Pulse width modulation generation strategy
The PWM generation method for iEESM in electric and hybrid vehicles balances magnetic flux and avoids duty cycle errors by applying half of the duty cycle at the start and end of a section period, improving transformer efficiency and dynamic response.
Patent Information
- Application Number
- JP2024231517
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2023-12-30
- Filing Date
- 2024-12-27
- Publication Date
- 2025-07-30
AI Technical Summary
Existing pulse width modulation (PWM) generation procedures for induction-wound field synchronous motors (iEESM) in electric and hybrid vehicles cause transformer saturation and duty cycle errors under dynamic conditions, leading to decreased energy transfer and dynamic response issues.
A PWM generation method where half of the duty cycle is applied at the start and half at the end of a defined section period, balancing the magnetic flux and avoiding duty cycle errors, using a phase shift full bridge converter (PSFBC) to supply bipolar rectangular voltage to the transformer.
This method prevents transformer saturation and duty cycle errors, ensuring balanced magnetic flux and maintaining efficient dynamic torque control without the need for additional corrections, thus enhancing the transformer's energy transfer efficiency.
Smart Images
Figure 2025111390000001_ABST
Abstract
Description
Technical Field
[0001] The present invention relates to the fields of electronics and electrical engineering, and more particularly to the field of electromechanics.
Background Art
[0002] Electromechanics can be used in electric and hybrid vehicles to provide torque to the vehicle's wheels. The motor can be, for example, a wound-field synchronous motor, i.e., an EESM. Since permanent magnet synchronous motors require the use of rare earth materials, EESM is an alternative to permanent magnet synchronous motors.
[0003] In an EESM, energy must be transferred from the non-rotating part of the electromechanics, i.e., the stator, to the rotating part, i.e., the rotor. Such power transfer can be either conductive or inductive, but inductive power transfer is preferred today because it causes no mechanical losses and is maintenance-free.
[0004] In an induction wound-field synchronous motor, i.e., an iEESM, the stator and rotor are associated with a converter, a transformer, and a rectifier. The converter supplies voltage to the transformer, and the rectifier is connected between the transformer and the rotor to supply the desired positive voltage to the rotor winding.
[0005] For dynamic and efficient torque control of an iEESM, it is necessary to change the rotor current according to the required torque and further boundary conditions (e.g., DC voltage, current limit, motor speed). The torque requirement usually follows the driver's request for the vehicle's accelerator pedal or brake pedal. Therefore, since the torque requirement changes over time, it is necessary to quickly change the torque of the electromechanics. As a result, the voltage supplied to the rotor winding of the iEESM must also be changed rapidly, which is why it is important to operate the converter correctly.
[0006] There are various pulse width modulation (PWM) generation procedures in the prior art for operating a converter. For example, it is known to generate a voltage pulse such that its center is placed within the PWM half-cycle. SUMMARY OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION
[0007] However, this generates a DC offset under dynamic operating conditions, i.e., when the duty cycle of the converter changes, and as a result, saturation occurs in the transformer under dynamic conditions. This saturation occurs, for example, because the transformer current does not stay within certain limits due to magnetic flux imbalance, leading to a decrease in the energy transferred from the primary side to the secondary side of the transformer.
[0008] Although other PWM generation procedures can correct the saturation problem, they may lead to duty cycle errors under dynamic conditions. Such duty cycle errors cause unexpected limitations in the output voltage of the transformer. This error can result in a decrease in the dynamic response in the current control loop, and since the requirements regarding the transient torque response characteristics are strict, this error must be avoided. MEANS FOR SOLVING THE PROBLEM
[0009] The present invention conforms to this context by providing a PWM generation procedure in which half of a given duty cycle of a converter is aligned with the start of a given time, and the other half of the duty cycle is aligned with the end of said time, thereby correcting the saturation problem and avoiding duty cycle errors.
[0010] In connection therewith, the present invention is a pulse width modulation (PWM) generation method for an electromechanical converter of an electric or hybrid vehicle, wherein the converter is operated with a phase shift associated with a duty cycle, the converter supplies a voltage to an electromechanical transformer, and the pulse width modulation generation method includes operating the converter with a duty cycle change request, a section period being defined between two duty cycle change requests, and the pulse width modulation generation method being characterized in that half of the duty cycle is applied at the start of the section period and half of the duty cycle is applied at the end of the section period.
[0011] The pulse width modulation generation procedure according to the present invention, i.e., the PWM generation procedure, is here used to operate a DC / AC converter incorporated in an electromechanical machine. Such an electromechanical machine is, more precisely, an induction-wound field synchronous motor or iEESM. The electromechanical machine comprises, together with the converter, various other components including a transformer and a rectifier.
[0012] The converter is a phase shift full bridge converter, i.e., PSFBC, and is operated by a PWM generation method using a phase shift associated with a duty cycle ratio. The converter supplies a bipolar rectangular voltage applied to the transformer, more precisely to the primary side of this transformer. The secondary side of the transformer comprises a rectifier required to supply a desired positive voltage to the rotor winding. The primary side of the transformer is in the non-rotating part of the electromechanical machine, and the secondary side of the transformer is in the rotating part of the electromechanical machine.
[0013] The PWM generation method consists of operating the converter with a duty cycle change request. The duty cycle change is a duty cycle update. The duty cycle request changes over time. Two consecutive duty cycle change requests define a section period. In this section period, half of the duty cycle is applied at the start and the other half is applied at the end. As a result, the duty cycle coincides with the start and end of a given section period.
[0014] In such a PWM generation method, flux balance is achieved both in the steady state and dynamic operating conditions, and thus saturation in the transformer is avoided.
[0015] Furthermore, since the duty cycle starts simultaneously with the segmentation period, no duty cycle error is generated. In other words, the duty cycle and the segmentation period are adjusted, and the required duty cycle coincides with the actual duty cycle under dynamic operating conditions without the need for PWM generation correction or special processing.
[0016] According to an optional feature of the present invention, the segmentation period corresponds to a multiple of one PWM half-cycle.
[0017] As an example, the segmentation period may be equal to one PWM half-cycle. In other words, the PWM half-cycle corresponds to the interval between the first duty cycle change request and the second duty cycle change request. Naturally, other integer multiples of one PWM half-cycle may alternatively be used to define the segmentation period.
[0018] According to an optional feature of the present invention, the converter generates at least one positive voltage pulse and at least one negative voltage pulse in a given segmentation period.
[0019] This enables the redistribution of the flux of the converter in a given segmentation period.
[0020] According to an optional feature of the present invention, in a PWM half-cycle, the flux of the positive voltage pulse is equal to the flux of the negative voltage pulse.
[0021] As an example, there are the same number of positive voltage pulses as negative voltage pulses within a given PWM half-cycle. In the PWM cycle, the sum value of the positive voltage pulses and the sum value of the negative voltage pulses cancel each other out. Thereby, the flux of the converter is balanced.
[0022] According to an optional feature of the present invention, when the divided period corresponds to one PWM half-period, in a given PWM half-period, a positive pulse corresponding to half of the duty cycle is generated at the start of the PWM half-period, and a negative pulse corresponding to half of the duty cycle is generated at the end of the PWM half-period.
[0023] In such a case, the positive pulse corresponds to the pulse aligned to the left within a given PWM half-period, and the negative pulse corresponds to the pulse aligned to the right.
[0024] According to an optional feature of the present invention, when the divided period corresponds to one PWM half-period, in a given PWM half-period, a negative pulse corresponding to half of the duty cycle is generated at the start of the PWM half-period, and a positive pulse corresponding to half of the duty cycle is generated at the end of the PWM half-period.
[0025] In such a case, the negative pulse corresponds to the pulse aligned to the left within a given PWM half-period, and the positive pulse corresponds to the pulse aligned to the right.
[0026] According to an optional feature of the present invention, when the divided period corresponds to one PWM period, in a given PWM period, a first negative pulse corresponding to half of the duty cycle is generated at the start of the PWM period, a positive pulse corresponding to the duty cycle is generated in the middle of the PWM period, and a second negative pulse corresponding to half of the duty cycle is generated at the end of the PWM period.
[0027] Therefore, the first negative pulse corresponds to the pulse aligned to the left within a given PWM period, and the second negative pulse corresponds to the pulse aligned to the right. When these are added, the duty cycles corresponding to the first negative pulse and the second negative pulse are equal to the duty cycle corresponding to the positive pulse.
[0028] According to an optional feature of the present invention, when the section period corresponds to one PWM period, in a given PWM period, a first positive pulse corresponding to half of the duty cycle is generated at the start of the PWM period, a negative pulse corresponding to the duty cycle is generated in the middle of the PWM period, and a second positive pulse corresponding to half of the duty cycle is generated at the end of the PWM period.
[0029] In this alternative form, the first positive pulse and the second positive pulse respectively correspond to the left-aligned pulse and the right-aligned pulse within a given PWM period. The combination of the duty cycle corresponding to the first positive pulse and the duty cycle corresponding to the second positive pulse is equal to the duty cycle corresponding to the negative pulse.
[0030] According to an optional feature of the present invention, the voltage pulse starts when the primary transformer current of the transformer is equal to 0.
[0031] This is the result of the transformer flux being balanced, which itself is the result of the positive and negative voltage pulses being balanced.
[0032] Other features, details, and advantages of the present invention will become clearer by reading the following description on the one hand and by referring to some examples of implementation given as illustration without limitation by reading the attached schematic diagrams on the other hand.
Brief Description of the Drawings
[0033]
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
[0034] The features, modifications, and different realizations of the present invention may be associated with each other in various combinations as long as they do not contradict each other or are not exclusive. In particular, a variant of the present invention that includes only a selected portion of the features described later from among the other features described is envisioned if the selected portion of the features provides technical advantages and / or is sufficient to distinguish the present invention from the prior art.
[0035] Throughout the drawings, like reference numerals refer to like elements.
[0036] FIGS. 1 and 2 are schematic diagrams of the electromechanical machine 1, or a part of the electromechanical machine 1. The electromechanical machine 1 is assumed to be mounted on a vehicle such as an electric vehicle or a hybrid vehicle and can be used to supply torque to the vehicle's wheels. Here, the electromechanical machine 1 is an induction-wound field-oriented synchronous motor, also known by its acronym iEESM.
[0037] As seen in FIG. 1, the electromechanical machine 1 includes a rotor 2 and a stator 4. In this example, the stator 4 is arranged around the rotor 2, but in other embodiments, the rotor 2 may be arranged around the stator 4. In an induction-wound field-oriented synchronous motor such as this electromechanical machine 1, power is transmitted to the windings attached to the rotor 2 using a transformer 6. The transformer 6 is more precisely a rotating high-frequency transformer. The transformer 6 includes a primary side 8 and a secondary side 10, and these two sides 8, 10 differ in that the primary side 8 is in the non-rotating part of the electromechanical machine 1, i.e., the stator, and the secondary side 10 is in the rotating part of the electromechanical machine 1, i.e., the rotor. In FIG. 1, the separation between the non-rotating part and the rotating part is shown as a dashed line.
[0038] The transformer 6 is associated with a rectifier 12. The rectifier 12 is a rotary rectifier and is thus arranged on the secondary side 10 of the transformer 6, i.e., on the rotating part of the electric machine 1. The role of the rectifier 12 is to supply only a positive voltage to the rotor 2. This rectifier 12 is implemented, together with the transformer 6, in a transmitter 14, here an inductive transmitter.
[0039] As can be seen in Figure 2, a converter 16 is connected to the primary side 8 of the transformer 6. The converter 16 is a phase-shifted full-bridge converter, i.e., a PSFBC. It operates at a variable pulse-width modulation frequency, i.e., a PWM frequency. The PWM frequency of the converter 16 can actually vary within a wide range, for example, from 5 kHz to 100 kHz, depending on the design of the converter 16 and / or the transformer 6 and / or the required current of the rotor 2.
[0040] The converter 16 is controlled by an electronic control unit of the electric machine, such as a microcontroller, a field-programmable gate array (FPGA), or a digital signal processor (DSP), to convert direct current from the vehicle battery into alternating current in order to supply an appropriate voltage to the primary side 8 of the transformer 6. The converter 16 may also include an input capacitor 17. As shown here, the converter 16 includes four transistors 18, two upper transistors 18A and two lower transistors 18B. Each upper transistor 18A is associated with a lower transistor 18B, and they are controlled in reverse. In particular, an additional interlock time is introduced into the electric machine 1 to avoid a short circuit of the input capacitor 17. This additional interlock time delays the switch-on command of the upper transistor 18A or the lower transistor 18B of one half-bridge compared to the switch-off command of the upper transistor 18A or the lower transistor 18B of one half-bridge.
[0041] The converter 16 operates according to the PWM generation method according to the present invention, that is, the PWM generation procedure, in order to perform dynamic and efficient torque control of the electric machine 1. For this purpose, the converter 16 is operated by phase shift, and each phase shift is included between 0° and 180°. Further, a duty cycle ratio is associated with each phase shift of the converter 16, and such a duty cycle ratio is included between 0 and 1.
[0042] Figure 3 shows the relationship between the phase shift, the duty cycle associated with these phase shifts, and the voltage pulses applied in relation to these duty cycles. For this purpose, Figure 3 shows five consecutive PWM periods, and the phase shift is changed by the change of each PWM period. The first PWM period T PWM During 1, the 180° phase shift between the first upper transistor 18A and the second upper transistor 18A corresponds to a duty cycle ratio of 1, and the application duration of the voltage pulse is the entire duration of this first PWM period T PWM 1. During the second PWM period T PWM 2, the 135° phase shift between the first upper transistor 18A and the second upper transistor 18A corresponds to a duty cycle ratio of 0.75. During the third PWM period T PWM 3, the 90° phase shift between the first upper transistor 18A and the second upper transistor 18A corresponds to a duty cycle ratio of 0.5, and the application duration of the voltage pulse is half of the duration of this third PWM period T PWM 3. During the fourth PWM period T PWM 4, the 45° phase shift between the first upper transistor 18A and the second upper transistor 18A corresponds to a duty cycle ratio of 0.25. During the fifth PWM period T PWM 5, the 0° phase shift between the first upper transistor 18A and the second upper transistor 18A corresponds to a duty cycle ratio of 0, and no voltage pulse is applied.
[0043] Converter 16 generates voltage waveform 20, and such waveform 20 can be seen in FIGS. 3, 4 and 6. The voltage waveform 20 can be divided into a plurality of PWM periods. Generally speaking, when the duty cycle ratio is equal to 1, a positive or negative voltage is continuously applied during all given PWM periods, there is no zero voltage value, and each positive or negative voltage is applied for half of the PWM period. However, for example, when the duty cycle ratio is equal to 0.5, positive and negative voltages are applied for a shorter duration during a given PWM period, a zero voltage value is supplied between them, and each positive or negative voltage is applied for half of the time taken when the duty cycle ratio is equal to 1.
[0044] To change the duty cycle, converter 16 operates in response to a duty cycle change request. Such duty cycle change requests are indicated by the white arrows in FIGS. 4 and 6, which represent the first embodiment and the second embodiment respectively. In these two figures, the required duty cycle is shown at the top, and the voltage pulses implemented according to the PWM generation method of the present invention to obtain the duty cycle are shown at the bottom.
[0045] Two consecutive duty cycle change requests define a section period 22. This section period 22 corresponds to one PWM period in the first embodiment of FIG. 4 and to a PWM half-period in the second embodiment of FIG. 6. That is, the duty cycle change request occurs every PWM period in the first embodiment and every PWM half-period in the second embodiment. More generally, the section period 22 may correspond to any integer multiple of the PWM half-period, which means that the duty cycle change request can occur every multiple of the PWM half-period. Preferably, the section period 22 corresponds to a control time. Such control time is defined by the time during which a specific voltage is obtained. As a result, the control time may be equal to the PWM period according to the first embodiment or the PWM half-period according to the second embodiment. Therefore, the duty cycle change request occurs between two consecutive control times.
[0046] According to the present invention, in the PWM generation method, half of the duty cycle is applied at the start of a given section period 22, and half of the duty cycle is applied at the end of the section period 22.
[0047] It should be noted that in both embodiments, the converter 16 generates at least one positive voltage pulse 24 and at least one negative voltage pulse 26 for each section period 22. More specifically, in the first embodiment, the converter 16 generates one positive voltage pulse 24 and two negative voltage pulses 26 for each section period 22 corresponding to one PWM period. On the other hand, in the second embodiment, the converter 16 generates one positive voltage pulse 24 and one negative voltage pulse 26 respectively for each section period 22 corresponding to one PWM half-period.
[0048] As described above, in the first embodiment of FIG. 4, that is, for the section period 22 equal to one PWM period, the duty cycle change request occurs for each such PWM period. In the case of a duty cycle of 0.25 required at the first time t0.5, 0.125, which is half of the duty cycle, is realized at the start of the PWM period at the first time t0.5, and the remaining half of the duty cycle, also 0.125, is realized at the end of the PWM period at the second time t1.5. Similarly, in the case of a duty cycle of 0.5 required at the second time t1.5, 0.25, which is half of the duty cycle, is realized at the start of the PWM period at the second time t1.5, and the remaining half of the duty cycle, also 0.25, is realized at the end of the PWM period at the third time t2.5.
[0049] Furthermore, in this first embodiment, since the section period 22 is one PWM period, there is an additional pulse at the center of the PWM period to supply an appropriate voltage during the PWM period to obtain the required duty cycle. As shown in FIG. 4, for each PWM period, a first pulse 26A corresponding to half of the duty cycle generated at the start of the PWM period, here a negative pulse 26A, then an additional pulse 24 of the opposite sign, here a positive pulse 24 corresponding to the duty cycle generated in the middle of the PWM period, and a second pulse 26B corresponding to half of the duty cycle generated at the end of the PWM period, here a second negative pulse 26B. Thus, after the first duty cycle change request of the first embodiment, the first negative pulse 26A corresponding to 0.125 duty cycle starts at the first time t0.5, the positive pulse 24 corresponding to 0.25 duty cycle is generated centered around the fourth time t1, and the second negative pulse 26B corresponding to 0.125 duty cycle is generated to end at the second time t1.5.
[0050] Considering two consecutive section periods 22 having a first section period 22 associated with a duty cycle requirement of 0.25 and a second section period 22 associated with a duty cycle requirement of 0.5, as a result of the procedure of the present invention, at the start of the first section period 22, a first pulse corresponding to a duty cycle of 0.125, here a negative pulse 26A, is generated, then at the middle of the first section period 22, a second pulse corresponding to a duty cycle of 0.25, here a positive pulse 24, is generated, then a third pulse corresponding to a duty cycle of 0.325, here another negative pulse 26B, is generated, and the 0.325 duty cycle corresponds to the 0.125 duty cycle at the end of the first section period 22 and the 0.25 duty cycle at the start of the second section period 22.
[0051] Although not shown here, a first positive pulse 24A corresponding to half of the duty cycle is generated at the start of the division period 22, a negative pulse 26 corresponding to the duty cycle is generated in the middle of the division period 22, and a second positive pulse 24B corresponding to half of the duty cycle is generated at the end of the division period 22. There may be a variant of the first embodiment of the present invention.
[0052] In the second embodiment of FIG. 6, that is, for the division period 22 equal to one PWM half-cycle, a duty cycle change request occurs every such PWM half-cycle, that is, twice as often as in the first embodiment. As can be seen in FIG. 6, for a duty cycle of 0.25 requested at the first time t0.5, 0.125, which is half of the duty cycle, is realized at the start of the PWM half-cycle at the first time t0.5, and the other half of the duty cycle, also 0.125, is realized at the end of the PWM half-cycle at the fourth time t1. Similarly, for a duty cycle of 0.5 requested at the fourth time t1, 0.25, which is half of the duty cycle, is realized at the start of the PWM cycle at the fourth time t1, and the remaining half of the duty cycle, also 0.25, is realized at the end of the PWM cycle at the second time t1.5.
[0053] As shown in FIG. 6, for the division period 22 of one PWM half-cycle, for each such PWM half-cycle, there is only one negative pulse 26 corresponding to half of the duty cycle generated at the start of the PWM half-cycle and one positive pulse 24 corresponding to half of the duty cycle generated at the end of the PWM half-cycle. Therefore, after the first duty cycle change request in the second embodiment, the negative pulse 26 corresponding to a 0.125 duty cycle starts at the first time t0.5, and the positive pulse 24 corresponding to a 0.125 duty cycle ends at the fourth time t1. In the second embodiment, there is no voltage pulse generated in the middle of the PWM half-cycle.
[0054] Similar to what has been described for the first embodiment, the second embodiment alternatively may consist of a positive pulse 24 corresponding to half of the duty cycle generated at the start of the section period 22 and a negative pulse 26 corresponding to half of the duty cycle generated at the end of the section period 22, without exceeding the scope of the present invention.
[0055] As a result of the above, in either the first embodiment of FIG. 4 or the second embodiment of FIG. 6, in a given section period 22, the positive voltage pulse 24 and the negative voltage pulse 26 are balanced. In other words, in one PWM period or each PWM half period of the first embodiment and the second embodiment, the magnetic flux of the positive voltage pulse 24 is equal to the magnetic flux of the negative voltage pulse 26.
[0056] FIG. 5 regarding the first embodiment where the section period 22 is equal to one PWM period shows different waveforms. These waveforms correspond, in order from the top, to the transformer input voltage waveform 28, the converter input current waveform 30, and the primary transformer current waveform 32 generated by the transformer 6. Here, "correspond" means that the transformer input voltage waveform 28, the converter input current waveform 30, and the primary transformer current waveform 32 are observed in the same PWM period.
[0057] As can be seen in this FIG. 5, there is no deflection in the waveforms, and in the transformer input voltage waveform 28, the converter input current waveform 30, and the primary transformer current waveform 32, the positive and negative pulses are balanced. More precisely, the primary transformer current waveform 32 is symmetric about the zero current line, which is a characteristic of a balanced primary transformer current. Such a balanced primary transformer current is desired because it means that saturation and / or magnetic flux imbalance of the transformer 6 are prevented.
[0058] In addition, there is no duty cycle offset in the waveforms, and in particular, there is no error between the required duty cycle and the realized duty cycle. As a result of the balanced primary transformer current, the voltage pulse starts when the primary transformer current of the transformer 6 is equal to 0. This avoids a decrease in the dynamic response of torque control.
[0059] Accordingly, the present invention encompasses a PWM generation procedure that generates a balanced transformer flux without a duty cycle error by aligning the duty cycle of the converter with the start and end of a given predetermined time.
[0060] Many modifications and other embodiments of the invention described herein will come to mind to those of ordinary skill in the art having the benefit of the teachings presented in the foregoing description and the related drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.
Claims
1. A pulse width modulation (PWM) generation method for a converter (16) of an electromechanical machine (1) of an electric or hybrid vehicle, wherein the converter (16) operates with a phase shift associated with a duty cycle, the converter (16) supplies a voltage (20) to a transformer (6) of the electromechanical machine (1), and the pulse width modulation generation method includes operating the converter (16) with a duty cycle change request, a section period (22) is defined between two duty cycle change requests, and in the pulse width modulation generation method, half of the duty cycle is applied at the start of the section period (22), and half of the duty cycle is applied at the end of the section period (22). A pulse width modulation generation method characterized by this.
2. The pulse width modulation generation method according to claim 1, wherein the section period (22) corresponds to a multiple of one PWM half cycle.
3. The pulse width modulation generation method according to claim 1 or 2, wherein the converter (16) generates at least one positive voltage pulse (24) and at least one negative voltage pulse (26) in a given section period (22).
4. The pulse width modulation generation method according to any one of claims 1 to 3, wherein in a PWM half cycle, the magnetic flux of the positive voltage pulse (24) is equal to the magnetic flux of the negative voltage pulse (26).
5. When the section period (22) corresponds to one PWM half cycle, in a given PWM half cycle, a positive pulse (24) corresponding to half of the duty cycle is generated at the start of the PWM half cycle, and a negative pulse (26) corresponding to half of the duty cycle is generated at the end of the PWM half cycle. The pulse width modulation generation method according to any one of claims 1 to 4 combined with claim 2.
6. When the section period (22) corresponds to one PWM half cycle, in a given PWM half cycle, a negative pulse (26) corresponding to half of the duty cycle is generated at the start of the PWM half cycle, and a positive pulse (24) corresponding to half of the duty cycle is generated at the end of the PWM half cycle. The pulse width modulation generation method according to any one of claims 1 to 4 combined with claim 2.
7. When the division period (22) corresponds to one PWM period, in a given PWM period, a first negative pulse (26A) corresponding to half of the duty cycle is generated at the start of the PWM period, a positive pulse (24) corresponding to the duty cycle is generated in the middle of the PWM period, and a second negative pulse (26B) corresponding to half of the duty cycle is generated at the end of the PWM period. The pulse width modulation generation method according to any one of claims 1 to 6 in combination with claim 2.
8. When the division period (22) corresponds to one PWM period, in a given PWM period, a first positive pulse (24A) corresponding to half of the duty cycle is generated at the start of the PWM period, a negative pulse (26) corresponding to the duty cycle is generated in the middle of the PWM period, and a second positive pulse (24B) corresponding to half of the duty cycle is generated at the end of the PWM period. The pulse width modulation generation method according to any one of claims 1 to 6 in combination with claim 2.
9. The pulse width modulation generation method according to any one of claims 1 to 8, wherein the voltage pulses (24, 26) start when the primary transformer current of the transformer (6) is equal to 0.