Soft-switching High Frequency AC Link Buck-Boost DC-DC Converters. By TAO WANG B.S., Shandong Agricultural University, China, 2013 THESIS

Transcription

1 Soft-switching High Frequency AC Link Buck-Boost DC-DC Converters By TAO WANG B.S., Shandong Agricultural University, China, 2013 THESIS Submitted as partial fulfillment of the requirements for the degree of Master of Science in Electrical and Computer Engineering in the Graduate College of the University of Illinois at Chicago, 2015 Defense Committee: Mahshid Amirabadi, Chair, Advisor Milos Zefran, Professor and Director of Graduate Studies Siavash Pakdelian, Research Assistant Professor

2 ACKNOWLEDGEMENTS I would like to show the heartiest gratitude to my supervisor Dr. Mahshid Amirabadi of Electrical Engineering for her valuable and patient help, and continuous encouragement in my research. I could not have a better professor like her. She helps me with the best method and attitude for my research. From her I can learn who is a good professor and life mentor. I would like to give my thanks for my parents Lixian Wang and Min Yu, who give me life support. Under the help of them, I can concentrate all my mind into research. Love from them can let me overcome any dilemma. From them, I can learn their attitude and experience of life. I could not love them more. Also I need to express thanks for my friends in our lab. They help me with problems during research. We can communicate with each other with problems we met in common life, which is a great help in learning and development. TW ii

3 To my parents, Lixian Wang and Min Yu To my sister, Yanli Wang iii

4 TABLE OF CONTENTS CHAPTER PAGE 1 LITERATURE REVIEW Parallel ac link universal power converter SOFT-SWITCHING AC LINK DC-DC CONVERTER (TOPOLOGIES AND PRINCIPLES OF THE OPERATION Hard-switching vs. soft-switching Principles Operation of the Soft-switching AC-link DC-DC Buck-Boost converter CONTROL OF SOFT-SWITCHING AC LINK DC-DC CONVERTER Control Method I PI controller combined with PWM Control Method II SIMULATIONS AND EXPERIMENTS Simulations using Control Method I Simulations using Control Method II Experimental results SUMMARY AND FUTURE WORK CITED LITERATURE PUBLICATION VITA iv

5 LIST OF TABLES TABLE PAGE I Polarity of the link current and voltage during different modes II Parameters of Designed and Simulated Unidirectional soft-switching dc-dc Ac link buckboost Converter III Parameters of prototype v

6 LIST OF FIGURES FIGURES PAGE 1 A modified the dc-dc buck-boost converter Dc-dc ac-link buck-boost converter Soft-switching bidirectional ac-ac ac link buck-boost converter Bidirectional soft-switching ac-link dc-dc converter Link current and voltage in dc-dc configuration One step down conversion with resonant Voltage and current in switch of soft-switching Soft-switching feature Behavior of the bidirectional soft-switching ac link converter when the power flows from V1 to V Behavior of the bidirectional soft-switching ac link converter when the power flows from V2 to V Unidirectional soft-switching ac link dc-dc converter Behavior of the unidirectional soft-switching ac link converter Simplified soft-switching ac link dc-dc converter Behavior of the simplified unidirectional soft-switching ac link converter Simplified step up soft-switching ac link dc-dc converter for step-up applicatoins Control Method I Control method II Output voltage and current (Vo and Io) of the proposed converters at full load (control method I) Input voltage and current (Vin and Iin) of the proposed converters at full load (control method I) vi

7 LIST OF FIGURES(continued) FIGURES PAGE 20 Link voltage and current (Vlink and Ilink) of the proposed converters at full load (control method I) Link voltage and current (Vlink and Ilink) of the proposed converters at half load (control method I) Output voltage and current (Vo and Io) of the proposed converters when the load changes (control method I) Link voltage and current (Vlink and Ilink) of the proposed converters when the load changes (control method I) Input voltage and current of the original configuratoin at full load Output voltage and current of the original configuration at full load Link voltage and current (Vlink and Ilink) of the original configuration at full load Output voltage and current (Vo and Io) of the proposed converters at full load (control method II) Link voltage and current (Vlink and Ilink) of the proposed converters at full load (control method II) Output voltage and current (Vo and Io) of the proposed converters when the load changes (control method II) Link voltage and current (Vlink and Ilink) of the proposed converters when the load changes (control method II) Link voltage and current (Vlink and Ilink) of the converter shown in Fig.13 during step down operation Input voltage and current (Vin and Iin) of the original configuration at full load (control method II) Output voltage and current (Vo and Io) of the original configuration at full load (control method II) vii

8 LIST OF FIGURES(continued) FIGURES PAGE 34 Link voltage and current (Vlink and Ilink) of the original configuration at full load (control method II) Input current (blue) and voltage (yellow) of converter operating at 400 W Load current (pink) and voltage (green) of converter of converter operating at 400 W Link voltage (pink) and link current (green) of converter operating at 400W viii

9 CHAPTER 1 LITERATURE REVIEW Ac link universal power converter is a relatively new class power converter with numerous advantages. In these converters the inputs and output can be either dc or ac, multi-phase or single-phase; therefore, these converters can be configured as dc to dc, dc to ac, ac to dc, or ac to ac [1]. Given their good performance, the ac link universal converters can be efficiently and widely used in variously applications, including electric vehicles (EV), solar, and wind power generation. This thesis focuses on dc-dc parallel ac-link universal converter, which is in essence a soft-switching ac-link buck-boost converter. There has been a significant amount of research focusing on different types of dc-dc converters, such as novel topologies, modeling, and control schemes [2-6]. The ac-link universal converter is an extension of the buck-boost converter, in which the inductor current does not have a dc component and the switches have soft-switching. A brief review on this new class of converters is present in this chapter. Also, novel dc-dc configurations, which are modifications of this converter, are introduced. 1.1 PARALLEL AC LINK UNIVERSAL POWER CONVERTER In this section parallel ac-link universal converter is introduced. It was first proposed in [7]. This converter is an extension of traditional dc-dc buck-boost converter. In this converter complementary switches are added and also the converter scheme is modified so that the switches benefit from the soft-switching. A link inductor in this converter is used as the main 1

10 2 energy storage component with ac link current. The parallel inductor is fully utilized with ac link current. The buck-boost converter with three extra switches is shown in figure 1. S1 I1 I2 S2 + V1 S3 IL L VL _ S4 V2 Figure 1 A modified the dc-dc buck-boost converter Current in the link inductor has a dc component. To make sure the inductor does not saturate, the dc component of the inductor current is eliminated, and the inductor current becomes ac. To realize ac current in the link, four more switches are added to this converter. Two switches are added to input side and two switches are added to the output side based on the converter as shown in Figure 2. During the first mode, switches S1 and S3 are turned on, charging the inductor in a positive direction. In mode 2, switches S2 and S4 are turned on, with link charging the output in a positive direction. In mode 3, the input V1 charges the link in a

11 3 reversed direction, with switches S5 and S7 turned on. In mode 4, switches S6 and S8 are turned on, and link charges the output V2 in a negative direction. S1 S5 S2 S6 IL S7 V1 S3 L S8 V2 S4 Figure 2 Dc-dc ac-link buck-boost converter To have more number of phases at the input or output more legs should be added. For ac sources and loads, each leg needs to have four switches. Similarly, for bidirectional dc-dc converters four switches are needed at each leg. To provide soft-switching a small capacitor is added to the link. The three-phase ac-ac and bidirectional dc-dc configurations are shown in figures 3 and 4, respectively. Soft switching is ensured by forming resonant ac-link during a switch transition. Switches are turned on at zero voltage and benefit soft turn-off. The converter, with a parallel inductor and capacitor, is named parallel ac link universal power converter.

12 4 AC L C AC Figure 3 Soft-switching bidirectional ac-ac ac link buck-boost converter S1 S13 S2 S14 S9 S5 S10 S6 L C S15 V1 S3 S16 V2 S4 S7 S11 S8 S12 Figure4 Bidirectional soft-switching ac-link dc-dc converter In a parallel ac link universal power converter, there is a special mode called resonant mode between each charging mode and discharging mode. Resonant mode facilitates zero voltage turn-on and soft turn-off of the switches. The resonant time is very short compared to the whole link cycle.

13 5 Link current and voltage of the dc-dc configuration is depicted in Figure 5. Inductor current is alternating with zero dc component. Figure 5 Link current and voltage in dc-dc configuration The parallel ac-link universal converters were first introduced in [7]. The application of this converter in renewable energy systems as well as its analysis and design were studied in [8-11]. Given the large number of switches, the control of the parallel ac-link universal converter is complex. In previous works including [8-11] digital control has been mainly used. The main candidates of digital control are FPGA (Field-Programmable Gate Array) and microcontroller. The use of a microcontroller limits the maximum link frequency due to its long execution time. Furthermore, the use of FPGA or microcontroller increases the cost of converters. Therefore, one the objectives of this thesis is simplifying the control scheme such that it can be implemented by analog circuits.

14 CHAPTER 2 SOFT-SWITCHING AC LINK DC-DC CONVERTER (TOPOLOGIES AND PRINCIPLES OF THE OPERATION) In this chapter, soft-switching ac-link buck-boost converter is introduced, and the principles of the operation of this converter are fully explained. 2.1 Hard-switching vs. soft-switching Increasing the power density of the power converters has been one of the active research areas in power electronics. One method to decrease the volume of the passive elements is increasing the frequency. However, increasing the frequency results in increasing the switching losses and reducing the efficiency. Switches are not ideal devices in reality, and when a switch is turned on its voltage does not drop to zero immediately. Therefore, there exists a fall time before voltage reaches zero. At the same time, switch current does not rise up to load current immediately when the switch is turned on, and there exists a rising time. During this time, there is an overlap area where voltage and current of the switch are both non-zero, leading to switching loss, which is called turn-on loss. Similarly, when a switch is turned off, its voltage does not rise up to the supply voltage immediately, and switch current does not drop to zero immediately. Therefore, during the switch turn off there will be switching loss also called turnoff loss. The hard-switching process is shown in Figure. 6. Total loss in switch is small and 6

15 7 negligible when switching frequency is under 1 KHz. However, in most power converters switching frequency are much higher than 1 KHz. The switching loss is too high to be negligible in high switching frequency operation. I-switch V-switch Ploss(On) Ploss(Off) Figure 6. Harding switching process To overcome this limitation in power converters, soft-switching configurations have been proposed. Compared to hard-switching, soft-switching is an important improvement to reach low switching loss. To have zero turn-off losses, switch is turned off when current reaches natural zero. To have zero turn-on losses switch is turned on when its voltage reaches zero. In soft-switching configurations there does not exist obvious overlap between voltage and current during turn-on and turn-off. Thus switching loss is greatly reduced. There are two soft-switching methods, zero voltage switching (ZVS) and zero current switching (ZCS). Voltage and current characteristics for switch with soft-switching are depicted in the Figure 7. Figure 7 depicted a ZVS in which the switch voltage is reduced to zero before current

16 8 start to increase. Figure 7 b shows ZCS, in which switch current is reduced to zero before voltage stat to rise. a.turn-on b.turn-off Figure 7 Voltage and current in switch of soft-switching In the soft-switching ac-link buck-boost converter, soft-switching is realized with the help of a parallel capacitor. The switches benefit from zero voltage turn-on. During turn-off, the link capacitor slows down the switch voltage rising, which results in negligible turn-off losses. Figure 8 shows the link voltage and current of a switch during turn-off.

17 9 Figure 8 Soft-switching feature 2.2 Principles of the operation of the Soft-switching AC-link DC-DC Buck-Boost converter The circuit diagram of the bidirectional soft-switching ac-link buck-boost converter is depicted in figure 4. In this configuration power can be transferred from V1 side to V2 side, as well as from V2 side to V1 side. Switches S1-S8 are used for power transfer from V1 to V2, and switches S9-S16 are used for transferring the power from V2 to V1. For each case, four switches are used during the first half link cycle that the link inductor is positive and the other four switches are used during the second half cycle that the inductor current is negative. The utilization of the inductor in the soft-switching ac-link buck-boost converter is increased due to its alternating current. Since power transfer from V1 side to V2 side is similar to power transfer from V2 to V1, only one case, which is power flowing from V1 to V2, is studied here. The behavior of the

18 10 converter for this case is shown in Fig. 9. In each link cycle, there are 8 modes of operation, including 4 power transfer modes and 4 resonant modes, occurring alternately. The link is charged from V1 during modes 1 and 5; it is discharged to V2 during modes 3 and 7; and resonates during mode 2, 4, 6, and 8. In all previous work relevant to parallel ac-link universal converters [6-10], the inductor operates at the boundary of the CCM and DCM. In this thesis operating in DCM is considered as well. First, the operation of the converter when operating at the boundary of CCM and DCM is presented. Mode 1(Energizing): During mode 1, switches S1 and S3 connect input V1 to ac link, charging the ac link inductor in a positive direction. In this mode, link voltage and current are both positive which are separately equal to input voltage and unfiltered current. Turning off switches S1 and S3 ends this mode. Mode 2(Resonant): In mode 2, there are no switches conducting and the ac link resonates. The link voltage decreases and its polarity changes. The link voltage decreases until it becomes equal to -V2. At this moment, switches S2 and S4 that have been previously turned on start to conduct. The link circuit in resonant mode acts as a simple LC circuit with initial link voltage equal to V1.

19 11 Mode 3(De-energizing): In mode 3, switches S2 and S4 conduct and ac link is connected to output side. Voltage across the ac link is V2. Link currents start to decrease since the ac link voltage is negative. In mode 3, link current is positive and link voltage is negative. At the end of mode 3, switches S2 and S4 are turned off. These two switches are turned off before the link is fully discharged. The energy left in the ac-link allows link voltage swing to -Vmax. V1 and V2 is slightly lower than Vmax. Allowing the link voltage to swing to Vmax makes it possible for the input side switches to have soft-switching regardless of the input and output voltage values. Mode 4(Resonant): In mode 4, ac link circuit has another resonant mode. At the beginning of mode 4, link voltage equals to V2 and then decreases to Vmax. When the link voltage reaches its negative peak value, -Vmax, link current becomes zero and its polarity changes to the opposite. Switches S5 and S7 are turned in this mode; however, they do not conduct. Mode 5(Energizing): In mode 5, switches S5 and S7 connect input V1 to the ac link, charging the ac link inductor in the negative direction. In this mode, link voltage and current are both negative and equals to input voltage and unfiltered current, respectively. At the end, switches 5 and 7 are turned off. Mode 7(De-energizing): In mode 7, switches S6 and S8 connect the link to the load, and voltage across the ac link is V2. Link current increases since the link voltage is positive. In

20 12 mode 7, link current is negative and link voltage is positive. At the end of mode 7, switches S6 and S8 are turned off. The two switches are turned off before the link is fully discharged to the output side. Thus, energy left in the link makes link voltage swing to V max. This allows the input side switches to have soft-switching no matter what input and output side voltage values are. Mode 6, 8(Resonant): In mode 6 and 8, link inductor and capacitor form a resonating circuit. Table I shows the polarity of the link voltage and current in the eight modes of operation when the converter operates at the border of DCM and CCM. The behavior of the converter during different modes of operation when the power flows from V2 to V1 is shown in Figure 10. Table I Polarity of the link current and voltage during different modes Mode Polarity of VLink Polarity of ILink Switches turend on S1 S3 2 + and - + None S2 S and- None S5 S7 6 -and + - None

21 C DC S6 S and + None S1 S13 S2 S14 S9 S5 S10 S6 S15 V1 S3 I_Link S16 V2 S4 S7 S11 S8 S12 a) Mode 1 S1 S13 S2 S14 S9 S5 I_Link S10 S6 S15 V1 S3 S16 V2 S4 S7 S11 S8 S12 b) Modes 2, 4

22 14 S1 S13 S2 S14 S9 S5 S10 S6 S15 S7 V1 S3 S11 I_Link S16 S8 V2 S4 S12 c) Mode 3 d) Link current in one full cycle Figure 9 Behavior of the bidirectional soft-switching ac link converter when the power flows from V1 to V2

23 C 15 S1 S13 S2 S14 S9 S5 S10 S6 ` S15 V1 S3 I_Link S16 V2 S4 S7 S11 S8 S12 a) Mode 1 S1 S13 S2 S14 S9 S5 S10 S6 S15 S7 V1 S3 S11 I_Link S16 S8 V2 S4 S12 b) Mode 3 Figure 10 Behavior of the bidirectional soft-switching ac link converter when the power flows from V2 to V1

24 16 S1 S5 S2 S6 S7 V1 S3 L C S8 S4 Figure 11 Unidirectional soft-switching ac link dc-dc converter For applications that bidirectional flow of power is not required the converter can be simplified as shown in figure11. In this converter power can only flow from V1 to output side. The principles of the operation of the unidirectional soft-switching ac link converter, is shown in Figure 12. There are eight modes in one switching cycle.

25 17 S1 S5 S2 S6 I_Link C S7 V1 S3 L S8 S4 a) Mode 1 S1 S5 S2 S6 I_Link C S7 V1 S3 L S8 S4 b) Mode 2,4

26 V1 18 S1 S5 S2 S6 I_Link C S7 V1 S3 L S8 S4 c) Mode 3 S1 S5 I_Link C S2 S6 S7 V1 S3 L S8 S4 d) Mode 5 S1 S5 I_Link C S2 S6 S7 S3 L S8 S4 e) Mode 6,8

27 V1 19 S5 S6 S1 I_Link C S2 S7 S3 L S8 S4 f) Mode 7 Figure 12 Behavior of the unidirectional soft-switching ac link converter To simplify the configuration and reduce the number of switches, a simplified configuration is proposed here. This converter is shown in Figure 13.As seen in this figure, only 5 switches used in this configuration. Switches S4, S6, S2 and S8 in Figure 11 are removed in this configuration. To have control over discharging mode, one switch, S9, is added to the output side. Link current and link voltage are same as those of the converter depicted in Figure11. Compared to the original converter shown in Fig. 11, this simplified configuration is more efficient and less costly due to the great reduction in switch count.

28 Load 7 V Figure 13 Simplified soft-switching ac link dc-dc converter Figure 14 shows the principles of the operation of the simplified unidirectional softswitching ac link converter. Similar to the original configuration, there are eight modes in one switching cycle, two charging mode, two discharging mode, and four resonant modes. Mode 1(Energizing): Similar to the original configuration during mode 1, switches S1 and S3 conduct and the link inductor is charged in the positive direction. Mode 2(Resonant): In mode 2, the ac link resonates and no switches or diodes conduct during this mode. The link voltage decreases and when it becomes negative switch S9 is turned on. However, since diodes D2, D4, D6, and D8 are all reverse-biased this switch does not conduct. The link voltage decreases until it is equals to -V2. At this point, diodes 2 and 4 become forward biased, and they start conducting. Switch S9 conducts, too.

29 21 Mode 3(De-energizing): In mode 3, D2, D4, and S9 conduct, and voltage across the link is equal to V2. Link current decreases in this mode since the link voltage is negative. In mode 3, link voltage is negative and link current is positive. At the end of mode 3, switch 9 is turned off to allow the link resonate during mode 4. Mode 4(Resonant): In mode 4, all switches are turned off and the ac link resonates. At the beginning of mode 4, link voltage is equal to V2 and then resonates to V max. At this point switches S5 and S7 are turned on; however, they do not conduct before the link voltage resonates to V1. Mode 5(Energizing): In mode 5, switches 5 and 7 conduct and when the link is sufficiently charged these switches are turned off to allow the link resonate during mode 6. Mode 6 (Resonant): the link resonates and when the link voltage becomes positive switch S9 is turned on; however, it does not conduct immediately. When the link voltage becomes equal to V2, mode 7 starts. Mode 7(De-energizing): In mode 7, diodes 6 and 8 are forward biased, allow switch S9 conduct. D6, D8, and S9 connect the load to the ac link.

30 V1 3 I_Link Load g) Mode V1 3 I_Link Load h) Mode 2

31 V1 3 I_Link Load i) Mode V1 3 I_Link Load j) Mode 4

32 V1 3 I_Link Load k) Mode V1 3 I_Link Load l) Mode 6

33 V1 3 I_Link Load m) Mode V1 3 I_Link Load n) Mode 8 Figure 14 Behavior of the simplified unidirectional soft-switching ac link converter

34 26 If the simplified unidirectional converter, shown in Figure. 13, is used as a step-up converter, i.e. V2 > V1, switch S9 can be removed. This configuration is shown in figure. 15. In this converter, the number of switches are half the number of switches needed in the original unidirectional configuration, shown in Figure 11. This greatly reduces the cost, power dissipation, and reliability. If this converter is used in step-down mode, i.e. V2 < V1, at the beginning of the resonating modes 2 and 6, link voltage will be higher than the output voltage; therefore, diodes at the output side become forward biased and start to conduct. This initiates the discharging mode at the wrong time. The output switch, S9 in Figure 13, is added to prevent this malfunction. However, when the converter is used as step up converter, this switch is not necessary. A similar method had been proposed in [12] for reducing the number of switches in a three-phase rectifier Load 7 V Figure 15 Simplified step up soft-switching ac link dc-dc converter for step-up applications

35 CHAPTER 3 CONTROL OF SOFT-SWITCHING AC LINK DC-DC CONVERTER In the previous chapter the principles of the operation of the soft-switching ac-link buckboost converter were introduced. It was assumed that regardless of the power level the converter works at the boundary of DCM and CCM. This allows link inductor to be small. The link and switching frequency will be different at different power levels. It can be shown that by decreasing the power level the link frequency increases. If the switching frequency is variable, Pulse Width Modulation (PWM) cannot be used. In this part, two control methods for soft-switching ac-link buck-boost converters are introduced. The first control method allows the converter to operate at a fixed switching frequency, whereas in the second control method the link and switching frequencies are variable. In bidirectional configuration two controllers are needed. One is used to regulate voltage for power conversion flowing from V2 to V1, and the other is used to regulate voltage when power flows from V1 to V Control Method I PI controller combined with PWM The first control method uses Pulse Width Modulation (PWM) technique. Similar to classic PWM control, it employs a PI controller to regulate the current or voltage. For this control 27

36 28 control method the switching frequency, and consequently the link frequency, need to constant at different power levels. Therefore, the converter cannot operate at the boundary of CCM and DCM at all power levels. The controller is designed such that at full power the converter operates at the boundary of CCM and DCM, and at lower power levels it operates in DCM. In the soft-switching configuration when none of the switches conduct, the link resonates. Therefore, the link inductor current will be sinusoidal instead of remaining zero. The task of the controller is to turn on and turn off the switches to regulate output voltage or output current. To simplify the control scheme we may use the polarity of the link current and voltage as listed in Table I to determine the switches that are supposed to be turned on during each mode. Thus, the switches are turned on based on the signal combination of polarity of link voltage and link current. To end each non-resonating mode, the conducting switches need to be turned off, and for the input-side switches this may be done by a PI controller that regulates the filtered load voltage. The block diagram of the first control method is shown in figure 16.

37 29 Vref Vo PI Limiter Carrier Triangle Wave Comparator Control Ilinksensed Comparator D Q Ilinkneg Vlinksensed Comparator D S ET Q Vlinkneg CLK Q Ilinkpos CLR Q Vlinkpos Control Control Vin Vlink softsw Control Ilinkpos Vlinkpos softsw To gates of 1 and 3 Figure. 16 Control Method I The controller can be implemented by an analog circuit. To generate the gate signal of the input-side switches a four ports AND logic gate whose inputs are the outputs of the PI controller, two signals that show the polarities of the link current and voltage, and a signal that prevents the input side switches from hard-switching. T In mode 1, switch S1 and S3 are turned on when the output of the PI controller is 1, the link current is positive, and the link voltage is positive. To ensure the link inductor is alternating, and each switch conducts only once in each link cycle, D flip flops are used. Furthermore, to make sure the soft-switching feature is not lost, the AND logic gate that commands the input side switches, should have another input signal that ensures the switches are reverse biased at the moment they are turned on. Details are shown in figure 16.

38 30 When the converter operates at the border of CCM and DCM, the link frequency is determined by [10]: = (, ) (1) where, P is the rated power of this converter, L is link inductance and I link,peak is the peak current of the link inductor when operating at the border of continuous and discontinuous conduction modes. When this converter operates at the boundary of continuous and discontinuous conduction modes the link peak current is given by [10]:, = (2) In (2), I1 and I2 are the input and output currents. It should be noted that in this converter the frequency of link current and the switching frequency are the same. As mentioned earlier the switching frequency in the first proposed control scheme is chosen such that at the rated load the circuit operates at the boundary of the CCM and DCM. 3.2 Control Method II In the second control scheme the converter works at the bounder of DCM and CCM at all power levels. Thus the switching frequency is variable. The switches are turned on according to Table I. The input side switches are turned off when the link is sufficiently charged, or in other words, the link peak current reaches its reference. Since the link frequency is not fixed, the PWM controller cannot be used here. Instead, the output of the PI controller is used as the

39 31 reference of the link peak current. Equation (2) shows that by increasing the output current the link peak current increases. The block diagram of this control method is presented in figure 17. At the time link current reaches the reference of the link peak current, the input-side switches are turned off so that the link current will not be increased further. To prevent the input side switches from being turned on again during mode 2, a D flip-flop should also be used. Similar to the control method I, the output side switches will be turned off when the link current is less than a certain value. Vref PI Limiter Ilinkpeakref Control Vo Ilinksensed Comparator Figure 17 Control method II

40 CHAPTER 4 SIMULATIONS AND EXPERIMENTS In this section, the performance of the bidirectional, unidirectional, simplified unidirectional soft-switching dc-dc ac-link buck-boost converter will be evaluated. The parameters of the simulated converters are listed in Table II. Moreover, experimental results corresponding to the configuration shown in figure 15, using the second control method implemented by a microcontroller are represented. TABLE II Parameters of Designed and Simulated Unidirectional soft-switching dc-dc Ac link buck-boost Converter Parameter Power Rating Link Inductance Link Capacitance Input dc Voltage Reference output Voltage Value 1500 W 20 μh 10 nf 100 V 200 V 4.1 Simulations using Control Method I 32

41 33 Figs depict the simulation results of the converters shown in Figs. 13 and 15. The frequency of the triangle waveform is khz. Fig. 18 represents the steady state output voltage and filtered output current when the converter works at full load. Fig. 19 represents the steady state filtered input current and input voltage. Link current and link voltage are depicted in Fig. 20. Since the output voltage is higher than the input voltage, the link is fully discharged in the discharging modes, and the absolute value of the link voltage decreases in mode 4. Fig. 21 shows the link current and voltage when the converter is working at half load. The link resonates during mode 4. Figs. 22 and 23 represent the transient response of the converter. The load resistance changes from 53.4 Ω to 26.7 Ω, from half load to full load. Figure. 18 output voltage and current (Vo and Io) of the proposed converters at full load (control method I)

42 34 Figure 19 input voltage and current (Vin and Iin) of the proposed converters at full load (control method I) Figure 20 Link voltage and current (Vlink and Ilink) of the proposed converters at full load (control method I)

43 35 Figure 21 Link voltage and current (Vlink and Ilink) of the proposed converters at half load (control method I) Figure 22 Output voltage and current (Vo and Io) of the proposed converters when the load

44 36 changes (control method I) Figure 23 Link voltage and current (Vlink and Ilink) of the proposed converters when the load changes (control method I) Simulation results of the original configuration, shown in Fig. 4, are shown in Figs Figure 24 depicts the steady state input voltage and filtered current. Steady state output voltage and filtered current are illustrated in figure 25. Figure 26 shows the link voltage and current (V link and I link).

45 37 Figure. 24 Input voltage and current of the original configuration at full load Figure. 25 Output voltage and current of the original configuration at full load

46 38 Figure 26 Link voltage and current (Vlink and Ilink) of the original configuration at full load 4.2 Simulations using Control Method II Figs show the simulation results based on the control method II. Fig. 27 represents the steady state filtered output voltage and current when the converter operates at full load. Fig. 28 illustrates the link current and link voltage at full load. Fig. 29 and Fig. 30 show the transient response of the converter when the load resistance changes from 53.4 Ω to 26.7 Ω. As seen in Fig. 30, when the load changes, the link frequency also changes so that the converter operates at the border of CCM and DCM regardless of the power level. Moreover, as expected, the link peak current increases when the load resistance decreases.

47 39 To verify the performance of the simplified unidirectional converter in step-down operation, the simulation is repeated for the case that the output voltage is 50 V. As discussed earlier, converter shown in Fig. 15 cannot be used in this case. Fig. 31represents the link current and voltage in step-down operation. To make sure that the input-side switches are turned on at zero voltage when the input voltage is higher than the output voltage, mode 3 should be finished before the link is fully discharged. This allows the link voltage to swing to a higher voltage before the input side switches are turned on. Figure 27 output voltage and current (Vo and Io) of the proposed converters at full load (control method II)

48 40 Figure.28 Link voltage and current (Vlink and Ilink) of the proposed converters at full load (control method II) Figure 29 output voltage and current (Vo and Io) of the proposed converters when the load changes (control method II)

49 41 Figure.30 Link voltage and current (Vlink and Ilink) of the proposed converters when the load changes (control method II) Figure 31 Link voltage and current (Vlink and Ilink) of the converter shown in Fig. 13 during step down operation

50 42 In this part, simulation results corresponding to the original configuration with control methods II are present. Figure 32 shows the steady state filtered input voltage and current. Figure 33depicts the steady state output voltage and filtered current. The link current and voltage of the converter when control method II is used is shown in Fig. 34. Figure 32 Input voltage and current (Vin and Iin) of the original configuration at full load (control method II)

51 43 Figure 33 Output voltage and current (Vo and Io) of the original configuration at full load (control method II) Figure.34 Link voltage and current (Vlink and Ilink) of the original configuration at full load (control method II) 4.3 Experimental results The experimental results corresponding to the converter shown in Fig. 11 are depicted in Figs In this case the second control method is implemented on a microcontroller, and the converter is operating at 400 W. Since a microcontroller is used the frequency of the link is much lower than the simulations. TABLE III Parameters of prototype

52 44 Parameter Power Rating Value 400 W Link Inductance 800 µh Link Capacitance Input dc Voltage Reference output Voltage 400 nf 200 V 100 V The input voltage and filtered current are shown in figure 35. The input current is about 2 A. Figure 36 shows the output voltage and filtered current. The link voltage and link current are depicted in figure 37. Figure 35 Input current (blue) and voltage (yellow) of converter operating at 400 W

53 45 Figure 36 Load current (pink) and voltage (green) of converter operating at 400 W Figure 37 Link voltage (pink) and link current (green) of converter operating at 400 W

54 CHAPTER 5 SUMMARY AND FUTURE WORK In this thesis, soft-switching high frequency ac-link dc-dc buck-boost converters that belong to a relatively new class of power converters, called parallel ac-link universal power converters, were studied in detail. The universal power converters have good performance with flexible inputs and outputs, since the converter can be configured as dc-dc, dc-ac, ac-dc, or ac-ac. Also, inputs and outputs can be either single phase or multiphase. With its flexibility, this converter can be used in various applications. In this converter the switches are all turned on at zero voltage and have a soft turn-off. The current of the inductor is ac with no dc component. This prevents the inductor from being saturated. In this dissertation two modified configuration for the unidirectional dc-dc conversioner are proposed. Proposed configurations have fewer switches, which results in lower cost and higher efficiency. Moreover, in this thesis two new control methods were proposed for the soft-switching high frequency ac-link dc-dc buck-boost converter. The first control scheme uses PWM. Thus, the switching frequency is fixed in this method. In the second control method the converter operated at the boundary of CCM and DCM regardless of the power level; therefore, the switching frequency is variable in this case. Details of these two control methods were 46

55 47 presented in this thesis. Suggested future works: Implementing the proposed control schemes through analog circuit and evaluate them experimentally, Employing Wide Band gap Semiconductor devices to operate the proposed converters at very high frequencies, Using the proposed converters for developing multilevel configurations

56 CITED LITERATURE 1. Asghar Karamat, High Frequency Inverter-Transfomer-Cycloconverter System for DC to AC (3-Phase) Power Conversion, Vol. IE - 31, No. 21 May S. Ćuk, A new zero-ripple switching DC-to-DC converter and integrated magnetics, Magnetics, IEEE Transactions on, vol. 19, pp , S. Inoue and H. Akagi, A Bidirectional Isolated DC DC Converter as a Core Circuit of the Next-Generation Medium-Voltage Power Conversion System, Power Electronics, IEEE Transactions on, vol. 22, pp , M. N. Kheraluwala, R. W. Gascoigne, D. M. Divan, and E. D. Baumann, Performance characterization of a high-power dual active bridge DC-to-DC converter, Industry Applications, IEEE Transactions on, vol. 28, pp , R. D. Middlebrook, Transformerless DC-to-DC converters with large conversion ratios, Power Electronics, IEEE Transactions on, vol. 3, pp , R. L. Steigerwald, High-Frequency Resonant Transistor DC-DC Converters, Industrial Electronics, IEEE Transactions on, vol. IE-31, pp , W.C. Alexander, Universal Power Converter, US patent 2008/ A1, Jan.17, M. Amirabadi, J. Baek, H. A. Toliyat, and W. C. Alexander, Soft-Switching AC-Link three-phase AC-AC Buck-Boost Converter, IEEE Transactions on Industrial Electronics, vol. 62, pp. 3-14, M. Amirabadi, A. Balakrishnan, H. Toliyat, and W. Alexander, High Frequency AC- Link PV Inverter, Industrial Electronics, IEEE Transactions on, vol. 61, pp , M. Amirabadi, H. A. Toliyat, and W. C. Alexander, "A Multi-Port AC Link PV Inverter with Reduced Size and Weight for Stand-Alone Application," Industry Applications, IEEE Transactions on, vol.49, no.5, pp ,

57 CITED LITERATURE (continued) 11. Mahshid Amirabadi, Hamid A. Toliyat, AC-Link Universal Power Converters: A New Class of Power Converters for Renewable Energy and Transportation in Power Electronics for Renewable Energy Systems, Transportation and Industrial Application, 1st ed., Haitham Abu-Rub, Mariusz Malinowski, Kamal Al-Haddad, Ed. New York, John Wiley & Sons, August 2014, pp M. Amirabadi, Extremely Sparse Parallel AC-Link Universal Power Converters, presented at IEEE Energy Conversion Congress and Exposition (ECCE), Farzad Sedaghati, Seyed Hossein Hosseini, Mehran Sabahi, Gevork B. Gharehpetian, Extended configuration of dual active bridge DC DC converter with reduced number of switches. IET Power Electronics, 7th September Nicholas.A. Denniston, High Gain Transformerless DC-DC Converters for Rewneable Energy Sources, May Tao Wang, Soumik Sen, and Mahshid Amirabadi, Soft Switching High Frequency AC-Link DC-DC Buck-Boost Converters March [2015] IEEE 49

58 PUBLICATION The outcome of this thesis is printed with permission from: Soft Switching High Frequency AC-Link DC-DC Buck-Boost Converters, by T. Wang, S. Sen, M. Amirabadi, presented at Applied Power Electronics Conference and Exposition (APEC), Charlotte, NC, March [2015] IEEE 50

59 VITA Name: Education: Tao Wang M. S., Electrical and Computer Engineering University of Illinois at Chicago, IL Aug B.S., Electrical and Mechanical Engineering Shandong Agricultural University, China June Experience: IEEE Applied Power Electronics Conference and Exposition (APEC) Technical Sessions Speaker Membership IEEE Student Member 51