EXPERIMENT 4 SWITCHED MODE DC/DC CONVERSION USING BUCK CONVERTER

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1 Introduction: YEDITEPE UNIERSITY ENGINEERING & ARHITETURE FAULTY INDUSTRIAL ELETRONIS LABORATORY EE 432 INDUSTRIAL ELETRONIS EXPERIMENT 4 SWITHED MODE D/D ONERSION USING BUK ONERTER In this experiment, characteristics of D/D switchedmode converters will be observed. An A/D converter using a linear regulator was constructed in experiment 1 and important characteristic values such as efficiency were measured. It was observed that the efficiency of linear regulator was very low. A similar A/D conversion using a switched mode converter will be performed in this experiment and it will be seen that how employment of solid state switching devices affect the efficiency of such a converter. Equipments: General Information: Table 1. List of equipments ariable Load Resistor (0100Ω) (two sets of resistors are needed) Transformer 45/90, 3N D Power Supply Reference ariable Generator Bridge rectifier Diode Load Power Electronics IGBT x 1000µF apacitors Isolation Amplifier (x 2) ontrol Unit PWM, PFM DW6060 A Power Wattmeter Metra Hit 25S Multimeter (x 2) Oscilloscope Electric and electronic devices and systems need to be supplied with power. If the power is supplied from the mains network, adaptation of the voltage amplitude and electrical isolation are usually required. These functions are performed by power supplies which are available in several topologies. Most of the linear power supplies consist of a transformer, rectifier, a charging capacitor and a linear regulator (as you have done in the second experiment). A stable current/voltage supply is achieved by the means of stabilization units. The EE432 Industrial Electronics, Fall 2011 Experiment 4, page 1/7 Last updated October 29, :25 PM by D. Yildirim

2 conventional stabilizer circuits consisting of linear regulators usually cause significant power loss (as was observed in the second experiment). On the other hand, the use of a proper switching circuitry instead of the linear regulator will be a much better solution with consideration to power loss. Before we proceed to the construction of the high efficiency A/D converter, let us examine how a switchedmode D/D onverter works. D/D onverters: The D/D converters are also known as D choppers where a fixed D voltage source is converted in to a variable voltage D source. A chopper can be considered as D equivalent to an A transformer with continuously variable turns ratio. Like transformer, it can be used to stepdown or stepup a D voltage source as well as inverting (negative) applied voltage. Operation Principles of StepDown onverters (Buck onverters): The principle of operation can be simply explained by Figure 1. When switch is in position 1 for a time D, the input voltage g appears across the load. If the switch is moved in to position 2 for a time (1D), the voltage across the load will be zero. The output voltage waveform v s (t) for a resistively loaded D chopper is shown in Figure 2. The chopper switch can be implemented by a power semiconductor switching device such as a MosFET, an IGBT, or a BJT. g 1 2 S i L (t) L v L (t) i c (t) v s (t) v s (t) g (a) D (1 D) t switch position: s = D g (b) Figure 1. (a) ircuit diagram of a resistive loaded buck converter and (b) output voltage waveform [4]. The average output voltage is given by, 1 ton avg = s () = g avg = g Ts Ts 0 v t dt D EE432 Industrial Electronics, Fall 2011 Experiment 4, page 2/7 Last updated October 29, :25 PM by D. Yildirim

3 oavg, and the average load current can be found by Ioavg, =, where D is the duty R cycle. The duty cycle D can be varied from 0 to 1 allowing us to change output voltage from 0 to g. By controlling D the power delivered to load can be controlled. The switching frequency f s (or chopping period ) is kept constant and on time D is varied in which the width of the pulse is varied and this type of control is known as pulse width modulation (PWM) control. We can notice from Figure 1b that the output of the D chopper with resistive load is discontinuous and contains harmonics. The ripple content is normally reduced by an L filter and power semiconductor switch implementation is illustrated in Figure 2. g v ds M i L (t) L v L (t) i c (t) D D Figure 2. Buck converter employing power semiconductor switches. Operation Principles of StepUp onverters (Boost onverters): If an output voltage higher than the input voltage is required, a boost converter can be employed as depicted in Figure 3. Operation principle is same as the Buck converter except that the location of switch, diode and inductor is changed. The average output voltage of Boost converter can be computed by averaging the inductor voltage waveform over one switching period and is given by, g avg = 1 D L i L (t) v L (t) i d D i c (t) g M v ds D Figure 3. Boost converter. Switched Mode Regulators: D choppers can be used as switching mode regulators to convert a D voltage to a variable D voltage normally unregulated to a regulated D output voltage. The regulation is normally achieved by pulse width modulation at a fixed frequency. EE432 Industrial Electronics, Fall 2011 Experiment 4, page 3/7 Last updated October 29, :25 PM by D. Yildirim

4 The basic elements of the switched mode regulators are control blocks in a typical closedloop system to regulate the output voltage as shown in Figure 4. L g H(s) gate driver compensator D(t) pulsewidth modulator v c G c (s) v e D m ref Procedure of Experiment: controller Figure 4. Main blocks of switched mode regulators. Note: When capturing oscilloscope screen and include in your report, you have to specify the time base ( ms/div) and scale of voltages/currents ( /div A/div). 1. FullWave Bridge Rectifier ircuit Setup: Assemble the circuit shown in Figure 5. A oscilloscope dc I out h1 h2 2U2 2U3 2U1 transformer sec f R load 0 I Deniz Yildirim Oct 31, 2010 e4_1.eps Figure 5. FullWave Bridge Rectifier Load resistor should be selected such that output current is 1A. Actually you are making an uncontrolled unregulated rectifier. You have already done this in your first experiment. Make the output voltage ripple as low as possible by changing the capacitor values. Obtain the time waveforms of the input and output voltages with and without capacitors and capture the oscilloscope screen. EE432 Industrial Electronics, Fall 2011 Experiment 4, page 4/7 Last updated October 29, :25 PM by D. Yildirim

5 Write down the values of output voltage, output current, and voltage ripple along with capacitor value in Table haracteristics of PWM ontrol Unit ircuit Setup: Set the circuit shown in Figure D power supply reference variable control unit PWM pulse Figure 6. ontrol Unit PWM oscilloscope h1 h2 Deniz Yildirim Oct 31, 2010 e4_1c.eps Set switching frequency to 5kHz. Observe the change in the squarewave for different reference voltage values adjusted from the reference variable. Obtain the characteristic of duty cycle values as a function of reference voltage (D ref plot). Use at least ten reference voltage values and write down values in Table 3. Plot the reference voltage versus duty cycle in your report. 3. Regulated Power Supply Using Buck (StepDown) onverter ircuit Setup: Set the circuit shown in Figure 7. Set the duty cycle from the reference variable such that the output voltage will be 15 olts. Load resistor should be selected such that output current is 1A. Obtain the time waveforms of the voltage across load resistor, diode and the current passing through the inductor. apture the oscilloscope screen. Write down the values of input power (P ac ), output voltage, output current, voltage ripple ( ), and current ripple ( I) in Table 4 (also write down switching frequency, capacitor and inductor values). Place a parallel capacitor from the load module to the resistor (4µF, 8µF and 16µF). What are the effects of these capacitors to the voltage ripples? Find the efficiency of the converter (P out /P ac ) at full load (15, 1A). EE432 Industrial Electronics, Fall 2011 Experiment 4, page 5/7 Last updated October 29, :25 PM by D. Yildirim

6 2U D power supply 2U U1 transformer reference variable sec control unit PWM pulse f dc o A I out Deniz Yildirim Oct 31, 2010 e4_buck.eps P ac Figure 7. Regulated power supply using buck converter P out onclusion: We have investigated the principles of operation for a switchedmode D chopper and used a Buck converter as a voltage regulator. As a conclusion, it is ask for the students to answer the following questions and submit results as a report. Simulate and analyze the boost converter shown in Figure 3 by the help of a computer design tool such as Pspice, PSIM or Proteus. It is required for you to plot the output voltage time ( t), output voltage duty cycle ( D) and efficiency duty cycle (η eff D) at full load condition. where: g =12 =30 Ω L=0.35 mh =33 µf Switching frequency = 20 khz References: Your goal is to make the output voltage equal to 24 olts. Find the appropriate duty cycle that fits these specifications. If you are not capable of reaching 24 volts, you may try to increase the value of the inductor, but it should not be any greater than 1 mh. [1] M. H. Rashid, Power Electronics; ircuits, Devices and Applications, 3 rd edition, Prentice Hall. [2] D. W. Hart, Introduction to Power Electronics, Prentice Hall, [3] B. K. Bose, Modern Power Electronics and A Drives, Prentice Hall [4] R. W. Erickson and D. Maksimovic, Fundamentals of Power Electronics, 2 nd ed., Kluwer Academic Publishers, EE432 Industrial Electronics, Fall 2011 Experiment 4, page 6/7 Last updated October 29, :25 PM by D. Yildirim

7 E X P E R I M E N T R E S U L T S H E E T This form must be filled in using a PEN. Use of PENIL IS NOT ALLOWED EXPERIMENT 3: SWITHED MODE D/D ONERSION USING BUK ONERTER STUDENT NO STUDENT NAME SIGNATURE DATE 1 2 INSTRUTOR APPROAL 3 4 output voltage () Table 2: Bridge rectifier output current filter capacitor, f (A) (µf) output voltage ripple, () Table 3: ariation of duty cycle with control voltage ariable Reference Duty ycle oltage () (%) output voltage () Table 4: Buck converter output current input power output current () (W) ripple, I (A) output voltage ripple, () switching frequency (khz) inductor value (mh) capacitor value (µf) EE432 Industrial Electronics, Fall 2011 Experiment 4, page 7/7 Last updated October 29, :25 PM by D. Yildirim