EEE3076 Power Electronics: PE1 & PE2 2016/2017
FACULTY OF ENGINEERING
LAB SHEET
EEE 3076 POWER ELECTRONICS
TRIMESTER 1, 2016-2017
PE 1 – Power Semiconductor Switches
PE 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.
Technical part of this lab sheet has been revised, there may be some grammatical errors. We will correct them in next version. Apologies for any inconveniences caused.
Experiment PE1
Power Semiconductor Switches
A. 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 is
LO1 - Select an appropriate power semiconductor switch for converters based on given input and output specifications. (cognitive – applying, level 3)
C. 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): 100kW or 2 x 50kW 1
6. Resistor: 10W/0.25W 1
7. Resistor: 100W/0.25W 1
8. Resistor: 1kW/0.25W 2
9. Resistor: 2kW/0.25W 1
10. Resistor: 10kW/0.25W 1
11. Resistor: 22kW/0.25W 1
12. Resistor: 1MW/0.25W 2
13. Resistor: 100W/2W 1
14. Ceramic disc capacitor: 1nF 2
15. Electrolytic capacitor: 47mF/(16V or above) 2
16. Electrolytic capacitor: 100mF/(16V or above) 1
17. Inductor: 100mH/0.29W, 0.79A 1
D. Introduction
1. Introduction of SCR
The 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.
2. Operation of SCR
Fig-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.
3. Current-Voltage Characteristics of an SCR
The 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.
4. Switching parameters of BJT
Bipolar 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.
E. Experiment
Experimental effort evaluation
Student is expected to use multimeter and oscilloscope for all measurement and monitoring during this experiments.
Part I: SCR switch
Know 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:
Table 1
No. / S1 / S2 / Multimeter Reading / State (On/Off)1 / Close / Open
2 / Close / Close
3 / Close / Open
4 / Open / Open
Questions:
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.
Section 2: Basic operation of an SCR
Results:
Table 2
Procedures:
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
Section 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?
Section 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 0W.
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 (DVAK) 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
(DVAK ~ 0.02V)
Questions:
i) Plot a graph IA versus VAK, where . Comment on your graph.
ii) From the graph, determine the holding current.
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 VS in the circuits in Fig-11.
Fig-10: Voltage regulator IC 7805 circuit for fixed +5V output.
Section 4: Temporal switching behaviour of BJT and effect of inductor
Note: 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.
Section 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:
1) 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 (5V/div, ground level: +2 division, trigger edge: +) to P1 and draw the waveform (keep this connected all the time),
ii) Connect CH2 (2V/div) to P2 and draw the waveform,
iii) Connect CH2 (2V/div) to P3 and draw the waveform,