Flying capacitor device and method for preloading a capacitor of such a device - Patents.com

Abstract

A flying capacitor device (1, 1'), such as an inverter arm (1) or a boost converter (1'), comprises a plurality of N switching cells (2, 2') nested together and a balancing circuit (50) configured to have a resonant frequency equal to the switching frequency of the transistors, said balancing circuit (50) being connected on the one hand to said midpoint (70) and to said terminal (71) of the capacitor of the last cell (2').

Description

Summary of the Invention

[0001] The present invention relates to flying capacitor devices such as inverters or boost converters.

[0002] In the field of electric motor vehicles, it is known that 800V electric traction systems impose constraints on semiconductors which are normally made for a cut-off voltage of 1200V.

[0003] However, using 650V components in series, via a multi-cell, or flying capacitor, topology, is known to have several advantages: -The use of lower voltage components, such as gallium nitride transistors, has a better figure of merit for operating at higher frequencies; -This is due, on the one hand, to the low intensity of the switching edges, and on the other hand, to the fact that the first harmonic resulting from switching is N*F SW This allows the output voltage to have a more favorable harmonic spectrum, since the harmonic spectrum is boosted to F SW where is the switching frequency of the transistor and N is the number of switching cells, this allows significant benefits in terms of filtering volume and in terms of the ability to control the output current.

[0004] Thus, multi-cell topologies are known from the prior art. Figure 1 from the prior art depicts in figure 1a an arm with two cells and in figure 1b its generalization to N cells.

[0005] On the one hand, the frequency of the first harmonic is the switching frequency of the transistor multiplied by the number of cells.

[0006] In the technical field, this performance is particularly advantageous for: - On-board single-phase charger for the boost system to compensate for fluctuations in instantaneous power by increasing the power to regulate the current and by using 650V components to cope with 800V networks; - Boost converters for powering networks with several sources of different voltages in parallel. Typically fuel cells and accumulators. In practice, the benefits in terms of filtering volume are significant, as well as the efficiency of using transistors with large gaps, for example gallium nitride transistors, as well as -An inverter for controlling the device of the present invention, for which a wide passband is necessary to distribute harmonic voltages that are multiples of the fundamental electrical frequency, while at the same time making it possible to filter out the spectrum resulting from switching.

[0007] However, such a topology requires a V dc / N, 2V dc / N, 3V dc This has the disadvantage that the stability of the voltage on the flying capacitor must be guaranteed with an average value over a period of at least 1 / Fsw, such as / N.

[0008] This balancing occurs naturally by assuming load current harmonics at the transistor switching frequency and at (N-1) times that frequency, where N is the number of nested switching cells.

[0009] However, this balancing is insufficient or even non-existent in two cases: - Preloading the high-voltage circuit prior to connecting the accumulator. In this case, the first capacitor charges while the multi-level arm does not draw current: natural balancing cannot occur, only the first capacitor is charged, which in fact causes a voltage V dc To force. In operation, if the load impedance is insufficient, the rebalancing of the flying capacitor voltages is too slow or even non-existent.

[0010] Thus, there is a need for a solution for balancing the flying capacitor voltages in the two cases outlined above.

[0011] To this end, a flying capacitor device is proposed, such as an inverter or boost converter, comprising a number N of switching cells nested together, each cell comprising, between its connecting terminals, two switchable transistors in series and a capacitor in parallel with said two transistors, said N cells being nested such that a second cell is nested in a first cell by being connected between the two transistors in series of the first cell, said nesting being repeated for said N nested cells, the Nth nested cell in turn comprising two transistors connected in series directly one after the other, the output voltage of said device being obtained from the potential difference between a midpoint located between the two transistors of the Nth nested cell and the terminals of the capacitor of the first cell.

[0012] The flying capacitor device comprises a balancing circuit having a resonant frequency equal to the switching frequency of the transistors, the balancing circuit being connected on the one hand to the midpoint between the two transistors of the Nth cell and on the other hand to the terminal of the capacitor of the first cell.

[0013] Advantageously, the balancing circuit comprises a series RLC resonant circuit, in other words a resonant circuit comprising a resistor, an inductor and a capacitor connected in series, thus making the device relatively cheap to manufacture and particularly reliable.

[0014] In particular, the device comprises only two switching cells.

[0015] Advantageously, the flying capacitor device comprises a bandpass filter and the balancing circuit comprises at least one capacitor shared with the bandpass filter, thus resources may be shared between the bandpass filter and the balancing circuit.

[0016] Advantageously, the balancing circuit comprises a switch connected in series with the RLC circuit. In this way, the duration of forced or natural balancing of the flying capacitors may be controlled.

[0017] Advantageously, the flying capacitor device comprises a bandpass filter whose passband is centred on the switching frequency of the transistor, the bandpass filter being connected in parallel with the switch.

[0018] In particular, the device comprises two switching cells.

[0019] Advantageously, the flying capacitor device further comprises a low pass filter, said balancing circuit comprising at least one inductor shared with said low pass filter.

[0020] The invention also relates to a power conversion system comprising at least one flying capacitor device according to any one of the preceding claims.

[0021] The invention also relates to a vehicle equipped with a power conversion system as described above. The invention also relates to a method for preloading a flying capacitor device as described above, in particular when said balancing circuit comprises a switch connected in series with said RLC resonant circuit, comprising: - closing said switch; - preloading said flying capacitor to switch the switchable transistor of each cell at the switching frequency of the transistor; - comparing the voltage of the capacitor of a first cell with a predetermined voltage; - opening the switch when the voltage on the capacitor of the first cell is substantially equal to the predetermined voltage; Includes. According to an alternative, the flying capacitor device comprises a bandpass filter whose passband is centred on the switching frequency of the transistor, when the bandpass filter is connected in parallel with the switch, the invention relates to a method for preloading a flying capacitor device, comprising: - measuring the voltage across the switch, filtered by the bandpass filter, when the switch is open; - closing said switch if said measured voltage exceeds a predetermined imbalance threshold; - preloading the flying capacitor to switch the switchable transistor of each cell at the switching frequency of the transistor; - comparing the voltage of the capacitor of the first cell with a predetermined voltage; - opening the switch when the voltage on the capacitor of the first cell is substantially equal to the predetermined voltage; Includes.

[0022] The present invention will be better understood upon reading the detailed description of the embodiments of the invention with reference to the figures. [Brief description of the drawings]

[0023] [Figure 1a] FIG. 1 is a schematic diagram of a device consisting of two flying capacitor cells known from the prior art. [Figure 1b]FIG. 1 is a schematic diagram of a generalization of a device from the prior art to N flying capacitor cells. [Diagram 2] FIG. 2 is another schematic diagram of a generalization of a device from the prior art including an electrical load to N flying capacitor cells. [Diagram 3] FIG. 2 is a diagram of the output voltage shape of a flying capacitor device according to the prior art. [Figure 4a] FIG. 4 is a schematic diagram of the shape of the voltage Voutput when balanced for a two-level 800V inverter arm with the same control duty cycle as in FIG. 3. [Figure 4b] FIG. 13 is a schematic diagram of the shape of the voltage Voutput when unbalanced for a two-level 800V inverter arm under the same control conditions. [Figure 5a] FIG. 2 is a schematic diagram of a flying capacitor device according to the present invention with a dedicated balancing circuit. [Figure 5b] FIG. 2 is a schematic diagram of a flying capacitor device according to the present invention with a balancing circuit including a shared inductor with a low pass filter that is necessary to control the low voltage current. [Figure 6] 4 is a schematic illustration of a method of operation of a circuit for detecting imbalance according to one embodiment of the present invention. [Figure 7a] 1 is a schematic drawing of a converter powered by a DC voltage source that may be connected according to one embodiment of the present invention. [Figure 7b] 1 is a schematic depiction of a boost converter according to one embodiment of the present invention. [Figure 8] 7 depicts, in the form of a time graph, a preloading protocol for a circuit for compensating power fluctuations according to one embodiment of the invention (FIG. 7a). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] 1a and 1b depict the topology of a two-level inverter arm with nested cells (or flying capacitors) and its generalization to N cells, respectively.

[0025] This topology, known to those skilled in the art, has several advantages: -The spectral content of the distributed voltage is much poorer since the harmonic spectrum of the chopping is N times smaller in magnitude and begins at N times the switching frequency of the transistors.

[0026] FIG. 3 illustrates the voltage generated for an arm having two cells supplied with 800V (for triangular voltage settings ranging from 0.1*800=80V to 0.9*800V=720V): the magnitude of the edges is 400V, while the frequency of the voltage pulses is twice the switching frequency of the transistors of the arm.

[0027] This modulation of the spectrum relative to the spectrum of the arm with switching cells having two levels makes it possible to significantly reduce the size of the filtering that is necessary to attenuate these harmonics linked to the switching of the transistors.

[0028] In this way, it is possible to place several transistors in series while at the same time ensuring that the voltage on each flying capacitor is equal to its balancing voltage (V dc / N, 2V dc / N,...(N-1)V dc / N), ensuring balancing of the voltages applied to each of the transistors.

[0029] In fact, at high voltages (typically 800V) it becomes possible to use lower voltage components (typically GaN-based HEMT - High Electron Mobility Transistor - components) which have excellent high-frequency switching capabilities.

[0030] These advantages allow to significantly improve high-voltage converters that require high regulation of the distributed current, the main ones being: -A converter that is charged by compensating for the instantaneous power fluctuations associated with a single-phase charger (where the instantaneous power pulsating at twice the network frequency generates a current ripple in the accumulator that is charged without compensation). - A boost converter, typically associated with several voltage sources in parallel, e.g. a fuel cell and a battery. - Inverters that require a wide passband of the distributed voltage to compensate for spatial variations that are inherent in the machines to be controlled, e.g. iron powder equipment.

[0031] For these advantages to remain effective, excellent balancing of the flying capacitor voltages must be permanently guaranteed, in particular: - Voltage V dc But during the preloading phase, which increases (slowly, via the preloading system) from, say, 0V to 800V, the vehicle is stationary and the multilevel converter does not draw any current that would jeopardize the natural balancing; - Operating voltage V dc in the case of strong transitions or when multiple flying capacitors vary in value (which should be the same value).

[0032] Therefore, the flying capacitors have a voltage of V in the two-cell case. dc / 2 or M*V in the case of the Mth cell when there are N nested switching cells. dc / N to balance properly.

[0033] When this balance is reached, each transistor is then at V dc / N.

[0034] During the preloading phase, the voltage V dcAs V increases slowly, it is necessary to ensure that the voltage on the flying capacitor increases at the same rate.

[0035] In this phase, referring to Fig. 1b and Fig. 2, the multicell arm does not draw any current, I load = 0, and the flying capacitor C i =1 to N-1 cannot be balanced.

[0036] Figure 2 shows the current I of a flying capacitor with index M. CM At a given moment, I is either zero or depends on the state of the transistors of cells M and M-1 surrounding the flying capacitor. load Or -I load or

[0037] If the load has too high an impedance, the current I CM is sufficient to counter the preloading force and the capacitor C M is insufficient to cause the voltage at the transistor to change, thus protecting the transistor from destructive overvoltages.

[0038] To permanently balance the voltages of the flying capacitors, a resonant circuit 50, also called the balancing circuit 50, is added. The resonant frequency of the balancing circuit 50 is set to the switching frequency F SW is equal to.

[0039] Thus, according to a main embodiment of the present invention, with reference to Figures 5a, 5b and 7a and 7b, a flying capacitor device 1, 1', such as an inverter 1 or boost converter 1', comprises a plurality of N switching cells 2, 2' nested together, 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'.

[0040] The N cells 2, 2' are nested such that the second cell 2 is nested in the first cell 2' by being connected between the two transistors 21', 22' in series of the first cell 2'.

[0041] This nesting is repeated for N nested cells, with the Nth nested cell containing its own two transistors 21, 22 connected in direct series one after the other.

[0042] The output voltage V of the devices 1 and 1' output is therefore obtained from the potential difference between a 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 capacitor of the first cell 2'.

[0043] The device 1, 1' comprises a balancing circuit 50, here an RLC resonant circuit 50, including a capacitor Ceq, an inductor Leq and a resistor Req connected in series. The balancing circuit 50 is configured to have a resonant frequency equal to the switching frequency of the transistors.

[0044] This balancing circuit 50 is connected on the one hand to a midpoint 70 between the two transistors of the Nth cell and to a terminal 71 of the capacitor of the first cell 2'.

[0045] This resonant circuit 50 thus generates a resonance at this particular frequency F SW This allows one to control the load impedance at the 100 MHz frequency band, in effect allowing one to balance multiple flying capacitors whatever the payload impedance.

[0046] According to one particular implementation of the invention, this circuit may be connected and disconnected by a switch.

[0047] FIG. 6 illustrates diagrammatically an RLC resonant circuit 50 according to the invention connected in series with a four-quadrant switch, which may be a mechanical or semiconductor switch.

[0048] Thus, during the preloading method implemented in accordance with the present invention, the voltage at the terminals of the high voltage circuit increases very slowly.

[0049] From the beginning of the preloading phase, the transistors are switched so that a current appears in the balancing circuit.

[0050] In one particular embodiment of the present invention relating to a power fluctuation compensator for a single-phase charger or inverter arm, FIG. 7a displays a diagram where the accumulator is on the high voltage side.

[0051] After the battery is connected by closing its contactor 75, the ratio V C / V bat A DC duty cycle is applied that is equal to V C is the storage capacitor C storage is the desired voltage to be preloaded, and V bat is the battery voltage at the moment the contactor closes.

[0052] In the case of a boost converter depicted in Figure 7b for a hybrid vehicle or fuel cell, the duty cycle applied during preloading is the ratio V bat / V dc Here, V bat is the voltage of the battery connected to the low-voltage side, and V dc is the voltage desired on the high voltage side, e.g. 800V.

[0053] In the case of the inverter depicted in FIG. 7a, the same duty cycle is applied to each arm that supplies power to a phase of the motor.

[0054] Thus, the switching frequency F SW Only one homopolar current of N times (where N is the number of nested cells in each arm) can be produced in the multiple phases of the motor without producing any torque.

[0055] The balancing circuit 50 may be disconnected by an electronic or electromechanical switch 501 (seen in Figures 5a and 5b) so that only the impedance of the converter's payload balances the multiple flying capacitors, as illustrated in Figure 6.

[0056] During the preloading phase, the resonant balancing circuit 50 is systematically connected and then disconnected from the beginning of the movement of the arm, in other words, from the time when the resonant balancing circuit 50 draws current.

[0057] Figure 4 shows the voltage V for a two-level 800V inverter arm when the flying capacitors are balanced (Figure 4a), or at 400V, and when the flying capacitors are unbalanced (Figure 4b), here at 300V for the same triangular modulant as depicted in Figure 3. output Draw the shape of.

[0058] In addition to the overvoltages applied to the two transistors, the switching frequency F SW The appearance of harmonics at 1000 Hz may be distinguished, while the first harmonic is 2*F in the balanced case. SW It is.

[0059] In reality, this frequency F SW The RLC resonant circuit 50 tuned to and connected in parallel with the load allows a dedicated current to flow exclusively to at least balance the load current.

[0060] 6 also shows the principle of a circuit for detecting imbalance by processing the measurement of the voltage 61 at the terminals of an open switch. If a certain imbalance threshold 63 is exceeded, i.e. if the load impedance is not suitable for naturally balancing the flying capacitors, the control of the system 64 can command the closing of switch 501 to ensure its rebalancing.

[0061] Measuring the voltage 61 at the terminals of the open switch 501 of the RLC balancing circuit 50 makes it possible to measure this voltage and to measure the frequency F SW A marker of imbalance, F SW This allows one to detect the intensity of the harmonics at (or within, for example, + / - 15%).

[0062] Imbalance is V dc 10% to 20% of / N (or the indicated V dc The imbalance is generally considered to be significant when it exceeds 100 V (40 to 80 V in the two-level case for =800 V).

[0063] This solution is advantageous because it requires only one simple analog circuit that is permanently referenced based on the inverter arm (as opposed to a direct measurement of the flying capacitor voltage).

[0064] 8 illustrates an example of a preloading protocol for a circuit for compensating power fluctuations. The balancing circuit is connected 80 from the beginning of preloading 81 as soon as a switching command is issued. An intermediate capacitor 86 charges with the current of the balancing circuit.

[0065] It can be seen that balancing 86 ceases when the circuit is disconnected 82 and resumes after the circuit is reconnected 83.

[0066] Thus, with this balancing phase in operation, the following advantages are realized:

[0067] In operation, imperfect balancing of the flying capacitors can generate significant currents in the balancing circuit for long periods of time (typically several seconds). In this case, it is advantageous to disconnect this circuit to allow natural balancing to occur via current harmonics in the load.

[0068] Thus, after the preloading phase 81 is performed, the balancing circuitry is disconnected 82 just before conversion begins.

[0069] If the unbalance threshold is exceeded during operation, the controller commands switch 501 to be closed to reconnect balancing circuit 50, as depicted in FIG. 6, for a given period of time before reopening the contacts.

[0070] The detection threshold as well as the duration of connection of the balancing circuit may be variable and may be a function of the voltage at the terminals of the contacts as well as other measured parameters such as the load current.

[0071] According to one embodiment of the present invention, components may be combined or shared between the RLC balancing circuitry and the filtering circuitry according to the present invention.

[0072] To this end, FIG. 5a depicts a dedicated balancing circuit, and FIG. 5b depicts a balancing circuit with a shared inductor and low-pass filtering 505 that is necessary to control the low-voltage current.

[0073] Since the inductor is the bulkiest and most dissipative passive component, sharing this component between the two functions can offer a relatively significant benefit in terms of volume.

Claims

1. A flying capacitor device (1, 1'), such as an inverter (1) or a boost converter (1'), comprises a plurality of N switching cells (2, 2') nested one inside the other, each cell (2, 2') comprising, between its connecting terminals, two switchable transistors (21, 22, 21', 22') in series and a capacitor (23, 23') in parallel with said two transistors (21, 22, 21', 22'), and a second cell (2) is connected between the two series transistors (21', 22') of a first cell (2') to thereby a flying capacitor device (1,1') in which the N cells (2,2') are nested such that the first cell (2') is nested within the second cell (2'), the nesting being repeated for the N nested cells, the Nth nested cell comprising two transistors (21,22) connected in series in series with one another, the output voltage of the device (1,1') being derived from the potential difference between a midpoint (70) located between the two transistors (21,22) of the Nth nested cell and a terminal (71) of the capacitor (23') of the first cell (2'), The device (1, 1') operates at the switching frequency (F ＳＷ ), the balancing circuit (50) having a resonant frequency equal to (N / N) / (N / N), the balancing circuit (50) being connected on the one hand to the midpoint (70) between the two transistors of the Nth cell and to the terminal (71) of the capacitor of the first cell (2'). Flying capacitor device (1, 1').

2. 2. The flying capacitor device (1, 1') according to claim 1, characterized in that the balancing circuit (50) comprises a series RLC resonant circuit (50).

3. 3. The flying capacitor device (1, 1') according to claim 2, characterized in that the balancing circuit (50) comprises a switch (501) connected in series with the RLC resonant circuit (50).

4. 4. The flying capacitor device (1, 1') of claim 3, further comprising a bandpass filter (62) whose passband is centered on the switching frequency of the transistor, the bandpass filter (62) being connected in parallel with the switch (501).

5. Flying capacitor device (1, 1') according to any one of claims 1 to 4, characterized in that the device (1, 1') comprises two switching cells (2, 2').

6. 5. The flying capacitor device of claim 1, further comprising a low-pass filter (505), wherein the balancing circuit (50) comprises at least one inductor shared with the low-pass filter.

7. A power conversion system for a vehicle comprising at least one flying capacitor device according to any one of claims 1 to 4.

8. A vehicle comprising the power conversion system of claim 7.

9. 5. The method for preloading a flying capacitor device according to claim 3 or 4, further comprising: - closing said switch (501); - a step (85) of preloading the flying capacitors (23, 23') to switch the switchable transistors (21, 22, 21', 22') of each cell (2, 2') at the switching frequency of the transistors; - comparing the voltage of the capacitor (23') of the first cell (2') with a predetermined voltage; - opening said switch (501) when said voltage on said capacitor of said first cell is substantially equal to said predetermined voltage; A method comprising:

10. 5. The method for preloading a flying capacitor device of claim 4, comprising: - measuring the voltage of the switch (501) filtered by the band-pass filter (62) when the switch (501) is open; - closing said switch (501) if said measured voltage exceeds a predetermined imbalance threshold; - a step (85) of preloading the flying capacitors (23, 23') to switch the switchable transistors (21, 22, 21', 22') of each cell (2, 2') at the switching frequency of the transistors; - comparing the voltage of the capacitor (23') of the first cell (2') with a predetermined voltage; - opening said switch (501) when said voltage on said capacitor of said first cell is substantially equal to said predetermined voltage; A method comprising:

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