Coupled Inductor-Based Parallel Operation of a qZ-Source Full-Bridge DC-DC Converter

Abstract

This study presents a novel transformer isolated parallel connected quasi Z-source (qZ-source) full-bridge DC-DC converter that uses a coupled inductor in both the qZ-source network and output filter inductor. Unlike traditional voltage-fed or current-fed converters, the proposed converter can be open- and short-circuited without damaging switching devices. Therefore, the desired buck and boost functions can be achieved and converter reliability can be significantly improved. All the bulky inductors in the qZ-source network and output filter can also be minimized with the proposed inductor structures. A 4 kW prototype DC-DC converter is built and tested to verify the performance of the proposed converter.

I. INTRODUCTION

Two types of full-bridge (FB) transformer-isolated DC-DC converters generally exist: a voltage-fed (V-fed) FB converter and a current-fed (C-fed) FB converter. Fig. 1 shows two traditional transformer-isolated DC-DC power conversion circuits. The V-fed FB converter [Fig. 1(a)] always has a buck (step-down) function, and the upper and lower switches that consist of a phase leg cannot be short-circuited [1]. Similarly, the C-fed FB converter [Fig. 1(b)] always has a boost (step-up) function, and the upper and lower switches that consist of a phase leg cannot be open-circuited.

Fig.1.Traditional FB DC-DC converters.

Thus, the two traditional DC-DC converters shown in Fig. 1 can only produce an output voltage that is smaller than or larger than the input voltage. Furthermore, they cannot be either short- or open-circuited. Consequently, traditional V-fed and C-fed converters are susceptible to electromagnetic interference (EMI) noise, which considerably impairs their system reliability Quasi Z-source (qZ-source) or Z-source converters or inverters are introduced to overcome the aforementioned problems [1]-[3]. Fig. 2 shows an example of a transformer-isolated qZ-source FB DC-DC converter.

Fig. 2.qZ-source FB DC-DC converter.

By using a qZ-source network that consists of a diode, two inductors, and two capacitors, the main switches (S1 to S4) can be short- and open-circuited without damaging the switching devices. Therefore, the desired buck and boost functions can be achieved, which is appropriate for converters with a having wide input voltage range.

Parallel-connected power converters operate via a general method that easily increases power level in the power electronics area. Through parallel operation, low current rating semiconductor devices can be used without over-sizing a converter module [4]-[6]. When multiple converters are connected in parallel, an interleaved pulse-width modulation (PWM) method is widely used to decrease input and output current ripples. Consequently, this method can reduce input and output capacitor volumes and increase converter efficiency.

Although a qZ-source converter is resistant to EMI noise because of the presence of a qZ-source network, the two inductors and two capacitors in the qZ-source network are heavy and bulky even if the two inductors L1 and L2 are coupled. The switching frequency of the converter should be increased to decrease the size of passive components. Increasing switching frequency, however, reduces converter efficiency.

This study presents a novel transformer-isolated and parallel-connected FB DC-DC converter a qZ-source network is used to achieve the desired buck and boost functions and increase converter reliability. A specially designed coupled inductor is also used in both the qZ-source network and output filter inductor to minimize inductor current ripple and volume significantly.

With this structure, the proposed DC-DC converter exhibits the following advantages over conventional converters.

This paper is organized as follows. The principal operation of the proposed converter is reviewed in Section II. A study on the comparative analysis of current ripples on the non-coupled inductor and coupled inductor is presented in Section III. Experiment results to verify are illustrated in Section IV. Lastly some conclusions and application of the proposed converter are given in Section V.

 

II. PRINCIPAL OPERATION OF THE PROPOSED PARALLEL CONNECTED QZ-SOURCE FB DC-DC CONVERTER

Fig. 3 shows the overall circuit configuration of the proposed DC-DC converter. Two identical transformer-isolated qZ-source FB DC-DC converters are connected in parallel to increase the power rating of the converter. Conventional two-phase interleaved PWM switching is applied. Therefore, the top and bottom converters are switched at 90° apart, which results in a 180° phase shift at the top and bottom qZ-source networks and output filter inductors because of the FB structure.

Fig. 3.Proposed parallel-connected qZ-source FB DC-DC converter that uses a coupled inductor.

As mentioned in Section I, the inductors in the qZ-source network are heavy and take a significant portion of the entire converter. All the inductors (L1 to L4) at the top and bottom qZ-source networks are coupled to decrease inductor size. The two output filter inductors (Lo1 and Lo2) are also coupled by following a similar method.

A. Buck Mode Operation ( Vi>Vo )

When Vi>Vo, the proposed converter operates in buck mode. No shoot-through interval occurs in buck mode, and thus, diodes Dz1 and Dz2 in the qZ-source networks are always conducting. No energy is stored in the qZ-source inductors; hence,

Consequently, the proposed converter operates as the conventional parallel-connected V-fed FB DC-DC converter that uses PWM control. Defining D as the effective duty ratio in the output filter side, the voltage gain of the proposed converter in this mode is expressed as follows:

where n is the transformer turns ratio.

B. Boost Mode Operation ( Vi

In the boost mode, a shoot-through interval occurs at the gate signal, and energy is stored in the qZ-source inductors. The well-known voltage equations in the shoot-through (boost) mode are as follows:

where Dsh is the switch shoot-through duty ratio of the converter that is defined as Dsh=1−D.

Therefore, the voltage gain of the proposed converter in the boost mode is expressed as follows:

Given that Dsh=1−D, Equation (5) can be rewritten as follow:

Fig. 5 shows the voltage gain plot of the proposed converter as D varies when n=1. As expected, the desired buck- and boost functions can be achieved by modulating D.

Fig. 4 shows the key waveforms of the proposed converter in both buck and boost modes. Given the interleaving effect, the output (or input) current ripple is reduced. However, all inductor current ripples are not reduced even with the interleaving action. Thus, they remain bulky and heavy. A coupled inductor is used in this study to decrease inductor current ripples.

Fig. 4.Key waveforms of the proposed parallel-connected qZ-source FB DC-DC converter.

Fig. 5.Voltage gain of the proposed converter (n=1).

 

III. THE COUPLED INDUCTOR AND ITS EFFECTS

This section discusses the effect of the coupled inductor on decreasing ripple currents. Fig. 3 shows that, the four inductors in the qZ-source networks and the two inductors in the output filter are coupled. The two output filter inductors (Lo1, Lo2) are first selected for the analysis because their operation is similar to that of the conventional two-phase interleaved buck converter [7]-[13].

The two main issues of interest in the interleaved buck converter is the inductor (phase) current ripple and output current ripple, which is the sum of the two inductor currents. The current ripples in a non-coupled inductor are first analyzed to compare the ripple reduction effect.

A. Non-Coupled Inductor

Fig. 6 shows the voltage waveforms across Lo1 and Lo2. Four operating states are observed. The detailed voltage and current direction in each state is summarized in Table I by assuming that vrec1 = vrec2 = nVpn.

Fig. 6.Voltage waveform of the output filter inductor.

TABLE IOUTPUT INDUCTOR CURRENT VARIATION OF A NON-COUPLEDINDUCTOR

Given that the two inductors are non-coupled, no interaction occurs between them and the inductor current ripple is calculated as follows:

If Lo1=Lo2=L,then the inductor current ripple in a non-coupled case is expressed as follows:

Considering that the output current is the sum of iL1 and iL2, then it increases in State 1 and decreases in State 2. This cycle is repeated in States 3 and 4. Consequently, the output current ripple occurs twice at a given period Ts. The equation for the output current ripple is as follows:

Equations (7) to (10) are valid when D<0.5. When D>0.5, the equations can be derived similarly as summarized in Table II [7]. The inductor and output current ripples of the two-phase interleaved converter are compared with those of its single-phase counterpart to determine the ripple current reduction effect of the former. The results are summarized in Table II. Unlike in the single-phase converter, the output current ripple in the two-phase case is reduced significantly and its maximum effect is reached when D approaches 0.5. The inductor current ripples, however, remain unchanged [8], [9].

TABLE IICURRENT RIPPLE COMPARISON

B. Coupled Inductor

Fig. 7 depicts the proposed coupled inductor structures for both the qZ-source networks and output filter inductors. The secondary side of the transformer of the proposed converter is redrawn (Fig. 8.) to analyze the coupled inductor. The coupled inductor is represented as an ideal 1:1 transformer, two leakage inductances (Ll1,Ll2), and a magnetizing inductance (Lm). Notably, the ideal transformer is connected out of phase with the polarity dots on the opposite ends [8]. Therefore,

Fig. 7.Coupled inductor design for the proposed converter

Fig. 8.Secondary side of the proposed converter with a coupled inductor.

Similar to in the non-coupled case, four operation states exist in the coupled case. Regardless of the operation states, the following relationships are always applied:

1) State 1 [0~DTs]: Based on Figs. 6 and 8, the following equation is applied in State 1:

From Equations (12) to (14), VLl1 and VLl2 are expressed as follows:

By assuming that Ll1=Ll2=Ll and substituting Equations (15) to (16) into Equation (13), VLm is expressed as follows:

Base on Equations (15) to (17), defining p=Lm/Ll yields the following:

The output current ripple is expressed as follows:

2) State 2 [DTs~Ts/2]: In State 2, VLo1=VLo2=-Vo. Thus,

When Equations (21) to (22) are substituted into Equation (13), then VLm=0. Hence,

3) State 3 [Ts/2~(Ts/2+DTs)]

State 3 is similar to State 1. The only difference is that vLo1=-Vo and vLo2=nVpn-Vo. Using methods similar to those in State 1, the inductor and output current ripple in State 3 are derived as follows:

4) State 4 [Ts/2+DTs)~Ts]: State 4 is exactly the same as State 2.

According to the state analysis of the coupled inductor, the output current ripple in the coupled case also occurs twice at a given period Ts. The coupling ratio p does not affect the output current ripple [7]. Equations (10) and (20) show that if Llis equal to the value of the individual inductor L in the non-coupled case, then the output current ripples in both the non-coupled and coupled inductors are the same.

Unlike those in the non-coupled case, the inductor current ripples in the coupled inductor change direction twice per cycle as the output current ripple. The two inductor currents become equal and in-phase as p increases to infinity (perfect coupling). Thus, the coupling ratio p has a significant effect on inductor current ripples, although it does not affect output current ripples [7].

Representing Lland Lm in the coupled inductor (or transformer) in terms of the coupling coefficient k rather than the coupling ratio p is more intuitive because we are more familiar with k. Based on the general coupled inductor theory, Lland Lm are represented as follows [14]:

where Ls the self-inductance of the coupled inductor.

Therefore, the relationship between p and k is as follows:

Using Equation (29), Equations (18) and (19) can be rewritten as shown in Table III. The inductor current ripple depends on the D and k values. Therefore, the ratio of the inductor current ripple of a coupled inductor (ΔiL,cp) normalized with respect to that of a non-coupled inductor(ΔiL,nc) is expressed as follows:

TABLE IIICURRENT RIPPLE OF THE FILTER INDUCTOR AND OUTPUT

The relationship in Equation (30) is plotted in Fig. 9. As shown in the figure, a small current ripple can be achieved with a strong coupling coefficient. The same analysis can be readily applied to the coupled inductor in the qZ-source network. The only difference is the voltage across the qZ-source network inductors.

Fig. 9.Inductor current ripple of a coupled inductor normalized with respect to a non-coupled inductor.

Fig. 7(a) shows, that the four inductors (L1 to L4) in the qZ-source network are coupled. L1 and L2 are wound on

the same leg. Similarly, L3 and L4 are wound on the other leg. Given that the operations of top and bottom qZ-source networks are the same as that of the output coupled inductor, the same ripple reduction effect occurs in the qZ-source inductor currents and in the input current ripple.

 

IV. EXPERIMENT RESULTS

A 4 kW prototype converter is built and tested to verify the performance of the proposed converter. Table IV illustrates the electrical specifications of the proposed converter, including the detailed parameters of the coupled inductors. The transformer turns ratio is set to 1:1.

TABLE IVELECTRICAL SPECIFICATIONS OF THE PROPOSED CONVERTER AND PARAMETERS OF THE COUPLED INDUCTORS

Fig. 10 shows the 4 kW proposed converter and the photo of the coupled inductor built for this study. similar core structures are used for both the qZ-source inductors and output filter inductors. Figs. 11 and 12 show the transformer voltage and current waveforms of the proposed converter that operates in buck and boost modes. Given that the transformer current is a reflection of the output filter inductor current, the transformer current waveforms of the coupled case follow the inductor current waveforms.

Fig. 10.Photos of the 4 kW prototype converter and the coupled inductors.

Fig. 11.Waveforms of transformer voltages and currents at buck mode. ( Vtr1,2[250V/div.],itr1,2[10A/div.],[10μs/div.] Vi=600V, Vo=400V, and Po=4kW )

Fig. 12.Waveforms of transformer voltages and currents at boost mode. ( Vtr1,2[250V/div.],itr1,2[10A/div.],[10μs/div.] Vi=200V, Vo=400V, and Po=4kW )

Fig. 13 shows the current waveforms of the output filter inductor and output currents for both non-coupled and coupled cases. As expected, the inductor current ripples are reduced significantly with the coupled inductor while maintaining the same output current ripple. The two inductor currents are nearly in phase.

Fig. 13.Waveforms of the output filter inductor and output current. ( [2A/div.],[5μs/div.], Vi=600V, Vo=400V, Po=4kW )

Fig. 14 shows the current waveforms of the qZ-source inductors and input currents for both non-coupled and coupled cases. The waveforms are similar to the case of output filter inductors.

Fig. 14.Waveforms of the qZ-source inductor and input current. ( [5A/div.],[5μs/div.], Vi=200V, Vo=400V, Po=4kW )

Fig. 9 shows that the coupled inductor exhibits the smallest current ripple at D=0.5, which can be identified in Fig. 15.

Fig. 15.Waveforms of the output filter inductor and output current. ( [2A/div.],[5μs/div.], Vi=600V, Vo=300V, D=0.5 )

Fig. 16 shows the efficiency curves of the proposed converter that is measured as input voltage and output power vary.

Fig. 16.Efficiency of the proposed converter.

 

V. CONCLUSIONS

In this study presents a parallel operation of a qZ-source FB DC-DC converter that uses a coupled inductor. With this configuration, the proposed converter exhibits the following features.

A 4 kW prototype that consist of two parallel qZ-source FB DC-DC converters is built and successfully tested to verify the operation principle of the proposed converter.

The converter is as valuable as a renewable energy source with a wide output range. It has an applied battery charge discharge system with a low current ripple. Extending power by parallel connection is also easy. By using buck and boost functions, reducing the current ripple, and realizing its parallel operation, the proposed converter can be a good candidate in such systems because requires wide input voltage variation and exhibits high reliability.