1.0 INTRODUCTION 1.1 Significance This thesis will take a better insight of design and implementation of single-phase controlled rectifier by applying thyristor, namely Silicon Controlled Rectifier (SCR). In addition to that, this thesis will also introduce an effective method of designing the controlled rectifier by means of applying a current shaping circuit at dc side of the rectifier to act as an active shaping element to the input line current. The active shaping element will consists of a step-up dc-dc converter applied at dc side of the rectifier to further improve the output signal generated, with lower distortion produced as well as better power factor. Also, we will look into more details on feedback control applied to generate feedback to the converter in order to compare the output generated with a reference value to minimize the error. YII MING LIONG 1
1.2 Background Power Electronics, in broad terms, can be defined as a task to process and control the flow of electric energy by supplying voltage and currents in a form that is optimally suited for user loads. Figure 1.1 below shows a typical power electronic system in the form of block diagram. Pow er input V in Iin Power Proccessor Control signals Load V o Power output Io Measurements Controller Re ference Figure 1.1: Block diagram of typical power electronic system As reference to the block diagram shown, the power input to the power processor is usually from the utility at line frequency of either 50Hz or 60Hz. The phase angle between the input voltage and the current are very much depended on the topology and the control of the processor. The processed output voltage, current as well as frequency will be as desired by the load. If the power processor s output can be regarded as a voltage source, the output current and the phase angle relationship between the output voltage and the current depend on the load characteristic. Also, normally, a feedback controller will be provided to perform comparison to the output of the power processor unit with a desired value, or as mentioned earlier, reference value, and the error between the two is minimized by the controller. (Mohan Ned, 1995) YII MING LIONG 2
Generally, design of converter that can be considered as a good design, should cover few essential aspects. One of them will have to be efficiency, which can be considered as the ultimate goal of design in power electronic. Besides that, a few current issues can directly and indirectly affected the course of a design of a converter. One of these significant issue is the line quality, whereby is critical to ensure that the utility lines and transformer would supply undistorted wave voltage to customers. Here, the source and line inductances play an important role in the line quality issue. With the presence of the reactive power to the line, it increased the volt-ampere rating. Thus, the input ac line voltage becomes distorted from the higher peak currents. As a result, high reactive components are being used. This is a drawback because a poor power factor causes heavy expenses to the user. Besides that, the growing concern regarding harmonic pollution of the power distribution system has create awareness for clean ac line current and a power factor close to unity. The phase angle of the fundamental harmonic current with respect to the line voltage is a very important parameter that determines the power factor. Above-mentioned issues are some of the critical aspects that should be taken into consideration when designing a good converter. 1.3 General Overview of the Project As mentioned earlier in earlier sections, the ultimate goal of the project is to implement a design of single-phase controlled rectifier by means of Thyristor Bridge. For an effective design, few aspects mentioned in section above should be considered as factors that will determine the quality of this design. Essentially, our aim is to produce a dc signal with less ripples as possible, less harmonic distortion, YII MING LIONG 3
high power factor and thus lead to a high efficiency Basically, the design consists of Thyristor Bridge, the triggering circuit, and the feedback controller applied with the step up converter as a current shaping component. Also, in the latter segments, we shall implement a practical design of single-phase rectifier by means of alternative method, and results obtained from design implemented in practical manner will be used as a reference to perform comparison with the design method applied in simulation manner. These are several important aspects that we will go into more details in sections that follow. 1.4 Organisation of Material The early section of this report focuses on brief discussion on project implemented with aims and method of implementation applied, in abstract. This will be followed by a brief discussion on the history of single phase controlled rectifier and also a more detail description of this project, elaborated in introduction to design and implementation of single-phase controlled rectifier. In chapter two, theoretical review, focus will be on theoretical analysis on controlled rectifier in terms of their operation analysis, as well as functionality of thyristor. Two main chapters, chapter 3 and 4, which is explanation of the whole development stages of simulation process and hardware implementation, will follow this respectively. Here, a better insight will be gain in terms of how the whole design process is achieved through three different development stages, to come out with the final design of the single-phase controlled rectifier. From here, the chapter that YII MING LIONG 4
follows explains in more detail how this final design implemented by means of simulation will be adapted as reference to implement it into hardware. Latter segments will be on comparison of results between output from simulations and output generated by hardware implemented, brief discussion on problems encountered, which will lead to suggestion for further developments and conclusion of the whole thesis, which will be outlined in more details in chapter 5, 6 and 7, respectively. YII MING LIONG 5
2.0 THEORETICAL REVIEW 2.1 Thyristor: Silicon Controlled Rectifier (SCR) Thyristor, or sometime also known as silicon controlled rectifier (SCR), are one of the oldest types of solid-state power device and was known famous because of its highest power-handling capability. They have a unique four-layer construction and are a latching switch that can be turned on by the control terminal (gate), but cannot be turned off by the gate. For better reference, the symbol for the thyristor is shown in figure 2.1. It is essentially the symbol of a diode with a third control terminal, the gate, added to it. Anode IA + VAK- Cathode IG Gate Figure 2.1: Circuit symbol for Thyristor The main current for thyristor flows from anode to cathode. In its off state, the thyristor can block a forward polarity voltage and not conduct. The thyristor can be triggered into the on state by applying a pulse of positive gate current for a short duration provided that the device is in its forward-blocking state. Both of this characteristics can been observed in the figure 2.2 and 2.3 shown below, stating the I-V characteristics and the idealized characteristics of the thyristor. (Mohan Ned, 1995) YII MING LIONG 6
Figure 2.2: I-V characteristics of thyristor Figure 2.3: Idealized characteristics of thyristor Once thyristor begins to conduct, it is latched on and the gate current can be removed. As mentioned earlier, the thyristor itself cannot be turned off by the gate, and the thyristor conducts as a diode, only when the anode current tries to go negative, under the influence of the circuit in which the thyristor is connected, does the thyristor turn off and the current goes to zero. This will allow the gate to regain control in order to turn the device on at some controllable time after it has again entered the forward-blocking state. YII MING LIONG 7
In a diode rectifier, power flows only from input to output and it cannot be controlled. As for the thyristor rectifier, it has additional capability whereby; the power can be converted from the dc to the ac side that is the inverter function. In deed, the fluctuation of power will take place naturally with fluctuation of supply voltage and load. In many applications, there is a need for such power flow control principle that delays the firing angle of the gate for the desired purpose. 2.2 SCR Triggering Circuit The angle delay of the gate, mentioned above can be archive by means of several methods. In PSIM simulation, alpha controller triggers thyristor. Figure 2.4 shows a delay angle alpha controller. Figure 2.4: Delay Angle Alpha Controller There are three inputs for the controller, namely: Alpha value, in degree Synchronization signal Enable/disable signal The alpha controller is enabled (or disabled) if the enable/disable signal is high (or low). The transition of the synchronization signal from low to high (from 0 to 1) provides the synchronization and this moment corresponds to when the delay angle alpha equals zero. A gating pulse with a delay of alpha degrees is generated and sent to the thyristors. The alpha value is updated instantaneously. (Power SIM Inc., 2003) YII MING LIONG 8
The triggering controller is shown in figure 2.5 below: Voltage sensor 2nd-order low pass filter Delay angle alpha controller step voltage source Comparator Firing angle Figure 2.5: Gate triggering controller Figure 2.5: Triggering control circuit Figure 2.6 shows the gating pulse with firing angle set to 30 : Figure 2.6: Gate-trigger Signal with Firing Angle = 30 In practical manner, versatile integrated circuits, such as the TCA780, are available to provide gate trigger signals to the thyristors. Figure 2.7 shown as follows represents a simplified block diagram of a gate trigger control circuit, implemented in hardware manner. (Mohan Ned, 1995) YII MING LIONG 9
Vst Vsynchronization Saw-tooth Generator Vcontrol Vst t + _ Comparator and Logic Gate-trigger signal Vcontrol t Figure 2.7: Gate trigger control circuit Figure 2.8 shows the waveform generated by the gate trigger control circuit. Referring to the waveform shown, the sawtooth waveform synchronized to the ac input is compare with the control signal V control and the firing angle α with respect to the positive zero crossing of ac line voltage is obtained in terms of V control and the peak of the sawtooth waveform is obtained as: Vcontrol α = 180 [2.1] V st Vsynchronization 0 t α ω α ω 0 t Gate-trigger signal 0 Vcontrol Figure 2.8: Waveforms generated by the gate trigger control circuit t YII MING LIONG 10
2.3 Single-phase Controlled Rectifier In this section, we will provide some theoretical review on single-phase controlled rectifier, in terms of their functionality as well as operation of rectifier and effects of different load implied to it, as well as with source inductance presented in ac side of the rectifier. Controlled rectifier has proven to be more effective compare to diode rectifier in terms of certain application such as battery chargers and a class of dc-motor and acmotor drive whereby it is desirable to have the dc voltage to be controllable. Also, in this section, more focus will be on continuous-current-conduction mode, as most of the controlled rectifier s application, the dc-side current I d flows continuously, though brief discussion on discontinuous-current conduction mode will be touched as well. The converter dc current I d cannot change direction, and hence this a type of converter that only involves two-quadrants operation. Figure 2.9 shows a typical single-phase controlled rectifier with a resistive load at dc side. With SCR as a rectifying component, the average dc-side voltage can be controlled from a positive maximum to a negative minimum value in a continuous manner, as the magnitude of the average output voltage in thyristor converter can be controlled by delaying the instants at which the thyristor are allowed to start conduction. YII MING LIONG 11
Iload is S1 S2 + Vs Vd Resistive load S3 S4 _ Figure 2.9: Single-phase bridge rectifier with resistive load For simplicity, it is preferable to state that the rectifier shown above has two pair of SCR, with S 1 and S 4 forming one pair and, S 2 and S 3 the other pair. When it is positive, SCRs S 1 and S 4 can be triggered and then current flows from V S through SCR S 1, load resistor R, SCR S 4 and back into the source. In the next half-cycle, the other pair of SCRs conducts. Even though the direction of current through the source alternates from one half-cycle to the other half-cycle, the current through the load remains unidirectional. The dc side output voltage is controlled by means of varying the firing angle. Consider the effect of applying gate current pulse that are delayed by an angle α, termed firing angle. Let V S = E x Sin ωt, with 0<ωt<360 o. If ωt = 30 o when S 1 and S 4 are triggered, then the firing angle is said to be 30 o. In this instance, the other pair is triggered when ωt = 210 o. YII MING LIONG 12
When V S changes from a positive to a negative value, the current through the load becomes zero at the instant ωt = π radians, since the load is purely resistive and the SCRs cease to conduct. After that there is no current flow till the other pair is triggered. Hence the conduction or current flow through the load is discontinuous. Note that for firing angle α = 180, it cannot be archived in practical manner, as the period o reverse bias of the thyristor is continually reducing as α approaches 180, sufficient time must be allow for the thyristor to turn off and regain forward blocking capability before forward voltage is reapplied, otherwise, commutation failure will occur. Normally, for a 60Hz system, the maximum firing angle is limited to about 160 to 175. Theoretically, the average value of output voltage V d can be obtained as follows: 2 2 V dα = VS cosα = 0.9VS cosα [2.2] π 2.4 Step-up Converter with Feedback Control One of the smoothing component used commonly are LC filters, namely inductance and capacitance been inserted in conjunction with rectifier bridge to improve the waveform of the current drawn from the source. As mentioned in section above, the source inductance, Ls together with the LC filtering applied at both ac side and dc side of the rectifier, respectively, can further improve the input current waveform. To further improve the output signal generated, by means of current shaping, it can be archived through step-up converter with feedback control applied at dc side of the YII MING LIONG 13
rectifier to replace to LC filtering component. With this kind of circuit arrangement, it is possible to shape the input current drawn by the rectifier bridge to be sinusoidal and in phase with the input voltage. For the purpose of better illustration, figure 2.10 shows the circuit configuration. As noticed from the figure shown in 2.10, the source inductance is not included in the figure, for the purpose of simplicity, as our main focus for this section is the theoretical analysis for the step-up converter. il id Iload is + Ld ic + Vs IVsI Cd Vd Step-up converter Figure 2.10: Step-up converter for current shaping At the input side, the input current i s is desired to be sinusoidal and in phase with input voltage Vs, also, at the full bridge rectifier output, i L and absolute value of Vs will have the same waveform as well. For the theoretical analysis below, the power loss of the rectifier bridge and the step-up converter will be neglect due to the fact that the losses are somewhat small. Thus, we have, for V S = 2V and S I S = 2 I S, the input power can be expressed as: P ( t) = V sinω t I sinω t = V I V I cos2ω t in S S S S S S [2.3] The average value of current I d can be expressed as: V I = I S S d = I load [2.4] Vd YII MING LIONG 14
Also, the current through the capacitor is: VS I S ic ( t) = cos2ω t = I d cos2ω t [2.5] V d From these expressions, the ripple in V d can be determined by means of estimation, which is shown as below: V 1 I d ( t) ic dt sin 2ω t C [2.6] 2ω C d, ripple = d The step-up converter in the figure shown is operating in current-regulated mode, as our main purpose is to shape the input current of the step-up converter. The feedback control, represented in block diagram, is shown in figure 2.11 below. As mentioned earlier, this feedback control serves the purpose of comparing the output generated with a reference value, in order to minimize the error between these two. d IVsI Vd* Vd (actual) PI Regulator Error Multiplier i L*(t) Current-Mode Control Switch control signal i L (measured) Figure 2.11: Feedback Control block diagram I L * shown in the figure is the reference value of the current i L in the step-up converter shown earlier. The amplitude of I L * should be such that as to maintain the output voltage at a reference level of V d *, in spite of the variation of load and the fluctuation of the line voltage from its nominal value. The waveform of I L * is obtained by means of measuring absolute value of Vs, by a resistive potential divider and multiplying it with the amplified error between the reference value V d * and the YII MING LIONG 15
actual measured value of V d. In other hand, the actual current I L is sensed, usually by measuring the voltage across a small resistor inserted in the return path pf I L. The status of the switch in the step-up converter is controlled by comparing the actual current I L and I L *. If constant frequency is applied for this feedback control, the ripple current can thus be expressed as: I rip ( V V d S S = [2.7] f L V S d ) V d In terms of maximum ripple current, it can be expressed as: I V d rip, max = [2.8] 4 f S Ld The step-up converter topology is well suited for the input current shaping method because when the switch is off, the input current directly feeds the output stage. YII MING LIONG 16
3.0 SIMULATION OF CONTROLLED RECTIFIER 3.1 Introduction In this chapter, focuses more on implementing the design by means of simulation. For simulation, we have chosen the Power Simulator as simulation software. Using this software, it was possible to simulate the operation to a small load, such as dc motor using the model presented. Furthermore, it also has its advantage of having the model to be examine prior to the extent of actually setting up the system. PSIM has the advantages of being user friendly as well. Another reason that PSIM is very convenient is due to the fact that it has the capability to measure the power factor as well as total harmonic distortion directly by means of VA-power factor meter and total harmonic distortion block, respectively. This will no doubt save a lot of trouble of calculating these parameters. 3.2 Circuit Design Development Stages For this section onwards, a detailed explanation will be main material, as this will explain in details the three main stages until completion of the final design by means of simulation. The 1 st stage will be on studies on fundamental design, while the 2 nd stage will touch more on effects of filtering component and finally, the implementation of a step-up converter with feedback control at dc side of the rectifier for the purpose of further improve the power factor as well as to reduce to total harmonic distortion. YII MING LIONG 17
3.2.1 Studies on Fundamental Design In this initial stage, design procedure is begun by performing several investigations on functionality of thyristor by means of studies on fundamental design. For studies of fundamental design, we focus on a single phase thyristor bridge only without any filtering component in order to observe the effects of firing angle have on output signal generated, as well as effects it have on power factor produced and total harmonic distortion. The circuit configuration to meet the specification is shown as in figure 3.1 below: VA-power factor meter Total harmonic distortion block Figure 3.1: Fundamental design of controlled rectifier Figure 3.2 shows a typical waveform displaying output waveforms corresponding to delay angle of 30, with triggering pulse. Also, figure 3.2(b) shows the waveform for power factor and total harmonic distortion, measured by the power factor meter and total harmonic distortion block. YII MING LIONG 18
Figure 3.2(a): Waveforms for input voltage, triggering pulse and output voltage YII MING LIONG 19
Figure 3.2(b): Waveforms showing power factor and THD measured Corresponding to the waveform displayed above, the total harmonic distortion is 22.6% while the power factor generated is close to unity. Which is 0.98. These results are expected, as we didn t imply any filtering component to the rectifier design. After series of observation done on simulation, by means attempting the simulation by varying the firing angle, from 0, 30, 45, 60, 90, 135, 150 and 180, several valuable points have been discovered, which are: As firing angle is increased from 0 to 180, the power factor of the converter will be worsened, and the power transferred to the dc load, YII MING LIONG 20
which in this case, is only a purely resistive load, will be reduce to zero (VI cosθ =0). When firing angle α = 135, this represent the condition where firing angle is increased beyond 90, in this case, load current can only flow I the load itself presents a negative voltage, which will occur in the case, for example, of a dc motor under overhauling load condition. When firing angle α = 180, the mean dc voltage has reached its maximum negative value, and the ac ripple content has decreased from its maximum at α = 90, to a minimum at α = 180. The SCR will remains in conduction for α = 180 and the SCR current and the resulting ac supply current will lag the source voltage by 180. 3.2.2 Effects of Filtering Component After investigation on fundamental circuit, further investigation were carried out on effects of LC filtering have on the rectifier, with a source inductance presented at ac side of the rectifier. Theoretically, the purpose of the filter is to produce an output voltage, which is close to purely dc. The capacitor holds the output voltage at a constant level, while the inductor smoothes the current from the rectifier and reduces the peak current in the diode. The combination of inductance and capacitance form a low pass filter. The circuit configuration implemented in PSIM to meet the specification of LC filtering and ac side inductance is shown in figure 3.3. YII MING LIONG 21
Figure 3.3: Controlled rectifier with filtering component In figure shown above, extra capacitance of 10µF is applied before the dc side capacitance to further improve the output generated. The capacitance is relatively small compare to the dc side capacitance. This allow large ripple in voltage across this capacitor resulting in improved input current waveform. This ripple will then been filtered out by the dc side low pass filter. Also, a quite large value of inductance and capacitance has been applied due to one of the drawbacks of singlephase rectifier, which is the fact that single-phase system can only be termed 2- pulse. In other words, the ratio of the fundamental dc voltage ripple frequency to that of input ac supply is two, resulting in high smoothing requirements is needed. Note that the greater the pulse number, the lower the smoothing requirements. In practical, three pulses or 6-pulse system is generally applied. YII MING LIONG 22
For reference, output waveform generated corresponding to firing angle of 30º is shown in figure 3.4. Figure 3.4: Output voltage and current waveform for firing angle=30º Also, the power factor and THD measured corresponding to firing angle of 30º is shown in figure 3.5 and 3.6, respectively. YII MING LIONG 23
Figure 3.5: Power factor corresponding to firing angle of 30º Figure 3.6: THD corresponding to firing angle of 30º YII MING LIONG 24
Corresponding to the output generated shown in two figures above, the parameters obtained are as follows: V dmax = 200V V dmin = 195.69V Ripple voltage = 4.31V % Ripple Voltage = 2.22% I dmax = 2A I dmin = 1.95A Ripple current = 0.043A THD = 49.6% Power factor = 0.807 Table 3.1 below shows the results of similar parameters as above as firing angle is varied from 0º to 135º: Parameters Firing angle αº 0º 30º 60º 90º Output voltage (V) 200V 195.9V 179.6V 158V Ripple voltage (V) 4.3V (2.15%) 4.31V (2.22%) 4.49V (2.5%) 3.95V (2.5%) Ripple current (A) 0.043A (2.15%) 0.0431A (2.22%) 0.0449A (2.5%) 0.0395A (2.5%) THD (%) 49% 49.6% 59% 87% Power Factor 0.81 0.807 0.746 0.52 Table 3.1: Results from simulation for varying firing angle YII MING LIONG 25
Observing from the results shown above, it clearly indicates that the validity of theoretical analysis holds as power factor got worsen as firing angle is increased from 0º to 135º. Also, the output voltage also decreased as due to the firing angle delay trigger by thyristor, which is the main component which results in controlled dc voltage in practical implementation. However, this design is rather not desirable as we look at the results corresponding to firing angle of 0º, which can be represented by diode rectifier, as the triggering angle of 0º is equivalent to operation of diode. Refer back to the results shown: Output voltage = 200V % of ripple = 2.15% THD = 49% Power Factor = 0.81 The efficiency generated is rather low as the input voltage is 240V, which lead to an 200(2) efficiency of only η % = = 69.44%. Also, implementing this kind of 240(2.4) circuit configuration, however, can only generate a highest power factor of 0.81, which is also rather not satisfying. For complete output waveforms for simulation without feedback control, refer to appendix A. In order to further improve the power factor and reduce the total harmonic distortion, a step-up converter with feedback control is applied at dc side of the circuit, as mentioned in sections earlier. YII MING LIONG 26
3.3 Complete Design: Step-up Converter at DC side In earlier sections of this thesis, some theoretical analysis has been touched, on stepup converter with its feedback control. Now, in this section, we shall focus on the complete design on the controlled rectifier with step-up converter at dc side to act as a current shaping circuit. 3.3.1 Circuit Configuration The feedback controller circuit configuration is shown in figure 3.7. Voltage sensor Multiplier Proportional-Integral Controller Summer Limiter Comparator 3.3.2 Simulation Results Absolute function block 3.3.3 Comparison DC voltage of Results source To be connect to current sensor To be connect to mosfet switch Triangular-wave voltage source Figure 3.7: Feedback controller circuit configuration The feedback controller shown above operates by comparing the output generated with a reference value set. In simulation, the actual output current is sensed by a current sensor connected at dc side of the rectifier, and this actual value of I d is sent to negative probe of the summer, as shown in figure above. The reference value of I d is transmitted through positive probe of summer. The comparison of signal will take place at the comparator, with signal generated from PI controller and triangular wave. YII MING LIONG 27
For better reference, figure 3.8 shows the complete design of single-phase controlled rectifier with step-up converter and feedback control implemented in PSIM. current sensor Step-up converter On-off switch controller Figure 3.8: Thyristor converter with step-up converter 3.3.2 Simulation Results Simulation is carried out based on similar parameters applied on rectifier design without step-up converter in order to observe the difference between these two designs. Similarly, for better illustration, simulation results corresponding to firing angle of 30º are shown as in figure 3.9. YII MING LIONG 28
Figure 3.9: Waveforms for output voltage and output current Figure 3.10 shows the waveform of I d (actual value) measured at the point of current sensor. This waveform corresponds to I d before been transmitted to feedback to be compare with reference value. YII MING LIONG 29
Figure 3.10: Waveform for I d measured at current sensor Figure 3.11 ad 3.12 shows the waveform for power factor produced as well as total harmonic distortion, respectively. Figure 3.11: Power factor corresponding to firing angle of 30 YII MING LIONG 30
Figure 3.12: THD produced corresponding to firing angle of 30 Table 3.2 shows performance parameters measured corresponding to firing angle of 0º, 30º, 60º and 90º. Parameters Firing angle αº 0º 30º 60º 90º Output voltage (V) 246.5V 248.3V 242.6V 198.3V Ripple voltage (V) 3V (1.21%) 4.6V (1.8%) 5.58V (2.3%) 5.3V (2.67%) Ripple current (A) 0.03A (1.21%) 0.046 (1.8%) 0.04 (2.3%) 0.053 (2.67%) THD (%) 21% 30.6% 52% 74% Power Factor 0.98 0.953 0.83 0.606 Table 3.2: Parameters measured corresponding to various firing angle YII MING LIONG 31
3.3.3 Comparison of Results In this section, we will look back at results for two types of rectifier design configuration to perform comparison between each other in order to determine which design is better than another. For comparison purpose, table 3.3 below shows some of the essential parameters corresponding to firing angle of 0 to determine which design configuration are superior to another. Parameters Types of rectifier configuration Without Step-up With Step-up converter and Converter feedback control Output voltage (V) 200V 246.5V Ripple voltage (V) 4.3V (2.15%) 3V (1.21%) THD (%) 49% 21% Power Factor 0.81 0.98 Table 3.3: Comparison of results Referring to results generated in table above, its obvious that thyristor converter with step-up converter and feedback control shows an improvement in overall aspects. In terms of THD, it increases from 49% to 21%. While as for power factor produced, it increase from 0.81 to 0.98. 3.3.4 Final Design This section will act as a conclusion the rectifier design implemented in simulation. Generally, the final design for simulation will be basically the thyristor controlled rectifier with step-up converter and feedback control. However, several variation has been made as required. Firstly, the input voltage is set to 20V rather than 240V, as our primary interest is low power application. Corresponding to input voltage of YII MING LIONG 32
20V, not much change have been made to the design parameters, with only variation on values of inductance and capacitance applied to the design. The values of inductance are shown in figure 3.13 below. Figure 3.13: Final design of controlled rectifier All waveforms are shown figures3.14, 3.15 and 3.16 that follow. YII MING LIONG 33
Figure 3.14: Output voltage and current waveform Figure 3.15: Waveform for power factor YII MING LIONG 34
Figure 3.16: Waveform for THD To summarize the whole design process by means of simulation manner, results generated for various firing angle is shown in table 3.4 below. Parameters Firing angle αº 0º 30º 60º 90º Output voltage (V) 37.70V 37.35V 33.55V 27V Ripple voltage (V) 1.24V (3.3%) 1.3V (3.48%) 1.54V (4.6%) 1.8V (6.6%) Ripple current (A) 0.0124A (3.3%) 0.013A (3.48%) 0.0154A (4.6%) 0.018A (6.6%) THD (%) 30.2% 36.2% 79% 140% Power Factor 0.96 0.94 0.77 0.52 Table 3.4: Performance parameters for final design For complete design development stages in simulation by PSIM by means results generated, please refer to Appendix A. YII MING LIONG 35
4.0 HARDWARE IMPLEMENTATION 4.1 Introduction As till chapter 3 in thesis, a flow of project milestone of implementing a design of controlled rectifier by means of thyristor and step-up converter has been shown. In this chapter, focuses will be on hardware implementation of single-phase controlled rectifier by an alternative approach. An early section, focuses will be on explanation of several important components applied in this practical design. This will followed by a detailed explanation on the design principles of the hardware as well as operation analysis. Finally, observations will be on results generated from this practical implementation. 4.2 Elements of Hardware In this section, a more detailed explanation on several important components will be touched on. For this hardware implementation, main components are IC LM723 and Darlington transistor pair. 4.2.1 LM723: Voltage Regulator The LM723, or in its full terms, MC1723C is a positive or negative voltage regulator designed primarily for series regulator applications to deliver load current up to 150mA. Output current capability can be increased to several amperes through use of one or more external pass transistors. MC1723C is specified for operation over the commercial temperature range of 0 to +70 C. (Motorola Analog IC device data) Several specification of LM723 is as follows: YII MING LIONG 36
Output Voltage Adjustable from 2.0 Vdc to 37 Vdc Output Current to 150 madc Without External Pass Transistors 0.01% Line and 0.03% Load Regulation Adjustable Short Circuit Protection Figure 4.1 below shows a side view of LM723 Figure 4.1: MC1723C For a detailed specification of MC1723C, specification sheet is available in Appendix B. Note that, the MC1723C is also useful in a wide range of other applications such as a shunt regulator, a current regulator or a temperature controller. For the purpose of reference, specification of every pin on MC1723C is shown as figure 4.2. YII MING LIONG 37
NC NC CURRENT LIMIT FREQUENCY COMP CURRENT SENSE +Vs INVERT INP UT NON-INVERT INP UT Vref -Vs Vc Vo Vz NC Figure 4.2: Pin connection of MC1723C 4.2.2 2SA966 & 2N3055: Darlington Pair Another important elements in the practical design that contributes to voltage regulation are TR1 and TR2, which is PNP transistor coded 2SA966 and NPN pass transistor 2N3055. A Darlington pair is generally used to amplify weak signals so that they can be clearly detected by another circuit. The first transistor s emitter feeds into the second transistor s base and as a result the input signal is amplified by the time it reaches the output. Figure 4.3 below shows the transistor configuration of Darlington pair TR1 2sA966 Darlington pair TR2 2N3055 Figure 4.3: Darlington Transistor pair YII MING LIONG 38
Figure 4.4 shows the overall view of TR1 2SA966. Pin 1-Emitter Pin 2-Collector Pin 3-Base Figure 4.4: Overall view of PNP transistor 2SA966 For detailed specification sheet of 2SA966, please refer to Appendix A. TR1 2SA966 has a simple operation principle of amplifying signal transmitted through it. Thus, it act as the first transistor in Darlington pair to amplify the signal sent towards it and pass through emitter to reach to NPN pass transistor, which is TR2 2N3055 to provide the amplified output signal. Figure 4.5 shows the outlook of TR2 2N3055. 2N3055 will act as second transistor to further amplify the signal transmitted from emitter of TR1 and transmit the output signal. In practical manner, 2N3055 is mounted on heat sink to effectively perform heat dissipations as 2N3055 usually operates under high current conditions. For full specification of 2N3055, please refer to Appendix B. Figure 4.5: Outlook of TR2 2N3055 YII MING LIONG 39
4.2.3 Other Elements of the practical Design Above mentioned two important elements that required for detailed explanation. The rest of the component applied in this design are some simple components such as Diode Bridge, diode, resistors and capacitors, with normal LED indicator and elements such as step down transformer and metering component. Diode Bridge used in this design is under the code of KP206G. Diodes used in this practical design are switching diode, under the code of IN4007 and IN539. The value of variable resistor applied for the purpose of controlling the voltage is 5KΩ. As for resistors and capacitors used in this practical design, it is as shown below. Capacitance C1 = 1000µF C2 = 0.01µF C3 = 470µF Resistance R1 = 10KΩ R2 = 3KΩ R3 = 12KΩ R4 = 10KΩ R5 = 7.5KΩ R6 = 330Ω R7 = 1KΩ R8 = 240Ω VR = 5KΩ Table 4.1: Values for capacitors and resistors YII MING LIONG 40
4.3 Design of Hardware In hardware manner, the variable dc output will be controlled by means of applying a regulator chip LM723 to control the output voltage generated. This design method is rather different than the design approach implement in simulation. One of the reasons for this is that an exact solution for hardware implementation based on design in simulation has not yet been found; these reasons will be outlined in more details in problems encountered and suggestions for further development section. For this practical design, in terms of rectifying component, Diode Bridge will be applied for this hardware implementation. For better illustration, the circuit diagram for the practical design is shown as in figure 4.6 below. Vs D1 D3 KP206G D2 D4 1000uF + _ IN539 LED indicator 10k VR 5K 3k 12k X X X 1 2 3 4 5 6 7 LM723 14 X 13 12 11 10 X 9 8 X 330 + 10k 7.5k 0.01uF _ 240R 1k TR1 2sA966 IN4007 Darlington pair TR2 2N3055 470uF + C3 + Output Figure 4.6: Circuit diagram of practical converter design For this particular design, the input voltage Vs is equal to 18V, which is stepped down by a transformer, not shown in figure. Diode D1, D2, D3 and D4 shown in figure forms the diode bridge, KP206G. Also, another diode, IN539 is applied at dc YII MING LIONG 41
side. One of the functions for this diode is to act as a feedback blocker, whereby it steers any current that might be coming from the device under power around the regulator to prevent the regulator from damages. These sorts of reverse current usually occur when the rectifier is been powered down. Component code and the specification for the component is all shown in figure above for easy reference purpose. LED indicator shown in figure is for the purpose of being a operation indicator. A detailed operation analysis will be elaborated in section that follows. 4.4 Operation Analysis Basically, this practical rectifier design applies a different approach as compare to design implement in simulation. Referring to the figure shown, as the transformer, to an ac secondary voltage of 18V, steps down the input voltage. The voltage rectified by the diode bridge to produce unfiltered dc output voltage. This unfiltered dc output voltage will contain big ripple and is pulsating. This pulsating output voltage will then been filtered by the capacitive filter of 1100µF capacitor in order for manageable for the regulator. As notice in figure shown, there s only capacitive filtering applied to the design, this is due to the fact that in low-power applications, the inductor required for rectification design could be a costly item, that s the main reason most low-power converters dispense with the inductor and apply an direct capacitive filtering method. With no load, the DC voltage across the terminals of the filter is going to be 18 to 30 volts. YII MING LIONG 42
The regulation is obtained using the Darlington pair (TR1 and TR2). They in turn are controlled by the 723 regulator The 723 have its own internal highly regulated voltage reference supply (pin 6). Internally the 723 compare this reference voltage to the output of the power supply and it is varied by means of variable resistor VR, shown in figure. This sets the output voltage. The regulation process evolves around pin 11 and pin 6 of LM723 regulator. Pin 11 of 723-regulator control voltage supply, and this V C will trigger the base of TR1, which is 2SA966, PNP transistor, which will act as a simple amplifier to increase the current available to drive the base of the pass transistor, i.e. TR2 2N3055.This explains the function of Darlington transistor pair applied. Capacitance of 0.01µF connected to frequency component (pin 13) of 723-regulator function as a transient response improver, which improves the response of the regulator when it is operating during high frequencies. Regulated dc output, will be filtered again by capacitance of 470µF to produce an output voltage that contains a minimum ripple and close to pure dc voltage. Necessary protective device are all been installed in this practical design such as fuses, but its not shown in the figure. Also, metering component for voltage and current have also been installed in this practical design for better reference. Note here, these metering component are meant for as a guideline as the accuracy of these meters might be a ±1V difference for the case of voltmeter. Therefore, for better accuracy, a multimeter should be used. Figure 4.7 shows an overall view of practical design. Notice in the figure TR2, 2N3055 are mounted on a huge heat sink. This step is necessary, as the heat sink YII MING LIONG 43
installed will helps to dissipate the massive flow of heat generated to the pass transistor. Figure 4.7(a): Main component arrangement of the practical design Figure 4.7(b): Overall view of practical design implemented YII MING LIONG 44
Figure 4.7(c): Overview of measurement process, with resistive load of 100Ω 4.5 Results The figure shown below shows several output waveforms captured on oscilloscope. Figure 4.8 shows the output waveform when the rectifier output voltage is regulated to a minimum level, which in this case, set at 1V, in order to observe to ripple voltage of the output. Figure 4.8: Waveform for output voltage (minimum level) YII MING LIONG 45
Observing from the output waveform shown above, results gathered from output generated are shown as below. V dmax = 1.08V V dmin = 960mV V d, avg = 1.02V Ripple voltage, V ripple = 120mV % Ripple Voltage = 11.76% Similarly, figure 4.9 shows the output waveform corresponding to the dc voltage when the rectifier output is regulated to produce a maximum voltage. Figure 4.9: Waveform for output voltage (maximum level) All measured results are shown as below. For comparison purpose, we take the results corresponding to maximum level of output voltage. For measurement purpose, the practical design is connected to a simple resistive load of 100Ω, which have a total power dissipation of 5W. These results will then be used as reference to YII MING LIONG 46
compare with results from design implemented in simulation, which will be touched in more details in next chapter. V dmax = 16.2V V dmin = 15.4V V d, avg = 15.8V Ripple voltage, V ripple = 800mV % Ripple Voltage = 5.06% I d, avg = 0.18A Ripple current, I ripple = 0. 032 A 2 2 I rms = I d + I ripple = 0.0324 + 0.001 = 0. 1828A [4.1] By applying the method of ac and dc side power balance, we can determine the inphase fundamental component of the ac current, which will be required for the calculations of Total harmonic distortion as well as power factor: V I ac ac I ac cosφ = V V cosφ = o I o o I o + V V + V diode ac diode I rms I rms (15.8)(0.18) + (0.7)(0.1828) = = 0.165A 18 [4.2] Because we are dealing with Diode Bridge, therefore I ac, which is represented by I 1, be found as: I I ac cosφ (0.165) = I ac = = 0. 165A cosφ cos (0 ) 1 = YII MING LIONG 47
Thus, the total harmonic distortion of the system can be calculated as: THD = THD = I 2 rms 2 I1 I 0.1828 2 1 2 0.165 100% 0.165 2 2 100% = 47.7 % [4.3] Current harmonic factor can be expressed as: I1 0.165 CHF = = = 0.90 [4.4] 0.1828 I rms Therefore, the power factor produced by calculation can be expressed as: PF = ( CHF) ( DPF) = (0.9)(cos0 ) = 0.90 [4.5] YII MING LIONG 48
5.0 COMPARISON WITH DESIGN IN SIMULATION 5.1 Introduction So far, this thesis has shown most of the work done for this project of simulation and implementation of single-phase rectifier. Until this extend, the work has involved theoretical review, design in simulations and design implementation in practical manner. In this chapter, prominence will be on analysis and comparison of two designs implemented in a different manner, as mentioned in section before. From there, similarities and difference will be listed out and results generated from both of the design method will be compare as well to observe which design is superior to another. YII MING LIONG 49
5.2 Comparison of Results As design implemented from hardware manner, which applied the diode bridge as main conversion component, for comparison purpose, we compare results from design in simulation corresponding to firing angle of 0. Table 5.1 below shows results from both of the design. Parameters Design method Design from Simulation Practical Design (firing angle = 0 ) Input Voltage (V ac ) 20V 18V Output Voltage (V d,avg ) 37.7V 15.8V Ripple voltage (V ripple ) 1.24V (3.3%) 800mV (5.06%) Output Current (I d ) 0.37A 0.18A Output power (W) 13.9W 2.84W THD (%) 30.2% 47.7% Power Factor 0.96 0.90 Table 5.1: Comparison of results Referring to results shown in table above, the design implemented in simulation use 20V as input voltage, as mentioned earlier, as main concern is on low power application. While in hardware design, the input voltage is in 18V, which is stepped down by a step down transformer. 18V secondary side voltage is one of the common rates of voltage used in terms of low voltage application. Results shown above clearly indicate that results generated from simulation have gain advantages over the practical design. These are shown through power factor produced and total harmonic distortion created. However, the difference in terms of YII MING LIONG 50
power factor and ripple voltage didn t show a big difference. The only major difference comes from the total harmonic distortion. Note that when comes to comparison of results from simulation and practical manner, some slight discrepancies should be taken into account. This is due to the fact that from simulation point of view, generally ideal components are used and so they have theoretical constraints associated with them, which is possible to differ slightly from the actual physical component. As for practical converter, there might be losses in the process of stepping down voltage as well as where conversion from ac to dc takes place. Besides that, each of the components, particularly resistors and diode used, all contribute to losses generated. Lastly, the comparison of results shown above is mainly for reference purpose as both design are implemented based on different design approach. However, if making a conclusion based on an overview of the results, the design approach from simulation generates a better output compare to outcome generated from the practical design. The sections that follows will touch in more details difference between these two design approaches. YII MING LIONG 51
5.3 Difference Between Two Design Obviously, the design of AC to DC converter is commonly used as power supply. Observing from the two designs shown so far, the design method used in simulation, which involves switching of thyristor is a switching power supply. While as for rectifier design implemented in hardware manner, by series regulator chip LM723, is obviously a linear power supply. There exist only one common similarity between these two designs, which is the fact that they both are designed for the aim of controlled dc output voltage, or in other words, output voltage produced is variable and can be controlled in a range of value. In the case of design implemented in PSIM simulation, we are actually dealing with design configuration that involves a dc voltage controlled by means of thyristor, which is triggered by a firing angle delay to produce a variable dc voltage, as desired by user. Also, this design applied a step-up converter at dc side with a feedback control to form a sensing circuit continuously monitors the output voltage, adjusting the switching duty cycle to maintain a constant voltage output. As for design implemented in practical manner, it utilizes the LM723 regulator chip to perform series regulation in order to control the output voltage under desired value. Therefore, this kind of design configuration can also be known as series regulated power supply design. From overall point of view, as well as based on results shown in earlier sections, plus some studies, it clearly indicates that Switching power supplies are generally more efficient than series-regulated supply, due to the fact that only little power is YII MING LIONG 52
dissipated in a switching transistor. Switching supplies are physically smaller than series-regulated types because components operating at the switching frequency (typically 20 khz) are much smaller than those used in a non-switching series regulated supply operating at 50 to 60 Hz. The advantages of higher frequencies include reduced component size; lower ripple voltage and higher power per unit volume. However, there are a few drawbacks for switching power supply despite all the advantages mentioned above. Some of the obvious drawbacks are as follows: Switching-type power supplies are electrically and sometimes audibly noisy. Thus, they are unsuitable for powering circuits that are sensitive to electrical noise unless those circuits are filtered and shielded. Limited response to dynamic load changes. Unlike series regulated supplies with very low output impedance, load voltage correction in a switcher takes place only after a full cycle of the oscillator. In addition, the control-loop time constant is set to integrate the output voltage change over several cycles to prevent continuous ringing. A well-designed linear power supply has an output noise level of less than 1mV peak-to-peak, compared with peak-to-peak voltage of 10mV for the same capacity switcher. Both conducted and radiated noise and switching frequency harmonics extend into the radio frequency spectrum. Switching power supplies are generally more costly than other power supplies. YII MING LIONG 53
6.0 PROBLEMS ENCOUNTERED & SUGGESTIONS FOR FURTHER DEVELOPMENT 6.1 Introduction From an overall point of view, the project of design and implementation of singlephase controlled rectifier has reached its project milestone. However, some problems have been encountered along the course of this project. These problems did, directly and indirectly interrupted the completion of this project. In this segments, focuses will be on problems faced during the course of project implementation. Also, in latter section of this chapter, some suggestion for further development will be elaborated in more details. Basically, there are two major problems that interrupted the completion of the project, which is simulation problem and practical design problem. These problems are highlighted in sections that follow. 6.2 Simulation Problem The most obvious problem faced regarding the simulation problem is due to the fact that the software employ for this project, PSIM, is only in student version. This implies that limitations have been set. For example, the total number of measuring component are limited to 8, it means that if number of waveform to display in SIMVIEW exceed the total of 8, the simulation process cannot take place. This indirectly slow down the process of simulation as for a configuration of circuit, its required to take several simulation turns to view all waveforms needed. YII MING LIONG 54