FR3053545A1 - NEW CASCADE TYPE DC / DC CONVERTER - Google Patents

Abstract

The invention relates to an elementary unit (UE) for converting a cascaded converter, an elementary unit comprising a transformer comprising: a primary winding having two input terminals connected to two output terminals of a conversion cell; input (CE), two input conversion cell input terminals forming two input terminals of the unitary unit, and • a first secondary winding of which two output terminals are connected to two input terminals of a first output conversion cell (CS1), the elementary conversion unit according to the invention is characterized in that the transformer also comprises a second secondary winding of which two output terminals are connected to two input terminals d a second output conversion cell (CS2), a first output terminal of the second output conversion cell (CS2) being connected to a second terminal of output of the first output conversion cell (CS1), a first output terminal of the first output conversion cell (CS1) and a second output terminal of the second output conversion cell (CS2) respectively forming a first and a second output terminal of the elementary conversion unit (UE). The invention also relates to a converter comprising p elementary units as described above, associated in cascade.

Description

Holder (s): SUPERGRID INSTITUTE Simplified joint-stock company.

Extension request (s)

Agent (s): INNOVATION COMPETENCE GROUP.

ù> 4 / NEW DC / DC CONVERTER OF THE CASCADE TYPE.

FR 3 053 545 - A1 © J The invention relates to an elementary conversion unit (UE) for a cascade type converter, elementary unit comprising a transformer comprising:

a primary winding, two input terminals of which are connected to two output terminals of an input conversion cell (CE), two input terminals of the input conversion cell forming two input terminals of the elementary unit, and a first secondary winding, two output terminals of which are connected to two input terminals of a first output conversion cell (CS1),

The elementary conversion unit according to the invention is characterized in that the transformer also comprises a second secondary winding, two output terminals of which are connected to two input terminals of a second output conversion cell (CS2), one being the first output terminal of the second output conversion cell (CS2) being connected to a second output terminal of the first output conversion cell (CS1), a first output terminal of the first output conversion cell (CS1) ) and a second output terminal of the second output conversion cell (CS2) respectively forming a first and a second output terminal of the elementary conversion unit (UE).

The invention also relates to a converter comprising p elementary units as described above, associated in cascade.

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New cascaded type DC / DC converter

Technical field and state of the art

The invention relates to a new DC / DC converter of cascaded type. Such a converter is generally used in power electronics to pass from a DC voltage level to another DC voltage level with very good conversion efficiency. They find an application in particular in the field of electrical networks of the smartgrid type where it is sought to connect to the same receiver (such as an electrical energy distribution network or an electrical energy consumer) energy sources (such as wind turbines, fuel cells, solar panels, ...) producing electrical energy at often very different voltage levels.

Such a converter is for example described in the document Dl = US5027264. It includes a plurality of elementary conversion units associated to form a cascade structure (fig. Lb). Each elementary conversion unit (fig. La) comprises a transformer comprising:

• a primary winding, two input terminals of which are connected to two output terminals of an input DC / AC conversion cell, two input terminals of the input DC / AC conversion cell forming two terminals d input of the elementary unit, and • a secondary winding, two output terminals of which are connected to two input terminals of an output AC / DC conversion cell.

The input DC / AC and output AC / DC conversion cells each consist of a conventional H bridge; they thus use semiconductor power switches (interconnected transistors) which operate all or nothing and freewheeling diodes connected in parallel on the switches. The transfer of power between the source and the receiver is regulated by the phase shift between the AC voltages generated by each AC / DC conversion cell. Conventionally, if n is the ratio between the number of turns of the secondary winding of the transformer and the number of turns of the primary winding of the transformer, then the nominal voltage at the output terminals of the first AC / conversion cell Output DC is equal to n times the nominal voltage at the input terminals of the input DC / AC converter cell. And, for a converter comprising p elementary conversion units associated in cascade according to the diagram in FIG. 1b, then the nominal voltage at the output terminals of the converter is equal to p * n times the nominal voltage at the input terminals of the converter. Thus, the use of a cascaded DC / DC structure makes it possible to work with high output voltage levels, far beyond the voltage that can be supported by a single semiconductor component.

However, it is sought to go towards ever higher voltage levels in order to produce higher voltage electrical networks, this in order to reduce the heat losses in the electrical lines.

Description of the invention

The invention proposes a new elementary conversion unit and a new converter using this elementary conversion unit, a new converter which has, compared to known converters such as that of Dl, either improved technical performance in terms of voltage and power level. , or a reduced cost and / or size for similar technical performance.

With these objectives in view, the invention proposes a new elementary conversion unit (UE) for a cascaded DC / DC type converter, elementary unit comprising a transformer comprising:

• a primary winding, two input terminals of which are connected to two output terminals of an input conversion cell (CE), two input terminals of the input conversion cell forming two input terminals of the elementary unit, and • a first secondary winding, two output terminals of which are connected to two input terminals of a first output conversion cell (CS1).

The elementary unit according to the invention is characterized in that the transformer also comprises a second secondary winding, two output terminals of which are connected to two input terminals of a second output conversion cell (CS2), a first terminal of the second output conversion cell (CS2) being connected to a second output terminal of the first output conversion cell (CS1), a first output terminal of the first output conversion cell (CS1) and a second output terminal of the second output conversion cell (CS2) respectively forming a first and a second output terminal of the elementary conversion unit (UE).

Thus, according to the invention, by adding a second secondary winding to the transformer and a second output conversion cell connected in series with the first output conversion cell (the first output terminal of the second output conversion cell being connected at the second output terminal of the first output conversion cell), between the first output terminal of the first output conversion cell and the second output terminal of the second conversion cell (respectively forming a first and a second output terminal of the elementary conversion unit) a nominal voltage equal to 2 * n times the nominal voltage at the input terminals of the input conversion cell, i.e. a nominal voltage twice greater than the nominal voltage output of a conversion unit of a conventional cascaded DC / DC converter, n is the ratio between the number of turns of a winding s secondary and the number of turns of a primary winding of the transformer.

In an elementary unit according to the invention, the transformer can more generally comprise m secondary windings with which are associated m output conversion cells (CS1, CS2, ... CSm); a row x output conversion cell comprises two input terminals connected to two output terminals of a secondary winding of the same row x; a first output terminal of an output conversion cell (CSx) of rank x is connected to a second output terminal of an output conversion cell (CSx-1) of rank x-1; a first output terminal of the output conversion cell (CS1) of rank 1 forms the first output terminal of the elementary unit (UE); a second output terminal of the output conversion cell (CSm) of rank x = m forms the second output of the elementary unit (UE); x is an integer between 2 and m.

With m secondary windings, each winding being coupled to an output conversion cell, and with all the output conversion cells associated in series (the first output terminal of the output conversion cell of rank x being connected to the second output terminal of the output conversion cell of rank x-1), one obtains, between the first output terminal of the first output conversion cell and the second output terminal of the m-th output conversion cell (respectively forming a first and a second output terminal of the elementary conversion unit) a nominal voltage at least equal to m * n times the nominal voltage at the input terminals of the input conversion cell, ie a voltage nominal at least m times greater than the nominal output voltage of a conversion unit of a conventional cascaded DC / DC converter, as will be seen better below.

To make a basic DC / DC type conversion unit according to the invention, a DC / AC conversion cell at the input (primary side of the transformer) and AC / DC conversion cells at the output (secondary side of the transformer) will be used. . An AC / DC conversion cell can be built on the same electronic diagram as a DC / AC conversion cell as will be seen more clearly below in examples, the input terminals and the output terminals are simply reversed.

An input conversion cell connected to the primary winding of the transformer can include:

• a simple H-bridge, • a half-bridge neutral point clamped converter, • a complete conversion bridge clamped by the neutral.

An output conversion cell connected to a secondary winding of the transformer can include:

• a simple H-bridge, • a half-bridge neutral point clamped converter, • a complete conversion bridge clamped by the neutral.

According to a preferred embodiment of a conversion cell according to the invention:

• the transformer comprises two secondary windings • a transformation ratio n is equal to 0.5, n being a ratio between a number of turns of a secondary winding and a number of turns of a primary winding • the conversion cell d input connected to the primary winding of the transformer is a complete conversion bridge clamped by the neutral and • the output conversion cells connected to the secondary windings of the transformer are half-bridge neutral point clamped conversion half-bridge converter).

The invention also provides a cascaded DC / DC converter, converter comprising p elementary units (UE1, ... UEp) of conversion as described above.

According to one embodiment, the inputs of the elementary units are connected in parallel and the outputs of the elementary units are connected in series. In other words: the first input terminals of the p elementary units are connected together and form a first input terminal of the converter; the second input terminals of the p elementary units are connected together and form a second input terminal of the converter; a first output terminal of the elementary unit (UEy) of rank y is connected to a second output terminal of the elementary unit (UEy-1) of rank y-1; a first output terminal of the elementary unit (UE1) of rank 1 forms a first output terminal of the converter; a second output terminal of the elementary unit (UEp) of rank y = p forms a second output terminal of the converter; y is an integer between 2 and p. Other embodiments of the converter are described below.

By making a converter with p elementary conversion units according to the invention associated in cascade, a converter is obtained having, at output, a nominal voltage at least equal to p * m * n times the nominal voltage at the input terminals of the converter. , as will be seen more clearly below in the examples. Or, for the same number p of elementary conversion units, the same transformer transformation ratio n and the same nominal input voltage, a nominal voltage is obtained at output at least m times greater than the nominal output voltage d 'a converter according to the prior art. Or, from another point of view, for the same transformation ratio n and the same nominal input voltage, it is possible to obtain a nominal output voltage at least equal to the nominal output voltage of a converter according to the prior art using p times fewer elementary conversion units.

Brief description of the figures

The invention will be better understood, and other characteristics and advantages of the invention will appear in the light of the following description of examples of an elementary conversion unit and of a converter according to the invention. These examples are given without limitation. The description should be read in conjunction with the accompanying drawings in which:

• the figure la shows a known elementary conversion unit, • the figure lb shows a known converter using conversion units according to the figure la, • the figures 2, 3, 4 show three embodiment of an elementary conversion unit according to the invention, and • Figures 5a, 5b, 5c show three embodiments of converters.

Description of embodiments of the invention

As described in the preamble, an elementary known UE unit of conversion (FIG. 1a) comprises a transformer 11 comprising:

• a primary winding 12, two input terminals of which are connected to two output terminals of a CE input conversion cell • a secondary winding 13, of which two output terminals, are connected to two input terminals of a cell output conversion CS, • two input terminals of the input conversion cell CE forming the two input terminals of the elementary conversion unit UE between which an input voltage Vin can be applied • two terminals output of the output conversion cell CS forming two output terminals of the elementary conversion unit UE between which an output voltage Vout is available.

In the example of FIG. La, the input conversion cell CE is a DC / AC conversion cell comprising a conventional H bridge; the output terminals of the H-shaped bridge are connected to the input terminals of the primary winding; the voltage Vin is applied to the input terminals of the H-bridge and a smoothing capacitor is connected between the input terminals of the H-bridge. In known manner, and as shown in FIG. 1 a, the H-bridge comprises four branches and each branch comprises a power switch operating in all or nothing mode (semiconductor transistor) and a freewheeling diode connected in parallel on the switch . The CS output conversion cell is an AC / DC conversion cell produced in a similar manner to the DC / AC conversion cell, the input terminals and the output terminals being simply inverted.

A basic conversion unit according to the invention (fig. 2) is distinguished in that it comprises m output conversion cells CS1, CS2, ..., CSm and a transformer 21 with a primary winding 22 and m secondary windings 23.1, 23.2, ..., 23.m, connected as follows:

• primary side, two input terminals of the primary winding 22 are connected to two output terminals of the CE input conversion cell and two input terminals of the CE input conversion cell form the two terminals input of the elementary conversion unit UE between which an input voltage Vin can be applied, • secondary side of the transformer: an output conversion cell CS1 of rank 1 comprises two input terminals connected to two terminals of output of the secondary winding 23.1 of the same rank 1, an output conversion cell CSx of rank x comprises two input terminals connected to two output terminals of a secondary winding 23.x of the same rank x, x is a whole number between 2 and m, a first output terminal of the output conversion cell (CS2) of rank 2 is connected to a second output terminal of the output conversion cell (CS1) of rank 1, a first so terminal rtie of the output conversion cell (CSx) of rank x is connected to a second output terminal of the output conversion cell (CSx-1) of rank x-1, a first output terminal of the conversion cell of output (CS1) of rank 1 forms the first output terminal of the elementary unit (UE), a second output terminal of the output conversion cell (CSm) of rank x = m forms the second output of the elementary unit (UE). In other words, the m output conversion cells CS1 to CSm have their inputs connected respectively to the outputs of the m secondary windings, and have their outputs connected in series. The output voltage Vout is available between the first output terminal of the row 1 output cell and the second output terminal of the row m output cell.

FIG. 2 details an embodiment of an input conversion cell CE of the DC / AC type, identical to the input conversion cell of the elementary conversion unit of FIG. The output conversion cells are not detailed, but they are of the AC / DC type and identical to the output conversion cell of the elementary unit in FIG. All the DC / AC and AC / DC conversion cells are therefore produced here by H-bridges. In this case, if n is the ratio between the number of turns of the primary winding of the transformer and the number of turns of a secondary winding of a transformer with m secondary windings, then:

• the nominal voltage at the output terminals of an output AC / DC conversion cell is equal to n times the nominal voltage Vin applied to the input terminals of the input DC / AC conversion cell, • the nominal voltage at the output terminals of the elementary conversion unit is equal to m * n times the nominal voltage at the input terminals of the input DC / AC input conversion cell: Vout = m * n * Vin, and • the nominal current lin flowing in the semiconductor components of the input DC / AC conversion cell is equal to m * n times the nominal current lout flowing in the semiconductor components of the output AC / DC conversion cell: lin = m * n * lout.

FIG. 3 shows another embodiment of an elementary conversion unit according to the invention. The transformer here includes for simplicity m = 2 secondary windings. The input DC / AC conversion cell and the output AC / DC conversion cells each consist of a half conversion bridge clamped by the neutral (or NPC type half bridge for Neutral Point Clamped converter) classic made according to a known electronic scheme; as in the previous example, each output AC / DC conversion cell is connected to a secondary winding of the transformer and all the output AC / DC conversion cells are associated in serial cascade output.

Note that, in the example of Figure 3, an NPC structure at three levels is shown. Structures with 5 levels, 7 levels or more can also be used to make multi-level AC / DC or DC / AC conversion cells.

In the example in Figure 3, as in the example in Figure 2:

• the nominal voltage at the output terminals of an output AC / DC conversion cell is equal to n times the nominal voltage Vin applied to the input terminals of the input DC / AC conversion cell, • the nominal voltage at the output terminals of the elementary conversion unit is equal to m * n times the nominal voltage at the input terminals of the input DC / AC input conversion cell: Vout = m * n * Vin, and • the nominal current flowing in the semiconductor components of the input DC / AC cell is equal to m * n times the nominal current flowing in the semiconductor components of the output AC / DC cell: lin = m * n * lout.

But if, to implement the elementary units of FIGS. 2 and 3, semiconductor components (transistors and diodes) having the same nominal blocking voltages are used, then it is possible to apply to the input of the elementary unit in FIG. 3 a voltage Vin twice greater than that which can be applied to the input of the elementary unit of FIG. 2 and the voltage Vout obtained at the output of the elementary unit of FIG. 3 is then twice greater than the voltage Vout obtained at the output of the elementary unit of FIG. 2. In other words, an elementary conversion unit according to FIG. 2 supports at input and at output a nominal voltage twice greater than the nominal voltage supported at input and output by an elementary conversion unit according to FIG. 2, if the semiconductor components used in the two elementary units individually have the same nominal voltages blocking nales.

FIG. 4 shows yet another embodiment of an elementary conversion unit according to the invention. The transformer also includes here for simplicity m = 2 secondary windings (but m can be chosen larger). The input DC / AC conversion cell consists this time of a complete conversion bridge clamped by the neutral or full NPC bridge produced according to a known electronic diagram; as in the example in figure 3, the output AC / DC conversion cells each consist of an NPC type conversion half-bridge, each output AC / DC conversion cell is connected to a secondary winding of the transformer and all output AC / DC conversion cells are associated as serial cascade output.

In the example in Figure 4:

• the nominal voltage at the output terminals of an output AC / DC conversion cell is equal to 2 * n times the nominal voltage Vin applied to the input terminals of the input DC / AC conversion cell, and • the nominal voltage Vout at the output terminals of the elementary conversion unit is equal to 2 * m * n times the nominal voltage at the input terminals of the input DC / AC input conversion cell: Vout = 2 * m * n * Vin, • the nominal current flowing in the semiconductor components of the input DC / AC cell CE is equal to m * n times the nominal current flowing in the semiconductor components of the output AC / DC cell CS1 , CS2.

Thus, it is possible to transmit more power (Vout * lout) with the cell in FIG. 4 than with the cell in FIG. 3, by choosing the same values of m and of n, and by choosing semiconductor components for the input cell able to withstand the nominal voltage and current. Or, it is possible to transmit the same power with the cell of FIG. 4 as with the cell of FIG. 3, by choosing semiconductor components of smaller dimensions (in terms of nominal current). For example, by choosing m = 2 and n = 0.5, it is possible to use the same semiconductor components (same nominal voltages and currents) to make the input cell CE and the output cells CS.

A DC / DC converter according to the invention is produced in a similar manner to a known converter by cascading p elementary units UE1, UE2, UEp for conversion.

A converter according to the invention differs from known converters by the construction of its elementary conversion units. In general, for a converter according to the invention with p elementary conversion units, the following performances are obtained:

• the nominal voltage at the output terminals of an elementary conversion unit according to the invention is at least equal to m * n times the nominal voltage at the input terminals of the input DC / AC conversion cell (in particular m * n times for the structure of figure 3 and 2 * m * n for the structure of figure 4, as we will see later), • the nominal current flowing in the semiconductor components of the input DC / AC cell of an elementary conversion unit is equal to m * n times the nominal current flowing in the semiconductor components of the output AC / DC cells ,.

Below is compared a known embodiment and different embodiments according to the invention of a DC / DC converter producing an output voltage Vs = 40kV from an input voltage Ve = 4kV.

Realization n ° 1: a known converter produced according to figure 5a and using elementary units produced according to figure la. Compared to the converter in FIG. 1b, the connections of the input terminals of the elementary units are modified: the elementary units are grouped two by two in series as input and the groups of two elementary units thus formed have their associated input terminals in parallel. The voltage Ve = 4kV is applied between the first input terminal of the elementary unit of rank 1 and the second input terminal of the elementary unit of rank 2. Thus, each elementary unit receives the voltage Vin = Ve / 2 on its input terminals. The voltage Vs is available between the first output terminal of the elementary unit of rank 1 and the second output terminal of the elementary unit of rank p. Compared to the converter in FIG. 1b, the converter in FIG. 5a supports a nominal input voltage twice greater than Vin, ie Ve = 4 kV instead of 2kV. Like the previous embodiment, it is necessary to associate p = 20 elementary units UE in cascade to obtain a nominal voltage Vs = 40 kV at output. Totaling :

• 20 * (8 diodes and 8 transistors), ie 160 diodes and 160 transistors, • 20 transformers with a primary winding and a secondary winding.

Realization n ° 2: a converter according to the invention produced according to figure 5b and using elementary units produced according to figure 2 with a transformer comprising m = 2 secondary and a transformation ratio η = 1; each DC / AC or AC / DC conversion cell includes an H-bridge, i.e. 8 semiconductor components: 4 diodes and 4 transistors (controlled switches). As in the example in FIG. 5a, the elementary units upstream of the transformers are grouped two by two in series at the input and the groups of two elementary units thus formed have their associated input terminals in parallel. This makes it possible to apply, as before, a voltage Ve = 4kV at the input of each group of two elementary units, each elementary unit individually supporting Vin = 2 kV. In this example, we have at the output of the converter Vs = p * m * n * Vin = p * m * n * Ve / 2. In the case where m = 2 (transformer with 2 secondary windings in the elementary units) and n = 1, then p = 10 elementary units are sufficient to obtain a voltage Vs = 40kV from a voltage Ve = 4 kV. Is :

• p = 10 elementary units each comprising 3 conversion cells, an input conversion cell and m = 2 output conversion cells; either 10 * 3 * (4 diodes + 4 transistors) or a total of 120 diodes and 120 transistors • 10 transformers with one primary winding and two secondary windings.

Thus, compared with a known converter (embodiment # 1), a converter according to the invention according to embodiment 2 uses the same DC / AC or AC / DC conversion cells with H-bridge but uses m times less elementary units to supply the same output voltage Vs from the same input voltage Ve. The number of semiconductor components and the number of transformer windings are also less.

Conversely, with a converter according to the invention similar to embodiment no. 2 but using the same number of elementary units UE as the converter according to embodiment no. 1 known, the voltage at the output of the converter according to the invention is m times higher than the output voltage of a known converter. We would thus have here with p = 20 elementary units an output voltage Vs = p * m * n * Ve / 2 = 20 * 2 * l * Ve / 2, i.e. a voltage Vs of 80 kV for an input voltage Ve 4 kV.

Realization n ° 3: a converter according to the invention produced according to figure 5c and using elementary units produced according to figure 3; each DC / AC or AC / DC conversion cell includes a half conversion bridge clamped by the neutral, ie 10 semiconductor components: 6 diodes and 4 transistors (controlled switches). If the semiconductor components are dimensioned to support a nominal voltage of 2 kV, then it is possible to apply a voltage Vin of 4kV between the input terminals of an elementary unit UE, therefore a voltage Ve = Vin = 4kV between the converter input terminals. In this example, we have at the output of the converter Vs = p * m * n * Ve. In the case where m = 2 (transformer with 2 secondary windings in the elementary units) and n = 1, then p = 5 elementary units are sufficient to obtain a voltage Vs = 40kV from a voltage Ve = 4 kV. Is :

• p = 5 elementary units each comprising an input conversion cell and m = 2 output conversion cells either 5 * 3 * (6 diodes + 4 transistors) or a total of 90 diodes and 60 transistors • 5 transformers at one primary winding and two secondary windings.

Thus, compared with a converter according to the invention using DC / AC or AC / DC cells with an H-bridge (embodiment n ° 2), a converter according to the invention using DC / AC or AC / DC cells comprising a ίο conversion half-bridge clamped by neutral (realization n ° 3) includes a smaller number of semiconductor components.

In addition, it is possible to further reduce the number of elementary conversion units and the number of semiconductor components by increasing the number m of secondary windings of the transformers used to make the DC / AC or AC / DC conversion cells.

Realization n ° 4: a converter according to the invention produced according to figure 5c and using elementary units produced according to figure 4; each output AC / DC conversion cell includes a half conversion bridge clamped by the neutral, ie 10 semiconductor components: 6 diodes and 4 transistors (controlled switches); each input DC / AC conversion cell includes a complete conversion bridge clamped by the neutral, ie 10 diodes and 8 transistors. If the semiconductor components are dimensioned to support a nominal voltage of 2 kV, then it is possible to apply a voltage Vin of 4kV between the input terminals of an elementary unit UE, therefore a voltage Ve = Vin = 4kV between the converter input terminals. In this example, we have at the output of the converter Vs = 2 * p * m * n * Ve. In the case where m = 2 (transformer with 2 secondary windings in the elementary units), then n = 0.5 and p = 5 elementary units are sufficient to obtain a voltage Vs = 40kV from a voltage Ve = 4 kV. Is :

• p = 5 elementary units each comprising an input conversion cell and m = 2 output conversion cells, ie 5 * l * (10 diodes + 4 transistors) + 5 * 2 * (6 diodes + 4 transistors) or one total of 110 diodes and 60 transistors • 5 transformers with one primary winding and two secondary windings.

Compared to realization n ° 3, a few more semiconductor components are used here but, because n = 0.5, the nominal voltage and the nominal current are the same for all the components, whether they are positioned upstream of the primary or downstream of a secondary of a transformer.

Claims (8)

1. Elementary unit (UE) of conversion for a cascaded type converter, elementary unit comprising a transformer comprising: • a primary winding, two input terminals of which are connected to two output terminals of an input conversion cell (CE), two input terminals of the input conversion cell forming two input terminals of the elementary unit, and • a first secondary winding, two output terminals of which are connected to two input terminals of a first output conversion cell (CS1), elementary unit characterized in that the transformer also comprises a second winding secondary of which two output terminals are connected to two input terminals of a second output conversion cell (CS2), a first output terminal of the second output conversion cell (CS2) being connected to a second output terminal output of the first output conversion cell (CS1), a first output terminal of the first output conversion cell (CS1) and a second output terminal of the second output cell output conversion (CS2) respectively forming a first and a second output terminal of the elementary conversion unit (UE). 2. elementary unit according to claim 1 wherein the transformer comprises m secondary windings, elementary unit comprising m output conversion cells (CS1, CS2, ... CSm), an output conversion cell of rank x comprising two terminals d input connected to two output terminals of a secondary winding of the same rank x, a first output terminal of an output conversion cell (CSx) of rank x being connected to a second output terminal of a output conversion (CSx-1) of rank x-1, a first output terminal of the output conversion cell (CS1) of rank 1 forming the first output terminal of the elementary unit (UE), a second terminal of the output conversion cell (CSm) of rank x = m forming the second output of the elementary unit (UE), x being an integer between 2 and m. 3. Elementary unit according to one of the preceding claims, in which the input conversion cell connected to the primary winding of the transformer is: • a simple H-bridge, • a half-bridge neutral point clamped converter, • a complete conversion bridge clamped by the neutral. 4. Elementary unit according to one of the preceding claims, in which an output conversion cell connected to a secondary winding of the transformer is: • a simple H-bridge, • a half-bridge neutral point clamped converter, • a complete conversion bridge clamped by the neutral. 5. Elementary unit according to one of the preceding claims, in which: • the transformer comprises two secondary windings • a transformation ratio n is equal to 0.5, n being a ratio between a number of turns of a secondary winding and a number of turns of a primary winding • the conversion cell d input connected to the transformer primary winding is a full bridge of 5 conversion clamped by neutral and • the output conversion cells connected to the secondary windings of the transformer are half-bridge neutral point clamped converter. 6. Cascaded modular converter, converter comprising p elementary units (UE1, ... UEp) of conversion according to one of claims 1 to 5. 7. Converter according to claim 6, the first input terminals of the p elementary units being connected together and forming a first input terminal of the converter, the second input terminals of the p elementary units being connected together and forming a second input terminal of the converter, a first output terminal of the elementary unit (UEy) of rank y being connected to a second output terminal of the elementary unit (UEy-1) of rank y-1, a first terminal of output of the elementary unit (UE1) of rank 1 forming a 15 first output terminal of the converter, a second output terminal of the elementary unit (UEp) of rank y = p forming a second output terminal of the converter, y being an integer between 2 and p. 1/7 EU cs T \ _