Constant common-mode voltage type three-level Buck photovoltaic direct-current converter and hybrid modulation method

Abstract

The invention discloses a constant common-mode voltage type three-level Buck photovoltaic direct-current converter and a hybrid modulation method, based on the constant common-mode voltage type three-level Buck photovoltaic direct-current converter, a circuit working range is divided into three intervals, 0 lt; dlt; when D is greater than or equal to Dth1 and less than or equal to Dth2, the interval I is the interval II, and Dth2lt is the interval II; dlt; when 1 is an interval III, an interlaced modulation strategy is adopted in the interval I and the interval III, and a synchronous modulation strategy is adopted in the interval II; d is the duty ratio, Dth1 is the lower limit of a synchronous modulation interval (interval II), and Dth2 is the upper limit of the synchronous modulation interval (interval II), on the basis of not influencing the differential mode characteristic, the common-mode voltage of the circuit is kept constant by cutting off a common-mode loop, adding a clamping structure and the like, the common-mode characteristic is improved, the leakage current is effectively reduced, and the reliability of the circuit is improved. Conduction and radiation interference, current harmonic waves and loss caused by high-frequency leakage current can be reduced, and the safety of personnel and equipment is protected.

Description

Technical Field

[0001] The present invention belongs to the technical field of power electronics, and mainly relates to a constant common-mode voltage type three-level Buck photovoltaic DC converter and a hybrid modulation method. Background Art

[0002] With the increase in photovoltaic installation capacity, multiple DC converter outputs are often connected in series in a photovoltaic low-voltage DC (PV-LVDC) system to provide a higher DC bus voltage. In this case, the three-level Buck converter has attracted much attention due to its simple topology, high efficiency, voltage balancing effect, low voltage stress of power switching tubes and diodes, low switching losses, and the ability to cut off the output of the photovoltaic panel and continuously adjust the output voltage from zero.

[0003] The traditional three-level Buck converter is a non-isolated structure. Due to the lack of a transformer, there is a lack of electrical isolation between the photovoltaic panel and the DC bus. There is a parasitic capacitance to the ground in the photovoltaic panel. The switching actions of power devices in the non-isolated TTL-Buck converter may generate a high-frequency time-varying voltage acting on this parasitic capacitance, thereby generating a leakage current i cm (also known as common-mode current or ground current) in the common-mode (CM) loop composed of the panel parasitic capacitance, the Buck converter, the DC bus, and the ground. The generation of high-frequency leakage current will bring conduction and radiation interference, current distortion, and increased losses, and even endanger the safety of personnel and equipment. Therefore, measures need to be taken to limit the leakage current.

[0004] It can be seen from the information disclosed in Chinese Patent CN202411182722.3 that for the traditional three-level Buck converter when adopting the synchronous modulation strategy, the common-mode characteristics are good, the common-mode voltage can be kept constant, and the leakage current can be effectively reduced. However, when adopting the synchronous modulation strategy, the differential-mode characteristics are poor. Summary of the Invention

[0005] In view of the problem of poor common-mode characteristics in the prior art when uniformly adopting the interleaved modulation method, the present invention provides a constant common-mode voltage type three-level Buck photovoltaic DC converter and a hybrid modulation method. Based on the constant common-mode voltage type three-level Buck photovoltaic DC converter, the circuit operating range is divided into three intervals. When 0 < D < D th1 it is Interval I, when D th1 ≤ D ≤ D th2 it is Interval II, and when D th2 < D < 1 it is Interval III. The interleaved modulation strategy is adopted in Interval I and Interval III, and the synchronous modulation strategy is adopted in Interval II; the D is the duty cycle, D th1 is the lower limit of the synchronous modulation interval (Interval II), D th2is the upper limit of the synchronous modulation interval (Interval II). Without affecting the differential-mode characteristics, the common-mode voltage of the circuit is maintained constant by cutting off the common-mode loop and adding a clamping structure, etc., improving the common-mode characteristics, effectively reducing the leakage current, which is beneficial to reducing the conduction and radiation interference and current harmonics and losses caused by the high-frequency leakage current, and protecting the safety of personnel and equipment.

[0006] To achieve the above object, the technical solution adopted by the present invention is: a constant common-mode voltage type three-level Buck photovoltaic DC converter, which includes at least eight switching tubes, four input-side DC capacitors, two energy transfer capacitors and four diodes.

[0007] The four input-side DC capacitors are connected in series and then connected in parallel with the input voltage source.

[0008] The positive terminal of the first input-side DC capacitor is connected to the negative terminal of the third diode, the positive terminal of the third diode is connected to the drain of the third switching tube, the negative terminal of the first input-side DC capacitor is connected to the drain of the fifth switching tube, the source of the fifth switching tube is connected to the source of the sixth switching tube, the drain of the sixth switching tube is connected to the source of the third switching tube, and the source of the third switching tube is connected to the positive terminal of the first energy transfer capacitor.

[0009] The negative terminal of the first energy transfer capacitor is connected to the positive terminal of the second energy transfer capacitor, the negative terminal of the second energy transfer capacitor is connected to the drain of the fourth switching tube, the source of the fourth switching tube is connected to the negative terminal of the fourth diode, the positive terminal of the fourth diode is connected to the negative terminal of the fourth input-side DC capacitor; the positive terminal of the fourth input-side DC capacitor is connected to the drain of the seventh switching tube, the source of the seventh switching tube is connected to the source of the eighth switching tube, and the drain of the eighth switching tube is connected to the drain of the fourth switching tube.

[0010] The positive terminal of the first energy transfer capacitor is connected to the drain of the first switching tube, the source of the first switching tube is connected to the positive terminal of the first diode, the negative terminal of the first diode is connected to the positive terminal of the second diode, the negative terminal of the second diode is connected to the drain of the second switching tube, and the source of the second switching tube is connected to the negative terminal of the second energy transfer capacitor.

[0011] As an improvement of the present invention, the four input-side DC capacitors are connected in parallel with the input voltage source. The positive terminal of the first input-side DC capacitor is connected to the positive pole of the input voltage source, the negative terminal of the first input-side DC capacitor is connected to the positive terminal of the second input-side DC capacitor, the negative terminal of the second input-side DC capacitor is connected to the positive terminal of the third input-side DC capacitor, the negative terminal of the third input-side DC capacitor is connected to the positive terminal of the fourth input-side DC capacitor, and the negative terminal of the fourth input-side DC capacitor is connected to the negative pole of the input voltage source.

[0012] As another improvement of the present invention, the connection point of the first energy transfer capacitor and the second energy transfer capacitor is connected to the connection point of the first diode and the second diode through a wire, and the positive terminal of the first diode and the negative terminal of the second diode are respectively connected to the positive terminal and the negative terminal of the LC filter.

[0013] As another improvement of the present invention, the LC filter at least includes a first filter inductor, a second filter inductor, a first filter capacitor and a second filter capacitor.

[0014] As yet another improvement of the present invention, one end of the first filter inductor serves as the positive terminal of the input side of the LC filter, and the other end is connected to the first filter capacitor and serves as the positive terminal of the output side of the LC filter; one end of the second filter inductor serves as the negative terminal of the input side of the LC filter, and the other end is connected to the second filter capacitor and serves as the negative terminal of the output side of the LC filter; the first filter capacitor and the second filter capacitor are connected.

[0015] To achieve the above object, the technical solution adopted by the present invention is also: a hybrid modulation method for a constant common-mode voltage type three-level Buck photovoltaic DC converter, which divides the circuit operating range into three intervals. When 0 < D < D th1 , it is interval I. When D th1 ≤ D ≤ D th2 , it is interval II. When D th2 < D < 1, it is interval III. An interleaved modulation strategy is adopted in interval I and interval III, and a synchronous modulation strategy is adopted in interval II; the D is the duty cycle, D th1 is the lower limit of the synchronous modulation interval, and D th2 is the upper limit of the synchronous modulation interval.

[0016] As an improvement of the present invention, when the first energy transfer capacitor supplies energy alone, the first switching tube is turned on, the second switching tube is turned off, the third and fourth switching tubes are turned off to cut off the common-mode loop, the fifth and sixth switching tubes are turned on, and the seventh and eighth switching tubes are turned off;

[0017] When the second energy transfer capacitor supplies energy alone, the first switching tube is turned off, the second switching tube is turned on, the third and fourth switching tubes are turned off to cut off the common-mode loop, the fifth and sixth switching tubes are turned off, and the seventh and eighth switching tubes are turned on.

[0018] As another improvement of the present invention, when the first energy transfer capacitor and the second energy transfer capacitor supply energy together, the first switching tube, the second switching tube, the third switching tube and the fourth switching tube are turned on, and the fifth switching tube, the sixth switching tube, the seventh switching tube and the eighth switching tube are turned off;

[0019] When the circuit is in the freewheeling mode, the first switch tube and the second switch tube are turned off to cut off the common-mode loop, the third switch tube and the fourth switch tube are turned on, and the fifth switch tube, the sixth switch tube, the seventh switch tube and the eighth switch tube are all turned off.

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows: The present invention proposes a constant common-mode voltage type three-level Buck photovoltaic DC converter and a hybrid modulation method. On the basis of not affecting the differential-mode characteristics, the common-mode voltage of the circuit is maintained constant by cutting off the common-mode loop and adding a clamping structure, etc., improving the common-mode characteristics, effectively reducing the leakage current, and being beneficial to reducing the conduction and radiation interference and current harmonics and losses caused by high-frequency leakage current, and protecting the safety of personnel and equipment. Brief Description of the Drawings

[0021] Figure 1 It is a schematic circuit structure diagram of the constant common-mode voltage type three-level Buck photovoltaic DC converter of the present invention;

[0022] Figure 2 It is a working mode diagram of the constant common-mode voltage type three-level Buck converter of the present invention;

[0023] Figure 3 It is a working timing diagram of the constant common-mode voltage type three-level Buck converter of the present invention;

[0024] Figure 4 It is a working waveform diagram of the constant common-mode voltage type Buck converter of the present invention when D = 0.5;

[0025] Figure 5 It is a schematic diagram of the hybrid modulation strategy of the novel three-level Buck photovoltaic DC converter with constant common-mode voltage of the present invention;

[0026] Figure 6 It is a schematic diagram of the inductor current pulsation curve of the constant common-mode voltage type three-level Buck converter in Embodiment 1 of the present invention;

[0027] Figure 7 It is a schematic diagram of the effective values of the currents of the switch tubes S3 and S4 of the constant common-mode voltage type three-level Buck converter in Embodiment 1 of the present invention when different modulation strategies are adopted

[0028] Figure 8 It is a schematic diagram of the differential-mode characteristics of different topological structures when different modulation strategies are adopted when D = 0.3 in Experimental Example 1 of the present invention;

[0029] Figure 9 It is a schematic diagram of the common-mode characteristics of different topological structures when different modulation strategies are adopted when D = 0.3 in Experimental Example 1 of the present invention;

[0030] Figure 10Schematic diagram of differential-mode characteristics of different modulation strategies for different topological structures when D = 0.7 in Experimental Example 2 of the present invention;

[0031] Figure 11 Schematic diagram of common-mode characteristics of different modulation strategies for different topological structures when D = 0.7 in Experimental Example 2 of the present invention;

[0032] Figure 12 Schematic diagram of differential-mode characteristics of different modulation strategies for different topological structures when D = 0.5 in Experimental Example 3 of the present invention;

[0033] Figure 13 Schematic diagram of common-mode characteristics of different modulation strategies for different topological structures when D = 0.5 in Experimental Example 3 of the present invention. Detailed implementation manners

[0034] The present invention will be further clarified below in conjunction with the accompanying drawings and detailed implementation manners. It should be understood that the following detailed implementation manners are only used to illustrate the present invention and not to limit the scope of the present invention.

[0035] Embodiment 1

[0036] A novel three-level Buck photovoltaic DC converter that can achieve a constant common-mode voltage, which can effectively reduce the leakage current. Based on the traditional three-level Buck converter, some switching tubes and diodes are added. The specific structure is as Figure 1 shown, and it includes at least eight switching tubes, four input-side DC capacitors, two energy transfer capacitors and four diodes. Among them, the first input-side DC capacitor C dc1 , the second input-side DC capacitor C dc2 , the third input-side DC capacitor C dc3 , the fourth input-side DC capacitor C dc4 are connected in series and then connected in parallel with the input voltage source U pv . The positive terminal of the first input-side DC capacitor C dc1 is connected to the positive electrode of the input voltage source U pv . The negative terminal of the first input-side DC capacitor C dc1 is connected to the positive terminal of the second input-side DC capacitor C dc2 . The negative terminal of the second input-side DC capacitor C dc2 is connected to the positive terminal of the third input-side DC capacitor C dc3 . The negative terminal of the third input-side DC capacitor C dc3 is connected to the positive terminal of the fourth input-side DC capacitor C dc4 . The negative terminal of the fourth input-side DC capacitor C dc4 is connected to the negative electrode of the input voltage source U pv . The positive terminal of the first input-side DC capacitor C dc1 is connected to the negative terminal of the third diode D3, and the positive terminal of the third diode D3 is connected to the drain of the third switching tube S3. The positive terminal of the first input-side DC capacitor Cdc1 The negative terminal is connected to the drain of the fifth switching transistor S5. The source of the fifth switching transistor S5 is connected to the source of the sixth switching transistor S6. The drain of the sixth switching transistor S6 is connected to the source of the third switching transistor S3. The source of the third switching transistor S3 is connected to the positive terminal of the first energy transfer capacitor C1. The negative terminal of the first energy transfer capacitor C1 is connected to the positive terminal of the second energy transfer capacitor C2. The negative terminal of the second energy transfer capacitor C2 is connected to the drain of the fourth switching transistor S4. The source of the fourth switching transistor S4 is connected to the negative terminal of the fourth diode D4. The positive terminal of the fourth diode D4 is connected to the negative terminal of the fourth input-side DC capacitor C dc4 negative terminal, and the positive terminal of the fourth input-side DC capacitor C dc4 is connected to the drain of the seventh switching transistor S7. The source of the seventh switching transistor S7 is connected to the source of the eighth switching transistor S8. The drain of the eighth switching transistor S8 is connected to the drain of the fourth switching transistor S4. The positive terminal of the first energy transfer capacitor C1 is connected to the drain of the first switching transistor S1. The source of the first switching transistor S1 is connected to the positive terminal of the first diode D1. The negative terminal of the first diode D1 is connected to the positive terminal of the second diode D2. The negative terminal of the second diode D2 is connected to the drain of the second switching transistor S2. The source of the second switching transistor S2 is connected to the negative terminal of the second energy transfer capacitor C2. The connection point of the first energy transfer capacitor C1 and the second energy transfer capacitor C2 is connected to the connection point of the first diode D1 and the second diode D2 by a wire. The positive terminal of the first diode D1 and the negative terminal of the second diode D2 are respectively connected to the first filter inductor L f1 and the second filter inductor L f2 and the first filter capacitor C f1 and the second filter capacitor C f2 at the positive and negative terminals of the LC filter composed of them.

[0037] The LC filter includes the first filter inductor L f1 and the second filter inductor L f2 and the first filter capacitor C f1 and the second filter capacitor C f2 . One end of the first filter inductor L f1 serves as the positive terminal on the input side of the LC filter, and the other end is connected to one end of the first filter capacitor C f1 and serves as the positive terminal on the output side of the LC filter. One end of the second filter inductor L f2 serves as the negative terminal on the input side of the LC filter, and the other end is connected to one end of the second filter capacitor C f2 and serves as the negative terminal on the output side of the LC filter. The other ends of the first filter capacitor C f1 and the second filter capacitor C f2 are connected.

[0038] Figure 2 , Figure 3 As shown in Figure 2When operating in the dual-capacitor energy supply mode as shown in (a), the first switching transistor S1 and the second switching transistor S2 are turned on simultaneously, the third switching transistor S3 and the fourth switching transistor S4 are turned on, and the fifth switching transistor S5, the sixth switching transistor S6, the seventh switching transistor S7, and the eighth switching transistor S8 are turned off. At this time, the common-mode voltage is U pv / 2; when the circuit operates in Figure 2 the upper-capacitor energy supply mode as shown in (b), the first switching transistor S1 is turned on, the second switching transistor S2 is turned off, the third switching transistor S3 and the fourth switching transistor S4 are turned off, cutting off the common-mode loop. The fifth switching transistor S5 and the sixth switching transistor S6 are turned on, clamping the sum of the voltage of the third diode D3 and the third switching transistor S3 to U pv / 4. The seventh switching transistor S7 and the eighth switching transistor S8 are turned off. At this time, the common-mode voltage is U pv / 2; when the circuit operates in Figure 2 the lower-capacitor energy supply mode as shown in (c), the first switching transistor S1 is turned off, the second switching transistor S2 is turned on, the third switching transistor S3 and the fourth switching transistor S4 are turned off, cutting off the common-mode loop. The fifth switching transistor S5 and the sixth switching transistor S6 are turned off, and the seventh switching transistor S7 and the eighth switching transistor S8 are turned on, clamping the sum of the voltage of the fourth diode D4 and the fourth switching transistor S4 to U pv / 4. At this time, the common-mode voltage is U pv / 2; when the circuit operates in Figure 2 the freewheeling mode as shown in (d), the first switching transistor S1 and the second switching transistor S2 are turned off simultaneously, cutting off the common-mode loop. The third switching transistor S3 and the fourth switching transistor S4 are turned on, and the fifth switching transistor S5, the sixth switching transistor S6, the seventh switching transistor S7, and the eighth switching transistor S8 are turned off. At this time, the common-mode voltage is U pv / 2. By cutting off the common-mode loop and adding a clamping structure, this circuit can maintain a constant common-mode voltage throughout the switching cycle, effectively reducing the leakage current.

[0039] It can be found from Figure 2 that when D = 0.5, the constant common-mode voltage type three-level Buck converter switches between the upper-capacitor energy transfer mode and the lower-capacitor energy transfer mode as shown in Figure 2 (b) and (c). The switching transistors S3 and S4 are completely turned off throughout the operating cycle. Although the switching transistors S5, S6, S7, and S8 can take over the conduction, at this time, the input-side photovoltaic power supply U pv cannot charge the first energy transfer capacitor C1 and the second energy transfer capacitor C2. The first energy transfer capacitor C1 and the second energy transfer capacitor C2 continuously discharge, and the capacitor voltage continuously drops, resulting in the inductor being unable to store enough energy and the inductor current also continuously decreasing, causing the control link to collapse and the output voltage to be unable to be maintained. At this time, the operating situation is as shown in Figure 4 .

[0040] To solve this problem, the present invention also proposes a hybrid modulation strategy, as shown inFigure 5 As shown, the circuit operating range is divided into three intervals. When 0 < D < D th1 , it is Interval I. When D th1 ≤ D ≤ D th2 , it is Interval II. When D th2 < D < 1, it is Interval III. The interleaved modulation strategy is adopted in Interval I and Interval III, and the synchronous modulation strategy is adopted in Interval II to ensure the normal operation of the circuit.

[0041] The range of Interval II is determined by two parts: the inductor size and the rms current of switches S3 and S4. Figure 6 The curves of the inductor current ripple of the constant common-mode voltage type three-level Buck converter using different modulation strategies are shown. It can be found that when the synchronous modulation strategy is adopted and D = 0.5, the required inductor size of the constant common-mode voltage type three-level Buck converter is the same as the maximum required inductor when the interleaved modulation strategy is adopted. Therefore, if it is desired to set the synchronous modulation interval near D = 0.5, in order to make the inductor current ripple meet the design requirements when D < 0.5 and the synchronous modulation strategy is adopted, that is, Δi Lf ≤ 20%I o , the inductor needs to be appropriately increased. Figure 7 The rms currents of switches S3 and S4 of the constant common-mode voltage type three-level Buck converter using different modulation strategies are shown. When the interleaved modulation strategy is adopted, if the converter D → 0.5, the rms current will increase sharply, posing high requirements on the performance of the switches. When D = 0.5, since S3 and S4 are completely turned off throughout the switching cycle, their rms currents drop to 0. When the synchronous modulation strategy is adopted, the rms currents of S3 and S4 are smaller near D = 0.5. Considering the requirements of both the change in the inductor value and the rms currents of switches S3 and S4, when the increase in the inductor is less than 10% and the reduction in the maximum rms current borne by switches S3 and S4 is greater than 50%, the operating range near D = 0.5 can be defined as the synchronous modulation interval. This hybrid modulation strategy can be implemented by DSP or FPGA.

[0042] Experimental Example 1: The input voltage U pv = 320V, the output voltage U o = ±48V, the duty cycle D = 0.3 < 0.5, Interval I

[0043] The traditional three-level Buck (Traditional Three Level Buck, TTL-Buck) converter is simulated based on Matlab / Simulink under three cases: adopting the interleaved modulation strategy and the synchronous modulation strategy, and the improved three-level Buck (Improved ThreeLevel Buck, ITL-Buck) converter adopting the hybrid modulation strategy. From the differential-mode voltage uDM and the inductor current i Lf Analyze the differential-mode characteristics of two three-level Buck converter topologies in three cases from two aspects, namely the common-mode voltage u CM and the leakage current i cm Analyze the common-mode characteristics of two three-level Buck converter topologies in three cases from two aspects.

[0044] Figure 8 and Figure 9 are the differential-mode and common-mode characteristics of the TTL-Buck converter using the interleaved modulation strategy and the synchronous modulation strategy respectively, and the ITL-Buck converter using the hybrid modulation strategy. And perform FFT analysis on the waveforms, and the results are summarized in Table 1.

[0045] Table 1

[0046]

[0047] Table 1 shows the comparison data of the FFT analysis results of the differential-mode and common-mode characteristics of different topologies using different modulation strategies when D = 0.3. It can be found from the above simulation results that when the TTL-Buck converter adopts the synchronous modulation strategy, the differential-mode voltage frequency is the same as the switching frequency f s , the amplitude of the differential-mode voltage at the switching frequency is the largest, and the pulsation of the inductor current is also the largest. And the amplitude of the common-mode voltage at the switching frequency is 0, that is, a constant common-mode voltage can be maintained, and the leakage current at the switching frequency is 0; when the TTL-Buck converter adopts the interleaved modulation strategy, the differential-mode voltage frequency is 2f s , the amplitude at the switching frequency is smaller than that when using the synchronous modulation, and the pulsation of the inductor current is also smaller. And the amplitude of the common-mode voltage at the switching frequency is larger, and a constant common-mode voltage cannot be maintained, and the amplitude at the switching frequency reaches 3.513A. It can also be seen from this that the TTL-Buck converter has poor differential-mode characteristics and good common-mode characteristics when adopting the synchronous modulation strategy; when adopting the interleaved modulation strategy, the differential-mode characteristics are good and the common-mode characteristics are poor.

[0048] When the ITL-Buck converter proposed in the present invention adopts the hybrid modulation strategy, the differential-mode voltage frequency is 2f s , the amplitude of the common-mode voltage and the pulsation of the inductor current at the switching frequency are not much different from those of the TTL-Buck converter when adopting the interleaved modulation strategy, and do not affect its good differential-mode characteristics; while the amplitude of the common-mode voltage and the leakage current amplitude at the switching frequency are very small, and are reduced by 99.05% and 99.15% respectively compared with the TTL-Buck converter when adopting the interleaved modulation strategy, greatly improving its common-mode characteristics.

[0049] Experimental Example 2: The input voltage U pv = 137V, the output voltage U o= ±48V, duty cycle D = 0.7 > 0.5, interval Ⅲ

[0050] For three cases of the TTL - Buck converter using the interleaved modulation strategy and the synchronous modulation strategy and the ITL - Buck converter using the hybrid modulation strategy, simulations are carried out based on Matlab / Simulink. From the differential - mode voltage u DM and the inductor current i Lf , the differential - mode characteristics of the two three - level Buck converter structures in three cases are analyzed. From the common - mode voltage u CM and the leakage current i cm , the common - mode characteristics of the two three - level Buck converter structures in three cases are analyzed.

[0051] Figure 10 , Figure 11 are the differential - mode characteristics and common - mode characteristics of the TTL - Buck converter using the interleaved modulation strategy and the synchronous modulation strategy and the ITL - Buck converter using the hybrid modulation strategy, respectively. And FFT analysis is carried out on the waveforms, and the results are summarized in Table 2.

[0052] Table 2

[0053]

[0054] Table 2 is the comparison data of the FFT analysis results of the differential - mode characteristics and common - mode characteristics of different topologies using different modulation strategies when D = 0.7. From the above simulation results, it can be found that when the TTL - Buck converter adopts the synchronous modulation strategy, the differential - mode voltage frequency is the same as the switching frequency f s . The amplitude of the differential - mode voltage at the switching frequency is the largest, up to 69.49V, and the pulsation of the inductor current is also the largest. While the amplitude of the common - mode voltage at the switching frequency is 0, that is, a constant common - mode voltage can be maintained, and the leakage current at the switching frequency is 0. When the TTL - Buck converter adopts the interleaved modulation strategy, the differential - mode voltage frequency is 2f s . The amplitude at the switching frequency is smaller than that when using the synchronous modulation, and the pulsation of the inductor current is also smaller. While the amplitude of the common - mode voltage at the switching frequency is larger, and a constant common - mode voltage cannot be maintained, and the amplitude at the switching frequency reaches 1.47A. It can also be seen from this that the TTL - Buck converter has poor differential - mode characteristics and good common - mode characteristics when adopting the synchronous modulation strategy; when adopting the interleaved modulation strategy, the differential - mode characteristics are good and the common - mode characteristics are poor.

[0055] And for the ITL - Buck converter proposed in the present invention, when adopting the interleaved modulation strategy, the differential - mode voltage frequency is 2f sAt the switching frequency, both the common-mode voltage amplitude and the inductor current ripple are not much different from those of the TTL-Buck converter using the interleaved modulation strategy, inheriting its good differential-mode characteristics. The common-mode voltage amplitude and the leakage current amplitude at the switching frequency are both very small, and are reduced by 97.26% and 97.28% respectively compared with the TTL-Buck converter using the interleaved modulation strategy, greatly improving its common-mode characteristics.

[0056] Experimental Example 3: Input voltage U pv = 192V, output voltage U o = ±48V, duty cycle D = 0.5, Interval II

[0057] Simulations are carried out based on Matlab / Simulink for three cases: the TTL-Buck converter using the interleaved modulation strategy and the synchronous modulation strategy, and the ITL-Buck converter using the hybrid modulation strategy. The differential-mode characteristics of the two three-level Buck converter structures in the three cases are analyzed from two aspects: the differential-mode voltage u DM and the inductor current i Lf . The common-mode characteristics of the two three-level Buck converter structures in the three cases are analyzed from two aspects: the common-mode voltage u CM and the leakage current i cm .

[0058] Figure 12 And Figure 13 are the differential-mode characteristics and common-mode characteristics of the TTL-Buck converter using the interleaved modulation strategy and the synchronous modulation strategy, and the ITL-Buck converter using the hybrid modulation strategy respectively. FFT analysis is carried out on the waveforms, and the results are summarized in Table 3.

[0059] Table 3

[0060]

[0061] Table 3 is the comparison data of the FFT analysis results of the differential-mode characteristics and common-mode characteristics of different topological structures using different modulation strategies when D = 0.5. When D = 0.5, for the TTL-Buck converter using the synchronous modulation strategy, the differential-mode voltage is a two-level voltage with a frequency of f s . The differential-mode voltage amplitude at the switching frequency is the largest, which is 125.00V, the inductor current ripple is relatively large, the common-mode voltage is constant, and the leakage current is basically 0. When using the interleaved modulation strategy, the differential-mode voltage is constant, the inductor current ripple is 0, the common-mode voltage is a two-level voltage with a frequency of f s . The common-mode voltage amplitude at the switching frequency is relatively large, which is 62.52V, the leakage current is relatively large, and the leakage current amplitude at the switching frequency reaches 2.65A.

[0062] The ITL-Buck converter adopts a hybrid modulation strategy. In interval II, it is synchronous modulation. At this time, the amplitude of the differential-mode voltage at the switching frequency is 120.9V, and the pulsation of the inductor current is 19.17%, which does not exceed the design requirements. The common-mode voltage is constant and the leakage current is 0, so the common-mode characteristics are greatly improved, and the normal operation of the circuit when D = 0.5 is ensured.

[0063] In summary, the present invention discloses a three-level Buck photovoltaic DC converter with a constant common-mode voltage and a hybrid modulation method. On the basis of not affecting the differential-mode characteristics, the common-mode voltage of the circuit is maintained constant by cutting off the common-mode loop and adding a clamping structure, etc., the common-mode characteristics are improved, the leakage current is effectively reduced, which is beneficial to reducing the conduction and radiation interference and current harmonics and losses caused by high-frequency leakage current, and protecting the safety of personnel and equipment.

[0064] It should be noted that the above content only illustrates the technical idea of the present invention and cannot limit the protection scope of the present invention. For those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and retouches can be made, and these improvements and retouches all fall within the protection scope of the claims of the present invention.

Claims

1. A constant common-mode voltage type three-level Buck photovoltaic DC converter, characterized in that : It includes at least eight switching tubes, four input-side DC capacitors, two energy transfer capacitors and four diodes. The four input-side DC capacitors are connected in series and then connected in parallel with the input voltage source. The positive terminal of the first input-side DC capacitor is connected to the negative terminal of the third diode. The positive terminal of the third diode is connected to the drain of the third switching tube. The negative terminal of the first input-side DC capacitor is connected to the drain of the fifth switching tube. The source of the fifth switching tube is connected to the source of the sixth switching tube. The drain of the sixth switching tube is connected to the source of the third switching tube. The source of the third switching tube is connected to the positive terminal of the first energy transfer capacitor. The negative terminal of the first energy transfer capacitor is connected to the positive terminal of the second energy transfer capacitor. The negative terminal of the second energy transfer capacitor is connected to the drain of the fourth switching tube. The source of the fourth switching tube is connected to the negative terminal of the fourth diode. The positive terminal of the fourth diode is connected to the negative terminal of the fourth input-side DC capacitor. The positive terminal of the fourth input-side DC capacitor is connected to the drain of the seventh switching tube. The source of the seventh switching tube is connected to the source of the eighth switching tube. The drain of the eighth switching tube is connected to the drain of the fourth switching tube. The positive terminal of the first energy transfer capacitor is connected to the drain of the first switching tube. The source of the first switching tube is connected to the positive terminal of the first diode. The negative terminal of the first diode is connected to the positive terminal of the second diode. The negative terminal of the second diode is connected to the drain of the second switching tube. The source of the second switching tube is connected to the negative terminal of the second energy transfer capacitor.

2. The constant common-mode voltage type three-level Buck photovoltaic DC converter according to claim 1, characterized in that: The four input-side DC capacitors are connected in parallel with the input voltage source. The positive terminal of the first input-side DC capacitor is connected to the positive pole of the input voltage source. The negative terminal of the first input-side DC capacitor is connected to the positive terminal of the second input-side DC capacitor. The negative terminal of the second input-side DC capacitor is connected to the positive terminal of the third input-side DC capacitor. The negative terminal of the third input-side DC capacitor is connected to the positive terminal of the fourth input-side DC capacitor. The negative terminal of the fourth input-side DC capacitor is connected to the negative pole of the input voltage source.

3. A constant common-mode voltage type three-level Buck photovoltaic DC converter according to claim 1 or 2, characterized in that: The connection point of the first energy transfer capacitor and the second energy transfer capacitor is connected to the connection point of the first diode and the second diode through a wire. The positive terminal of the first diode and the negative terminal of the second diode are respectively connected to the positive terminal and the negative terminal of the LC filter.

4. A constant common-mode voltage type three-level Buck photovoltaic DC converter according to claim 3, characterized in that: The LC filter includes at least a first filter inductor, a second filter inductor, a first filter capacitor and a second filter capacitor.

5. A constant common-mode voltage type three-level Buck photovoltaic DC converter according to claim 4, characterized in that: One end of the first filter inductor serves as the positive terminal of the input side of the LC filter, and the other end is connected to the first filter capacitor and serves as the positive terminal of the output side of the LC filter. One end of the second filter inductor serves as the negative terminal of the input side of the LC filter, and the other end is connected to the second filter capacitor and serves as the negative terminal of the output side of the LC filter. The first filter capacitor and the second filter capacitor are connected.

6. A hybrid modulation method for a constant common-mode voltage type three-level Buck photovoltaic DC converter, using the DC converter as described in claim 1, characterized in that: The operating range of the circuit is divided into three intervals. When 0 < D < D th1 , it is interval I. When D th1 ≤ D ≤ D th2 , it is interval II. When D th2 < D < 1, it is interval III. The interleaved modulation strategy is adopted in intervals I and III, and the synchronous modulation strategy is adopted in interval II; where D is the duty cycle, D th1 is the lower limit of the synchronous modulation interval, D th2 is the upper limit of the synchronous modulation interval.

7. The hybrid modulation method of a constant common-mode voltage type three-level Buck photovoltaic DC converter according to claim 6, characterized in that: When the first energy transfer capacitor supplies energy alone, the first switching tube is turned on, the second switching tube is turned off, the third and fourth switching tubes are turned off to cut off the common-mode loop, the fifth and sixth switching tubes are turned on, and the seventh and eighth switching tubes are turned off. When the second energy transfer capacitor supplies energy alone, the first switching tube is turned off, the second switching tube is turned on, the third and fourth switching tubes are turned off to cut off the common-mode loop, the fifth and sixth switching tubes are turned off, and the seventh and eighth switching tubes are turned on.

8. The hybrid modulation method of a constant common-mode voltage type three-level Buck photovoltaic DC converter according to claim 6, characterized in that: When the first energy transfer capacitor and the second energy transfer capacitor supply energy together, the first switch tube, the second switch tube, the third switch tube, and the fourth switch tube are turned on, and the fifth switch tube, the sixth switch tube, the seventh switch tube, and the eighth switch tube are turned off; When the circuit is in the freewheeling mode, the first switch tube and the second switch tube are turned off to cut off the common-mode loop, the third switch tube and the fourth switch tube are turned on, and the fifth switch tube, the sixth switch tube, the seventh switch tube, and the eighth switch tube are all turned off.

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