COMPARISON BETWEEN LS-PWM AND LPS-PWM FOR SWITCHING-CAPACITOR ( SC ) INVERTER

ABSTRACT: A two modulation methods of the switching-capacitor inverter are proposed. The inverter output voltage is larger than the input voltage by connect switching-capacitor in series and in parallel respectively. The level output voltages are decided by number of the capacitor. Five capacitors are used to obtain eleven level in output voltage waveform. The proposed inverter is driven by the LS-PWM and LPS-PWM methods. The circuit configuration, states operation and simulation results by using MATLAB/SIMULINK are presented in this paper. The simulation results demonstrated best LPS-PWM method compared with LS-PWM in terms THD of output voltage.


INTRODUCTION
AC-DC converters with rise voltage gain are desired in considerable industrial applications such as front-end parts for neat energy sources, Uninterruptible Power Supplies (UPS), Electrical Vehicles (EVs), and Distributed Generation (DG) systems (Yoo et al., 2008;Lu et al., 2017).EVs are distinguished by motor drive equipments, where power electronic is used in electric power conversion such as Pulse Width Modulation (PWM) inverters and DC/DC converters (Emadi et al., 2006).
The inverter with a step-up transformer, the Z-source inverter, and single stage flyback inverter are used to obtain high input voltage/ output voltage ratio (Hu et al., 2017;Voglitsis et al., 2017).But the transformer or the inductor creates the inverter which is big because they have large size magnetic cores to bear the high power (Chandrasekaran and Gokdere, 2004).
The multilevel inverter techniques are used a lot of attention because the attractive solution for high power applications.The output voltage of The multilevel inverter is graduate waveform, lower common-mode voltages, reduced (dv/dt) voltage and lower Total Harmonic Distortion (THD) (Franquelo et al., 2008).The multilevel inverter has numerous topologies; such as, the cascade H-bridge (CHB) inverter (Karasani et al., 2017), the Neutral Point Clamped PWM (NPC-PWM) inverter (Rodriguez et al., 2009) and the flying capacitor inverter (Kou et al., 2004).These topologies have a major number of switching device or numerous input voltage sources.
A Switched Capacitor (SC) inverter outputs have higher outputs and large gain ratio in a similar way to the charge pumps (Ardashir et a., 2017).A SC inverter using series and parallel conversion is based on the boost converter and a full-wave bridge inverter (Hinago, and Koizumi, 2010).
The SC inverter has the lower number of devices contrast to the conventional multilevel inverters (Fong et al., 2017).There are numerous modulation methods to drive a SC inverter: the multicarrier PWM (McGrath and Holmes, 2002) , the space vector modulation (Das and Narayanan, 2012), the hybrid modulation and the selective harmonic elimination (Agelidis et al., 2008).The Level and Phase Shifted PWM (LPS-PWM) method is incorporation of the Level Shifted PWM (LS-PWM) and Phase Shifted PWM (PS-PWM) (Hinago and Koizumi 2011).
In this paper, four stages of SC inverter driven by LS-PWM and PS-PWM methods are proposed.The voltage ripple and the losses in this inverter are less comparable with conventional inverters.The operation stage principle and simulation results are shown and compared based on a newly four stage SC inverter.

MODEL DESCRIPTION AND STATE OF OPERATION IN LS-PWM METHOD
A model configuration of the four layer switching-capacitor (SC) inverter is shown in Fig. 1 (Hinago, and Koizumi, 2010).The one phase circuit, switches Sai, Sbi, and Sci (I=1,2,3,4) are used which switch for the capacitors Ch (h=1,2,3,4,5) that connected in parallel to charge and in series to discharge so that the output of the inverter a higher voltage than the input voltage EIN.
The switches Sai are utilized for series attachment of capacitors and switches Sbi and Sci are utilized for parallel attachments of capacitors.The condenser C3 couple in parallel to the supply Ein can be rejected when using ideal voltage source performs.The full-bridge inverter used switches S1-S4 in the output stage and low pass filter is constructed from an inductor L and capacitor C with resistive or inductive loads can be connected in the output terminal of the inverter.
In LS-PWM technique, five stages operation in each half cycle of reference voltage can be described in the following( referred to Fig. 2).Where VC1 is the voltage across the capacitor C1.Therefore, the output voltage Vout is charging between Ein and Ein+ VC1.

Stage 3
When the period (t) satisfied (t2≤ t ≤ t3) in Fig. 2. The switches Sa2, Sb2, and Sc2 are switched on while Sa1, Sb1, and Sc1 are kept on state however Sai (i=3,4) is kept turn on while Sbi and Sci (i=3,4) are keep turn off.The condenser C2 is charged by the negative current iC2 as shown in Fig. 3d and discharged by the positive current iC2.
The output voltage Vout is Where VC2 is the potential difference on the capacitor C2.Thus, the output voltage is charging between Ein + VC1 and Ein + VC1+ VC2 alternately.

Stage 4
In the interval (t3≤ t ≤ t4), the full-bridge inverter switches S1 and S4 is turn on while S2 and S3 are turn off.The switches Sai (i=1,2) are turned on while Sbi and Sci (i=1,2) are turn off.However, switches Sa3 , Sb3 and Sc3 are turned on or off alternately and the switches Sa4 is turning off while Sb4 and Sc4 are turning on.So the capacitor C1 and C2 are discharging and capacitor C4 is charging and discharging alternately while capacitor C5 is charging as shown in Fig. 3e.In these states, the output voltage Vout change between Ein + VC1+ VC2 and Ein + VC1+ VC2+VC4 respectively.

Stage 5
This stage can be described during the interval which is (t4≤ t ≤ t5), the full-wave inverter switches keep of the states and switches Sai (i=1,2,3) are turned on while switches Sbi and Sci (i=1,2,3) are turned off.The switches Sa4, Sb4 and Sc4 are turned on or off alternately as shown in Fig. 3f.
After time (t5), the five states are iterative by a half cycle of reference voltage and illustrated in

OPERATION PRINCIPLE OF SC INVERTER IN LPS-PWM METHOD
Fig. 4 illustrates the basic pulse in the LPS-PWM method, the reference waveforms eref is splited into five levels.With triangle waveform tri1 is employed between 3Bc and 5Bc, another triangle waveform tri3 is employed between Bc and 3Bc, and triangle tri5 is employed between -Bc and Bc, and tri7 is employed between -3Bc and -Bc , and tri9 is employed between -5Bc and -3Bc respectively.Which are coincide in the same phase at the carrier frequency fref and their magnitude of Bc.The other triangle waveforms tri2, tri4, tri6, tri8, and tri10 are put on each level but their phases are shifted by 180 degree with respect to the previous group so it is called to this modulation as level and phase shifted (LPS).
The operation principle can be described by the following: 1-when eref > tri6 so S1 is on and S2 is off but when eref > tri5 so S3 is on and S4 is off.

SIMULATION BLOCKS OF LS-PWM AND LPS-PWM TECHNIQUES
This part is devoted to the simulation block of the switched-capacitor inverter.Simulation diagram is divided into three blocks (as shown in Fig. 5): carrier and reference block, derive circuit and power circuit.The power circuit is common for both techniques (as shown in Fig. 6), while each drive circuit is specified for certain techniques.The first drive circuit generation pulse for the power circuit using LS-PWM(as shown in Fig. 7), whereas the pulse generation of the second drive circuit is based on LPS-PWM(as shown in Fig. 8).In addition, each technique has its own reference and carrier block.

SIMULATION RESULT
The switched-capacitor inverter is modeled in SIMULINK/MATLAB using power system set.Compared to the results of the two methods in terms of THD shows that the results of the first method best for the modulation index range from 2.5 to 4.5.
The voltage ripple of the capacitor C1 was reduced when the proposed LPS-PWM method was applied compared with LPS-PWM.Therefore, the losses caused by the voltage reduction of the switched capacitors were low and the efficiency of the inverter was improved.

Fig
Fig. 1.Proposed diagram of switching capacitor circuit.

2
and Sci are derived by the same pulses.The switches S1 and S2 are turned on or off alternately while the switches S3 is maintained off states and S4 is preserved on states.Therefore, the states shown in Fig.3a and bare transform alternately and the output voltage Vout occupy between 0 and Ein.StageWhen the condition for time duration is satisfied (t1≤ t ≤ t2) in Fig.2.The upper switches Sa1, Sb1, and Sc1 are switched alternative by the gate-source voltage VGSa1, VGSb1, and VGSc1, respectively.While the state lower switches Sai (i=2,3,4) is turned off andSbi and Sci (i=2,3,4)    are maintained turn on.The condenser C1 is charged by the negative current iC1 as shown in Fig.3(b) and discharged by positive current iC1 as shown in Fig. 3c.So the output voltage satisfied the following relation Vout = Ein + VC1 1

Table 1 .
For the positive half cycle of the reference wave and Table.2 in the negative half cycle of the reference wave by purpose VC1=VC2=VC4= VC5=Ein.