Three-phase CLLC resonant converter synchronous rectification control method based on extended harmonic waves

Abstract

The invention provides a three-phase CLLC resonant converter synchronous rectification control method based on extended harmonic waves, a three-phase CLLC resonant converter comprises a primary side inversion network, a resonant network and a secondary side rectification network, and the resonant network comprises a resonant capacitor, a resonant inductor and a high-frequency transformer. The three-phase CLLC resonant converter can work in a three-phase mode, a full-bridge mode and a half-bridge mode through a phase cutting strategy. According to the method, firstly, a secondary side MOSFET and a primary side MOSFET are fixed to be synchronously turned on, the phase difference between input square wave signals and secondary side resonance current in different modes is calculated by analyzing extended harmonic impedance models in different modes, and the turn-on time of the secondary side MOSFET can be further calculated, so that the turn-off time of the secondary side MOSFET in different modes is determined. According to the method, synchronous rectification of all working modes after a phase cutting strategy of the three-phase CLLC resonant converter is realized based on the extended harmonic impedance model, the calculation precision of the model at a resonance deviation point is improved, the efficiency of the converter in a full-load range is improved, and additional components do not need to be added.

Description

Technical Field

[0001] The present invention relates to the field of resonant converters, and in particular to a three-phase CLLC resonant converter synchronous rectification control method based on extended harmonics. Background Art

[0002] The three-phase CLLC resonant converter is favored in the application of electric vehicle on-board chargers due to its excellent characteristics such as automatic current sharing, high power density and low output ripple. In addition, the three-phase CLLC resonant converter can cover the three stages of battery charging (pre-charging stage, constant current charging stage, constant voltage charging stage) by switching off one or two bridge arms when the load current decreases through the phase cutting strategy, solving the problem of reduced efficiency under light load.

[0003] The topology of the three-phase CLLC resonant converter is as follows: Figure 1 As shown in the figure, the circuit consists of a primary-side inverter network, a secondary-side rectifier network and a resonant network. The transformer is connected in star shape, and the resonant capacitor is connected in triangle shape to form a resonant network. The primary and secondary sides are electrically isolated through a three-phase transformer. Due to the large voltage drop of the MOSFET body diode, when the resonant current flows through the secondary side, a certain loss will be generated. In order to further improve the efficiency of the converter and realize the bidirectional transmission of energy, synchronous rectification control can be used. That is, when the resonant current passes through the zero point, the secondary-side MOSFET switch is controlled so that the current does not pass through the diode. Because the on-resistance of the MOSFET is low, the loss generated when the current flows through is low, thereby improving the efficiency of the converter.

[0004] Traditionally, fundamental wave analysis is widely used in modeling resonant converters and has high accuracy when close to the resonant frequency. For CLLC resonant converters, when the parameters of the primary and secondary LC resonant circuits do not match, the resonant frequency is no longer unique, resulting in a decrease in the accuracy of the fundamental wave equivalent circuit model, but the extended harmonic impedance model still has high accuracy when it deviates from the resonant point. However, there are still few studies on the synchronous rectification control strategy for three-phase CLLC resonant converters, and the synchronous rectification control scheme that adapts to different working modes under the phase-cutting strategy has not been widely studied, especially based on the extended harmonic synchronous rectification control algorithm. At the same time, the phase-cutting strategy changes the topology of the resonant converter, which also brings challenges to the synchronous rectification control. Summary of the invention

[0005] In view of the shortcomings of the prior art, the present invention provides a synchronous rectification control method for a three-phase CLLC resonant converter based on extended harmonics. Without the need for additional sensors, the on-time of the secondary-side MOSFET is accurately calculated in a calculation method that takes extended harmonics into consideration. The method is applicable to all operating modes after the converter switches off phase, and can effectively improve the efficiency and accuracy of the converter in the full load range.

[0006] The present invention is achieved through the following technical solutions:

[0007] A three-phase CLLC resonant converter synchronous rectification control method based on extended harmonics, the resonant converter includes a primary-side inverter network, a resonant network and a secondary-side rectification network. The primary and secondary sides of the three-phase transformer in the resonant network are connected in star shape, and the resonant capacitor is connected in triangle shape, which can realize automatic current sharing in the three-phase operation mode, and because the resonant capacitor has no DC bias, it is also helpful to realize soft start, characterized in that: the three-phase CLLC resonant converter synchronous rectification control method based on extended harmonics includes the following steps:

[0008] Step 1: Sample the input and output voltage and current in real time;

[0009] Step 2: Determine the working mode of the converter by sampling the voltage and current data;

[0010] Step 3: Determine the turn-on time of the secondary-side MOSFET;

[0011] Step 4: According to different modes, based on the extended harmonic equivalent circuit model, the input signal can be split into multiple periodic signals, and Fourier series expansion is performed on them;

[0012] Step 5: According to different modes, based on the extended harmonic analysis method, the phase difference between the input square wave and the secondary side resonant current is calculated, and the zero crossing point of the secondary side MOSFET current is further obtained;

[0013] Step 6: Calculate the conduction time of the secondary side MOSFET;

[0014] Step 7: Determine the turn-off time of the secondary-side MOSFET;

[0015] Furthermore, in step 2, the output power can be calculated by sampling data to determine the current operating mode of the resonant converter.

[0016] Furthermore, in step 3, the secondary-side MOSFET and the corresponding primary-side MOSFET are turned on synchronously.

[0017] Further, taking the three-phase operation mode as an example, the step 4 specifically includes the following steps:

[0018] (1) The input step square wave signal V AN_3ph_k (t) is split into two periodic square wave signals V with the same frequency and amplitude AN1 (t) and V AN2 (t);

[0019] (2) V AN1 (t) and VAN2 (t) can be expanded by Fourier series to obtain the following formula:

[0020]

[0021] V AN_3ph_k (t) = V AN1 (t)+V AN2 (t) (2)

[0022] Further, taking the three-phase operation mode as an example, the step 5 specifically includes the following steps:

[0023] (1) Calculate the equivalent resistance R eq_3ph_k , the calculation formula is:

[0024]

[0025] Where R eq_3ph_k is the equivalent output load in three-phase operation mode, R o is the output load, n is the transformer ratio;

[0026] (2) Calculate the load impedance Z 1k , the calculation formula is:

[0027]

[0028] in,

[0029]

[0030] In the formula, ω s is the converter operating frequency, L rs is the secondary side resonant inductor, C rs is the secondary side resonant capacitor;

[0031] (3) Calculate the load impedance Z 1k The impedance angle θ 1κ , the calculation formula is:

[0032]

[0033] (4) Calculate the load impedance Z 2k , the calculation formula is:

[0034] Z 2k =jkωL m ||Z 1k =a 2k +jb 2k (7)

[0035] in,

[0036]

[0037] Where, L m is the converter excitation inductance;

[0038] (5) Calculate the load impedance Z 2k The impedance angle θ 2κ , the calculation formula is:

[0039]

[0040] (6) Calculate the load impedance Z 3k , the calculation formula is:

[0041]

[0042] in,

[0043]

[0044] Where, L rp is the primary side resonant inductor, C rp is the primary side resonant capacitor;

[0045] (7) Calculate the load impedance Z 3k The impedance angle θ 3κ , the calculation formula is:

[0046]

[0047] (8) Calculate the resonant current i on the secondary side s '(t), assuming that the phase of the step wave voltage is 0°, then:

[0048]

[0049] Where θ κ =θ 2κ -θ 3κ -θ 1κ , is the phase angle of the resonant current, so the secondary resonant current in the time domain can be obtained as:

[0050]

[0051] Where I s is the effective value of the resonant current;

[0052] In order to solve the phase difference θ between the input signal and the resonant current, we can s '(t) = 0 and use MATLAB to calculate its zero crossing point.

[0053] Furthermore, the step 6 specifically includes the following steps:

[0054] (1) Calculate the dead time t dead To ensure that MOSFET can achieve ZVS, it is necessary to consider that the charge of MOSFET parasitic capacitance can be completely released during the dead time, so:

[0055] t dead >18L m C oss f max (15)

[0056] Where, t dead is the dead time, C oss is the switch junction capacitance, f max is the maximum operating frequency of the converter;

[0057] (2) Calculate the on-time t on , let the phase difference between the zero-crossing point of the secondary-side resonant current and the turn-on time of the secondary-side MOSFET be Then we can calculate:

[0058]

[0059] In the formula, f s is the converter operating frequency.

[0060] Furthermore, in step 7, the turn-off time of the secondary side MOSFET is the turn-on time of the secondary side MOSFET plus the conduction time t on .

[0061] Furthermore, when the converter switches to the full-bridge operation mode, according to its topological structure, its primary and secondary side resonant capacitors become half of the original, the primary and secondary side resonant inductors become twice of the original, and the excitation inductance becomes twice of the original, and:

[0062]

[0063] Where R eq_fb_k is the equivalent output load in full-bridge operation mode.

[0064] Furthermore, when the converter switches to the full-bridge operation mode, according to its extended harmonic impedance model, the input signal is expanded by Fourier series to obtain:

[0065]

[0066] Where V AN_fb_k (t) is the input signal of full-bridge operation mode.

[0067] Furthermore, through the above three-phase mode calculation steps, the secondary side MOSFET conduction time t of the full-bridge mode can be calculated in the same way. on, and then the turn-off time of the secondary-side MOSFET in full-bridge mode is obtained.

[0068] Furthermore, when the converter switches to the half-bridge operation mode, according to its topological structure, its primary and secondary side resonant capacitors become half of the original, the primary and secondary side resonant inductors become twice of the original, and the excitation inductance becomes twice of the original, and:

[0069]

[0070] Where R eq_hb_k is the equivalent output load in half-bridge operation mode.

[0071] Furthermore, when the converter switches to the half-bridge operation mode, the input signal is split into two periodic signals with the same amplitude according to its extended harmonic impedance model. After the input signal is expanded by Fourier series, it can be obtained:

[0072]

[0073] Where V AN_hb_k (t) is the input signal of half-bridge operation mode.

[0074] Furthermore, through the above three-phase mode calculation steps, the secondary side MOSFET conduction time t of the half-bridge mode can be calculated in the same way. on , and then the turn-off time of the secondary-side MOSFET in full-bridge mode is obtained.

[0075] Compared with the prior art, the present invention has the following beneficial effects:

[0076] (1) The present invention adopts digital control and only needs to sample the DC voltage and current on the input and output sides. Therefore, the present invention does not add other hardware auxiliary circuits, thereby reducing the cost of the resonant converter.

[0077] (2) The synchronous rectification control algorithm of the present invention can meet all the working modes of the phase cutting strategy of the three-phase CLLC resonant converter, thereby improving the efficiency of the three-phase CLLC resonant converter in the full load range.

[0078] (3) The present invention adopts an extended harmonic analysis method, which, compared with the fundamental wave analysis method, still has higher accuracy when deviating from the resonance point, thereby improving the calculation accuracy of the model. BRIEF DESCRIPTION OF THE DRAWINGS

[0079] Figure 1 It is a block diagram of a three-phase CLLC resonant converter and synchronous rectification control;

[0080] Figure 2 It is the topological structure and driving waveform diagram of three-phase operation mode;

[0081] Figure 3 Expanded harmonic equivalent circuit diagram for three-phase operation mode;

[0082] Figure 4 It is the key waveform diagram of synchronous rectification at different frequencies in three-phase operation mode;

[0083] Figure 5 It is the topology structure and driving waveform of full-bridge operation mode;

[0084] Figure 6 Expanded harmonic equivalent circuit diagram for full-bridge operation mode;

[0085] Figure 7 Key waveforms of synchronous rectification at different frequencies in full-bridge operation mode;

[0086] Figure 8 It is the topology structure and driving waveform of half-bridge operation mode;

[0087] Fig. 9 Extended harmonic equivalent circuit diagram for half-bridge operation mode;

[0088] Fig.10 It is the key waveform diagram of synchronous rectification at different frequencies in half-bridge operation mode;

[0089] Fig.11 Input voltage waveform and its split waveform diagram for three-phase operation mode;

[0090] Fig.12 It is the input voltage waveform after Fourier series expansion in three-phase operation mode;

[0091] Fig.13 Input voltage waveform and its split waveform diagram for half-bridge operation mode; DETAILED DESCRIPTION

[0092] The following is a clear and complete description of the technical solutions in the embodiments of the present invention in combination with the accompanying drawings and specific embodiments. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. The accompanying drawings are only used to assist in explaining the content and purpose of the present invention, so the drawings are relatively simple and do not use precise proportions for operation.

[0093] Figure 1 The three-phase CLLC resonant converter and synchronous rectification control block diagram consists of MOSFET switch tube Q 1 ~Q 6 The primary side inverter network is formed, and the MOSFET switch tube SR 1 ~SR 6A secondary side rectification network is formed, and a resonant network is formed by a resonant inductor, a resonant capacitor and a transformer. The primary and secondary sides of the three-phase transformer in the resonant network are connected in star shape, and the resonant capacitor is connected in triangle shape, which can realize automatic current sharing in the three-phase operation mode, and because the resonant capacitor has no DC bias, it is also helpful to realize soft start. The three-phase CLLC resonant converter synchronous rectification control method based on extended harmonics includes the following steps:

[0094] Step 1: Sample the input and output voltage and current in real time;

[0095] Step 2: Determine the working mode of the converter by sampling the voltage and current data;

[0096] The working mode of the converter can be determined by sampling data, calculating the output power, and determining the current working mode of the resonant converter.

[0097] Step 3: Determine the turn-on time of the secondary-side MOSFET;

[0098] Since the current flowing into the secondary-side MOSFET is approximately a sinusoidal waveform with a small amplitude at the beginning and when the current crosses zero, the effect of a slight turn-on delay or early turn-off on the converter efficiency can be ignored. Therefore, the secondary-side MOSFET can be turned on simultaneously with the corresponding primary-side MOSFET.

[0099] Step 4: According to different modes, based on the extended harmonic equivalent circuit model, the input signal can be split into multiple periodic signals, and Fourier series expansion is performed on them;

[0100] Fig.11 The input voltage waveform and its split waveform diagram of the three-phase operation mode are shown in Figure 1. The split signal waveform and fundamental wave after Fourier series expansion are compared with the original step wave V AN_3ph_k (t) comparison, we can get Fig.12 The input voltage waveform after Fourier series expansion of the three-phase operation mode is shown. It can be seen that as the number of harmonic components increases, the reconstructed signal gradually approaches the original square wave signal, indicating that the introduction of more high-order harmonics can more accurately restore the original square wave signal. Therefore, compared with the calculation of the fundamental wave equivalent model, it still has higher accuracy when it deviates from the resonance point. Taking the three-phase operation mode as an example, the step 4 specifically includes the following steps:

[0101] (1) The input step square wave signal V AN_3ph_k (t) is split into two periodic square wave signals V with the same frequency and amplitude AN1 (t) and V AN2 (t);

[0102] (2) V AN1 (t) and V AN2(t) can be expanded by Fourier series to obtain the following formula:

[0103]

[0104] V AN_3ph_k (t) = V AN1 (t)+V AN2 (t) (2)

[0105] Step 5: According to different modes, based on the extended harmonic analysis method, the phase difference between the input square wave and the secondary side resonant current is calculated, and the zero crossing point of the secondary side MOSFET current is further obtained;

[0106] The topology and drive waveform of the three-phase operation mode are as follows: Figure 2 As shown, according to its topological structure, we can get Figure 3 The extended harmonic equivalent circuit shown in the figure takes the three-phase operation mode as an example for analysis. The specific steps of step 5 are as follows:

[0107] (1) Calculate the equivalent resistance R eq_3ph_k , the calculation formula is:

[0108]

[0109] Where R eq_3ph_k is the equivalent output load in three-phase operation mode, R o is the output load, n is the transformer ratio;

[0110] (2) Calculate the load impedance Z 1k , the calculation formula is:

[0111]

[0112] in,

[0113] a 1k =R eq_3ph_k

[0114]

[0115] In the formula, ω s is the converter operating frequency, L rs is the secondary side resonant inductor, C rs is the secondary side resonant capacitor;

[0116] (3) Calculate the load impedance Z 1k The impedance angle θ 1κ , the calculation formula is:

[0117]

[0118] (4) Calculate the load impedance Z2k , the calculation formula is:

[0119] Z 2k =jkωL m ||Z 1k =a 2k +jb 2k (7)

[0120] in,

[0121]

[0122] Where, L m is the converter excitation inductance;

[0123] (5) Calculate the load impedance Z 2k The impedance angle θ 2κ , the calculation formula is:

[0124]

[0125] (6) Calculate the load impedance Z 3k , the calculation formula is:

[0126]

[0127] in,

[0128]

[0129] Where, L rp is the primary side resonant inductor, C rp is the primary side resonant capacitor;

[0130] (7) Calculate the load impedance Z 3k The impedance angle θ 3κ , the calculation formula is:

[0131]

[0132] (8) Calculate the resonant current i on the secondary side s '(t), assuming that the phase of the step wave voltage is 0°, then:

[0133]

[0134] Where θ κ =θ 2κ -θ 3κ -θ 1κ , is the phase angle of the resonant current, so the secondary resonant current in the time domain can be obtained as:

[0135]

[0136] Where I s is the effective value of the resonant current;

[0137] In order to solve the phase difference θ between the input signal and the resonant current, we can s '(t) = 0 and use MATLAB to calculate its zero crossing point.

[0138] Step 6: Calculate the conduction time of the secondary side MOSFET;

[0139] The key waveforms of synchronous rectification at different frequencies in three-phase operation mode are as follows: Figure 4 As shown, the specific steps for calculating the conduction time of the secondary side MOSFET are as follows:

[0140] (1) Calculate the dead time t dead To ensure that MOSFET can achieve ZVS, it is necessary to consider that the charge of MOSFET parasitic capacitance can be completely released during the dead time, so:

[0141] t dead >18L m C oss f max (15)

[0142] Where, t dead is the dead time, C oss is the switch junction capacitance, f max is the maximum operating frequency of the converter;

[0143] (2) Calculate the on-time t on , let the phase difference between the zero-crossing point of the secondary-side resonant current and the turn-on time of the secondary-side MOSFET be Then we can calculate:

[0144]

[0145] In the formula, f s is the converter operating frequency.

[0146] Step 7: Determine the turn-off time of the secondary-side MOSFET;

[0147] The turn-off time of the secondary MOSFET is the turn-on time of the secondary MOSFET plus the calculated on-time t on .

[0148] Through the above steps, synchronous rectification of the three-phase CLLC resonant converter in three-phase mode can be achieved.

[0149] When the converter switches to full-bridge operation, the topology and drive waveforms are as follows: Figure 5As shown, according to the topological structure, the extended harmonic equivalent circuit of the full-bridge mode can be obtained as follows Figure 6 As shown, it can be seen that the primary and secondary side resonant capacitance becomes half of the original, the primary and secondary side resonant inductance becomes twice of the original, the excitation inductance becomes twice of the original, and:

[0150]

[0151] Where R eq_fb_k is the equivalent output load in full-bridge operation mode.

[0152] When the converter switches to full-bridge operation mode, according to its extended harmonic impedance model, Figure 6 As shown, after the input signal is expanded by Fourier series, we can get:

[0153]

[0154] Where V AN_fb_k (t) is the input signal of full-bridge operation mode.

[0155] Through the above three-phase mode calculation steps, the secondary side MOSFET conduction time t of the full-bridge mode can be calculated in the same way. on , and then get the turn-off time of the secondary side MOSFET in full-bridge mode, and realize the synchronous rectification in full-bridge mode. The key waveform of synchronous rectification is as follows Figure 7 shown.

[0156] When the converter switches to half-bridge operation mode, the topology and drive waveform are as follows: Figure 8 As shown, according to the topological structure, the extended harmonic equivalent circuit of the half-bridge mode can be obtained as follows Fig. 9 As shown, it can be seen that the primary and secondary side resonant capacitance becomes half of the original, the primary and secondary side resonant inductance becomes twice of the original, the excitation inductance becomes twice of the original, and:

[0157]

[0158] Where R eq_hb_k is the equivalent output load in half-bridge operation mode.

[0159] When the converter switches to half-bridge operation mode, according to its extended harmonic impedance model, Fig. 9 As shown in Figure 2, the input signal is split into two periodic signals with the same amplitude. The input voltage waveform in the half-bridge operation mode and its split waveform are shown in Figure 2. Fig.13 As shown, after the input signal is expanded by Fourier series, we can get:

[0160]

[0161] Where V AN_hb_k(t) is the input signal of half-bridge operation mode.

[0162] Through the above three-phase mode calculation steps, the secondary side MOSFET conduction time t of the half-bridge mode can be calculated in the same way. on , and then get the turn-off time of the secondary side MOSFET in the half-bridge mode, and realize the synchronous rectification in the half-bridge mode. The key waveform of the synchronous rectification is as follows Fig.10 shown.

[0163] The beneficial effects of the present invention are:

[0164] (1) The present invention adopts digital control and only needs to sample the DC voltage and current on the input and output sides. Therefore, the present invention does not add other hardware auxiliary circuits, thereby reducing the cost of the resonant converter.

[0165] (2) The synchronous rectification control algorithm of the present invention can meet all the working modes of the phase cutting strategy of the three-phase CLLC resonant converter, thereby improving the efficiency of the three-phase CLLC resonant converter in the full load range.

[0166] (3) The present invention adopts an extended harmonic analysis method, which, compared with the fundamental wave analysis method, still has higher accuracy when deviating from the resonance point, thereby improving the calculation accuracy of the model.

[0167] The above specific embodiments have described the principles and implementation methods of the present invention in detail, but the present invention is not limited to the above specific embodiments. Those skilled in the art can make corresponding changes in the specific implementation methods by referring to the ideas of the present invention within the scope of their knowledge.

Claims

1. A three-phase CLLC resonant converter synchronous rectification control method based on extended harmonics, the resonant converter includes a primary side inverter network, a resonant network and a secondary side rectifier network. The primary and secondary sides of the three-phase transformer in the resonant network are connected in star shape, and the resonant capacitor is connected in triangle shape, which can realize automatic current sharing in the three-phase operation mode, and because the resonant capacitor has no DC bias, it is also helpful to realize soft start, characterized in that: The three-phase CLLC resonant converter synchronous rectification control method based on extended harmonics comprises the following steps: Step 1: Sample the input and output voltage and current in real time; Step 2: Determine the working mode of the converter by sampling the voltage and current data; Step 3: Determine the turn-on time of the secondary-side MOSFET; Step 4: According to different modes, based on the extended harmonic equivalent circuit model, the input signal can be split into multiple periodic signals, and Fourier series expansion is performed on them; Step 5: According to different modes, based on the extended harmonic analysis method, the phase difference between the input square wave and the secondary side resonant current is calculated, and the zero crossing point of the secondary side MOSFET current is further obtained; Step 6: Calculate the conduction time of the secondary side MOSFET; Step 7: Determine the turn-off time of the secondary-side MOSFET.

2. The method for controlling synchronous rectification of a three-phase CLLC resonant converter based on extended harmonics according to claim 1, characterized in that: In step 2, the output power can be calculated by sampling data to determine the current working mode of the resonant converter.

3. The method for controlling synchronous rectification of a three-phase CLLC resonant converter based on extended harmonics according to claim 1, characterized in that: In step 3, the secondary-side MOSFET and the corresponding primary-side MOSFET are turned on synchronously.

4. The method for controlling synchronous rectification of a three-phase CLLC resonant converter based on extended harmonics according to claim 1, characterized in that: Taking the three-phase operation mode as an example, step 4 specifically includes the following steps: (1) The input step square wave signal V AN_3ph_k (t) is split into two periodic square wave signals V with the same frequency and amplitude AN1 (t) and V AN2 (t); (2) V AN1 (t) and V AN2 (t) can be expanded by Fourier series to obtain the following formula: V AN_3ph_k (t)=V AN1 (t)+V AN2 (t) (2) 5. The method for controlling synchronous rectification of a three-phase CLLC resonant converter based on extended harmonics according to claim 1, characterized in that: Taking the three-phase operation mode as an example, step 5 specifically includes the following steps: (1) Calculate the equivalent resistance R eq_3ph_k , the calculation formula is: Where R eq_3ph_k is the equivalent output load in three-phase operation mode, R o is the output load, n is the transformer ratio; (2) Calculate the load impedance Z 1k , the calculation formula is: in, In the formula, ω s is the converter operating frequency, L rs is the secondary side resonant inductor, C rs is the secondary side resonant capacitor; (3) Calculate the load impedance Z 1k The impedance angle θ 1k , the calculation formula is: (4) Calculate the load impedance Z 2k , the calculation formula is: Z 2k =jkωL m ||Z 1k =a 2k +jb 2k (7) in, Where, L m is the converter excitation inductance; (5) Calculate the load impedance Z 2k The impedance angle θ 2k , the calculation formula is: (6) Calculate the load impedance Z 3k , the calculation formula is: in, Where, L rp is the primary side resonant inductor, C rp is the primary side resonant capacitor; (7) Calculate the load impedance Z 3k The impedance angle θ 3k , the calculation formula is: (8) Calculate the resonant current i on the secondary side s '(t), assuming that the phase of the step wave voltage is 0°, then: Where θ κ =θ 2κ -θ 3κ -θ 1κ , is the phase angle of the resonant current, so the secondary resonant current in the time domain can be obtained as: Where I s is the effective value of the resonant current; In order to solve the phase difference θ between the input signal and the resonant current, we can s '(t) = 0 and use MATLAB to calculate its zero crossing point.

6. The method for controlling synchronous rectification of a three-phase CLLC resonant converter based on extended harmonics according to claim 1, characterized in that: The step 6 specifically comprises the following steps: (1) Calculate the dead time t dead To ensure that MOSFET can achieve ZVS, it is necessary to consider that the charge of MOSFET parasitic capacitance can be completely released during the dead time, so: t dead >18L m C oss f max (15) Where, t dead is the dead time, C oss is the switch junction capacitance, f max is the maximum operating frequency of the converter; (2) Calculate the on-time t on , let the phase difference between the zero-crossing point of the secondary-side resonant current and the turn-on time of the secondary-side MOSFET be Then we can calculate: In the formula, f s is the converter operating frequency.

7. The method for controlling synchronous rectification of a three-phase CLLC resonant converter based on extended harmonics according to claim 1, characterized in that: In step 7, the turn-off time of the secondary side MOSFET is the turn-on time of the secondary side MOSFET plus the conduction time t on .

8. The method for controlling synchronous rectification of a three-phase CLLC resonant converter based on extended harmonics according to claim 1, characterized in that: When the converter switches to full-bridge operation mode and half-bridge operation mode, according to its topological structure, its primary and secondary side resonant capacitors become half of the original, the primary and secondary side resonant inductors become twice of the original, the excitation inductance becomes twice of the original, and: Where R eq_fb_k is the equivalent output load in full-bridge operation mode, R eq_hb_k is the equivalent output load in half-bridge operation mode.

9. The method for controlling synchronous rectification of a three-phase CLLC resonant converter based on extended harmonics according to claim 1, characterized in that: When the converter switches to full-bridge operation mode and half-bridge operation mode, according to its extended harmonic impedance model, the input signal is expanded by Fourier series to obtain: Where V AN_fb_k (t) is the input signal of full-bridge operation mode, V AN_hb_k (t) is the input signal of half-bridge operation mode.

10. The three-phase CLLC resonant converter synchronous rectification control method based on extended harmonics according to claims 1, 2, 3, 4, 5, 6, and 7, characterized in that: Through the above three-phase mode calculation steps, the secondary side MOSFET conduction time t in full-bridge mode and half-bridge mode can be calculated in the same way. on , and then the turn-off time of the secondary side MOSFET in full-bridge mode and half-bridge mode is obtained.

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