S4b6甲32p4D6甲甲 ⅩXX E5.7 Three-phase rectifier output compared to the input signals. The input signals as well as the labels are those referenced to Fig. 5.6. The three-phase inputs with the associated rectifier output voltage are shown in Fig. 5. 7 as they would appear without the low-pass filter section. The three-phase bridge rectifier has a reduced ripple content of 4%as diodes that conduct are also shown at the top of the figure. This output waveform assumes a purely resistive load connected as shown in Fig. 5.6. Most loads(motors, transformers, etc. and many sources(power grid) clude some inductance, and in fact may be dominated by inductive properties. This causes phase shifts between the input and output waveforms. The rectifier output may thus vary in shape and phase considerably from that shown in Fig. 5.7 (Kassakian et al, 1991]. When other types of switches are used in these circuits the inductive elements can induce large voltages that may damage sensitive or expensive components. Diodes are used regularly in such circuits to shunt current and clamp induced voltages at low levels to protect expensive One variation of the typical rectifier is the Cockroft Walton circuit used to obtain high voltages without the necessity of providing a high-voltage transformer. The uit in Fig. 5. 8 multiplies the peak secondary voltage y a factor of six. The steady-state voltage level at each filter capacitor node is shown in the figure. Adding s, max s ma 2vs.max additional stages increases the load voltage further. As in other rectifier circuits, the value of the capacitors will determine the amount of ripple in the output FIGURE 5.8 Cockroft-Walton circuit used for voltage actors in a lower voltage stage than in the next highest voltage stage Defining Terms Bipolar device: Semiconductor electronic device that uses positive and negative charge carriers to conduct electric current Diode: Two-terminal solid-state semiconductor device that presents a low impedance to current flow in one direction and a high impedance to current flow in the opposite direction. Pn-junction: Metallurgical interface of two regions in a semiconductor where one region contains impurity elements that create equivalent positive charge carriers(p-type) and the other semiconductor region ontains impurities that create negative charge carriers(n-type) Ripple: The ac(time-varying) portion of the output signal from a rectifier circuit. hottky diode: A diode formed by placing a metal layer directly onto a unipolar semiconductor substrate Uncontrolled rectifier: A rectifier circuit employing switches that do not require control signals to operate them in their“on”or“off” states. e 2000 by CRC Press LLC
© 2000 by CRC Press LLC The three-phase inputs with the associated rectifier output voltage are shown in Fig. 5.7 as they would appear without the low-pass filter section. The three-phase bridge rectifier has a reduced ripple content of 4% as compared to a ripple content of 47% in the single-phase bridge rectifier [Milnes, 1980]. The corresponding diodes that conduct are also shown at the top of the figure. This output waveform assumes a purely resistive load connected as shown in Fig. 5.6. Most loads (motors, transformers, etc.) and many sources (power grid) include some inductance, and in fact may be dominated by inductive properties. This causes phase shifts between the input and output waveforms. The rectifier output may thus vary in shape and phase considerably from that shown in Fig. 5.7 [Kassakian et al., 1991]. When other types of switches are used in these circuits the inductive elements can induce large voltages that may damage sensitive or expensive components. Diodes are used regularly in such circuits to shunt current and clamp induced voltages at low levels to protect expensive components such as electronic switches. One variation of the typical rectifier is the CockroftWalton circuit used to obtain high voltages without the necessity of providing a high-voltage transformer. The circuit in Fig. 5.8 multiplies the peak secondary voltage by a factor of six. The steady-state voltage level at each filter capacitor node is shown in the figure. Adding additional stages increases the load voltage further. As in other rectifier circuits, the value of the capacitors will determine the amount of ripple in the output waveform for given load resistance values. In general, the capacitors in a lower voltage stage should be larger than in the next highest voltage stage. Defining Terms Bipolar device: Semiconductor electronic device that uses positive and negative charge carriers to conduct electric current. Diode: Two-terminal solid-state semiconductor device that presents a low impedance to current flow in one direction and a high impedance to current flow in the opposite direction. pn-junction: Metallurgical interface of two regions in a semiconductor where one region contains impurity elements that create equivalent positive charge carriers (p-type) and the other semiconductor region contains impurities that create negative charge carriers (n-type). Ripple: The ac (time-varying) portion of the output signal from a rectifier circuit. Schottky diode: A diode formed by placing a metal layer directly onto a unipolar semiconductor substrate. Uncontrolled rectifier: A rectifier circuit employing switches that do not require control signals to operate them in their “on” or “off” states. FIGURE 5.7 Three-phase rectifier output compared to the input signals. The input signals as well as the conducting diode labels are those referenced to Fig. 5.6. FIGURE 5.8 Cockroft-Walton circuit used for voltage multiplication
Related Topics 22.2 Diodes.30.1 Power Semiconductor Devices References R.G. Hoft, Semiconductor power electronics, New York: Van Nostrand Reinhold. 1986. J.G. Kassakian, M.E. Schlecht, and G.C. Verghese, Principles of Power Electronics, Reading, Mass.: Addison K.G. McKay, Avalanche breakdown in silicon, "Physical Review, voL 94, P. 877, 1954 A.G. Milnes, Semiconductor Devices and Integrated Electronics, New York: Van Nostrand Reinhold, 1980 J.L. Moll, Physics of Semiconductors, New York: McGraw-Hill, 1964 N.F. Mott, Note on the contact between a metal and an insulator or semiconductor, Proc. Cambridge philos. Soc,vol.34,p.568,1938 W. Schottky,"Halbleitertheorie der Sperrschicht, " Naturwissenschaften, vol 26, P. 843, 1938 W. Shockley, The theory of p-n junctions in semiconductors and p-n junction transistors, "Bell System Tech Jvol.28,p.435,1949 Further information A good introduction to solid-state electronic devices with a minimum of mathematics and physics is Solid State Electronic Devices, 3rd edition, by B.G. Streetman, Prentice-Hall, 1989. A rigorous and more detailed discussion is provided in Physics of Semiconductor Devices, 2nd edition, by S M. Sze, John Wiley Sons, 1981. Both of these books discuss many specialized diode structures as well as other semiconductor devices. Advanced material on the most recent developments in semiconductor devices, including diodes, can be found in technical journals such as the IEEE Transactions on Electron Devices, Solid State Electronics, and Journal of Applied Physics. a good summary of advanced rectifier topologies and characteristics is given in Basic Principles of Power Electronics by K. Heumann, Springer-Verlag, 1986. Advanced material on rectifier designs as well as other power electronics circuits can be found in IEEE Transactions on Power Electronics, IEEE Transactions on Industry Applications, and the EPE Journal. Two good industry magazines that cover power devices such as diodes and power converter circuitry are Power Control and Intelligent Motion(PCIM) and Power Technics. 5.2 Limiters Theodore F. Bogart, r Limiters are named for their ability to limit voltage excursions at the output of a circuit whose input may undergo unrestricted variations. They are also called clipping circuits because waveforms having rounded peaks that exceed the limit(s)imposed by such circuits appear, after limiting, to have their peaks flattened, or"clipped off. Limiters may be designed to clip positive voltages at a certain level, negative voltages at a different level, or to do both. The simplest types consist simply of diodes and dc voltage sources, while more elaborate designs incorporate operational amplifiers Limiting Circuits Figure 5.9 shows how the transfer characteristics of limiting circuits reflect the fact that outputs are clipped at certain levels In each of the examples shown, note that the characteristic becomes horizontal at the outpr level where clipping occurs. The horizontal line means that the output remains constant regardless of the input level in that region. Outside of the clipping region, the transfer characteristic is simply a line whose slope equals Excerpted from T. Bogart, Jr, Electronic Devices and Circuits, 3rd ed, Columbus, Ohio: Macmillan/Merrill, 1993, Pp. 689-697. with permission e 2000 by CRC Press LLC
© 2000 by CRC Press LLC Related Topics 22.2 Diodes • 30.1 Power Semiconductor Devices References R.G. Hoft, Semiconductor Power Electronics, New York: Van Nostrand Reinhold, 1986. J.G. Kassakian, M.F. Schlecht, and G.C. Verghese, Principles of Power Electronics, Reading, Mass.: AddisonWesley, 1991. K.G. McKay, “Avalanche breakdown in silicon,” Physical Review, vol. 94, p. 877, 1954. A.G. Milnes, Semiconductor Devices and Integrated Electronics, New York: Van Nostrand Reinhold, 1980. J.L. Moll, Physics of Semiconductors, New York: McGraw-Hill, 1964. N.F. Mott, “Note on the contact between a metal and an insulator or semiconductor,” Proc. Cambridge Philos. Soc., vol. 34, p. 568, 1938. W. Schottky, “Halbleitertheorie der Sperrschicht,” Naturwissenschaften, vol. 26, p. 843, 1938. W. Shockley, “The theory of p-n junctions in semiconductors and p-n junction transistors,” Bell System Tech. J., vol. 28, p. 435, 1949. Further Information A good introduction to solid-state electronic devices with a minimum of mathematics and physics is Solid State Electronic Devices, 3rd edition, by B.G. Streetman, Prentice-Hall, 1989. A rigorous and more detailed discussion is provided in Physics of Semiconductor Devices, 2nd edition, by S.M. Sze, John Wiley & Sons, 1981. Both of these books discuss many specialized diode structures as well as other semiconductor devices.Advanced material on the most recent developments in semiconductor devices, including diodes, can be found in technical journals such as the IEEE Transactions on Electron Devices, Solid State Electronics, and Journal of Applied Physics. A good summary of advanced rectifier topologies and characteristics is given in Basic Principles of Power Electronics by K. Heumann, Springer-Verlag, 1986. Advanced material on rectifier designs as well as other power electronics circuits can be found in IEEE Transactions on Power Electronics, IEEE Transactions on Industry Applications, and the EPE Journal. Two good industry magazines that cover power devices such as diodes and power converter circuitry are Power Control and Intelligent Motion (PCIM) and Power Technics. 5.2 Limiters1 Theodore F. Bogart, Jr. Limiters are named for their ability to limit voltage excursions at the output of a circuit whose input may undergo unrestricted variations. They are also called clipping circuits because waveforms having rounded peaks that exceed the limit(s) imposed by such circuits appear, after limiting, to have their peaks flattened, or “clipped” off. Limiters may be designed to clip positive voltages at a certain level, negative voltages at a different level, or to do both. The simplest types consist simply of diodes and dc voltage sources, while more elaborate designs incorporate operational amplifiers. Limiting Circuits Figure 5.9 shows how the transfer characteristics of limiting circuits reflect the fact that outputs are clipped at certain levels. In each of the examples shown, note that the characteristic becomes horizontal at the output level where clipping occurs. The horizontal line means that the output remains constant regardless of the input level in that region. Outside of the clipping region, the transfer characteristic is simply a line whose slope equals 1 Excerpted from T.F. Bogart, Jr., Electronic Devices and Circuits, 3rd ed., Columbus, Ohio:Macmillan/Merrill, 1993, pp. 689–697. With permission
(a) Positive clippi (b) Negative clipping (c)Positive and negative clipping FIGURE 5.9 Waveforms and transfer characteristics of limiting circuits. Source: T.E. Bogart, Jr, Electronic Devices and Circuits, 3rd ed, Columbus, Ohio: Macmillan/Merrill, 1993, p 676. With permission. the gain of the device. This is the region of linear operation. In these examples, the devices are assumed to have unity gain, so the slope of each line in the linear region is 1. Figure 5.10 illustrates a somewhat different kind of limiting action. Instead of the positive or negative peaks being clipped, the output follows the input when the signal is above or below a certain level. The transfer characteristics show that linear operation occurs only when certain signal levels are reached and that the output remains constant below those levels. This form of limiting can also be thought of as a special case of that shown in Fig. 5.9. Imagine, for example, that the clipping level in Fig. 5.9(b) is raised to a positive value; then the result is the same as Fig. 5.10(a) Limiting can be accomplished using biased diodes. Such circuits rely on the fact that diodes have very low impedances when they are forward biased and are essentially open circuits when reverse biased. If a certain point in a circuit, such as the output of an amplifier, is connected through a very small impedance to a constant voltage, then the voltage at the circuit point cannot differ significantly from the constant voltage. We say in this case that the point is clamped to the fixed voltage. An ideal, forward-biased diode is like a closed switch if it is connected between a point in a circuit and a fixed voltage source, the diode very effectively holds the boint to the fixed voltage. Diodes can be connected in operational amplifier circuits, as well as other circuits, e 2000 by CRC Press LLC
© 2000 by CRC Press LLC the gain of the device. This is the region of linear operation. In these examples, the devices are assumed to have unity gain, so the slope of each line in the linear region is 1. Figure 5.10 illustrates a somewhat different kind of limiting action. Instead of the positive or negative peaks being clipped, the output follows the input when the signal is above or below a certain level. The transfer characteristics show that linear operation occurs only when certain signal levels are reached and that the output remains constant below those levels. This form of limiting can also be thought of as a special case of that shown in Fig. 5.9. Imagine, for example, that the clipping level in Fig. 5.9(b) is raised to a positive value; then the result is the same as Fig. 5.10(a). Limiting can be accomplished using biased diodes. Such circuits rely on the fact that diodes have very low impedances when they are forward biased and are essentially open circuits when reverse biased. If a certain point in a circuit, such as the output of an amplifier, is connected through a very small impedance to a constant voltage, then the voltage at the circuit point cannot differ significantly from the constant voltage. We say in this case that the point is clamped to the fixed voltage. An ideal, forward-biased diode is like a closed switch, so if it is connected between a point in a circuit and a fixed voltage source, the diode very effectively holds the point to the fixed voltage. Diodes can be connected in operational amplifier circuits, as well as other circuits, FIGURE 5.9 Waveforms and transfer characteristics of limiting circuits. (Source: T.F. Bogart, Jr., Electronic Devices and Circuits, 3rd ed., Columbus, Ohio: Macmillan/Merrill, 1993, p. 676. With permission.)
E FIGURE 5. 10 Another form of clipping. Compare with Fig. 5.9.(Source: T.E. Bogart, Jr, Electronic Devices and Circuits, 3rd ed, Columbus, Ohio: Macmillan/Merrill, 1993, P 690. With permission. +10=VD Vo>0 vvD=v-9 FIGURE 5.11 Examples of biased diodes and the signal voltages vi required to forward bias them.(Ideal diodes are ssumed.)In each case, we solve for the value of v, that is necessary to make Vp >0.(Source: T.E. Bogart, Jr, Electronic Devices and Circuits, 3rd ed, Columbus, Ohio: Macmillan/Merrill, 1993, P. 691. with permission. in such a way that they become forward biased when a signal reaches a certain voltage. When the forward-biasing level is reached, the diode serves to hold the output to a fixed voltage and thereby establishes a clipping level a biased diode is simply a diode connected to a fixed voltage source. The value and polarity of the voltage ource determine what value of total voltage across the combination is necessary to forward bias the diode Figure 5.11 shows several examples. (In prac ies resistor would be connected in each circuit to limit current flow when the diode is forward biased. In each part of the figure, we can write Kirchhoffs voltage law e 2000 by CRC Press LLC
© 2000 by CRC Press LLC in such a way that they become forward biased when a signal reaches a certain voltage.When the forward-biasing level is reached, the diode serves to hold the output to a fixed voltage and thereby establishes a clipping level. A biased diode is simply a diode connected to a fixed voltage source. The value and polarity of the voltage source determine what value of total voltage across the combination is necessary to forward bias the diode. Figure 5.11 shows several examples. (In practice, a series resistor would be connected in each circuit to limit current flow when the diode is forward biased.) In each part of the figure, we can write Kirchhoff’s voltage law FIGURE 5.10 Another form of clipping. Compare with Fig. 5.9. (Source: T.F. Bogart, Jr., Electronic Devices and Circuits, 3rd ed., Columbus, Ohio: Macmillan/Merrill, 1993, p. 690. With permission.) FIGURE 5.11 Examples of biased diodes and the signal voltages vi required to forward bias them. (Ideal diodes are assumed.) In each case, we solve for the value of vi that is necessary to make VD > 0. (Source: T.F. Bogart, Jr., Electronic Devices and Circuits, 3rd ed., Columbus, Ohio: Macmillan/Merrill, 1993, p. 691. With permission.)
12b 12v FIGURE 5. 12 Examples of parallel clipping circuits. ( Source: T.E. Bogart, Jr, Electronic Devices and Circuits, 3rd ed, Columbus, Ohio: Macmillan/Merrill, 1993, p. 692. with permission. around the loop to determine the value of input voltage v, that is necessary to forward bias the diode. Assuming that the diodes are ideal (neglecting their forward voltage drops), we determine the value v; necessary to forward bias each diode by determining the value v; necessary to make vp >0. when v, reaches the voltage necessary to make Vp>0, the diode becomes forward biased and the signal source is forced to, or held at, the dc source voltage. If the forward voltage drop across the diode is not neglected, the clipping level is found by determining the value of vi necessary to make Vp greater than that forward drop(e.g ,Vp>0.7 V for a silicon diode Figure 5. 12 shows three examples of clipping circuits using ideal biased diodes and the waveforms that result when each is driven by a sine-wave input. In each case, note that the output equals the dc source voltage when ne input reaches the value necessary to forward bias the diode. Note also that the type of clipping we showed in Fig. 5. 9 occurs when the fixed bias voltage tends to reverse bias the diode, and the type shown in Fig. 5.10 occurs when the fixed voltage tends to forward bias the diode. When the diode is reverse biased by the input signal, it is like an open circuit that disconnects the dc source, and the output follows the input. These circuits are called parallel clippers because the biased diode is in parallel with the output. Although the circuits behave the same way whether or not one side of the dc voltage source is connected to the common(low) side of the input and output, the connections shown in Fig. 5. 12(a)and(c)are preferred to that in(b), because the latter Figure 5. 13 shows a biased diode connected in the feedback path of an inverting operational amplifier. The diode is in parallel with the feedback resistor and forms a parallel clipping circuit like that shown in Fig. 5.12 In an operational amplifier circuit, v = v, and since v+=0V in this circuit, v is approximately oV(virtual ground). Thus, the voltage across R is the same as the output voltage v, Therefore, when the output voltage reaches the bias voltage E, the output is held at E volts. Figure 5.13(b)illustrates this fact for a sinusoidal input. So long as the diode is reverse biased, it acts like an open circuit and the amplifier behaves like a conventional inverting amplifier. Notice that output clipping occurs at input voltage-(R/R)E, since the amplifier inverts and has closed-loop gain magnitude R/R, The resulting transfer characteristic is shown in Fig. 5.13(c) In practice, the biased diode shown in the feedback of Fig. 5.13(a) is often replaced by a Zener diode in series with a conventional diode. This arrangement eliminates the need for a floating voltage source. Zener diodes e 2000 by CRC Press LLC
© 2000 by CRC Press LLC around the loop to determine the value of input voltage vi that is necessary to forward bias the diode. Assuming that the diodes are ideal (neglecting their forward voltage drops), we determine the value vi necessary to forward bias each diode by determining the value vi necessary to make vD > 0. When vi reaches the voltage necessary to make VD > 0, the diode becomes forward biased and the signal source is forced to, or held at, the dc source voltage. If the forward voltage drop across the diode is not neglected, the clipping level is found by determining the value of vi necessary to make VD greater than that forward drop (e.g., VD > 0.7 V for a silicon diode). Figure 5.12 shows three examples of clipping circuits using ideal biased diodes and the waveforms that result when each is driven by a sine-wave input. In each case, note that the output equals the dc source voltage when the input reaches the value necessary to forward bias the diode. Note also that the type of clipping we showed in Fig. 5.9 occurs when the fixed bias voltage tends to reverse bias the diode, and the type shown in Fig. 5.10 occurs when the fixed voltage tends to forward bias the diode. When the diode is reverse biased by the input signal, it is like an open circuit that disconnects the dc source, and the output follows the input. These circuits are called parallel clippers because the biased diode is in parallel with the output. Although the circuits behave the same way whether or not one side of the dc voltage source is connected to the common (low) side of the input and output, the connections shown in Fig. 5.12(a) and (c) are preferred to that in (b), because the latter uses a floating source. Figure 5.13 shows a biased diode connected in the feedback path of an inverting operational amplifier. The diode is in parallel with the feedback resistor and forms a parallel clipping circuit like that shown in Fig. 5.12. In an operational amplifier circuit, v– ª v+, and since v+ = 0 V in this circuit, v– is approximately 0 V (virtual ground). Thus, the voltage across Rf is the same as the output voltage vo . Therefore, when the output voltage reaches the bias voltage E, the output is held at E volts. Figure 5.13(b) illustrates this fact for a sinusoidal input. So long as the diode is reverse biased, it acts like an open circuit and the amplifier behaves like a conventional inverting amplifier. Notice that output clipping occurs at input voltage –(R1/Rf )E, since the amplifier inverts and has closed-loop gain magnitude Rf /R1. The resulting transfer characteristic is shown in Fig. 5.13(c). In practice, the biased diode shown in the feedback of Fig. 5.13(a) is often replaced by a Zener diode in series with a conventional diode. This arrangement eliminates the need for a floating voltage source. Zener diodes FIGURE 5.12 Examples of parallel clipping circuits. (Source: T.F. Bogart, Jr., Electronic Devices and Circuits, 3rd ed., Columbus, Ohio: Macmillan/Merrill, 1993, p. 692. With permission.)