In 2002, Z-source inverter (ZSI) was firstly introduced to power electronics area as a new voltage-fed topology and has been proven experimentally^[1]. Z-source inverters have several advantages and disadvantages, which have been extensively reviewed in Ref.[2-4]. The key notion of ZSI is established by using a diode in the front end, and two inductors and two capacitors connected in the X-shaped. In 2008, Professor F. Z. Peng proposed the improved version of ZSI, that is, quasi-Z-source inverter (qZSI)^[5]. Along with the advantages of ZSI, qZSI has several merits of their own, such as reduced passive component ratings, continuous input current, a common dc rail between the source and inverter, etc^[6]. Many topologies have been derived from the primary Z-source power converter, such as Z/quasi-Z-source AC-AC converter, Z/quasi-Z-source DC-DC converter, Y-source converter, X-source converter, etc. From then on, quasi-Z-source inverter has been widely applied in lots of areas such as electric vehicle, photovoltaic generation, wind power generation, etc.

The conventional bidirectional DC-DC power converters, such as bidirectional Buck-Boost power converter and Cuk bidirectional DC-DC power converter are introduced in Ref.[6-7]. In the bidirectional Buck-Boost converter, it can invert the polarity of the voltage, and can either increase or decrease the voltage magnitude. But it is only applicable to small and medium-sized power applications. As for the Cuk bidirectional DC-DC power converter, one of its principal advantages over conventional converters is that both the input and output currents are nonpulsating, but due to the complicated circuit, the efficiency of the power convertion is not high enough. However, with the development of science and technology, quasi-Z-source inverter has been proposed. It inherits all the advantages of the traditional ZSI and has several more advantages, including reduced passive component stress and continuous input current features^[8,9,10,11]. These advantages make it easier to be applied in bidirectional power converters.

In this manuscript, a DC-DC circuit based on the existing quasi-Z-source converter topology is proposed. This proposed circuit topology contains all the benefits of existing quasi-Z-source-based power converters, such as buck-boost function of the single topological structure, reversal of the polarity of the input/output voltage and high reliability owing to their immunity from shoot-through and open-circuit problems. In addition to that, the proposed circuit topology can be applied to DC-DC power conversion which has more freedom for increasing voltage gain by regulating the duty cycle D. So, it can be widely used in a variety of electric power supply systems, including cars, ships, aircraft, computers, even in the microgrid.

This paper focuses on the construction of DC-DC power converter topology based on quasi-Z-source. The rest of this manuscript is structured as follows. In Section 2, the operating principle and equivalent circuits are introduced and verified. Then, the results of simulation and experiments are shown in Section 3, followed by the conclusion in Section 4.

The circuit topology of a quasi-Z-source-based DC-DC converter is shown in Fig.1. Apparently, its main circuit is similar to the quasi-Z-source-based converter, where two capacitors C₁、C₂ and two inductors L₁、L₂ are used. As for the power switches, two IGBT modules or power MOSFETs are face-to-face connected, each of them is conventionally composed of a power transistor and an anti-paralleled diode, and this will be convenient to use the existing IGBT module or power MOSFET. Due to the power switches, the circuit can transmit bi-directional power flow.

Same to the conventional Z-source converters, the quasi-Z-source-based converter has a symmetrical network in which C₁ = C₂, L₁ = L₂. The switches S₁ and S₂ are considered as a pair of complementary working switches. Apparently, the diode and switch S₁ are on and off at the same time, the diode and switch S₂ are on and off at the same time, too. In this circumstance, no control signal is applied to switches and , so they are both at the off state. Given that the switching cycle is T, the turn-on time of S₂ is T₀, the turn-off time is T₁, apparently T = T₀ + T₁, so the duty cycle of S₂ is D = T₀/T.

(1) When the power flow is transmitted from the left side to the right side, the equivalent circuit is shown in Fig.2. There are two operating modes in this case, one is the switch S₁ is on and switch S₂ is off, and the other is switch S₁ is off and switch S₂ is on. In the former case, the inductors L₁、L₂ and L₃ are charged, the capacitors C₁ and C₂ are discharged, the equivalent circuit is shown in Fig.2a.

From the equivalent circuit diagram, Fig.2a, one has

（1）

Where U_in is the input dc voltage and U_out is the output dc voltage.

In the latter case, that is, when the switch S₂ is on and the switch S₁ is off. In this case, the inductors L₁、L₂ and L₃ are discharged, the capacitors C₁ and C₂ are charged, the equivalent circuit is shown in Fig.2b.

From the equivalent circuit diagram, Fig.2b, one has

（2）

At steady state, the average voltage of the inductors over one switching period should be zero. Hence, from Equ. (1) and Equ. (2), we have

（3）

（4）

（5）

Where T₀ is the duration of S₁ at the on state and T₁ is the duration of it at the off state.

From Equ. (3)～(5), the voltage of the capacitors C₁ and C₂ can be expressed as

（6）

（7）

Hence, we have

（8）

（9）

（10）

Where D is the duty cycle of the switch S₂.

(2) When the power flow is transmitted from the right side to the left side, the equivalent circuit is shown in Fig.3. The power switches and is considered as a pair of complementary working switches. Apparently, diode VD₁ and switch are on and off at the same time, diode VD₂ and switch are on and off at the same time. In this circumstance, no control signals are applied to switches S₁ and S₂, so they are at the off state. When the switch is on, and the switch is off, the equivalent circuit is shown in Fig.3a; When the switch is on, and the switch is off, the equivalent circuit is shown in Fig.3b.

From the above discussion in (1), we can have the similar conclusion

（11）

（12）

Where D is the duty cycle of the switch .

The plot of voltage gain G versus duty cycle D is shown in Fig.4, which makes it clear that the relationship between voltage and duty cycle D. In this figure, the case is when the power flow is transmitted from the left side to the right side. When D varies from 0 to 0.5, the output voltage is with the input voltage polarity, the circuit operates in boost mode; when D varies from 0.5 to 1, the polarity of output voltage and input voltage are reversal. Wherein, when D varies from 0.5 to 0.667, the circuit operates at reverse boost mode; when D varies from 0.667 to 1, it operates at reverse buck mode. From the above analysis, it is quite clearly that the output voltage can be adjusted by adjusting the duty cycle D of switch S₂( ). So the proposed quasi-Z-source-based DC-DC power converter topology can be easily and widely used in lots of areas.

The simulation is performed using the software Saber. A prototype for the proposed DC-DC power converter is fabricated. The simulation diagram of the proposed DC-DC power converter is shown in Fig.5 and the simulation results are shown in Fig.6. System parameters for simulation are given as follows:

(1) DC input voltage U_in = 24V.

(2) Inductors and capacitors of the quasi-Z-source: L₁ = L₂ = L₃ = 1mH, C₁ = C₂ = C₃ = 500µF.

(3) System switching frequency f = 20kHz.

(4) The load resistor value R = 10Ω.

When the quasi-Z-source-based converter is applied to DC-DC power conversion, and the power flow is transmitted from the left side to the right side, the simulation results are shown in Fig.6a. Setting the input voltage on 24V, and adjusting the duty cycle of switch S₂ to 0.2. According to Equ. (10), the theoretical value of output voltage is 32V. It can be seen from the simulation waveform in Fig.6a that the output voltage is 32.038V. The voltage of capacitor C₁ is 7.954 5V, which theoretical value is 8V; The voltage of capacitor C₂ is 31.967V, which theoretical value is 32V; The currents of the inductors are all flat, which verify the accuracy of the above theoretical analysis. It is clearly that the simulation results are nearly perfect to verify the conclusion in Section 2.

When the power flow is transmitted from the right side to the left side, the simulation results are shown in Fig.6b. The input voltage is also 24V, and the duty cycle of switch S₂ is still 0.2. According to Equ. (12), the theoretical value is 18V. It can be seen from the simulation waveform in Fig.6b that output voltage is 18.002V. The voltage of capacitor C₁ is 6.003 4V, which theoretical value is 6V; The voltage of capacitor C₂ is 23.997V, which theoretical value is 24V; The currents of the inductors are also flat, which could also verify the accuracy of the above theoretical analysis.

Finally, from the above simulation results, it can be seen that when the power flow is transmitted from the left side to the right side, the quasi-Z-source-based converter can achieve boost function and maintain output voltage stability. On the other hand, when the power flow is transmitted from the right side to the left side, the quasi-Z-source-based converter can achieve buck function. All these waveforms was confirmed that the proposed circuit operated following the theory, which can indeed transmit bidirectional power flow in a single topology with a good accuracy by regulating the duty cycle D.

The prototype has been constructed in lab that is shown in Fig.7. The same parameters are used in the experimental work which is shown in the above section. DSP TMS320F2812 is used as a tool for generating signals to control the switches. It has all necessary circuits, which includes PWM generator; This DSP chip can generates four routes of signals, which can be divided into two groups to control S₁、S₂ and switches 、 . The switches are not turned on at the same time, that is, when switch S₁ is on, switch S₂ is off; when switch S₁ is off, switch S₂ is on. For the switches 、 , there has the same rule. Due to the amplitude of signals which DSP generates is only 3.3V, it can’t drive IGBT, so the driven chip KA962D is adopted as the driven part. KA962D can amplify signals vary from -8V to +15V, so the switches can work efficiently. At the end of this section, the experimental results of the proposed DC-DC power converter are shown in Fig.8.

As can be seen in Fig.8, when the power flow is transmitted from the left side to the right side, the quasi-Z-source-based DC-DC converter can boost the voltage from 24V to the average voltage 32.2V; When the power flow is transmitted from the right side to the left side, the quasi-Z-source-based DC-DC converter can buck the voltage from 24V to the average voltage 18.1V. All of these experimental results are in good agreement with the previous analysis. The peak voltage of the output voltage waveform was caused by several aspects including EMI, existence of nonlinear components and switches instant action. It can be restricted by adding the embedded absorption circuit on the switches.

The simulation and experimental results validate the advantages of the proposed quasi-Z-source-based DC-DC power converter topology. It can either boost or buck the voltage in a single topology, and the output voltage is stable with high accuracy and a small ripple.

This paper has presented a comprehensive study on the DC-DC power converter topology based on quasi-Z-source concept. Both simulation and experiments confirmed that the proposed DC-DC power converter topology can transmit bidirectional power flow in a single topology by PWM control. It can also extend the regulating range to more than that can be achieved in the conventional DC-DC converters. The circuit can not only provide buck-boost function, but also reverse polarity of the input/output voltage. Due to the uniqueness of this circuit structure, it can increase efficiency and reliability, overcome the conceptual and theoretical barriers and limitations of the conventional power converters and provide a novel all-purpose power converter topology. With these merits, it can broad the application of power converters based on Z-source/quasi-Z-source.

This paper describes the operating principle, analyzes the circuit characteristics, the simulation and experimental results, which presented to verify the rationality and superiority of the circuit topology. Due to the page limit, the simulation and experimental results will be provided in the future papers to demonstrate the features of the AC-AC power converter topology based on quasi-Z-source in the same topology.