Floating capacitance device and method for pre-charging the capacities of such a device.

Abstract

A floating-capacitor device (1, 1'), such as an inverter arm (1) or a step-up DC-DC converter (1'), comprising a plurality of N nested switching cells (2, 2') and a balancing circuit (50) adapted to have a resonant frequency equal to the switching frequency of the transistors, said balancing circuit (50) being mounted on one side at said midpoint (70) and on the other at said terminal (71) of the capacitor of the last cell (2'). (See abstract figure: Figure 5a)

Description

Title of the invention: Floating capacitor device and method for precharging the capacitors of such a device.

[0001] The present invention relates to a floating capacitor device, such as an inverter or a DC-DC voltage boost converter, also called Boost.

[0002] In the field of electric motor vehicles, it is known that 800 V electric traction systems impose constraints on semiconductors which are usually sized at 1200 V blocking voltage.

[0003] However, it is known that the series use of 650V components via a multi-cell topology, or with floating capacitor, has several advantages:

[0004] - Using lower voltage components has a better figure of merit to operate at high frequency, such as gallium nitride transistors;

[0005] - This makes it possible to have an output voltage whose harmonic spectrum is more favorable, on the one hand because of the lower amplitude of the switching edges and on the other hand because the first harmonic resulting from the switching is pushed back to N*FSW, where Fsw is the switching frequency of the transistors and N the number of switching cells, which allows a significant gain in terms of filtering volume and dynamics of output current control.

[0006] Thus, a multicellular topology is known in the prior art. Figure 1 of the prior art represents, [Fig.1a], a 2-cell arm and, [Fig.1b], its generalization to N cells.

[0007] While the frequency of the 1st harmonic is the switching frequency of the transistors multiplied by the number of cells.

[0008] In the technical field, these performances are advantageous in particular for:

[0009] - On-board single-phase chargers for boost compensation systems instantaneous power fluctuation, increasing current regulation dynamics and addressing the 800V network with 650V components;

[0010] - DC / DC Boost converters for powering the network by several parallel sources of different voltage. Typically a fuel cell and a battery. Indeed, the gain in filtering volume is significant, as is the efficiency with the use of wide gap transistors such as Gallium Nitride transistors for example; as well as

[0011] - Inverters to drive innovative machines where a large bandwidth is necessary to deliver multiple harmonic voltages of the fundamental electrical frequency while being able to filter the spectrum resulting from switching.

[0012] However, such a topology has the disadvantage of having to ensure the stability of the voltage of the floating capacitors at an average value, over a period of at least 1 / Fsw, of Vdc / N, 2 Vdc / N, 3 Vdc / N etc. (which can be seen in figure 1 of prior art).

[0013] This balancing is done naturally by allowing current harmonics to circulate at the switching frequency of the transistors and its (Nl) multiples (N being the number of nested switching cells).

[0014] However, this balancing is insufficient or even non-existent in 2 cases:

[0015] - Precharging the high voltage circuit, prior to connecting the battery. In In this case, the 1st capacity is charged while the multi-level arm is not delivering any current: natural balancing cannot take place and only the 1st capacity is charged, thereby imposing the Vdc voltage on the transistors of the 1st cell, with a high risk of destruction by overvoltage.

[0016] - During operation. If the load impedance is insufficient, the re balancing of floating capacitor voltages is too slow or even non-existent.

[0017] Also, there is a need for a solution to ensure the balancing of the voltages of the floating capacitors in the two cases described above.

[0018] To this end, a floating capacitor device is proposed, such as an inverter or a DC-DC voltage boost converter, comprising a plurality of N switching cells nested within each other, each cell comprising between its connection terminals two switchable transistors in series and a capacitor in parallel with the two transistors, the N cells being nested so that a second cell is nested in a first cell by being mounted between the two transistors in series of the first cell; this nesting being repeated for the N nested cells, the Nth nested cell comprising its two transistors mounted in series directly next to each other, an output voltage of said device being obtained by the potential difference between the midpoint located between the two transistors of the Nth nested cell and a terminal of the capacitor of the first cell.

[0019] The floating capacitor device comprises a balancing circuit having a resonant frequency equal to the switching frequency of the transistors, said balancing circuit being mounted on the one hand at said midpoint between the two transistors of the Nth cell and at said terminal of the capacitor of the first cell.

[0020] Advantageously, said balancing circuit comprises a series RLC resonant circuit. In other words, a resonant circuit comprising an electrical resistance, an inductance and a capacitor connected in series. Thus, this device is relatively inexpensive to manufacture and is particularly reliable.

[0021] In particular, the device comprises only two switching cells.

[0022] Advantageously, the floating capacitor device comprises a bandpass filter, and said balancing circuit comprises at least one capacitance shared with said bandpass filter. Thus, resources can be shared between the bandpass filter and the balancing circuit.

[0023] Advantageously, the balancing circuit comprises a switch connected in series with the RLC circuit. Thus, the periods of forced or natural balancing of the floating capacitors can be controlled.

[0024] Advantageously, the floating capacitor device comprises a bandpass filter whose bandwidth is centered at the switching frequency of the transistors, the bandpass filter being connected in parallel to the switch.

[0025] In particular, the device comprises two switching cells.

[0026] Advantageously, the floating capacitor device further comprises a low-pass filter, and said balancing circuit comprises at least one inductor shared with said low-pass filter.

[0027] The invention also relates to a power conversion system comprising at least one floating capacitor device as described previously.

[0028] The invention also relates to a vehicle comprising a power conversion system as described above.

[0029] The invention also relates to a method for precharging a floating capacitor device as described above, in particular when the balancing circuit comprises a switch connected in series with said RLC resonant circuit, comprising: - a step of closing the switch; - a step of precharging the floating capacities in order to switch the switchable transistors of each cell at the switching frequency of the transistors, - a comparison step between the voltage of the capacity of the first cell and a predetermined voltage; - a step of opening the switch when said voltage of the capacity of the first cell is substantially equal to the predetermined voltage. According to an alternative, when the floating capacitor device comprises a band-pass filter whose bandwidth is centered at the switching frequency of the transistors, the band-pass filter being connected in parallel to the switch, the invention relates to a method for precharging a floating capacitor device comprising: - a step of measuring the voltage of the switch, in its open state, filtered by the band-pass filter; - a step of closing the switch if the measured voltage exceeds a predetermined imbalance threshold; - a step of precharging the floating capacitors so as to switch the switchable transistors of each cell at the switching frequency of the transistors, - a comparison step between the voltage of the capacity of the first cell and a predetermined voltage; - a step of opening the switch when said voltage of the capacity of the first cell is substantially equal to the predetermined voltage.

[0030] The invention will be better understood upon reading the detailed description of an embodiment of the invention with reference to the figures:

[0031] [Fig. 1a] is a schematic view of a device composed of two floating capacitor cells known from the prior art;

[0032] [Fig.lb] is a schematic view of a generalization of a device with N floating capacitor cells of the prior art;

[0033] [Fig.2] is another schematic view of a generalization of an N device prior art floating capacitor cells comprising an electrical charge;

[0034] [Fig.3] is a view of the output voltage shape of a capacitor device floating according to the prior art;

[0035] [Fig.4a] is a schematic view, for a 2-level 800 V inverter arm with a control duty cycle identical to [Fig.3], of the shape of the voltage Vouttie when there is balance;

[0036] [Fig.4b] is a schematic view, for a 2-level 800 V inverter arm in the same driving conditions, of the form of the voltage Voutielwhen there is unbalance;

[0037] [Fig.5a] is a schematic view of a floating capacitor device according to the invention comprising a dedicated balancing circuit;

[0038] [Fig.5b] is a schematic view of a floating capacitor device according to the invention comprising a balancing circuit comprising an inductor shared with a low-pass filter necessary for controlling the low-voltage current;

[0039] [Fig.6] is a schematic representation of the operating method of a circuit imbalance detection according to one embodiment of the invention;

[0040] [Fig.7a] is a schematic representation of a converter powered by a direct voltage source connectable according to an embodiment of the invention;

[0041] [Fig.7b] is a schematic representation of a DC-DC converter voltage booster, called Boost, according to one embodiment of the invention; and

[0042] [Fig.8] represents in the form of a time graph a precharge protocol for a power fluctuation compensation circuit ([Fig.7a]) according to one embodiment of the invention.

[0043] Figures 1a and 1b respectively represent the topology of a two-level inverter arm with nested cells (or floating capacity) and its generalization to N cells.

[0044] This topology, known to those skilled in the art, has several advantages:

[0045] - The spectral content of the supplied voltage is much less rich because the spectrum harmonic of the switching starts at N times the switching frequency of the transistors for an amplitude N times lower.

[0046] [Fig.3] represents the voltage produced for a 2-cell arm supplied with 800V (for a triangular voltage setpoint ranging from 0.1*800 = 80V to 0.9*800V = 720V): the amplitude of the edges is 400V while the frequency of the voltage square waves is twice the switching frequency of the arm's transistors.

[0047] This modification of the spectrum compared to that of an arm with a two-level switching cell makes it possible to significantly reduce the size of the filtering necessary to attenuate these harmonics linked to the switching of the transistors.

[0048] It thus makes it possible to put several transistors in series while guaranteeing balancing of the voltage applied to each of them, to the extent that the voltage of each floating capacitor is stabilized at its equilibrium voltage (Vdc / N, 2 Vdc / N, .... (Nl) Vdc / N).

[0049] In fact, it becomes possible to use under high voltage (typically 800V) lower voltage components but with better switching capabilities at high frequency (typically HEMT components - "High Electron Mobility Transiter" in English - based on GaN).

[0050] These advantages make it possible to significantly improve high voltage converters which require high regulation dynamics of the control of the delivered current, the main ones are:

[0051] - Converters responsible for compensating for power fluctuation ins tantalized for single-phase chargers (where the instantaneous power which is pulsed at 2 times the network frequency produces current ripples in the battery under charge without compensation),

[0052] - Boost converters, typically for associating in parallel several voltage sources, a fuel cell and a battery for example.

[0053] - Inverters for which a wide bandwidth of the delivered voltage is required for compensate for spatial deformations intrinsic to the machine to be controlled. For example, an iron powder machine.

[0054] For these advantages to remain valid, it is necessary to constantly guarantee good balancing of the voltage of the floating capacitors, and in particular:

[0055] - In the precharge phase, where the Vdc voltage increases (slowly via a system of precharge) from 0 to 800V for example, the vehicle is stationary and the multilevel converters do not supply current which compromises natural balancing; and

[0056] - During operation, in the event of a strong transient on the Vdc voltage, or in the event dispersion of the value of floating capacities (which must be of the same value).

[0057] Thus the floating capacitors are suitably balanced when their voltage is at Vdc / 2 in the case of 2 cells or M*Vdc / N in the case of the Mth cell when there are N nested switching cells.

[0058] When this balance is reached, each transistor then blocks the same voltage equal to Vdc / N.

[0059] During the precharge phase, when the voltage Vdc increases slowly, it is necessary to ensure that the voltage of the floating capacitors evolves in the same ratio.

[0060] Knowing that in this phase, with reference to [Fig. 1b] and 2, the multicellular arm does not deliver current, Icharge = 0, the floating capacities C;=l to Nl cannot balance.

[0061] [Fig.2] shows that the current ICM of a floating capacitor of index M can be at a given instant either zero or equal to Icharge or -Icharge depending on the state of the transistors of the cells M and Ml which surround it.

[0062] When the load has too high an impedance, the ICM current is insufficient to vary the voltage of the CM capacitor with sufficient dynamics compared to that of the precharge, and therefore to protect the transistors from a destructive overvoltage.

[0063] To ensure permanent balancing of the voltage of the floating capacitors, a resonant circuit 50, also called balancing circuit 50, is added. The resonant frequency of the balancing circuit 50 is equal to the switching frequency Fsw of the transistors.

[0064] Thus according to the main embodiment of the invention the device 1, 1', with reference to figures 5a, 5b and 7a and 7b, with floating capacitors, such as an inverter 1 or a DC-DC voltage boost converter 1', comprises a plurality of N switching cells 2, 2' nested within each other, each cell 2, 2' comprising between its connection terminals two switchable transistors 21, 22, 21', 22' in series and a capacitor 23, 23' in parallel with the two transistors 21, 22, 21', 22'.

[0065] The N cells 2, 2' are nested so that a second cell 2 is nested in a first cell 2' by being mounted between the two series transistors 21', 22' of the first cell 2'.

[0066] This nesting is repeated for the N nested cells, the Nth nested cell comprising its two transistors 21, 22 connected in series directly next to each other.

[0067] The output voltage Voutie of said device 1, 1' is thus obtained by the potential difference between the midpoint 70 located between the two transistors 21, 22 of the Nth nested cell, here the second cell 2, and the terminal 71 of the capacitance of the first cell 2'.

[0068] This device 1,1' comprises a balancing circuit 50, here a resonant circuit RLC 50, comprising a capacitance Ceq, an inductance Leq and a resistance Req connected in series. The balancing circuit 50 is adapted to have a resonant frequency equal to the switching frequency of the transistors.

[0069] This balancing circuit 50 is mounted on the one hand at the midpoint 70 between the two transistors of the Nth cell and at the terminal 71 of the capacitor of the first cell 2'.

[0070] This resonant circuit 50 thus makes it possible to control the load impedance at this particular frequency Fsw and in fact ensure the balancing of the floating capacitors whatever the payload impedance.

[0071] According to a particular implementation of the invention, this circuit can be connected and disconnected by a switch.

[0072] [Fig.6] schematically represents a resonant circuit RLC 50 according to the invention connected in series by a four-quadrant switch which can be mechanical or semiconductor.

[0073] Thus, during a precharging process, implemented by the invention, the voltage at the terminals of the high-voltage circuit increases quite slowly.

[0074] From the start of the precharge phase, the transistors are switched so that a current appears in the balancing circuit.

[0075] In a particular embodiment of the invention for a power fluctuation compensator for a single-phase charger or an inverter arm, [Fig.7a] shows the diagram where the battery is on the high-voltage side.

[0076] Once the battery is connected via the closure of its contactor 75, a continuous duty cycle equal to the ratio Vc / Vbat is applied where Vc is the voltage at which the storage capacity Cstorage is desired to be precharged and Vbat the battery voltage at the moment the contactor closes.

[0077] In the case of a DC / DC step-up converter, also called Boost, shown in [Fig.7b], for a hybrid or fuel cell vehicle, the duty cycle applied during precharging will be the ratio Vbat / Vdc where Vbat is the voltage of the battery connected to the low voltage side and Vdc the desired voltage on the high voltage side, for example 800V.

[0078] In the case of an inverter, as shown [Fig.7a], an identical duty cycle is applied to each arm supplying a phase of the motor.

[0079] Thus, only a homopolar current at N times (N being the number of nested cells of each arm) the switching frequency Fsw is created in the phases of the motor without creating torque.

[0080] The balancing circuit 50 can be disconnected by an electronic or electromechanical switch 501 (visible in FIGS. 5a and 5b), so that only the impedance of the converter's payload ensures the balancing of the floating capacitors, which is illustrated by [Fig.6].

[0081] During the precharge phase, the resonant balancing circuit 50 is systematically automatically connected and then disconnected as soon as the arm starts operating, in other words as soon as it delivers current.

[0082] Figure 4 represents, for a 2-level 800 V inverter arm, the shape of the voltage Vout when the floating capacitors are balanced ([Fig.4a]), i.e. a floating capacitor at 400 V, and when the floating capacitors are unbalanced ([Fig.4b]) here a floating capacitor at 300 V for the same triangular modulator as represented in [Fig.3].

[0083] In addition to the overvoltage applied to 2 transistors, we distinguish the appearance of a harmonic at the switching frequency Fsw, while the 1st harmonic is at 2*FSW in the case of equilibrium.

[0084] In fact, a resonant circuit RLC 50 tuned to this frequency Fsw connected in parallel with the load makes it possible to circulate exclusively a current dedicated to balancing with a minimum current load.

[0085] [Fig.6] also shows the principle of an imbalance detection circuit via the processing of the measurement of the voltage 61 at the terminals of the open switch. Beyond a certain imbalance threshold 63, that is to say if the load impedance is unsuitable for ensuring the natural balancing of the floating capacitors, the control of the system 64 can order the closing of the switch 501 to ensure this rebalancing.

[0086] Measuring the voltage 61 across the terminals of the open switch 501 of the balancing RLC circuit 50 makes it possible to measure this voltage and, via a bandpass filter 62 centered on the frequency Fsw, to detect the amplitude of the harmonic at Fsw (or close to + / - 15% for example) which is an unbalance marker.

[0087] The imbalance is generally considered significant when it exceeds 10% to 20% of Vdc / N (i.e. 40 to 80 V in the case of 2 levels for Vdc = 800V which is presented).

[0088] This solution is advantageous because it only requires a simple analog circuit permanently referenced to the base of the inverter arm (unlike a direct measurement of the voltage of the floating capacitors).

[0089] [Fig.8] represents an example of a pre-charging protocol for a power fluctuation compensation circuit. The balancing circuit is connected 80 from the start of the pre-charging 81 at the same time as the switching orders are launched. The intermediate capacitor 86 is charged thanks to the current in the balancing circuit.

[0090] It is noted that the balancing 86 ceases as soon as the circuit is disconnected 82 and resumes after its reconnection 83.

[0091] Thus, through this balancing phase in operation, the following advantages are noted.

[0092] During operation, imperfect balancing of floating capacitors can produce a significant current in the balancing circuit for a long period of time. (typically several seconds). It is advantageous in this case to disconnect this circuit in order to allow natural balancing to take place via the current harmonics in the load.

[0093] Thus, once the precharge phase 81 has been carried out, the balancing circuit is disconnected 82 just before the start of the conversion.

[0094] If an imbalance threshold is exceeded during operation, the control orders the closing of the switch 501 in order to reconnect the balancing circuit 50, as shown [Fig.6], for a given duration before reopening the contact.

[0095] The detection threshold as well as the connection duration of the balancing circuit can be variable and depend on other measured parameters, in addition to the voltage across the contact terminals, such as the load current.

[0096] According to one embodiment of the invention, components can be combined and shared between the RLC balancing circuit according to the invention and a filtering circuit.

[0097] For this purpose, [Fig.5a] represents a dedicated balancing circuit and [Fig.5b] represents a balancing circuit comprising an inductor shared with the low-pass filtering 505 necessary for controlling the low-voltage current.

[0098] Since inductors are the largest and most dissipative passive components, sharing this component between the two functions can present a relatively significant gain in volume.

Claims

Claims

1. Device (1, 1') with floating capacitors, such as an inverter (1) or a DC-DC voltage boost converter (1'), comprising a plurality of N switching cells (2, 2') nested within each other, each cell (2, 2') comprising between its connection terminals two switchable transistors (21, 22, 21', 22) in series and a capacitor (23, 23') in parallel with the two transistors (21, 22, 21', 22'), the N cells (2, 2') being nested so that a second cell (2) is nested in a first cell (2') by being mounted between the two series transistors (21', 22') of the first cell (2');this nesting being repeated for the N nested cells, the Nth nested cell comprising its two transistors (21, 22) mounted in series directly next to each other, an output voltage of said device (1, 1') being obtained by the potential difference between the midpoint (70) located between the two transistors (21, 22) of the Nth nested cell and a terminal (71) of the capacitance (23') of the first cell (2'); the device (1, 1') comprising a balancing circuit (50) having a resonant frequency equal to the switching frequency (Fsw) of the transistors, said balancing circuit (50) being mounted on the one hand at said midpoint (70) between the two transistors of the Nth cell and at said terminal (71) of the capacitance of the first cell (2'); said balancing circuit (50) comprises an RLC resonant circuit (50) in series and a switch (501) connected in series with said RLC resonant circuit (50);the device (1, 1') being characterized in that it further comprises a band-pass filter (62) whose passband is centered at the switching frequency of the transistors, the band-pass filter (62) being connected in parallel to the switch (501).;

2. Floating capacitor device (1, 1') according to claim 1, characterized in that the device (1, 1') comprises two switching cells (2, 2').

3. A floating capacitor device according to claim 1 or 2, further comprising a low-pass filter (505), characterized in that said balancing circuit (50) comprises at least one inductor shared with said low-pass filter.

4. A power conversion system for a vehicle, comprising at least one floating capacitor device according to any one of the re- previous claims.

5. A vehicle comprising a power conversion system according to claim 4.

6. Method for precharging a floating capacitor device according to any one of claims 1 to 3, characterized in that it comprises: - a step of measuring the voltage of the switch (501), in its open state, filtered by the bandpass filter (62); - a step of closing the switch (501) if the measured voltage exceeds a predetermined imbalance threshold; - a precharging step (85) of the floating capacitors (23, 23') so as to switch the switchable transistors (21, 22, 21', 22') of each cell (2, 2') at the switching frequency of the transistors, - a step of comparison between the voltage of the capacitance (23') of the first cell (2') and a predetermined voltage; - a step of opening the switch (501) when said voltage of the capacitance of the first cell is substantially equal to the predetermined voltage.

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