes the voltage at the 44S output of the operational amplifier, • RI, R2, R3 and R4 respectively denote the values ​​of the first resistance RI, the second resistor R2, the third resistor R3, and the fourth resistor R4, • V+ denotes the voltage on the first input 44E1, and • U- denotes the voltage on the second input 44E2.

[0136] The example of reading unit 34 described above is not limiting.

[0137] In particular, the presence of the amplifier 42 is justified here by the implementation illustrated with currents supplied by the current sources 32_1 to 32_N, which are relatively low. Its presence is optional when a sufficiently strong current is present to be read.

[0138] Furthermore, the reading unit 34 can be any circuit allowing the sum of the currents of the current sources 32_1 to 32_N to be read in the form of a voltage.

[0139] The operation of the switching system 14 is now described with reference to several use cases.

[0140] In operation, the control units 26 and 28 control the gates T1G and T2G of the transistors T1 or T2 to change their state from a conducting state to a non-conducting state blocked or vice versa.

[0141] In the conducting state, transistor T1 or T2 allows current to flow between the source T1S or T2S and the drain T1D or T2D, whereas in the blocked state, transistor T1 or T2 interrupts the flow of current from the drain T1D or T2D to the source T1S or T2S, but part of the current may also pass through diode 24.

[0142] With the head-to-tail arrangement, the control of the two transistors Tl or T2 of each arm 18_1 to 18_N ensures that, in the conducting state, the current flows in the arm 18_1 to 18_N (closed arm configuration) while, in the blocked state, no current can flow, the current being blocked by one of the transistors Tl or T2 according to the direction of current flow (open arm configuration).

[0143] When one of the two transistors T1 or T2 is short-circuited or has a failure, the arm 18_1 to 18_N will therefore be in a malfunctioning situation corresponding to the fact that, in the closed arm configuration, no current flows in the arm 18_1 to 18_N or that in the open arm configuration, part of the current flows in the arm 18_1 to 18_N.

[0144] In the event of a malfunction of an arm 18_1 to 18_N, the test circuit 30 can detect a difference in behavior between the measured current flow and the expected current flow due to the command sent to the gates T1G or T2G of the two transistors T1 and T2 of the arm 18_1 to 18_N.

[0145] Indeed, the voltage value measured at the level of the biasing resistor 40 reflects the number of arms 18_1 to 18_N closed or open.

[0146] With an appropriate command sent by the first control unit 26 and the second control unit 28, it is possible to test the transistors T1 (of the set El) and the transistors T2 (of the set E2).

[0147] Measuring the sum of the currents allows us to determine the number of functional transistors T1 (of the set El) and the number of functional transistors T2 (of the set E2).

[0148] The test circuit 30 thus makes it easier to test a plurality of arms 18_1 to 18_N in parallel comprising contactors 20_1 and 20_2.

[0149] The number of components involved in the test function is reduced while still allowing a complete test.

[0150] In fact, the test circuit 30 makes it possible to determine the presence of contactor 20_l and 20_2 in short circuit or faulty by indicating the number of arms 18_1 to 18_N impacted.

[0151] Other applications of the test circuit 30 can be envisaged.

[0152] The test circuit 30 can thus be used to test a switching system allowing a battery energy storage system to be isolated from an electrical grid.

[0153] A battery energy storage system is a system comprising gen- raally a plurality of benches, each bench bringing together a set of batteries linked to a respective conversion system.

[0154] Such a storage system is more often referred to as a BESS system in reference to the corresponding English name of “Battery Energy Storage System”.

[0155] The switching system 14 is more generally usable to isolate a high power electrical source from any system to be powered involving the use of high power.

[0156] Herein, a high-power electrical source is understood to mean any source capable of supplying a power output greater than 1 kW.

[0157] The switching system 14 is nevertheless capable of ensuring its switching functionality for much higher powers, typically several tens of kW.

Claims

Demands

1. Test circuit (30) of a switching system (14) of a high-power electrical source (12), the switching system (14) comprising a plurality of arms (18_1, 18_2, 18_3, 18_N) arranged in parallel, each arm (18_1, 18_2, 18_3, 18_N) having contactors (20_1, 20_2) placed back-to-back and separated by an intermediate point (PI, P2, P3, PI, PN), each contactor (20_1, 20_2) comprising a transistor (22_1, 22_2), the test circuit (30) comprising: - for each intermediate point (PI, P2, P3, PI, PN), a respective current source (32_1, 32_1, 32_N) connected to the intermediate point (PI, P2, P3, PI, PN), each current source (32_1, 32_I, 32_N) being suitable for supplying a current, and - a reading unit (34) suitable for delivering a voltage image of the sum of the currents supplied by each current source (32_1, 32_I, 32_N).

2. Test circuit according to claim 1, wherein each transistor (22_1, 22_2) is an insulated gate field-effect transistor.

3. Test circuit according to claim 1 or 2, wherein the reading unit (34) includes a resistor (40).

4. Test circuit according to any one of claims 1 to 3, wherein the reading unit (34) includes an amplifier (34).

5. Test circuit according to any one of claims 1 to 4, wherein the current source (32_1, 32_I, 32_N) comprises a field-effect transistor with junction (36).

6. Test circuit according to any one of claims 1 to 5, wherein the contactors (20_l) connected to an output (12S) of the high power electrical source (12) form a first set (El), the first set (El) being controlled by a first control unit (26).

7. Test circuit according to claim 6, wherein the contactors (20_2) connected to an input (16E) of the system to be powered (16) form a second set (E2), the second set (E2) being controlled by a second control unit (28), the second control unit (28) being separate from the first control unit (26).

8. System for cutting off (14) a high-power electrical source (12) comprising a plurality of arms (18_1, 18_2, 18_3, 18_N) arranged in parallel, each arm (18_1, 18_2, 18_3, 18_N) comprising contactors placed head-to-tail and separated by an intermediate point (PI, P2, P3, PI, PN), each contactor (20_1, 20_2) comprising a transistor (22_1, 22_2), the switching system (14) comprising a test circuit (30) according to any one of claims 1 to 7.

9. High power power source (12) equipped with a cut-off system (16) according to claim 8, the high power power source (12) preferably being a battery (12).

10. Vehicle comprising a high power electrical source (12) according to claim 9, the vehicle being selected from the list consisting of an aircraft, a means of rail transport, a means of road transport, a means of maritime or river transport.

11. A method for testing a switching system (14) of a high-power electrical source (12), the switching system (14) comprising a plurality of arms (18_1, 18_2, 18_3, 18_N) arranged in parallel, each arm (18_1, 18_2, 18_3, 18_N) having contactors (20_1, 20_2) placed back-to-back and separated by an intermediate point (PI, P2, P3, PI, PN), each contactor (20_1, 20_2) comprising a transistor (22_1, 22_2), the test method being implemented by a test circuit (30) comprising: - for each intermediate point (PI, P2, P3, PI, PN), a respective current source (32_1, 32_1, 32_N) connected to the intermediate point (PI, P2, P3, PI, PN), and - a reading unit (34), the process comprising a step of: - supplying a current by each current source (32_1, 32_I, 32_N), and - delivering by the reading unit (34) a voltage image of the sum of the currents supplied by each current source (32_1, 32_I, 32_N).

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