WO2022053146A1

WO2022053146A1 - Multi-phase modular composite bidirectional dc/dc converter with reduced number of power switches

Info

Publication number: WO2022053146A1
Application number: PCT/EP2020/075536
Authority: WO
Inventors: Johan De Backer; Tuan-Dat MAI
Original assignee: Toyota Motor Europe
Priority date: 2020-09-11
Filing date: 2020-09-11
Publication date: 2022-03-17

Abstract

A multiphase DC-DC converter (1) comprising an input (2) comprising a first and second input poles (21, 22) intended to be connected to a power supply such as a battery, an output (3) comprising a first and a second output poles (31, 32) intended to be connected to an inverter, and at least one converter module (4) comprising a first terminal (41) electrically connected to the first input pole (21), a second terminal (42) electrically connected to the second input pole (22) and to the second output pole (32), a third terminal (43) electrically connected to the first output pole (31), and a transformer (44) which comprises a primary coil (441) and a secondary coil (442). The primary coil (441) and the secondary coil (442) of the transformer (44) of said at least one converter module (4) comprise each a first connection (4410, 4420) electrically coupled to the first input pole (41). And said at least one converter module (4) further comprises only a first and a second power switches (45, 46), the first power switch (45) being electrically coupled between a second connection (4415) of the primary coil (441) and the second terminal (42) of the converter module (4), and the second power switch (46) being electrically coupled between a second connection (4425) of the secondary coil (442) and the third terminal (43) of said converter module (4).

Description

Title of the invention

MULTI-PHASE MODULAR COMPOSITE BIDIRECTIONAL DC/DC CONVERTER WITH REDUCED NUMBER OF POWER SWITCHES

Background of the invention

The present application concerns a DC-DC converter with at least two operating modes and a method for operating such a DC-DC converter, an electrical power supply line and an electric vehicle.

It relates to a DC-DC converter for supplying power especially in a high-voltage range, DC standing for direct current. In a normal mode (first operating mode), there is usually no voltage amplification or the output voltage can be slightly lower than the input voltage due to the minimal voltage drops within the converter. In a second operating mode of the DC-DC converter, also called boost mode, a voltage amplification should take place, which makes it possible for a certain period of time (eg. a few seconds or longer) to provide increased power. In this operating mode, the DC-DC converter is a boost converter.

Such DC-DC converters can be used, for example in electric cars, to accelerate the vehicle by increasing power (to overtake another vehicle for example, or during higher power driving such as high speed driving or while climbing an uphill slope) through higher voltage and thus limiting required current to optimize efficiency. This is a "boost", i.e. an increase in voltage, from the lower voltage side (e.g. battery side) to the higher voltage side (e.g. traction side).

Essentially, two types of DC-DC converters can be distinguished from one another in the prior art.

In a first type illustrated on figure 1 and disclosed in the article "Electrified Automotive Powertrain Architecture Using Composite DC-DC Converters" from Hua Chen published in IEEE transactions on power electronics vol.32, No. 1, January 2017, a DC-DC converter 100 comprises an input 102 comprising two terminals intended to be coupled to a battery and an output 103 comprising two terminals intended to be coupled to an inverter input. One of the input terminals is coupled, through an electromagnetic component such as a inductance 104, to a first terminal of a first controlled switch 105 and a first terminal of a second controlled switch 106, while the other input terminal is coupled to a second terminal of the first controlled switch 105 and to one of the output terminals, the other output terminal being coupled to a second terminal of the second controlled switch 106. A capacity 107 is also coupled in parallel to the output 103.

In a normal mode, the DC current flows from the battery through the input 102 via the inductance 104 and a closed switch 105 or 106 to the output 103 to which the inverter is coupled. This is a conventional half-bridge converter. The input voltage corresponds to the output voltage in normal mode. The disadvantage of this structure is that the entire current flows through the inductance 104 which implies using a large inductance to be able to handle all the current and that at least an additional buck-boost stage is required.

Moreover, semiconductor switches cause high losses, especially at high currents. In addition, the half-bridge and thus the permanently loaded upper switch must be placed on the output voltage. The required dimensioning of throttle and switch is costly and requires a lot of space.

In a second type, the input and the output are completely galvanically separated from one another, i.e. electrically isolated from each other. The energy transfer takes place exclusively inductively via a transformer.

A variation of such converters is illustrated on figure 2 and disclosed in document EP 2 985 898. In such converters, the DCDC- converter 110 comprises an input 112 comprising two terminals intended to be coupled to a battery and an output 113 comprising two terminals intended to be coupled to an inverter input. One of the input terminals is coupled to a first terminal of a first switching device 114 and a first terminal of a second switching device 115, while the other input terminal is coupled to a second terminal of the first switching device 114 and to one of the terminals of the output 113. The other terminal of the output 113 is coupled to a second terminal of the second switching device 115. The two switching devices 114 and 115 are also coupled through a transformer having a primary winding 117 coupled between two pairs of controlled switches of the first switching device 114 and a secondary winding 118 coupled between two pairs of controlled switches of the second switching device 118.

However, such converter device comprises eight switches altogether for each transformer and has a tight voltage adjustment range.

Document EP 3 024 130 discloses a DC-DC converter with a cascade and buck-boost topology but with a major part of the power transferred between input and output which does not pass through the power electronic conversion stage.

Document US 7 518 886 discloses a converter with a full-bridge topology but which only offers a uni-directional power flow and uses too many switches as it has twelve switches and six diodes for three modules. Moreover such converter presents the disadvantage of having all the power going through the transformer

Object and summary of the invention

The invention aims to offer a higher power density DC-DC converter applicable to a high voltage DC-DC booster stage of an electric powertrain.

An object of the present invention proposes a multiphase DC-DC converter having at least two modes and comprising a DC input comprising a first and second input poles intended to be connected to a power supply, a DC output comprising a first and a second output poles intended to be connected to an inverter, and at least one converter module comprising a first terminal electrically connected to the first input pole, a second terminal electrically connected to the second input pole and to the second output pole, a third terminal electrically connected to the first output pole, and a transformer which comprises a primary coil and a secondary coil.

According to a general feature of this DC-DC converter, the primary coil and the secondary coil of the transformer of said at least one converter module comprise each a first connection electrically coupled to the first input pole, and said at least one converter module further comprises only a first and a second power switches, said first power switch being electrically coupled between a second connection of the primary coil and the second terminal of the converter module, and said second power switch being electrically coupled between a second connection of the secondary coil and the third terminal of said converter module.

Hence, the structure of the DC-DC converter according to the invention is capable of delivering bidirectional power flow and has a fly-back topology with a transformer having its primary winding driven by a power switch coupled to the input.

A converter with a flyback topology is a converter that uses mutually coupled inductors to store energy when current passes through and releasing the energy when the power is removed, therefore forming a flyback transformer which is capable of storing energy when a typical transformer does not have this ability. Flyback converters are similar to booster converters in architecture and performance. However, the primary winding of the transformer replaces the inductor of the booster converter while the secondary provides the output. In the flyback configuration, the primary and secondary windings are utilized as two separate inductors.

Unlike conventional transformers, a flyback transformer is not fed with a signal of the same waveshape as the intended output current. A convenient side effect of such a transformer is the considerable energy which is available in its magnetic circuit.

When the current flowing through an inductor is cut off, the energy stored in the magnetic field is released by a sudden reversal of the terminal voltage. Each power switch can comprise a diode referred to as a flyback diode. This arrangement has the interesting property of transferring energy to the secondary side of the power supply only when the first power switch is off, i.e. the switch on the primary side, or primary switch.

The flyback topology of the converter allows using a relatively small number of components. The primary switch chops the input DC voltage and the energy in the primary is transferred to the secondary through the transformer. The diode in the second power switch rectifies the voltage.

The flyback topology offers the following advantages. It has a primary circuit isolated from the output. It is able to supply multiple output voltages all isolated from the primary, and has the ability to regulate the multiple output voltages with a single control, and operate on a wide range of input ranges and use very few components.

The structure of the converter according to the invention also allows combining as many converter modules as possible together. The number of converter modules depends on the power scale of the high power density converter. Each converter module steps down the input voltage to the output voltage or vice-versa. The high-side voltage is hence equivalent to the sum of low-side voltage and auxiliary-voltage.

Moreover, the modular structure of the converter enables fault tolerance and partial load capability.

The converter according to the invention globally offers a higher power density for a non-galvanic isolated bidirectional DC-DC converter with only two switches per converter module what reduces the number of power electronic switches in comparison to known bi-directional half-bridge or fullbridge DC-DC converters.

Moreover, the output voltage of the DC-DC converter according to the invention can be easily adjusted because of the energy storage in magnetic components what allows the converter to operate as a buck-boost converter.

In an embodiment of the multiphase DC-DC converter, the first power switch can comprise a drain pin or a collector pin which is electrically connected to the second connector of the primary coil, and the second power switch can comprise a source pin or an emitter pin electrically connected to the second connector of the secondary coil.

In another embodiment of the multiphase DC-DC converter, the converter can comprise a buffering capacity electrically connected between the third terminal of said at least one converter module and the first output pole.

In another embodiment of the multiphase DC-DC converter, the converter can comprise an input filter capacity electrically coupled in parallel to the poles of the DC input, and an output filter capacity electrically coupled in parallel to the poles of the DC output.

Another object of the invention proposes an electric powertrain comprising a high voltage DC-DC booster stage comprising a DC-DC converter as defined here above, a power supply electrically connected to the input of the DC-DC converter, and an inverter electrically connected to the output of the DC-DC converter.

In an embodiment of the electric powertrain, the power supply comprises a renewable energy source.

In an embodiment of the electric powertrain, the power supply comprises an energy storage system.

Another object of the invention proposes a vehicle comprising an electric powertrain as defined here above.

Brief description of the drawings

The invention will be better understood by reading here after, as examples and in a non-limitative way, in reference to the enclosed drawings on which:

- Figure 1 shows schematically a DC-DC converter according to a first prior art; - Figure 2 shows schematically a DC-DC converter according to a second prior art;

- Figure 3 shows schematically a DC-DC converter according to an embodiment of the invention;

- Figure 4 shows a detailed scheme of the DC-DC converter of Figure 3.

Detailed description of the embodiments

The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. Where the term "comprising" is used in the present description and claims, it does not exclude other elements or steps. Where an indefinite or definite article is used when referring to a singular noun e.g. "a" or "an", "the", this includes a plural of that noun unless something else is specifically stated.

The term "comprising", used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. Thus, the scope of the expression "a device comprising means A and B" should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B.

Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

In figure 3 is illustrated schematically a multiphase DC-DC converter according to an embodiment of the invention.

In the embodiment illustrated on Figure 3, the converter 1 comprises an input 2, an output 3, two converter modules 4, a buffering capacity 5, an input filter capacity 6 and an output filter capacity 7. The input 2 comprises a first input pole 21 and a second input pole 22 both intended to be connected to a power supply such as a battery. The output 3 comprises a first output pole 31 and a second output pole 32 both intended to be connected to an inverter.

Both converter modules 4 comprise each a first terminal 41, a second terminal 42 and a third terminal 43. The first terminal 41 is electrically connected to the first input pole 21. The second terminal 42 is electrically connected to the second input pole 22 and to the second output pole 32. And the third terminal 43 is electrically connected to the first output pole 31.

In figure 4 is illustrated schematically detailed scheme of the DC-DC converter of Figure 3.

As illustrated, each converter module 4 further includes a transformer 44 comprising a primary winding 441 and a secondary winding 442. The primary winding 441 of the transformer 44 of each converter module 4 comprises a first connection 4410 and a second connection 4415. The secondary winding 442 of the transformer 44 of each converter module 4 comprises a first connection 4420 and a second connection 4425.

The first connections 4410 and 4420 of both windings 441 and 442 of each converter module 4 are both electrically coupled to the first input pole 41.

Each converter module 4 comprises only two power switches 45 and

46. In each converter module 4, a first power switch 45 is electrically coupled between the second connection 4415 of the primary winding 441 and the second terminal 42 of the converter module 4, and a second power switch 46 is electrically coupled between a second connection 4425 of the secondary winding 442 and the third terminal 43 of the converter module 4.

Each power switch 45 and 46 of each converter module 4 comprises a transistor 47 and a diode 48 electrically connected in parallel to the transistor

47. The transistor 47 of each power switch 45 and 46 can be a bipolar transistor, an insulated gate bipolar transistor, a thyristor, or a power MOSFET.

The transistor 47 of the first power switch 45 comprises a drain pin or a collector pin, depending on the type of transistor, which is electrically connected to the second connector 4415 of the primary winding 441.The transistor 47 of the second power switch 46 comprises a source pin or an emitter pin, depending on the type of transistor, which is electrically connected to the second connector 4425 of the secondary winding 442. The buffering capacity 5 is electrically connected between the third terminal 43 of both converter modules 4 and the first output pole 21. The input filter capacity 6 is electrically coupled in parallel to the first input pole 21 and the second input pole 22. And the output filter capacity 7 is electrically coupled in parallel to the first output pole 31 and to the second output pole 32.

When current flows in the converter 1 from the input 2 to the output 3, i.e. from the battery side to the inverter side when the converter 1 is connected between a battery and an inverter, the transistors 47 connected to the secondary windings 442, i.e. the transistors 47 of the second power switch 46, of each converter module 4 are off, i.e. non conducting, so that current is drawn from the transformers 44 of the converter modules 4 towards the inverter via the free-wheeling diode 48 of the second power switch 46, the transformers 44 being considered as the low-side voltage terminal and the inverter being considered as the high-side voltage terminal. This operation is considered as a voltage step up. Although the converter module 4 acts as a step down converter in this operation, the high-side final voltage corresponds to the sum of the low-side voltages (eg. battery) and the auxiliary voltage (or called deviated potential voltage), where the voltage conversion ratio is less than 1.

When the transistor 47 connected to the primary winding 441, i.e. the transistor 47 of the first power switch 45, is on, i.e. is conducting current, each converter module 4 operates as a flyback converter. At this state, the voltage across the primary winding 441 each converter module 4 corresponds to the low-side voltage, but no current is drawn from the transistor 47 of the second power switch 46 on the secondary side of the transformer 44 due to the reverse-bias of the free-wheeling diode 48 of the second power switch 46, and magnetic energy is stored in the magnetic core of the transformer 44.

When the transistor 47 of the first power switch 45 is off, i.e. non conducting, an inductive voltage appears in the secondary winding 442 of the transformer 44, thus allowing the diode 48 of the second power switch 46 to conduct and transfer power to the high-side voltage terminal, i.e. to the inverter coupled to the output poles 31 and 32.

When current flows in the converter 1 from the output 3 to the input 2, i.e. from the inverter side to the battery side when the converter 1 is connected between a battery and an inverter, the transistor 47 connected to the primary windings 441, i.e. the transistor 47 of the first power switch 45, of each converter module 4 are off, i.e. non conducting, so that the primary winding 441 of the transformer 44 of each converter module 4 is float. Thus the transistor 47 of the second power switch 46 of each converter module 4 operates as an inductor. This operation is considered as a voltage step down.

Under the voltage step-down operation which appears for example during urgent regeneration braking mode, the transistors 47 of the second power switch 46 can simultaneously operate, thus extending the return power in a very short period. If the converter is placed between the battery and inverter's DC-link, the transformer can operate as a buck converter to return wheel rotating energy to battery under the slow-down generation braking.

In this voltage step down mode, the converter module is equivalent to a buck converter during a step-down voltage due to the avoidance of any transistor high-impedance on the inverter side.

The converter presents a converter module with a flyback structure and only one of the two power switches operated during stepping-up operation.

The DC-DC converter according to the invention offers a higher power density for a non-galvanic isolated bidirectional DC-DC converter with only two switches per converter module what reduces the number of power electronic switches in comparison to known bi-directional half-bridge or fullbridge DC-DC converters. Indeed, the structure of the converter module 4 is characterized by the fact that only one power switch is modulated during each voltage step-up operation and step-down operation. Therefore, the total number of switches or transistors per converter module is less than that in the prior art such as the converters disclosed in EP2985898 and EP3024130.

Because losses only exist in DC/DC converters which deliver a minor power in comparison to the entire power flow, the efficiency of this topology is slightly higher than the isolated-converter topologies. For example: a 400/200V galvanic isolated converter has a constant efficiency of 92%. If this converter is implemented according to the invention to perform a 24kW, 400/600V nonisolated converter, the equivalent efficiency is equal to: (24)/(24*(200/600)/0.92 + 24* (400/600)/ 1.00) = 97.2%. Similarly, if the galvanic 400/200 efficiency gains to 95%, the equivalent efficiency becomes 98.27%. The structure of the converter makes it equivalent to a buck converter during generating and braking operation due to the avoidance of a transistor high impedance on the output side. The output voltage of the DC-DC converter according to the invention can be easily adjusted because of the energy storage in magnetic components what allows the converter to operate as a buck-boost converter.

Furthermore, the converter modules 4 allow sufficient time for heat dissipation of both transformer and switches. In addition, the modular topology allows equally distributing the heat sources (switches and transformer) to the heat sink surface. Finally the element converter can be designed to operate in quasi-resonant, which reduces the switching losses.

The converter according to the invention avoids the use of an additional inductor as it is the case in the prior art illustrated in Figure 1 while enabling a voltage ratio variation.

The invention topology helps reducing the transformer size by having the converter working in the discontinuous-conduction mode (DCM).

The power flow contains a major direct power delivery without using switch-converters, and a minor auxiliary power which is delivered by each converter module. The high-side voltage equals to the low-side voltage (eg. battery) and the auxiliary voltage (or called deviated potential voltage, and corresponding to a small part which, due to operation of power converters, is smaller than input voltage), where the voltage conversion ratio is less than 1. A minor part of power is delivered to the output of the converter 1 via power electronics conversion state, while the unity efficient power is transferred at the same input voltage. The overall efficiency is hence improved. In other words, most of the power is thus transferred to the output without going through the switched of the conversion state, improving therefore the overall efficiency.

By directly extracting most of the power to the output, the volume of non-galvanic DC-DC conversion part becomes smaller than the traditional design, which also gains the compactness of (automotive) power control units PCUs, such as DC/DC converters and DC/ AC converters.

The minor benefit is the usage of battery (UL) state-of-charge level (from full-charge to almost-depleted level). When all power switches are open, voltage at the input and output are almost identical. Power flow is unidirectional from low-side voltage terminal to high-side voltage terminal via free- wheeling diodes of the second power switches 46 of each converter module 4. Therefore very low voltage is dropped between two power terminals.

The structure of the converter module is an interleaved topology which offers modularity and is interconnected in parallel, which means the PCU can be stacked with multiple converter modules for different inverter size. In addition, the current ripple is less than other prior arts as it has a lower voltage conversion ratio of switched converter eDCm

Claims

1. A multiphase DC-DC converter (1) comprising an input (2) comprising a first and second input poles (21, 22) intended to be connected to a power supply such as a battery, an output (3) comprising a first and a second output poles (31, 32) intended to be connected to an inverter, and at least one converter module (4) comprising a first terminal (41) electrically connected to the first input pole (21), a second terminal (42) electrically connected to the second input pole (22) and to the second output pole (32), a third terminal (43) electrically connected to the first output pole (31), and a transformer (44) which comprises a primary coil (441) and a secondary coil (442), characterized in that the primary coil (441) and the secondary coil (442) of the transformer (44) of said at least one converter module (4) comprise each a first connection (4410, 4420) electrically coupled to the first input pole (41), and that said at least one converter module (4) further comprises only a first and a second power switches (45, 46), the first power switch (45) being electrically coupled between a second connection (4415) of the primary coil (441) and the second terminal (42) of the converter module (4), and the second power switch (46) being electrically coupled between a second connection (4425) of the secondary coil (442) and the third terminal (43) of said converter module (4).

2. The multiphase DC-DC converter (1) according to claim 1, wherein the first power switch (45) comprises a drain pin or a collector pin which is electrically connected to the second connector (4415) of the primary coil (441), and the second power switch (46) comprises a source pin or an emitter pin electrically connected to the second connector (4425) of the secondary coil (442).

3. The multiphase DC-DC converter (1) according to claim 1 or 2, comprising a buffering capacity (5) electrically connected between the third terminal (43) of said at least one converter module (4) and the first output pole (21).

4. The multiphase DC-DC converter (1) according to any of claims 1 to 3, comprising an input filter capacity (6) electrically coupled in parallel to the poles (21, 22) of the input (2), and an output filter capacity (7) electrically coupled in parallel to the poles (31, 32) of the output (3).

5. An electric powertrain comprising a high voltage DC-DC booster stage comprising a DC-DC converter (1) according to any of claims 1 to 4, a power supply electrically connected to the input (2) of the DC-DC converter (1), and an inverter electrically connected to the output (3) of the DC-DC converter.

6. The electric powertrain according to claim 5, wherein the power supply comprises a renewable energy source.

7. The electric power train according to claims 5 or 6, wherein the power supply comprises an energy storage system.

8. A vehicle comprising an electric powertrain according to any of claims 5 to 7.