Power Electronics Power semiconductor devices Dr. Firas Obeidat 1
Table of contents 1 Introduction 2 Classifications of Power Switches 3 Power Diodes 4 Thyristors (SCRs) 5 The Triac 6 The Gate Turn-Off Thyristor (GTO) 7 Insulated Gate-Commutated Thyristor (IGCT) 8 The MOS-Controlled Thyristor 9 Bipolar Junction Transistor (BJT) 10 MOSFET 11 IGBT 2
Introduction Electronic switches capable of handling high voltage and current operations at high frequency (HF) are the most important devices needed in the design of energy conversion systems that use power electronic. An ideal power electronic switch can be represented as a three terminals device as shown in the figure; The input, the output, and a control terminal that imposes ON/OFF conditions on the switch. A switch is considered ideal When the switch is open, it has zero-current through it and can handle infinite voltage. When the switch is closed it has zero-voltage across it and can carry infinite current. An ideal switch changes condition instantly, which means that it takes zero-time to switch from ON-to-OFF or OFF-to-ON. exhibits zero-power dissipation, carries bidirectional current, and can support bidirectional voltage. By definition, an ideal switch can operate in all four quadrants. 3
Introduction Practical or real switches do have their limitations in all of the characteristics explained in an ideal switch. When a switch is on, it has some voltage across it, known as the on-voltage and it carries a finite current. During the off stage, it may carry a small current known as the leakage current while supporting a finite voltage. The switching from ON-to-OFF and vice versa does not happen instantaneously. There is voltage and current across the non-ideal switch at all times, which will result in two types of losses The first loss occurs during the on and off-states and is defined as the conduction loss. The second loss is defined as the switching loss which occurs just as the switch changes state as either opening or closing. The switch losses result in raising the overall switch temperature. 4
Classifications of Power Switches There are three classes of power switches 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. 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. 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 IGCT, the GTO thyristor, and the MOS-controlled thyristor (MCT). 5
Power Diodes A diode is the simplest electronic switch. It is uncontrollable in that the on and off conditions are determined by voltages and currents in the circuit. Diode terminals are known as Anode (A) and cathode (K) as shown in fig. a. The diode is forward-biased (ON) when the current i D is positive while supporting a small voltage (0.2V to 3V) - (quadrant I in fig. b). The diode current varies exponentially with the diode voltage. When the diode is reversed-biased (OFF), it supports a negative voltage and carries a negligible current (leakage current from μa to ma) - (quadrant III in fig. b). When the negative voltage exceeds a certain limit, known as the breakdown voltage, the leakage current increases rapidly while the voltage remains at the breaking value, which potentially damages the device. The ideal diode characteristics are shown in fig. c. During the ONstate, the diode has zero-voltage across it and carries a positive current. During the OFF state, the diode carries zero-current and supports a negative voltage. 6
Types of Power Diodes Line frequency (general purpose) On state voltage: very low (below 1V). Large t rr (about 25us) (very slow response). Very high current ratings (up to 6kA). Very high voltage ratings(8kv). Used in line-frequency (50/60Hz) applications such as rectifiers. Fast recovery Very low t rr (<1μs). Power levels at several hundred volts and several hundred amps. Normally used in high frequency circuits. Schottky Very low forward voltage drop (typical 0.3V). Limited blocking voltage (50-100V). Used in low voltage, high current application such as switched mode power supplies. 7
Thyristors 8
Thyristors ( Silicon Controlled Rectifiers SCRs ) Thyristors are electronic switches used in some power electronic circuits where control of switch turn-on is required. But once it turns ON, the control terminal becomes ineffective and the thyristor behaves similar to a diode. Thyristors are capable of large currents and large blocking voltages for use in high-power applications, but switching frequencies cannot be as high as when using other devices such as MOSFETs. The thyristor current, I A, flows from the anode (A) to the cathode (K) and the voltage V Ak across the thyristor is positive when the anode is at higher voltage than the cathode. 9
Thyristors ( Silicon Controlled Rectifiers SCRs ) In quadrant I, in the absence of a gate current, the device is OFF in the forward blocking region and supports a positive voltage. If a gate current is applied, the device switches to the ON-state region and the device has a i-v characteristic similar to that of a diode. In quadrant III, the device is OFF and the region is known as the reverse blocking region and supports a negative voltage. The characteristics are similar to those of a diode. Comparing the switching characteristics of a diode and a thyristor, it appears that when the thyristor is OFF, it can block large positive or negative voltage, which is a fundamental feature that is important in circuit applications. thyristor can be considered to carry an unidirectional current and supports a bidirectional voltage. 10
Thyristors ( Silicon Controlled Rectifiers SCRs ) Thyristors can only be turned on with three conditions: The device must be forward biased, i.e., the anode should be more positive than the cathode. A positive gate current (I g ) should be applied at the gate. The current through the thyristor should be more than the latching current. Once conducting,the anode current is LATCHED (continuously flowing). SCR rating Surge Current Rating (IFM) The surge current rating (IFM) of an SCR is the peak anode current an SCR can handle for a short duration. Holding Current (IL) A minimum anode current must flow through the SCR in order for it to stay ON initially after the gate signal is removed. This current is called the latching current (IL). Latching Current (IH) After the SCR is latched on, a certain minimum value of anode current is needed to maintain conduction. If the anode current is reduced below this minimum value, the SCR will turn OFF. Peak Repetitive Reverse Voltage (VRRM) The maximum instantaneous voltage that an SCR can withstand, without breakdown, in the reverse direction. Peak Repetitive Forward Blocking Voltage (VDRM) The maximum instantaneous voltage that the SCR can block in the forward direction. If the VDRM rating is exceeded, the SCR will conduct without a gate voltage. Nonrepetitive Peak Reverse Voltage (VRSM) The maximum transient reverse voltage that the SCR can withstand. Maximum Gate Trigger Current (IGTM) The maximum DC gate current allowed to turn the SCR ON. Minimum Gate Trigger Voltage (VGT) The minimum DC gate-to-cathode voltage required to trigger the SCR. Minimum Gate Trigger Current (IGT) The minimum DC gate current necessary to turn the SCR ON. 11
Thyristors ( Silicon Controlled Rectifiers SCRs ) Thyristor TURN-OFF Thyristor cannot be turned off by applying negative gate current. It can only be turned off if I A goes negative (reverse). This happens when negative portion of the of sine-wave occurs (natural commutation), Another method of turning off is known as forced commutation, The anode current is diverted to another circuitry. Types of Thyristors Phase controlled - Rectifying line frequency voltage and current for ac and dc motor drives. - Large voltage (up to 7kV) and current (up to 5kA) capability. - Low on-state voltage drop (1.5 to 3V). Inverter grade - Used in inverter and chopper. - Quite fast. Can be turned-on using force-commutation method. Light activated - Similar to phase controlled, but triggered by pulse of light. - Normally very high power ratings. TRIAC - Dual polarity thyristors. 12
The Triac (triode for alternating current) The Triac is a member of the thyristor family which can conduct in both directions (bidirectional semi-controlled device). Thus a Triac is similar to two back to back (anti parallel) connected thyristosr but with only three terminals. The Triac extensively used in residential lamp dimmers, heater control and for speed control of small single phase series and induction motors. The conduction of a triac is initiated by injecting a current pulse into the gate terminal. The triac turns off only when the current through the main terminals become zero. As the Triac can conduct in both the directions the terms anode and cathode are not used for Triacs. The three terminals are marked as MT 1 (Main Terminal 1), MT 2 (Main Terminal 2) and the gate by G. 13
The Triac (triode for alternating current) Since a Triac is a bidirectional device and can have its terminals at various combinations of positive and negative voltages, there are four possible electrode potential combinations as given below MT 2 positive with respect to MT 1, G positive with respect to MT 1. MT 2 positive with respect to MT 1, G negative with respect to MT 1. MT 2 negative with respect to MT 1, G negative with respect to MT 1. MT 2 negative with respect to MT 1, G positive with respect to MT 1. In trigger mode-1 the gate current flows mainly through the P 2 N 2 junction like an ordinary thyristor. When the gate current has injected sufficient charge into P 2 layer the Triac starts conducting through the P 1 N 1 P 2 N 2 layers like an ordinary thyristor. 14
The Gate Turn-Off Thyristor (GTO) The GTO thyristor is a device that operates similar to a normal thyristor except the device physics, design and manufacturing features allow it to be turned-off by a negative gate current which is accomplished through the use of a bipolar transistor. Turning off is difficult. Need very large reverse gate current (normally 1/5 of anode current). I g I a A (Anode) + V ak _ K (Cathode) GTO: Symbol I a Gate drive design is very difficult due to very large reverse gate current at turn off. V r I h I bo I g >0 I g =0 Ratings: Highest power ratings switch: Voltage: V ak <5kV; Current: I a <5kA. Frequency<5KHz. V bo v-i characteristics V ak 15
Insulated Gate-Commutated Thyristor (IGCT) Conducts like normal thyristor (latching), but can be turned off using gate signal, similar to IGBT turn off; 20V is sufficent. I a Power switch is integrated with the gate-drive unit. I + V ak _ IGCT g K (Cathode) Ratings: Voltage: V ak <6.5kV; Current: I a <4kA. Frequency<1KHz. Currently 10kV device is being developed. Very low on state voltage: 2.7V for 4kA device 16
The MOS-Controlled Thyristor The MCT is a hybrid or double mechanism device that was designed to have a control port of a MOSFET and a power port of a thyristor. The characteristics of MCT are similar to the GTO, except that the gate drive circuitry for the MCT is less complicated than the design for a GTO as the control circuit of the MCT uses a MOSFET instead of a transistor. 17
Transistors 18
Bipolar Junction Transistor (BJT) For the npn-type BJT shown in fig. a, the Base (B) is the control terminal, where the power terminals are the Collector (C) and the Emitter (E). The real v-i characteristics of device are shown in fig. b. The device operates in quadrant I and is characterized by the plot of the collector current I C versus the collector to emitter voltage v CE as shown in fig. b. BJT is a current-controlled device. The device has three regions two of them where the device operates as a switch and the third is where the device operates as a linear amplifier. The device is OFF in the region below i B =0 and is ON in the region where v CE is less than v CE(Sat). Neglecting the middle region, the idealized device characteristics as a switch are shown in fig. c. During the ON state the device carries a collector current I C >0 with v CE =0. In the OFF-state, the device supports positive v CE >0 with I C = 0. Therefore, the BJT is unidirectional current and voltage device. 19
The Metal Oxide Semiconductor Field Effect Transistor (MOSFET) For the n-channel MOSFET shown in fig. a, the control terminal known as the Gate (G) and the power terminals are the Drain (D) and Source (S). The device is controlled by supplying a voltage (v GS ) between the gate and the source. This makes it a voltage controlled device compared to the BJT, which is a current-controlled device. The real v-i characteristics of device are shown in fig. b. Similar to the BJT, the MOSFET operates within three operating regions. Two of the regions are used when the device is operated as a switch, and the third is when the device is used as an amplifier. To maintain the MOSFET in the off-state, v GS must be less than a threshold voltage known as v T, which is the region below the line marked OFF. And when the device is ON it act as resistance determined by the slope of the line marked ON. The idealized characteristics of a MOSFET switch are shown in fig. c. When the device is ON, it has zero v DS and carriers a current I D >0 and when the device is OFF it supports a positive v DS and has zero drain current (I D = 0). 20
The Insulated Gate Bipolar Transistor (IGBT) The IGBT (its symbol shown in fig. a) is a hybrid or also known as double mechanism device. Its control port resembles a MOSFET and its output or power port resembles a BJT. Therefore, an IGBT combines the fast switching of a MOSFET and the low power conduction loss of a BJT. (b) The control terminal is labeled as gate (G) and the power terminals are labeled as collector (C) and emitter (E). The i-v characteristics of a real IGBT are shown in fig. b, which shows that the device operates in quadrants I and III. The ideal characteristics of the device are shown in fig. c. The device can block bidirectional voltage and conduct unidirectional current. An IGBT can change to the ON-state very fast but is slower than a MOSFET device. Discharging the gate capacitance completes control of the IGBT to the OFF-state. IGBT s are typically used for high power switching applications such as motor controls and for medium power PV and wind PE. 21
Comparison between GTO, IGCT and IGBT Item GTO IGCT IGBT Maximum switch power (V I ) 36MVA 36MVA 6MVA Active di/dt and dv/dt control No No Yes Active short circuit protection No No Yes Turn-off (dv/dt) snubber Required Not required No required Turn-on (di/dt) snubber Required Required No required Parallel connection No No Yes Switching speed Slow Moderate Fast Behavior after destruction Shorted Shorted Open in most cases On-state losses Low Low High Switching losses High Low Low Gate Driver Complex, Complex, Simple, separate integrated compact Gate Driver Power Consumption High High Low 22
Device Rating V (V) 12000 10000 SCR 12000V/1500A (Mitsubishi) SCR: GTO/GCT: IGBT: 27MVA 36MVA 6MVA 8000 6000 4000 2000 6500V/600A (Eupec) 4500V/900A (Mitsubishi) 6500V/1500A (Mitsubishi) 7500V/1650A (Eupec) 3300V/1200A (Eupec) GTO/GCT 2500V/1800A (Fuji) IGBT 6000V/3000A (ABB) 1700V/3600A (Eupec) 6500V/4200A (ABB) 4800V 5000A (Westcode) 6000V/6000A (Mitsubishi) 0 0 1000 2000 3000 4000 5000 6000 I (A) 23
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