EEE3076 Power Electronics: PE1 & PE2 2017/2018

FACULTY OF ENGINEERINGLAB SHEETEEE 3076 POWER ELECTRONICS TRIMESTER 1, 2017-2018PE 1 – Power Semiconductor SwitchesPE 2 –DC-DC Buck Converter Note: On-the-spot evaluation will be carried out during or at the end of the experiment. Students are advised to read through this lab sheet before doing experiment. Your performance, teamwork effort, and learning attitude will count towards the marks. EEE3076 Power Electronics: PE1 & PE2 2017/2018 Experiment PE1 POWER SEMICONDUCTOR SWITCHESA. OBJECTIVES: 1) To demonstrate a practical go/no-go method of testing an SCR with a multimeter 2) To study the turn-on/turn-off states of the SCR 3) To study the effects of gate current on SCR and determine the minimum holding current to keep the SCR conducting 4) To study the switching parameters of an npn BJT B. LEARNING OUTCOME OF SUBJECTS:This experiment will help student to achieve one of the learning of outcomes of the subject which isLO1 - Select an appropriate power semiconductor switch for converters based on given input and output specifications. (cognitive – applying, level 3)Gobbi.R..Rev 170717 2 of 32 EEE3076 Power Electronics: PE1 & PE2 2017/2018C. MATERIALS REQUIRED: a) Equipment 1. DC Power Supply (variable 0 to 15V) 1 2. Digital multimeter 1 3. Dual channel oscilloscope 1 4. Function generator 1 5. Breadboard 1 b) Electronic components 1. SCR: C106D (could be different model) 1 2. npn BJT: BC548 or BC547 or equivalent 1 3. Voltage regulator IC 7805 (+5V, 1A) 1 4. Signal diode: 1N4148 or equivalent 2 5. Single turn potentiometer (linear): 100k or 2 x 50k 1 6. Resistor: 10/0.25W 1 7. Resistor: 100/0.25W 1 8. Resistor: 1k/0.25W 2 9. Resistor: 2k/0.25W 1 10. Resistor: 10k/0.25W 1 11. Resistor: 22k/0.25W 1 12. Resistor: 1M/0.25W 2 13. Resistor: 100/2W 1 14. Ceramic disc capacitor: 1nF 2 15. Electrolytic capacitor: 47F/(16V or above) 2 16. Electrolytic capacitor: 100F/(16V or above) 1 17. Inductor: 100H/0.29, 0.79A 1D. INTRODUCTION1. Introduction of SCRThe silicon-controlled rectifier (SCR) is a four-layer pnpn bipolar semiconductor device with three terminals, as shown in Fig-1. The SCR belongs to the thyristor family of electronic devices, which operates on the principle of current conduction when the break over voltage is reached. An SCR has an anode, a cathode and a gate terminal. A gate terminal can also trigger the device into conduction below the break over voltage level. It operates similar to a normal diode, where current flows only in the forward-biased condition but must be triggered into conduction by the gate terminal. Once the SCR is triggered into conduction, it acts like a latched switch, and the gate no longer has control of the current flow through the SCR.Anode (A) Metal Anode (A) Gate (G) case + A P Cathode (C) N device circuit Cathode Plastic Gate (G) Anode P structure G symbol case N Gate - C Gate (G) Plastic Cathode (C) Anode (A) case Cathode (C) Practical packages Fig-1: Device structure, circuit symbol and practical packages Gobbi.R..Rev 170717 3 of 32 EEE3076 Power Electronics: PE1 & PE2 2017/20182. Operation of SCRFig-2 shows schematically the basic operation of a SCR. The anode is connected through a series-limiting resistor RL to a positive voltage. The cathode is connected to ground via switch S2 and the gate is connected to switch S1, which is connected to ground. Under this configuration as in Fig-2(a), junction 1 and 3 (i.e. J1 and J3) are forward-biased but junction 2 (J2) is reverse biased, which prevents any appreciable current from flowing through the SCR. When S1 is moved up to the bottom side of RA as in Fig-2(b), a small gate current flows into the gate (electrons flow out from the gate). This introduces holes into the p-type gate region, which induce electron-injection across J3 into the p-type gate layer. The electrons will diffuse across the p-type layer and be swept across J2 by the localized field at J2 into the upper n-layer. These electrons in the n-layer will induce hole-injection across J1 into the upper n-layer. The holes will diffuse across the n-layer and be swept across J2 into the p-type gate layer. A new cycle of induced process will begin but the holes are generated internally, not by the gate current. This cyclic process is called regenerative process, which speeds up the SCR into conduction state without the help of the gate current anymore. The SCR is in heavy minority carrier injection and brings J2 to forward-bias (saturation condition). Now, the gate can be set back to ground via S1 and RG as in Fig-2(c), the large current flowing through the SCR is on or latched. The SCR can only be turned off if this main current flowing from the anode to the cathode is reduced below its minimum holding current (IH). This can be accomplished by momentarily opening switch S2 in the cathode lead of the circuit. The SCR can be considered reset or off. The SCR can be turned on again by the gate current triggering. V =+12V (b) (c) (a) AA V =+12V V =+12V AA AAR =100 L R =100 R =100 L L R =22k A Anode (A) R =22k R =22k A Anode (A) A Anode - - - P ------(A) + + + I P P - - - J1 fwd G + + + J1 fwd + + + N N------J1 fwd + + + biasedJ2 rev - - - -N - - biased Y Gate (G) - - - Y Gate (G) J2 fwd Gate (G) J2 fwd P +P + + Y +P + + +- -+ - + J3 fwd + + + + + + X S1 N X - - - J3 fwd X - - - J3 fwd biased S1 N S1 N + + + + + + + + + biased R =10k Cathode (C) Cathode (C) G R =10k R =10k Cathode (C) Y G Y G S2 S2 Y S2 X X X Fig-2: Operation of an SCR: (a) off condition, (b) triggering on; (c) on condition without triggeringGobbi.R..Rev 170717 4 of 32 EEE3076 Power Electronics: PE1 & PE2 2017/20183. Current-Voltage Characteristics of an SCRThe SCR operates similar to a normal diode when in the reverse biased condition, as shown in Fig-3(a). The SCR exhibits very high internal impedance, with perhaps a slight reverse blocking current. However, if the reverse breakdown voltage is exceeded, the reverse current rapidly increases to a large value and may destroy the SCR. In the forward bias condition (gate is grounded), the internal impedance of the SCR is very high with a small current flowing called the forward blocking current. When the forward voltage (+VF) is increased beyond the forward break over voltage point, an avalanche breakdown occurs and the current from the cathode to anode increases rapidly. A regenerative action occurs with the conduction of p-n junctions and the internal impedance of the SCR decreases. This results in a decrease in voltage across the anode and the cathode as verified by Ohm's law where V = IR. When R is small, so is the voltage drop across it. The forward current flowing through the SCR is limited primarily by the impedance of the external circuit, and the SCR will remain on as long as this current does not fall below the holding current. If the gate current is allowed to flow as shown in Fig-3(b), the forward break over point will be smaller. The larger the gate current flows, the lower the point at which forward break over will occur, as illustrated in Fig-3(c). Normally, SCRs operated with applied voltage lower than the forward break over voltage point (with no gate current flowing) and the gate triggering current is made sufficiently large to ensure complete turn on.+V I High current F R (on condition) L Holding Current + A Reverse break over voltage G -V Regenerative action - C -V R R R L V G + A F G Forward blocking current (off condition) - C Reverse blocking current (off condition) R Forward break over voltage G -I (a) R+V (variable) +V I R F L (b) I I A A A + I > I > I G2 G1 G0 V G AC R C G I + V G - F V V G AK - (c)Fig-3: I-V characteristics for an SCR, (a) I-V curves, (b) test for gate current, (c) gate current curves.Gobbi.R..Rev 170717 5 of 32 EEE3076 Power Electronics: PE1 & PE2 2017/20184. Switching parameters of BJTBipolar junction transistors (BJT) are moderate speed switches in among the power semiconductor switches. It is because carriers (electrons and holes) are collected at the BE junction during on state. During switching off, these carriers have to be removed before the depletion layer at the BE junction starts to develop and turn off the BJT. During switching on, carriers have also to be collected at the junction before the BJT starts to turn on. Finite times are required for the BJT to fully turn on and fully turn off. Below are four defined switching parameters, which can be used to characterize the BJT switching characteristics for a given test circuit with conditions. td is the turn-on delay time, tf the fall time of vCE, ts the storage time and tr the rise time of vCE. The switching-on time is tsw-on = td + tf and switching-off time is tsw-off = ts + tr. tPW is the negative-going pulse-width of vCE. V I (max)+V CC 0.1V v I (max) I R 0V C V v CE(max) R CE B 0.9V DUT CE(max) V I (max) 0.5V t 0V CE(max) P at DC, t & t r f 0.1V (b) t CE(max) t PW 0V d t t t r f s (a) tsw-on t sw-of f Fig-4: (a) Typical v and v waveforms of npn BJT. (b) A simple test circuit for npn BJT. I CEE. EXPERIMENT Gobbi.R..Rev 170717 6 of 32 EEE3076 Power Electronics: PE1 & PE2 2017/2018Experimental effort evaluation Student is expected to use multimeter and oscilloscope for all measurement and monitoring during this experiments. Part I: SCR switchKnow your SCR before starting the SCR experiments (refer to Appendices)Section 1: Testing an SCR with a digital multimeter (diode test mode)Procedures:1) Connect the circuit as shown in Fig-7. 2) Set the multimeter in DIODE TEST MODE. 3) Set the position of the switches S1 and S2 as indicated in sequence no.1 in Table 1. Record the meter reading for each sequential setting of the switches as shown in Table 1. Meter reading (in diode test mode): A 3- or 4-digit number means the device is conducting current with voltage drops in V or mV (depended on meter used). A “1” displayed at the left means the device is not conducting current. 4) Complete table 1 by repeating step (3). Follow the sequence from No 1 to 4.Results: S2 Table 1 S1 Multimeter State + No. S1 S2 A Reading (On/Off) Multimeter 1 Close Open G - 2 Close Close K 3 Close Open 4 Open Open Fig-7: Go/no-go testing of SCRQuestions: i) Compare the measured readings in Table 1 and briefly explain how the observations of these readings relate to the conduction states of the SCR. V =12V AAR =22k R =100/2W A L A Section 2: Basic operation of an SCR R =1k Y G G + K S1 V AK X + S2 V - R =10k GK Gobbi.R..Rev 170717 G - 7 of 32 Fig-8: Test circuit of an SCR EEE3076 Power Electronics: PE1 & PE2 2017/2018Results:Table 2 Sequence No. S1 S2 V /V V /V State AK GK 1 X close 2 Y close 3 X close 4 X open 5 X closeProcedures:1) Connect the circuit shown in Fig-8. 2) Set the position of the switches S1 and S2 as indicated in sequence no.1 in Table 2 and then apply power to the circuit. Record the readings of VGK and VAK and indicate the states of the SCR for each sequential setting as in Table 2. 3) Complete table 2 by repeating step (2). Follow the experimental sequence from 1 to 5.Questions: i) Before firing (triggering), what is the VAK? Give reason to support your answer. ii) What is the VAK when the SCR is conducting? Give reason to support your answer.Section 3: Current control of an SCR Gobbi.R..Rev 170717 8 of 32 EEE3076 Power Electronics: PE1 & PE2 2017/2018V =12V AAR = B R =100/2W L R = 22k A A 2 x 1M R =1k Y G G + K S1 V + I AK X G - V GK -(a)V =12V AAR =2 x 50k H or 100kR =2k L R = B I 22k A A + + V Multimeter S1 G AK K - - R =1k G + R =100/ + S V Multimeter 0.25W RS - - (b)Fig-9: Current control of an SCR, (a) gate current control, (b) holding current controlSection 3.1 Gate Current Control Procedures:1) Remove power supply and modify the circuit in Fig-8 as shown in Fig-9(a). 2) Set the switch S1 at position X and then apply power to the circuit. Record the voltage VGK and VAK. 3) Move Switch S1 to position Y. Record VGK and VAK. 4) Turn-off the SCR and repeat step 2) and 3) to confirm your results. Results: Step 2) VGK = ______, VAK = ______Step 3) VGK = ______, VAK = ______Questions: i) What is the current flowing through the gate (IG) in step 2)? Is the SCR on or off? Why? ii) What is the current flowing through the gate (IG) in step 3)? Is the SCR on or off? Why?Gobbi.R..Rev 170717 9 of 32 EEE3076 Power Electronics: PE1 & PE2 2017/2018Section 3.2 Holding Current Control Procedures:1) Remove the power supply and modify the circuit in Fig-9(a) as shown in Fig-9(b).2) Set the wipers of the potentiometers RH so that the resistance is 0. 3) Ensure that the switch is opened. 4) Make firm connections to the multimeters as shown in Fig-9(b). 5) Set the multimeter in DC 2V range. 6) Apply power supply to the circuit and close the switch S1 and then open it again. Record the voltage VAK and VRS in Table 3. 7) Slowly adjust RH and record VAK and VRS in Table 3 with VAK change (VAK) at approximately 0.02V (Note: VAK will decrease and then increase again). The record ends when the reading in VAK suddenly jumps to ~12V. Results:Table 3 V /V AK V /V RS(VAK ~ 0.02V)Questions:VRS i) Plot a graph IA versus VAK, where I A  . Comment on your graph. R S ii) From the graph, determine the holding current. Gobbi.R..Rev 170717 10 of 32 EEE3076 Power Electronics: PE1 & PE2 2017/2018 Part II: BJT switch Before starting these experiments: 1. Test your BJT and diodes. 2. Check your voltage probes, oscilloscope and function generator.Fixed +5V power supply Construct a fixed +5V voltage source as shown in Fig-10. This output will be the V S in the circuits in Fig-11. +9V to +10V IN OUT +5V 7805+ COM + From adjustable output 47F 47F Constant output voltage as V in Fig-11. DC power supply SFig-10: Voltage regulator IC 7805 circuit for fixed +5V output. Section 4: Temporal switching behaviour of BJT and effect of inductorNote: Waveforms must be drawn on a common time axis as shown in the figure in each section. Each waveform has its own vertical scale with its ground level (channel position) at one of the vertical major grid position, e.g. at +2 division means at 2 divisions above centre of the vertical axis. Use a single page graph paper to draw all the waveforms.V = S R =100/2W P4 +5V SR =1k P2 C C = + P1 B1 P3 I B Q (npn) 100F R = B2 E C =1nF B 100 D =1N4148 B (a)Fig-11: A simple BJT switching circuitSection 4.1: BJT temporal switching behavior  Caution when using the electrolytic capacitor: The polarity of the capacitor must be connected correctly, otherwise, explosion may occur.  Caution when using the function generator: Never short-circuit the output, which may burn the output stage of the function generator.Procedures:Gobbi.R..Rev 170717 11 of 32 EEE3076 Power Electronics: PE1 & PE2 2017/20181) Construct the circuit as shown in Fig-11.2) Oscilloscope settings: You must use VOLTS/DIV and TIME/DIV values as mentioned in each part, if any. Channel POSITION must be put at one of the vertical major grid position. Set AC/GND/DC input coupling switches at DC. Make sure the VARIABLE knobs for CH1, CH2 and time base at the CAL positions. 3) Function generator settings: Select square-wave mode and set frequency at 40kHz. Connect the function generator output to CH1. Set the output voltage amplitude to 5V (or peak-to-peak to 10V). Set the Oscilloscope settings (volt/div, time/div accordingly to get a stable waveform)4) Skill to draw voltage waveforms: The HORIZONTAL position knob should not be moved before all the waveforms, which share a common time axis, are drawn. Draw the waveform by using one-to-one scale, i.e. 1 cm on the graph paper is equivalent to 1 division on the oscilloscope screen. Draw the ground level and locate some of the important points, e.g. maximum and minimum points, turning points, points where the waveform cuts through the ground level and the major grids. Connect the points together by a smooth curve. 5) Using graph paper (Note: this section and next section use the same time axis. Start P1 waveform at top-left corner of the graph paper), draw the voltage waveforms at P2, P3 and P4 with respect to the reference voltage waveform at P1 to show the detailed v(t) and t relationships among them. To do this: i) Connect CH1 to P1 and draw the waveform (keep this connected all the time), ii) Connect CH2 to P2 and draw the waveform, v iii) Connect CH2 to P3 and draw the waveform, P1 t iv) Connect CH2 to P4 and draw the waveform. v P2 v) Now, remove capacitor CB from the circuit, Connect CH2 to P2 With t v and draw the waveform. (w/o CB) C P3 B t v Note: You must draw the waveforms as shown at the left and P4 t indicate the ground level of each waveform. State the volt/div and w/o v C P2 time/div used on the graph paper. B tQuestions: i) vP2 waveform with CB: Explain why there are positive spike and negative spike? ii) vP3 waveform with CB: Comment and explain on the waveform. iii) vP4 waveform with CB: Comment and explain on the waveform. iv) vP2 waveform without CB: Comment on this waveform to the vP2 waveform with CB.Lab report format, evaluation and submissionGobbi.R..Rev 170717 12 of 32 EEE3076 Power Electronics: PE1 & PE2 2017/2018 The report should consists of a).Results and answers for all the questions (Please write down the corresponding step or procedure number as the identification of your answer in appropriate order), b).Discussion and Conclusion. c).Lab report to be submitted to the lab technician within one week from the experiment date.End of Lab sheetTables Sheet (To be included in lab report)Section 1: Testing an SCR with a digital multimeter in diode test mode Gobbi.R..Rev 170717 13 of 32 EEE3076 Power Electronics: PE1 & PE2 2017/2018Table 1 Multimeter State No. S1 S2 Reading (On/Off) 1 Close Open 2 Close Close 3 Close Open 4 Open OpenSection 2: Basic operation of an SCR Table 2 Sequence No. S1 S2 V /V V /V State AK GK 1 X close 2 Y close 3 X close 4 X open 5 X closeSection 3.1 Gate Current Control Step 2) VGK = ______, VAK = ______Step 3) VGK = ______, VAK = ______Section 3.2 Holding Current Control Table 3V /V AK V /V RSSection 4.1: BJT temporal switching behavior Please sketch the following on a graph paper. To be verified by Lab Supervisor (Stamp and v Signature) P1 t v P2 With t v C P3 B t v P4 t ……………………………………………… w/o v C P2 B t 1s/divExperiment PE2 DC-DC BUCK CONVERTERGobbi.R..Rev 170717 14 of 32 EEE3076 Power Electronics: PE1 & PE2 2017/2018A. OBJECTIVES:1) To study the operation principle of a dc-dc converter 2) To provide hand-on design experience on a basic converter circuitB. LEARNING OUTCOME OF SUBJECTS:This experiment will help student to achieve three of the learning of outcomes of the subject which are:LO3 - Design a Buck/Boost converter for resistive-inductive loads. (cognitive – creating, level 6)C. MATERIAL REQUIRED: a) Equipment 1. DC power supply (variable 0 – 15V, fixed +5V) 1 2. Digital multimeter 1Gobbi.R..Rev 170717 15 of 32 EEE3076 Power Electronics: PE1 & PE2 2017/20183. Dual channel oscilloscope 1 4. Function generator 1 5. Breadboard 2 b) Electronic components 1. npn BJT: BC548 or BC547 or equivalent 1 2. pnp BJT: 2N2905 or 2N4403 or BFY64 or equivalent 1 3. Voltage regulator IC 7805 (+5V, 1A) 1 4. Schottky diode: 1N5817/18/19 or equivalent 1 5. Signal diode: 1N4148 or equivalent 1 6. Resistor: 100/0.25W 3 7. Resistor : 100/02W 1 8. Resistor: 1k/0.25W 1 9. Resistor: 47/1W 1 10. Resistor: 22/2W 1 11. Resistor: 33/2W 1 12. Ceramic disc capacitor: 1nF 1 13. Electrolytic capacitors: 47F/(16V or above) 2 14. Electrolytic capacitors: 100F/(16V or above) 2 15. Inductor: 100H/0.29, 0.79A 1 16. LED 1 17. DC Motor Fan (12V) 1D. INTRODUCTION + + 1. Basic Switching Converter V VO R S L V In a switching converter circuit, the transistor operates- as an electronic switch by beingO completely on or off. This circuit is known as a dc chopper circuit. Assuming- the switch is t ideal in Fig-1, the output is the same as the input when the switch is closed, and the output ison zero when the switch is open. VS+ + closed open closed t + + V VO V S RL S VO RL T - VO- - - DT (1-D)T + ton + V V S O (a)RL VS (b) - VO - Fig-1 : (a) basic dc-dc switching t on + (c) closed open converter,closed (b) switchingt equivalent, (c) + V output voltage V S S V RL T - O + closed open closed t + - DT (1-D)T V The periodic openings and closings of the switch S V RL T - results inO the pulsed output shown in Fig-1(c). The average - or dc DT (1-D)T component (using capital letter) of the output is 1 T 1 DT V  v (t)dt  v (t)dt  V D (Eq-1) o T 0 o T 0 s sGobbi.R..Rev 170717 16 of 32 EEE3076 Power Electronics: PE1 & PE2 2017/2018The dc component of the output is controlled by adjusting the duty cycle ratio D, which is the fraction of the period that the switch is closed, i.e. ton ton D    ton f (Eq-2) ton  toff T where f is the switching frequency in hertz. The dc component of the output will be less than or equal to the input for this circuit.The power absorbed by the idea switch is zero. When the switch is open, there is no current in it; when the switch is closed, there is no voltage across it. Therefore, all power is absorbed by the load, and the energy efficiency is 100%. Losses will occur in a real switch because the voltage across will not be zero when it is on, and the switch must pass through the linear region when making from one state to the other. + V L -In practice, switching is done using various method dependingSwitch on theL type of switches used,iR iL i + e.g. MOSFET or BJT. In this experiment, BJT is used hence, the+ switching pulseC (as base current) is given to the base terminal of BJT. The+ pulse is produced separately by a pulse V V S X C V RL circuit (Fig-8) - O - -2. The Buck Converter Low Pass FilterVL = VS - VO + - Switch L + + + V V = V S X S C V RL - O - -(a) (b) + V L -Switch L iR iL iC + + + V V S X C V RL - O - -Low Pass FilterVL = VS - VO + - Switch L + + + V V = V S X S C V RL - O - -Gobbi.R..Rev 170717 17 of 32 EEE3076 Power Electronics: PE1 & PE2 2017/2018VL = - VO + - Switch L + + + V V = 0 S X C V RL - O - - (c)Fig-2: (a) Buck dc-dc converter, (b) Equivalent circuit for switch closed, (c) Equivalent circuit for switch open. Controlling the dc component of a pulsed output of the type in Fig-1(c) may be sufficient for some applications, but often the objective is to produce an output that is purely dc. One way of obtaining a dc output from the circuit of Fig-1(a) is to insert a low-pass filter after the switch. Fig-2(a) shows an inductor-capacitor (L-C) low-pass filter added to the basic converter. The diode provides path for the inductor current when the switch is opened and is reverse biased when the switch is closed. The circuit is called a buck converter or a down converter because the output voltage is less than the input. a) Voltage and Current RelationshipsIf the low-pass filter is ideal, the output voltage is the average of the input voltage to the filter. The input to the filter, vx in Fig-2(a), is Vs when the switch is closed and is zero when the switch is open, provided that the inductor current remains positive, keeping the diode on. If the switch is closed periodically at a duty ratio D, the average voltage at the filter input is Vs D, as seen by Eq-1.This analysis assumes that the diode remains forward biased for the entire time that the switch is open, implying that the inductor current remains positive. An inductor current that remains positive throughout the switching period is known as continuous current. Conversely, discontinuous current is characterized by the inductor current returning to zero during each period.The buck converter (and dc-dc converters in general) has the following properties when operating in the steady state:1. the inductor current is periodic: iL (t  T )  iL (t) (Eq-3) 2. The average inductor voltage is zero: 1 t T V  v ()d  0 (Eq-4) L T t L 3. The average capacitor current is zero: 1 t T I  i ()d  0 (Eq-5) C T t C 4. The power supplied by the source is the same as the power delivered to the load. For non-ideal components, the source also supplies the losses:Ps = Po (ideal) andPs = Po + losses (non-ideal) (Eq-6)Gobbi.R..Rev 170717 18 of 32 EEE3076 Power Electronics: PE1 & PE2 2017/2018Analysis of the buck converter of Fig-2(a) begins by making these assumptions:1. The circuit is operating in the steady state. 2. The inductor current is continuous (always positive) 3. The capacitor is very large, and the output voltage is held constant at voltage V o. This restriction will be relaxed later to show the effects of finite capacitance. 4. The switching period is T; the switch is closed for the time DT & open for time (1- D)T 5. The components are ideal.The key to the analysis for determining the output Vo is to examine the inductor current and inductor voltage first for the switch closed and then for the switch open. The net change in inductor current over one period must be zero for steady-state operation. The average inductor voltage is zero. b) Analysis for the switch closed When the switch is closed in the buck converter circuit of Fig-2(a), the diode is reverse- biased and Fig-2(b) is an equivalent circuit. The voltage across the inductor is di v  V  V  L L L s o dt Rearranging, di V V L  s o with switch closed dt L Since the derivative of the current is a positive constant, the current increases linearly, as shown in Fig-3(b). The change in current while the switch is closed is computed by modifying the preceding equation: di i i V  V V  V L  L  L  s o (i )  ( s o )DT (Eq-7) dt t DT L L closed L c) Analysis for the switch open When the switch is open, the diode becomes forward biased to carry the inductor current, and the equivalent circuit of Fig-2(c) applies. The voltage across the inductor when the switch is open is di v  V  L L L o dt Rearranging, di V L   o with switch open dt LThe derivative of current in the inductor is a negative constant value, and the current decreases linearly, as shown in Fig-3(b). The change in inductor current when the switch is open isGobbi.R..Rev 170717 19 of 32 EEE3076 Power Electronics: PE1 & PE2 2017/2018i i V L  L   o t (1 D)T L V (i )  ( o )(1  D)T (Eq-8) L open LSteady-state operation requires that the inductor current at the end of the switching cycle be the same as that at the beginning, meaning that the net change in inductor current over one period is zero. This requires (iL )open  (iL )closed  0 Using equation Eq-7 and Eq-8, V  V V  ( s o )DT   o (1 D)T  0 L  L Solving for Vo, Vo  VsD (Eq-9)This gives the same result as equation Eq-1. The bulk converter produces an output, which is less than or equal to the input.An alternative derivation of the output voltage is based on the inductor voltage, as shown in Fig-3(a). Since the average inductor voltage is zero for periodic operation,VL  (Vs  Vo )DT  (Vo )(1 D)T  0 Solving the preceding equation for V0 yields the same result as equation Eq-9, Vo= VsD. v L V -V s o (a) t -V o Fig-3: (a) Inductor voltage, v , i DT T L I L (b) Inductor current, i , max I i L I R L (c) Capacitor current, i . min C t (b) i Ci (c) t LNote that the output voltage depends only on the input and the duty ratio D. If the input voltage fluctuates, the output voltage can be regulated by adjusting the duty ratio appropriately. A feedback loop is required to sample the output voltage, compare it to a reference and set the duty cycle of the switch accordingly.The average inductor current must be the same as the average current in the load resistor, since the average capacitor current must be zero for steady-state operation (IL = IC + IR):V I  I  o (Eq-10) L R RGobbi.R..Rev 170717 20 of 32 EEE3076 Power Electronics: PE1 & PE2 2017/2018Since the change in inductor current is known from equations Eq-7 and Eq-8, the maximum and minimum values of the inductor current are computed asi I  I  L max L 2Vo 1 Vo   1 (1 D)    (1 D)T  Vo    (Eq-11) R 2  L   R 2Lf  i I  I  L min L 2Vo 1 Vo   1 (1 D)    (1 D)T   Vo    (Eq-12) R 2  L   R 2Lf  where f = 1/T is the switching frequency in hertz.Equation Eq-12 can be used to determine the combination of L and f that will result in continuous current. Since I min= 0 is the boundary between continuous and discontinuous current. 1 (1 D) Imin  0  Vo    (Eq-13)  R 2Lf  (1 D)R (Lf )  (Eq-14) min 2 v L i V – V L s o 0  T  T DT 1 2 V o TFig-4: Discontinuous current d) Discontinuous current operationFig-4 shows the regions for iL > 0 (during DT and 1T) and iL = 0 (2T). During the interval2T, the power to the load is supplied by the filter capacitor alone. Again, equating the integral of the inductor voltage over one T to zero (equation Eq-4) yields(Vs – Vo)DT + (-Vo)1T = 0V D o  (Eq-15) Vs D  1 e) Output voltage RippleIn the preceding analysis, the capacitor was assumed to be very large to keep the output voltage constant. In practice, the output voltage cannot be kept perfectly constant with a finite capacitance. The variation in output voltage, or ripple, is computed from the voltage current relationship of the capacitor. The current in the capacitor is iC  iL  iR , as shown inFig-5(a), where IC  I L  I R . Assuming I L  I R  Vo / R , then IC  I L . Gobbi.R..Rev 170717 21 of 32 EEE3076 Power Electronics: PE1 & PE2 2017/2018While the capacitor current is positive, the capacitor is charging. From the definition of Q capacitance, Q  CVo Q  CVo V  o C i C i /2 Q L t Fig-5: (a) Capacitor current, i , T/2 (a) C v (b) capacitor ripple voltage o V V o o t (b) The change in charge, Q is the area of the triangle above the time axis;1 T i  Ti Q  L  L 2  2  2  8 Thus, Ti V  L o 8CUsing equation Eq-8 for iL, TVo Vo V  (1 D)T  (1 D) (Eq-16) o 8CL 8CLf 2In this equation, V0 is the peak-to-peak ripple voltage at the output, as shown in Fig-5(b). it is also useful to express the ripple as a fraction of the output voltage:2 V (1 D) 2  f  o  c 2 = (1  D)  (Eq-17) Vo 8CLf 2  f  1 where f c  is the corner frequency of the low pass LC filter. 2 LC If the ripple is not large, the assumption of a constant output is reasonable and the preceding analysis is essentially valid.3. Design ConsiderationsMost buck converters are designed for continuous-current operation. The choice of switching frequency and inductance to give continuous current is given by equation Eq-14, and the output ripple is described by equation Eq-17. Note that as the switching frequency increases, the minimum size of the inductor to produce continuous current and the minimum size of the capacitor to limit output ripple both decrease. Therefore, high switching frequencies are desirable to reduce the size of both the inductor and the capacitor.The trade-off for high switching frequencies is increased power loss in the switches. Increased power loss for the switches, decreases the converter’s efficiency, and the large heat sink required for the transistor switch offsets the reduction in size of the inductor and capacitor. Typical switching frequencies are in the 20-50kHz range. As switching devices improve, switching frequencies will increase.Gobbi.R..Rev 170717 22 of 32 EEE3076 Power Electronics: PE1 & PE2 2017/2018The inductor wire must be rated at the rms current, and the core should not saturate for peak inductor current. The capacitor must be selected to limit the output ripple to the design specifications, to withstand peak output voltage, and to carry the required rms current.The switch and diode must withstand maximum voltage stress when off and maximum current when on. The temperature ratings must not be exceeded, possibly requiring a heat sink.Gobbi.R..Rev 170717 23 of 32 EEE3076 Power Electronics: PE1 & PE2 2017/2018E. Experiment: Buck ConverterStudent must show multimeter reading, oscilloscope display, etc to lab experiment supervisor before proceeding to the next section. The experimental effort evaluation is performed within 3-hour lab session only. Before starting experiments: 1. Test your BJTs and diodes. 2. Check your voltage probes, oscilloscope and function generator.Procedures1) Construct a fixed +5V voltage source as shown in Fig-6. This output (terminal X1 and Y1) will be the VS in the circuits in Fig-7 and 8. +9 V to +10 V IN OUT 7805 X1 from DC Constant 5 V power supply 47µF 47µF outputY1Fig-6: Voltage regulator IC 7805 circuit for fixed +5V output. 2) Measure voltage across X1-Y1 and make sure voltage value is 5 V.3) Construct the Pulse Circuit (Fig 7) on a breadboard. X1R1 100 B1R2 100 R R4 3 P1 1 k 100  C B From Function Q2 GeneratorD1 E IN4148 C 1 nF Y1Fig-7: Pulse circuitGobbi.R..Rev 170717 24 of 32 EEE3076 Power Electronics: PE1 & PE2 2017/20184) Function generator settings: Select square-wave mode and set frequency at 62.5 kHz. DO NOT RELY ON READING AT THE FUNCTION GENERATOR; YOU NEED TO CHECK THE PERIOD (T) AT THE OSCILOSCOPE (EXAMPLE, FOR FREQUENCY 62.5 kHz, PERIOD IS 1/62.5kHz = 16 s). Make sure the duty cycle is 50% (RAMP/PULSE knob is pushed in). Connect the function generator output to CH1, check if the square wave pulse is correct. Set output voltage amplitude to 5V or peak- to-peak 10V 5) Connect terminal P1 to channel 1 (DO NOT REMOVE THIS CONNECTION THORUGHOUT THIS EXPERIMENT). Connect terminal B1 to Channel 2. Verify the results. You may check with the lab supervisor.6) Construct the Buck converter on a separate breadboard. Circuit diagram for the circuit is shown in Fig-8. Caution when using the electrolytic capacitor for buck converter circuit, the polarity of the capacitor must be connected correctly, otherwise, explosion may occurL E Q1 C A1 X1 100µH B + B1 100µF D F C VO RL Schottky 22  /2 W diode -Y1 A2 Fig-8: Buck converter circuit7) Connect terminals B1 (from the pulse circuit) to B1 (at the buck converter circuit). 8) Connect terminal A1 to channel 2. This is to capture the output voltage signal. Verify the results. You may check with the lab supervisor.9) Plot both the signals from terminals P1 and A1 on a graph paper. Make sure the time and volt division information are stated on the graph paper. Gobbi.R..Rev 170717 25 of 32 EEE3076 Power Electronics: PE1 & PE2 2017/2018SECTION 1: Study of Effect of Switching Frequency on the output current rippleIn this section, effect of switching frequency on the output current ripple will be studied. At the end of this section, students shall be able to relate the effect of switching frequencies on average output current and output current ripple.1) Measure the Vmin and Vmax from oscilloscope, and Vaverage using Multimeter. Record in table 1. [Since the output load is resistor, hence output current can be calculated by equation V/RL]. Table 1Measurement Calculation Output current Switching Vmin Vmax Vaverage Iout max Iout min  Iout Iaverage ripple Frequency (V) (V) (V) (Vmax/RL) (Vmin/RL) (Iout max - (Vaverage/R ( Iout/ (kHz) *Use (A) (A) Iout min) L) multimeter (A) (A) Iaverage) (%) 62.5 40 202) Set switching frequencies to other value as indicated in Table 1 and repeat step 1.3) Plot graph switching frequencies versus output current ripple. Analyze the graph.SECTION 2: Effect of duty cycle on output voltage V0 and its ripple V0 This section is to observe the effect of the duty cycle on the output voltage and the output voltage ripple. The experimental results will be compared to the theoretical values. From these comparisons, you may reveal some of the non-ideal components that affect the efficiency of the Buck converter.Procedures: 1) Set Function generator: 100kHz (T=10s) and 5V amplitude. Connect CH1 to P1, CH2 to A1 and multimeter test leads across RL (Across Terminals A1 and A2). 2) By changing ton (adjust the function generator RAMP/PULSE knob with knob pulled out), measure the corresponding V0 and V0 values as shown in table below. Here, ton is the Q1 on-state time duration (seen from terminal P1), V0 the average output voltage (read from multimeter) and V0 the peak-to-peak voltage at A1 (read from CH2). Table 2Gobbi.R..Rev 170717 26 of 32 EEE3076 Power Electronics: PE1 & PE2 2017/2018 ton /s** 3 4 5 6 7 8V0 /VV0 /mV**The frequency may be changed when ton is changed. Adjust the frequency adjustment knob to return to 100kHz if necessary. SECTION 3: Application of Buck Converter as a LED Dimmer and DC Motor Speed Controller. This section is to observe the dimming effect of a LED, when variable DC voltage is supplied from the Buck converter. The variable voltage is achieved by varying the duty cycle. The same concept it used to control speed of a dc motor.Procedures: 1) Function generator settings : 100 kHz (T=10s) and 5 V amplitude. 2) Connect terminal A1 and A2 to a LED in series to a resistor, R as shown in Fig-9. 3) Vary the duty cycle to 20%, 50% and 80% and observe changes in light intensity from the LED.L E Q1 C A1 X1 100µH B + B1 R 100µF 100 /0.5 W D F C VO Schottky LED diode -Y1 A2Fig-9: Buck converter circuit with LED load4) Now, remove the LED and resistor from terminal A1 and A2. 5) Connect a DC motor fan to the terminals (A1 and A2) 6) Repeat step 3 and observe changes in fan speed.Tables Sheet (To be included in lab report)Gobbi.R..Rev 170717 27 of 32 EEE3076 Power Electronics: PE1 & PE2 2017/2018Table 1Measurement Calculation Output current Switching Vmin Vmax Vaverage Iout max Iout min  Iout Iaverage ripple Frequency (V) (V) (V) (Vmax/RL) (Vmin/RL) (Iout max - (Vaverage/R ( Iout/ (kHz) *Use (A) (A) Iout min) L) multimeter (A) (A) Iaverage) (%) 62.5 40 20Table 2 ton /s** 3 4 5 6 7 8V0 /VV0 /mV- Write comments based on your observation on the LED with the Buck converter. ………………………………………………………………………………………………………… ………………………………………………………………………………………………………… ………………………………………………………………………………………………………… ……....- What happen to speed of the fan when duty cycle is varied? ………………………………………………………………………………………………………… ………………………………………………………………………………………………………… ………………………………………………………………………………………………………… ……....To be verified by Lab Supervisor (Stamp and Signature)………………………………………………Gobbi.R..Rev 170717 28 of 32 EEE3076 Power Electronics: PE1 & PE2 2017/2018Lab report format, evaluation and submission The report should consists of the following (Do not attached any other documents other the following)1. Faculty Lab Sheet front page 2. The following results and answer in orderSection 1, Step 5 : Graph Paper and Write-up based on the graph paper Section 1, Table 1 : Fully filled up (using the Table Sheet), With stamp and signature of Lab Supervisor Section 1, Step 8 : Graph Paper and Write-up based on analysis Section 2, Table 2 : Fully filled up (using the Table Sheet), With stamp and signature of Lab Supervisor Section 3 Step 3, Write-up based on observation Section 3 Step 6, Write-up based on observation3. Discussion and Conclusion. 4. Lab report to be submitted to the lab technician within one week from the date of the experimentEnd of Lab sheetGobbi.R..Rev 170717 29 of 32 EEE3076 Power Electronics: PE1 & PE2 2017/2018APPENDICESComponent pin layout Pin layout7805IN COM OUTThe Resistor color code Capacitance chart ABC .abcAB x 10C pF 0.abc FPotentiomete rBreadboard internal 25 holes connectedconnection horizontally 25 holes connected horizontally5 holes connected vertically5 holes connected verticallyDiodes 1. The packaging of 1N4007 and 1N5819 is the same, check diode code written on the diode. 2. The packaging of 1N4148 and 1N5231 is the same, check diode code written on the diode.Gobbi.R..Rev 170717 30 of 32 EEE3076 Power Electronics: PE1 & PE2 2017/2018COMPONENT AND EQUIPMENT CHECKS The go/no-go method of testing is used. Always do these checks before start your experiment. Diode and Zener diode checks Use multimeter in “diode test” mode. A good diode will give a reading for forward-biased test and no reading for reverse-biased test. “Diode test” mode: “COM” terminal is negative “–“. “V, , mA” terminal is positive “+”. Forward bias displays reading in mV, V or other unit. Reversed bias displays a symbol. All these depend on the multimeter used. The test current is in mA (said 0.1mA, 1.5mA, etc). “Ohm mode” test: This mode cannot be used for most of the multimeter. SCR check Use multimeter in “diode test” mode. A good SCR will only give forward-biased readings for GK (gate-to-cathode) test (gate is connected to “+” and cathode is connected to “–“) and AK (anode-to-cathode) test after gate triggering (connect anode to gate momentarily). Some SCRs (non-sensitive gate type) give forward-biased readings for KG test due to the internal resistor connected in between the gate and the cathode (cathode short). BJT check 1) Use multimeter in “diode test” mode. A good npn BJT will only give forward-biased readings for BE test and BC test. A good pnp BJT will only give forward-biased readings for EB test and CB test. Some BJTs (for inductive loads) give forward-biased readings for npn EC test or pnp CE test due to the internal diode connected in between the collector and the emitter. 2) Use multimeter in “hFE test” mode. A good npn or pnp BJT will give an hFE reading within the specification range of the BJT.“hFE test” mode: An hFE test socket labeled with pnp and npn is provided in some multimeter panel. Plug in the BJT being tested into the corresponding holes of the socket. The reading is the DC current gain IC/IB. The test current for IB is in A (e.g 10A). Oscilloscope voltage probe check Use oscilloscope calibration (CAL) terminal. A good probe will give a waveform of positive square wave with 2V peak-to-peak and about 1 kHz. Oscilloscope channel check Use oscilloscope calibration (CAL) terminal and a good voltage probe. A good input channel will give the corresponding waveform of the CAL terminal. Function generator check Check the output waveform by oscilloscope. A good function generator will give a stable waveform on the oscilloscope screen. Caution: Never short-circuit the output to ground, this can burn the output stage of the function generator. Resistors, capacitors and inductors Resistors are hard to fail. Capacitors are hard to fail except over-voltage or wrong connections of polar capacitors. Inductors are hard to fail except coil burned by over-current. Gobbi.R..Rev 170717 31 of 32 EEE3076 Power Electronics: PE1 & PE2 2017/2018OSCILLOSCOPE INFORMATIONBelow are the functions of switches/knobs/buttons:INTENSITY knob: control brightness of displayed waveforms. Make sure the intensity is not too high. FOCUS knob: adjust for clearest line of displayed waveforms.TRIG LEVEL knob: adjust for voltage level where triggering occur (push down to be positive slope trigger and pull up to be negative slope trigger). Trigger COUPLING switch: Select trigger mode. Use either AUTO or NORM. Trigger SOURCE switch: Select the trigger source. Use either CH1 or CH2.HOLDOFF knob: seldom be used. Stabilize trigger. Pull out the knob is CHOP operation. This operation is used for displaying two low frequency waveforms at the same time. X-Y button: seldom be used. Make sure this button is not pushed in. POSITION (Horizontal) knob: control horizontal position of displayed waveforms. Make sure that it is pushed in (pulled up to be ten times sweep magnification). POSITION (vertical) knobs: control vertical positions of displayed waveforms. Pulled out CH1 POSITION knob leads to alternately trigger of CH1 and CH2. Pulled out CH2 POSITION knob leads to inversion of CH2 waveform. Time base: TIME DIV: provide step selection of sweep rate in 1-2-5 step. VARIABLE (for time div) knob: Provides continuously variable sweep rate by a factor of 5. Make sure that it is in full clockwise (at the CAL position, i.e. calibrated sweep rate as indicated at the time div knob).Vertical deflection: VOLTS DIV: provide step selection of deflection in 1-2-5 step. VARIABLE (for volts div) knob: A smaller knob located at the center of VOLTS DIV knob. Fine adjustment of sensitivity, with a factor of 1/3 or lower of the panel-indicated value. Make sure that it is in full clockwise (at the CAL position). Pulled out knob leads to increase the sensitivity of the panel-indicated value by a factor of 5 (x 5 MAG state). Make sure that it is pushed down. AC/GND/DC switches: select input coupling options for CH1 and CH2. AC: display AC component of input signal on oscilloscope screen. DC: display AC + DC components of input signal on oscilloscope screen. GND: display ground level on screen, incorporate with AUTO trigger COUPLING selection). CH1/CH2/DUAL/ADD switch: select the operation mode of the vertical deflection. CH1: CH1 operates alone. CH2: CH2 operates alone. DUAL: Dual-channel operates with CH1 and CH2 swept alternately. This operation is used for displaying two high frequency waveforms at the same time. Note: Keep the oscilloscope ON. The oscilloscope needs an amount of warm up time for stabilization. CAUTION: Never allow the INTENSITY of the displayed waveforms too bright. This can burn the screen material of the oscilloscope. Gobbi.R..Rev 170717 32 of 32

EEE3076 Power Electronics: PE1 & PE2 2017/2018

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