https://doi.org/10.6113/JPE.2018.18.4.1015
ISSN(Print): 1598-2092 / ISSN(Online): 2093-4718
Family of Dual-Input Dual-Buck Inverters Based on Dual-Input Switching Cells
Fan Yang*, Hongjuan Ge*,**, Jingfan Yang*, Runyun Dang*, and Hongfei Wu
†
†,*College of Automation Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing, China
**College of Civil Aviation, Nanjing University of Aeronautics and Astronautics, Nanjing, China
Abstract
A family of dual-DC-input (DI) dual-buck inverters (DBIs) is proposed by employing a DI switching cell as the input of traditional DBIs. Three power ports, i.e. a low voltage DC input port, a high voltage DC input port and an AC output port, are provided by the proposed DI-DBIs. A low voltage DC source, whose voltage is lower than the peak amplitude of the AC side voltage, can be directly connected to the DI-DBI. This supplies power to the AC side in single-stage power conversion. When compared with traditional DBI-based two-stage DC/AC power systems, the conversion stages are reduced, and the power rating and power losses of the front-end Boost converter of the DI-DBI are reduced. In addition, five voltage-levels are generated with the help of the two DC input ports, which is a benefit in terms of reducing the voltage stresses and switching losses of switches. The topology derivation method, operation principles, modulation strategy and characteristics of the proposed inverter are analyzed in-depth. Experimental results are provided to verify the effectiveness and feasibility of the proposed DI-DBIs.
Key words: Circuit topology, Dual-Buck, Dual-input, High efficiency, Inverter
Manuscript received Feb. 15, 2017; accepted Mar. 1, 2018
Recommended for publication by Associate Editor Sangshin Kwak.
†Corresponding Author: wuhongfei@nuaa.edu.cn Tel: +86-25-84896005, Fax: +86-25-84896005, Nanjing University of Aeronautics and Astronautics
*College of Automation Engineering, Nanjing University of Aeronautics and Astronautics, China
**College of Civil Aviation, Nanjing University of Aeronautics and Astronautics, China
Ⅰ. INTRODUCTION
Inverters are key elements for the interface an AC load or grid and various DC sources. They have been widely used in renewable energy power systems, micro-grids, uninterruptable power supplies, air-plane power systems and so on [1], [2]. Despite their long history, inverter topologies are still evolving. The search for new inverter topologies with a higher efficiency and a high reliability has never stopped in an effort to meet the requirements of various new applications. As a result, new topological variations and innovations have been continuously emerging [3], [4].
Generally speaking, inverter topologies can be classified into three types: Buck-type, Boost-type and Buck-Boost type. Among them, the buck type inverters, e.g. full-bridge inverters, are more popular due to their simplicity and high efficiency. However, full-bridge inverters, whose switching legs are composed of two active switches in an in-series connection, have the potential for a dangerous short-through [5]. The reverse recovery problem of the body-diodes of active switches is another threat to the reliability and efficiency of full-bridge inverters [6]. The reverse-recovery problem can be avoided by adopting SiC diodes. However, the forward-voltage of a SiC diode is much higher than that of a Si diode or the body-diode of a Si switch. Therefore, it is very difficult to make the current commutate to a SiC diode by paralleling a SiC diode with a Si switch. In addition, the short-through problem cannot be solved when SiC diodes are used in full-bridge inverters. In order to overcome these drawbacks, dual-buck inverters (DBIs) were proposed by splitting the switching-leg in the full-bridge inverter into two buck-type switching-legs [7]-[11]. Since the switching-leg of a DBI is composed of a series-connected active switch and diode. The problems of short-through and reverse-recovery of the body diodes can be solved perfectly. Therefore, DBIs have attracted a lot of attention for use in high reliability and high efficiency applications, such as airplane power systems and the grid-tied inverters for renewable power systems [12]-[14].
Since the DBI is a buck type converter, another Boost converter has to be used as the front-end DC-DC converter if the voltage of the DC input is lower than the peak amplitude of the AC output voltage. As a result, all of the input power has to be processed by both the front-end Boost converter and the downstream DBI, which hurts the overall efficiency of the DC-AC power system. Although a lot of efforts have been made to improve the efficiency of DBI topologies [15], [16], the power rating and power losses of the front-end Boost converter cannot be reduced. When the DC input voltage is lower than the peak amplitude of the AC output voltage, a Boost type or buck-boost type inverter is more promising in terms of reducing the conversion stages [17]-[20]. However, even though single-stage power conversion can be achieved with a Boost or Buck-Boost type inverter, the conversion efficiency of the Boost or Buck-Boost type inverter is usually lower than that of a buck-type inverter due to the high voltage/current stresses of the devices as well as their complicated topologies and controls.
It is noted that the AC side voltage of an inverter periodically varies between zero and its peak value, which means that the instantaneous value of the AC voltage is not always higher than the voltage of the low voltage DC energy source. This suggests that a low voltage DC input source can directly supply power to an inverter in single-stage power conversion if a low voltage DC input port can be introduced to the inverter. The major contribution of this paper is to propose a new family of dual-DC-input dual-buck inverters (DI-DBIs) to improve the overall conversion efficiency of DC-AC power systems. The structures of the proposed DI-DBI-based and traditional DBI-based DC-AC power conversion systems are shown in Fig. 1. It is important to note that the proposed inverter is referred to as a dual-input dual-buck inverter because both VL and VH are DC inputs for the inverter stage. However, in terms of the entire DC-AC power system, the VH port is an intermediate DC-bus, not a real DC source, and that only the VL port is a DC source. In practice, like the intermediate DC-bus in a conventional two-stage DC-AC power system, the VH port for the DI-DBI is provided by a front-end Boost converter. The VL and VH ports are the input and output of the Boost converter, respectively. With the proposed DI-DBIs, the power conversion stage is reduced and the efficiency of the DC-AC power system is improved. In addition, the power stress of the front-end Boost is significantly reduced. Therefore, the power losses, power rating and cost of the Boost converter are all reduced as well. Moreover, the multi-level characteristic is realized with the proposed DI-DBIs, which is beneficial for the reduction of switching loss.
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Ⅱ. DERIVATION OF DI-DBI TOPOLOGIES
The proposed DI-DBI is derived by cascading a dual-input switching cell and a traditional DIB topology. The general structure of the proposed DI-DBI is shown in Fig. 2, where it can be seen that the dual-input switching cell is used as the input of the DBI. Therefore, the dual-input switching cell is the key for the generation of DI-DBI topologies.
Fig. 2. Structure of the proposed DI-DBI.
Four types of dual-input switching cells are illustrated in Fig. 3. It can be seen that the dual-input switching cell is composed of a high voltage DC source VH, a low voltage DC source VL, a active switch SH and a diode DL or active switch SL. The two voltage sources can share a common ground, as shown in Fig. 3(a) and Fig. 3(b), or can be connected in series, as shown in Fig. 3(c) and Fig. 3(d). For the switching cells in Fig. 3(a) and Fig. 3(b), the voltage of VH must be greater than VL. Meanwhile, there are no limitations on the voltages of VH and VL for the switching cells in Fig. 3(c) and Fig. 3(d). In Fig. 3(a) and Fig. 3(c), the diode DL is in-series with VL, which means that VL can only output power to the downstream DBI. In Fig. 3(b) and Fig. 3(d), an active switch SL is used to connect VL. Therefore, the power flows of both VL and VH can be bidirectional.
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The operation principles of the four dual-input switching cells are all similar. Take the dual-input switching cell in Fig. 3(a) as an example. Since the voltage VH is greater than VL, the diode DL is reverse-biased when SH is ON. Therefore, once SH is ON, the high voltage DC source VH supplies power to the downstream DBI. When SH is OFF, DL conducts so that VL supplies power to the inverter. Obviously, by regulating the ON/OFF states of SH and DL, the two DC inputs can alternatively supply power to the inverter.
The topologies of the proposed DI-DBI can be derived by cascading the dual-input switching cell in Fig. 3 and a traditional single-input DBI. Following this principle, a family of DI-DBI topologies, which can simultaneously interface a low voltage DC source and a high voltage DC source, can be derived. Three examples of the proposed DI-DBIs are shown in Fig. 4. These topologies are obtained based on the dual-input switching cell shown in Fig. 3(a). The three single-input DBIs used in Fig. 4(a), Fig. 4(b) and Fig. 4(c) can be found in [7], [8] and [9], respectively. It should be noted that the DI-DBIs are not limited to the ones shown in Fig. 4. The topology derivation method is also effective for the three dual-input cells shown in Fig. 3(b)-(d). In addition, the single-input DBI used to construct a DI-DBI can be another.
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In practical applications, the low voltage DC source VL can be made up of photovoltaic arrays, fuel cells, batteries, low- voltage DC buses or other low-voltage DC sources. The high voltage DC source VH can be the output of a Boost converter, whose input is a low voltage DC source VL. When the low voltage source VL fully meets the needs of the output voltage vo, all of the output power can be provided by VL. When the low voltage source VL cannot meet the requirements of the output voltage vo, the output power is provided by VL and VH alternatively. Since part of the input power is directly fed to the inverter from VL, the power losses and power rating of the Boost converter are significantly reduced, which is helpful for the improvement of the overall efficiency.
Ⅲ. OPERATION PRINCIPLE, MODULATION STRATEGY AND CHARACTERISTICS
The operation principles and characteristics of the proposed DI-DBIs are similar to each other. The DI-DBI shown in Fig. 4(a) is taken as an example for analysis to explain the operation principles and characteristics.
A. Operation Principle
As mentioned above, the dual-input switching cell in a DI-DBI is used as the input stage of a traditional dual-buck inverter. However, the DI-DBI is not a simple combination of two circuits. After the dual-input switching cell and a dual- buck inverter are connected as an integrated inverter topology, both the switches in the dual-input switching cell and the switches in the original dual buck inverter can be used to regulate the output voltage/current/power of the AC side. Since the dual-input switching cell is the input stage of the converter, the switch SH and the diode DL work regardless of whether the converter operates in the positive half-cycle or negative half-cycle of the AC output voltage. On the other hand, the two buck converters in the original dual-buck inverter operate alternatively in the positive and negative half-cycles of the AC output voltage. More specifically, for the DI-DBI shown in Fig. 4(a), the switch S3 is kept ON, S1 and D1 are operated at a high frequency, while S2, D2 and S4 are kept in the OFF state if the DI-DBI operates in the positive half-cycle of the output voltage vo. On the other hand, S1, D1 and S3 are always OFF and S2, D2 and S4 are modulated in the negative half-cycle of the output voltage vo.
Depending on the switching states of these switches, there are a total of six switching states for the DI-DBI shown in Fig. 4(a). The equivalent circuits of these switching states are shown in Fig. 5.
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State I [Fig. 5(a)]: The switches SH, S1 and S3 conduct, the other switches and diodes are off. The voltage stress of the diode DL is clamped by the voltage difference (VH﹣VL). The high voltage source VH supplies energy to the AC side. The voltage vAB between the mid-points of the two switching legs is clamped by the high side voltage VH, i.e. vAB=VH. The output voltage vo is in its positive half cycle with vo>0.
State II [Fig. 5(b)]: The switches S1 and S3 and the diode DL are ON, while the other switches and diodes are OFF. The voltage stress of the switch SH is clamped by voltage difference (VH﹣VL). The low voltage source VL is used as the input of the dual-buck inverter and supplies energy to the load. vAB=+VL and vo>0.
State III [Fig. 5(c)]: The switch S3 and the diode D1 are ON, and the other switches and diodes are OFF. The current of the inductor L1 freewheels through the switch S3 and the diode D1. The voltage stress of the switch S1 is VL. Neither the high voltage source VH nor the low voltage source VL supply energy to the load. vAB=0 and vo>0.
State IV [Fig. 5(d)]: The switches SH, S2 and S4 are ON, while the other switches and diodes are OFF. The voltage stress of the diode DL is clamped by the voltage difference (VH﹣VL). The high voltage source VH supplies energy to the load. vAB=﹣VH and vo<0.
State V [Fig. 5(e)]: The switches S2 and S4 and the diode DL are ON, and the other switches and diodes are OFF. The voltage stress of the switch SH is clamped by the voltage difference (VH﹣VL). The low voltage source VL supplies energy to the load. vAB=﹣VL and vo<0.
State VI [Fig. 5(f)]: The switch S4 and the diode D2 are ON, while the other switches and diodes are OFF. The current of the inductor L2 freewheels through the switch S4 and the diode D2. The voltage stress of the switch S1 is VL. Neither the high voltage source VH nor the low voltage source VL supply energy to the load. vAB=0 and vo<0.
According to the six switching states, the relationship of the switching state, the energy source and the mid-points voltage vAB can be summarized and listed in Table I, where 1 and 0 show when the corresponding switch is in the ON and OFF state, respectively.
From Table I, it can be seen that five voltage levels, i.e. +VH, +VL, 0, ﹣VL and﹣VH, are derived from the mid-points of the switching bridge. When the mid-points voltage vAB is +VH or﹣VH, the high voltage source VH supplies energy to the load. When the mid-points voltage vAB is +VL or﹣VL, the low voltage source VL supplies energy to the load. Neither VH nor VL supply energy to the load when vAB is 0. The switches S3 and S4 conduct in the positive and negative half-cycles of the output voltage vo, respectively. As a result, the two switches work at a low frequency. With the proper control method, the mid-points voltage vAB is able to switch between two adjacent voltage levels, so that there is only one switch working at a high- frequency at any given time.
B. Modulation Strategy
The above analysis indicates that the low voltage source VL can directly supply energy to the load. It also shows that five voltage levels can be obtained from the mid-points of the switching bridge. The multi-level characteristic is helpful to reduce the voltage stress of the switches and the volume/loss of the output filter. It should be noted that unlike traditional multi-level inverters, which generate balanced multiple voltage- levels, the five voltage-levels generated by the DI-DBI are asymmetrical. The low voltage level is not fixed and is determined directly by the low-voltage DC source VL.
In practical applications, the low voltage source VL of a DI-DBI can be the output of a renewable energy source, a low voltage battery, a DC voltage bus and so on. In other words, the voltage of VL is directly determined by the DC energy source and can be lower than the peak amplitude of the AC output voltage. However, the high voltage source VH has to be designed to satisfy the modulation index and the peak voltage of the AC output voltage vo. The voltage VH can be generated by the VL through the front-end Boost converter. In order to reduce the power conversion stages and the power losses of the Boost converter, it is necessary to design a modulation strategy to maximize the input power from the VL port of the DI-DBI.
Due to the symmetrical configuration of the circuit, the operation of the negative half-cycle is similar to that of the positive half-cycle. Therefore, the positive half-cycle of vo is taken as an example to explain the modulation strategy and characteristics of the converter. According to the relationship between VL and the instantaneous value of the output voltage vo, the inverter has two operation modes in the positive half cycle of vo. As shown in Fig. 6, the inverter works in mode I when the low voltage source VL is greater than the instantaneous value of the output voltage vo. In addition, it can be seen that the DI-DBI works in mode II when the low voltage source VL is lower than the instantaneous value of the output voltage vo.
Fig. 6. Operation mode.
Mode I: VL≥vo
In this mode, the low voltage source VL can fully meet the needs of the output voltage vo. Therefore, the output power of the inverter can be provided directly from the low voltage source VL. In this mode, the mid-point voltage vAB only needs to be switched between the low voltage level VL and zero. In other words, the inverter only needs to switch between state II and state III.
Mode II: VL≤vo
In this mode, the low voltage source VL cannot meet the needs of the output voltage vo. There are two possible modulation strategies. In the first strategy, all of the output power is provided by the high voltage source VH. The mid- point voltage vAB is switched between the high voltage level VH and zero. In other words, the inverter only needs to switch between state I and state III. In the second strategy, the output power is provided by both the high voltage source VH and the low voltage source VL. The mid-points voltage vAB is switched between the high voltage level VH and the low voltage level VL. In other words, the inverter switches between state I and state II. Obviously, the second modulation strategy increases the input power of the VL port and reduces the switching loss of the high-frequency switches. Therefore, the second modulation strategy is preferred.
A digital controller is used in this paper. Since there is only one carrier in a digital signal processor, two modulation waves are used to generate the driving signals. The modulation strategies of the DI-DBI are illustrated in Fig. 7, where Fig. 7(a) is a control block diagram for the implementation of the modulation strategy, while Fig. 7(b) shows key waveforms. In Fig. 7, voref is the reference of the output voltage, vos is the sampled value of the output voltage vo, vPL is the output of the voltage regulator, vNL is obtained from the reverse of vPL, ΔVC is the peak-to-peak value of the carrier vc, vPH and vNH are obtained by subtracting ΔVC from vNL and vPL, respectively, vgSH and vgS1~vgS4 are the driving signals of the switches SH and S1~S4, respectively, and vAB is the voltage between the mid-points of the two switching bridges.
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The driving signals vgS1 and vgS2 are obtained by comparing vPL and vNL with the carrier vc. Since the switch SH is shared by positive and negative Buck converters, the drive signal vgSH is generated by comparing vPH and vNH together with the carrier vc. In the positive half cycle, when vPL is smaller than ΔVC, vPH is a negative value and the DI-DBI works in Mode I. In this mode, S1 switches at a high frequency, while SH is kept off. When vPL is greater than ΔVC, vPH is a positive value and the DI-DBI works in Mode II. In this mode, SH switches at a high frequency, while S1 is kept on. Like conventional PWM, the peak value of the modulation wave vPH should satisfy the modulation index and be lower than the peak value of the carrier vc:
where VPH_peak is the peak value of vPH. Therefore, the relationship between the peak value of vPL and ΔVC should satisfy:
where VPL_peak is the peak value of vPL.
C. Characteristics and Analysis
The energy directly transferred from VL to the load is called direct transmitted energy. The energy transferred to the load through the VH port is called indirect energy. The more direct energy, the higher the system efficiency. The relationship between the direct energy and a low voltage VL will be analyzed. In order to simplify the analysis, it is assumed that the output voltage vo is in phase with the output current io. Due to the symmetrical configuration of the circuit, the following only analyzes the relationship in the positive half cycle.
According the above analysis, when VL≥vo, all of the energy of the load is provided by the low voltage source VL. Therefore, the power supplied by the low voltage source VL is expressed as:
where pL is the instantaneous power supplied by the low voltage source VL, and po and Po are the instantaneous and average values of the output power, respectively.
When VL≤vo, the energy of the load is provided by both the high voltage source VH and the low voltage source VL. Therefore:
where pH and pL are the average power provided by the high voltage source VH and the low voltage source VL in every switching period. Therefore, pH and pL can be expressed as:
(5)where dSH is the duty cycle of the switch SH. In addition, according to the volt-second balance of the filter inductor L1, the duty cycle dSH can be obtained as:
Substituting equation (6) into (4) and (5) yields:
For better understanding, the direct power ratio kP of the average power provided by the low voltage source VL relative to the total output power in every period of the output voltage is calculated as follows:
The curves of the direct power ratio kP are illustrated in Fig. 8 with the conditions of 
V and VH =180V. It can be seen that kP increase with an increase of VL. This indicates that as VL increases, the power transmitted directly from VL to the load increases, which helps improve the efficiency of the system. When VL is greater than or equal to the peak amplitude of the output voltage, all of the output power is directly provide by VL without being processed by the Boost converter. That is to say, all of the power is single-stage power conversion.
Fig. 8. Direct power transfer ratio.
Therefore, from the point of view of reducing the conversion stages and power losses of the front-end Boost converter, a higher value of VL is better. However, the value of VL is determined by the demand of practical applications.
Ⅳ. EXPERIMENTAL RESULTS
Based on aeronautical applications, a 1kW prototype was built and tested based for the DI-DBI topology shown in Fig. 4 (a). As mentioned above, the advantage of the DI-DBI is its ability to improve the efficiency of a system by reducing the indirect power and increasing the direct power. To evaluate the effectiveness of this method, a traditional prototype of the DBI has also been built. A Boost converter is used as the front-end DC-DC converter for both the DI-DBI and the DBI. Schematics of the DI-DBI and DBI-based DC-AC systems are shown in Fig. 9. For the DI-DBI-based DC-AC system, the low voltage source VL and high voltage source is VH are connected to the input and output port of Boost, respectively. By regulating the Boost, VH is maintained at about 180V. For the DBI-based DC-AC system, the DBI is cascaded with the Boost directly. VL is connected to the input port of the Boost while VH is the DC bus voltage between Boost and DBI. It should be noted that the parameters, devices and PCB boards of the two DC-AC systems are exactly the same expect for the DI cell. The parameters and devices are listed in Table II.
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| Parameter |
Value |
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VH |
180V |
| VL |
(60-150)V |
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| vo |
115V/400Hz |
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| SH |
IPB320N20N3G |
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| S1-S4 |
IRFB4137PbF |
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| DL |
SBR20A200CTB |
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| D1&D2 |
SBR10A300CTB |
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| L1&L2 |
0.3mH |
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| Co |
1uF |
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| Switching frequency |
50kHz |
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| Boost |
SB |
IXFH69N30P |
| DB |
DPG30C300HB |
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| LB |
320uH |
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| Switching frequency |
100kHz |
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A. Experimental Waveforms and Analysis
Fig. 10 shows experimental waveforms of vgSH, vgS1, vgS3 and vo of the DI-DBI with VL= 60V. The positive and negative voltages of the driving signals are +15V and -9V, respectively. In the positive half cycle of vo, the switch S3 remains conductive, while the switches sH and s1 are alternately operated at a high frequency. When the switch sH is operated at a high frequency, the switch s1 remains on. When the switch s1 is operated at a high frequency, the switch sH remains off. In the negative half cycle of vo, the switches s1 and s3 are kept off while the switch sH is still operated at a high frequency. In the positive half cycle of vo, when the switch sH is in high frequency operation, VL≤vo. Meanwhile, when the switch s1 is in high frequency operation, VL≥vo. Obviously, when VL = 60V, because the voltage of VL is relatively low, the high frequency working time of the switch s1 is relatively short and the high frequency working time of the switch sH is relatively long. The experimental waveforms are consistent with the theoretical analysis. Fig. 11(a) and Fig. 11 (b) show the waveforms of iL1, iL2, vAB and vo with VL = 90V and VL = 150V, respectively, where iL1 and iL2 represent the current through the inductors L1 and L2, respectively. It can be seen that the inductors L1 and L2 operate in the positive and negative half cycles of vo, respectively. Five voltage levels are obtained from the mid-points of the switching bridge. Unlike the usual five-level characters, the low voltage level VL is variable. When VL = 90V, the five levels are symmetrical while they are asymmetrical when VL = 150V. In addition, it can be seen form Fig. 11(b) that the high frequency working time of the switch s1 is relatively long since the voltage of VL is relatively high.
Fig. 10. Waveforms of driving and output voltages with VL=60V.
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Fig. 12(a) shows dynamic waveforms of the DI-DBI with a load step-up when the output voltage of vo is regulated. It can be seen that the output voltage can be well controlled. Fig. 12(b) shows dynamic waveforms of the DI-DBI with VL increases when the output voltage of vo is regulated. It can also be seen that the output voltage can be well controlled.
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B. Comparative Analysis
To help the design tradeoff and topology selection, a comparison between DC-AC systems based on the proposed DI-DBI and the traditional DBI-based is necessary.
The efficiency and THD are tested by a power analyzer WT1806 of YOKOGAWA. Fig. 13 shows a wiring diagram for the efficiency and THD tests, where IB and UB are the input current and voltage of the Boost converter, IL and VL are the input current and voltage of the VL port of the DI-DBI, IH and VH are output current and voltage of the Boost converter, and Io and Vo are output current and voltage of the DI-DBI, respectively. In the tests, the efficiencies of the Boost, the DI-DBI and the entire DC-AC system are defined as follows:
Fig. 13. Wiring diagram for efficiency and THD tests.
Efficiency comparison results between the proposed DI-DBI and the traditional DBI are shown in Fig. 14, where Fig. 14(a) shows the tested efficiency and Fig. 14(b) shows the calculated power loss distribution of the DI-DBI. It can be seen in Fig. 14(a) that the efficiency of the DI-DBI is higher than that of the DBI. In addition, the value of VL has little impact on efficiency. It can be seen in Fig. 14(b) that although more conductive losses are caused by the addition of SH and DL, the switching loss of the proposed DI-DBI is reduced significantly thanks to its multilevel characteristics. The total losses of the proposed DI-DBI are lower than those of the traditional DBI, which shows good agreement with the tested results.
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Fig. 15 shows an efficiency comparison between DI-DBI- based and traditional DBI-based DC-AC systems, where Fig. 15(a) and Fig. 15(b) show the DC-AC system’s tested efficiency results, and Fig. 15(c) shows the calculated power loss distribution between the front-end Boost and the down-stream inverter. It can be seen from Fig. 15(a) and Fig. 15(b) that the efficiency improvement of the proposed DI-DBI-based DC-AC system increases with an increase of VL, and that the efficiency is improved by 1.2% to 1.8%. It can be seen from Fig. 15(c) that the power loss of the frond-end Boost in the DI-DBI-based DC-AC system is smaller than that of the traditional DBI-based DC-AC system and that this advantage becomes more obvious when VL increases. This is because the power and loss of the frond-end Boost in the proposed system are greatly reduced. Together with the efficiency improvement of the inverter, the efficiency improvement of the proposed DI-DBI-based DC-AC system is more obvious, which coincides with the tested results. The efficiency results verify the effectiveness of the DI-DBI in improving the efficiency of the system.
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Thanks to the multi-level characteristics, the harmonics of vAB and can be improved when compared with the traditional DBI. Fig. 16 shows comparison results of the THD of vAB. It can be seen that the THD of vAB in the DI-DBI is much better than that of the DBI in the whole range of VL. In addition, the calculated and simulated results coincides with the tested results very well.
Fig. 16. Comparison of the THD of vAB.
The tested results are a little lower than the calculated and simulated results. This is caused by the bandwidth limitation of the power analyzer. Fig. 17 shows the harmonic spectrum analysis of vAB with VL =100V and VL =150V. It can be seen that the harmonic spectrum is distributed around the switching frequency and the integer times of the switching frequency. In addition, the harmonic component when VL =150V is higher than that when VL =100V. Fig. 18 shows the THD of the output current io in the DI-DBI. After filtering, the value of the THD is about 2% in the whole range of VL, which indicates the effectiveness of the DI-DBI in improving THD.
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Fig. 18. THD of output current io in DI-DBI.
It is well known that the DBI has one more inductor when compared with the traditional full-bridge inverter which seems to be bad for the power density and cost. A volume comparison of the inductor between DI-DBI-based and traditional full- bridge-based DC-AC systems are listed in Table III. Fig. 19 shows a picture of the inductors in the two system, where L1 and L2 are the inductors of the DI-DBI, LFB is the inductor of the full-bridge and LB is the inductor of the Boost. The values of inductors of the two systems are calculated according to the current ripple requirements. Thanks to the multi-level effects, the single inductor value of the DI-DBI is smaller than that of the full-bridge. Secondly, the RMS value of the current though the inductor of the DI-DBI is smaller than that of the full- bridge since the inductor only works in half a cycle. Thirdly, the power of front-end Boost is largely reduced in the DI-DBI- based DC-AC system. As a result, the current through the inductor of the front-end Boost in the proposed DC-AC system is smaller than that in the traditional full-bridge-based DC-AC system. Therefore, although one more inductor is used in the proposed DC-AC system, the total volume of the two systems is about the same.
Fig. 19. Picture of the inductors in DI-DBI-based and full-bridge- based DC-AC systems.
The power density of an inverter is determined by the inductors. In addition, it is also affected a lot by the thermal stress, heat sink, mechanical stress, etc. Since higher efficiency is achieved, the thermal stress is reduced. In addition, a lot of work has been done to reduce the volume and weight of the output filtering inductor [21]-[23] which can be used to further optimize the inductor of the DI-DBI.
Ⅴ. CONCLUSIONS
A family of dual-DC-input dual-buck inverters (DI-DBIs) has been proposed for efficiency improvements. A low DC-input port is introduced to a traditional DBI by employing a dual-input switching cell as the input of the DBI. As a result, a low voltage source, whose voltage is lower than the peak amplitude of the AC output voltage, can be directly connected to the DBI to supply power to the AC side in single-stage power conversion. The advantages of high efficiency and high reliability of a traditional DBI can be achieved with the proposed DI-DBIs. In addition, theoretical analysis and experimental verifications indicate that the proposed DI-DBIs have a number of advantages. 1) The conversion stages of a DI-DBI-based DC-AC power system are reduced, since part of the DC input power can be directly fed to the AC side within a single power conversion stage. 2) The power rating and power losses of the front-end Boost converter are reduced. 3) Multi-level characteristics are achieved with the DI-DBIs, which can help to reduce the switching losses and the volume of the output filter. All these features make the proposed DI-DBIs good candidates for high-efficiency and high-reliability DC-AC power systems.
ACKNOWLEDGMENT
This work is supported by National Natural Science Foundation of China (51677085), the Fok Ying-Tong Education Foundation of China, Six talent peaks project in Jiangsu Province (2016-XNYQC-008) and the Fundamental Research Funds for the Central Universities (NO. NE2018 102).
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Fan Yang was born in Shanxi Province, China, in 1985. She received her B.S. degree in Electrical Engineering from Nanjing Normal University, Nanjing, China, in 2007; and her M.S. degree in Electrical Engineering from the Nanjing University of Aeronautics and Astronautics (NUAA), Nanjing, China, in 2013, where she is presently working towards her Ph.D. degree in Electrical Engineering and Power Drives. Her current research interests include the topologies and control of DC-DC and DC-AC converters.
Hongjuan Ge was born in Jiangsu Province, China, in 1966. She received her B.S. and M.S degrees in Engineering from Southeast University, Nanjing, China, in 1985 and 1988, respectively. She received her Ph.D. degree in Electrical Engineering from the Nanjing University of Aeronautics and Astronautics (NUAA), Nanjing, China, in 2007. She joined the Faculty of Electrical Engineering, NUAA, in 1988, where she is presently working as a Professor in the Department of Electrical Engineering, College of Automation Engineering. Her current research interests include the topologies and control for AC-AC matrix converters, and PMSM drives and control. She has authored more than 30 technical papers published in journals and conference proceedings.
Jingfan Yang was born in Jiangsu Province, China, in 1993. She received her B.S. degree in Electrical Engineering from the Nanjing University of Aeronautics and Astronautics (NUAA), Nanjing, China, in 2015, where she is presently working towards her M.S. degree in Electrical Engineering. Her current research interests include the topologies and control of DC-AC converters.
Runyun Dang was born in Gansu Province, China, in 1992. She received her B.S. degree in Electrical Engineering from Nanjing Normal University, Nanjing, China, in 2014. She is presently working towards her M.S. degree in Electrical Engineering at the Nanjing University of Aeronautics and Astronautics (NUAA), Nanjing, China. Her current research interests include the topologies and control of DC-AC converters.
Hongfei Wu (S’11, M’13) was born in Hebei Province, China, in 1985. He received his B.S. and Ph.D. degrees in Electrical Engineering, Power Electronics and Power Drives from the Nanjing University of Aeronautics and Astronautics (NUAA), Nanjing, China, in 2008 and 2013, respectively. From June 2012 to July 2012, he was a guest Ph.D. student at the Institute of Energy Technology, Aalborg University, Aalborg, Denmark. Since 2013, he has been with the Faculty of Electrical Engineering at NUAA, where he is presently working as an Associate Professor in the College of Automation Engineering. He has authored or co-authored more than 100 peer-reviewed papers published in journals and conference proceedings. He is the holder of more than 20 patents. His current research interests include power converters and distributed power generation systems. Dr. Wu was the recipient of an Outstanding Reviewer Award of the IEEE Transactions on Power Electronics (2013). He was a recipient of a Changkong Scholar Award of NUAA, in 2017.