Symmetrical high voltage gain half-bridge inverter based double-Y-source networks with reduced voltage stress

Received Dec 15, 2018 Revised Mar 1, 2019 Accepted Jul 8, 2019 A single-stage symmetrical high voltage gain half-bridge (HB) DC-AC converter is proposed in this paper. Using two Y-source impedance networks, the following key features are utilized from the proposed topology: single stage inverter with very high voltage gain compared to conventional HB inverter, symmetrical output voltage waveform, low voltage stress across the passive components because it is distributed across two impedance networks, and only two switching devices are needed for the converter. Furthermore, important merit of the proposed topology is that the current drawn by the Y-coupled inductors is symmetrical around the X-axis which helps to prevent the Y-network cores from reaching the saturation state. And the last compelling feature is a virtual neutral point for the load connection is inherited in the proposed double Y-source impedance networks converter with no need for DC-Link capacitors. For low voltage sources such as photovoltaic (PV) and fuel cell, the converter is designed to achieve continuous input current operation. The operation modes and principles of the inverter are analyzed and discussed deeply in this paper. A detailed mathematical equations system is derived and verified for the presented converter. Finally, PSpice simulation tools are used to simulate the converter and verify the derived mathematical formulas.


INTRODUCTION
Renewable energy resources such as photovoltaic, wind turbines, and fuel cells are widely being demanded worldwide. The maximum supply voltage by those resources is low compared to AC power grid voltage standards. The output voltage also changes according to weather penetration. So that, wide range input voltage DC-AC power converters with high voltage gain are needed for clean energy applications. Conventional voltage source inverters (half and full bridge converters) are not promising topologies for renewable energy application because they have no voltage gain in basic operating principles.
High voltage gain inverters were proposed based on using different source impedance configurations as explored in next. In [1], a full-bridge single-phase inverter was proposed using Z-source impedance. The converter consists of four switching devices with a Z-source impedance to achieve high voltage gain with the discontinuous input current. Reference [2] proposed a switched-boost single phase HB inverter using two Z-source impedance networks. In addition to the voltage gain limitation of Z-source impedance inverters (the maximum output gained voltage is equal to twice its input to the Z-network), two additional DC-Link capacitors are needed for the HB converter to create a neutral point for the load connection. The voltage stress across the two capacitors is equal to twice the peak value of the output AC gained voltage, which means DC-Link capacitors with high voltage rating are required in such topology. A novel quasi-Z-source HB DC-DC converter is introduced in [3]. The proposed topology has a high-frequency transformer with a voltage doubler to obtain high voltage gain as well as circuit isolation. With Asymmetrical voltage waveform generated by the HB inverter, DC voltage component would exist in the voltage waveform. The challenges of selecting a blocking capacitor to prevent the DC component from passing through the high frequency transformer makes the converter more complicated. Otherwise, the highfrequency transformer would be exposed to the saturation problem. A single-phase high voltage gain DC-DC converter was presented for a half-bridge photovoltaic inverter system in [4]. The main disadvantage of the proposed converter is that many switching devices are required for the boost converter, resulting in low power converter reliability, and high power loss. Also, high voltage stress is experienced across the output capacitors of the proposed boost converter. An isolated transformer boost half-bridge micro-inverter for a single-phase PV system was explored in [5]. Using an isolated transformer helps to increase the voltage gain of the converter, but it increases the power conversion stages and more switching devices to achieve the required grid voltage level. In so doing that, the reliability of the converter is reduced as many components are needed to reach the grid voltage level. A Z-source inverter with high voltage gain capability is proposed in [6]. The disadvantage of this topology is that in case of failure of either one of the input power supplies (or in case of unequal voltage supply), asymmetrical output voltage waveform is expected to appear across the load. In addition to that, the extension of the proposed topology needs many components to be used to achieve the desired voltage gain. In [7], a high gain switched-boost half-bridge inverter is presented. The converter has a lower voltage gain than the proposed topology in this paper. A novel quasi-Z-source halfbridge DC-DC converter was introduced in [8]. The output AC voltage of quasi-Z-source inverter should be equal to the half value of the DC gained voltage, which is countered what is mentioned the operation waveforms of the proposed topology, according to the conventional half-bridge inverter operation. In [9], a Z-source half-bridge inverter is proposed, which requires two DC-Link capacitors with high voltage rating because the stress voltage across those capacitors are high. Although the stress voltage of the capacitors is reduced-based HB inverter, but the voltage gain is still not higher than other Y-source impedance networks converters [10]. In addition to that, the proposed topology is applicable only for two symmetrical input voltage supplies. Quasi-Z-source DC converter for high power photovoltaic applications is shown in [11]. The main drawback of the topology is that multi-power-conversion units are used to achieve high voltage gain, which is reduced the converter reliability. A novel single-input-dual output impedance network converter is presented in [12]. The disadvantage of the designed converters is that the DC-Link capacitors are suffered from high voltage gain in addition to the reduction of the converter reliability because of the decreasing in the DC-Link capacitors life cycle. A single-phase impedance source inverters are introduced in [13]. The presented topologies with high frequency transformer have lower voltage gain compared to Ysource-based inverters. A new magnetically coupled Z-source impedance networks was proposed in [14]. The proposed topology discussed how to reduce the saturation problem of the Y-impedance core. In [15]- [24], single Y-source impedance network converters were presented to achieve high voltage gain for power inverters with continuous and/or discontinues input current capability. A new HB DC-DC converter for wide range input voltage applications was presented in [25]. High duty cycle ratio is used (expanded to 100%) to achieve only twice the voltage gain of the conventional HB inverter. On the other side, HB inverters with Ysource networks can achieve way higher than twice the voltage gain of the traditional HB inverters with the same number of active circuit components.
This paper presents a one stage DC-AC conversion unit with a simple control method, low voltage stress across the components, and a wide variety of output voltage level with extra voltage gain -based double Y HB inverter. It is very useful for renewable energy application and especially photovoltaic systems. Detailed information about the converter design and operation are described in the next sections. The rest of the paper is organized as follows. Section 2 represents inverter design and operation. The concept of the topology components selection is presented in 3. In section 4, simulation results are explored and discussed. The conclusion is finally given in section 5.

INVERTER DESIGN AND OPERATION
The proposed inverter, as shown in Figure 1, consists of double Y-impedance networks, switching devices, power diodes, input inductors, and capacitors. The main merits of using double-Y-source networks are to provide a neutral point for the load connected with no need to use DC-Link capacitors, higher voltage gain compared to using single Y-impedance, and to achieve symmetrical output AC voltage waveform. Small cores can be used for the Y-inductors to achieve high enough voltage gain; compared to using a single Y- Hence, symmetrical AC current flows through all the windings of the Y-impedance networks as shown in the operating principles of the topology in next. In so doing that, the saturation problem of the Y-impedance cores is addressed out.

Non-shoot-through-state (NSTS)
In the NSTS, there are two operating states. The first one is when SW1 is ON, while SW2 is OFF, as shown in Figure 2 (a), a positive voltage appears across the load which is contributed by Y1. The AC load voltage level is equal to the half gained voltage of Y1 (VY1/2) as shown in Figure 3. The second operating state is that when SW2 is ON and SW1 is OFF, as shown in Figure 2 (b), in the opposite of the first operating state, a negative output AC voltage can be noted across the load in this state which is (-VY2/2). During this state, the Y-capacitors are charging up. Generally speaking, both diodes are only conducted during the NSTS. Using the loop voltage equations, the system equations for the NSTS are shown in below

Shoot-through-state (STS)
Both switches (SW1 & SW2) are ON in this state, as shown in Figure 2(c), while both diodes are in reverse blocking operation mode (OFF state). The output voltage of both Y-networks (Vo) is zero; so zero AC voltage appears across the load as shown in Figure 3. During the STS, the energy is being released from the capacitors and stored in the Y-inductors to achieve high voltage gain during the NSTS. With increasing the STS duration (ts or DS), more energy would be stored in the Y-inductors which results in higher voltage gain. The loop voltage equations of the STS are described below By applying volt-second balance for the Y-inductor (L) using equ. (4) and equ. (6), the voltage of the capacitors is derived as follows ( 7 ) By also applying volt-second balance for the input inductor (Lin) using equ. (1)  While in principle operation of the conventional HB inverter, the RMS load voltage is equal to (12) where , and The relationship between the peak-peak load voltage gain and the shoot-through duty cycle (Ds) is plotted for different (M) values, as shown in Figure 4. The plot shows that the shoot-through duty cycle (DS) region is restricted by the selected M value.
In principles operation of buck-boost converters, traditional buck converter has a voltage gain (Vo/Vin) which is less than or equal to 1, while the boost converter has a voltage gain greater than 1. The proposed source-impedance network converter works as a boost converter with a wide variety of the output AC voltage level. It can be noticed from the gain curves, as shown in Figure 4, that there are two operation quarters: positive and negative boost operation quarters. For the positive boost operation region, the shootthrough duty cycle (DS) is (0.5 0). In the negative boost operation region, the shoot-through duty cycle is (1 0.5).

RESULTS AND DISCUSSION
The proposed topology is simulated in detailed using PSpice simulation tools. The simulation tests conditions are listed in Table 1, both Y-impedance networks components are identical. The simulation results verified the operational concept of the proposed topology and the derived equations. The gate voltage of the switching devices is shown in Figure 5(a). It shows that an overlapped pulse width modulation (PWM) strategy is used where the shoot-through occurs on both sides of the gate pulses. This simple overlapped method is used to achieve symmetrical output AC waveform instead of using single side overlapped PWM which results in asymmetrical AC output voltage. The gained voltage by both Y-impedance networks is shown in Figure 5(b). The first Y-network voltage is positive which is VY1, while the second Y-network voltage is negative which is VY2. The difference between both impedance networks (VY2-VY1) represents the load voltage (Vo) as shown in Figure 5(c). It can be noted that high voltage (Vo) is gained from 24 V input voltage. Both diodes share the same amount of voltage stress as shown in Figure 6(b). The small oscillation in the diode's voltage represents the parasitic elements of the simulation circuit. Almost an equal voltage stress across the input capacitors and Y-networks capacitors are shown in Figure 6(c) and Figure 6(d), respectively. The simulation current results are shown in Figure 7. The input current, as shown in Figure 7(a), is continuous current. The diode and switching devices currents are shown in Figure 7(b) and Figure 7(c), respectively. The load current is shown in Figure 7(d) based on a resistive load. The Y-network currents (iL1, iL2, and iL3) are shown in Figure 8

CONCLUSION
One stage high voltage half-bridge inverter is designed and analyzed using double Y-source networks. The simulation results show promising features for renewable energy applications such as high voltage gain, low voltage stress across the circuit elements, continuous input current, and symmetrical current waveforms in all Y-inductors which helps to prevent the Y-cores saturation. The other most important features of the HB converter are symmetrical AC load voltage and an inherited virtual neutral point for the load connection. The operation principles are discussed and studied deeply in this paper based on derived the formulas. The circuit components are selected based on mathematical equations for better power inverter design. Furthermore, PSpice simulation tools are used to validate the proposed converter operation modes and detailed derived mathematical formulas.