Electrical power conversion stage protected against battery reversal, charging device, motor vehicle and associated control process
The use of GaN transistors with blocking voltage application in motor vehicle battery charging systems addresses transistor damage from polarity inversion, reducing size and thermal losses, and improving efficiency.
Patent Information
- Application Number
- FR2023001282
- Authority / Receiving Office
- FR · FR
- Patent Type
- Patents
- Current Assignee / Owner
- Filing Date
- 2023-02-10
- Publication Date
- 2025-01-10
- Estimated Expiration
- 2043-02-10
AI Technical Summary
Existing motor vehicle battery charging systems using MOSFET transistors are prone to damage due to high intensity currents during polarity inversion, requiring space-consuming protection switches and generating thermal losses.
Implementing switching arms with heterostructure field effect transistors made of gallium nitride (GaN) and applying a continuous blocking voltage between the gate and source of each transistor to prevent current flow during polarity inversion, eliminating the need for protection switches and reducing thermal losses.
Prevents transistor damage during polarity inversion without additional switches, reduces size and thermal losses, and enhances efficiency by using GaN transistors with modified threshold voltages.
Smart Images
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Abstract
Description
Title of the invention: Battery inversion protected electric power conversion stage, charging device, motor vehicle and associated control method
[0001] The present invention relates to devices for charging motor vehicle batteries.
[0002] The present invention relates more particularly to an electrical power conversion stage for charging an auxiliary battery of a motor vehicle with thermal, electric or hybrid propulsion, the conversion stage being produced from heterostructure field effect transistors made of gallium nitride (GaN).
[0003] The invention further relates to a charger comprising such a conversion stage, a vehicle comprising such a charger, and a method for controlling such a conversion stage.
[0004] Generally, a motor vehicle with electric, hybrid or thermal propulsion includes an auxiliary battery having, for example, a voltage of 14 volts at its terminals, connected to a reversible electric power converter to charge the battery.
[0005] The converter generally comprises a first conversion stage connected to a high voltage battery of an electrically powered vehicle, for example having a voltage of 400 volts at its terminals, or to an alternator of a thermally powered vehicle or to a starter alternator of a hybrid vehicle, and a second conversion stage connected to the auxiliary battery.
[0006] The conversion stages are generally made from metal oxide gate field effect transistors known by the English acronym MOSFET "Metal Oxide Semiconductor Field Effect Transistor".
[0007] The first and second stages are connected to each other by a transformer providing galvanic isolation between said stages.
[0008] When the auxiliary battery is empty, it is known to directly connect the terminals of said battery to a functional auxiliary battery of another vehicle to charge the empty auxiliary battery.
[0009] If the faulty and functional auxiliary batteries are connected together so that terminals of the batteries of opposite polarity are connected together (polarity reversal), a high intensity current flows in the second stage and makes a low threshold internal diode of each MOSFET transistor conductive so that the high intensity current is likely to damage and destroy the MOSFET transistors of the second stage.
[0010] In order to prevent the destruction of the transistors of the second stage during the circulation of said current, it is known to connect the negative polarity terminal of the battery to the second stage by means of a controlled switch so that it is in the on state when the battery voltage is positive and in the open state during the polarity reversal between the auxiliary batteries, the switch generally comprising a MOSFET transistor.
[0011] However, the implementation of the switch requires space, means for controlling the switch detecting a polarity reversal.
[0012] In addition, the MOSFET transistor of the switch in the on state crossed by a current generates thermal losses.
[0013] It is therefore proposed to overcome all or part of the aforementioned drawbacks.
[0014] In view of the above, the invention relates to a conversion stage of reversible secondary electrical power comprising at least two input connections intended to be connected to an alternating voltage network, at least two output connections intended to be connected to an auxiliary battery for a motor vehicle, and at least two switching arms each comprising a first end connected to a first output connection and a second end connected to a second output connection, each switching arm comprising two heterostructure field effect transistors made of gallium nitride, the drain of a first transistor being connected to the first end, the source of the first transistor being connected to the drain of the second transistor, the source of the second transistor being connected to the second end, the source of the first transistor of each arm being further connected to an input connection different from the two input connections, and blocking means.
[0015] The blocking means are configured to apply a continuous blocking voltage of predetermined negative value between the gate and the source of each transistor when the voltage across the terminals of the auxiliary battery is less than or equal to a predetermined failure voltage, the blocking voltage being equal to the value of the potential applied to the gate of each transistor minus the value of the potential applied to the source of said transistor.
[0016] Preferably, the secondary electrical power conversion stage further comprises a filter comprising an inductor connected between the first end of each switching arm and the first output connection, and comprising a capacitor comprising a first end connected to the first output connection and a second end connected to the second output connection.
[0017] The invention also relates to a motor vehicle comprising a reversible secondary electrical power conversion stage as defined above, an alternating voltage network connected to the input connections, an auxiliary battery connected to the output connections, and means for measuring the voltage at the terminals of the auxiliary battery.
[0018] The invention also relates to a charging device for an auxiliary battery of a motor vehicle, comprising a reversible secondary electrical power conversion stage as defined above, the secondary electrical power conversion stage further comprising a third switching arm identical to the first and second switching arms, the first end of the third switching arm being connected to the first output connection and the second end of the third switching arm being connected to the second output connection, the source of the first transistor of the third arm being further connected to a third input connection, each input connection being intended to be connected to a different phase of an alternator of the motor vehicle forming the alternating voltage network.
[0019] The invention also relates to a motor vehicle comprising a charging device as defined above, an auxiliary battery connected to the output connections, an alternator, and means for measuring the voltage at the terminals of the auxiliary battery, each phase of the alternator being connected to a different input connection.
[0020] The invention also relates to a charging device for an auxiliary battery of a motor vehicle, comprising two switching arms each comprising a first end and a second end intended to be connected to a different terminal of a propulsion battery and a connection point, a transformer comprising a primary circuit comprising two ends each connected to a different connection point and comprising a secondary circuit comprising two ends, and a reversible secondary electrical power conversion stage as defined above, the input connections of the secondary electrical power conversion stage each being connected to a different end of the secondary circuit, and the two output connections of the secondary electrical power conversion stage being intended to be connected to the auxiliary battery.
[0021] The invention also relates to a charging device for an auxiliary battery of a motor vehicle, comprising a reversible primary electrical power conversion stage comprising two output terminals intended to be connected to a propulsion battery of the motor vehicle and two connection points, a transformer comprising two secondary circuits each comprising two ends, and a reversible secondary electrical power conversion stage as defined above, each end of a first secondary circuit being connected to a different connection point and each end of the second secondary circuit being connected to a different input connection of the secondary electrical power conversion stage, the propulsion battery, the primary power conversion stage and the transformer forming the alternating voltage network.
[0022] The invention also relates to a motor vehicle comprising a charging device as defined above, an auxiliary battery connected to the output connections, a propulsion battery connected to the output terminals, and means for measuring the voltage at the terminals of the auxiliary battery.
[0023] Preferably, the measuring means comprise a voltage sensor.
[0024] The invention also relates to a method for controlling a reversible secondary electrical power conversion stage comprising at least two input connections connected to an alternating voltage network, at least two output connections connected to an auxiliary battery of a motor vehicle, and at least two switching arms each comprising a first end connected to a first output connection and a second end connected to a second output connection, each switching arm comprising two heterostructure field effect transistors made of gallium nitride, the drain of a first transistor being connected to the first end, the source of the first transistor being connected to the drain of the second transistor, the source of the second transistor being connected to the second end, the source of the first transistor of each arm being further connected to a different input connection.
[0025] A DC blocking voltage of predetermined negative value is applied between the gate and the source of each transistor when the voltage across the battery is less than or equal to a predetermined fault voltage, the blocking voltage being equal to the value of the potential applied to the gate of each heterostructure field effect transistor minus the value of the potential applied to the source of said transistor.
[0026] Other aims, characteristics and advantages of the invention will appear on reading the following description, given solely by way of non-limiting example, and made with reference to the appended drawings in which:
[0027] [Fig-1] illustrates an electric motor vehicle according to the invention;
[0028] [Fig.2] illustrates a first example of embodiment of a charging device according to the invention;
[0029] [Fig.3] illustrates an example of the embodiment of a power conversion stage secondary electrical according to the invention;
[0030] [Fig.4] illustrates a second example of embodiment of the charging device according to the invention; and
[0031] [Fig.5] illustrates a hybrid motor vehicle according to the invention.
[0032] [Fig.l] illustrates an electric motor vehicle 1 comprising a battery charging device 2, a propulsion battery 3 of the vehicle 1, and an auxiliary battery 4.
[0033] The propulsion battery 3 powers an electric propulsion motor (not shown) of the vehicle 1.
[0034] When the auxiliary battery 4 is operational, the voltage at its terminals is for example equal to 14 volts.
[0035] The device 2 comprises input terminals 5, 6, 7 connected to a three-phase supply voltage source 8, and output terminals 9, 10, 11, 12.
[0036] A first and second output terminals 9, 10 are connected to the terminals of the propulsion battery 3, and a third and fourth output terminals 11, 12 are connected respectively to the positive and negative terminals of the auxiliary battery 4.
[0037] The device 2 charges the propulsion battery 3 from the source 8, and charges the auxiliary battery 4 from the propulsion battery 3.
[0038] Alternatively, the device 2 is powered by a single-phase supply voltage source.
[0039] [Fig.2] illustrates a first example of embodiment of the charging device 2.
[0040] The device 2 comprises an input stage 13, a power conversion stage primary electric 14 reversible connected to the propulsion battery 3, a secondary electric power conversion stage 15 reversible connected to the auxiliary battery 4, and a transformer 16 comprising a primary circuit 17 and two secondary circuits 18, 19.
[0041] The input stage 13 comprises five switching arms 20, 21, 22, 23, 24, a filtering capacitor 25 and three filtering inductors 26, 27, 28.
[0042] Each switching arm 20, 21, 22, 23, 24 comprises a first end 20a, 21a, 22a, 23a, 24a connected to a first end of the capacitor 25, and a second end 20b, 21b, 22b, 23b, 24b connected to a second end of the capacitor 25.
[0043] Each switching arm 20, 21, 22, 23, 24 further comprises two identical field effect transistors T1, T2, for example of the MOSFET type.
[0044] The drain of a first transistor T1 is connected to the first end 20a, 21a, 22a, 23a, 24a of the switching arm 20, 21, 22, 23, 24, the source of the first transistor T1 is connected to the drain of the second transistor T2, the source of the second transistor T2 is connected to the second end 20b, 21b, 22b, 23b, 24b of the switching arm 20, 21, 22, 23, 24, and the gate of the first and second transistors T1, T2 is connected to control means of the device 2 comprising for example a processing unit 29.
[0045] The source of the first transistor T1 of a first switching arm 20 is connected to a first input terminal 5 via a first inductance 26.
[0046] The source of the first transistor T1 of a second switching arm 21 is connected to a second input terminal 6 via a second inductance 27.
[0047] The source of the first transistor T1 of a third switching arm 22 is connected to the third input terminal 7 via the third inductance 28.
[0048] A connection point 23c between the source of the first transistor T1 and the drain of the second transistor T2 of a fourth switching arm 23 is connected to a first end of the primary circuit 17, and a connection point 24c between the source of the first transistor T1 and the drain of the second transistor T2 of the fifth switching arm 24 is connected to a first end of the primary circuit 17.
[0049] The primary electrical power conversion stage 14 comprises two switching arms 30, 31 and a filtering capacitor 32.
[0050] Each switching arm 30, 31 comprises a first end 30a, 31a connected to a first end of the capacitor 32 and to the first output terminal 9, and a second end 30b, 31b connected to a second end of the capacitor 32 and to the second output terminal 10.
[0051] Each switching arm 30, 31 further comprises two identical field effect transistors T3, T4, for example of the MOSFET type.
[0052] The drain of a first transistor T3 is connected to the first end 30a, 31a of the switching arm 30, 31, the source of the first transistor T3 is connected to the drain of the second transistor T4, the source of the second transistor T4 is connected to the second end 30b, 31b of the switching arm 30, 31, and the gate of the first and second transistors T3, T4 is connected to the processing unit 29 of the device 2.
[0053] A connection point 30c between the source of the first transistor T3 and the drain of the second transistor T3 of a first switching arm 30 is connected to a first end of a first secondary circuit 18 of the transformer 16, and a connection point 31c between the source of the first transistor T3 and the drain of the second transistor T4 of the second switching arm 31 is connected to a second end of the first secondary circuit 18.
[0054] The secondary electrical power conversion stage 15 comprises two input connections 33, 34 each connected to a different end of the second secondary circuit 19 of the transformer 16, two output connections 35, 36 connected respectively to the third and fourth output terminals 11, 12.
[0055] The secondary electrical power conversion stage 15 further comprises two switching arms 37, 38 each comprising a first end 37a, 38a connected to a first output connection 35 and a second end 37b, 38b connected to a second output connection 36.
[0056] Each switching arm 37, 38 comprises two identical field effect transistors T5, T6 made with heterostructure in gallium nitride GaN.
[0057] The drain of a first transistor T5 is connected to the first end 37a, 38a, the source of the first transistor T5 is connected to the drain of the second transistor T6, the source of the second transistor T6 is connected to the second end 37b, 38b.
[0058] The source of the first transistor T5 of the first switching arm 37 is further connected to a first input connection 33, and the source of the first transistor T5 of the second switching arm 38 is further connected to the second input connection 34.
[0059] The secondary electrical power conversion stage 15 further comprises blocking means 39.
[0060] The blocking means 39 compare the voltage at the terminals of the auxiliary battery 4 to a predetermined failure voltage, for example equal to 6 volts.
[0061] The predetermined failure voltage is representative of the failure of the battery 4 such that a connection of the battery 4 with a functional battery is probable.
[0062] The voltage at the terminals of the auxiliary battery 4 is measured by measuring means comprising, for example, a voltage sensor 40.
[0063] When the voltage across the terminals of the auxiliary battery 4 is less than or equal to the predetermined fault voltage, the blocking means 39 apply a continuous blocking voltage of predetermined negative value between the gate and the source of each transistor T5, T6 of the secondary electrical power conversion stage 15.
[0064] The blocking voltage is equal to the value of the potential applied to the gate of each transistor T5, T6 minus the value of the potential applied to the source of said transistor T5, T6.
[0065] The gate-source voltage VGs of each transistor T5, T6 of the secondary electrical power conversion stage 15 is then equal to the blocking voltage.
[0066] The GaN transistors T5, T6 subjected to the negative voltage VGs behave respectively like diodes D5, D6 each having a threshold voltage higher than the threshold voltage of the internal diode of a MOSFET transistor known from the state of the art, making it possible to prevent the circulation of a current in the switching arms 37, 38 of the secondary electrical power conversion stage 15 when a functional battery is connected with a polarity inversion to the faulty auxiliary battery 4.
[0067] Transistors T5, T6 are not damaged or destroyed without the addition of a protection switch connecting battery 4 to secondary electrical power conversion stage 15 known from the state of the art during polarity inversion.
[0068] The elimination of the protection switch also makes it possible to eliminate the means for controlling said switch and to eliminate the losses by thermal dissipation generated by the passage of a current in the switch.
[0069] The removal of the protection switch and the control means of said in switch allows space to be freed up in the secondary electrical power conversion stage 15 to reduce the size of the conversion stage.
[0070] [Fig.3] schematically illustrates the secondary electrical power conversion stage 15 when the gate-source voltage VGs of each transistor T5, T6 is equal to the DC blocking voltage.
[0071] The anode of diode D5 equivalent to transistor T5 when the DC blocking voltage is applied is connected to the cathode of diode D6 equivalent to transistor T6 when the DC blocking voltage is applied.
[0072] The cathode of diode D5 is connected to the first end 37a, 38a.
[0073] The anode of diode D6 is connected to the second end 37b, 38b.
[0074] As long as the voltage VDS between the drain and the source of each transistor T5, T6 ([Fig.2]) is not lower than a threshold depending on the characteristics of the transistor and no current flows between the drain and the source of each transistor T5, T6, the transistors T5, T6 are controlled by the voltage:
[0075] VGD=-VDS+VGS (1)
[0076] where VGD is the voltage between the gate and the drain of each transistor T5, T6.
[0077] Each diode D5, D6 is turned on when
[0078] VDS<-Vth+VGS (2)
[0079] where Vth is the control threshold voltage of transistors T5 and T6.
[0080] The application of a continuous blocking voltage of negative value makes it possible to modify the threshold of each diode D5, D6 equivalent to the transistor T5, T6.
[0081] For example, if the voltage Vgs is equal to -10V and the threshold Vth is equal to 1.5V, each diode D5, D6 makes it possible to block a voltage equal to -10V+1.5V=-8.5V, the diodes of each switching arm 37, 38 making it possible to block a voltage of -17V.
[0082] The blocking means 39 comprise, for example, a charge pump and control means controlling the charge pump according to the result of the comparison of the voltage at the terminals of the auxiliary battery 4 with the predetermined fault voltage.
[0083] The control means comprise for example the processing unit 29.
[0084] As long as the voltage across the terminals of the auxiliary battery 4 is higher than the predetermined fault voltage, the blocking means 39 do not apply the blocking voltage between the gate and the source of each transistor T5, T6 of the secondary electrical power conversion stage 15, and furthermore if the input stage 13 is connected to the source 8, the processing unit 29 controls the gates of the transistors T1, T2 of the input stage 13 and the gates of the transistors T3, T4 of the primary electrical power conversion stage 14 to recharge the propulsion battery 3 from the source 8.
[0085] When the input stage 13 is not connected to the source 8 and the voltage across the terminals of the auxiliary battery 4 is higher than the predetermined fault voltage, the processing unit 29 controls the gates of the transistors T3, T4 of the primary electrical power conversion stage 14 and the gates of the transistors T5, T6 of the secondary electrical power conversion stage 15 to charge the auxiliary battery 4 from the propulsion battery 3, the propulsion battery 3, the primary power conversion stage 14 and the transformer 16 forming an alternating voltage network.
[0086] The secondary electrical power conversion stage 15 may further comprise a filter 41 (shown in [Fig.2]) comprising an inductor 42 connected between the first end 37a, 38a of each switching arm 37, 38 and the first output connection 35, and a capacitor 43 comprising a first end connected to the first output connection 35 and a second end connected to the second output connection 36.
[0087] [Fig.4] illustrates a second example of embodiment of the charging device 2.
[0088] We find the propulsion battery 3, the secondary electrical power conversion stage 15, the auxiliary battery 4 connected to the third and fourth output terminals 11, 12, and the processing unit 29.
[0089] The device 2 further comprises the switching arms 30, 31 of the primary electrical power conversion stage 14, the first end 30a, 31a of the switching arms 30, 31 being connected to a first terminal of the propulsion battery 3 and the second end 30b, 31b of the switching arms 30, 31 being connected to a second terminal of the propulsion battery 3. The connection points 30c, 31c of the arms 30, 31 are connected to a different end of a primary circuit 48 of a second transformer 47.
[0090] A secondary circuit 49 of the second transformer 47 is connected to the input connections 33, 34 of the secondary electrical power conversion stage 15.
[0091] [Fig. 5] illustrates a hybrid motor vehicle 50 comprising a thermal engine 51, a starter alternator 52 mechanically connected to said engine 51, and another embodiment of the device 2 comprising another embodiment of the secondary electrical power conversion stage 15, the auxiliary battery 4 connected to the two output connections 35, 36, and the processing unit 29.
[0092] The secondary electrical power conversion stage 15 comprises as many input connections 33, 34, 53 as there are phases of the starter alternator 52.
[0093] It is assumed that the starter alternator 52 is of the three-phase type.
[0094] The secondary electrical power conversion stage 15 comprises the switching arms 37, 38 and the sensor 40 connected as described previously.
[0095] The secondary electrical power conversion stage 15 further comprises a third switching arm 54 identical to the switching arms 37, 38 having a first end 54a connected to the first output connection 35 and a second end 54b connected to the second output connection 36.
[0096] The source of the first transistor T5 of the third switching arm 54 is connected to the third input connection 53.
[0097] As long as the voltage across the terminals of the battery 4 is higher than the predetermined fault voltage, the processing unit 29 drives the gates of the transistors T5, T6 of the switching arms 37, 38, 54 so that the starter alternator 52 recharges the battery 4 when the engine 51 is running or so that the battery 4 supplies the starter alternator 52 to start the engine 51 when it is stopped.
[0098] When the voltage across the terminals of the battery 4 is lower than the predetermined failure voltage, the blocking means 39 apply the blocking voltage to each transistor T5, T6 of the switching arms 37, 38, 54.
[0099] Alternatively, the starter alternator 52 is replaced by an alternator, the vehicle being a thermal propulsion vehicle.
Claims
Claims
1. Reversible secondary electrical power conversion stage (15) comprising at least two input connections (33, 34) intended to be connected to an alternating voltage network (16, 47, 52), at least two output connections (35, 36) intended to be connected to an auxiliary battery (4) for a motor vehicle (1, 50), and at least two switching arms (37, 38) each comprising a first end (37a, 38a) connected to a first output connection (35) and a second end (37b, 38b) connected to a second output connection (36), each switching arm comprising two heterostructure field effect transistors (T5, T6) made of gallium nitride, the drain of a first transistor (T5) being connected to the first end, the source of the first transistor (T5) being connected to the drain of the second transistor (T6), the source of the second transistor (T6) being connected to the second end,the source of the first transistor (T5) of each arm being further connected to an input connection different from the two input connections (33, 34), and blocking means (39), characterized in that the blocking means (39) are configured to apply a continuous blocking voltage of predetermined negative value between the gate and the source of each transistor (T5, T6) when the voltage across the terminals of the auxiliary battery (4) is less than or equal to a predetermined failure voltage, the blocking voltage being equal to the value of the potential applied to the gate of each transistor (T5, T6) minus the value of the potential applied to the source of said transistor (T5, T6).,
2. A secondary electrical power conversion stage according to claim 1, further comprising a filter (41) comprising an inductor (42) connected between the first end (37a, 38b) of each switching arm (37, 38) and the first output connection (35), and comprising a capacitor (43) having a first end connected to the first output connection (35) and a second end connected to the second output connection (36).
3. Charging device (2) for an auxiliary battery (4) of a motor vehicle (1, 50), comprising a reversible secondary electrical power conversion stage (15) according to one of claims 1 or 2, the secondary electrical power conversion stage further comprising a third switching arm (54) identical to the first and second switching arms (37, 38), the first end (54a) of the third switching arm being connected to the first output connection (35) and the second end (54b) of the third switching arm being connected to the second output connection (36), the source of the first transistor (T5) of the third arm being further connected to a third input connection (53), each input connection being intended to be connected to a different phase of an alternator (52) of the motor vehicle forming the alternating voltage network.
4. Charging device for an auxiliary battery of a motor vehicle (1), comprising two switching arms (30, 31) each comprising a first end (30a, 31a) and a second end (30b, 31b) intended to be connected to a different terminal of a propulsion battery (3) and a connection point (30c, 31c), a transformer (47) comprising a primary circuit (48) comprising two ends each connected to a different connection point and comprising a secondary circuit (49) comprising two ends, and a reversible secondary electrical power conversion stage (15) according to one of claims 1 or 2, the input connections (33, 34) of the secondary electrical power conversion stage (15) each being connected to a different end of the secondary circuit (49), and the two output connections (35,36) of the secondary electrical power conversion stage (15) being intended to be connected to the auxiliary battery (4).,
5. Charging device for an auxiliary battery of a motor vehicle (1), comprising a reversible primary electrical power conversion stage (14) comprising two output terminals (9, 10) intended to be connected to a propulsion battery (3) of the motor vehicle and two connection points (30c, 31c), a transformer (16) comprising two secondary circuits (18, 19) each comprising two ends, and a reversible secondary electrical power conversion stage (15) according to one of claims 1 or 2, each end of a first secondary circuit (18) being connected to a different connection point (30c, 31c) and each end of the second secondary circuit (19) being connected to a different input connection (33, 34) of the secondary electrical power conversion stage (15), the propulsion battery, the primary power conversion stage and the transformer forming the alternating voltage network.
6. Motor vehicle comprising a reversible secondary electrical power conversion stage according to one of claims 1 or 2, a alternating voltage network (8) connected to the input connections (33, 34), an auxiliary battery (4) connected to the output connections (35, 36), and means for measuring the voltage at the terminals of the auxiliary battery (4).
7. Motor vehicle comprising a charging device according to claim 3, an auxiliary battery (4) connected to the output connections (35, 36), an alternator (52), and means for measuring the voltage at the terminals of the auxiliary battery, each phase of the alternator being connected to a different input connection.
8. Motor vehicle comprising a charging device according to one of claims 4 or 5, an auxiliary battery (4) connected to the output connections (35, 36), a propulsion battery (3) connected to the output terminals (9, 10), and means for measuring the voltage at the terminals of the auxiliary battery.
9. Motor vehicle according to one of claims 6 to 8, in which the measuring means comprise a voltage sensor (40).
10. Method for controlling a reversible secondary electrical power conversion stage comprising at least two input connections (33, 34) connected to an alternating voltage network, at least two output connections (35, 36) connected to an auxiliary battery (4) of a motor vehicle (1, 50), and at least two switching arms (37, 38) each comprising a first end (37a, 38a) connected to a first output connection (35) and a second end (37b, 38b) connected to a second output connection (36), each switching arm comprising two heterostructure field effect transistors (T5, T6) made of gallium nitride, the drain of a first transistor (T5) being connected to the first end, the source of the first transistor (T5) being connected to the drain of the second transistor (T6), the source of the second transistor (T6) being connected to the second end,the source of the first transistor (T5) of each arm being further connected to a different input connection, characterized in that a continuous blocking voltage of predetermined negative value is applied between the gate and the source of each transistor (T5, T6) when the voltage across the battery is less than or equal to a predetermined failure voltage, the blocking voltage being equal to the value of the potential applied to the gate of each hetero-structure field effect transistor minus the value of the potential applied to the source of said transistor.,