AC battery system
By using series-connected battery modules and H-bridge circuits in an AC battery system, combined with a controller to control switch switching, the problems of high cost and high failure rate are solved, and efficient, low-cost AC power conversion and fault tolerance are achieved.
Patent Information
- Application Number
- CN202510289928.4
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Priority Date
- 2024-03-13
- Filing Date
- 2025-03-12
- Publication Date
- 2025-09-16
AI Technical Summary
Existing AC battery systems require high-cost, high-power switching components and multiple switches, resulting in expensive systems with high failure rates and difficulty in providing effective AC power supply.
Multiple battery modules are connected in series, each module contains series and parallel switches, and the switch switching is controlled by an H-bridge circuit and a controller to achieve DC power to AC power conversion, reduce the number of switches and improve system flexibility.
It reduces system costs, reduces failure rates, improves power conversion efficiency and system reliability, and provides higher flexibility and safety.
Smart Images
Figure CN120657888A_ABST
Abstract
Description
Technical Field
[0001] The present disclosure relates to an AC battery system, a method for operating the system, and a controller for the AC battery system. Background Art
[0002] Conventional battery systems include arrays of battery cells arranged in series and parallel to provide a DC power supply at a desired rated voltage and current. However, power may often be required in the form of an AC power supply, such as for 240 or 120 volts AC for domestic or industrial use, or to power electric motors at varying voltages and frequencies, such as for vehicle propulsion. Therefore, battery storage systems are often combined with power electronic converters configured to convert the DC power supply from the battery to the desired AC output power supply. The converter is also typically capable of operating in reverse, converting the AC power supply to a DC power supply for charging the battery.
[0003] A problem with conventional battery systems, particularly for high-power applications, is the need for costly and high-power switching components to convert between AC and DC power. These components may, for example, be based on SiC-based semiconductor technology. Another issue is the need for many individual battery cells to be combined to provide the required power. If any individual battery cell or group of battery cells fails, it can lead to failure of the entire battery.
[0004] Another type of battery storage system is known as an AC battery. Like a DC battery, an AC battery comprises numerous individual battery cells arranged in series and parallel to provide an output power supply. However, instead of using a converter connected to the battery to provide AC:DC power conversion, semiconductor switches are provided that are selectively controlled to switch each battery module to adjust the output power supply, thereby providing the desired AC power supply directly from the battery itself. This provides a greater degree of flexibility in terms of the ability to select battery modules based on their state of charge and, if necessary, their fault status. The battery can also be charged directly from the AC supply by operating the switch arrangement synchronously with the AC supply.
[0005] A problem with existing AC battery systems is the need to use many switches, typically high-current FET switches. To enable the battery to output AC power, each battery module would require at least four switches, allowing the output to be switched to provide positive or negative output and connect or bypass the module. This makes existing AC battery systems more expensive and, as the number of switches increases, the likelihood of one or more of them failing increases, leading to an increased FIT (failures in Time) metric. Summary of the Invention
[0006] According to a first aspect, there is provided a battery system comprising:
[0007] a plurality of battery modules connected in series, each battery module comprising a plurality of battery cells connected in series, a first switch connected in series with the plurality of battery cells, and a second switch connected in parallel with the plurality of battery cells;
[0008] an H-bridge circuit connected across the plurality of battery modules and having first and second output terminals; and
[0009] A controller is configured to control the H-bridge circuit and switching of the first and second switches in each battery module to transfer power between the first and second output terminals and the plurality of battery modules.
[0010] The controller may be configured to supply a control signal to each of the plurality of battery modules to open or close the first switch and to close or open the second switch accordingly.
[0011] The controller may be configured to convert DC power from the plurality of battery modules into AC power at the first and second output terminals within each consecutive AC cycle by:
[0012] providing a first series of control signals to first and second switches in the plurality of battery modules to provide a first varying power supply to the H-bridge circuit when the H-bridge circuit connects the first and second output terminals across the plurality of battery modules in a first direction during a first half cycle; and
[0013] providing a second series of control signals to first and second switches in the plurality of battery modules to provide a second varying power supply to the H-bridge circuit when the H-bridge circuit connects the first and second output terminals across the plurality of battery modules in a second opposite direction during a second half cycle,
[0014] Wherein the controller is configured to switch the H-bridge circuit from the first direction to the second direction between first and second half cycles.
[0015] The first and second series of control signals may be identical.
[0016] The first and second switches may be FET switches.
[0017] The windings of the electric motor may be connected between the first and second output terminals.
[0018] Each of the plurality of battery modules may include a control input connected to the first switch via a non-inverting amplifier and to the second switch via a first inverting amplifier.
[0019] Each of the plurality of battery modules may include a third switch connected to the control input via a second inverting amplifier.
[0020] A multi-phase battery system comprising a plurality of battery systems according to claim 1 is also provided, wherein the controller of each battery system is configured to provide switching signals to the battery modules and the H-bridge circuit to operate each battery system at a different AC phase.
[0021] The controllers of each battery system are integrated into a common controller.
[0022] According to a second aspect, there is provided a method of operating the battery system according to the first aspect, the method comprising:
[0023] the controller providing a first series of control signals to first and second switches in the plurality of battery modules to provide a first varying power supply to the H-bridge circuit when the H-bridge circuit connects the first and second output terminals across the plurality of battery modules in a first direction during a first half cycle; and
[0024] the controller providing a second series of control signals to first and second switches in the plurality of battery modules to provide a second varying power supply to the H-bridge circuit when the H-bridge circuit connects the first and second output terminals across the plurality of battery modules in a second opposite direction during a second half cycle;
[0025] wherein the controller switches the H-bridge circuit from the first direction to the second direction between first and second half cycles.
[0026] The first and second series of control signals may be identical.
[0027] The controller may supply a control signal to each of the plurality of battery modules to open or close the first switch and to close or open the second switch accordingly.
[0028] Each of the plurality of battery modules may include a control input connected to the first switch via a non-inverting amplifier and to the second switch via a first inverting amplifier.
[0029] According to a third aspect, there is provided a controller for the battery system according to the first aspect, the controller being configured to:
[0030] providing a first series of control signals to first and second switches in the plurality of battery modules to provide a first varying power supply to the H-bridge circuit when the H-bridge circuit connects the first and second output terminals across the plurality of battery modules in a first direction during a first half cycle; and
[0031] providing a second series of control signals to first and second switches in the plurality of battery modules to provide a second varying power supply to the H-bridge circuit when the H-bridge circuit connects the first and second output terminals across the plurality of battery modules in a second opposite direction during a second half cycle,
[0032] Wherein the controller is configured to switch the H-bridge circuit from the first direction to the second direction between first and second half cycles.
[0033] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS
[0034] Embodiments will be described, by way of example only, with reference to the accompanying drawings, in which:
[0035] Figure 1 is a schematic circuit diagram of an example battery system;
[0036] Figure 2 yes Figure 1 another schematic circuit diagram of an exemplary battery system of the type shown;
[0037] Figure 3 is used for Figure 2 A circuit diagram of an example battery module of a battery system;
[0038] Figure 4 is a schematic diagram of an example three-phase battery system connected to the windings of a three-phase motor;
[0039] Figure 5 yes Figure 4 Schematic graphs of output voltage and switching sequence of an example battery system;
[0040] Figure 6 is a schematic diagram of an exemplary three-phase battery system connected to the windings of a three-phase electric machine and having an auxiliary battery charging system; and
[0041] Figure 7 is a flow chart illustrating a series of operations performed by an example controller of a battery system.
[0042] It should be noted that the drawings are diagrammatic and not drawn to scale. For clarity and convenience in the drawings, the relative sizes and proportions of the various parts of these drawings have been shown in an exaggerated or reduced manner. The same reference numerals are used throughout to refer to corresponding or similar features in modified and different embodiments. DETAILED DESCRIPTION
[0043] Figure 1is a schematic diagram of an example battery system 100. Battery system 100 includes a plurality of series-connected battery modules 101a-c, an H-bridge circuit 105, and a controller 108. Each battery module 101a-c includes a plurality of series-connected battery cells 102a-c, a first switch 103a-c connected in series with the plurality of battery cells 102a-c, and a second switch 104a-c connected in parallel with the plurality of battery cells 102a-c. Each of the first switches 103a-c and the second switches 104a-c may be a FET switch.
[0044] The H-bridge circuit 105 is connected across the plurality of battery modules 101a-c between a first battery terminal 1091 and a second battery terminal 1092. A first output terminal 106a and a second output terminal 106b of the H-bridge circuit 105 are provided for connection to a winding 107 of an electric machine. The battery modules 101a-c and the H-bridge circuit 105 may form part of a multi-phase battery system having additional similarly arranged battery modules and H-bridges, as further described below.
[0045] The controller 108 is configured to control the switching of the H-bridge circuit 105 and the first and second switches 103a-c, 104a-c in each battery module 101a-c to transfer power between the motor windings 107 and the plurality of battery modules 101a-c. Power can be transferred from the battery modules 101a-c to the windings 107 to power the motors when operating as motors, or from the windings 107 to the battery modules 101a-c when the motors are operating as generators. Alternatively, the first and second output terminals 106a, 106b can be connected to another type of AC power load or a generator.
[0046] The H-bridge circuit 105 includes first, second, third and fourth switches 110 1-4 , shown in this example as MOSFETs, with a reverse biased diode connected across each MOSFET shown. Alternatively, the switch 110 1-4 The controller 108 operates the H-bridge circuit 105 to open and close the switch 110. 1-4 , thereby connecting and disconnecting the battery modules 101a-c to the output terminals 106a, 106b. The controller 108 selects the polarity of the connection to the output terminals 106a, 106b by: in a first direction, when the second switch 1102 and the third switch 1103 are closed, selecting the first switch 1101 and the fourth switch 1104 to be open; and in a second direction, when the second switch 1102 and the third switch 1103 are open, selecting the first switch 1101 and the fourth switch 1104 to be closed.
[0047] In order to transfer power from the plurality of battery modules 101a-c to the output terminals 106a, 106b, the controller 108 selectively controls the switching of the first switch 103a-c and the second switch 104a-c of each of the battery modules 101a-c. Figure 1 In the example shown, three battery modules 101a - c are shown for simplicity, but in practice more battery modules connected in series may be used to allow for higher voltage output and finer control of the DC voltage across the battery terminals 1091 , 1092 .
[0048] Figure 2 Shown are 19 individual battery modules 201 1-19 In the example battery system 200, each battery module may be similar to Figure 1 A similar H-bridge circuit 205 is connected between battery terminals 2091 and 2092, which in this case are a positive terminal BP and a negative terminal BN. Although the operation of the H-bridge circuit 205 can switch the polarity of the output terminals 206a and 206b, the output terminals 206a and 206b can also be considered as a positive terminal MP and a negative terminal MN.
[0049] The controller 208 sends a signal to the battery module 201 1-19 Each of them provides a control signal ctl <1> -ctl <19> , to selectively switch or bypass each of the modules and to the H-bridge circuit switch 210 1-4 Provide control signal hctl <1> -hctl <4> to connect the battery terminals 2091, 2092 to the output terminals 206a, 206b in either the positive or negative direction.
[0050] Figure 3 Shows that it can be constructed Figure 1 and 2 Battery modules 101a-c, 201 1-19 An example battery module 301 of one of the battery modules 301. A series connection arrangement of such battery modules 301 can be formed by connecting the positive terminal vp and the negative terminal vn to other similar battery modules. The battery module 301 includes a plurality of battery cells 302 1-6 In this example, six battery cells are shown. The first switch 303 is connected to the plurality of battery cells 302. 1-6 The second switch 304a is connected in series with the plurality of battery cells 302 1-6 A third switch 304b may also be provided, which is also connected to the plurality of battery cells 302 1-6 A third switch 304b may be provided in the event of a failure of the second switch 304a, so that a circuit may be formed to bypass the battery cell 302. 1-6Reliable short circuit.
[0051] The operation of the two switches 303, 304a (and the third switch 304b, if provided) is controlled by a control signal ct1 at a control input 311. The control signal ct1 is provided to the first switch 303 via a non-inverting amplifier 312, and to the second switch 304a (and the third switch 304b) via inverting amplifiers 313a, 313b. Thus, when the control signal ct1 closes the first switch 303, the second switch 304a and the third switch 304b are opened, and when the control signal ct1 opens the first switch 303, the second switch 304a and the third switch 304b are closed.
[0052] Figure 4 Schematically shown with connections to three AC batteries 401 1-3 The controller 108 of the battery system 400, each AC battery includes a plurality of battery modules and an H-bridge circuit, as described above with respect to Figure 1 AC battery 401 1-3 The output terminal of each of the three-phase motors is connected to the corresponding winding 107 1-3 Connection. Winding 107 1-3 In an alternative embodiment, a star arrangement may be used, both types of connection arrangements being known to the skilled person.
[0053] The controller 108 controls the AC battery 401 1-3 The first and second switches in each of the battery modules and the AC battery 401 1-3 The switches in the H-bridge circuit are switched so that currents of appropriate magnitude and phase pass through the winding 107. 1-3 The magnitude and phase of the current are determined by the connection to the AC battery 401 1-3 Each of the H-bridge circuits is controlled by the number of battery modules bypassed therefrom.
[0054] Figure 5 Shown across Figure 4 AC Battery 401 1-3 An example graph of the voltage at each of the battery terminals versus phase is shown for three time-varying voltage supplies 503. 1-3 、505 1-3 , and the corresponding control signal 504 1-3Considering only the first phase signal 5011, a first series of control signals is provided to the first switches 103a-c and the second switches 104a-c in the plurality of battery modules 101a-c to provide a first variable power supply 5031 when the H-bridge circuit 105 connects the first output terminal 106a and the second output terminal 106b across the plurality of battery modules 101a-c in a first direction (e.g., positive) during the first half-cycle 502a. In this example, the switching control is indicated by the high state of the control signal 5041. A second series of control signals is provided to the first switches 103a-c and the second switches 104a-c in the plurality of battery modules 101a-c to provide a second variable power supply 5051 when the H-bridge circuit 105 connects the first output terminal 106a and the second output terminal 106b across the plurality of battery modules 101a-c in a second, opposite direction (e.g., negative) during the second half-cycle 502b. In this example, the switching control is indicated by the low state of the control signal 5041. This has the effect of inverting the second varying supply 5051 across the output terminals 106a, 106b, thereby providing the negative half of each complete AC cycle. Corresponding control signals 5042, 5043 are provided to the second phase 5012 and the third phase 5013 to provide corresponding first supplies 5032, 5033 and second supplies 5052, 5053, wherein the second supplies are inverted in the second half of each cycle by the control signals 5042, 5043. In this way, a three-phase motor can be powered by an AC battery, with the phases operating 120 degrees out of phase.
[0055] The controller 108 is configured to switch the H-bridge circuit between the positive and negative connections between the battery terminals and the output terminals only when the voltage across the battery terminals is zero or close to zero (i.e., within a predetermined band around zero). This allows the switching losses from the H-bridge circuit to be minimized, resulting in only conduction losses during each half cycle. By switching the polarity of the H-bridge within each half cycle, the number of switches required in each battery module can be reduced from a typical minimum of four or five to a minimum of only two or three, thereby significantly saving cost and reducing the FIT value of the entire battery system. Another advantage is that when the battery system is inoperable, there is no voltage on the battery terminals because the switches in the battery modules will all be disconnected, providing a safe state when the battery system is inactive.
[0056] Figure 6 An example three-phase battery system 600 for vehicle applications is shown in FIG. 6 , incorporating three AC batteries 601. 1-3 and auxiliary battery charging system 605. AC battery 601 1-3It operates under the control of control signals p1ctl<19:1>, p1hctl<4:1> for respectively controlling the battery module and H-bridge switches of the first AC battery 6011, control signals p2ctl<19:1>, p2hctl<4:1> for respectively controlling the battery module and H-bridge switches of the second AC battery 6012, and control signals p3ctl<19:1>, p3hctl<4:1> for respectively controlling the battery module and H-bridge switches of the third AC battery 6013.
[0057] When driving the vehicle, the AC battery 601 1-3 to the corresponding winding 607 of the motor 1-3 Provides power. When the motor is stationary, each AC battery 601 1-3 The H-bridge circuit in the circuit can be disconnected to allow winding 607 1-3 The power is supplied to the auxiliary battery connected between the auxiliary battery terminal 603 and the chassis terminal 604. 1-3 Delivered to the auxiliary battery, the auxiliary charging module 602 1-3 Power is transferred from each of the three AC batteries to the auxiliary battery terminal 603 .
[0058] When the motor is running at low power, the H-bridge circuit can be operated using PWM control to allow a DC voltage to be present across the battery module to power the motor and provide charge to the auxiliary battery.
[0059] The battery system configuration allows for the detection of various failure modes and, if necessary, the ability to overcome or avoid them. Various measurements can be made on the battery modules, for example by measuring the impedance of each module to determine if a module is faulty. A faulty module can be isolated by adjusting the control signal provided to the AC battery to bypass it while allowing the battery system to continue operating (optionally providing a warning flag to indicate the need for repair).
[0060] In electric vehicle power applications, during normal operation, when the motor is running, the average battery voltage is typically high enough to allow the auxiliary battery to be charged. In this case, the output waveform of each AC battery is a rectified sine wave, which can have a frequency in the range of 20 to 1000 Hz. In stop mode, the H-bridge in each AC battery is disconnected to disconnect the motor. The battery output voltage can be set to approximately half of the full voltage and power provided to charge the auxiliary battery. The batteries can be cycled to ensure that all battery modules are used equally. In slow run mode, the battery system can be operated at an intermediate voltage, where the output H-bridge can be operated at a low frequency using PWM control. The DC voltage used can be set to provide a compromise between efficient motor operation and auxiliary power charging.
[0061] In a specific example embodiment, the nominal maximum DC voltage of each AC battery is 400V. Using battery cells with a nominal DC voltage of 3.6V and 6 battery cells per module, 19 modules will provide the required maximum voltage. FET switches with an on-resistance of approximately 0.5mΩ can be used in the battery modules. IGBTs can be used in the H-bridge circuit.
[0062] When charging a battery system, a 3-pole relay can be used to connect the battery to a common ground. The AC battery can be switched to match the incoming rectified AC waveform and provide a constant charging current to the battery module. When the motor is connected, an auxiliary relay can be used to isolate the battery from the connector.
[0063] Figure 7 is a schematic flow chart illustrating a series of operations performed by an example controller for operating a battery system of the type described herein. In a first step 701, the controller operates switches in the H-bridge 105 to connect the battery terminals across the output terminals in the positive direction. In step 702, a first series of control signals is then provided to the plurality of battery modules over a first half cycle. At the end of the first half cycle, in step 703, the controller operates switches in the H-bridge to connect the battery terminals across the output terminals in the negative direction. In step 704, a second series of control signals is then provided to the plurality of battery modules over a second half cycle. The method is then repeated by returning to step 701 for a subsequent cycle. As described above, the H-bridge connection is switched only when the voltage across the output terminals is approximately zero.
[0064] After reading this disclosure, the skilled person will find other changes and modifications. Such changes and modifications may involve equivalent other features that are already known to us in the field of battery systems and can be used as a substitute or supplement to the features already described herein.
[0065] Although the appended claims are directed to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel feature or any novel combination of features disclosed herein, whether explicitly or implicitly, or any generalization thereof, regardless of whether it relates to the same invention as claimed in any claim currently or whether it alleviates the same technical problem as any or all of the technical problems alleviated by the present invention.
[0066] Features described in the context of separate embodiments may also be provided in combination in a single embodiment. Conversely, various features described in the context of a single embodiment for the sake of brevity may also be provided separately or in any suitable subcombination. Applicants hereby caution that new claims may be formulated based on such features and / or combinations of such features during the prosecution of this application or any further application derived therefrom.
[0067] For the sake of completeness, it is also stipulated that the term "comprising" does not exclude other elements or steps, the term "a / an" does not exclude a plurality, a single processor or other unit may perform the functions of several components recited in the claims, and the reference signs in the claims should not be construed as limiting the scope of the claims.
Claims
1. A battery system (100), characterized in that: include: A plurality of battery modules (101a-c) connected in series, each battery module (101a-c) comprising a plurality of battery cells (102a-c) connected in series, a first switch (103a-c) connected in series with the plurality of battery cells (102a-c), and a second switch (104a-c) connected in parallel with the plurality of battery cells (102a-c); an H-bridge circuit (105) connected across the plurality of battery modules (101a-c) and having a first output terminal (106a) and a second output terminal (106b); and A controller (108) is configured to control the H-bridge circuit (105) and switching of the first switch (103a-c) and the second switch (104a-c) in each battery module (101a-c) to transfer power between the first output terminal (106a) and the second output terminal (106b) and the plurality of battery modules (101a-c).
2. The battery system (100) according to claim 1, characterized in that The controller (108) is configured to supply a control signal to each of the plurality of battery modules (101a-c) to open or close the first switch (103a-c) and to close or open the second switch (104-c) accordingly.
3. The battery system (100) according to claim 1, characterized in that The controller (108) is configured to convert DC power from the plurality of battery modules (101a-c) into AC power at the first output terminal (106a) and the second output terminal (106b) within each consecutive AC cycle by: providing a first series of control signals to the first switches (103a-c) and the second switches (104a-c) in the plurality of battery modules (101a-c) to provide a first varying power supply to the H-bridge circuit (105) when the H-bridge circuit (105) connects the first output terminal (106a) and the second output terminal (106b) across the plurality of battery modules (101a-c) in a first direction within a first half cycle (502a); as well as providing a second series of control signals to the first switches (103a-c) and the second switches (104a-c) in the plurality of battery modules (101a-c) to provide a second varying power supply to the H-bridge circuit (105) when the H-bridge circuit (105) connects the first output terminal (106a) and the second output terminal (106b) across the plurality of battery modules (101a-c) in a second opposite direction within a second half cycle (502b), The controller (108) is configured to switch the H-bridge circuit (105) from the first direction to the second direction between the first half cycle (301) and the second half cycle (302).
4. The battery system (100) according to claim 3, characterized in that The first and second series of control signals are identical.
5. The battery system (100) according to any of the preceding claims, characterized in that The first switch (103a-c) and the second switch (104a-c) are FET switches.
6. The battery system (100) according to any of the preceding claims, characterized in that The winding (107) of the motor is connected between the first output terminal (106a) and the second output terminal (106b).
7. The battery system (100) according to any of the preceding claims, characterized in that Each of the plurality of battery modules (101a-c, 301) includes a control input (311) connected to the first switch (303) via a non-inverting amplifier (312) and to the second switch (304a) via a first inverting amplifier (313a).
8. A multi-phase battery system (400) comprising a plurality of battery systems according to claim 1, characterized in that: The controller (108) of each battery system is configured to provide switching signals to the battery modules (101a-c) and the H-bridge circuit (105) to operate each battery system (100) at a different AC phase.
9. A method of operating the battery system (100) according to claim 1, characterized in that: The method comprises: The controller (108) provides a first series of control signals to the first switches (103a-c) and the second switches (104a-c) in the plurality of battery modules (101a-c) to provide a first varying power supply to the H-bridge circuit (105) when the H-bridge circuit (105) connects the first output terminal (106a) and the second output terminal (106b) across the plurality of battery modules (101a-c) in a first direction within a first half cycle (502a); and The controller (108) provides a second series of control signals to the first switches (103a-c) and the second switches (104a-c) in the plurality of battery modules (101a-c) to provide a second varying power supply to the H-bridge circuit (105) when the H-bridge circuit (105) connects the first output terminal (106a) and the second output terminal (106b) across the plurality of battery modules (101a-c) in a second opposite direction within a second half cycle (502b). The controller (108) switches the H-bridge circuit (105) from the first direction to the second direction between the first half cycle (301) and the second half cycle (302).
10. A controller (108) for the battery system (100) according to claim 1, characterized in that: The controller (108) is configured to: providing a first series of control signals to the first switches (103a-c) and the second switches (104a-c) in the plurality of battery modules (101a-c) to provide a first varying power supply to the H-bridge circuit (105) when the H-bridge circuit (105) connects the first output terminal (106a) and the second output terminal (106b) across the plurality of battery modules (101a-c) in a first direction within a first half cycle (502a); as well as providing a second series of control signals to the first switches (103a-c) and the second switches (104a-c) in the plurality of battery modules (101a-c) to provide a second varying power supply to the H-bridge circuit (105) when the H-bridge circuit (105) connects the first output terminal (106a) and the second output terminal (106b) across the plurality of battery modules (101a-c) in a second opposite direction within a second half cycle (502b), The controller (108) is configured to switch the H-bridge circuit (105) from the first direction to the second direction between the first half cycle (301) and the second half cycle (302).