1. Title: - Design of Buck ZCS Resonant-Switch Dc-Dc Converter Using SCR 2. Project Description This project is about a design of a power electronic product. That product should convert 100V dc into 12V dc and should have 1kW capacity with high efficiency. It will be useful in electrical vehicles and places where the dc transmission lines are available. Participants Mr. Vijayakumar K., Email: - vijay@iee.org MEPE student, DEEE, KU, Nepal Dr. P. Freere, Email: - pfreere@ku.edu.np Supervisor, DEEE, KU, Nepal. Mr. K. Gurung, Email: - krishna@ku.edu.np Supervisor, DEEE, KU, Nepal.
3. Background Normally in the dc-dc converters the entire load current is turned on and off during each switching. The Figure 3.1 shows that how the voltage across the switch, current through the switch and switching power losses at the switch change as a result of these types of switching.
(a) Turn on and turn off of switch
(b) Vs , Is and Ps during turn on and turn off of a switch Figure 3.1 Turn on and turn off of switch 1
Due to these types of switching, it is obvious that the following shortcomings can occur, • High switching stresses • High switching losses • Electromagnetic interference due to high di and dv dt dt In the case of converter designs , high frequency switching are used to reduce the size of the converter, but the switching losses are linearly proportional to the switching frequency. Therefore it becomes important to minimize the aforementioned shortcomings. If the turn on and turn off of a switch in the converter are done when the current through the switch is zero or the voltage across the switch is zero or both are zero, we can minimize these shortcomings. Therefore the efficiency of the converter can be increased. In my project by using zero current switching (ZCS) resonant-switch, the 100/12V dc-dc converter is going to be designed. The basis circuit which is going to be used for my project is shown in Figure 3 .2.
Figure 3.2 Basic circuit of ZCS resonant-switch converter By means of using the theory described in Appendix A, we can select the proper values for Lr and Cr to make the current i T come to zero and turn on and off the switch when current i T is zero. Not only that, the output voltage can be controlled by varying the switching frequency. It will be useful to regulate the output voltage. As the expected output voltage and power are 12V dc and 1kW respectively, the output current will be very high. Therefore it is better to use thyristor as switch because thyristors have high current ratings. Not only that if we give the gate pulse to the thyristor, it will turn on but we need not to worry about the turn off. Because when the current iT comes to zero thyristor will turn off itself. However, it is necessary to design the protection circuit if the current flows more than rated value of thyristor. This converter product can be used in electric vehicles and places where the dc transmission lines are available. In the latter case this will function as dc step down transformer with 1 kW capacity. As the switching losses are reduced considerably, its efficiency will increase. Therefore these types of product will help to save the energy.
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4. Statement of Goals and Objectives In this project, the aim is to design a buck ZCS resonant-switch dc-dc converter. That product should satisfy the following objectives, • • • • • • The product should be able to convert 100V dc into 12V dc. The rated power of the converter should be around 1kW. The efficiency is expected considerably high. The product should be made using ZCS resonant-switch. The proper protection system should also be available. The thesis work and presentation should be done according to the work carried out.
5. Methodology Based on the objectives, to reach the aim, the following works are going to be carried out • Theoretical study about buck ZCS resonant-switch dc-dc converter should be made using the relevant materials such as books, journals, etc. During this study it is important to understand the operation of each component to reach the project aims. It is important to simulate the circuit using PSIM software to reach required output for proper input and switching frequency. The important waveforms got from simulation should be compared with the waveforms collected from theoretical study. After that, it becomes necessary to make an analytical report about the operation of the converter. This report should include the limitation of the converter design. Next step is that again simulate the circuit using the limitation got in analytical report. Then the circuit should be implemented to reach the proper output and other requirements mentioned in the objectives. Next, the written work should be done on the basis of work carried out during the project and thesis should be submitted. This should include the theory used for this project, practical works done and problems faced during the project and the result got in the project. Finally, the presentation should be made according to project works.
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6. Tools Some special hard wares and soft wares which are going to be used in my project are listed below and under their title it is mentioned that for what purpose it was selected. 6.1 SCR, Electronic Components and Instruments In my design, the scr which is normally known as thyristor is going to be used as switching purpose because it will automatically turn off when current though the scr comes to zero. Therefore it is useful to use for ZCS resonant-switch converter. Not only that, thyristors are available at high current ratings. The other components and instruments which are available in Power Electronic Laboratory are going to be used for my project work. 6.2 PSIM In my project work, PSIM is going to be used as simulation package as it is easy to use and fast to operate. But, it only deals with ideal devices as ideal switches. Hence it is limited when the circuit is dependent on the detailed parameters of the device. However, for testing basic circuit configurations, it is very useful. Not only that, it has some other benefits as mentioned below. 1. The user can specify the component values 2. Simulation can be run with user determined time step 3. Simulation results can be got graphically 4. More than one graph can be compared at the same time 5. Simulation can be run at desired frequencies 6. Software library has all the components needed for my project 6.3 Matlab It is going to be used to plot the curve according to the results found out in the theoretical study and to solve any mathematical function when it is needed. 6.4 Ms Visio, Ms Word and Ms PowerPoint These tools are going to be used only for thesis work and presentation. Ms Visio is useful to draw the drawings and circuit diagrams easily and quickly, because it has large library. Ms Word and Ms PowerPoint are normally used for writing thesis work and presenting the work done respectively.
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7. Work-plan The planned work schedule for my project work is as shown in the Table 7.1 below. Planned period Planned work
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Jul
2 3 4 1
Aug
2 3 4 1
Sep
2 3 4 1
Oct
2 3 4 1
Nov
2 3 4
Theoretical study about ZCS resonant-switch dc-dc converter Simulation of the circuit using PSIM software Making an analytical report Implementation of the circuit Writing a thesis report Table 7.1 The proposed project schedule
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Appendices Appendix A Series-Resonant Circuit with a Capacitor-Parallel Load
Figure A.1 Series-resonant circuit with capacitor parallel load [1] The series-resonant circuit with a capacitor in parallel with a current Io represented as load is shown in Figure A.1. Here Vd and Io are dc quantities and their initial conditions at time t0 are IL0 and Vc0 respectively. vc = Vd - Lr diL (A-1) dt iL - ic = Io (A-2) By differentiating equation A-1 ic = Cr dvc = -LrCr d2iL (A-3) dt dt2 Substitute ic from equation A-3 into equation A-2 d2iL + ? 20iL= ? 20 Io (A-4) dt2 Here ? 0 is the angular resonance frequency and defined as follows ? 0=2p f0= (A-5) vLrCr The solution of these equations for t = t0 is as follows: iL(t) = Io + (IL0-Io )cos ? 0(t-t0) + Vd-Vc0sin ? 0(t-t0 ) Z0
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(A-6)
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and vc (t) = V d -(Vd-Vc0)cos ? 0(t-t0) + Z0(IL0-Io) sin ? 0 (t-t0) (A-7)
Here ? 0 is the angular frequency and Z0 is the characteristic impedance as mentioned in equation A-8. Z0 = Lr (A-8) Cr If the initial case at t=t0 Vc0=0 and IL0=Io iL(t) = Io + Vd sin ? 0 (t-t0) (A-9) Z0 vc (t) = Vd [1 cos ? 0(t-t0 )] (A-10) The waveforms of vc and iL Vs t when the Vco=0, IL0=Io =0.5 per unit is as shown in Figure A.2.
Figure A.2 vc and iL Vs t when the Vco=0, IL0=Io=0.5 [1]
ZCS Resonant-Switch Converter The ZCS resonant-switch converter is shown in Figure A.3. If the inductance Lf is selected as large, the current Io can be assumed as constant. At steady state the waveforms of iT Vs t and vc Vs t will be as shown in Figure A.4.
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These wave forms can be explained using 4 modes of operation as shown in Figure A.5 (a)-(d).
Figure A.4 Waveforms of iT Vs t and vc Vs t at steady state When the diode is conducting , if the switch is turned on at time t0, the voltage across inductor Lr becomes Vd. Therefore, the current iT will start to increase. At the same time the current through the diode decreases at the same rate because Io is constant. At this time the circuit will be as shown in Figure A.5 (a). This mode will continue until iT reaches Io at time t1. From time t0 to t1 the voltage vc will be zero because diode (Assume diode is ideal) conducts.
(a)
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At time t1 the circuit becomes as shown in Figure A.5 (b), because the current through the diode comes to zero and excess current, (iT-Io) will charge the capacitor Cr. Compare this circuit with the Figure A.1. At initial state t=t1 Vc0=0 and IL0=Io, therefore the equation A-9 and A-10 will be re-written as follows iL(t) (A-11) vc (t) (A-12) In this mode, iT Vs t and vc Vs t will show sinusoidal variation. The maximum current will be (Io+V d/Zo) at time t’1 . At the same time the voltage wave reach Vd. At time t’’1 the voltage will reach its 2Vd as its maximum. At the same time the current will come back to Io. If Io < Vd the current will come to zero. Zo This depends on the value selected for Z0. If we use thyristor as switch there will be no reverse current. Now the current Io will be supplied by the capacitor. The circuit will be as shown in Figure A.5 (c). When capacitor discharges voltage across it will decrease. If the voltage reaches zero the diode will start to conduct as shown in Figure A.5 (d). If the switch is turned on again process will repeat. Appendix B References [1] N. Mohan, T.M. Undeland, and W.P.Robbins, Power Electronics, John Willey & Sons, Inc., Singapore, 2004. [2] J. Vithayathil, Power Electronics, McGraw-Hill, Inc., USA, 1995 [3] P.S.Bimbhra, Power Electronics, Khanna Publishers, 3 rd edition, Delhi, 2004 [4] M.H.Rashid, Power Electronics Hand Book, Academic press, Canada, 2001 9 Z0 = Vd [1 cos ? 0(t-t1 )] = Io + Vd sin ? 0 (t-t1)
[5] P. Horowitz, and W. Hill, The Art of Electronics, Cambridge University Press, USA, 1991 [6] P. Freere, Practical Dc-Dc Converter, Lecture notes, Kathmandu University, 2005 [7] P. Freere, Recent Development in Power Electronics, Lecture notes, Kathmandu University, 2005
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