A Dual-Input Variable-Structure Multi-Mode DC-DC Converter and Its Control Method

Abstract

The present invention discloses a dual-input variable-structure, multi-operating-mode DC-DC converter and its control method, belonging to the technical field of power electronic converters. The present invention includes DC input sources U1 and U2, power switching tubes S1, S2, and S3, diodes D1 and D0, boost inductors L1 and L2, filter capacitors C, and load resistors R. The amplitudes and characteristics of the two DC input sources U1 and U2 can be the same or different; the power switching tubes S1, S2, and S3 can be turned on simultaneously or staggered at a certain angle. The converter of the present invention adopts a dual-input power supply and utilizes different combinations of variable-structure switch groups to achieve six operating conditions: DC input source U1 working alone, DC input source U2 working alone, DC input sources U1 and U2 working in full parallel, DC input sources U1 and U2 working in staggered parallel, DC input sources U1 and U2 working in series with boosting, and DC input sources U1 and U2 working in normal series, thereby improving the stability, reliability, and flexibility of the distributed power generation system.

Description

Technical Field

[0001] The present invention relates to the technical field of power electronic converters, and in particular to a dual-input variable-structure multi-operating-mode DC-DC converter and a control method thereof. Background Art

[0002] With the depletion of fossil resources like oil, coal, and natural gas, and the growing environmental impact of these issues, the use of renewable energy sources, such as photovoltaics, fuel cells, and geothermal energy, is gaining increasing attention. However, these renewable energy sources suffer from poor power quality and significant environmental impact. Therefore, the use of a multi-energy system can effectively improve the reliability and quality of power supply systems.

[0003] In a multi-energy combined power supply system, each input source can use a single-input DC-DC converter to regulate voltage, but this results in a relatively complex system structure. Using a single dual-input DC converter instead of two single-input DC converters can simplify the system structure and reduce costs. For example, Patent Application No. 2013100188444 discloses a variable-structure dual-input DC converter. This converter comprises an inductor, a high-frequency power switch, a high-frequency diode, a boost capacitor, and an output filter capacitor. It also includes a low-frequency switching circuit consisting of a first low-frequency switch, a second low-frequency switch, and a low-frequency bypass diode. This variable-structure dual-input converter is capable of both dual-input and single-input high-gain operation, achieving different functions using the same circuit structure. Specifically, a low-frequency switching circuit is introduced within the multi-input converter. This circuit can adapt its structure based on changing input conditions by controlling the low-frequency switch, enabling both multi-input combined power supply and single-input high-gain operation. However, the operating conditions supported by this application are relatively limited, and the structure is relatively complex. Summary of the Invention

[0004] 1. Technical problem to be solved by the invention

[0005] The present invention addresses the problems existing in the above-mentioned prior art and provides a dual-input variable-structure, multi-operating-mode DC-DC converter and its control method. The converter of the present invention has high integration, simple structure, and low cost. Using dual-input power supplies, it can realize six operating conditions: DC input source U1 operating alone; DC input source U2 operating alone; DC input sources U1 and U2 operating in full parallel; DC input sources U1 and U2 operating in staggered parallel; DC input sources U1 and U2 operating in series with boosted voltage; and DC input sources U1 and U2 operating in normal series, thereby improving the stability, reliability, and flexibility of the power supply system.

[0006] 2. Technical solution

[0007] In order to achieve the above object, the technical solution provided by the present invention is:

[0008] A dual-input variable-structure multi-operating-mode DC-DC converter of the present invention includes DC input sources U1 and U2, power switches S1, S2, and S3, diodes D1 and D0, boost inductors L1 and L2, a filter capacitor C, and a load resistor R; wherein:

[0009] The negative electrode of the DC input source U1 is connected to one end of the inductor L1, and the other end of the inductor L1 is connected to terminal 2 of the power switch tube S2 and terminal 1 of the power switch tube S3 respectively; the positive electrode of the DC input source U1 is connected to the anode of the diode D1, and the cathode of the diode D1 is connected to terminal 1 of the power switch tube S1 and the anode of the diode D0 respectively, and the cathode of the diode D0 is connected to the positive electrode of the filter capacitor C and one end of the load resistor R respectively;

[0010] The positive electrode of the DC input source U2 is connected to one end of the inductor L2, and the other end of the inductor L2 is respectively connected to terminal 2 of the power switch tube S1 and terminal 1 of the power switch tube S2; the negative electrode of the DC input source U2 is respectively connected to terminal 2 of the power switch tube S3, the negative electrode of the filter capacitor, and the other end of the load resistor R.

[0011] Furthermore, two ends of the power switch tubes S1, S2, and S3 are connected with anti-parallel diodes.

[0012] Furthermore, the amplitudes and characteristics of the input sources U1 and U2 may be the same or different.

[0013] Furthermore, the three power switches S1 , S2 , and S3 can be driven simultaneously or at a certain angle.

[0014] Furthermore, the two DC input sources U1 and U2 can work independently or simultaneously. Simultaneous operation includes the following situations: staggered parallel operation, full parallel operation, series boost operation and ordinary series operation.

[0015] Furthermore, the driving signals for the six working conditions are as follows:

[0016] The DC input source U1 works alone, the switch tube S3 is always off, and the switches S1 and S2 are turned on and off at the same time;

[0017] The DC input source U2 works alone, the switch tube S1 is always off, and the switches S2 and S3 are turned on and off at the same time;

[0018] DC input sources U1 and U2 operate in full parallel. When the voltage values of U1 and U2 are different, and t0, t1, and t2 are different times within a working cycle, switches S1, S2, and S3 are simultaneously turned on at t0, switch S1 is turned off at t1, and switches S2 and S3 are simultaneously turned off at t2. When the voltage values of U1 and U2 are the same, switches S1, S2, and S3 are simultaneously turned on and off.

[0019] The DC input sources U1 and U2 are staggered and operated in parallel, that is, the switches S1 and S3 are driven 180 degrees apart, and the switch S2 is always turned on;

[0020] The DC input sources U1 and U2 are connected in series to boost the voltage, the switch tube S2 is always on, and the switches S1 and S3 are turned on and off at the same time;

[0021] The DC input sources U1 and U2 operate in series normally, the switch tubes S1 and S3 are always turned off at the same time, and the series operation is achieved by controlling the switch tube S2.

[0022] A control method for a dual-input variable-structure multi-operating-mode DC-DC converter according to the present invention is characterized in that the power switches S1, S2, and S3 in six operating conditions are controlled as follows:

[0023] The DC input source U1 works alone, controlling the switch tube S3 to be always off, and the switches S1 and S2 are turned on and off at the same time;

[0024] The DC input source U2 works alone, controlling the switch tube S1 to be always off, while the switches S2 and S3 are turned on and off at the same time;

[0025] The DC input sources U1 and U2 are fully connected in parallel. When the voltage values of U1 and U2 are different, t0, t1, and t2 are different times within a working cycle. The control switches S1, S2, and S3 are turned on at the same time at t0, the switch S1 is turned off at t1, and the switches S2 and S3 are turned off at the same time at t2. When the voltage values of U1 and U2 are the same, the switches S1, S2, and S3 are turned on and off at the same time.

[0026] The DC input sources U1 and U2 are staggered and operated in parallel, controlling the driving of the switch tubes S1 and S3 with a phase difference of 180 degrees, and the switch tube S2 is always kept on;

[0027] The DC input sources U1 and U2 are connected in series to boost the voltage, controlling the switch tube S2 to be always on, while the switches S1 and S3 are turned on and off at the same time;

[0028] The DC input sources U1 and U2 operate in series normally, and the switch tubes S1 and S3 are controlled to be turned off at the same time, and the series operation is achieved by controlling the switch tube S2.

[0029] 3. Beneficial effects

[0030] Compared with the existing known technologies, the technical solution provided by the present invention has the following significant effects:

[0031] (1) The present invention provides a dual-input variable-structure, multi-operating-mode DC-DC converter that allows for two energy inputs, with the input sources being either the same or different in amplitude and characteristics. The two input sources can supply power to the load simultaneously or separately, and the input voltage range is wide, thereby improving the stability, reliability, and flexibility of the system. This converter achieves comprehensive energy utilization and has the advantages of simple structure, compact size, and low cost.

[0032] (2) The dual-input variable structure multi-operating condition DC-DC converter of the present invention has six different operating conditions. By utilizing different combinations of variable structure switch groups, when one DC input source fails to operate normally, the converter can still operate normally. When two DC input sources operate normally, by utilizing different combinations of variable structure switch groups, the converter can operate in full parallel condition, staggered parallel condition, series boost condition, and ordinary series condition, thereby achieving the purpose of reducing output voltage ripple and improving converter gain. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 2 is a schematic diagram of the circuit structure of embodiment 1 of the present invention.

[0034] Figure 2 Schematic diagram of driving waveforms of each power switch tube in embodiment 1 of the present invention.

[0035] Figure 3 (a) and (b) are modal schematic diagrams of the first working condition of the present invention.

[0036] Figure 4 (a) and (b) are modal schematic diagrams of the second working condition of the present invention.

[0037] Figure 5 (a), (b) and (c) are modal schematic diagrams of the third working condition of the present invention.

[0038] Figure 6 (a), (b), (c) and (d) are modal schematic diagrams of the fourth working condition of the present invention.

[0039] Figure 7 (a) and (b) are modal schematic diagrams of working condition 5 of the present invention.

[0040] Figure 8 (a) and (b) are modal schematic diagrams of working condition six of the present invention.

[0041] Figure 9(a) and (b) are waveform diagrams of steady-state simulation experiments when one input source works alone.

[0042] Figure 10 (a) and (b) are waveform diagrams of the steady-state simulation experiment when two input sources work in parallel.

[0043] Figure 11 (a) and (b) are waveform diagrams of the steady-state simulation experiment when two input sources work in series. DETAILED DESCRIPTION

[0044] In order to further understand the content of the present invention, the present invention is described in detail with reference to the accompanying drawings and embodiments. It will be understood that the specific embodiments described herein are only used to explain the relevant inventions, rather than to limit the invention. It should also be noted that, for ease of description, only the parts related to the invention are shown in the accompanying drawings. The first, second, etc. terms described in the present invention are provided for the convenience of describing the technical solution of the present invention and do not have a specific limiting effect. They are all general terms and do not constitute a limiting effect on the technical solution of the present invention. It should be noted that, in the absence of conflict, the embodiments in this application and the features in the embodiments can be combined with each other.

[0045] Power switches S1, S2, and S3 can use IGBTs or other power switches such as MOSFETs. When using IGBTs, terminals 1, 2, and 3 of power switches S1, S2, and S3 correspond to the collector, emitter, and base of power switches S1, S2, and S3, respectively. When using MOSFETs, terminals 1, 2, and 3 of power switches S1, S2, and S3 correspond to the drain, source, and gate of power switches S1, S2, and S3, respectively.

[0046] Example 1

[0047] A dual-input variable-structure multi-operating-mode DC-DC converter of the present invention includes DC input sources U1 and U2, power switches S1, S2 and S3, diodes D1 and D0, boost inductors L1 and L2, filter capacitor C and load resistor R;

[0048] The negative electrode of the DC input source U1 is connected to one end of the inductor L1, and the other end of the inductor L1 is connected to terminal 2 of the power switch tube S2 and terminal 1 of the power switch tube S3 respectively. The positive electrode of the DC input source U1 is connected to the anode of the diode D1, and the cathode of the diode D1 is connected to terminal 1 of the power switch tube S1 and the anode of the diode D0 respectively. The cathode of the diode D0 is connected to the positive electrode of the filter capacitor C and one end of the load resistor R respectively.

[0049] The positive electrode of the DC input source U2 is connected to one end of the inductor L2, and the other end of the inductor L2 is respectively connected to terminal 2 of the power switch tube S1 and terminal 1 of the power switch tube S2. The negative electrode of the DC input source U2 is respectively connected to terminal 2 of the power switch tube S3, the negative electrode of the filter capacitor C, and the other end of the load resistor R.

[0050] Among them, two ends of the power switch tubes S1, S2, and S3 are connected with anti-parallel diodes.

[0051] Example 2

[0052] In this embodiment, the six working conditions of the switch tube drive signal waveforms formed by using different combinations of variable structure switch groups are shown in the following figure: Figure 2 As shown, D1 is the duty cycle of the drive signal of the power switch tube S1, D2 is the duty cycle of the drive signal of the power switch tube S2, and D3 is the duty cycle of the drive signal of the power switch tube S3. The drive signals of the six working conditions are as follows: Figure 2 As shown:

[0053] 1. The DC input source U1 works alone, the switch tube S3 is always off, and the switch tubes S1 and S2 are turned on and off at the same time.

[0054] 2. The DC input source U2 works alone, the switch tube S1 is always turned off, and the switch tubes S2 and S3 are turned on and off at the same time.

[0055] 3. The DC input sources U1 and U2 operate in parallel. This embodiment analyzes a relatively complex situation where the two voltage values are different. The switches S1, S2, and S3 are turned on simultaneously at time t0, the switch S1 is turned off at t1, and the switches S2 and S3 are turned off simultaneously at time t2.

[0056] 4. The DC input sources U1 and U2 are staggered and operated in parallel, that is, the switching tubes S1 and S3 are driven 180° apart, and the switching tube S2 is always kept on.

[0057] 5. The DC input sources U1 and U2 are connected in series to boost the voltage, the switch tube S2 is always on, and the switch tubes S1 and S3 are turned on and off at the same time.

[0058] 6. The DC input sources U1 and U2 operate in series normally, and the switch tubes S1 and S3 are always turned off at the same time. The series operation is achieved by controlling the switch tube S2.

[0059] Example 3

[0060] In this embodiment, a dual-input variable structure multi-operating state DC-DC converter utilizes different combinations of variable structure switch groups. The proposed converter has six different operating states, each of which includes different operating modes. Figure 3 , the working principle of the converter is analyzed in detail.

[0061] First, the working mode of working condition 1 is analyzed in detail.

[0062] Mode 1

[0063] like Figure 3 As shown in (a), the switch tube S3 is always turned off. When the power switch tubes S1 and S2 are turned on at the same time, the input power supply U1 charges the inductor L1 through the power switch tubes S1, S2 and diode D1.

[0064] Mode 2

[0065] like Figure 3 As shown in (b), the switch tube S3 is always turned off. When the power switch tubes S1 and S2 are turned off at the same time, the input power supply U1 and the inductor L1 are discharged at the same time, and energy is provided to the capacitor C and the load through the anti-parallel diode of the power switch tube S3, diodes D1 and D0.

[0066] Secondly, the working mode of working condition 2 is analyzed.

[0067] Mode 1

[0068] like Figure 4 As shown in (a), the switch tube S1 is always turned off. When the power switch tubes S2 and S3 are turned on at the same time, the input power supply U2 charges the inductor L2 through the power switch tubes S2 and S3.

[0069] Mode 2

[0070] like Figure 4 As shown in (b), when the switch tube S1 is always turned off and the power switch tubes S2 and S3 are turned off at the same time, the input power supply U2 and the inductor L2 discharge at the same time, and energy is provided to the capacitor C and the load through the anti-parallel diode of the power switch tube S1 and the diode D0.

[0071] Then the working mode of working condition three is analyzed.

[0072] Mode 1

[0073] like Figure 5 As shown in (a), when the power switches S1, S2, and S3 are turned on at the same time, the input power supply U1 charges the inductor L1 through the power switches S1, S2, and diode D1. The input power supply U2 charges the inductor L2 through the power switches S2 and S3. The capacitor C discharges to provide energy to the load.

[0074] Mode 2

[0075] like Figure 5As shown in (b), when the power switch tube S1 is turned off and the power switch tubes S2 and S3 are turned on at the same time, the input power supply U1 and the inductor L1 are discharged at the same time, and energy is provided to the capacitor and the load through the anti-parallel diode of the power switch tube S3, diodes D1 and D0; the input power supply U2 charges the inductor L2 through the power switch tubes S2 and S3.

[0076] Mode Three

[0077] like Figure 5 As shown in (c), when the power switches S1, S2, and S3 are turned off at the same time, the input power supply U1 and inductor L1 are discharged at the same time, and energy is provided to the capacitor and the load through the anti-parallel diode of the power switch S3, diodes D1, and D0; the input power supply U2 and inductor L2 are discharged at the same time, and energy is provided to the capacitor C and the load through the anti-parallel diode of the power switch S1 and diode D0.

[0078] Next, the working mode of working condition four is analyzed.

[0079] Mode 1

[0080] like Figure 6 As shown in (a), when the power switches S1, S2, and S3 are turned on at the same time, the input power supply U1 charges the inductor L1 through the power switches S1, S2, and diode D1, and the input power supply U2 charges the inductor L2 through the power switches S2 and S3.

[0081] Mode 2

[0082] like Figure 6 As shown in (b), when the switch tube S3 is turned off and the power switch tubes S1 and S2 are turned on, the input power supply U1 charges the inductor L1 through the power switch tubes S1, S2, and diode D1. The input power supply U2 and inductor L2 discharge at the same time, and energy is provided to the capacitor C and the load through the anti-parallel diode of the power switch tube S1 and diode D0.

[0083] Mode Three

[0084] like Figure 6 As shown in (c), when the power switches S1, S2, and S3 are turned on at the same time, the input power supply U1 charges the inductor L1 through the power switches S1, S2, and diode D1, and the input power supply U2 charges the inductor L2 through the power switches S2 and S3.

[0085] Mode Four

[0086] like Figure 6As shown in (d), when the power switch tube S1 is turned off and the power switches S2 and S3 are turned on, the input power supply U1 and the inductor L1 are discharged at the same time, and energy is provided to the capacitor and the load through the anti-parallel diode of the power switch tube S3, diodes D1 and D0; the input power supply U2 charges the inductor L2 through the power switches S2 and S3.

[0087] Then the working mode of working condition five is analyzed.

[0088] Mode 1

[0089] like Figure 7 As shown in (a), the power switch tube S2 is always on. When the power switch tubes S1 and S3 are turned on at the same time, the input power supply U1 charges the inductor L1 through the power switch tubes S1, S2 and diode D1, and the input power supply U2 charges the inductor L2 through the power switch tubes S2 and S3.

[0090] Mode 2

[0091] like Figure 7 As shown in (b), when the power switch tube S2 is turned on and S1 and S3 are turned off at the same time, the input power supplies U1 and U2, the inductors L1 and L2 discharge at the same time, and power is supplied to the capacitor and the load through the power switch tube S2 and the diodes D1 and D0.

[0092] Finally, the working mode of working condition six is analyzed.

[0093] Mode 1

[0094] like Figure 8 As shown in (a), the power switches S1 and S3 are always off. When the power switch S2 is on, the input power sources U1 and U2 supply power to the capacitor and the load through the power switch S2 and the diodes D1 and D0.

[0095] Mode 2

[0096] like Figure 8 As shown in (b), the power switches S1, S2, and S3 are always off, and the capacitor supplies power to the load.

[0097] To simplify the converter analysis, the following assumptions are made:

[0098] (1) All semiconductor power devices are ideal devices, and diode D0 is an anti-reverse diode.

[0099] (2) The capacitor C is large enough to keep its voltage constant during one switching cycle.

[0100] (3) The currents of inductors L1 and L2 operate in continuous conduction (CCM) mode.

[0101] Applying the volt-second balance principle of inductors to L1 and L2 respectively, the input and output voltage relationships under various working conditions can be obtained:

[0102] Working condition 1:

[0103] Working condition 2:

[0104] Working condition three:

[0105] Working condition four:

[0106] Working condition five:

[0107] Working condition six: U0=U1+U2⑹

[0108] Example 4

[0109] Figure 9 This is the waveform diagram of the steady-state simulation experiment when one input source of the present invention works alone. The simulation waveform when the power supply U1 works alone is as follows Figure 9 (a) is the simulation waveform when U1=100V, U2=0V, D1=D2=0.75, D3=0. It can be seen from the figure that when U0=400V, the current of inductor L2 is zero, and the current of inductor L1 is greater than zero. The simulation waveform when power supply U2 works alone is as follows Figure 9 (b) is the simulation waveform when U1 = 0V, U2 = 100V, D1 = 0, D2 = D3 = 0.75. It can be seen from the figure that when U0 = 400V, the current of inductor L1 is zero, and the current of inductor L2 is greater than zero. They satisfy formulas (1) and (2) respectively.

[0110] Figure 10 This is the waveform diagram of the steady-state simulation experiment when the two input sources of the present invention work in parallel. The simulation waveform when the two power sources work in full parallel is as follows Figure 10 (a) is the simulation waveform when U1=100V, U2=80V, D1=0.75, D2=0.8, D3=0.8. It can be seen from the figure that when U0=400V, the current of inductors L1 and L2 are both greater than zero. The simulation waveform when the two power supplies are staggered in parallel is as follows: Figure 10 (b) is the simulation waveform when U1 = 100V, U2 = 80V, D1 = 0.75, D2 = 1, and D3 = 0.8. It can be seen from the figure that when U0 = 400V, the currents of inductors L1 and L2 are both greater than zero, satisfying formulas (3) and (4) respectively.

[0111] Figure 11 This is the waveform diagram of the steady-state simulation experiment when the two input sources of the present invention are connected in series. Figure 11 (a) is the simulation waveform when U1=100V, U2=100V, D1=0.75, D2=1, D3=0.75. It can be seen from the figure that when U0=800V, the current of inductors L1 and L2 are both greater than zero. The simulation waveform when the two power supplies are normally connected in series is as follows: Figure 11 (b) is the simulation waveform when U1 = 100 V, U2 = 100 V, D1 = 0, D2 = 1, D3 = 0. It can be seen from the figure that U0 = 200 V. Formulas (5) and (6) are satisfied respectively.

[0112] The simulation results show that regardless of whether the input voltage of the converter proposed in this patent is equal, the six working conditions proposed can meet the corresponding theoretical analysis, thus verifying its feasibility.

[0113] The converter of the present invention has high integration, simple structure and low cost. It adopts dual input power supplies and can realize six working conditions: DC input source U1 working alone, DC input source U2 working alone, DC input sources U1 and U2 working in full parallel, DC input sources U1 and U2 working in staggered parallel, DC input sources U1 and U2 working in series with boosting, and DC input sources U1 and U2 working in ordinary series, thereby improving the stability, reliability and flexibility of the power supply system.

[0114] The above is a schematic description of the present invention and its embodiments, which is not restrictive. The drawings show only one embodiment of the present invention, and the actual structure is not limited thereto. Therefore, if a person skilled in the art is inspired by this and, without departing from the purpose of the present invention, designs a structure and embodiment similar to this technical solution without inventiveness, they shall fall within the scope of protection of the present invention.

Claims

1. A dual-input variable-structure multi-operating-mode DC-DC converter, characterized by: It includes DC input sources U1 and U2, power switches S1, S2, and S3, diodes D1 and D0, boost inductors L1 and L2, filter capacitor C, and load resistor R. The negative electrode of the DC input source U1 is connected to one end of the inductor L1, and the other end of the inductor L1 is connected to terminal 2 of the power switch tube S2 and terminal 1 of the power switch tube S3 respectively; the positive electrode of the DC input source U1 is connected to the anode of the diode D1, and the cathode of the diode D1 is connected to terminal 1 of the power switch tube S1 and the anode of the diode D0 respectively, and the cathode of the diode D0 is connected to the positive electrode of the filter capacitor C and one end of the load resistor R respectively; The positive electrode of the DC input source U2 is connected to one end of the inductor L2, and the other end of the inductor L2 is connected to terminal 2 of the power switch tube S1 and terminal 1 of the power switch tube S2 respectively; the negative electrode of the DC input source U2 is connected to terminal 2 of the power switch tube S3, the negative electrode of the filter capacitor, and the other end of the load resistor R respectively; The power switch tubes S1, S2, and S3 can use IGBT switch tubes or MOSFET switch tubes; when using IGBTs, the 1st, 2nd, and 3rd terminals of the power switch tubes S1, S2, and S3 respectively correspond to the collector, emitter, and base of the power switch tubes S1, S2, and S3; when using MOSFETs, the 1st, 2nd, and 3rd terminals of the power switch tubes S1, S2, and S3 respectively correspond to the drain, source, and gate of the power switch tubes S1, S2, and S3.

2. The dual-input variable-structure multi-operating-mode DC-DC converter according to claim 1, characterized in that: Anti-parallel diodes are connected at both ends of the power switch tubes S1, S2, and S3.

3. The dual-input variable-structure multi-operating-mode DC-DC converter according to claim 2, characterized in that: The amplitudes and characteristics of the input sources U1 and U2 may be the same or different.

4. The dual-input variable-structure multi-operating-mode DC-DC converter according to claim 3, characterized in that: The three power switch tubes S1, S2, and S3 can be driven simultaneously or at a certain angle.

5. The dual-input variable-structure multi-operating-mode DC-DC converter according to claim 4, characterized in that: The two DC input sources U1 and U2 can work individually or simultaneously. The simultaneous working includes the following situations: staggered parallel operation, full parallel operation, series boost operation and ordinary series operation.

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