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 Cockroft￾Walton 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