Pulse width modulation method for soft switching of swiss rectifier high frequency power tube
By controlling the high-frequency power transistors of the SWISS rectifier using a new pulse width modulation method, the soft-switching problem during low-power operation is solved, achieving high power density and low current distortion, thus improving the overall performance of the rectifier.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- NANJING UNIV OF AERONAUTICS & ASTRONAUTICS
- Filing Date
- 2023-05-17
- Publication Date
- 2026-07-03
AI Technical Summary
Existing SWISS rectifiers struggle to achieve soft switching of high-frequency power transistors during low-power operation, and the required filter inductor structure affects converter size and losses, leading to reduced power density and efficiency.
A novel pulse width modulation method is adopted to control the operation of the high-frequency power transistors of the SWISS rectifier by calculating the conduction timing and duration of the high-frequency power transistors. This enables the switching of three levels of the high-frequency bridge output voltage, avoids the use of filter inductors, and directly controls the three-phase input current.
It achieves soft switching of high-frequency power transistors under both high and low power conditions, reduces duty cycle loss and input current distortion, improves rectifier efficiency and power density, and reduces total harmonic content of input current.
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Figure CN116545279B_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of three-phase AC / DC converters, and more particularly to a pulse width modulation method for soft switching of high-frequency power transistors in a SWISS rectifier. Background Technology
[0002] Three-phase pulse width modulation (PWM) rectifiers employ fully controllable power devices, enabling excellent control over the input current waveform and phase, thus achieving a high power factor. They are now widely researched and applied. The SWISS rectifier is a new type of step-down three-phase PWM rectifier, featuring high efficiency, high power factor, no input current surge, single-stage isolated step-down conversion, and a wide output voltage range. It shows great promise in a wide range of applications, including uninterruptible power supplies, variable frequency drives, electric vehicle charging, high-power lighting, and aviation power supplies.
[0003] Existing SWISS rectifiers employ pulse width modulation (PWM) using common carrier intersection or space vector modulation schemes. The basic principle is as follows: a modulation wave is generated using the three-phase current as a reference value; the modulation wave intersects with the carrier to generate a PWM signal that drives the rectifier's high-frequency power transistors. However, this modulation method suffers from drawbacks at low power operation. Due to the low leakage inductance energy, the high-frequency power transistors are insufficient to completely transfer charge from the junction capacitance of the high-frequency power transistors. Therefore, soft switching of the high-frequency power transistors is difficult at low power levels, hindering rectifier efficiency. Furthermore, implementing this modulation method requires a filter inductor structure at the rectifier output, which increases the converter's size and losses, negatively impacting high power density and conversion efficiency. A mismatch between the filter inductor current and the transformer leakage inductance current can lead to duty cycle loss, affecting not only the output voltage but also input current distortion and a reduced power factor. Summary of the Invention
[0004] Purpose of the invention: The purpose of this invention is to provide a pulse width modulation method for soft switching of high-frequency power transistors in SWISS rectifiers, which can achieve soft switching and effectively improve rectifier efficiency and power density.
[0005] Technical solution: The pulse width modulation method of the present invention is based on the three-phase input current peak value I. m * The turn-on timing and duration of the high-frequency power transistor are calculated. The specific implementation steps are as follows:
[0006] S1, the controller adjusts the three-phase input voltage u of the SWISS rectifier in each control cycle. a u b u c DC output voltage u o Sampling is performed, and the sampled voltage u is... a u b uc The peak input voltage U is obtained through phase-locked loop calculation. m With phase θ; based on the sampled DC output voltage u o Based on the transformer turns ratio N, the DC output voltage referred to the primary side of the transformer, u, is calculated. s =Nu o ;
[0007] S2, based on the peak input voltage U m The first high-frequency bridge output voltage u during the control cycle is calculated using phase θ. H With the output voltage u of the second high-frequency bridge L :
[0008]
[0009] S3, based on the peak reference value I of the three-phase input current m * With phase θ, the first current modulation target i of the bus within this control cycle is calculated. H * With the second current modulation target i L * :
[0010]
[0011] S4, based on the transformer leakage inductance value L r Switching period T s The first high-frequency bridge output voltage u H With the output voltage u of the second high-frequency bridge L and the first current modulation target i H * With the second current modulation target i L * The first modulation time t of the high-frequency power transistor within this control cycle is calculated. H With the second modulation time t L for:
[0012]
[0013] S5, determine the direction of the y-bus current based on the phase θ;
[0014] S6, according to the first modulation time t H With the second modulation time t L The generated high-frequency power transistor control signal controls the operation of the high-frequency power transistor in the SWISS rectifier.
[0015] Furthermore, within a high-frequency switching cycle time T s Inside, the SWISS rectifier first outputs voltage u from the first high-frequency bridge.H Energy is transferred, and then the second high-frequency bridge outputs voltage u. L Energy is transferred, and then the freewheeling phase begins. The transformer's forward magnetization ends, after which the leakage current decreases to zero and becomes intermittent, remaining in this intermittent state until T. s / 2;
[0016] In T s / 2 after that time, the SWISS rectifier first outputs voltage u from the first high-frequency bridge. H Energy is transferred, and then the second high-frequency bridge outputs voltage u. L Energy is transferred, and then the freewheeling phase begins. The transformer reverse magnetization ends, and the leakage inductance current remains discontinuous until the end of the control cycle.
[0017] Furthermore, the SWISS rectifier operates in a high-frequency AC tank current discontinuous state, within half a control cycle, u H The energy transfer time is t H u L The energy transfer time is t L The duration of the follow-through phase is t. D And satisfy: t H +t L +t D <T s / 2.
[0018] Furthermore, by setting appropriate high-frequency transistor switching timing and conduction time, the AC power side high-frequency bridge output voltage u of the SWISS rectifier can be increased. ab u appearing in sequence H u L Three voltage levels: 0, 1, and 0. The amplitude of the secondary side of the high-frequency isolation transformer is an alternating square wave of the DC output voltage. The transformer leakage inductance L... r The left end is a three-level voltage of "high-low-zero".
[0019] Furthermore, in step S5, if the y bus current is positive, then control u... L Voltage u during energy transfer ab Provided by the Y and Z buses; if the Y bus current is negative, then control u. L Voltage u during energy transfer ab Provided by the x and y buses.
[0020] Compared with the prior art, the significant advantages of this invention are as follows:
[0021] 1. In the modulation method of the present invention, when the rectifier is working, the leakage inductance energy is directly determined by the difference between the high-frequency bridge output voltage and the DC output voltage. When the power is low, the leakage inductance can still store a certain amount of energy, which is enough to completely discharge the junction capacitance of the power tube to 0. This enables the high-frequency power tube of the SWISS rectifier to achieve soft switching under both high output power and low output power conditions.
[0022] 2. In this invention, the secondary current of the transformer changes with the primary current, and there is no mismatch between the primary and secondary currents. Therefore, there is no loss of duty cycle, which reduces the distortion of the three-phase input current.
[0023] 3. This invention eliminates the need for a filter inductor at the rectifier output, effectively improving the efficiency and power density of the SWISS rectifier;
[0024] 4. This invention directly controls the three-phase input current, achieving a lower total harmonic distortion (THD) and a higher power factor. Attached Figure Description
[0025] Figure 1 This is a circuit diagram of a SWISS rectifier;
[0026] Figure 2(a) shows the waveform diagram of the SWISS rectifier input voltage circuit.
[0027] Figure 2(b) shows the ideal current waveform of the SWISS rectifier bus.
[0028] Figure 3 This is a schematic diagram of the modulation method of the present invention;
[0029] Figure 4(a) shows the positive magnetization of the transformer. H Schematic diagram of the working state during the power transfer stage.
[0030] Figure 4(b) shows the positive magnetization of the transformer. L Schematic diagram of the working state during the power transfer stage.
[0031] Figure 4(c) is a schematic diagram of the transformer's operation during the forward magnetization freewheeling stage.
[0032] Figure 4(d) is a schematic diagram of the intermittent current operation state after the transformer has finished forward magnetization.
[0033] Figure 4(e) shows the negative magnetization of the transformer. H Schematic diagram of the working state during the power transfer stage.
[0034] Figure 4(f) shows the negative magnetization of the transformer. L Schematic diagram of the working state during the power transfer stage.
[0035] Figure 4(g) is a schematic diagram of the transformer's operation during the negative magnetization freewheeling stage.
[0036] Figure 4(h) is a schematic diagram of the intermittent current operation state after the transformer has finished negative magnetization;
[0037] Figure 5 This is a flowchart of the modulation method of the present invention;
[0038] Figure 6(a) is a schematic diagram of the high-frequency power transistor in mode 0 operating state.
[0039] Figure 6(b) is a schematic diagram of the high-frequency power transistor in mode 1 operation.
[0040] Figure 6(c) is a schematic diagram of the high-frequency power transistor in mode 2 operation.
[0041] Figure 6(d) is a schematic diagram of the high-frequency power transistor in mode 3 operation.
[0042] Figure 6(e) is a schematic diagram of the high-frequency power transistor in mode 4 operation.
[0043] Figure 6(f) is a schematic diagram of the high-frequency power transistor in mode 5.
[0044] Figure 6(g) is a schematic diagram of the high-frequency power transistor in mode 6 operating state.
[0045] Figure 6(h) is a schematic diagram of the high-frequency power transistor in mode 7 operating state.
[0046] Figure 6(i) is a schematic diagram of the high-frequency power transistor in mode 8 operating state.
[0047] Figure 6(j) is a schematic diagram of the operating state of the high-frequency power transistor in mode 9.
[0048] Figure 6(k) is a schematic diagram of the operating state of the high-frequency power transistor in mode 10.
[0049] Figure 6(l) is a schematic diagram of the working state of the high-frequency power transistor mode 11;
[0050] Figure 7 This is a schematic diagram of the experimental waveforms of the voltage across the transformer and the leakage current.
[0051] Figure 8 For high-frequency power transistor S x2 Schematic diagram of voltage and current operating waveforms. Detailed Implementation
[0052] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.
[0053] Figure 1The diagram shows the topology of a Swiss rectifier, consisting of a three-phase input filter, a voltage expansion circuit, and a high-frequency isolation converter. The corresponding nodes of the three buses at the output of the voltage expansion circuit are defined as x, y, and z, respectively, and the current flowing through them is i. x i y i z This invention achieves the desired AC power supply side high-frequency bridge output voltage u by setting appropriate high-frequency transistor switching timing and conduction time. ab The first high-frequency bridge output voltage u appears sequentially. H The second high-frequency bridge output voltage u L Three voltage levels, 0, and 1, are used to generate an alternating square wave on the secondary side of the high-frequency isolation transformer with an amplitude equal to the DC output voltage value, thereby realizing the transformer leakage inductance L. r The left end represents the "high-low-zero" three-level voltage, and the right end represents the transformer secondary voltage u. cd Voltage u referred to the primary side s The voltage difference across the leakage inductor causes the leakage inductor current to rise or fall, thereby controlling the energy transfer.
[0054] As shown in Figure 2(a), the bidirectional switching transistors in the voltage expansion circuit switch on and off according to the three-phase voltage phase θ. One grid cycle can be divided into 12 sectors, and the voltage u xy with u yz It is a triangular wave-like phenomenon. Through the alternating conduction of bidirectional switching transistors, the three-phase input current flows into the three buses of the expanded circuit. Taking sector 1 as an example, the bus current i... x i y i z Corresponding to the three-phase current i a i b i c Therefore, the bus current can be controlled by using the three-phase input current as the modulation target. The ideal current waveform of the SWISS rectifier bus is shown in Figure 2(b).
[0055] Depending on the different conduction combinations of the power transistors in the high-frequency isolation converter circuit, the output voltage u of the high-frequency bridge will be... ab There can be three voltage combinations, namely:
[0056] (1) When a and b are connected to x and z respectively, the transformer is positively magnetized; when a and b are connected to z and x respectively, the transformer is negatively magnetized. The voltage across a and b in this combination is defined as the output voltage u of the first high-frequency bridge. H ;
[0057] (2) When a and b are connected to x and y respectively, the transformer is positively magnetized; when a and b are connected to y and x respectively, the transformer is negatively magnetized; or when a and b are connected to y and z respectively, the transformer is positively magnetized; when a and b are connected to z and y respectively, the transformer is negatively magnetized. The voltage across a and b in this combination is defined as the output voltage u of the second high-frequency bridge. L ;
[0058] (3) When a and b are connected, the leakage inductance current of the transformer continues to flow, and the voltage across a and b is 0. The secondary voltage u of the transformer... cd The voltage referred back to the primary side is defined as u. s u s The amplitude is Nu o Square wave.
[0059] Figure 3 This is a schematic diagram of the modulation method of the present invention. In one control period T... s Inside, the SWISS rectifier first outputs voltage u from the first high-frequency bridge. H Energy is transferred, and then the second high-frequency bridge outputs voltage u. L Energy is transferred, and then the freewheeling phase begins. The transformer's forward magnetization ends, after which the leakage inductance current is discontinuous until T. s / 2; in T s / 2 seconds later, the rectifier first outputs voltage u from the first high-frequency bridge. H Energy is transferred, and then the second high-frequency bridge outputs voltage u. L Energy is transferred, and then the freewheeling phase begins. The transformer reverse magnetization ends, after which the leakage inductance current is discontinuous until the end of the control cycle. The first high-frequency bridge output voltage u... H The average current flowing into the bus from the transformer leakage inductance during energy transfer is the first modulation current i. H The second high-frequency bridge output voltage u L The average current flowing into the bus from the transformer leakage inductance during energy transfer is the second modulation current i. L .
[0060] The SWISS rectifier operates in a discontinuous state of high-frequency AC tank circuit (transformer primary side) current (also transformer leakage inductance current), within half a control cycle, u H The energy transfer time is the first modulation time t. H u L The energy transfer time is the second modulation time t L The duration of the follow-through phase is t. D , satisfying: t H +t L +t D <T s / 2.
[0061] It should be noted that, due to the y bus current iy The direction is definite each time energy is transferred, therefore, in one complete energy transfer cycle, u L Voltage u during energy transfer ab The current is supplied only by the x and y buses or the y and z buses. The detailed operation of the SWISS rectifier under the modulation method of this invention when the y bus current is positive is as follows: Figures 4(a)-4(h) As shown.
[0062] Figure 5 The flowchart shown is a description of the modulation method of the present invention, which is explained in detail below:
[0063] 1) A controller, consisting of a digital signal processor or embedded computer, adjusts the three-phase input voltage u of the SWISS rectifier in each controller cycle. a u b u c and DC output voltage u o Sampling is performed, and the sampled voltage u is... a u b u c The peak input voltage U is obtained through phase-locked loop calculation. m With phase θ, based on DC output voltage u o Based on the transformer turns ratio N, the DC output voltage referred to the primary side of the transformer, u, is calculated. s =Nu o ;
[0064] 2) Based on the peak input voltage U m The first high-frequency bridge output voltage u during the control cycle is calculated using phase θ. H With the output voltage u of the second high-frequency bridge L for:
[0065]
[0066] 3) Based on the reference value I of the three-phase input current peak value m * With phase θ, the ideal first current modulation target i of the bus current within this control cycle is calculated. H * With the second current modulation target i L * for:
[0067]
[0068] 4) Based on the transformer leakage inductance value L r Switching period T s The first high-frequency bridge output voltage u H With the output voltage u of the second high-frequency bridge L and the first current modulation target iH * With the second current modulation target i L * The first modulation time t within this control cycle is calculated. H With the second modulation time t L for:
[0069]
[0070] 5) Determine the direction of the y-bus current based on the phase θ. If the y-bus current is positive, control the output voltage u of the first high-frequency bridge. L Voltage u during energy transfer ab This is handled by the y and z buses; if the y bus current is negative, it controls the output voltage u of the second high-frequency bridge. L Voltage u during energy transfer ab It is borne by the x and y buses.
[0071] 6) Based on the first modulation time t H With the second modulation time t L The system generates control signals for eight high-frequency power transistors to control the operation of the high-frequency power transistors in the SWISS rectifier.
[0072] The following analysis uses a three-phase voltage phase θ∈[π / 6, π / 3] and the transformer is positively magnetized (as shown in Figures 4(a)-(d)) as an example to illustrate the working process of the SWISS rectifier under the modulation method of this invention:
[0073] (c1)u H Energy transfer: As shown in Figure 4(a), the power transfer time in this mode is the first modulation time t. H The average current i flowing into the bus in this mode can be obtained. H for:
[0074]
[0075] (c2)u L Energy transfer: As shown in Figure 4(b), the power transfer time in this mode is the second modulation time t. L The average current i flowing into the bus in this mode can be obtained. L for:
[0076]
[0077] (c3) Freewheeling phase: As shown in Figure 4(c), the power transfer time in this mode is t. D The average leakage inductance current i of the transformer in this mode can be obtained. D for:
[0078]
[0079] (c4) Discontinuous leakage inductance current phase: As shown in Figure 4(d), this mode continues until T s / 2, the leakage current of the transformer is 0 at this stage.
[0080] Taking the negative magnetization of the transformer (as shown in Figure 4(e)-(h)) as an example, the working process of the SWISS rectifier under the modulation method of the present invention is analyzed:
[0081] (c5)u H Energy transfer: As shown in Figure 4(e), the power transfer time in this mode is the first modulation time t. H The average current i flowing into the bus in this mode can be obtained. H for:
[0082]
[0083] (c6)u L Energy transfer: As shown in Figure 4(f), the power transfer time in this mode is the second modulation time t. L The average current i flowing into the bus in this mode can be obtained. L for:
[0084]
[0085] (c7) Freewheeling phase: As shown in Figure 4(g), the power transfer time in this mode is t. D The average leakage inductance current i of the transformer in this mode can be obtained. D for:
[0086]
[0087] (c8) Intermittent leakage current stage: As shown in Figure 4(h), this mode continues until the end of the control cycle, during which the transformer leakage current is 0.
[0088] The modulation method of this invention uses the three-phase input current as the modulation target to control the bus current. When θ∈[π / 6, π / 3], the current i L The i-phase flows into phase b through the y-bus. H The current i flows into phase a through bus x, i.e., the current is modulated based on the ideal current of phase b. L The modulation current i is based on the ideal current of phase a. H The ideal modulation target can then be expressed as:
[0089]
[0090] During the switching period T s Transformer leakage inductance value L rGiven a specific condition, by combining equations (4)-(6), u within a control cycle can be calculated. H with u L The modulation time is:
[0091]
[0092] The same applies when θ is in other intervals. Therefore, the modulation method of this invention only needs to obtain the peak value U of the three-phase input voltage. m With phase θ, u within a control cycle can then be calculated. H u L i H * with i L * Then, combined with the known switching period T s With transformer leakage inductance value L r The modulation time t within this control period can then be obtained. H With t L .
[0093] The following analysis describes the soft-switching implementation of the high-frequency power transistor in the SWISS rectifier under the modulation method of this invention. Figures 6(a)-6(l) The diagram shows the operating modes of the high-frequency power transistor when θ∈[π / 6, π / 3]. Taking the forward magnetization of the transformer as an example:
[0094] Mode 0: As shown in Figure 6(a), the duration of this mode is the first modulation time t. H Before the mode begins, the power transistor S z2 It is already in the on state, power transistor S x1 With S y1 When activated, since the leakage current was previously 0, therefore S x1 With S y1 Turn on with zero current.
[0095] Mode 1: The duration of this mode is the dead time DT, as shown in Figure 6(b), S x1 When switched off, leakage current flows through power transistor S. y1 S z2 and power transistor S x1 S y2 S z1 The junction capacitance. Before the end of mode 1, S... x1 Junction capacitance discharge to u xy S z1 The junction capacitance is charged to u yz S y2 The junction capacitance is discharged to 0.
[0096] Mode 2: As shown in Figure 6(c), the duration of this mode is the second modulation time t. L Sy2 Activation, due to the previous mode S y2 The junction capacitance has been discharged to 0, S y2 The body diode conducts naturally, therefore S y2 Turn on at zero voltage.
[0097] Mode 3: As shown in Figure 6(d), the duration of this mode is DT, and the power transistor S... y1 Close. S before the end of Mode 3. z1 Junction capacitance discharged to 0, S x1 The junction capacitance is charged to u xy +u yz S y1 The junction capacitance is charged to u yz .
[0098] Mode 4: As shown in Figure 6(e), S z1 Activation, due to the previous mode S z1 The junction capacitance has been discharged to 0, S z1 The body diode conducts naturally, S z1 Zero-voltage turn-on. This mode is the freewheeling phase.
[0099] Mode 5: As shown in Figure 6(f), in this mode, the leakage inductance current decreases to 0, the forward magnetization of the transformer ends, and S z2 Off, S z1 The rectifier remains conducting to prepare for energy transfer in the next mode; the DC output current of the rectifier is supplied by the DC-side capacitor C. o supply.
[0100] Mode 6: As shown in Figure 6(g), the duration of this mode is the first modulation time t. H Before the mode begins, the power transistor S z1 It is already in the on state, power transistor S x2 With S y3 When activated, since the leakage current was previously 0, therefore S x2 With S y3 Turn on with zero current.
[0101] Mode 7: The duration of this mode is the dead time DT, as shown in Figure 6(h), S x2 When switched off, leakage current flows through power transistor S. y3 S z1 and power transistor S x2 S y4 S z2 The junction capacitance. Before the end of mode 7, S... x2 The junction capacitance is charged to u xy S z2 Junction capacitance discharge to u yz S y4 The junction capacitance is discharged to 0.
[0102] Mode 8: As shown in Figure 6(i), the duration of this mode is the second modulation time t. L S y4 Activation, due to the previous mode S y4 The junction capacitance has been discharged to 0, S y4 The body diode conducts naturally, therefore S y4 Turn on at zero voltage.
[0103] Mode 9: As shown in Figure 6(j), the duration of this mode is DT, and the power transistor S... y3 Close. S before the end of Mode 9. z2 Junction capacitance discharged to 0, S x2 The junction capacitance is charged to u xy +u yz S y3 The junction capacitance is charged to u yz .
[0104] Mode 10: As shown in Figure 6(k), S z2 Activation, due to the previous mode S z2 The junction capacitance has been discharged to 0, S z2 The body diode conducts naturally, S z2 Zero-voltage turn-on. This mode is the freewheeling phase.
[0105] Mode 11: As shown in Figure 6(l), in this mode, the leakage inductance current decreases to 0, the forward magnetization of the transformer ends, and S z1 Off, S z2 The rectifier remains conducting to prepare for energy transfer in the next mode; the DC output current of the rectifier is supplied by the DC-side capacitor C. o supply.
[0106] The same applies when θ is in other intervals. It can be proven that the modulation method of this invention enables soft switching of all high-frequency power transistors in the SWISS rectifier.
[0107] In traditional carrier interleaving or space vector modulation schemes, the leakage inductance current is the value of the output current referred to the primary side of the transformer. When the SWISS rectifier is operating at low power, the output current is low, and the leakage inductance current is also low. During high-frequency mode switching, the leakage inductance energy is insufficient to completely discharge the junction capacitance of the power transistor to 0, making it difficult to achieve soft switching of the power transistor. However, in the modulation method of this invention, when the rectifier is operating, the leakage inductance energy is directly determined by the difference between the high-frequency bridge output voltage and the DC output voltage. Even when operating at low power, the leakage inductance can still store a certain amount of energy, which is sufficient to completely discharge the junction capacitance of the power transistor to 0, thereby achieving soft switching of the high-frequency power transistor.
[0108] In traditional modulation schemes, the secondary current of the transformer is essentially constant. However, due to the time required for the primary current to change from positive (or negative) to negative (or positive) during the forward and reverse magnetization of the transformer, the primary current is lower than the secondary current, and the secondary current remains in a freewheeling state. This results in a loss of duty cycle, preventing the primary side from transferring power to the secondary side and further distorting the input current. In the modulation method of this invention, the secondary current of the transformer changes with the primary current, and there is no mismatch between the primary and secondary currents, thus eliminating the problem of duty cycle loss.
[0109] Traditional modulation schemes require a filter inductor structure at the rectifier output, and the inductance value is generally large. This results in a large inductor structure, accounting for a large proportion of the overall rectifier volume, which is not conducive to improving the rectifier's power density. At the same time, the filter inductor itself has energy loss, and a part of the rectifier's energy will be consumed by the filter inductor, making it difficult to improve efficiency. In contrast, the modulation method of this invention does not require a filter inductor structure at the rectifier output, which will effectively improve the efficiency and power density of the SWISS rectifier.
[0110] Meanwhile, in the modulation method of the present invention, the modulation time is calculated and the three-phase input current is directly controlled, which is beneficial to adjust the three-phase input current value in real time as needed, so as to achieve a lower total harmonic content of the input current.
[0111] Figure 7 This is an experimental waveform diagram showing the voltage across the transformer and the leakage inductance current. The leakage inductance current rises or falls due to the voltage difference across the leakage inductor, and the voltage u... ab The modulation method is high at the beginning and low at the end, which meets the design requirements of the modulation method of this invention.
[0112] Figure 8 The high-frequency power transistor S is used in the SWISS rectifier during low-power stable operation. x2 The measured operating waveforms of voltage and current show that the high-frequency power transistor drives u... GS_Sx2 Before being set high, the voltage u between the power transistor's drain and source (DS) is... DS_Sx2 It has dropped to zero, achieving zero-voltage switching.
Claims
1. A pulse width modulation method for soft switching of high-frequency power transistors in a Swiss rectifier, characterized in that... The SWISS rectifier includes a three-phase input filter, a voltage expansion circuit, and a high-frequency isolation converter circuit. The corresponding nodes of the three buses at the output of the voltage expansion circuit are defined as x, y, and z, respectively, and the current flowing through them is i. x i y i z According to the reference value I of the three-phase input current peak value m * The turn-on timing and duration of the high-frequency power transistor are calculated. The specific implementation steps are as follows: S1, the controller adjusts the three-phase input voltage u of the SWISS rectifier in each control cycle. a u b u c DC output voltage u o Sampling is performed, and the sampled voltage u is... a u b u c The peak input voltage U is obtained through phase-locked loop calculation. m With phase θ; based on the sampled DC output voltage u o Based on the transformer turns ratio N, the DC output voltage referred to the primary side of the transformer, u, is calculated. s =Nu o ; S2, based on the peak input voltage U m The first high-frequency bridge output voltage u during the control cycle is calculated using phase θ. H With the output voltage u of the second high-frequency bridge L : S3, based on the three-phase input current peak reference value I m * With phase θ, the first current modulation target i of the bus within this control cycle is calculated. H * With the second current modulation target i L * : S4, based on the transformer leakage inductance value L r Switching period T s The first high-frequency bridge output voltage u H With the output voltage u of the second high-frequency bridge L and the first current modulation target i H * With the second current modulation target i L * The first modulation time t of the high-frequency power transistor within this control cycle is calculated. H With the second modulation time t L for: S5, determine the direction of the y-bus current based on the phase θ; S6, according to the first modulation time t H With the second modulation time t L The generated high-frequency power transistor control signal controls the operation of the high-frequency power transistor in the SWISS rectifier.
2. The pulse width modulation method for soft switching of high-frequency power transistors in a SWISS rectifier according to claim 1, characterized in that, In a high-frequency switching cycle time T s Inside, the SWISS rectifier first outputs voltage u from the first high-frequency bridge. H Energy is transferred, and then the second high-frequency bridge outputs voltage u. L Energy is transferred, and then the freewheeling phase begins. The transformer's forward magnetization ends, after which the leakage current decreases to zero and becomes intermittent, remaining in this intermittent state until T. s / 2; In T s / 2 after that time, the SWISS rectifier first outputs voltage u from the first high-frequency bridge. H Energy is transferred, and then the second high-frequency bridge outputs voltage u. L Energy is transferred, and then the freewheeling phase begins. The transformer reverse magnetization ends, and the leakage inductance current remains discontinuous until the end of the control cycle.
3. The pulse width modulation method for soft switching of high-frequency power transistors in a SWISS rectifier according to claim 2, characterized in that, The SWISS rectifier operates in a high-frequency AC tank current discontinuous state, within half a control cycle, u H The energy transfer time is t H u L The energy transfer time is t L The duration of the follow-through phase is t. D And satisfy: t H +t L +t D <T s / 2.
4. The pulse width modulation method for soft switching of high-frequency power transistors in a SWISS rectifier according to claim 2, characterized in that, By setting appropriate high-frequency transistor switching timing and conduction time, the AC power side high-frequency bridge output voltage u of the SWISS rectifier can be adjusted. ab u appearing in sequence H u L Three voltage levels: 0, 1, and 0. The amplitude of the secondary side of the high-frequency isolation transformer is an alternating square wave of the DC output voltage. The transformer leakage inductance L... r The left end is a "high-low-zero" three-level voltage.
5. The pulse width modulation method for soft switching of high-frequency power transistors in a SWISS rectifier according to claim 1, characterized in that, In step S5, if the y bus current is positive, then control u. L Voltage u during energy transfer ab Provided by the Y and Z buses; if the Y bus current is negative, then control u. L Voltage u during energy transfer ab Provided by the x and y buses.