Extended Boost Quasi-Z-Source Inverters : Possibilities and Challenges

Quasi-Z-source inverter (qZSI) is a new promising power conversion technology perfectly suitable for interfacing of renewable (i.e., photovoltaic, wind turbines) and alternative (i.e., fuel cells) energy sources [1-3]. The qZSI has the following advantages:  Boost-buck function by the one-stage conversion;  Continuous input current (input current never drops to zero, thus featuring the reduced stress of the input voltage source, which is especially topical in such demanding applications as power conditioners for fuel cells and solar panels);  Excellent reliability due to the shoot-through withstanding capability;  Low or no in-rush current during start up;  Low common-mode noise. However, the efficiency and voltage gain of the qZSI are limited and comparable with the conventional system of a voltage source inverter with the auxiliary step-up DC/DC converter in the input stage [4]. The concept of extending the qZSI gain without increasing the number of active switches was recently proposed by several authors [5-8]. These new converter topologies are commonly referred to as the extended boost qZSI or cascaded qZSI and could be generally classified as capacitor assisted and diode assisted topologies [5]. In this paper different extended boost qZSIs with continuous input current will be presented, analyzed and compared. Moreover, some problematic issues of these converters will be pointed out and discussed.


Introduction
Quasi-Z-source inverter (qZSI) is a new promising power conversion technology perfectly suitable for interfacing of renewable (i.e., photovoltaic, wind turbines) and alternative (i.e., fuel cells) energy sources [1][2][3].The qZSI has the following advantages:  Boost-buck function by the one-stage conversion;  Continuous input current (input current never drops to zero, thus featuring the reduced stress of the input voltage source, which is especially topical in such demanding applications as power conditioners for fuel cells and solar panels);  Excellent reliability due to the shoot-through withstanding capability;  Low or no in-rush current during start up;  Low common-mode noise.However, the efficiency and voltage gain of the qZSI are limited and comparable with the conventional system of a voltage source inverter with the auxiliary step-up DC/DC converter in the input stage [4].The concept of extending the qZSI gain without increasing the number of active switches was recently proposed by several authors [5][6][7][8].These new converter topologies are commonly referred to as the extended boost qZSI or cascaded qZSI and could be generally classified as capacitor assisted and diode assisted topologies [5].In this paper different extended boost qZSIs with continuous input current will be presented, analyzed and compared.Moreover, some problematic issues of these converters will be pointed out and discussed.

Extended Boost qZSI -Basic Topologies
It should be noted here that this paper provides a general coverage of the extended boost voltage-fed qZSIs with continuous input current.All the topologies to be discussed and analyzed have a common property -the input inductor L 1 that buffers the source current.It means that during the continuous conduction mode (CCM) the input current of the converter never drops to zero, thus featuring the reduced stress of the input voltage source.
Capacitor Assisted Extended Boost qZSI.The investigated topologies are presented in Fig. 1.The basic topology of a capacitor assisted extended boost qZSI (CAEB qZSI, Fig. 1a) could be derived by the adding of one diode (D 2 ), one inductor (L 3 ) and two capacitors (C 3 and C 4 ) to the traditional qZSI with continuous input current [6][7][8].The modified topology of a capacitor assisted extended boost qZSI (MCAEB qZSI) could be derived from the CAEB qZSI simply by the changing of the connection points of capacitors C 2 and C 3 , as shown in Fig. 1b.

Steady state analysis of extended boost qZSI topologies
Generally, the topologies shown in Figs. 1 and 2 could be represented by the PWM inverter coupled with the cascaded qZS-network.In the same manner as the traditional qZSI, the extended boost qZSI has two types of operational states at the dc side: the non-shoot-through states (i.e., the six active states and two conventional zero states of the traditional three-phase voltage source inverter (VSI)) and the shoot-through state (i.e., both switches in at least one phase conduct simultaneously) [7][8][9].To simplify our analysis the inverter bridge was replaced by a switch S (Fig. 3).When the switch S is closed, the shoot-through state occurs and the converter performs the voltage boost action.When the switch S is open, the active (non-shootthrough) state emerges and previously stored magnetic energy in turn provides the boost of voltage seen on the load terminals.( (1) could also be represented as where D A and D S are the duty cycles of an active and shoot-through states, correspondingly.
Capacitor Assisted Extended Boost qZSI.Fig. 4 shows the equivalent circuits of the CAEB qZSI operating in the CCM for the shoot-through (a) and active (b) states.
From the equivalent circuit of the CAEB qZSI during the shoot-through state (Fig. 4a), the voltage of the inductors can be represented as: During the active (non-shoot-through) state (Fig. 3b), the voltage of the inductors can be represented as At the steady state the average voltage of the inductors over one operating period is zero.Considering that fact and defining the shoot-through duty cycle as D S and the non-shoot-through duty cycle as (1-D S ), the voltages of capacitors C 1 …C 4 can be found The boost ratio of the input voltage is From the equivalent circuit of the MCAEB qZSI during the shoot-through state (Fig. 5a), the voltage of the inductors can be represented as: During the active (non-shoot-through) state (Fig. 5b) the voltage of the inductors can be represented as: The peak DC-link voltage across the switch S is The boost ratio of the input voltage of the MCAEB topology is the same as of CAEB qZSI (10).
Diode Assisted Extended Boost qZSI.Fig. 6 shows the equivalent circuits of the DAEB qZSI operating in the CCM for the shoot-through (a) and active (b) states.
From the equivalent circuit of the DAEB qZSI during the shoot-through state (Fig. 6a), the voltage of the inductors can be represented as: During the active (non-shoot-through) state (Fig. 6b), the voltage of the inductors can be represented as: The voltages of capacitors The peak DC-link voltage is The boost ratio of the input voltage is Fig. 7 shows the equivalent circuits of the MDAEB qZSI operating in the CCM for the shoot-through (a) and active (b) states.From the equivalent circuit of the modified MDAEB qZSI during the shoot-through state (Fig. 7a), the voltage of the inductors could be represented as: During the active (non-shoot-through) state (Fig. 7b), the voltage of the inductors could be represented as: The The peak DC-link voltage is The boost ratio of the input voltage of the MDAEB topology is the same as of DAEB qZSI (25).

Comparison of operating properties of different extended boost qZSI topologies
In order to verify theoretical assumptions a of simulations were performed by the help of PSIM simulation software.The proposed extended boost qZSI topologies were evaluated with the switching frequency f=30 kHz and shoot-through duty cycle D S =0.167.The input voltage was set at 30 V and the load resistor selected was 5 Ω.To simplify the analysis losses in the components were neglected.Capacitors and inductors selected for the qZS-networks have the following parameters: Capacitor Assisted Extended Boost qZSI.Fig. 8 shows the general operating waveforms of the CAEB and MCAEB qZSI topologies.As expected, both converters operate normally, producing twofold boost of the input voltage (U IN =30 V, U DC =60 V, Fig. 8a and Fig. 8b).Moreover, the topologies ensure the continuous input current (Fig. 8a) in the CCM.The average values of capacitor voltages of the CAEB and MCAEB qZSI topologies obtained during the simulations are compared in Table 1.It shows that by changing the interconnection points of capacitors C 2 and C 3 , as in Fig. 1b, the operating voltages of the capacitor C 3 were reduced by five times.Moreover, the voltages of capacitors C 2 …C 4 are equalized now.Diode Assisted Extended Boost qZSI.Fig. 9 shows the general operating waveforms of the DAEB and MDAEB qZSI topologies.Both converters operate normally, producing the demanded boost of the input voltage (U IN =30 V; U DC =57 V, Figs.9a and 9b).As expected, both topologies ensure the continuous input current (Fig. 9a) in the CCM.
The average values of capacitor voltages of the DAEB and MDAEB qZSI topologies obtained during the simulations are compared in Table 2.It shows that by changing the interconnection points of the capacitor C 3 (Fig. 2b), its operating voltage could be decreased by more than six times.Comparison of Idealized Boost Properties of Proposed Topologies.By the help of simulations it was verified (Fig. 11) that the capacitor assisted and diode assisted qZSI have common advantages, such as continuous input current and increased boost factor of the input voltage for the same value of the shoot-through duty cycle as with the traditional qZSI [1][2][3].Moreover, in the lossless approach the CAEB and MCAEB qZSI topologies could ensure up to 1.25 times higher boost than that of DAEB and MDAEB qZSI topologies (Fig. 11).

Experimental Results
In order to verify theoretical assumptions the laboratory setups corresponding to the investigated topologies were assembled.600 V/200 A IGBT with extra low saturation voltage was selected for S (see Fig. 3 for the details).Components used in the qZS-networks had the following properties: In the first experiment, main operating waveforms of the proposed topologies were acquired and compared.The shoot-through duty cycle was set at 0.167.As seen from Fig. 12b, due to the losses in the components of the qZSnetwork both of the capacitor assisted topologies could provide only a 90% of a theoretically predicted input voltage gain (DC-link voltage amplitude 54 V instead of 60 V).In the case of diode assisted topologies (Fig 13b) the practical voltage gain for the same operating conditions was reduced by 9% in comparison with the theoretically predicted (DC-link voltage amplitude 52 V instead of 57 V).Due to the input inductor L 1 all the proposed topologies feature the continuous input current in the CCM, as predicted in the analysis.During the second experiment the boost properties of the proposed topologies were experimentally verified and compared with theoretical results.In the conditions of fixed input voltage (30 V) and constant load (5 Ω), the shoot-through duty cycle of the converters was increased step by step from 0 to 0.25.Fig. 14 shows that all of the presented topologies suffer from the boost factor reduction, which is mostly caused by the losses in the inductors and diodes of the qZS-network as well as by voltage drops in the interconnection wires.In order to achieve higher possible voltage gain and efficiency these problematic issues should be first addressed during the design routine of extended boost qZS-converters.Another interesting fact is that all four topologies have demonstrated identical boost properties within the shootthrough duty cycle range of 0…0.15, which could be considered during proper topology selection for different applications.

Conclusions
In this paper four extended boost qZSI topologies were proposed, discussed and compared.These topologies were classified as capacitor assisted and diode assisted.A steady state analysis of topologies operating in the continuous conduction mode was performed.Theoretical study was validated by the simulations and experiments.
It was experimentally stated that in similar operating conditions the discussed topologies provide an identical boost factor of the input voltage within the shoot-through duty cycle range of 0…0.15.Thanks to the presence of the input inductor L 1 all the discussed extended boost qZSIs have continuous input current in the CCM, thus featuring the reduced stress of the input voltage source.

Fig. 1 .Fig. 2 .
Fig. 1.Capacitor assisted extended boost qZSI topologies: basic CAEB qZSI (a) and MCAEB qZSI (b) Diode Assisted Extended Boost qZSI.The investigated topologies are presented in Fig. 2. The basic topology of a diode assisted extended boost qZSI (DAEB qZSI, Fig. 2a) could be derived by the adding of one capacitor (C 3 ), one inductor (L 3 ) and two diodes (D 2 and D 3 ) to the traditional qZSI with continuous input current.The modified topology of a diode assisted extended boost qZSI (MDAEB qZSI) could be derived from the DAEB qZSI simply by the

Fig. 3 .
Fig. 3. Simplified power circuit of the extended boost qZSI used in the analysis The operating period of the qZS-converter in the CCM basically consists of a shoot-through state t S and an active state t A S A t t T   .(1)

Fig. 4 .
Fig. 4. Equivalent circuits of the CAEB qZSI: during the shootthrough state (a) and during the active (non-shoot-through) state (b)

Fig. 5 .
Fig.5shows the equivalent circuits of the modified

Fig. 7 .
Fig. 7. Equivalent circuits of the MDAEB qZSI: during the shootthrough state (a) and during the active (non-shoot-through) state (b)

Fig. 8 .
Fig. 8. Simulated waveforms of the CAEB and MCAEB qZSI topologies: input voltage and current (a) and DC-link voltage and current (b)

Fig. 11 .
Fig. 11.Comparison of idealized boost properties of capacitor assisted and diode assisted extended boost qZSIs

Fig. 12 .Fig. 13 .
Fig. 12. Experimental waveforms of input voltage and current (a) and DC-link voltage and current (b) of the CAEB and MCAEB qZSIs

Fig. 14 .
Fig. 14.Theoretical and practical boost properties of the compared topologies

Table 1 .
Comparison of operating voltages of capacitors in CAEB and MCAEB qZSI topologies

Table 2 .
Comparison of operating voltages of capacitors in DAEB and MDAEB qZSI topologies