CHAPTER 11
POWER ELECTRONICS

11.1 INTRODUCTION

Power electronics is the application of solid-state electronics to the control and conversion of electric power.

The first high-power electronic devices were mercury-arc valves. In modern systems, power conversion is performed with semiconductor switching devices such as diodes, thyristors, and transistors, pioneered by R. D. Middlebrook and others beginning in the 1950s. An AC/DC converter (rectifier) is the most typical power electronics device found in many consumer electronic devices, e.g., television sets, personal computers, battery chargers. The power range is typically from tens of watts to several hundred watts. In industry, a common application is the variable speed drive (VSD) that is used to control an induction motor. The power range of VSDs starts from a few hundred watts and end at tens of megawatts. Power electronics started with the development of the mercury-arc rectifier. It was used to convert AC into DC.

Power electronics are important in applications to power systems. Power electronic devices include high-power semiconductor devices used in rapid switching operations in protection devices like relays and circuit breakers. In the generation system of a power system, they are components of protection devices used in protecting generators and transformers. In transmission systems, they are key components of flexible alternating current transmission system (FACTS) devices used in increasing the power-transfer capability of the transmission network, increasing the loading capability of the lines to their thermal capabilities, providing direct control of power flow over assigned transmission line routes, and facilitating loss reduction. In distribution systems, they are components of devices used for power-actor correction and in protection of transformers. In renewable-energy sources when integrating with the grid or stand-alone, converters are needed for both integration and applications such as the use of DC to AC converters in the conversion of renewable DC sources such as PV to AC sources in microgrid and smart-grid systems. They are also the choice of technology for AC/DC/AC conversion needed in different loads. Power electronic devices are used in the decentralization of renewable-energy resources in microgrids and smart grids.

This chapter discusses the elements of power electronics and provides a comparative study of power electronic devices. Different power electronic devices, such as power diodes and power transistors, are discussed with respect to their principles of operation and characteristics and a comprehensive performance analysis. Different applications of power electronics in utilities, smart grid, and ship systems are discussed.

11.2 POWER SYSTEMS WITH POWER ELECTRONICS ARCHITECTURE

Figure 11.1 shows the architecture of a power system with renewable-energy sources. The power electronic devices are applied in protection, transmission lines capacity enhancement, power quality enhancement, and power conversion. The protection devices include circuit breakers and relays, fuses, and isolators for the protection of generators, transformers, and other devices or machines. FACTS devices that have thyristors and other power electronic devices as key components include static synchronous compensators (STATCOMs), static var compensators (SVC), and unified power flow controllers (UPFCs) used to enhance or boost the power-transmission capability of the transmission lines. Also, an application of power electronic device is the use of thyristor-controlled reactors (TCRs) and thyristor-switched capacitors (TSCs), which help power-factor improvement of the distribution system. Lastly, renewable-energy sources can be integrated into the network using converters (AC to DC, DC to DC, DC to AC, and AC to AC) for various applications.

Block diagram shows transformer attached with PE protection devices, generator with PE protection devices, transmission line with PE FACTS devices, motor or load with PE power quality enhancement devices, PE converters with solar PV, wind source, fuel cell, and storage.

Figure 11.1 Architecture of power system with power electronic devices.

11.3 ELEMENTS OF POWER ELECTRONICS

Power electronics and converters utilizing them made a head start when the first silicon-controlled rectifier (SCR) device was proposed by Bell Laboratories and commercially produced by General Electric in the earlier 1950 s. Mercury-arc rectifiers were well in use by that time, and the robust and compact SCR started replacing it in rectifiers and cyclo-converters. The necessity arose of extending the application of SCRs beyond the line-commutated mode of action, which called for external measures to circumvent its turn-off incapability via its control terminals. Various turn-off schemes were proposed and their classifications suggested but it became increasingly obvious that a device with turn-off capability was desirable to permit wider application.

The power metal oxide semiconductor field-effect transistor (MOSFET) became successful commercially in the late 1970 s. This device represented the first successful marriage between modern integrated circuit and discrete power semiconductor manufacturing technologies. Its voltage-drive capability—giving it again a higher gain, ease of paralleling, and, most importantly, much higher operating frequencies reaching up to a few MHz—saw it replace the bipolar at the sub-10 kW range, mainly for switch mode power supply applications. Extension of very-large-scale integration manufacturing facilities for the MOSFET reduced its price vis-à-vis the bipolar. Different versions of gate turn-off thyristor, were proposed by various manufacturers. Requirements for an extremely high turn-off control current via the gate and the comparatively high cost of the device restricted its application only to inverters rated above a few hundred kva. Brief comparison of power semiconductor devices commonly used for energy processing are given in Table 11.1.

TABLE 11.1 Comparison of Power Semiconductor Devices for Energy Processing

Semiconductor Device Description Symbol(s) Applications
Diodes Electrical device that allows current to move through it in one direction with far greater ease than in the other Diagram shows symbols for PN diode and Zener diode.
  • limit circuits
  • clamp applications
  • rectifier circuits
  • Zener diodes, used in voltage regulation
Bipolar Junction Transistor (BJT) Active semiconductor device formed by two P–N junctions whose function is amplification of an electric current; uses both electron and hole charge carriers Diagram shows symbols for PNP type bipolar junction transistor and NPN type bipolar junction transistor. It shows base, emitter, and collector terminals of devices.
  • high-speed digital logic
  • amplifiers
  • temperature sensors
  • logarithmic converters
Field Effect Transistor (FET) Unipolar devices that operate only with one type of charge carrier; voltage controlled, where the voltage between two of the terminals (gate and source) controls the current through the device Diagram shows symbols for n-channel field effect transistor and n-channel field effect transistor. It shows gate, drain, and source terminals of devices.
  • amplifiers (for example, due to large input resistance and low output resistance, is effective as a buffer in common-drain (source follower) configuration)
Insulated Gate Bipolar Transistor (IGBT) Combines features of BJTs and MOSFETs; has high input impedance like MOSFETs and low on-state conduction losses like BJTs Diagram shows symbol for insulated gate bipolar transistor. It shows gate, emitter, and collector terminals.
  • switching internal combustion engine ignition coils, where fast switching and voltage blocking capabilities are important
  • medium-power applications such as DC and AC motor drives, power supplies, solid-state relays, and contactors
Metal Oxide Semiconductor Field Effect Transistor (MOFET) Voltage-controlled devices with very high input impedance; used for switching and amplifying electronic signals in electronic devices; can operated in a circuit in two configurations: depletion type or enhancement type

Diagram shows symbols for n-channel enhancement-type mosfet and p-channel enhancement-type mosfet. It shows drain, source, and gate terminals.

Diagram shows symbols for n-channel depletion-type mosfet and p-channel depletion-type mosfet. It shows drain, source, and gate terminals.
  • digital-integrated circuits, such as microprocessors
  • calculators, memories, and logic complementary metal–oxide–semiconductor gates
  • analog switches
  • amplifiers
  • applications of power electronics and switch-mode power supplies
  • oscillators in radio systems
  • automobile sound systems and in sound reinforcement systems
Thyristor Bistable semiconductor device made of three or more junctions that can be switched from the off state to the on state and vice versa

Diagram shows symbol for thyristor. It shows anode, gate, and cathode.
  • controls power to many kinds of loads: a simple lamp, power supplies, voltage regulators, and industrial motors

11.4 POWER SEMICONDUCTOR DEVICES

11.4.1 Diodes

Diodes are made from semiconductor material, silicon or germanium, in which half is positively doped and is known as the p region and the other half is negatively doped and referred to as the n region. The regions are separated at the p–n junction by a depletion region. The p region is the anode while the n region the cathode. Figure 11.2 shows the schematic representation of a diode.

Diagram shows symbol for diode which includes triangle on left representing anode and narrow vertical frame on right representing cathode.

Figure 11.2 Schematic symbol of a diode.

Typically, the anode (A) and cathode (C) are identifiable on a diode, depending on the physical configurations. A band or some other feature usually identifies the cathode.

Diode Operation

  1. Forward biasing: To bias a diode, a DC voltage is applied across it. Forward bias is when current is allowed to flow through the p–n junction. A DC-voltage source is connected by conductive material (contacts and wires) across a diode to produce forward bias. This external-bias voltage is designated as VBIAS. A resistor is used in the arrangement to limit the forward current to a value that will not damage the diode. The negative side of the voltage source is connected to the n region of the diode and the positive side is connected to the p region. This is one requirement for forward bias. A second requirement is that the bias voltage, VBIAS, must be greater than the barrier potential.
  2. Reverse biasing: Reverse bias is the condition that essentially inhibits current through the diode. A DC-voltage source is connected across a diode to produce reverse bias. This external bias voltage is known as VBIAS just as in forward bias. The positive side of the DC voltage VBIAS is connected to the n region of the diode and the negative side is connected to the p region. The depletion region is much wider than in forward bias or equilibrium.
  3. Reverse breakdown: Normally the reverse current is so small that it can be neglected. However, if the external reverse-bias voltage is increased to a value called the breakdown voltage, the reverse current will drastically increase. This multiplication of conduction electrons is termed the avalanche effect, and reverse current can increase dramatically if steps are not taken to limit the current. When the reverse current is not limited, the resulting heating will permanently damage the diode. Most diodes are not operated in reverse breakdown, but if the current is limited (by adding a series-limiting resistor, for example), there is no permanent damage to the diode.
  4. V–I characteristic for forward bias: When a forward-bias voltage is applied across a diode, current flows to the circuit. This current is called the forward current and is designated IF. V–I characteristic for forward bias shows what happens when a forward-bias voltage is increased positively. A resistor is used to limit the forward current to prevent overheating and damage to the diode.

    With 0 V across the diode, there is no forward current. As the forward-bias voltage is gradually increased, the forward current and the voltage across the diode gradually increase. A portion of the forward-bias voltage is dropped across the limiting resistor. When the forward-bias voltage is increased to a value where the voltage across the diode reaches approximately the barrier potential, the forward current begins to increase rapidly, as illustrated in Figure 11.3.

    With further increase in the forward-bias voltage, the current continues to increase very rapidly, but the voltage across the diode increases only gradually. This small increase in the diode voltage above the barrier potential is due to the voltage drop across the internal dynamic resistance of the semiconductive material.

  5. V–I characteristic for reverse bias: When a reverse-bias voltage is applied across a diode, there is only an extremely small reverse current (IR) through the p–n junction. With 0 V across the diode, there is no reverse current. As you gradually increase the reverse-bias voltage, there is a very small reverse current and the voltage across the diode increases. When the applied bias voltage is increased to a value where the reverse voltage across the diode (VR) reaches the breakdown value (VBR), the reverse current begins to increase rapidly. A continuous increase in the bias voltage causes the current to continue increasing very rapidly, but the voltage across the diode increases very little above VBR. Breakdown, with exceptions, is not a normal mode of operation for most p–n junction devices.
  6. Reverse recovery characteristics: Once a diode is in a forward-conduction mode and its forward current is reduced to zero (due to the natural behavior of the diode circuit or by applying a reverse voltage), the diode continues to conduct due to minority carriers that remain stored in the p–n junction and the bulk semiconductor material. The minority carriers require a certain time to recombine with opposite charges to be neutralized. This is called reverse recovery time of the diode (see Figure 11.3(a) and (b) [5]) and it is given by
    (11.1)numbered Display Equation

    where ta is due to charge storage in the depletion region and represents the time between zero crossing and the peak reverse current Irr and tb is due to charge storage in the bulk semiconductor material. The ratio is known as a softness factor (SF).

    The peak reverse current can be expressed in reverse di/dt as

    (11.2)numbered Display Equation

    where ta = t2 − t1is the period from t1 to t2 (as seen in Figures 11.2(a) and 11.3(b)) and is the rate of change of current.

Current versus time graph shows curve initially constant at I sub(F), decreasing to negative value I sub(RR) during time t sub(a), and increasing to zero value during t sub(b). Graph on right shows similar curve with ripples.

Figure 11.3 (a) Soft reverse-recovery characteristics. (b) Abrupt reverse-recovery characteristics.

As seen in Figure 11.4 [5], when the switch is open, the diode is forward biased by current source I0, which makes it output a maximum current of I0. When the switch closes at t = t0, the diode is forced to start turning off and so the current starts reducing from t0 to t1. At this interval, the diode current is positive, and the forward-diode voltage is small and is constant. At t1, diode current is zero and it is expected that the diode will turn off, but that is not the case. The diode still conducts until t2 because of the excess minority carriers that have to be removed from the diode's p–n junction for its reverse voltage to begin to rise. The point at which it gets to t2 is the IRR, which is a function of and the t1 to t2 interval.

Circuit diagram shows voltage source V sub(in), switch S, diode with non-ideal turn off, and current source I sub(0). Diode current versus time graph shows curve initially constant at I sub(0), declining to minus I sub(RR), and then increases to zero.

Figure 11.4 Typical diode-switching characteristics. (a) Switching circuit with S closed at t = t0. (b) Diode current.

From interval t2 to t3, the diode current rises exponentially to zero at t3, which is caused by the charge stored in the bulk of the semiconductor material. At t3, the diode is fully switched off.

For most practical purposes, a diode can be regarded as an ideal switch, whose characteristics are shown in Figure 11.5(b). The V–I characteristics shown in Figure 11.5(a) can be expressed by the Shockley diode equation, given as

Current versus voltage graph on left shows curve increasing after threshold voltage in first quadrant and curve increasing in negative direction after breakdown voltage in third quadrant. Graph on right shows I sub(D) along y axis and reverse voltage V sub(D) along negative x axis.

Figure 11.5 V–I characteristics of a diode. (a) Practical characteristics. (b) Ideal characteristics.

where ID is the current through the diode, VD is the diode voltage with anode positive with respect to cathode V, IS is the leakage (or reverse-saturation) current, VT is a constant called thermal voltage given by

(11.4)numbered Display Equation

k is Boltzmann's constant: 1.3806 × 10−23 J/K, T is the absolute temperature in Kelvin (K = 273 + °C), and q is the electron charge: 1.6022 × 10−19 Coulomb (C).

Basic Diode Waveform Illustrations

From Figure 11.6(a) [5] and assuming the diode is ideal, during the positive half cycle of the input waveform, the diode acts in reverse-bias mode, so no current flows through it. It acts as an open circuit, meaning the full voltage is impressed on the diode and therefore the voltage dropped on the resistor is zero as seen in the output waveform. During the negative half cycle of the input waveform, the diode acts in the forward-bias, mode making current flow through the resistor. Therefore, there is voltage (full voltage) dropped on the resistor, as seen in the output waveform in Figure 11.6(a). Because there is conduction in the negative cycle, the output voltages appear in the negative cycle.

Diagram shows circuit with diode and parallel resistor. Sinusoidal input applied at negative terminal of diode creates output with positive half cycles clipped and input applied at positive terminal of diode creates output with negative half cycles clipped.

Figure 11.6 Diode waveform illustrations. (a) Positive clipper. (b) Negative clipper.

From Figure 11.6(b) [5] and assuming the diode is ideal, during the positive half cycle of the input waveform, the diode acts in forward-bias mode, making current flow through the resistor. Therefore, there is voltage (full voltage) dropped on the resistor as seen in the output waveform in Figure 11.6(b). During the negative half-cycle of the input waveform, the diode acts in reverse bias mode and so no current flows through it. It acts as an open circuit, meaning the full voltage is impressed on the diode and therefore the voltage dropped on the resistor is zero as seen in the output waveform. Because there is conduction in the positive cycle, the output voltages appear in the positive cycle.

Example 1

The forward-voltage drop of a power diode is VD = 1.2 V at ID = 350 A. Assuming that n = 3 and VT = 25.8 mV, find the saturation current IS.

Solution:

From Equation 11.3

numbered Display Equation

Power Diodes

Silicon-power diodes are the successors of selenium rectifiers, having significantly improved forward characteristics and voltage ratings. They are classified mainly by their turn-off (dynamic) characteristics, as seen in Figure 11.7. The minority carriers in the diodes require finite time - trr (reverse recovery time) to recombine with opposite charges and neutralize. Large values of Qrr = (Q2 + Q2), the charge to be dissipated as a negative current when the diode turns off, and trr = t2 − t0), the time it takes to regain its blocking features, impose strong current stresses on the controlled device in series. Also, a “snappy” type of recovery of the diode effects high voltages on all associated power devices in the converter because of load or stray inductances present in the network. There are broadly three types of diodes used in power electronic applications:

Circuit diagram shows ac supply, switch, transformer, and four diodes connected in ring pattern with two junctions connected to ground in which one through filter capacitor.

Figure 11.7 Diode as a rectifier.

  • Line-frequency diodes: These PN diodes with general-purpose, rectifier-type applications, are available at the highest voltage (∼5 kV) and current ratings (∼5kA) and have excellent over-current (surge rating about six times average current rating) and surge-voltage withstanding capability. They have relatively large Qrr and trr specifications.
  • Fast-recovery diodes: Fast-recovery diffused diodes and fast-recovery epitaxial diodes have significantly lower Qrr and trr (∼1.0 sec). They are available at high powers and are mainly used in association with fast-controlled devices, such as freewheeling or DC–DC choppers and rectifier applications. They are used in DC–DC and DC–AC converter circuits where the speed of recovery is of critical importance. Fast-recovery diodes also find application in induction heating, uninterruptible power systems, and traction.
  • Schottky diodes: These are the fastest rectifiers, being majority carrier devices without any Qrr. However, they are available with voltage ratings up to one hundred volts only, though current ratings may be high. Their conduction voltages specifications are excellent (∼0.2 V). The freedom from minority-carrier recovery permits reduced snubber requirements. Schottky diodes face no competition in low-voltage, self-powered microsystem applications and in instrumentation.

Applications of Diodes

  • Rectifier circuits: Because of their ability to conduct currents in one direction and block currents in the other direction, diodes are used in circuits called rectifiers that convert AC voltage into DC voltage. Rectifiers are found in all DC power supplies that operate from an AC voltage source. A power supply is an essential part of each electronic system from the simplest to the most complex. Figure 11.7 and Figure 11.8 shows the circuit diagram for diode as a rectifier and its respective output curve.
  • Clamping applications: Sometimes it is necessary to limit the range of signal (for instance not to exceed certain voltage limits and not destroy a device). The circuit shown in Figure 11.9 accomplishes this. The diode prevents the output from exceeding 5.6 V, with no effect on voltages smaller than this, including negative voltages. The only limitation is that the input must not be so negative that the reverse-breakdown voltage is exceeded. Diode clamps are the standard equipment on all inputs in the complementary metal oxide semiconductor family of digital logic. Without them, the delicate input circuits are easily destroyed by static electricity.
  • Limiter circuits: Diode limiters Figure 11.10 are often used as input protection for high-gain amplifiers. For large-voltage amplifiers, the input must always be near zero voltage, otherwise the output is in a state of saturation. For an op amp with a gain of 1,000, the amplifier operates with supply voltage ± 15 V; sometimes it can be ± 12 V to ± 18 V. It will never give output voltage larger than the supply voltage, i.e., ± 15 V, so the input signal ± 15 mV ( ± 15 V/1,000) or bigger will saturate the output.
Voltage versus time graph shows positive half cycles of sine wave as unfiltered rectifier output and curve drawn through peak points of half cycles as filtered output power. Ripple voltage of filtered output is indicated.

Figure 11.8 Resultant output waveform.

Circuit diagram shows 1N914 diode connected parallel with 1.0 kilo-ohm resistor. Plus-5 volt dc is applied at negative terminal of diode.

Figure 11.9 Clamping configuration.

Circuit diagram shows resistor R and two parallel diodes in opposite directions inserted between input and output terminal.

Figure 11.10 Diode-limiter circuit.

11.4.2 Bipolar Junction Transistors

Bipolar junction transistors (BJTs) are constructed with three doped semiconductor regions separated by two p–n junctions. The three regions are called the emitter, base, and collector. Physical representations of the two types of BJTs are shown in Figures 11.11 and 11.12. One type consists of two n regions separated by a p region (npn), and the other consists of two p regions separated by an n region (pnp). Bipolar refers to the use of both holes and electrons as current carriers in the transistor structure. In order for a BJT to operate properly as an amplifier, the two p–n junctions must be correctly biased with external DC voltages. In this section, we mainly use the npn transistor for illustration. The operation of the pnp is the same as for the npn except that the roles of the electrons and holes, bias-voltage polarities, and current directions are all reversed.

Circuit diagram shows npn transistor which has base-emitter junction forward biased by dc voltage V sub(BB) and base-collector junction reverse biased by dc voltage V sub(CC). It shows resistors R sub(B) at base and R sub(C) at collector.

Figure 11.11 npn transistor.

Circuit diagram shows pnp transistor which has base-emitter junction forward biased by dc voltage V sub(BB) and base-collector junction reverse biased by dc voltage V sub(CC). It shows resistors R sub(B) at base and R sub(C) at collector.

Figure 11.12 pnp transistor.

Biasing

The emitter current is slightly greater than the collector current because of the small base current that splits off from the total current injected into the base region from the emitter.

When a transistor is connected to DC-bias voltages, as shown in Figures 11.11 and 11.12 for both npn and pnp types, VBB forward biases the base-emitter junction and VCC reverse biases the base-collector junction. VCC is normally taken directly from the power-supply output and VBB (which is smaller) can be produced with a voltage divider.

Circuit Analysis

Before analyzing the BJT characteristics, it is important to talk about its modes of operation (Figure 11.13):

  • Common base (CB) mode has current gain but no voltage gain.
  • Common emitter (CE) mode has voltage gain but no current gain.
  • Common collector (CC) mode has both current and voltage gain.
Diagram shows three different operation modes of BJT such as common base, common emitter, and common collector. In common base mode, base terminal is common to input and output and similarly for other modes.

Figure 11.13 Circuit operation modes of the BJT.

CB Characteristics

  • Input characteristics: Consider a pnp transistor where the input current is the emitter current (designated as IE) and the input voltage is the collector-base voltage (designated as VCB). Since the emitter-base junction is forward biased, input current IE increases for a fixed emitter-base voltage VEB when VCB increases. Figure 11.14 shows that the graph of IE versus VEB is akin to the forward-bias characteristics of a p–n diode.
  • Output characteristics: The output characteristics show the relation between output voltage and output current. IC is the output current and collector-base voltage and the emitter current IE is the input current and works as the parameters. Figure 11.15 shows the output characteristics for a pnp transistor in CB mode. As we know for pnp transistors, IE and VEB are positive and IC, IB, VCB are negative. These are the curve-active, saturation, and cutoff regions. The active region is the region where the transistor operates normally. Here the emitter junction is reverse biased. Now the saturation region is the region where both the emitter and collector junctions are forward biased. Finally, the cutoff region is where both the emitter and collector junctions are reverse biased.
Emitter current versus emitter-base voltage graph shows set of steadily increasing curves representing different collector-base voltage values.

Figure 11.14 CB input characteristics.

I sub(C) versus V sub(CB) graph shows set of curves representing I sub(E) values that are initially rising and become constant afterward. Constant value region is labeled as active region and region below zero I sub(E) curve is labeled as cut-off region.

Figure 11.15 CB output characteristics.

CE Characteristics

  • Input characteristics: IB (base current) is the input current; VBE (base-emitter voltage) is the input voltage for CE mode. So, the input characteristics for CE will be the relation between IB and VBE with VCE as parameter. The typical CE input characteristics are similar to that of a forward-biased p–n diode. But as VCB increases, the base width decreases. The characteristics are shown in Figure 11.16.
  • Output characteristics: Output characteristics for CE mode are the curve or graph between collector current (IC) and collector-emitter voltage (VCE) when the base current IB is the parameter. The characteristics are shown in Figure 11.17.

    Like the output characteristics of a common-base transistor, CE mode has also three regions: active, cutoff, and saturation. The active region has a collector region reverse biased and the emitter junction forward biased. For the cutoff region the emitter junction is slightly reverse biased and the collector current is not totally cut off. And finally, for the saturation region both the collector and the emitter junction are forward biased.

    The current gain of a transistor is the ratio of the collector current (IC) to the base current (IB) and is designated as beta (β).

    Typical values of β range from less than 20 to 200 or higher. The ratio of the DC collector current (IC) to the DC emitter current (IE) is alpha (α). The alpha is a less-used parameter than beta in transistor circuits.

    Typically, values of α range from 0.95 to 0.99 or greater, but α is always less than one. This is because IC is always slightly less than IE by the amount of IB. For example, if IE = 100A and IB = 1 A, then IC = 99A and α = 0.99.

Base current versus base-emitter voltage graph shows set of steadily increasing curves representing different collector-emitter voltage values.

Figure 11.16 CE input characteristics.

Collector current versus collector-emitter voltage graph shows set of exponentially increasing curves.

Figure 11.17 CE output characteristics.

CC Characteristics

  • Input characteristics: The input characteristics of a CC configuration are quite different from the CB and CE configurations because the input voltage VBC is largely determined by VEC level. Here,
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Input characteristics are obtained between the input current IB and the input voltage VCB at constant output voltage VEC. Keep the output voltage VEC constant at different levels, vary the input voltage VBC for different points, and record the IB values for each point. Figure 11.18 shows the graph of IB versus VCB.

  • Output characteristics: The output characteristics shows the relation between output voltage VEC and output current IE at constant input current IB. In the operation of CC circuit, if the base current is zero then the emitter current also becomes zero. As a result, no current flows through the transistor. Figure 11.19 shows the output characteristics for a transistor in CC mode.
Base current versus collector-base voltage graph from 0 to 100 micro-ampere and 0 to 6 volt respectively shows slightly two negative sloped lines representing collector-emitter voltage values.

Figure 11.18 CB input characteristics.

Collector current versus collector-emitter voltage graph shows set of curves that are initially rising and later become constant. It shows negative sloped line passing through saturation region, Q-point in active region and cut-off region.

Figure 11.19 CC output characteristics.

Saturation and Cutoff

Assume a transistor is connected in a CB configuration, VBB is set to produce a certain value of IB, and VCC is zero. For this condition, both the base-emitter junction and the base-collector junction are forward biased, while the emitter and the collector are at 0 V. The base current is through the base-emitter junction because of the low impedance path to ground and therefore IC is zero. When both junctions are forward biased, the transistor is in the saturation region of its operation. Saturation is the state of a BJT in which the collector current has reached a maximum and is independent of the base current.

As VCC is increased, VCE increases as the collector current increases. IC increases as VCC is increased due to the forward-biased base-collector junction.

Ideally, when VCE exceeds a stipulated base voltage, the base-collector junction becomes reverse biased and the transistor goes into the active or linear region of its operation. Once the base-collector junction is reverse biased, IC levels off and remains essentially constant for a given value of IB as VCE continues to increase. Actually, IC increases very slightly as VCE increases due to widening of the base-collector depletion region. This results in fewer holes for recombination in the base region, which effectively causes a slight increase in β. When VCE reaches a sufficiently high voltage, the reverse-biased base-collector junction goes into breakdown and the collector current increases quickly. A transistor should never be operated in this breakdown region.

When IB is equal to zero, the transistor is in the cutoff region, although there is a very small collector leakage current. Cutoff is the nonconducting state of a transistor.

Figure 11.20 [26] shows the circuit diagram of a simple inverter circuit with BJT application of CE configuration. The voltage v1 is the base-driving voltage, which has V1 and V2 as the positive polarity for driving base current into the base, and negative polarity for discharging the base current. At t = t0, v1applies V1, the collector current starts to flow after a delay time td, and it flows exponentially until it gets to its peak—this period is the rise time tr. The combination of td and tr give ton. When a negative-base voltage is applied, the BJT is turned off but it does not do so immediately, as the current remains at maximum because of ts, which is the storage time (caused by the delay in the removal of the stored saturation charge). After this, the current exponentially reduces to zero. The combination of ts and tf gives toff.

Circuit diagram shows transistor configured in common-collector mode, input voltage applied across collector-emitter junction and output taken across emitter-base junction. It shows input and output waveforms as function of time.

Figure 11.20 Switching characteristics of BJT. (a) Circuit. (b) Switching waveform

Example 2

The reverse-saturation current of an npn transistor in a CB configuration is 135 A. If the emitter current is 200 A and collector current is 184 A, calculate:

  1. current gain
  2. base current

Solution:

  1. Given
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    Therefore, the current gain is given by

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  2. The base current is given as
    numbered Display Equation

Example 3

For a certain transistor in a CC circuit, IC = 5.5 A, IB = 50 A, and saturation current is given as ISAT = 5 A. Determine the values of

  1. α, β, and emitter current IE
  2. new level of base current IB required to make IC = 10 A

Solution:

  1. The emitter current is given by IE = IB + IC
    numbered Display Equation

    Also, the collector current is given by

    numbered Display Equation

    therefore

    numbered Display Equation
  2. IC = βIB + (1 + β)ISAT&apt#160;5.5 = 100IB + (1 + 100)5
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11.4.3 MOSFET

The MOSFET is a semiconductor device used for switching and amplifying electronic signals in electronic devices. It is one type of FET transistor. In these transistors, the gate terminal is electrically insulated from the current-carrying channel so that it is also called an insulated gate FET (IG-FET). Due to the insulation between gate and source terminals the input resistance of MOSFET may be very high, such as in megaohms (MΩ). The MOSFET is the core of integrated circuit and it can be designed and fabricated in a single chip because of these very small sizes. The MOSFET is a four-terminal device with source (S), gate (G), drain (D), and body (B) terminals. The body of the MOSFET is frequently connected to the source terminal, making it a three-terminal device like a field-effect transistor. The MOSFET is by far the most common transistor and can be used in both analog and digital circuits. MOSFET can operate in a circuit in two configurations (Figure 11.21):

  • depletion type
  • enhancement type
Diagram shows symbols for n-channel enhancement-type mosfet, p-channel enhancement-type mosfet, n-channel depletion-type mosfet, and p-channel depletion-type mosfet. It shows drain, source, and gate terminals of devices.

Figure 11.21 Circuit symbols of enhancement-type and depletion-type MOSFETs. (a) Enhancement-type MOSFET (b) Depletion-type MOSFET.

Depletion-Type MOSFET

Depletion-type MOSFETs, normally known as switched-on devices, are usually closed when there is no-bias voltage at the gate terminal. If the gate voltage increases in the positive, then the channel width increases in depletion mode. As a result, the drain current ID through the channel increases. If the applied gate voltage is more negative, then the channel width is less and MOSFET may enter into the cutoff region. The depletion-mode MOSFET is a rarely used type of transistor in electronic circuits. When there is no voltage on the gate, the channel shows its maximum conductance. As the voltage on the gate is either positive or negative, the channel conductivity decreases.

Enhancement-Type MOSFET

Enhancement-type MOSFETs are a commonly used type of transistor, equivalent to a normally open switch because it does not conduct when the gate voltage is zero. If the positive voltage ( + VGS) is applied to the N-channel gate terminal, then the channel conducts and the drain current flows through the channel. If this bias voltage increases to more positive then channel width and drain current through the channel increase. But if the bias voltage is zero or negative (−VGS), then the transistor may switch off and the channel is in a nonconductive state. So now we can say that the gate voltage of the enhancement-type MOSFET enhances the channel.

As shown in Figure 11.21, within depletion-type and enhancement-type MOSFETs are:

  • P-channel MOSFETs: MOSFET that has P channel region between source and gate is called P-channel MOSFET. The drain and source are in a heavily doped p + region and the body or substrate is n-type. The flow of current is positively charged holes. When negative-gate voltage is applied, the electrons present under the oxide layer are pushed downward into the substrate with a repulsive force. The deflection region is populated by the bound positive charges, which are associated with the donor atoms. The negative-gate voltage also attracts holes from the p + source and drain region into the channel region.
  • N-channel MOSFETs: MOSFET that has n channel region between source and gate is called N-channel MOSFET. The drain and source are in a heavily doped n + region and the substrate or body is P-type. The current flow is as a result of negatively charged electrons. At application of the positive-gate voltage, the holes present under the oxide layer are pushed downward into the substrate with a repulsive force. The deflection region is populated by the bound negative charges, which are associated with the acceptor atoms. The electron-reach channel is formed. The positive voltage also attracts electrons from the n + source and drain regions into the channel.

Steady-State Characteristics

MOSFETS are voltage-controlled devices with very high input impedances. The gate draws a very small leakage current, on the order of nano-amperes. The current gain, which is the ratio of drain current ID, to input gate current IG, is typically on the order of 109. However, the current gain is not an important parameter. The trans-conductance, which is the ratio of drain current to gate voltage, defines the transfer characteristics and is a very important parameter. Transfer characteristics of MOSFETs are shown in Figure 11.22 [5].

Drain current versus gate-source voltage plots show increasing curve crossing y axis for n channel depletion, decreasing curve crossing y axis for p channel depletion, increasing curve in first quadrant for n channel enhancement, and decreasing curve in second quadrant for p channel enhancement.

Figure 11.22 Transfer characteristics of MOSFETs.

Due to high-drain current and low-drain voltage, the power MOSFETs are operated in the linear region for switching actions. In the saturation region, the drain current remains almost constant for any increase in the value of VDS and the transistors are used in this region for voltage amplification. It should be noted that saturation has the opposite meaning to that for bipolar transistors. The steady-state model is the same for both depletion-type and enhancement-type MOSFETs. The trans-conductance gm is defined as

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The output resistance is defined as

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Example 4

Two MOSFETs connected in parallel carry a total current of IT = 20 A. The drain-to-source voltage of MOSFET M1 is VDS1 = 2.5 V and that of MOSFET M2 is VDS2 = 3 V. Determine the drain current of each transistor and the difference in current sharing if the current-sharing series resistances are (a) RS1 = 0.3Ω and RS2 = 0.2Ω.

Solution:

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therefore

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Application of MOSFETs

MOSFET devices have a variable number of applications, including linear-amplifier stages, analog switches, basic chopper amplifiers, and switched-capacitor circuits.

11.4.4 Insulated Gate Bipolar Transistor

An insulated gate bipolar transistor (IGBT) combines the advantage of BJTs and MOSFETs, as shown in Figure 11.23 [3]. An IGBT has high input impedance, like MOSFETs, and low on-state conduction losses, like BJTs. But there is no second breakdown problem like BJTs. By design and structure, the equivalent drain-to-source resistance RDS is controlled to behave like that of a BJT. An IGBT is made of four alternate pnpn layers and could latch like a thyristor given the necessary condition that αnpn + αpnp > 1.

Diagram shows symbol for IGBT which includes gate, collector, and emitter terminals. Circuit diagram shows set of capacitances, drift region resistance, pnp transistor, npn transistor, and body region resistance.

Figure 11.23 Circuit symbol and equivalent circuit for an IGBT.

Steady-State Characteristics of IGBTs

The VI characteristics of an n-channel IGBT are shown in Figure 11.24. They appear qualitatively like those of a logic-level BJT except that the controlling parameter is not a base current but the gate-emitter voltage.

I sub(C) versus V sub(CE) graph shows set of curves representing different gate-emitter voltage values, saturation, active, and cut off regions, load line, and fault load line. I sub(C) versus V sub(gE) graph shows curve with slope g sub(fs) increasing after threshold voltage.

Figure 11.24 Static characteristics of an IGBT. (a) Output characteristics (b) Transfer characteristics.

An IGBT is a voltage-controlled device similar to a power MOSFET. It has lower switching and conducting losses while sharing many of the appealing features of power MOSFETs, such as ease of gate drive, peak current, capability, and ruggedness. An IGBT is inherently faster than a BJT. However, the switching speed of IGBTs is inferior to that of MOSFETs. The current rating of a single IGBT can be up to 400 A, 1,200 V, and the switching frequency can be up to 20 kHz.

A general comparison between BJTs, MOSFETs and IGBTs is given in Table 11.2.

TABLE 11.2 Properties of BJTs, MOSFETs, and IGBTs

Device Characteristic Power BJT Power MOSFET IGBT
Voltage Rating High < 1kV High < 1kV Very high > 1kV
Current Rating High < 500A Low < 200A High > 500A
Input Drive Current, hFE 20–200 Voltage, VGS 3–10V Voltage, VGE 4–8V
Input Impedance Low High High
Output Impedance Low Medium Low
Switching Speed Slow (uS) Fast (nS) Medium
Cost Low Medium High

11.4.5 Thyristors

A thyristor is a bistable semiconductor device made of three or more junctions that can be switched from the off state to the on state and vice versa. Electronically controlled switches use thyristors in industrial, military, aerospace, commercial, and consumer applications. Thyristors can control power to many kinds of loads, such as a simple lamps, power supplies, voltage regulators, and industrial motors. Circuits using AC or DC can use thyristors for power control.

The most commonly used thyristors for power control are:

  • silicon-controlled rectifier (SCR), a reverse blocking/unidirectional triode, gate-triggered, three-terminal thyristor that has positive anode to cathode voltages and exhibits a reverse blocking state for negative anode to cathode voltages
  • triode for alternating current (TRIAC), a bidirectional triode, gate-triggered, three-terminal thyristor that switches for either positive anode to cathode voltages or negative anode to cathode voltages

SCR Characteristics

An SCR conducts when the anode to cathode is forward biased and a gate-trigger signal is applied. After an SCR is turned on, it remains on as long as it has a minimum anode current. This minimum anode current that maintains conduction is the holding current. Holding current (IH) is the minimum current required to keep an SCR conducting.

Figures 11.25 and 11.26 [5] show the voltage/current characteristics of an SCR with no gate current applied and with a forward- and reverse-blocking voltage rating. When the forward-blocking voltage is reached, the SCR turns itself on. The anode-to-cathode voltage decreases, and the anode current increases. Reverse-blocking voltage is the maximum reverse voltage that may be applied without turning on an SCR. Forward-blocking voltage is the maximum forward voltage that may be applied without turning on an SCR.

Top graph shows forward breakover voltage, holding current, forward blocking region, latching current, maximum reverse voltage, reverse blocking region, and reverse avalanche region. Bottom graph shows forward current carrying on, forward voltage blocking off, and reverse blocking.

Figure 11.25 Switching characteristics of SCR. (a) I–V characteristics and symbolic representation (b) Ideal-switching characteristics.

Circuit diagram shows voltage V sub(s) applied across series combination of SCR and resistor R sub(L). I sub(T) versus V sub(AK) plot shows gate triggered, forward breakover voltage, forward volt-drop, latching, holding, and leakage currents, and reverse breakdown voltage.

Figure 11.26 SCR circuit and characteristics. (a) Circuit. (b) v-i characteristics.

Internally, the SCR is similar to two cross-connected transistors.

As seen in Figure 11.27, a gate-trigger signal turns on transistor Q2. Q2 then forward biases the Q1 base-emitter junction, and both transistors turn on, providing a current path from the anode to the cathode. Both transistors are then latched on until the anode current is removed. SCR can change AC into DC and at the same time can control the amount of power fed to the load. Thus SCR combines the features of a rectifier and a transistor.

Circuit diagram shows two cross-connected transistors Q1 and Q2 in which base of Q1 connected to collector Q2, base of Q2 connected to collector of Q1, emitter of Q1 connected to dc voltage source through resistor R and emitter of Q2 connected to ground.

Figure 11.27 SCR equivalent circuit.

An SCR has two states, i.e., either it does not conduct or it conducts heavily. There is no state in between. Therefore, SCR behaves like a switch. There are two ways to turn on the SCR. The first method is to keep the gate open and make the supply voltage equal to the break-over voltage. The second method is to operate SCR with supply voltage less than break-over voltage and then turn it on by means of a small voltage (typically 1.5 V, 30 mA) applied to the gate. Applying small positive voltage to the gate is the normal way to close an SCR because the break-over voltage is usually much greater than supply voltage. To open the SCR (i.e., to make it nonconducting), reduce the supply voltage to zero.

Breakover voltage is defined as the minimum forward voltage, when the gate is open, at which the SCR is turned on. This implies that if the breakover voltage of an SCR is 187 V, SCR remains turned off as long as the supply voltage is less than 187 V.

Holding current is defined as the maximum anode current, when the gate is open, at which SCR is turned off. It is the anode current when SCR turn-off is affected.

SCR Turn-On and Turn-Off

To facilitate SCR turn-on, the gate voltage is increased to a minimum value to initiate triggering. This minimum value of gate voltage at which SCR turn-on is achieved is called the trigger voltage. The resulting gate current is called trigger current.

  • DC-gate bias: Here, a positive, gate-triggering voltage is applied to the gate to turn on the SCR. A resistance is shunt connected in the circuit to facilitate noise suppression and improve the turn-on time. The primary factor on which the turn-on time depends is the magnitude of the gate-current AC. A higher gate current implies shorter turn-on time.
  • AC trigger method: During the positive half-cycle of the AC gate current, the SCR is turned on.

SCR turn-off is more complex than turn-on. This is because the gate loses control once turn-on is achieved. There are two methods of thyristor turn-off:

  • anode current reduction, when the anode current is reduced below the holding current and the SCR is turned off
  • forced commutation, which discharges a capacitor in parallel with an SCR

SCR Half-Wave, Full-Wave, and Full-Wave-Bridge Waveforms

Figure 11.28(a) shows a half-wave SCR with resistor R representing the load. Figure 11.28(b) shows the output voltage waveform and the trigger-pulse waveform. When the input voltage is in the positive half cycle, the thyristor is forward biased. The SCR will only start conducting after the gate signal α has been applied, which will conduct from α to π, the output voltage as seen in Figure 11.28(b). At ωt = α, the SCR will be naturally commutated since, at that point, the output voltage and current are zero. In the negative half cycle, the output will be zero since the SCR will be in reverse-bias mode. In the next positive half cycle, the SCR will conduct if the gate has been fired, and this continues periodically.

Circuit diagram shows voltage source e, SCR, and resistor R. Graph shows input sine wave in dotted lines and output wave follows input pattern from angle alpha to angle pi. It also shows current pulse i sub(g) at angle alpha.

Figure 11.28 SCR half-wave rectifier and waveform. (a) SCR half-wave rectifier. (b) Waveform and trigger pulse.

Figure 11.29(a) shows a full-wave SCR with a load and Figure 11.29(b) shows the output voltage waveform. When the input voltage is in the positive half cycle, the thyristor SCR1 is forward biased while SCR2 is reverse biased. The SCR1 will only start conducting after the gate signal α has been applied, which will conduct from α to π, the output voltage as seen in Figure 11.29(b), in which SCR1 will allow load current flow through. In the negative half cycle, the thyristor SCR2 is forward biased while SCR1 is reverse biased. The SCR2 will only start conducting after the gate signal α has been applied, which will conduct from α to π, the output voltage as seen in Figure 11.29b, in which SCR2 will allow load current flow through. In the next positive half cycle, the process continues periodically as previously.

Circuit diagram shows ac supply, transformer, two SCRs, and load. Graph shows output voltage of circuit as function of phase angle. Output waveform includes positive half cycle of sine wave between angles alpha and pi.

Figure 11.29 SCR full-wave rectifier and waveform.

As shown in Figure 11.30, during the positive half cycle of the input-voltage waveform, thyristors SCR1 and SCR4 are forward biased and thus conducting while SCR2 and SCR3 are in reverse-bias mode. During the negative half cycle of the input-voltage waveform, thyristors SCR2 and SCR3 are forward biased and thus conducting, while those of SCR1 and SCR4 are reverse biased. The output waveform is like that of the full-wave rectifier shown in Figure 11.29(b).

Circuit diagram shows full-wave bridge rectifier consisting of four SCRs along with load resistor and input ac voltage connected across alternate pair of SCRs.

Figure 11.30 SCR full-wave bridge rectifier.

Application of Thyristors

Power thyristors are used in applications where high voltages and currents are present. They are generally used in order to control AC. Power thyristors can also be used as the control elements for phase-fired controllers. They are commonly used in inverter circuits for converting direct power to alternating power of specified frequency. These are also used in converters to convert an alternating power into alternating power of different amplitude and frequency.

11.4.6 TRIAC

A TRIAC is a three-terminal, bidirectional thyristor that, unlike SCR, is capable of conducting in either direction between main terminals. A TRIAC can be triggered in the positive or negative half cycle of the AC voltage source by applying a positive or negative gate signal, respectively. Similar to SCR, TRIAC requires minimum current to remain in conduction. This minimum current is the holding current. The i–v characteristics and ideal-switching characteristics with symbolic representation of TRIAC are shown in Figure 11.31.

I sub(A) versus v sub(AK) graph shows s-shaped curve in first quadrant representing forward current carrying and s-shaped curve in third quadrant representing reverse current carrying. Ideal off and on states coincide with the current and voltage axes.

Figure 11.31 Switching characteristics of a TRIAC. (a) I–V characteristics and symbolic representation (b) Ideal-switching characteristics.

11.5 APPLICATIONS OF POWER ELECTRONICS DEVICES TO MACHINE CONTROL

Choppers

A chopper is a static-power electronic device that converts fixed DC voltage or power to variable DC voltage or power. It is an on/off switch that produces a chopped voltage across a load when connecting the load to and disconnecting the load from a DC source. The two types of choppers are the step-up chopper and step-down chopper.

Figure 11.32 shows a step-down chopper with resistive load. The thyristor in the circuit acts as a switch. The supply voltage appears across the load when thyristor is on and the voltage across the load will be zero when the thyristor is off. The output voltage and current waveforms are shown in Figure 11.33.

Circuit diagram shows dc voltage source V, thyristor, and resistor R. Current flowing in circuit is I sub(0) and voltage taken across R is V sub(0).

Figure 11.32 Step-down chopper with resistive load.

Graph shows rectangular waveforms of same pattern representing current I sub(0) and voltage v sub(0) and horizontal lines representing dc values. Waves have high value during time t sub(ON) and zero value during t sub(OFF).

Figure 11.33 Step-down choppers: output voltage and current waveforms.

 

  • Vdc = average value of output or load voltage
  • Idc = average value of output or load current
  • tON = time interval for which SCR conducts
  • tOFF = time interval for which SCR is off
  • T = tON + tOFF = period of switching or chopping period
  • frequency of chopper switching or chopping frequency.

Average output voltage

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(11.11)numbered Display Equation

(11.12)numbered Display Equation

Average output current

(11.13)numbered Display Equation

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Example 5

A chopper circuit is operating on time-ratio control at a frequency of 2.5 kHz on a 415 V supply. If the load voltage is 300 V, calculate the conduction period of the thyristor in each cycle.

Solution:

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Chopping period

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Output voltage

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Conduction period of thyristor

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Example 6

A 250 V, DC motor fed by a chopper runs at 1,200 rpm with a duty ratio of 0.9, Figure 11.34. What must be the on time of the chopper if the motor must run at 800 rpm? The chopper operates at 120 Hz.

Circuit diagram shows input dc voltage sV, thyristor, current I sub(a), resisor R sub(a), and motor with voltage E sub(b). Voltage across R sub(a) and motor is V sub(dc).

Figure 11.34 DC Motor Chopper Circuit.

Solution:

Speed of motor N1 = 1,200 rpm

Duty ratio d1 = 0.9, f = 120 Hz

We know that back emf of motor Eb is given by

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where N is the speed in rpm, φ is the flux/pole in Wb, Z is the number of armature conductors, P is the number of poles, and A is the number of parallel paths.

Therefore

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where Ia is the armature current and Ra is the armature resistance. Since Ra is not given, IaRa drop is neglected.

Therefore

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Supply,

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This implies that 250 ∝ 1,200, As speed changes, d also changes.

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Taking the ratio of to , we have that

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But

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Chopping frequency f = 120 Hz

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On time of chopper

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11.6 APPLICATIONS OF POWER ELECTRONICS DEVICES TO POWER SYSTEM DEVICES

Circuit Breaker

Figure 11.35 [27] shows a DC circuit breaker. It is made up of two thyristors SCR1 and SCR2. The circuit breaker is closed when thyristor SCR1 is conducting and this makes the full DC voltage appear across both the load and the capacitor. Thyristor SCR2 is turned on when the circuit breaker is to be open. Therefore, capacitor C discharges through SCR1 and SCR2, reversing the current through SCR1 and leading SCR1 to be turned off. This operation makes the circuit breaker trip off. Simultaneously, capacitor C will be charged in the reverse direction, causing the current through SCR2 to be turned off. This makes this breaker ideal for high-speed applications.

Circuit diagram shows voltage V applied across input, load and SCR 1 along first vertical line, resistor R and SCR 2 along second vertical line, and capacitor C connecting vertical lines.

Figure 11.35 Solid-state DC circuit breaker.

Solid-State Relays

A solid-state relay (SSR) is an electronic switching device that switches on or off when a small external voltage is applied across its control terminals. An SSR (Figure 11.36 [27]) shows that the control signal from a control circuit turns the thyristor on or off, which in turn controls the main circuit (by actuating the circuit breaker to open, protecting equipment and consumers from harm).

image

Figure 11.36 Solid-state relay.

11.7 APPLICATIONS OF POWER ELECTRONICS TO UTILITY, AEROSPACE, AND SHIPPING

Applications of power electronics range in size from a switched-mode power supply in an AC adapter, battery chargers, and fluorescent-lamp ballasts, through variable-frequency drives and DC-motor drives used to operate pumps, fans, and manufacturing machinery, and up to gigawatt-scale, high-voltage, DC-power-transmission systems used to interconnect electrical grids. Power electronic systems are found in virtually every electronic device.

Motor drives are found in pumps, blowers, and mill drives for textile, paper, cement, and similar facilities. Drives may be used for power conversion and for motion control [19]. For AC motors, applications include variable-frequency drives, motor soft starters, and excitation systems [20].

In hybrid electric vehicles, power electronics are used in two formats: series hybrid and parallel hybrid. The difference is the relationship of the electric motor to the internal combustion engine. Devices used in electric vehicles consist mostly of DC–DC converters for battery charging and DC–AC converters to power the propulsion motor. Electric trains use power electronic devices to obtain power, as well as for vector control using pulse-width modulation (PWM) rectifiers. The trains obtain their power from power lines. Another new use for power electronics is in elevator systems. These systems may use thyristors, inverters, permanent magnet motors, or various hybrid systems that incorporate PWM systems and standard motors [21].

Smart Grid

A smart grid is a modernized electrical grid that uses information and communications technology to gather and act on information, such as information about the behaviors of suppliers and consumers, in an automated fashion to improve the efficiency, reliability, economics, and sustainability of the production and distribution of electricity [22], [23].

Electric power generated by wind turbines and hydroelectric turbines by using induction generators can cause variances in the frequency at which power is generated. Power electronic devices are utilized in these systems to convert the generated AC voltages into high-voltage direct current (HVDC). The HVDC power can be more easily converted into three-phase power that is coherent with the power associated to the existing power grid. Through these devices, the power delivered by these systems is cleaner and has a higher associated power factor. Wind-power systems’ optimum torque is obtained either through a gearbox or direct-drive technologies that can reduce the size of the power electronics device [24].

Electric power can be generated through photovoltaic cells by using power electronic devices. The produced power is usually then transformed by solar inverters. Inverters are divided into three different types: central, module-integrated, and string. Central converters can be connected either in parallel or in series on the DC side of the system. For photovoltaic “farms,” a single central converter is used for the entire system. Module-integrated converters are connected in series on either the DC or AC side. Normally several modules are used within a photovoltaic system, since the system requires these converters on both DC and AC terminals. A string converter is used in a system that utilizes photovoltaic cells that are facing different directions. It is used to convert the power generated to each string, or line, in which the photovoltaic cells are interacting [24].

Grid Voltage Regulations

Power electronics can be used to help utilities adapt to the rapid increase in distributed residential/commercial solar-power generation. Germany and parts of Hawaii, California, and New Jersey require costly studies to be conducted before approving new solar installations. Relatively small-scale ground- or pole-mounted devices create the potential for a distributed-control infrastructure to monitor and manage the flow of power. Traditional electromechanical systems, such as capacitor banks or voltage regulators at substations, can take minutes to adjust voltage and can be distant from the solar installations where the problems originate. If voltage on a neighborhood circuit goes too high, it can endanger utility crews and cause damage to both utility and customer equipment. Further, a grid fault causes photovoltaic generators to shut down immediately, spiking demand for grid power. Smart grid-based regulators are more controllable than far more numerous consumer devices.

In another approach, a group of sixteen western utilities called the Western Electric Industry Leaders called for mandatory use of “smart inverters.” These devices convert DC to household AC and can also help with power quality. Such devices could eliminate the need for expensive utility-equipment upgrades at a much lower total cost [25].

11.8 FACTS

FACTS have been evolving to a mature technology with high power rating. It has widespread applications and has become a top-rate, reliable technology, based on power electronics. The main purpose of these systems is to supply the network as quickly as possible with inductive or capacitive-reactive power that is adapted to its particular requirements, while also improving transmission quality and the efficiency of the power transmission system.

With the progression and development in power electronics applications comes not only improved performance of AC systems but also feasibility for long-distance applications. FACTS can also help solve technical problems in interconnected power systems. FACTS are available in:

  • parallel connection: static var compensators (SVCs) or static synchronous compensators (STATCOM)
  • series connection: fixed-series compensation (FSC) or thyristor-controlled/protected series compensation (TCSC/TPSC)

11.8.1 Parallel Compensation

Parallel compensation is any type of reactive power compensation, employing either switched or controlled units that are connected in parallel to the transmission network at a power system node.

Mechanically Switched Capacitors/Reactors (MSCs/MSRs)

Most economical reactive power compensation devices are mechanically switched devices. Mechanically switched capacitors are a simple but low-speed solution for voltage control and network stabilization under heavy load conditions. Their utilization has almost no effect on the short-circuit power but it increases the voltage at the point of connection. Mechanically switched reactors have exactly the opposite effect and are therefore preferable for achieving stabilization under low-load conditions. An advanced form of mechanically switched capacitor is the MSCDN, an MSC with an additional damping circuit for avoidance of system resonances.

Static var Compensators (SVCs)

Static var compensators are a fast and reliable means of controlling voltage lines and system nodes. Switching or controlling reactive power elements connected to the secondary side of the transformer changes the reactive power. Each capacitor bank is switched on and off by a thyristor valve. When system voltage is low, the SVC supplies capacitive reactive power and raises the network voltage. When system voltage is high, the SVC generates inductive reactive power and reduces the system voltage.

Static var compensators perform tasks such as:

  • improvement in voltage quality
  • dynamic reactive power control
  • increase in system stability
  • damping of power oscillations
  • increase in power transfer capability
  • unbalance control (option)

The design and configuration of an SVC, including the size of the installation, operating conditions, and losses, depend on the system condition (weak or strong), the system configuration (meshed or radial), and the tasks to be performed.

11.8.2 Series Compensation

Series compensation is defined as insertion of a reactive power element into transmission lines. The most common application is the fixed-series capacitor. Thyristor-controlled series capacitors thyristor-protected series capacitors may also be installed.

Fixed-Series Capacitor

The simple and most cost-effective type of series compensation is provided by a fixed-series capacitor (FSC). FSCs comprise the actual capacitor banks, and for protection purposes, parallel arresters (metal oxide varistors or MOVs), spark gaps and a bypass switch for isolation purposes. Among the advantages of an FSC:

  • increase in transmission capacity
  • reduction in transmission angle

Thyristor-Controlled Series Capacitor

Reactive power compensation by means of a thyristor-controlled series capacitor (TCSC) can be adapted to a wide range of operating conditions. It is also possible to control the current and thus the load flow in parallel transmission lines, which simultaneously improves system stability. Further applications for TCSCs include power-oscillation damping and mitigation of sub-synchronous resonances, a crucial issue in case of large thermal generators.

Additional benefits of thyristor-controlled series compensation:

  • damping of power oscillations (POD)
  • load-flow control
  • mitigation of sub-synchronous resonances
  • increase in system stability

Thyristor-Protected Series Capacitor

When high-power thyristors are used, there is no need to install conventional spark gaps or surge arresters. Due to the very short cooling-down times of the special thyristor valves, thyristor-protected series capacitors (TPSCs) can be quickly returned to service after a line fault, allowing the transmission lines to be utilized to their maximum capacity. TPSCs are the first choice whenever transmission lines must be returned to maximum carrying capacity as quickly as possible after a failure.

Illustrative Problems and Examples

  1. Evaluate the following for a CB transistor:
    1. If it has α = 0.96 and IE = 2 mA, find its current through the collector IC and the base current IB.
    2. If it has β = 20A and IE = 2 mA. Compute the value of α, IC and IE.

    Solution:

    1. IC = αIE = 0.96 × 2 mA = 1.92 mA

      IB = IE − IC = 2 mA –1.92 mA

      IB = 0.08 mA

    2. IC = αIE = 0.9524 × 2 = 1.905 mA

  2. During a power electronics laboratory session, a student records the following measurements of a transistor: IB = 5.202 mA, IC = 50 m A, ICBO = 2 mA.

    Using these measurements as a base, calculate:

    1. α, β, and IE
    2. new value of IB required to make IC = 10 mA

      Solution:

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  3. Answer the following:
    1. What are the different operation regions of the SCR?
    2. Define latching current and holding current.
    3. Explain why thyristors are referred to as charge controlled device.
    4. What is the relation between the gate signal and forward break-over voltage (VBO)?
    5. List the losses that occur in thyristors during operation.
    6. What are the advantages of speed control using a thyristor.
    7. What happens when a capacitor is connected to a generator load?
    8. Briefly explain why capacitors work on AC only.
  4. An npn silicon transistor is connected as a CE amplifier with load RL and source resistance RS. The h parameters are: hie = 1.1k, hre = 2.5 × 10 − 4, hfe = 50, and hoe = 25μS. If RL = 10k and RS = 1k, find the various gains and input and output impedances. Derive the relations.
  5. For npn transistor as shown in Figure 11.11, calculate the exact value of the emitter current and VCE using the values of RC = 5kΩ, VCC = 12V and IB = 9 ×10−5A. Assume the transistor to be silicon and β = 100.
  6. An SCR has a break-over voltage of 400 V, trigger current of 10 mA, and holding current of 10 mA. What do you infer from this? What will happen if the gate current is made 15 mA ?
  7. An SCR in a circuit is subjected to a 50 A surge that lasts for 12 ms. Determine whether this surge will destroy the device, given that the circuit-fusing rating is 90 A2s.
  8. An SCR has a circuit-fusing rating of 50 A2s. The device is being used in a circuit where it could be subjected to a 100 A surge. Determine the maximum allowable duration of such a surge.
  9. A 220 Ω resistor is connected in series with the gate of an SCR. The gate current required to fire the SCR is 7 mA. What is the input voltage (Vin) required to fire the SCR?
  10. What is the difference between an SCR and an ordinary rectifier?
    1. Explain clearly why SCRs are typically turned on by gate currents.
    2. Why can an SCR not be used as a bidirectional switch?
    3. How does SCR control the power fed to the load?
    4. Why are SCRs usually used in AC applications?
    5. Turning off the SCR is usually more problematic than turning it on. This is because once the device is on, the gate no longer controls it. With the aid of diagrams, explain two turn-off methods.
    6. Why is SCR turned on by high-frequency radiation?
  11. SCRs are usually found useful in switching and control applications. This is because of their ability to control large currents in a load through a slight gate current. Explain explicitly the application of an SCR:
    1. as a static contactor
    2. for power control
    3. for speed control of a DC shunt motor

11.9 CHAPTER SUMMARY

Power electronics are an important subject of research in electronic and electrical engineering, which deals with the design, control, computation, and integration of nonlinear, time-varying, energy-processing electronic systems with fast dynamics. In this chapter, the elements of power electronics were treated, along with technical details of electronic components. The fundamental issues of modeling, computation steps, and applications of different electronic system/services were discussed. To help the reader, a summary of different types of electronic devices and applications to space, ship, and utility systems were given.

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