DC-DC Converters for Electric Aircraft Propulsion Systems

Abstract

The present invention relates to a DC-DC converter (209) for an electric aircraft propulsion system, the DC-DC converter (209) being designed to be connected in series with an electric energy storage unit of the electric propulsion system. The DC-DC converter (209) comprises an inverter (401), a transformer (403) and a rectifier (405), further comprising a current source (417) connected to the rectifier (405) and configured to control the power passing through the DC-DC converter (209). The transformer (403) comprises a primary coil (403a) and two secondary coils (403b, 403c), the two secondary coils (403b, 403c) sharing a common terminal (407) designed to be connected to a high voltage DC bus (207) of the electric propulsion system (201) and two other terminals (409, 411) connected to the rectifier (405). The rectifier (405) comprises two arms each comprising at least two transistors (415a, 415b, 415c, 415d) in series and connected on the one hand to the two other terminals (409, 411) of the transformer (403) and on the other hand to a current source (417).

Description

[Technical field]

[0001] The present invention relates to the field of electric aircraft propulsion systems, and more particularly to DC-DC converters for such networks. [Background technology]

[0002] Hybrid aircraft propulsion architectures, commonly referred to as series hybridization, are known from the prior art that make use of a shared source thermoelectric and an electrical energy storage unit connected to the same High Voltage Direct Current (HVDC) bus that supplies the aircraft loads.

[0003] In the following, the initials DC stand for "direct current" and AC for "alternating current".

[0004] As shown in FIG. 1, a hybrid propulsion architecture 101 generally comprises: an internal combustion engine 103 controlled by a control unit 105; a generator 107 coupled to the internal combustion engine 103 such that, in operation, the internal combustion engine 103 drives the generator 107; a rectifier 109 connected to the generator 107 and configured to convert the alternating current provided by the generator 107 (in this case a three-phase current) into a direct current; - DC-AC converters (which convert alternating current into direct current) 113a and 113b; - a high voltage DC bus 111 connecting the rectifier 109 to the DC-AC converters 113a and 113b; electric motors 115a and 115b connected to the DC-AC converters 113a and 113b such that, in operation, the DC-AC converters 113a and 113b supply the electric motors 115a and 115b with an alternating current; In operation, the electric motors 115a and 115b are coupled to the propellers 117a and 117b to drive the propellers 117a and 117b. Equipped with.

[0005] The architecture 101 further comprises an electric energy storage unit 119, also known as HVDC storage, meaning a high voltage DC storage such as a battery. This electric storage unit can perform the following functions: absorb excess electric energy from the HVDC bus, provide additional electric energy during a transitoire phase, or act as a main energy source together with the combustion engine or as a replacement for the combustion engine, for example in case of a failure. In particular, when a return of electric energy occurs on the HVDC bus, the storage unit 119 absorbs this excess electric energy in order to protect the components of the HVDC bus.

[0006] In such an architecture, the internal combustion engine 103, generator 107, and electric propulsion chain consisting of DC-AC converters 113a and 113b, electric motors 115a and 115b, and propellers 117a and 117b enable the aircraft to fly with multiple rotors using fossil fuel sources.

[0007] An aircraft with such a hybrid propulsion architecture is a multi-rotor that allows for additional freedom in terms of aircraft controllability, e.g., braking, avoidance maneuvers, changing direction, or tilting of the rotors, compared to conventional aircraft.

[0008] Other examples of this type of architecture are described in patent EP-B1-3519294 and patent application WO-A1-2019186042, in which multiple intermediate power generators are connected in parallel to the HVDC bus. Furthermore, in various known architectures, the HVDC bus may further be multiple or connected in a ring network.

[0009] In either case, such architecture may be used for VTOL (vertical take-off and landing) type aircraft or for CTOL (conventional take-off and landing) type aircraft.

[0010] In this type of architecture, where the loads connected to the HVDC bus are typically electric motors driving aircraft propellers operating at constant power, it is essential to maintain optimal control of the HVDC bus to avoid unstable conditions or even a complete loss of the bus due to a voltage collapse.

[0011] It is therefore known to use electrical energy storage units, as mentioned above, to enable this control.

[0012] Furthermore, this electrical energy storage unit may or may not be associated with a DC-DC converter, especially responsible for adapting the voltage and current levels delivered by the storage unit.

[0013] In the first approach, the electric energy storage unit is connected to the rest of the electric propulsion network without the use of a DC-DC converter – this is known as a direct connection.

[0014] The direct connection of one or more batteries (i.e. electrical energy storage units) allows the mass of the entire electrical network to be minimized. Indeed, the addition of a DC-DC converter has a cost not only in terms of mass, but also in terms of volume, efficiency, heat dissipation and control complexity.

[0015] However, due to the electrochemical nature of the batteries, this configuration imposes limitations to protect the batteries, since the main voltage is imposed by the battery(s) which all "see" the same voltage.

[0016] Furthermore, the voltage level of the network affects the state of charge of the battery(ies): when the latter changes, the charge of the battery changes accordingly.

[0017] Finally, in a direct connection configuration, the state of charge of batteries connected to the same electrical network is identical. This means that when additional batteries are connected to the electric propulsion system, the voltages must be balanced and high currents may flow between the batteries during a period known as the transition regime, which may interrupt or degrade the operation of the elements of the network. The impact of this transition regime can be limited by the use of dedicated systems, but these systems are potentially heavy and bulky.

[0018] The second approach is to use a DC-DC converter at the interface between the electric energy storage unit and the rest of the electric propulsion system.

[0019] The addition of such a DC-DC converter offers several advantages. First, the voltage of the electric energy storage unit can be decoupled from the voltage of the rest of the electric propulsion system, allowing to maintain different voltage levels between the energy storage unit and the voltage level of the HVDC bus. Second, the DC-DC converter can be step-up and / or step-down, i.e., the DC-DC converter can make it possible to increase or decrease the voltage at its output with respect to the voltage that the electric energy storage unit provided as input. Third, when the DC-DC converter is of the reversible current type, it can also control the energy level, also known as the state of charge (SOC), of the energy storage unit.

[0020] This allows the parallel use of several storage units, each with its own DC-DC converter, independent of each other and each protected from current fluctuations. It further allows easy connection of new energy storage units without the risk of triggering potentially harmful transition regimes.

[0021] Such a DC-DC converter at the interface between the electric energy storage unit and the rest of the electric propulsion system further allows, where appropriate, the addition of galvanic isolation properties (i.e. the absence of a conductive link between two parts of the electric network).The DC-DC converter then has a so-called full bridge structure, for example a controlled full bridge structure known as DAB, standing for "Dual Active Bridge".

[0022] However, as mentioned above, taking all these characteristics into account comes at a cost in terms of mass, volume, performance and control complexity. In particular, known DC-DC converters are designed to transfer the full power of the electrical energy storage unit, which necessarily implies a large amount of passive components and a high rated current / voltage (i.e. ability to transfer current / voltage) depending on the application. Moreover, the efficiency of such systems rarely exceeds 90%. [Prior art documents] [Patent documents]

[0023] [Patent Document 1] European Patent No. 3519294 [Patent Document 2] International Publication No. 2019 / 186042 Summary of the Invention [Problem to be solved by the invention]

[0024] The present invention proposes a solution to these disadvantages. [Means for solving the problem]

[0025] To this end, according to a first aspect, the invention provides a DC-DC converter for an electric aircraft propulsion system, the DC-DC converter being designed to be connected in series with an electric energy storage unit of said electric propulsion system and comprising an inverter configured to provide a first AC voltage from a DC input voltage coming from the electric energy storage unit, a transformer configured to provide at least one second AC voltage from the first AC voltage and a rectifier configured to provide a DC output voltage from said at least one second AC voltage, the DC-DC converter further comprising a current source coupled to the rectifier and configured to control power passing through the DC-DC converter; the transformer comprises a primary coil and two secondary coils, the two secondary coils having a common terminal designed to be connected to a high voltage DC (HVDC) bus of the electric propulsion system and two other terminals connected to a rectifier, and the reference level of the DC-DC converter and the reference level of the HVDC bus are connected to each other through a current source; The rectifier comprises two arms, each of which comprises at least two transistors in series, connected on the one hand to the two other terminals of the transformer and on the other hand to a current source; The present invention relates to a DC-DC converter.

[0026] A DC-DC converter according to the invention may include one or more of the following features, either isolated from one another or combined with one another: The current source comprises an inductor and an electric energy storage unit. The inverter comprises several transistors, preferably four transistors of the MOSFET or IGBT type. The transistors have a switching frequency of more than a few tens of kHz, advantageously of the order of magnitude of or more than a hundred kHz. the transformer is configured to step down said at least one second AC voltage with respect to the first AC voltage; - The transformer may be of planar or wound type. the transformer is configured to provide galvanic isolation between a primary coil and two secondary coils of said transformer; The rectifier comprises four transistors, two connected in series to each secondary coil of the transformer, said transistors being of the MOSFET type. - The gates of the two transistors connected in series with each secondary coil are common. - The gates of the two transistors connected in series with each secondary coil are separate. a diode connected in parallel to the transistor is arranged to protect said transistor from overvoltages; At least one filter, preferably of the RC filter type, is connected between the transformer and the rectifier. - a filter, preferably of the RC filter type, is connected in parallel with the transistor to the terminals of each arm of the rectifier.

[0027] According to a second aspect, the invention further relates to an electric aircraft propulsion system comprising at least one thermoelectric supply source and an electric energy storage unit configured to supply electric energy to a high-voltage DC bus designed to supply an electric charge, said electric aircraft propulsion system further comprising a DC-DC converter according to the first aspect.

[0028] The electric propulsion system according to the invention may include one or more of the following features, either in isolation or in combination with one another: The electrical energy storage unit is an electrolytic current source, such as a supercapacitor, or an electrochemical current source, such as a battery. The electric propulsion system further comprises a contactor configured, when activated, to directly connect a high potential of the high-voltage DC bus with a high potential of the electric energy storage unit.

[0029] Finally, in a third aspect, the invention relates to an aircraft comprising an electric propulsion system according to the second aspect.

[0030] The invention will be better understood and other details, features and advantages of the invention will become more apparent on reading the following description of illustrative and non-limiting examples of the invention with reference to the attached drawings, in which: [Brief description of the drawings]

[0031] [Figure 1] FIG. 1 shows a schematic representation of an embodiment of a hybrid propulsion architecture for an aircraft in the prior art. [Diagram 2] FIG. 1 shows a schematic representation of an embodiment of an electric aircraft propulsion system according to the present invention. [Diagram 3] FIG. 1 shows a schematic representation of an embodiment of an electric aircraft propulsion system according to the present invention. [Figure 4] FIG. 2 shows a schematic representation of an embodiment of a DC-DC converter of an electric aircraft propulsion system according to the present invention. [Diagram 5] FIG. 2 shows a schematic representation of an embodiment of a DC-DC converter of an electric aircraft propulsion system according to the present invention. [Figure 6] FIG. 2 shows a schematic representation of an embodiment of a DC-DC converter of an electric aircraft propulsion system according to the present invention. [Figure 7] FIG. 2 shows a schematic representation of an embodiment of a DC-DC converter of an electric aircraft propulsion system according to the present invention. [Figure 8] FIG. 2 shows a schematic representation of an embodiment of a DC-DC converter of an electric aircraft propulsion system according to the present invention. [Figure 9] FIG. 2 shows a schematic representation of an embodiment of a DC-DC converter of an electric aircraft propulsion system according to the present invention. [Figure 10] 2 shows an example of a transistor control sequence of an inverter of a DC-DC converter according to the present invention and an AC voltage generated by the inverter in response to the control sequence. [Figure 11] FIG. 11 shows an example of the voltages obtained at the terminals of the two arms of the rectifier of a DC-DC converter according to the invention from the voltages generated by the example inverter shown in FIG. 10 . DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0032] Elements having the same function in different embodiments have the same reference numbers in the drawings.

[0033] 2 and 3, an electric aircraft propulsion system 201 in accordance with the present invention will now be described.

[0034] The electric propulsion system 201 comprises at least one thermoelectric supply 203 and an electric energy storage unit 205 configured to supply electric energy to a high voltage direct current (HVDC) bus 207 .

[0035] In the example shown, the thermoelectric supply 203 comprises an internal combustion engine 203a, a generator 203b coupled to the internal combustion engine 203a, and a rectifier 203c connected to the generator 203b. The thermoelectric supply 203 delivers direct current to the HVDC bus 207.

[0036] The HVDC bus 207 is designed to supply loads (not shown), such as the electric motors of an aircraft. The aircraft concerned may be VTOL or CTOL aircraft. The invention is particularly well suited for aircraft weighing less than 5 tonnes, with an onboard mechanical power of between 50 kW and 2000 kW.

[0037] The electric aircraft propulsion system 201 further comprises an electric energy storage unit 205 (also referred to as “HVDC storage”), at the interface between the HVDC bus 207 and the DC-DC converter 209 (for direct current / direct current) connected in series with the electric energy storage unit 205.

[0038] One or more capacitors 211 are typically provided at the output of the thermoelectric generator 203 and the DC-DC converter 209. These capacitors ensure that all of the loads connected to the HVDC bus, i.e. the inverters and the motors driving the propellers, are actually voltage sources.

[0039] In various embodiments of the electric propulsion system 201, the electric energy storage unit 205 can be an electrolytic current source, such as a supercapacitor, or an electrochemical current source, such as a battery. In both cases, the energy storage unit behaves as an energy source (i.e., in a source mode) or as a charge (i.e., in a charge mode) during different operation phases. Furthermore, the invention also applies to energy storage units that comprise exclusively non reversible energy sources, such as solar panels or cells.

[0040] Supercapacitor type sources have the characteristic that they can provide powerful power during power peaks, i.e. very short periods of time. For example, supercapacitors are very useful for responding in transient regimes to high power demands that may be associated with the take-off phase of an aircraft. However, they are not suitable for long-term power requirements. On the other hand, supercapacitors can withstand a very large number of charge / discharge cycles.

[0041] Batteries, on the other hand, are not suited to responding to power peaks. They can deliver average power over long periods of time. They offer a large energy autonomy. However, they can only withstand fewer charge / discharge cycles.

[0042] In addition to the several elements constituting the DC-DC converter 209 grouped together in an assembly 215, described in detail below with reference to Figures 4 to 9, the DC-DC converter 209 according to the invention comprises an inductor 213 connected on the one hand to the output of this assembly 215 and on the other hand to the positive terminal of the storage unit 205, in order to control the current between the storage unit 205 and the HVDC bus 207.

[0043] In particular, the inductor 213 forms together with the assembly 215 a so-called Partial Power Converter (PPC) which, depending on the application, is able to prevent all of the power released by the storage unit 205 from being transferred to the HVDC bus 207.

[0044] 2 and 3, it is the combination of an inductor 213 connected to the storage unit 205 that controls the level of current flowing between the storage unit 205 and the HVDC bus 207. However, in other embodiments (not shown), this function may be obtained by this same inductor 213 associated with the output of an assembly 215 with another energy storage unit of the electric propulsion system 201.

[0045] In all cases, the inductor and the energy storage unit, coupled together, perform the function of controlling the current flowing between the storage unit 205 and the HVDC bus 207 .

[0046] Furthermore, the use of this inductor means that during the phase in which the storage unit 205 is charging, most of the power moving from the HVDC bus to the storage unit can move via said inductor. In this case, the efficiency of the assembly is close to unity.

[0047] Additionally, in certain embodiments, the electric propulsion system 201 may further include a contactor (not shown) configured to directly connect the high potential of the HVDC bus 207 with the high potential of the electric energy storage unit 205 when activated.

[0048] Advantageously, this contactor allows the electrical energy storage unit 205 to be directly connected to the HVDC bus 207 so that any failure of the DC-DC converter 209 does not result in a loss of the electrical energy storage unit 205 or of the HVDC bus 207.

[0049] An embodiment of a DC-DC converter for an electric aircraft propulsion system will now be described with reference to Figure 4. The described DC-DC converter 209 is designed to be connected in series with an electric energy storage unit of an electric propulsion system, such as the electric energy storage unit 205 of the electric propulsion system 201 described with reference to Figure 3.

[0050] In the example shown, the DC-DC converter 209 comprises an inverter 401 configured to provide an AC voltage from a DC voltage, known as the input voltage, provided by an electrical energy storage unit.

[0051] The inverter 401 is a single-phase inverter, i.e. it receives an alternating current on a transmission line consisting of two parallel wires, in other words a line with transistors 413a and 413c and a line with transistors 413b and 413d, respectively.

[0052] In the example shown, the inverter comprises four transistors 413a, 413b, 413c, and 413d distributed across two lines of two transistors in series, however, those skilled in the art will understand that the invention applies to inverters comprising several transistors, more than two, on each of the two lines.

[0053] Examples include MOSFET (Metal Oxide Semiconductor Field Effect Transistor) type transistors or IGBT (Insulated Gate Bipolar Transistor) type transistors.

[0054] In an advantageous embodiment, the transistors 413a, 413b, 413c and 413d of the inverter shown in Fig. 4 are made from a material that makes it possible to obtain high switching frequencies, such as SiC (Silicon Carbide) or GaN (Gallium Nitride). In this way, the size of the magnetic components (transformers and inductances) is advantageously smaller and the volume of the inverter is minimized.

[0055] Furthermore, the use of high switching frequencies, i.e. typically more than several tens of kHz or even of the order of or even more than a hundred kHz, makes it possible to increase the frequency of the chopped signals (current or voltage at the output of the inverter and rectifier stages) and thus improve their control (current and voltage control).

[0056] For example, with IGBT type transistors switching frequencies of up to 30 or 40 kHz can be used, with transistors made from large gap materials such as SiC or GaN it is possible to go even higher, for example switching frequencies of the order of 100-200 kHz can be obtained.

[0057] The DC-DC converter 209 further includes a transformer 403 configured to generate at least one AC voltage from the AC voltage provided at the output of the inverter 401 .

[0058] In the example shown, the transformer 403 is referred to as a step-down transformer in that it is configured to generate a lower voltage at its output than the voltage at its input, because the voltage from the electrical energy storage unit used is much higher than the voltage required to ensure control of the current flowing between the electrical energy storage unit and the HVDC bus, which is one of the purposes for which DC-DC converters are used as described below.

[0059] However, in certain embodiments of the invention, so-called step-up transformers can be used, depending in particular on whether the used electrical energy storage unit to which the DC-DC converter is connected is reversible or not. In fact, typically, the electrical energy storage unit has a nominal voltage higher than the voltage of the network, making it easier to control the flow of power from the storage unit to the network. However, in some cases of conventional monthly use, the source has a lower voltage and needs to be coupled with a step-up transformer, allowing a correct transfer of power from the source to the network. In all cases, the architecture is fixed and the type of transformer used is determined in advance.

[0060] Furthermore, in various embodiments of the DC-DC converter, the transformer may be planar or wound.

[0061] Furthermore, in the shown example of the DC-DC converter 209, the transformer 403 comprises a primary coil 403a and two secondary coils 403b and 403c. Advantageously, the transformer may be configured to provide galvanic isolation between the primary coil 403a and the two secondary coils 403b and 403c.

[0062] The two secondary coils 403b and 403c have a common terminal 407 designed to be directly connected to the HVDC bus 207 and two other terminals 409 and 411 connected to two arms of a rectifier 405 described below.

[0063] The DC-DC converter 209 further includes a rectifier 405 configured to provide a DC output voltage from one or more AC voltages coming from the transformer 403 .

[0064] As will be seen in more detail below, rectifier 405 is a so-called four-quadrant rectifier. In particular, this means that the architecture, together with the way it is (or can be) controlled, has the ability to output negative or positive voltages (i.e., stepped down or stepped up), and further has the ability to pass current from the electrical energy storage unit to the HVDC bus, or vice versa.

[0065] In other words, any degree of control is possible, allowing the storage unit to be charged or discharged at a battery voltage lower or higher than the voltage of the HVDC bus.

[0066] In the example shown, the rectifier 405 comprises four transistors 415a, 415b, 415c and 415d, which may also be, for example, of MOSFET or IGBT type. Similar to an inverter, the rectifier shown comprises two wires (two arms) of two transistors in series. However, the invention also applies to rectifiers with a higher number of transistors per wire.

[0067] Finally, as explained above with reference to Figures 2 and 3, the DC-DC converter 209 further comprises a current source 417 connected to the two arms of the rectifier 405 (for example via a common terminal of these two arms), which consists, on the one hand, of an inductor and, on the other hand, of either the storage unit 205 or another storage unit.

[0068] Thus, in the example shown in Fig. 3, the current source consists of the inductor 213 and the storage unit 205. Furthermore, as can also be seen in Fig. 3, the reference level (and therefore the reference voltage) of the DC-DC converter 209 and the reference level of the HVDC bus 207 are connected to each other via the current source (formed by the inductor 213 and the storage unit 205). In other words, the outputs of the two rectifier arms 405 are connected to the current source 417, which in turn is connected to the reference level of the HVDC bus 207.

[0069] It is this current source 417 that controls the power flowing from the storage unit 205 to the HVDC bus 207. In particular, the use of the current source 417 makes it possible to impose a voltage at the output of the DC-DC converter, thereby regulating the current passing between the electrical storage unit and the HVDC bus, and maintaining, if necessary, different voltage levels between the electrical energy storage unit and the HVDC bus. The DC-DC converter 209 is therefore referred to as a partial power converter, due to its ability to transfer only a portion of the power provided by the storage unit connected to it.

[0070] As a result, the DC-DC converter can control the state of charge of the electric energy storage unit independently of the HVDC bus voltage and control the transition current to the terminals of the electric energy storage unit. This last point protects the integrity of the electric energy storage unit and prevents thermal runaway that could damage the electric energy storage unit.

[0071] Furthermore, the invention also applies to electric propulsion systems comprising multiple parallel electric energy storage units, each having a DC-DC converter such as the one described.

[0072] Advantageously, such a DC-DC converter may be sized to the operating range of the electrical energy storage unit, i.e. according to the rated current and voltage.

[0073] Advantageously, the DC-DC converter further enables the voltage of the electric energy storage unit to be equalized to the voltage of the HVDC bus while regulating the current flowing between the electric energy storage unit and the HVDC bus. In this way, the electric propulsion system can be reconfigured (i.e. electric energy storage units can be added or removed) without damaging or stressing electric energy storage units already in use.

[0074] Secondly, when the voltage of the electric energy storage unit and the voltage of the HVDC bus are closed, in this case no current flows between the electric energy storage unit and the HVDC bus, so the efficiency of the DC-DC converter is close to 100%.

[0075] 4 through 9 show various embodiments of a DC-DC converter for an electric aircraft propulsion system in accordance with the present invention. In particular, these figures illustrate in more detail some example architectures of the rectifier 405 of the DC-DC converter 209.

[0076] 4, transistors 415a and 415b of rectifier 405 are connected in series with secondary coil 403b, and transistors 415c and 415d of rectifier 405 are connected in series with secondary coil 403c of transformer 403. Furthermore, in this example, the gates of the two transistors connected in series with each secondary coil are common. Advantageously, this configuration makes it possible to limit the number of signals required to control the rectifiers.

[0077] In contrast, in the example shown in Figure 5, the gates of the two transistors connected in series to each secondary coil are separate. Advantageously, this configuration can increase the control options of the rectifier (by allowing individual control of each transistor).

[0078] 6 and 7, a diode 419 connected in parallel to the transistors 415a, 415b, 415c, and 415d is configured to protect the transistors 415a, 415b, 415c, and 415d from any overvoltage. Furthermore, in the first case, the gates of the two transistors connected in series to each secondary coil are common, and in the second case, the gates of the two transistors connected in series to each secondary coil are separate.

[0079] Finally, in the examples shown in Figures 3, 8 and 9, one or more filters 421 are connected to the terminals of the rectifier.

[0080] For example, one or more filters may be connected between the transformer and the rectifier, or may be connected in parallel with the transistors between the two drains of the two MOSFET transistors on each arm of the rectifier.

[0081] These filters 421 remove certain unwanted frequencies from the voltage reaching the rectifier 405. In this case the filters used are RC type filters, i.e. using a combination of a resistor and a capacitor connected in series to filter certain frequencies.

[0082] 10 and 11 show examples of a control applied to a DC-DC converter according to the invention and the voltage obtained at the output of the converter from such a control.

[0083] In particular, Figure 10 shows, on the left side, an example of a sequence for controlling the transistors of an inverter of a DC-DC converter such as the DC-DC converter shown in Figure 6. The control sequence corresponds to a sequence of voltages applied as a function of time (x-axis) to the four transistors of the inverter. For example, the voltages shown can be sent to terminals G (gate) and S (source) of a transistor type MOSFET, or to terminals G and E (emitter) of a transistor type IGBT.

[0084] In particular, reference characters Q1, Q2, Q3, and Q4 correspond, respectively, to transistors 413a, 413b, 413c, and 413d in Figure 6. Each control voltage is a square wave signal that varies from a value of zero to a positive value (normalized to 1 in this example) and causes the transistor to behave like an open or closed switch, respectively.

[0085] The right hand side of FIG. 10 shows the voltage resulting from this control sequence and applied to the primary coil of the transformer of the DC-DC converter (called the primary voltage).

[0086] FIG. 11 shows an example of voltages obtained at the terminals of two arms of a rectifier of a DC-DC converter, such as the DC-DC converter described with reference to FIG. 6, from the voltages generated by an inverter according to the example given in FIG. 10.

[0087] In particular, the top curve shows the primary voltage resulting from the control sequence applied to the inverter described with reference to FIG.

[0088] The middle and bottom curves show the voltages obtained from this primary voltage at the terminals of the two arms of the rectifier of the DC-DC converter (respectively connected to the two secondary coils of the converter's transformer). In particular, voltage K1 corresponds to the voltage obtained at the terminals of the lines of the two transistors 415a and 415b in series, while voltage K2 corresponds to the voltage obtained at the terminals of the lines of the two transistors 415c and 415d in series.

[0089] Finally, thanks to the type of DC-DC converter used at the interface between the electric energy storage unit and the rest of the electric propulsion system, the electric propulsion system is able to generate high average power over long periods of time, generate high instantaneous electrical output, and be as compact as possible in terms of weight and volume.

Claims

1. A DC-DC converter (209) for an electric aircraft propulsion system (201), the DC-DC converter (209) being designed to be connected in series with an electric energy storage unit (205) of the electric propulsion system (201), the DC-DC converter comprising: an inverter (401) configured to provide a first AC voltage from a DC input voltage coming from the electric energy storage unit (205); a transformer (403) configured to provide at least one second AC voltage from the first AC voltage; and a rectifier (405) configured to provide an output DC voltage from the at least one second AC voltage; the DC-DC converter (209) further comprises a current source (417) connected to the rectifier (405) and configured to control power flow through the DC-DC converter (209); the transformer (403) comprises a primary coil (403a) and two secondary coils (403b, 403c), the two secondary coils (403b, 403c) having a common terminal (407) designed to be connected to a high voltage DC bus (207) of the electric propulsion system (201) and two other terminals (409, 411) connected to a rectifier (405), the reference level of the DC-DC converter (209) and the reference level of the HVDC bus (207) being connected to each other through a current source (417); The rectifier (405) comprises two arms, each of which comprises at least two series transistors (415a, 415b, 415c, 415d), connected on the one hand to the two other terminals (409, 411) of the transformer (403) and on the other hand to a current source (417); A DC-DC converter (209).

2. The DC-DC converter (209) of any preceding claim, wherein the current source (417) comprises an inductor (213) and an electrical energy storage unit.

3. DC-DC converter (209) according to claim 1, wherein the inverter (401) comprises a plurality of transistors (413a, 413b, 413c, 413d), preferably four transistors of the MOSFET or IGBT type.

4. DC-DC converter (209) according to claim 3, wherein the transistors (413a, 413b, 413c, 413d) have a switching frequency greater than several tens of kHz, advantageously of the order of or greater than a hundred kHz.

5. The DC-DC converter (209) of any preceding claim, wherein the transformer (403) is configured to step down the at least one second AC voltage relative to the first AC voltage.

6. The DC-DC converter (209) of claim 1, wherein the transformer (403) is of a planar type or a wound type.

7. 2. The DC-DC converter of claim 1, wherein the transformer is configured to provide galvanic isolation between the primary coil and two secondary coils of the transformer.

8. 2. The DC-DC converter of claim 1, wherein the rectifier comprises four transistors, two connected in series to each secondary coil of the transformer, said transistors being of the MOSFET type.

9. The DC-DC converter (209) of claim 8, wherein the gates of the two transistors connected in series with each secondary coil are common.

10. The DC-DC converter (209) of claim 8, wherein the gates of the two transistors connected in series with each secondary coil are separate.

11. The DC-DC converter (209) of claim 8, wherein a diode (419) connected in parallel with the transistor is configured to protect said transistor from overvoltage.

12. DC-DC converter (209) according to claim 1, wherein at least one filter (421), preferably of the RC filter type, is connected between the transformer and the rectifier.

13. 2. The DC-DC converter (209) of claim 1, wherein a filter (421), preferably of the RC filter type, is connected in parallel with a transistor (415a, 415b, 415c, 415d) to the terminals of each arm of the rectifier (405).

14. 14. An electric aircraft propulsion system (201) comprising at least one thermoelectric power supply (203) and an electric energy storage unit (205) configured to supply electric energy to a high-voltage DC bus (207) designed to supply an electric charge, the electric aircraft propulsion system (201) further comprising a DC-DC converter (209) according to any one of claims 1 to 13.

15. The electric aircraft propulsion system of claim 14, wherein the electrical energy storage unit is an electrolytic current source, such as a supercapacitor, or an electrochemical current source, such as a battery.

16. 15. The electric aircraft propulsion system of claim 14, further comprising a contactor configured, when activated, to directly connect a high potential of the high voltage DC bus with a high potential of the electric energy storage unit.

17. An aircraft comprising an electric propulsion system (201) according to claim 14.

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