Yuba Raj Kafle*, Saad Ul Hasan, and Graham E. Town

(School of Engineering, Macquarie University, NSW 2109, Australia)

Abstract: This paper presents a quasi-Z-source based isolated bidirectional DC-DC converter (qZIBDC) for renewable energy applications. The converter utilizes a dual active bridge circuit with a quasi-Z-source network on both sides, so the converter works as buck/boost converter from either side. It has a wider input/output voltage operating range, soft-switching capabilities without additional devices, and higher boost capability than a traditional dual active bridge circuit. Apart from that, shoot-through states are incorporated in its operating cycle to boost the input voltage resulting in high reliability of the proposed converter. Due to the symmetrical structure of the circuit, there is no defined high voltage or low voltage side as in traditional isolated bidirectional DC-DC converter. The operating principle and control strategy of the proposed converter are presented. Simulation and experimental results are provided to verify the effectiveness of the proposed converter topology and its control strategy.

Keywords: Bidirectional DC-DC converter, dual active bridge, impedance source, quasi-Z-source DC-DC converter

1 Introduction1

Bidirectional DC-DC converter has become a promising choice in applications where bidirectional power flow is required, such as dc motor drive systems, battery charging and discharging systems and auxiliary supplies for hybrid and fuel-cell vehicles[1-3]. Bidirectional converters are essentially divided into two types: isolated and non-isolated converters, depending on the application requirements. The non-isolated converters are attractive solutions for improving the efficiency,size and weight, while isolated converters are needed to obtain the voltage matching and galvanic isolation between source and load for protecting equipment and operations[4-5].

In bidirectional DC-DC converter, isolation is usually provided by a high-frequency transformer. An isolated bidirectional DC-DC converter consists of buck/boost mode converters interfacing low/high voltage sides linked with high frequency transformer shown in Fig. 1. A dual active bridge (DAB),originally proposed in [6,7], is a prominent candidate for an isolated DC-DC bidirectional converter which can obtain high efficiency and high-power density.Moreover, a DAB converter has additional advantages in ease of realizing soft switching, high power-handling capability, modular and symmetrical structure[8-10]. The power transfer of a DAB converter operates by utilizing the leakage inductance of a high-frequency transformer which serves as an instantaneous energy storing element,and energy is transferred by phase shifting the output voltage of each inverter[11]. The converter, however,may lose its soft switching in light-load conditions,requiring a complex control and limited regulation range of voltage. Also, the upper and lower legs of the converter cannot be gated simultaneously, which leads to a limited voltage-regulation ability and lack of flexibility[12]. The proposed converter is intended to overcome such drawbacks.

Fig. 1 Isolated bidirectional DC-DC converter configuration

Impedance source (Z-source) inverter was first introduced by F. Z. Peng in 2003[13]to overcome the limitation of current and voltage source converter which can only be boost type and buck type respectively. Also, switching devices are vulnerable to EMI as misgating-on causes short-circuit across the inverter bridge and destroy the switch. The Z-source converter overcomes such limitation and provides large voltage gain from zero to infinity. An improvement of ZSI, quasi-Z-source(qZS) inverter is proposed in [14,15]. qZS converter inherit all the advantage of Z-source converter and have additional advantages such as drawing continuous input current and low voltage stress on capacitors[16]. Most of the impedance-source based converter in the literature are unidirectional[12-15]and this research propose a novel isolated bidirectional qZS based converter.

This paper presents a novel configuration of a DAB where a quasi-Z-source network is added on both ends of the DAB. Due to high frequency transformer, the converter size gets reduced and ground leakage current is negligible so is applicable as bidirectional converter for distributed power generation systems. These systems usually benefit from storage,which requires a bidirectional converter, such as presented here. Apart from that, the converter can also be used for EV charger, fuel cell applications in distributed generations, solid-state transformer etc. The converter is symmetric on either side and it can work in both buck/boost modes with the PWM shoot-through control method (Fig.2). The control method is simple as it requires controlling of only four switches on the source-side converter, while the body diode on the other side is used as synchronous rectification. This reduces the complexity of control of all eight switches to that of a normal DAB. Also, “on resistance” is much smaller in synchronous rectifier which further improves the efficiency. Furthermore, the source connected to the input can either voltage or current source. The operating principle, theoretical analysis and control method of the proposed topology are presented.

Fig. 2 qZIBDC

This paper is structured as follows: Section 2 presents operation mode of proposed topology. Section 3 describe the operating analysis and its control strategy. The simulation and experimental results are presented and discussed in section 4, followed by the conclusion in section 5.

2 Operation mode of qZIBDC

The power flow from U1to U2is defined as forward power flow, and in the opposite direction is called reverse power flow. Because of the complete symmetry of the circuit, the circuit operation is the same either way, and so here we discuss the analysis of forward power transfer only. In addition, the DC source/ or load can be either current or voltage source/or load due to impedance source network[13,19]. When transferring power from U1to U2, Si(i = 1, 2, 3, 4) and SW2are turned on and all the other switches are turned off. The power flow thus is controlled by S1,…,S4like a VSI with SW1open and SW2closed. The diode VD1is used to prevent current backflow in the forward-power transfer mode and is turned off with reverse power flow. SW1is turned off in the forward power-flow mode and turned on in the reverse power flow mode. Similarly, SW2is turned off in the reverse power-transfer and turned on in the forward power-transfer mode. The antiparallel diode of switches (Q1,…,Q4) operates as full bridge rectifier,while the passive elements (L3, L4, C3and C4) of the qZS network perform as a low-pass filter. The switching states in reverse power flow can be analyzed in a similar manner. The converter goes through three operational states: active state, zero stage and shoot-through state. Unlike DAB, this converter has additional shoot-through state to allow extra boost apart from transformer turns ratio, so it has a wider regulation range of voltage. The operational modes will be described in more detail below.

2.1 Active state

In the active state, the complementary pair of switches turns on and power is transferred from the source to the load as shown in Fig. 3a. In the active positive state S1, S4provide positive current across the transformer primary while antiparallel diodes of Q1-Q4provide AC-DC rectification. In the active negative state S2-S3provide negative current to the transformer primary winding, while the antiparallel diodes of Q2-Q3provide AC-DC conversion.

Fig. 3 Equivalent circuit

2.2 Shoot-through state

Fig. 3b shows the equivalent circuit during the shoot-through state. During the shoot-through state,the upper and lower switches of any one phase leg, or both legs, are gated at the same time and there is no current transferred from the primary to the secondary side of the transformer. This shoot-through state contributes the voltage boost feature of the converter which is forbidden in a traditional VSI as it causes a short circuit across the DC-link and damages the converter[13,17]. This short circuit causes the diode VD1to reverse bias; and returns to conduction during the next active state. On the secondary side, the power is transferred from C2and C3into U2. Also, inductors L3,L4supplement the consumed energy to U2.

2.3 Zero state

During the zero state, the primary winding of the transformer is shorted, either through the upper switches S1-S3or through the lower switches S2-S4,while SW1is open which provides the freewheeling current path shown in Fig. 3c. The state of the circuit on the secondary side is the same as the shoot-through state discussed earlier.

3 Operating analysis and control strategy

3.1 Operating analysis of qZIBDC in steady state

Assume that T0is the shoot-through state interval and T1is the non-shoot-through (zero and active states)interval, with T= T0+T1as a switching cycle.

From Fig. 3b, in the shoot-through state, the following equations can be derived as

Similarly, during the non-shoot-through state for an interval T1with the equivalent circuit shown in Figs. 3a and 3c.

Applying the volt-second balance of inductor voltage, the average voltage of the inductors over one switching period (T) should be zero in the steady state.From (1) and (3), it can be derived as

Similarly, applying charge balance to capacitors over a switching period, the average currents of the capacitors over a switching period should be zero in the steady state. From (2) and (4), it can be derived as

From equations (5) to (8), the following equations can be derived as

The relation between the boost factor and the shoot-through duty cycle is graphically represented in Fig. 4. The boost ability can be improved not only by the transformer turns ratio but also by the shoot-through state, hence it has higher boost ability than traditional isolated bidirectional DC-DC converter. For stable operation, the shoot-through state should never exceed 0.5 and in practice Dstshould be less than 0.33[18]. It is important to note that in qZS converter the sum of three switching state is unity, i.e.DA+ DZ+ Dst= 1, where DZand DAare the zero state and active state duty cycle respectively.

Fig. 4 Relationship of gain B with shoot-through duty cycle Dst

The relationship between the input and output DC voltages in forward power flow is expressed as

From (11), when the input voltage is high enough,the shoot-through state can be eliminated, thus the converter performs like a traditional VSI, and when the input voltage is low, the converter utilizes shoot-through state, performing a boost operation. The traditional VSI is a buck type converter and buck operation of impedance source converter is further described in [13,21]. Thus, the qZIBDC realizes both buck and boost functions without any additional switches. In this work we are operating the converter in boost mode and buck mode of operation.

3.2 Control method of qZIBDC

The PWM shoot-through control method is applied to regulate the output voltage of the qZIBDC as shown in Fig. 5.

In the proposed modulation, the converter goes into 6 operating states (t0～ t6) in one switching period Ts: two active (t1～t2and t4～t5), two zero (t2～t3and t5～t6)and two shoot-through states (t0～t1and t3～t4). The shoot-through state is independent of the active state and both states are independently controllable for output-voltage regulation and compensation. The shoot-through state is applied to both sides of the bridge to distribute the switching stress equally, and there are two shoot-through states per switching period.The zero state is generated from the top and bottom switches alternately.

Fig. 5 Shoot-through PWM control

The gating-signal of generation logic of the proposed modulation technique is shown in Fig. 6. The saw-tooth carrier signal is used to generate the PWM signal and the shoot-through signal.

Fig. 6 Gating-signal generation logic

Unlike DAB converter which requires control of all eight switches, this converter can be controlled just by using four switches, so it has relatively easy control method. This control method is generic which can be implemented to any impedance source DC-DC converter. The control logic is simple and easily implemented in a digital controller such as a microcontroller, FPGA, digital signal processer etc.In this work, we implemented the digital control logic in a Xilinx Spartan 6 FPGA and the gating signals are shown in Figs. 7a and 7b for Dst=0.1 and 0.0 respectively. During boost-mode operation S1and S2are operated at the switching frequency while S3and S4are operated at twice the switching frequency,while during the non-boost mode all the switches are operated at the switching frequency. The buck mode operation is possible by controlling the duty cycle of the active state DAkeeping the shoot-through state duty cycle Dstzero which can be easily implemented from the proposed control structure as shown in Fig.6. During buck mode, power flow can be from high voltage to low voltage.

Fig. 7 Gating signals of S1～S4

4 Simulation and experimental results

To verify the proposed topology and modulation technique, a simulation is done in Matlab/Simulink and a 300 W prototype has been built in the laboratory.

The operating parameters for both simulation and experiments are presented in Tab. 1. The design of inductors and capacitors of impedance network is selected based on the current and voltage ripples during shoot-through and active states as described in[20,22]. The bi-directional DC-DC converter has a two operational mode- buck and boost mode. Utilization of shoot-through state leads to the boost mode of operation while controlling active state keeping zero shoot through state leads to the buck mode of operation. The output voltage can be stepped up and down not only by the transformer but also by the proposed shoot-through modulation method, so it has higher regulation range of voltage. Simulation and experiment are done for buck and boost mode of operation.

Tab. 1 Systems parameters

4.1 Buck mode of operation

For buck mode of operation, shoot-through state is kept zero and duty cycle of active state DAshould be less than 50%. The switching cycle thus consists of only active and zero state. Figs. 8a and 8b show the buck operation when the DA= 0.4, DZ= 0.6 and Dst= 0 where output voltage obtained is 42 V for the input voltage of 50 V. The voltage spikes at the input voltage waveform arises due to sudden change in the voltage level due to switching transitions. The spikes can be minimized using larger electrolytic capacitor at the input side.

4.2 Boost mode of operation

During boost mode of operation, the simulation and experiments are done with three shoot-through state, Dst= 0, 0.1, and 0.2. During boost mode, the output voltage is boosted by the factor of B as indicated in (12) and Fig. 4.

Fig. 8 Simulation and experimental results of qZIBDC for buck mode(DA = 0.4, DZ =0.6, Dst = 0.0)

4.2.1 Case1: Dst= 0.0

The first experiment was performed when the input voltage was 50 V with no shoot-through state,Dst=0. In this configuration, the operating frequency of all the switches is the switching frequency fs,reducing the switching loss. Figs. 9a and 9b show the simulation and experimental results of the input/output voltage, transformer voltages of the converter in the zero shoot-through state respectively, resulting in an output equal to the input voltage which is consistent with the theoretical results from (12). The converter can operate in this state when the input voltage is equal to or greater than the desired output voltage.

4.2.2 Case2: Dst= 0.1

In this case the shoot-through duty cycle was set to be 10%, and the simulation and experimental results shown in Figs. 10a and 10b. The qZIBDC works in the boost conversion mode, where a 50 V input is boosted to 62 V. In this operating mode the operating frequency of the first-leg transistors are at fswith 2fsfor the second-leg transistors.

Fig. 9 Simulation and experimental results of qZIBDC with Dst = 0.0

4.2.3 Case3: Dst= 0.2

The third experiment was performed with the shoot-through duty cycle of 20%, where the input voltage is boosted to 83 V. The transformer voltage is symmetric along the x-axis so that the average transformer voltage is zero over a period preventing transformer core from saturation and reduce the output-voltage ripple. The simulation and experimental results of the input/output voltages and the transformer primary voltage under the conditions is shown in Figs.11a and 11b.

Fig. 10 Simulation and experimental results of qZIBDCwith Dst = 0.1

Fig. 11 Simulation and experimental results of qZIBDCwith Dst = 0.2

The proposed modulation technique achieves soft switching on all the switches as shown in Figs.12 and 13. All the switches are operated with zero-voltage switching improving the overall efficiency of the converter and reducing EMI. Unlike DAB in which the soft switching is limited for heavy load condition,this modulation scheme provides ZVS on all switches irrespective of the load. Switches S2and S4follow the same switching pattern as S1and S3respectively.Although the switches S3and S4switched twice the switching frequency, the switching loss is minimum due to its soft-switching capability.

Fig. 12 Drain-to-source voltage/current of S1 and transformer primary voltage

Fig. 13 Drain-to-source voltage/current of S3 and transformerprimary voltage

Fig. 14 shows how the efficiency of the proposed converter varied with the load for a 10% shoot-through duty cycle. The efficiency is calculated from 0～300 W and the efficiency increases as load increases to the rated value. In the experiment carried out, the peak efficiency of 94% was observed.

The losses across various active components of the converter add to the efficiency pattern of the converter as shown in above figure. These are, but not limited to, the conduction losses, switching losses,core losses (across the magnetic components) and stray losses. A detailed overview and mathematical investigation of these losses is presented in [23].

5 Conclusions

This paper proposed a novel configuration for an isolated bidirectional DC-DC converter based on two switched quasi-Z-source networks and its control strategy. Due to the symmetric structure, the operation with forward power flow and reverse power flow is identical. Compared with a dual active bridge this converter has a wide input/output voltage operational range and high reliability. The converter can work either in buck or boost mode, and both ports are identical which improves the flexibility for installation of power sources. Also, the source connecting the dc terminal can be either a voltage or a current source.Simulation and experimental results demonstrated the suitability of the proposed topology and modulation method.