Elec581 chapter 2 - fundamental elements of power eletronics

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Elec581 chapter 2 - fundamental elements of power eletronics

  1. 1. Chapter 2 Fundamental Elements of Power Electronics
  2. 2.  To understand the operation of electronic circuits, it is useful to imagine that individual terminals have a potential level with respect to a reference terminal.  The reference terminal is any convenient point chosen in a circuit it is assumed to have zero electric potential.  The potential level of all other points is then measured with respect to this zero reference. 2 Introduction Potential level Advanced Electric Machines and Drives
  3. 3. 3 Advanced Electric Machines and Drives Potential level Introduction
  4. 4. 4 Voltage across some circuit elements Advanced Electric Machines and Drives Sources  By definition, ideal ac and dc voltage sources have zero internal impedance. We suppose that nothing that happens in a circuit can modify these levels.
  5. 5. 5 Voltage across some circuit elements Advanced Electric Machines and Drives Potential across a switch  When a switch is open, the voltage across its terminals depends exclusively upon the external elements that make up the circuit.  On the other hand, when the switch is closed the potential level of both terminals.  This simple rule also applies to idealized transistors, thyristors and diodes, because they behave like perfect switches
  6. 6. 6 Voltage across some circuit elements Advanced Electric Machines and Drives Potential across a resistor  If no current flows in a resistor. its terminals 3, 4 must be at the same potential, because the IR drop is zero  On the other hand, if the resistor carries a current I, the IR drop produces a corresponding potential difference between the terminals.
  7. 7.  The terminals of a coil are at the same potential only during those moments when the current is not changing.  If the current varies, the potential difference is given by. 7 Voltage across some circuit elements Advanced Electric Machines and Drives Potential across an inductance
  8. 8.  The terminals of a capacitor are at the same potential only when the capacitor is completely discharged.  the potential difference between the terminals remains unchanged during those intervals when the current f is zero 8 Voltage across some circuit elements Advanced Electric Machines and Drives Potential across an capacitor
  9. 9.  A final rule regarding potential levels is worth remembering. Unless we know otherwise. we assume the following initial conditions:. a) All currents in the circuit are zero and none are in the process of changing. b) All capacitors are discharged. 9 Voltage across some circuit elements Advanced Electric Machines and Drives Initial Potential Level
  10. 10.  A diode is an electronic device possessing two terminals, respectively called anode (A) and cathode (K)  Although it has no moving parts, a diode acts like a high-speed switch whose contacts open and close according to the following rules: 10 The Diode and Diode Circuits Diode Advanced Electric Machines and Drives
  11. 11. Rule 1. When no voltage is applied across a diode. it acts like an open switch. The circuit is therefore open between terminals A and K Rule 2. If we apply an inverse voltage E2 across the diode so that the anode is negative with respect to the cathode, the diode continues to act as an open switch. We say that the diode is reverse biased. 11 Advanced Electric Machines and Drives The Diode and Diode Circuits Diode
  12. 12. Rule 3. If a momentary forward voltage E1 of 0.7 V or more is applied across the terminals so that anode A is slightly positive with respect to the cathode, the terminals become short-circuited. The diode acts like a closed switch and a current I immediately begins to flow from anode to cathode. We say that the diode is forward biased. 12 Advanced Electric Machines and Drives The Diode and Diode Circuits Diode
  13. 13. Rule 3. In practice, while the diode conducts, a small voltage drop appears across its terminals. However, the voltage drop has an upper value of about 1.5V, so it can be neglected in most electronic circuits. It is precisely because the voltage drop is small with respect to other circuit voltages that we can assume the diode is essentially a closed switch when it conducts. 13 Advanced Electric Machines and Drives The Diode and Diode Circuits Diode
  14. 14. Rule 4. As long as current flows, the diode acts like a closed switch. However, if it stops flowing for even as little as 10 µs, the ideal diode immediately returns to its original open state 14 Advanced Electric Machines and Drives The Diode and Diode Circuits Diode
  15. 15.  Diodes have many applications, some of which are found again and again, in one form or another, in electronic power. 15 Diode circuit- Battery charger circuit with series resistor Advanced Electric Machines and Drives The Diode and Diode Circuits Circuit Waveforms
  16. 16. 16 Diode circuit- Battery charger circuit with series inductor Advanced Electric Machines and Drives The Diode and Diode Circuits WaveformsCircuit
  17. 17. 17 Diode circuit- Single phase rectifier Advanced Electric Machines and Drives The Diode and Diode Circuits Circuit Waveforms
  18. 18.  The rectifier circuits we have studied so far produce pulsating voltages and currents.  In some types of loads, we cannot tolerate such pulsations, and filters must be used to smooth out the valleys and peaks.  The basic purpose of a dc filter is to produce a smooth power flow into a load.  Consequently, a filter must absorb energy whenever the dc voltage or current tends to rise, and it must release energy whenever the voltage or current tends to fall.  In this way the filter tends to maintain a constant voltage and current in the load. 18 The Diode and Diode Circuits Filters Advanced Electric Machines and Drives
  19. 19.  The most common filters are inductors and capacitors.  Inductors store energy in their magnetic field. They tend to maintain a constant current; consequently. they are placed in series with the load.  Capacitors store energy in their electric field. They tend to maintain a constant voltage; consequently, they are placed in parallel with the load 19 The Diode and Diode Circuits Filters Advanced Electric Machines and Drives Circuits
  20. 20.  The greater the amount of energy stored in the filter, the better is the filtering action. In the case of a bridge rectifier using an inductor, the peak-to-peak ripple in percent is given by:  ripple peak-to-peak current as a percent of the dc current [%]  WL = dc energy stored in the smoothing inductor [J]  P = dc power drawn by the load [W]  f = frequency of the source [Hz] 20 The Diode and Diode Circuits Filters Advanced Electric Machines and Drives
  21. 21. We wish to build a 135 V, 20 A dc power supply using a single-phase bridge rectifier and an inductive filter. The peak-to-peak current ripple should be about 10%. If a 60 Hz ac source is available, calculate the following: a) The effective value of the ac voltage b) The energy stored in the inductor c) The inductance of the inductor d) The peak-to-peak current ripple 21 The Diode and Diode Circuits Exercise Advanced Electric Machines and Drives
  22. 22.  An example of a 3-phase rectifier composed of 3 diodes connected in series with the secondary windings of a 3-phase, delta-wye transformer is shown in the following figure.  The line-to-neutral voltage has a peak value Em. A filter inductance L is connected in series with the load, so that current Id remains essentially ripple-free. 22 The Diode and Diode Circuits Three phase rectifier Advanced Electric Machines and Drives Circuit
  23. 23. The Diode and Diode Circuits Three phase rectifier Waveforms 23 Advanced Electric Machines and Drives
  24. 24. The Diode and Diode Circuits Three phase rectifier  The sudden switch over from one diode to another called commutation. When the switchover takes place automatically, it is called natural commutation, or line commutation because it is the line voltage that forces the transfer of current from one diode to the next.  Voltage EKN across the load and inductor pulsates between 0.5 and Em. The ripple voltage is therefore smaller than that produced by a single-phase bridge rectifier. Moreover, the fundamental ripple frequency is three times the supply frequency, which makes it easier to achieve good filtering. 24 Advanced Electric Machines and Drives
  25. 25.  Consider the circuit of a 3-phase rectifier  The 6 diodes constitute what is called a 3-phase, 6-pulse rectifier. It is called 6-pulse because the currents flowing in the 6 diodes start at 6 different moments during each cycle of the line frequency. However, each diode still conducts for only 120°. The Diode and Diode Circuits Three-phase, 6-pulse rectifier Circuit 25 Advanced Electric Machines and Drives
  26. 26. The Diode and Diode Circuits Three-phase, 6-pulse rectifier 26 Advanced Electric Machines and Drives
  27. 27. The Diode and Diode Circuits Three-phase, 6-pulse rectifier Waveforms 27 Advanced Electric Machines and Drives
  28. 28.  The average dc output voltage is given by  The approximate peak-to-peak current ripple in percent is given by The Diode and Diode Circuits Three-phase, 6-pulse rectifier 28 Advanced Electric Machines and Drives
  29. 29.  A 3-phase bridge rectifier has to supply power to a 360 kW, 240 V dc load. If a 600 V, 3-phase, 60 Hz feeder is available, calculate the following: a) Voltage rating of the 3-phase transformer b) DC current per diode c) PIV across each diode d) Peak-to-peak ripple in the output voltage and its frequency e) Calculate the inductance of the smoothing choke required, if the peak-to-peak ripple in the current is not to exceed 5 percent. 29 The Diode and Diode Circuits Exercise Advanced Electric Machines and Drives
  30. 30.  A thyristor is an electronic switch, similar to a diode, but wherein the instant of conduction can be controlled. Like a diode, a thyristor possesses an anode and a cathode, plus a third terminal called a gate.  To initiate conduction, two conditions have to be met: 30 The Thyristor and Thyristor Circuits What is a thyristor? a) The anode must be positive. b) A current must flow into the gate for at least a few microseconds. In practice, the current is injected by applying a short positive voltage pulse to the gate Advanced Electric Machines and Drives
  31. 31.  As soon as conduction starts, the gate loses all further control. Conduction will only stop when anode current I falls to zero, after which the gate again exerts control.  Basically, a thyristor behaves the same way a diode does except that the gate enables us to initiate conduction precisely when we want to.  This enables us not only to convert ac power into dc power, but also to do the reverse: convert dc power into ac power. 31 The Thyristor and Thyristor Circuits What is a thyristor? Advanced Electric Machines and Drives
  32. 32.  As soon as conduction starts, the gate loses all further control. Conduction will only stop when anode current I falls to zero, after which the gate again exerts control.  Basically, a thyristor behaves the same way a diode does except that the gate enables us to initiate conduction precisely when we want to.  This enables us not only to convert ac power into dc power, but also to do the reverse: convert dc power into ac power. The Thyristor and Thyristor Circuits What is a thyristor? Advanced Electric Machines and Drives 32
  33. 33.  Consider the circuit of a thyristor and a resistor connected in series across an ac source. A number of short positive pulses Eg is applied to the gate, of sufficient amplitude to initiate conduction, provided the anode is positive. These pulses may be generated by a manual switch or an electronic control circuit. 33 Advanced Electric Machines and Drives The Thyristor and Thyristor Circuits Principle of gate firing
  34. 34. 34 Advanced Electric Machines and Drives The Thyristor and Thyristor Circuits Principle of gate firing
  35. 35.  When a voltage pulse is applied to the gate, a certain gate current flows. Because the pulses last only a few microseconds, the average power supplied to the gate is very small, in comparison to the average power supplied to the load.  The ratio of the two powers, called power gain, may exceed one million. Thus, an average gate input of only I W may control a load of 1000 k W.  An SCR does not, of course, have the magical property of turning one watt into a million watts. The large power actually comes from an appropriate power source, and the SCR gate only serves to control the power flow. 35 Advanced Electric Machines and Drives The Thyristor and Thyristor Circuits Power gain of thyristor
  36. 36. 36 Advanced Electric Machines and Drives  A thyristor ceases to conduct and the gate regains control only after the anode current falls to zero.  The current may cease flowing quite naturally at the end of each cycle or we can force it to zero artificially. Such forced commutation is required in some circuits where the anode current has to be interrupted at a specific instant.  The availability of GTOs, MOSFETs, and IGBTs has largely eliminated the need to use thyristors in such force-com mutated applications. For this reason, in the following discussion of thyristor power circuits, we consider only those involving line commutation. The Thyristor and Thyristor Circuits Current interruption and forced commutation
  37. 37.  Consider a circuit in which a thyristor and a load resistor R are connected in series across a dc source E.  If we apply a single positive pulse to the gate, the resulting dc load current I will flow indefinitely thereafter.  We can stop conduction in the SCR in one of 3 ways: a) Momentarily reduce the dc supply voltage E to zero. b) Open the load circuit by means of a switch. c) Force the anode current to zero for a brief period. 37 Advanced Electric Machines and Drives The Thyristor and Thyristor Circuits Current interruption and forced commutation
  38. 38.  Another technique consists of using 2 thyristors.  A load R can be switched on and off by alternately firing thyristors Q 1 and Q2. 38 Advanced Electric Machines and Drives The Thyristor and Thyristor Circuits Current interruption and forced commutation
  39. 39. Basic thyristor power circuits  Thyristors are used in many different ways.  However, in power electronics, six basic circuits cover about 90 percent of all industrial applications. These circuits, and some of their applications, are: 1. Controlled rectifier supplying a passive load 2. Controlled rectifier supplying an active load 3. Line-commutated inverter supplying an active ac load 4. AC static switch 5. Cycloconverter 6. Three-phase converter 39 Advanced Electric Machines and Drives Current interruption and forced commutation
  40. 40.  By definition, a passive load is one that contains no inherent source of energy (i.e., the resistor).  The following figure shows a resistive load and a thyristor connected in series across a single-phase source. The source produces a sinusoidal voltage having a peak value Em.  The gate pulses are synchronized with the line frequency and, in our example. they are delayed by an angle of 90°. 40 Basic thyristor power circuits Controlled rectifier supplying a passive load Advanced Electric Machines and Drives
  41. 41.  It is seen that the current lags behind the voltage because it only flows during the final 90◦.  This lag produces the same effect as an inductive load. Consequently, the ac source has to supply reactive power Q in addition to the active power P.  If the SCR is triggered at zero degrees (the start of the cycle), no reactive power is absorbed by the rectifier. Basic thyristor power circuits Controlled rectifier supplying a passive load Advanced Electric Machines and Drives 41
  42. 42.  The following figure shows an ac source Em and a dc load connected by an SCR in series with an inductor.  The load (represented by a battery) receives energy because when the thyristor conducts, current I enters the positive terminal.  Smoothing inductor L limits the peak current to a value within the SCR rating.  Gate pulses Eg initiate conduction at an angle θ1 Basic thyristor power circuits Controlled rectifier supplying an active load Advanced Electric Machines and Drives 42
  43. 43.  Using terminal 1 as a zero reference potential, it follows that the potential of terminal 2 lies Ed volts above it.  Furthermore, the potential of terminal A oscillates sinusoidally above and below the level of terminal 1.  If the SCR were replaced by a diode, conduction would begin at angle θ0 because this is the instant when the anode becomes positive.  In our example, conduction only begins when the gate is fired at θ1 degrees. 43 Basic thyristor power circuits Controlled rectifier supplying an active load Advanced Electric Machines and Drives
  44. 44.  An inverter, by definition, changes dc power into ac power. It performs the reverse operation of a rectifier, which converts ac power into dc power. There are two main types of inverters: 1. Self-commutated inverters (also called force commutated inverters) in which the commutation means are included within the power inverter 2. Line-commutated inverters, wherein commutation is effected by virtue of the line voltages on the ac side of the inverter Basic thyristor power circuits Line-commutated inverter Advanced Electric Machines and Drives 44
  45. 45.  In this chapter we examine the operating principle of a line- commutated inverter.  The circuit of such an inverter is identical to that of a controlled rectifier, except that the battery terminals are reversed. 45 Basic thyristor power circuits Line-commutated inverter Advanced Electric Machines and Drives
  46. 46. 46  An ac static switch is composed of two thyristors connected in anti parallel (back- to-back), so that current can flow in both directions.  The ac current flowing in the load resistor R can be precisely controlled by varying the phase angle α of gates g1 and g2. Thus, if the gate pulses are synchronized with the line frequency, a greater or lesser ac current will flow in the load. Basic thyristor power circuits AC static switch Advanced Electric Machines and Drives
  47. 47.  However, such delayed firing will draw reactive power from the line, even if the load is purely resistive. The reason is that the current is displaced behind the voltage.  If the gates are fired at 0° and 180° respectively, the static switch is in the fully closed position.  On the other hand, if neither gate is fired, the switch is in the open position.  Thus, a static switch ca n be used to replace a magnetic contactor.  In contrast to magnetic contactors, an electronic contactor is absolutely silent and its contacts never wear out. 47 Basic thyristor power circuits AC static switch Advanced Electric Machines and Drives
  48. 48.  A cycloconverter produces low-frequency ac power directly from a higher-frequency ac source.  A simple cycloconverter is shown in the following figure 48 Basic thyristor power circuits Cycloconverter Advanced Electric Machines and Drives
  49. 49.  It consists of three groups of thyristors, mounted back-to-back and connected to a 3-phase source. They jointly supply single- phase power to a resistive load R. 49 Basic thyristor power circuits Cycloconverter Advanced Electric Machines and Drives
  50. 50.  Suppose all thyristors are initially blocked  Then, for an interval T, the gates of thyristors Q1, Q2, and Q3 are triggered by 4 successive pulses g1, g2, g3, g1, in such a way that the thyristors function as if they were ordinary diodes. 50 Advanced Electric Machines and Drives Basic thyristor power circuits Cycloconverter
  51. 51.  During the next interval T, thyristors Q4, Q5, Q6, are fired by 4 similar pulses g4, g5, g6, g4. This makes terminal 4 negative with respect to N. The firing process is then repeated for the Q1, Q2, Q3 thyristors, and so on. 51 Advanced Electric Machines and Drives Basic thyristor power circuits Cycloconverter
  52. 52.  Compared to a sine wave, the low-frequency waveshape is rather poor. It is flat-topped and contains a large 180 Hz ripple when the 3-phase frequency is 60 Hz.  Assuming a 60 Hz source, we can show that each half-cycle corresponds to 540º on a 60 Hz base. The duration of T is, therefore, (540/360)x(1/60) =0.025s, which corresponds to a frequency of 1/(2 x 0.025) = 20 Hz. 52 Advanced Electric Machines and Drives Basic thyristor power circuits Cycloconverter
  53. 53.  Obviously, by repeating the firing sequence g1, g2, g3, g1, ... , we could keep terminal 4 positive for as long as we wish, followed by an equally long negative period, when g4, g5, g6, g4 ... are fired.  In this way we can generate frequencies as low as we want.  This cycloconverter can supply a single-phase load from a 3- phase system, without unbalancing the 3-phase lines. 53 Basic thyristor power circuits Cycloconverter Advanced Electric Machines and Drives
  54. 54.  The 3-phase, 6-pulse thyristor converter is one of the most widely used rectifier/inverter units in power electronics.  Three-phase, 6-pulse converters have 6 thyristors connected to the secondary winding of a 3-phase transformer 54 Advanced Electric Machines and Drives 3-phase, 6-pulse controllable converter Basic thyristor power circuits
  55. 55.  Because we can initiate conduction whenever we want, the thyristors enable us to vary the dc output voltage when the converter operates in the rectifier mode.  The converter can also function as an inverter, provided that a dc source is used in place of the load resistor R. 55 Advanced Electric Machines and Drives 3-phase, 6-pulse controllable converter Basic thyristor power circuits
  56. 56. 56  The converter is fed from a 3-phase transformer. The gates of thyristors Q1 to Q6 are triggered in succession at 60-degree intervals.  The load is represented by a resistor in series with an inductor L.  The inductor is assumed to have a very large inductance, so that the load current Id remains constant.  The two thyristors Q1, Q5 are conducting. A moment later, the thyristors Q2, Q4 conduct. The other thyristors are similarly switched, in sequence. When these steps have been completed, the entire switching cycle repeats.  The switching sequence is similar to that of the diode bridge rectifier Advanced Electric Machines and Drives 3-phase, 6-pulse controllable converter Basic thyristor power circuits
  57. 57. 57 3-phase, 6-pulse controllable converter Basic thyristor power circuits Advanced Electric Machines and Drives
  58. 58. 58 3-phase, 6-pulse controllable converter Basic thyristor power circuits Advanced Electric Machines and Drives Delayed triggering - rectifier mode
  59. 59. 59 Advanced Electric Machines and Drives 3-phase, 6-pulse controllable converter Basic thyristor power circuits Delayed triggering - rectifier mode
  60. 60. 60 3-phase, 6-pulse controllable converter Basic thyristor power circuits Delayed triggering - rectifier mode Advanced Electric Machines and Drives
  61. 61. Exercise  The 3-phase converter of the following figure is connected to a 3-phase 480V, 60 Hz source. The load consists of a 500 V dc source having an internal resistance of 2Ω. Calculate the power supplied to the load for triggering delays of (a) ]5° and (b) 75°. 61 Advanced Electric Machines and Drives
  62. 62.  If triggering is delayed by more than 90°, the voltage Ed developed by the converter becomes negative.  This does not produce a negative current because SCRs conduct in only one direction. Consequently, the load current is simply zero. 62 Delayed triggering – inverter mode Basic thyristor power circuits Triggering sequence and waveforms with a delay angle of 105°. Advanced Electric Machines and Drives
  63. 63.  However, we can force a current to flow by connecting a dc voltage of proper magnitude and polarity across the converter terminals. This external voltage E0 must be slightly greater than Ed in order for current to flow.  The load current is given by 63 Advanced Electric Machines and Drives Basic thyristor power circuits
  64. 64.  Because current flows out of the positive terminal of E0, the load is actually a source, delivering a power output P= E0Id.  Part of this power is dissipated as heat in the circuit resistance R and the remainder is delivered to the secondaries of the 3-phase transformer. 64 Delayed triggering – inverter mode Basic thyristor power circuits Advanced Electric Machines and Drives
  65. 65.  The original rectifier has now become an inverter, converting dc power into ac power. The transition from rectifier to inverter is smooth, and requires no change in the converter connections.  In the rectifier mode, the firing angle lies between 0° and 90°, and the load may be active or passive. In the inverter mode, the firing angle lies between 90° and 180°, and a dc source of proper polarity must be provided. 65 Delayed triggering – inverter mode Basic thyristor power circuits Advanced Electric Machines and Drives
  66. 66. Semiconductor switches  Apart from their important gate turn-off feature, GTOs are very similar to ordinary thyristors. The characteristics of both these devices in the on and off states are illustrated in the following figures.  Thus, in the off state, when the current is zero the thyristor can withstand both forward and reverse blocking voltages EAK, up to the maximum limits bounded by the cross-hatched bands 66 Thyristor and GTO Basic Characteristics Advanced Electric Machines and Drives
  67. 67. 67 Semiconductor switches  During the on state, when the thyristor conducts, the figure shows that the EAK voltage drop is about 2 V, and the upper limit of the anode current IAK is again indicated by the crosshatched band. These bands merely indicate the broad-brush maximum values that are currently available. Thyristor and GTO Basic Characteristics Advanced Electric Machines and Drives
  68. 68. 68 Semiconductor switches  The figure shows that GTOs are able to withstand forward voltages but not reverse blocking voltages. Furthermore, the voltage drop is about 3 V compared to 2 V for thyristors.  As in the case of a thyristor, conduction in a GTO is initiated by injecting a positive current pulse into the gate. In order to keep conducting, the anode current must not fall below the holding current of the GTO. Thyristor and GTO Basic Characteristics Advanced Electric Machines and Drives
  69. 69. Semiconductor switches  However, the GTO is a device in which the anode current can be blocked by injecting a strong negative current into the base for a few microseconds. To ensure extinction, the amplitude of the gate pulse has to be about one third the value of the anode current.  GTOs are high-power switches, some of which can handle currents of several thousand amperes at voltages of up to 4000 V. Thyristor and GTO Basic Characteristics Advanced Electric Machines and Drives
  70. 70. 70 Semiconductor switches  The transistor has three terminals named collector C, emitter E, and base B. BJT Basic Characteristics  The collector current IC that flows from collector to emitter is initiated and maintained by causing a sustained current IB to flow into the base. When operated as a switch, the base current must be large enough to drive the BJT into conduction. Advanced Electric Machines and Drives
  71. 71. 71 Semiconductor switches  Under these conditions, the voltage between the collector and emitter is about 2 to 3 volts, at rated collector current. Conduction ceases as soon as the base current is suppressed. BJT Basic Characteristics  The characteristics of the BJT in the on and off states are shown in the upper figure. together with the approximate limits of the collector-emitter voltage ECE and collector current Ic. Note that the transistor cannot tolerate negative values of ECE. Power transistors can carry currents of several hundred amperes and withstand ECE voltages of about 1kV. To establish collector currents of 100A the corresponding base current is typically about 1A. Advanced Electric Machines and Drives
  72. 72. 72 Semiconductor switches  The power MOSFET is a voltage-controlled three-terminal device having an anode and cathode, respectively called drain D, source S, and gate G. MOSFET Basic Characteristics  The drain current ID is initiated by applying and maintaining a voltage EGS of about 12V between the gate and the source. Conduction stops whenever EGS falls below a threshold limit (about 1 V). Advanced Electric Machines and Drives
  73. 73. 73 Semiconductor switches  The gate currents are extremely small; consequently, very little power is needed to drive this electronic switch. The characteristics in the on and off states are shown in the following figure, together with typical maximum BJT Basic Characteristics  limits of drain voltage EDS and drain current ID.  The MOSFET cannot tolerate negative values of EDS To meet this requirement. it has incorporated within it a reverse-biased diode, as shown in the symbol for the device. Advanced Electric Machines and Drives
  74. 74. 74 Semiconductor switches  Power MOSFETs can carry drain currents of about a hundred amperes and withstand ECE voltages of about 500 V. At rated current, when driven into saturation, the EDS voltage drop ranges from about 2 V to 5 V. BJT Basic Characteristics Advanced Electric Machines and Drives
  75. 75. 75 Semiconductor switches  The IGBT is also a voltage- controlled switch whose terminals are identified the same way as those in a transistor, namely collector. emitter, and base. The characteristics in the on and off states are shown in the following figure together with the limiting voltages and current.  The collector current in an IGBT is much higher than in a MOSFET. Consequently, the IGBT can handle more power. IGBT Basic Characteristics Advanced Electric Machines and Drives
  76. 76.  Compared to GTOs, an important feature of BJTs, MOSFETs, and IGBTs is their fast turn-on and turn-off times. This enables these switches to be used at much higher frequencies. As a result, the associated transformers. inductors, and capacitors are smaller and cheaper. Typical maximum frequencies are shown in Figs. The following. Another advantage of high-speed switching is that the semiconductor switches can generate lower-frequency voltages and currents whose waveshapes and phase can be tailored to meet almost any requirement. 76 Semiconductor switches IGBT Basic Characteristics Advanced Electric Machines and Drives
  77. 77. 77  In some power systems there is a need to transform DC power from one DC voltage level to either a higher or lower dc level.  In alternating current systems the voltage step-up or step-down can easily be done with a transformer.  In DC systems, an entirely different approach is required. It  involves the use of a dc-to-dc switching converter.  sometimes called a chopper. Semiconductor switches IGBT Basic Characteristics Advanced Electric Machines and Drives
  78. 78. DC-to-DC Switching Converter  Suppose that power has to be transferred from a high-voltage dc source Es to a lower-voltage dc load E0.  One solution is to connect an inductor between the source and the load and to open and close the circuit periodically with a switch 78 Advanced Electric Machines and Drives
  79. 79. DC-to-DC Switching Converter 79 Advanced Electric Machines and Drives
  80. 80. DC-to-DC Switching Converter  When the switch is closed, the voltage across the inductor is eL=ES-E0.  The inductor accumulates volt-seconds, and the resulting current i increases at a constant rate given by:  After time T1, the current is:  The corresponding magnetic energy stored in the inductor is 80
  81. 81. 81  When the switch opens the current collapses and all the stored energy is dissipated in the arc across the switch. At the same time, a high voltage eL is induced across the inductor because the current is collapsing so quickly.  The polarity of this voltage is opposite to what it was when the current was increasing.  The high negative voltage indicates that the inductor is rapidly discharging the volt-seconds it had previously accumulated. As a result, the current decreases very quickly. DC-to-DC Switching Converter Advanced Electric Machines and Drives
  82. 82. 82  We can prevent the energy loss every time the switch opens and closes by adding a diode to the circuit.  When the switch closes, the current rises to Ia as before. The diode has no effect because its cathode is positive with respect to the anode and so the diode does not conduct. When the switch opens, current i again begins to fall, inducing a voltage eL.  The current eventually becomes zero after a time T2. We can calculate T2 because the volt-seconds accumulated during the charging period T1 must equal the volt-seconds released during the discharge interval Referring DC-to-DC Switching Converter Advanced Electric Machines and Drives
  83. 83. 83 DC-to-DC Switching Converter Advanced Electric Machines and Drives
  84. 84. DC-to-DC Switching Converter  When the current is zero, the inductor will have delivered all its stored energy to load E0. Simultaneously, the diode will cease to conduct. We can therefore reclose the switch for another interval T1 and repeat the cycle indefinitely.  Consequently, this circuit enables us to transfer energy from a high voltage dc source to a lower-voltage dc load without incurring any losses.  In effect, the inductor absorbs energy at a relatively high voltage (ES-E0) and delivers it at a lower voltage E0. 84 Advanced Electric Machines and Drives
  85. 85. DC-to-DC Switching Converter  The diode is sometimes called a freewheeling diode because it automatically starts conducting as soon as the switch opens and stops conducting when the switch closes.  The switch is actually a GTO, MOSFET, or IGBT, whose on/off state is controlled by a signal applied to the gate. The combination of the electronic switch, inductor, and diode constitutes what is known as a step-down dc-to-dc converter, or buck chopper. 85 Advanced Electric Machines and Drives
  86. 86. DC-to-DC Switching Converter  Referring to the following figure, the switch is closed for an interval Ta and open during an interval Tb.  When the switch is open, the load current falls from its peak value Ia to a lower value Ib.  During this interval, current flows in the inductor, the load, and the freewheeling diode. 86 Advanced Electric Machines and Drives
  87. 87. DC-to-DC Switching Converter  When the current has fallen to a value Ib, the switch recloses. Because the cathode of the diode is now (+), the current in the diode immediately stops flowing, and the source now supplies current Ia. The current then builds up and when it reaches the value Ia (after a time Ta ), the switch reopens.  The freewheeling diode comes into play and the cycle repeats. 87 Advanced Electric Machines and Drives
  88. 88. DC-to-DC Switching Converter  The current supplied to the load fluctuates between Ia and Ib. Its average or DC value I0 is given by:  The average current Is during one cycle (time T) is:  That is, 88 Advanced Electric Machines and Drives
  89. 89. DC-to-DC Switching Converter  Turning our attention to the power aspects, the de power drawn from the source must equal the dc power absorbed by the load because, ideally, there is no power loss in the switch, the inductor, or the freewheeling diode. We can, therefore, write:  E0 can be controlled simply by varying the duty cycle D. Thus, the converter behaves like a highly efficient DC transformer in which the "turns ratio" is D. 89 Advanced Electric Machines and Drives
  90. 90. DC-to-DC Switching Converter  For a given switching frequency, this ratio can be changed as needed by varying the on time of the switch.  In practice, the mechanical switch is replaced by an electronic switch, such as an IGBT. It can be turned on and off at a frequency that may be as high as 50 kHz.  If more power is required. a OTO is used, wherein the frequency could be of the order of 300 Hz. 90 Advanced Electric Machines and Drives
  91. 91. Exercise1  The switch in the figure below opens and closes at a frequency of 20 Hz and remains closed for 3ms per cycle. A dc ammeter connected in series with the load E0 indicates a current of 70 A. a) If a dc ammeter is connected in series with the source. what current will it indicate? b) What is the average current per pulse? 91 Advanced Electric Machines and Drives
  92. 92. 92 Exercise2 We wish to charge a 120 V battery from a 600 V dc source using a dc chopper. The average battery current should be 20 A, with a peak-to-peak ripple of 2 A. If the chopper frequency is 200 Hz, calculate the following: a) The dc current drawn from the source b) The duty cycle c) The inductance of the inductor Advanced Electric Machines and Drives
  93. 93. Basic 2-quadrant dc-to-dc converter  Consider the following figure in which two mechanical switches S1 to S2 are connected across a dc voltage source EH .  The switches open and close alternately in such a way that when S1 is closed, S2 is open and vice versa.  The time of one cycle is T, and S1 is closed for a period Ta  It follows that the duty cycle of S1 is D = Ta/T, while that of S2 is (1-D). 93 Advanced Electric Machines and Drives
  94. 94. Basic 2-quadrant dc-to-dc converter  When S1 is closed, EL=E12=EH  When S2 is closed, E12=0.  The output voltage oscillates, therefore, between EH and zero and its average dc value E1 is given by  By varying D from zero to 1, we can vary the magnitude of EL from zero to EH 94 Advanced Electric Machines and Drives
  95. 95. Basic 2-quadrant dc-to-dc converter  Suppose we want to transfer dc power from terminals E12 to a load such as a battery, whose dc voltage E52 has a value E0  An inductor is used as filter  We assume that the load has a small internal resistance R.  Suppose that both the voltage source EH and the duty cycle D are fixed. Consequently, the dc component EL between points 1 and 2 is constant. 95 Advanced Electric Machines and Drives
  96. 96. Basic 2-quadrant dc-to-dc converter  If E0 is less than EL, a dc current IL will flow from terminal 1 into terminal 5. Its magnitude is given by  In this mode of operation, with less than EL , the converter acts like the step-down (buck) chopper.  On the other hand, if E0 is greater than EL , a dc current IL will flow out of terminal 5 and into terminal 1.  Its magnitude is:  Power now flows from the low-voltage battery side E0 to the higher voltage side EH.  In this mode of operation, with E0 greater than EL, the converter acts like a step-up (boost) chopper 96 Advanced Electric Machines and Drives
  97. 97. Four-quadrant dc-to-dc converter  A four quadrant converter consists of two identical 2-quadrant converters arranged as shown in the following figure.  Switches Q1, Q2 in converter arm A open and close alternately, as do switches Q3, Q4 in converter arm B.  The switching frequency (assumed to be 100 kHz) is the same for both. 97 Advanced Electric Machines and Drives
  98. 98. Four-quadrant dc-to-dc converter  The switching sequence is such that Q1 and Q4 open and close simultaneously.  Similarly, Q2 and Q3 open and close simultaneously. Consequently, if the duty cycle for Q1 is D, it will also be D for Q4. It follows that the duty cycle for Q2 and Q3 is (1 - D). 98 Advanced Electric Machines and Drives
  99. 99. Four-quadrant dc-to-dc converter  The dc voltage appearing between terminals A, 2 is given by  The dc voltage EB, between terminals B, 2 is  The dc voltage EB between terminals A and B is the difference between EA and EB,:  thus 99 Advanced Electric Machines and Drives
  100. 100. Four-quadrant dc-to-dc converter  the dc voltage is zero when D is 0.5. Furthermore, the voltage changes linearly with D, becoming + EH when D = I, and –EH when D = O.  The polarity of the output voltage can therefore be either positive or negative.  Moreover, if a device is connected between terminals A, B, the direction of dc current flow can be either from A to B or from B to A. 100 Advanced Electric Machines and Drives
  101. 101. Four-quadrant dc-to-dc converter  The following figure shows the wave shape when D =0.5 101 Advanced Electric Machines and Drives
  102. 102. Four-quadrant dc-to-dc converter 102  The following figure shows the wave shape when D =0.8 Advanced Electric Machines and Drives
  103. 103.  All semiconductor switches such as GTOs, MOSFETs, and IGBTs have losses that affect their temperature rise and switching efficiency. The switches all function essentially the same way, but to focus our analysis we assume the switching device is a GTO. The switching operation involves four brief intervals:  Turn-On time T1  On-state time T2  Turn-off time T3  Off-state time T4  The sum of T1 + T2 + T3 + T4 is equal to the period T of one cycle which, in turn, is equal to fc where fc is the switching frequency. Switching losses 103 Advanced Electric Machines and Drives
  104. 104. Switching losses  During each interval the instantaneous power dissipated in the GTO is equal to the product of the instantaneous voltage across it times the instantaneous current that flows through it.  The average power is equal to the energy dissipated in the GTO during one complete cycle, divided by T 104 Advanced Electric Machines and Drives
  105. 105. 105 Switching losses  Turn-On time T1  On-state time T2  Turn-off time T3  Off-state time T4 Advanced Electric Machines and Drives
  106. 106. Switching losses  The following figure shows a GTO with its anode, cathode, and gate. In addition to the circuit that is being switched (not shown), a snubber is connected to the GTO.  A snubber is an auxiliary circuit composed of R, L, C components (usually including semiconductor devices) that control the magnitude and rate of rise of the anode voltage EAK as well as the anode current I. The purpose of a snubber is to aid commutation and to reduce the losses in the GTO. 106 Advanced Electric Machines and Drives
  107. 107. Switching losses  It can be seen that the dissipation increases with the switching frequency fc and the duty cycle D.  It can be seen also that the dissipation can be reduced if the turn-on and turn-off times are shorter. 107 Advanced Electric Machines and Drives
  108. 108. DC-TO-AC SWITCHING CONVERTERS  We have studied the 2-quadrant and 4-quadrant dc-to-dc switching converters. In this following, we will examine the 4- quadrant converter as a dc-to-ac converter.  We have seen that the converter is able to transform the dc voltage into a rectangular ac voltage.  The rectangular wave can have any frequency 108 Advanced Electric Machines and Drives Dc-to-ac rectangular wave converter
  109. 109. 109 DC-TO-AC SWITCHING CONVERTERS  Consider the 4-quadrant dc-to-dc converter of the following figure, which is operating at a constant switching frequency, fc of several kilohertz.  Fc is called carrier frequency. Advanced Electric Machines and Drives Dc-to-ac converter with pulse with modulation
  110. 110. DC-TO-AC SWITCHING CONVERTERS  Suppose that the duty cycle is set at 0.8. The average value of ELL. is, therefore,  If D is set to 0.5, the average output voltage ELL becomes zero  if D = 0.2, we find that the average value of ELL is -0.6 EH 110 Advanced Electric Machines and Drives Dc-to-ac converter with pulse with modulation
  111. 111. DC-TO-AC SWITCHING CONVERTERS  Suppose now, that D is varied periodically, switching suddenly between D = 0.8 and D = 0.2 at a frequency f that is much lower than the carrier frequency fc.  As a result, the output voltage ELL will fluctuate continually between +0.6 EH and -0.6 EH.  The filtered output voltage is therefore a rectangular wave having a frequency f 111 Advanced Electric Machines and Drives
  112. 112. DC-TO-AC SWITCHING CONVERTERS  The big advantage over the rectangular wave of slide 108 is that the magnitude of Eo, as well as its frequency f can be controlled at will. 112 Advanced Electric Machines and Drives
  113. 113. DC-TO-AC SWITCHING CONVERTERS  Consider now the following figure wherein the duty cycle is varied gradually between 0.8 and 0.2, following a triangular pattern. This causes the filtered output voltage ELL to vary between +0.6 EH and -0.6 EH, faithfully reproducing the triangular wave. 113 Advanced Electric Machines and Drives
  114. 114. DC-TO-AC SWITCHING CONVERTERS  We need to determine the duty cycle pattern to generate a desired output voltage  We already know that  from which we immediately deduce 114 Advanced Electric Machines and Drives Dc-to-ac converter with pulse with modulation
  115. 115. DC-TO-AC SWITCHING CONVERTERS  Consequently, knowing EH (whose value is fixed) and knowing. the desired value of ELL as a function of time, the pattern of D can be programmed.  For example, suppose we want to generate an output voltage E given by  D is given by  The ratio EmlEH is called amplitude modulation ratio, designated by the symbol m. Consequently, the duty cycle pattern to generate a sine wave can be expressed as: 115 Advanced Electric Machines and Drives
  116. 116. Exercise  A 200 V dc source is connected to a 4-quadrant switching converter operating at a carrier frequency of 8 kHz. It is desired to generate a sinusoidal voltage having an effective value of 120 V at a frequency of 97 Hz and phase angle of 35° lagging. Calculate the value of the amplitude modulation ratio and derive an expression for the duty cycle . 116 Advanced Electric Machines and Drives

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