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Chapter 2 Power Semiconductors Devices the Concept of the Ideal Switch Is Important When Evaluating Circuit Topologies

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Chapter 2 Power Semiconductors Devices the Concept of the Ideal Switch Is Important When Evaluating Circuit Topologies Chapter 2 Power Semiconductors Devices The concept of the ideal switch is important when evaluating circuit topologies. Assumptions of zero-voltage drop, zero-leakage current, and instantaneous transitions make it easier to simulate and model the behavior of various electrical designs. Today’s power semiconductor devices are almost exclusively based on silicon material and can be classified as follows: • Diode • Thyristor or silicon-controlled rectifier (SCR) • Triac • Gate turn-off thyristor (GTO) • Bipolar junction transistor (BJT or BPT) • Power MOSFET • Static induction transistor (SIT) • Insulated gate bipolar transistor (IGBT) • MOS-controlled thyristor (MCT) • Integrated gate-commutated thyristor (IGCT) Using the characteristics of an ideal switch, there are three classes of power switches: 1. Uncontrolled switch: The switch has no control terminal. The state of the switch is determined by the external voltage or current conditions of the circuit in which the switch is connected. A diode is an example of such switch. 2. Semi-controlled switch: In this case the circuit designer has limited control over the switch. For example, the switch can be turned-on from the control terminal. However, once ON, it cannot be turned-off from the control signal. The switch can be switched off by the operation of the circuit or by an auxiliary circuit that is added to force the switch to turn-off. A thyristor or a SCR is an example of this switch type. 3. Fully controlled switch: The switch can be turned-on and off via the control terminal. Examples of this switch are the BJT, the MOSFET, the IGBT, the GTO thyristor, and the MOS-controlled thyristor (MCT). 1. Power diode (Uncontrolled switch) The switch has no control terminal. The state of the switch is determined by the external voltage or current conditions of the circuit in which the switch is connected. A diode is an example of such switch. Power diodes play an important role in power electronics circuits. They are used mainly in uncontrolled rectifiers to convert AC to fixed DC voltages and as freewheeling diodes to provide a path for the current flow in inductive loads. Power diodes are similar in function to ordinary PN junction diodes; however, power diodes have larger power-, voltage-, and current-handling capabilities. a) The PN Junction Diode: Diode has two terminals an anode A terminal (P junction) and a cathode K terminal (N junction). When the anode voltage is more positive than the cathode, the diode is said to be forward-biased, and it conducts current readily with a relatively low voltage drop. When the cathode voltage is more positive than the anode, the diode is said to be reverse-biased, and it blocks the current flow. The arrow on the diode symbol shows the direction of conventional current flow when the diode conducts. (b) The Voltage-Current Characteristic of a Diode: When forward-biased, the diode begins to conduct current as the voltage across its anode (with respect to its cathode) is increased. When the voltage approaches the so-called knee voltage, about 1 V for silicon diodes, a slight increase in voltage causes the current to increase rapidly. This increase in current can be limited only by resistance connected in series with the diode. the V-I characteristic of a diode When the diode is reverse-biased, a small amount of current called the reverse leakage current flows as the voltage from anode to cathode is increased; this simply indicates that a diode has a very high resistance in the reverse direction. This large resistance characteristic is maintained with increasing reverse voltage until the reverse breakdown voltage is reached. At breakdown, a diode allows a large current flow for a small increase in voltage. Again, a current-limiting resistor must be used in series to prevent destruction of the diode. (c) The Ideal Diode: Switch-equivalent circuits of an ideal diode. An ideal diode characteristic. Figure (a) shows a forward-biased diode, and Figure (b) shows its switch-equivalent circuit. When a diode’s anode is more positive than its cathode, it can be considered to act like a closed switch. Figure (c) shows a reverse-biased diode, and Figure 2.4(d) shows its switch equivalent. When the diode’s anode is more negative than its cathode, it can be considered to act like an open switch. (d) Power diode types: Power diodes can be classified as: (1) General purpose diodes. (2) Fast recovery diodes. (3) Schottky Diodes. 1.General purpose diodes: These diodes have pretty high reverse recovery time about 25m s. they are used in low frequency applications, e.g., line commutated converters etc. their rating cover a very wide range current about 1A to 2000 A or so and voltage 50 V to 5 kV. 2. Fast Recovery diodes: As the name indicates their recovery time is very low, generally less than 5ms. their current ratings are from about 1 A to hundreds of amperes and voltage ratings from 50 V to about 3 kV. 3. The Schottky diode: The Schottky diode is a low-voltage, high-speed device that works on a different principle from that of the PN junction diode. It is constructed without the usual PN junction. Instead, a thin barrier metal (such as chromium, platinum, or tungsten) is interfaced with the N-type semiconductor. This construction results in a low on-state voltage (about 0.6 V) across the diode when it conducts. Furthermore, it can turn off much faster than a PN junction diode, so switching frequency can be high. However, the reverse leakage current is much higher, and the reverse breakdown voltage is lower compared with that of a PN junction diode. Schottky diodes are therefore used as rectifiers in low-voltage applications where the efficiency of conversion is important. These diodes are also widely used in switching power supplies that operate at frequencies of 20 kHz or higher. (e) Principal Ratings for Diodes 1. Peak Inverse. Voltage (PIV): The peak inverse voltage rating of a diode is the maximum reverse voltage that can be connected across a diode without breakdown. If the PIV rating is exceeded, the diode begins to conduct in the reverse direction and can be immediately destroyed. PIV ratings extend from tens of volts to several thousand volts, depending on the construction. The PIV rating is also called the peak reverse voltage (PRV) or breakdown voltage V(BR). 2. Maximum Average Forward Current: The maximum average forward current is the maximum current a diode can safely handle when forward-biased. Power diodes are presently available in ratings from a few amperes to several hundred amperes. If a diode is to be used economically, it must be operated near its maximum forward current rating. 3. Reverse Recovery Time (trr): The reverse recovery time of a diode is of great significance in high-speed switching applications. A real diode does not instantaneously switch from a conduction to a non- conduction state. Instead, a reverse current flow for a short time and the diode does not turn off until the reverse current decays to zero, as shown in Figure. The diode initially conducts a current IF when the diode is reverse-biased, this current decreases and reverse current IR flows. The time interval during which reverse current flows is called the reverse recovery time. During this time, charge carriers that were stored in the Reverse recovery characteristics. junction when forward conduction terminated are removed. Diodes are classified as “fast recovery” or “slow recovery” types based on their reverse recovery times. Recovery times range from a few microseconds in a PN junction diode to several hundred nanoseconds in a fast-recovery diode like a Schottky diode. The PN junction diode is normally sufficient for rectification of a 60 Hz AC signal. Fast-recovery diodes with low trr are used in high- frequency applications such as inverters, choppers, and uninterruptible power supplies (UPS). 4. Maximum Junction Temperature: This parameter defines the maximum junction temperature that a diode can withstand without failure. The rated temperatures of silicon diodes typically range from -40°C to +200°C. Operation at lower temperatures generally results in better performance. Diodes are usually mounted on heat sinks to improve their temperature rating. 5. Maximum Surge Current: The IFSM (forward surge maximum) rating is the maximum current that the diode can handle as an occasional transient or from a circuit fault. (f) Diode Protection: 1. Overvoltage: A reverse-biased diode acts like an open circuit. If the voltage across the diode exceeds its breakover voltage, it breaks down, resulting in a large current flow. With this high current and large voltage across the diode, it is quite likely that the power dissipation at the junction will exceed its maximum value, destroying the diode. It is a common practice to select a diode with a peak reverse voltage rating that is 1.2 times higher than the expected voltage during normal operating conditions. 2. Overcurrent: Manufacturer’s data sheets provide current ratings based on the maximum junction temperatures produced by conduction losses in diodes. In a given circuit, it is recommended that the diode current be kept below this rated value. Overcurrent protection is then accomplished by using a fuse to ensure that the diode current does not exceed a level that will increase the operating temperature beyond the maximum value. 3. Transients: Transients can lead to higher-than-normal voltages across a diode. Protection against transients usually takes the form of an RC series circuit connected across the diode. This arrangement, shown in Figure, snubs or reduces the rate of change of voltage and is commonly called a snubber circuit.
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