WO2021130464A1

WO2021130464A1 - Power supply unit

Info

Publication number: WO2021130464A1
Application number: PCT/GB2020/052571
Authority: WO
Inventors: Stephen Greetham
Original assignee: Dyson Technology Limited
Priority date: 2019-12-23
Filing date: 2020-10-14
Publication date: 2021-07-01
Also published as: GB2590676A; GB201919241D0

Abstract

A power supply unit comprising a first battery unit and a second battery unit connected in series, a transformer having a tapped primary winding, a first switch connected between the positive terminal of the first battery unit and a first end of the primary winding, a second switch connected between the negative terminal of the second battery unit and a second end of the primary winding, and a controller for controlling the first switch and the second switch. The tapped terminal of the primary winding is connected to a node located between the two battery units. The power supply unit further comprises a first return path that provides unidirectional current flow from a first node to a second node, and a second return path that provides unidirectional current flow from a third node to a fourth node. The first node is located between the negative terminal of the second battery unit and the second switch, the second node is located between the first switch and the first end of the primary winding, the third node is located between the second end of the primary winding and the second switch, and the fourth node is located between the positive terminal of the first battery unit and the first switch.

Description

POWER SUPPLY UNIT

Field of the Invention

The present invention relates to a power supply unit.

Background of the Invention

A power supply may comprise a number of battery cells that output a DC voltage. In the case of an electric vehicle, the power supply may comprise several thousand cells. A problem with such power supplies is that individual cells may age at different rates. The likely cause for this is temperature variations within the power supply but other factors may also contribute to different aging rates, such as differences in the quality of the individual cells. As a cell ages, its capacity decreases. Older cells will therefore reach full charge and full discharge more quickly than younger cells. In the absence of a scheme for balancing the voltages of the cells, different aging will reduce the overall capacity of the power supply.

Summary of the Invention

The present invention provides a power supply unit comprising: a first battery unit and a second battery unit connected in series, each battery unit comprising a positive terminal and a negative terminal; a transformer comprising a primary winding and secondary winding, wherein the primary winding comprises a first end terminal, a second end terminal, and a tapped terminal located between the first end terminal and the second end terminal, and the tapped terminal is connected to a battery node located between the negative terminal of the first battery unit and the positive terminal of the second battery unit; a first switch connected between the positive terminal of the first battery unit and the first end terminal of the primary winding; a second switch connected between the negative terminal of the second battery unit and the second end terminal of the primary winding; a first return path connecting a first node to a second node, wherein the first node is located between the negative terminal of the second battery unit and the second switch, the second node is located between the first switch and the first end terminal of the primary winding, and the first return path comprises a first rectification device such that current flows in a direction from the first node to the second node; a second return path connecting a third node to a fourth node, wherein the third node is located between the second end terminal of the primary winding and the second switch, the fourth node is located between the positive terminal of the first battery unit and the first switch, and the second return path comprises a second rectification device such that current flows in a direction from the third node to the fourth node; and a controller arranged to control the first switch and the second switch.

Advantageously, the power supply unit is capable of balancing the voltages of the battery units in a cost-effective manner. In comparison to known power supplies that achieve voltage balancing, the power supply unit of the present invention requires fewer components. In particular, by introducing a tapped primary winding, it is possible to manage independently the power transfer of two battery units using a single transformer. The topology removes the need for a primary-side snubber whilst still retaining a single switch per battery unit. Additionally, energy stored in the leakage inductance of the transformer is returned to the battery units rather than being dissipated in a snubber resistor. As a result, the efficiency of the system is improved. The clamping action of the rectification devices also enables the use of switches having a lower voltage rating and therefore a lower on-state resistance, further improving efficiency.

In addition to providing voltage balancing, the power supply unit is simultaneously capable of outputting a voltage on the secondary-side of the transformer. The power supply unit is therefore capable of outputting two different voltages: a first voltage on the primary side and a second voltage on the secondary side.

In use, the controller may be arranged to configure the switches between: a first configuration in which one of the switches is closed and the other of the switches is open such that the primary winding is partially energised by current drawn from one only of the first battery unit and the second battery unit; a second configuration in which both switches are closed such that the primary winding is fully energised by current drawn from the first battery unit and the second battery unit; and a third configuration in which both switches are open such that energy stored in the transformer is transferred to the secondary winding. In the third configuration, energy stored in the transformer due to the leakage inductance of the primary winding is returned to the battery units via the return paths. The present invention also provides a power supply system comprising a plurality of power supply units as described above, wherein the battery units are connected in series to provide a primary voltage bus and each of the secondary windings is connected to a common secondary voltage bus.

The present invention further provides an electric vehicle comprising a traction motor, one or more auxiliary systems, and a power supply system as described above, wherein the traction motor is connected to the primary voltage bus, and the auxiliary systems are connected to the secondary voltage bus.

Brief Description of the Drawings

In order that the invention may be more readily understood, reference will now be made by way of example only to the accompanying drawings in which:

Figure 1 shows a circuit diagram of a power supply unit;

Figure 2 shows a circuit diagram of a power supply system comprising three of the power supply units of Figure 1;

Figure 3 shows a schematic depiction of an electric vehicle;

Figure 4 shows a schematic depiction of some of the systems and components of the electric vehicle;

Figure 5 shows a circuit diagram of a second power supply unit;

Figure 6 shows a circuit diagram of a third power supply unit; and

Figure 7 shows a circuit diagram of a fourth power supply unit. Detailed Description of the Invention

Figure 1 shows a power supply unit 1 that comprises a first battery unit 2 and a second battery unit 3. The first battery unit 2 comprises a positive terminal 4 and a negative terminal 5. Similarly, the second battery unit 3 comprises a positive terminal 6 and a negative terminal 7. The first battery unit 2 and the second battery unit 3 are connected in series such that the positive terminal 6 of the second battery unit 3 is connected to the negative terminal 5 of the first battery unit 2. In the Figures, each battery unit 2,3 is shown as comprising just one cell. However, each battery unit 2,3 may comprise any number of cells. For example, each battery unit may a string of parallel-connected cells, or a plurality of series-connected strings, each string comprising a plurality of parallel-connected cells.

The power supply unit 1 further comprises a transformer 10 which comprises a primary winding 20 and a secondary winding 30. The primary winding 20 comprises a first end terminal 21, a second end terminal 22 and a centre-tapped terminal 23, which is located between the first end terminal 21 and the second end terminal 22. The primary winding 20 comprises a first portion 24 located between the first end terminal 21 and the centre-tapped terminal 23, and a second portion 25 located between the centre-tapped terminal 23 and the second end terminal 22. The centre-tapped terminal 23 is connected to a battery node 8 located between the negative terminal 5 of the first battery unit 2 and the positive terminal 6 of the second battery unit 3.

The power supply unit 1 further comprises a first switch 40 and a second switch 41. The first switch 40 is connected between the positive terminal 4 of the first battery unit 2 and the first end terminal 21 of the primary winding 20. The second switch 41 is connected between the second end terminal 22 of the primary winding 20 and the negative terminal 7 of the second battery unit 3.

The power supply unit 1 comprises a first return path 50 and a second return path 60. The first return path 50 connects a first node 51 to a second node 52 and includes a first rectification device 53, which is a diode in this case. The first node 51 is located between the second switch 41 and the negative terminal 7 of the second battery unit 3, whilst the second node 52 is located between the first switch 40 and the first end terminal 21 of the primary winding 20. The first rectification device 53 allows current to flow from the first node 51 to the second node 52 but prevents current to flow from the second node 52 to the first node 51. The second return path 60 connects a third node 61 to a fourth node 62 and includes a second rectification device 63, also a diode. The third node 61 is located between the second end terminal 22 of the primary winding 20 and the second switch 41, whilst the fourth node 62 is located between the positive terminal 4 of the first battery unit 2 and the first switch 40. The second rectification device 63 allows current to flow from the third node 61 to the fourth node 62 but prevents current to flow from the fourth node 62 to the third node 61.

The power supply unit 1 also comprises a controller 70 for controlling the operation of the power supply unit 1. More specifically, the controller 70 controls the opening and closing of the first and second switches 40,41.

The power supply unit 1 comprises a pair of output terminals 82 for supplying an output voltage to a load. The output terminals 82 are connected to the secondary winding 30 via a rectification device 80, again a diode in this case. A smoothing capacitor 81 is then connected across the output terminals 82 to smooth the output voltage.

During operation, the first and second battery units 2,3 ideally have the same nominal voltage. However, a voltage difference between the two battery units 2,3 will inevitable arise through successive cycles of charging and discharging. For the sake of the following discussion, it is assumed that the voltage of the first battery unit 2 is slightly greater than the voltage of the second battery unit 3.

The operation of the power supply unit 1 begins with both switches 40,41 open. The controller 70 then closes the first switch 40 such that current flows from the first battery unit 2, via the first switch 40 to the primary winding 20. Current in the first portion 24 of the primary winding 20 then increases, in accordance with the voltage applied by the first battery unit 2 and the inductance of the first portion 24 of the primary winding 20. The current exits the primary winding 20 via the centre-tapped terminal 23 and returns to the first battery unit 2 via the battery node 8. A voltage is induced across the secondary winding 30. However, the voltage is negative in polarity and thus the rectification device 80 is reversed biased and prevents current from flowing in the secondary winding 30. The capacitor 81 therefore discharges in response to a load on the output terminals 82.

After a pre-determined period of time, the controller 70 closes the second switch 41 such that the second battery unit 3 is also connected to the primary winding 20. With both switches 40,41 closed, current now flows through both the first portion 24 and the second portion 25 of the primary winding 20. However, since the first portion 24 was energised prior to the second portion 25, the magnitude of the current flowing through the first portion 24 is greater. Current through the primary winding 20 therefore follows two paths. A first current flows from the first battery unit 2, through the first portion 24 and returns to the first battery unit 2 via the centre-tapped terminal 23 and the battery node 8. A second current flows from the battery units 2,3 through both the first portion 24 and the second portion 25 via the first switch 40, and returns to the second battery unit 3 via the second switch 41. Again, the voltage induced in the second winding 30 is negative in polarity and thus no current flows in the secondary winding 30 and the capacitor 81 continues to discharge.

After a further predetermined period of time, the controller 70 opens both the first switch 40 and the second switch 41. The voltage induced across the secondary winding 30 is now positive, which forward biases the rectification device 80 and allows current to flow from the secondary winding 30. As a consequence, the capacitor 81 is charged. It will be apparent to those skilled in the art that the transformer 10 operates as a flyback converter, with energy being stored in the transformer 10 when the primary-side switches 40,41 are closed and energy being released when the primary-side switches 40,41 are opened.

Owing to the leakage inductance of the primary winding 20, not all of the energy stored in the transformer 10 is transferred to the secondary winding 30. This leakage energy is then returned to the battery units 2,3 via the first and second return paths 50,60. Again, since the first portion 24 of the primary winding 20 was energised prior to the second portion 25, the first portion 24 stores a higher leakage energy. The leakage energy stored in the primary winding 20 is therefore returned to the battery units 2,3 via two different paths. A first return current flows from the first end terminal 21 of the primary winding 20, through the first portion 24 to the positive terminal 6 of the second battery unit 3 via the centre-tapped terminal 23 and the battery node 8. From there the current flows from the negative terminal 7 of the second battery unit 3 to the first end terminal 21 of the primary winding 20 via the first return path 50. A second return current flows from the first end terminal 21 of the primary winding 20 through the first and second portions 24,25 to the second end terminal 22. From there, the second return current flows to the positive terminal 4 of the first battery unit 1 via the second return path 60. The second return current then flows through the second battery unit 3 and to the first end terminal 21 of the primary winding 20 via the first return path 50.

It will be understood that the power supply unit 1 may be operated in this way so as to balance the voltages of the first and second battery units 2,3. In the case discussed above, the voltage of the first battery unit 2 is greater than that of the second battery unit 3. Accordingly, the controller 70 begins by closing the first switch 40 such that the primary winding 20 is first energised by the first battery unit 2. More energy is therefore extracted from the first battery unit 2. If, conversely, the voltage of the first battery unit 2 were less than that of the second battery unit 3, the controller would instead begin by closing the second switch 41. The primary winding 20 would then be first energised by the second battery unit 3 and thus more energy would be extracted from the second battery unit 3. In all other respects, the control of the switches 40,41 is unchanged. That is to say that after closing the second switch 41, the controller 70 waits a predetermined period of time before closing both switches 40,41, and then waits a further predetermined period of time before opening both switches 40,41.

It will be appreciated that, when the second portion 25 of the primary winding 20 is first energised, the paths taken by the current will differ slightly from that described above. In particular, when only the second switch 41 is closed, current flows from the second battery unit 2, via the battery node 8 to the centre-tapped terminal 23 of the primary winding 20. Current then increases in the second portion 24 of the primary winding 20 and returns to the second battery unit 2 via the second switch 41. Upon closing both switches 40,41, current flows through both the first portion 24 and the second portion 25 of the primary winding 20. However, since the second portion 25 was energised prior to the first portion 24, the magnitude of the current flowing through the second portion 25 is greater. Consequently, a first current flows from the second battery unit 3, through the second portion 24 and returns to the second battery unit 2 via the second switch 41. A second current flows from the battery units 2,3 through both the first portion 24 and the second portion 25 via the first switch 41, and returns to the second battery unit 3 via the second switch 41. Upon opening both switches 40,41, a first return current flows from the centre-tapped terminal 23 of the primary winding, through the second portion 25 to the positive terminal 4 of the first battery unit 2 via the second end terminal 22 and the second return path 60. From there the current flows from the negative terminal 5 of the first battery unit 2 to the centre-tapped terminal 23 of the primary winding 20 via the battery node 8. A second return current flows form first end terminal 21 of the primary winding 20, through the first and second portions 24,25 to the positive terminal 4 of the first battery unit 2 via the second return path 60. The second return current then flows through the second battery unit 3 and to the first end terminal 21 of the primary winding 20 via the first return path 50.

It can thus be seen that the power supply unit 1 may be operated so as to balance the voltages of the battery units 2,3. Importantly, the power supply unit 1 is capable of achieving voltage balancing in a cost-effective manner. In particular, by employing a centre-tapped primary winding 20, it is possible to manage independently the energy transfer of two battery units 2,3 using a single transformer 10. Moreover, the topology of the power supply unit 1 removes the need for a primary-side snubber, whilst still retaining a single switch 40,41 per battery unit 2,3. The leakage energy of the transformer 10 is returned, via the return paths 50,60, to the battery units 2,3 rather than being dissipated in a snubber resistor. As a result, the efficiency of the power supply unit 1 is improved. The clamping action of the rectification devices 53,63 also enables the use of switches 40,41 having a lower voltage rating and therefore a lower on-state resistance, further improving efficiency.

Figure 2 shows a power supply system 100 that comprises a plurality of power supply units 1, 1 ’ and 1 ”, each as described above with reference to Figure 1. In the example shown in Figure 2, the power supply system 100 comprises three power supply units. However, the power supply system 100 may comprise any number of power supply units. The batteries units 2, 3, 2’, 3’, 2”, 3” of the power supply units are connected in series to provide a primary voltage bus 101. The output terminals of each of the power supply units are connected to a common secondary voltage bus 102. Figure 3 shows a schematic depiction of an electric vehicle 200 comprising the power supply system 100 of Figure 2. As noted above, the power supply system 100 may comprise any number of power supply units 1, and the battery units 2,3 of each power supply unit 1 may comprise any number of cells. The electric vehicle 200 may comprise several thousand cells, arranged as X series-connected strings, each string having Y parallel-connected cells. The power supply system 100 might then comprise X/2 power supply units, and each battery unit might comprise a string of Y parallel connected cells.

Figure 4 shows a schematic depiction of some of the systems and components of the electric vehicle 200, which includes at least one traction motor 210 and a plurality of auxiliary systems 220. The traction motor 210 and the auxiliary systems 220 are electrically connected to the power supply system 100. More specifically, the traction motor 210 is connected to the primary voltage bus 101, and the auxiliary systems 220 are connected to the secondary voltage bus 102. The auxiliary systems 220 may comprise a 12 V battery, which provides a 12 V supply voltage on the secondary voltage bus 102. In this instance, energy transferred from the battery units 2,3 to the output terminals 82 of the power supply units 1 is then used to provide average power to the auxiliary systems, whilst the 12 V battery provides momentary peak power to the auxiliary systems. It will be appreciate that, if a 12 V battery is provided, the secondary-side capacitor 81 of each power supply unit 1 may be omitted or can be of relatively low capacitance.

The power supply unit 1 discussed above with reference to Figure 1 comprises a circuit topology that operates as a flyback converter. This then has the advantage that energy may be transferred using a fewer number of components (e.g. switches, rectifiers, inductors). However, a disadvantage is that energy transfer is discontinuous. As a result, the transformer 10 is required to store more energy at any one time. Additionally, if the power supply unit 1 is required to output a relatively smooth voltage at the output terminals 82, a capacitor 82 of higher capacitance will be required.

Figure 5 shows an alternative power supply unit 501 that operates as a forward converter. In addition to the components of the power supply unit of Figure 1, the power supply unit 501 comprises a third switch 502 located between the battery node 8 and the centre-tapped terminal 23, as well as a further rectification device 503 and an inductor 504 on the secondary side. The controller 70 is not shown in Figure 5 for the sake of clarity. In contrast to the power supply unit of Figure 1, the transformer 10 now operates as a conventional transformer and is no longer required to store energy. Instead, the inductor 504 acts as the energy storage element. This then has the benefit that a continuous output current can be achieved. Additionally, a relatively smooth voltage at the output terminals 82 can be achieved with a capacitor 81 of lower capacitance. The disadvantage is that additional components are required to implement the power supply unit.

Figure 6 shows a further alternative power supply unit 601 that operates as a forward-flyback converter. This power supply unit 601 may therefore be thought of as a compromise or balance of the two power supply units shown in Figures 1 and 5. In contrast to the power supply units of Figures 1 and 5, the secondary winding 30 of the power supply unit 601 is a centre-tapped winding like that of the primary winding 20. As a result, the power supply unit 601 is capable of operating as a forward converter, and separately as a flyback converter. Like the power supply of Figure 5, the power supply unit 601 is capable of delivering a continuous output current on the secondary side. However, since the transformer 10 now stores energy when operating in flyback mode, a smaller inductor may be employed on the secondary side.

Figure 7 shows yet a further power supply unit 701 that comprises an additional battery unit 702. Rather than having a centre-tapped terminal 23, the primary winding 20 now comprises two intermediate-tapped terminals 703,704. As a consequence, the primary winding 20 comprises a third portion 706 in addition to the first and second portions 24,25. Each of the tapped terminals 703,704 is then connected, via a switch 710,711, to a battery node 8,708 located between pairs of battery units. Again, the controller is not shown for the sake of clarity.

The power supply unit 701 of Figure 7 has the advantage of being able to balance the voltages of an odd number of battery units. However, where a power supply system comprises an even number of battery units then the power supply unit 1 of Figure 3 will be more cost effective. By way example, if the power supply system comprises a total of six battery units then the power supply system may comprise three of the power supply units of Figure 1 or two of the power supply units of Figure 7. In the former, the power supply system would comprise a total of six switches. In the latter, the power supply system would comprise a total of eight switches.

Whilst various embodiments of a power supply unit have been thus far been described, it will be appreciated that there is a basic topology on the primary side that is common to all of the power supply units. In particular, each power supply unit comprises: a first battery unit and a second battery unit connected in series; a transformer having a tapped primary winding, with the tapped terminal being connected (potentially via a switch) to a node located between the two battery units; a first switch connected between a positive terminal of the first battery unit and a first end of the primary winding; a second switch connected between the negative terminal of the second battery unit and a second end of the primary winding; a first return path that provides unidirectional current flow from a first node to a second node, the first node being located between the negative terminal of the second battery unit and the second switch, and the second node being located between the first switch and the first end of the primary winding; and a second return path providing unidirectional current flow from a third node to a fourth node, the third node being located between the second end of the primary winding and the second switch, and the fourth node being located between the positive terminal of the first battery unit and the first switch. The power supply unit may comprise other circuit topologies, beyond those described above, that include this basic topology on the primary side.

The above embodiments are to be understood as illustrative examples of the invention. It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.

Claims

1. A power supply unit comprising: a first battery unit and a second battery unit connected in series, each battery unit comprising a positive terminal and a negative terminal; a transformer comprising a primary winding and secondary winding defining a flyback converter, wherein the primary winding comprises a first end terminal, a second end terminal, and a tapped terminal located between the first end terminal and the second end terminal, and the tapped terminal is connected to a battery node located between the negative terminal of the first battery unit and the positive terminal of the second battery unit; a first switch connected between the positive terminal of the first battery unit and the first end terminal of the primary winding; a second switch connected between the negative terminal of the second battery unit and the second end terminal of the primary winding; a first return path connecting a first node to a second node, wherein the first node is located between the negative terminal of the second battery unit and the second switch, the second node is located between the first switch and the first end terminal of the primary winding and the first return path comprises a first rectification device such that current flows in a direction from the first node to the second node; a second return path connecting a third node to a fourth node, wherein the third node is located between the second end terminal of the primary winding and the second switch, the fourth node is located between the positive terminal of the first battery unit and the first switch, and the second return path comprises a second rectification device such that current flows in a direction from the third node to the fourth node; and a controller for controlling the first switch and the second switch.

2. A power supply system comprising a plurality of power supply units as claimed in claim 1, wherein the battery units are connected in series to provide a primary voltage bus and each of the secondary windings is connected to a common secondary voltage bus.

3. An electric vehicle comprising a traction motor, one or more auxiliary systems, and a power supply system as claimed in claim 2, wherein the traction motor is connected to the primary voltage bus, and the auxiliary systems are connected to the secondary voltage bus.