A comparison of single phase standalone square waveform solar inverter topologies: Half bridge and full bridge

Received Apr 10, 2019 Revised Jan 12, 2020 Accepted Feb 2, 2020 In stand-alone photovoltaic installations the photovoltaic inverter allows transforming the DC power produced by the photovoltaic modules into an AC power. Depending on the shape of the AC output voltage generated by the inverter there exist three main types of stand-alone PV inverters: pure sine waveform inverters, modulated sine waveform inverters and square waveform inverters and each type of these inverters is also divided into different topologies. In this paper we will be interested and study the square waveform stand-alone inverter topologies which are the half bridge and the full bridge inverter topologies.


INTRODUCTION
With the revolution and the advance of the use of photovoltaic installations all around the word, many designs and conceptions of photovoltaic devices have been developed [1][2][3], especially the conception and the design of DC-DC and DC-AC converters circuits [4][5][6]. Because of the low efficiency and the high cost of solar modules in photovoltaic systems [7], the effiecicy and the cicuits simplicity of the other devices of photovoltaic system have to be the most higher possible. A Stand-alone photovoltaic installation is an off grid installation, in which the photovoltaic modules are the only source that generates the electric power to feed a DC or an AC load [8]. Photovoltaic modules generate a DC electric power thus to feed an AC load in stand-alone photovoltaic installation a stand-alone photovoltaic inverter has to be used to convert the DC power produced by the photovoltaic modules into an AC power [9][10][11][12].
Depending on the shape of the AC output voltage generated by the inverter there exist three main types of single phase stand-alone photovoltaic inverters: pure sinewaveform inverters, modulated waveform inverters and square waveform inverters [13][14][15] and each type of these inverters is also divided into different topologies: half bridge and full bridge for the square waveform inverters and multilevel (bridges) for the modulated waveform and the pure sinewave inverters. Square waveform inverters are designed to feed loads which have an important inductance [6] as motors because inductive loads allow having relatively pure sinewave current. Looking for the most efficient and the simplest cicuit design of stand alone single phase solar inveter to feed AC motors and inductive loads, in this paper we will be interested by the square waveform single phase inverters doing a comparative study of the efficiency and the simplicity between the half bridge and full bridge topologies simulating them on ISIS Proteus software.  Figure 1 presents a single phase half bridge square waveform photovoltaicinverter which is composed of two stages: dc-dc and dc-ac stage. The dc-dc stage is a boost converter which is used to convert the low voltages produced by the PV modules (12-24V) into V 440 to have an RMS voltage value equal to 220V by the half bridge inverter. The output of the boost converter is connected in parallel with tow capacitors dividers to get = 2 ⁄ when : 2 is on and 3 is off and get = − 2 ⁄ when 3 is on and 2 is off [16][17][18][19][20]. The swiches of this topology would be driven by simple square pulses.  Figure 2 presents a single phase full bridge square waveform solar inverter which is also composed of two stages as the half bridge one: a dc-dc and a dc-ac stage. The dc-dc stage is a boost converter which is used to convert the low voltages produced by the PV modules (12-24V) into a Vdc equal to 220 to have an RMS voltage value equal to 220V by the full bridge inverter. The output of the boost converter is connected in parallel with two legs and each leg is composed of two inverting switches connected in series. The output voltage of the inverter equal to when 2 and 5 are on and 3 and 4 are off and equal to − when 3 and 4 are on and 2 and 5 are off [21][22][23][24][25]. The swiches would be driven by simple square pulses.  Figure 3 shows the basic configuration of a boost converter, its composed of an inductor, a switch: IGBT or a MOSFET transistor, a diode and a capacitor. In this part of the paper we will presents how to size and calculate the parameters of each one of its components which depend on the output voltage and current [26][27][28].

Duty cycle
Where, : Is the voltage coming from the PV module which is equal to 12V or 24V.
: Is the desired voltage 220 for the full bridge inverter and 440V for the half bridge one. : The inverter efficiency.

Inductor selection
Equation 2 [26] presents a good estimation for the right inductor: Where, Δ : The estimated inductor ripple current, a good estimation for the inductor ripple current is 20% to 40% of the output current.
: Minimum switching frequency of the converter.

Inductor ripple current
The inductor ripple current is calculated by the expression [26]: Where, ( ) : The maximum output current.

Output capacitor selection
Capacitors are used to minimize the ripple on the output voltage. The following equation can be used to adjust the output capacitor values for a desired output voltage ripple [26]:

SIMULATION PROCEDURES
In this part two stand-alone square waveform inverters topologies have been simulated on ISIS Proteus software: a half and a full bridge inverter. Each one of them has been designed to deliver an output power equal to 1000 wat and an output voltage equal to 220V with 50HZ of frequency. The PWMs used to control the circuit switches have been generated and controlled by the PIC 16F877A.

The half bridge inverter circuit description
This inverter is composed of two stages: a dc-dc and a dc-ac stage as shown in Figure 4. The dc-dc stage Figure 5 is a boost converter that converts the panel voltage Vp=24V into Vb=440V. An IGBT (IRG4PC50S) is used as a switch controlled by the PIC 16F877A and the IR211 driver. The output of the boost converter is connected in parallel with tow capacitors connected in series and a leg of two IGBTs (IRG4PC50KD) connected also in seriesto get an output voltage Vo= Vb/2=220V when t=T/2 and Vo=-Vb/2=-220 when t=T Figure 6. Table 1 presents the boost parameters, they were determined using the (1-4).

The full bridge inverter circuit description
This inverter is also composed of two stages: a dc-dc and a dc-ac as shown in Figure 7 .The dc-dc stage Figure 8 is a boost converter that converts the panel voltage Vp=24V into Vb=220V. An IGBT (IRG4PC50S) is used as a switch controlled by the IR211 driver. The output of the boost converter is connected in parallel with two capacitors in series and two legs, each leg contains two IGBTs (IRG4PC50KD) in series to give an output voltage Vo=(+Vb/2+Vb/2=+Vb=220V) when t=T/2 and Vo=(-Vb/2-Vb/2=-Vb=-220V) when t=T. Table 2 presents the Boost converter parameters of the full bridge inverter.

SIMULATION RESUTS AND COMPARISON
As presented in Figues 4 and 7, the output voltages of boost converters are different: 220V for the one used in the full bridge inverter and 440V for that one used in the half bridge inverter that make theircomponents parameters different using the same switching frequency and the input voltage, the main differences between the two inverters cited in the points below indicate that the half bridge inverter has less conduction losses than the full bridge inverter , due to the high output voltage of its boost converter, whereas, this high voltage could makes a risk on operators and the system, that leads to the use of an important isolation. Also the half bridge inverter has a simpler circuit design than the full bridge inverter wich reduces the cost and the time of asemblage

Desired output voltage ripple
As shown in Figures 9 and 10 the output voltage ripple of the boost converter used in half bridge inverter is more important than that of full bridge output voltage ripple they are the double because 3% of 220V equal to 6.6 and 3% of 440V equal to 13.2.

The output voltage in the half bridge inverter
As has been sized, the boost converter of the half bridge inverter has to deliver an output voltage equal to 440V this high voltage could be a risk on operators and the system which leads to the use of an important isolation, however,the advantage of this topology voltage is to reduce the current following the IGBTs into the half and by consequently the conduction losses in the IGBTs with 50% (200% less than that in the IGBTs of the full bridge inverter).

Duty cycle and conduction losses
As presented in Tables 1 and 2 the duty cycle of the boost used in the half bridge inverter topology is higher than that used in the full bridge topology with 5 % (10 % higher than a full bridge inverter) that enhances the conduction losses in the IGBT with the same amount, because the conduction losses are present during the period of the duty cycle [15,19].

The boost inductor and output capacitor
As shown in Table 1, the boost converter used in the full bridge inverter topology has a higher capacitor capacitancethan that one used in the half bridge inverter topology but the contrary for the inductor inductance: the boost converter used in the half bridge inverter has a higher inductor inductance than that one of the full bridge converter.

The component number
The full bridge inverter contains more components than the half bridge inverter due to the presence of one more leg of switches which makes its circuit more complex. Also the production cost of the full bridge inverter is higher than that of half bridge inverter because of the time and the effort of the assemblage.

CONCLUSION
In this paper a comparative study between tow topologies of stand-alone square waveform photovoltaic inverters has been done, simulating them on ISIS Proteus software. This study shows that the pefomance and the simplicity of half bridge inverter are higher than those of full bidge inveter wich make it the best solution in stand alone photovoltaic insallations that feed inductive loads and motors.  ISSN: 2088-8708