introduction to power electronics, Lecture notes of Power Electronics

Download introduction to power electronics and more Lecture notes Power Electronics in PDF only on Docsity! Lecture 2. Power semiconductor devices (Power switches) Power semiconductor switches are the work-horses of power electronics (PE). There are several power semiconductors devices currently involved in several industrial applications. PE switches works in two states only: Fully on (conducting, and Fully off (blocking) . Switches are very important and crucial components in power electronic systems • What is a good power switch: » No power loss when ON » No power loss when OFF » No power loss during turning ON or OFF » Little power required to turn it ON or OFF » Bi- directional? » Adequate voltage and current ratings » Low Turn-on and Turn-off times • Basic ratings: » carrying current, blocking voltage. » Speed. Any real device requires a definite time to switch. » Second-order ratings: di/dt, dv/dt, momentary capabilities. » Power loss » Thermal ratings –from power switching devices to heat sink » Control ratings: how to operate the switch TYPES OF POWER SEMICONDUCTOR SWITCHES The main types of power semiconductor switches in common use are 1. Power Diodes 2. Thyristor devices 2.a. Silicon controlled rectifier (SCR) 2.b. Static induction thyristor (SITH) 2.c. Triac (Triode ac switch) 2.d. Gate turn-off thyristor (GTO) 2.e. Mos- controlled thyristor (MCT) 9 2.f. integrated gated-commutated thyristor (IGCTs) 3. Power transistors 2.g. Bipolar junction transistor (BJT) 2.h. Metal oxide semiconductor field effect transistor (MOSFET) 2.i. Insulated gate bipolar transistor (IGBT) 2.j. Static induction transistor (SIT) 1. Power Diodes: These are two terminal switches, as shown in Fig. 2.1 -a, formed of a pn junction. It is not controllable and its operating states are determined by the circuit operating point. When diode is forward biased, it conducts current, i.e a forward positive voltage Vo will turn it on. When it reversed biased (a reverse negative current from Cathode to Anode) will turn it off. With forward biasing a small forward voltage (VF) will appear across it (0.2-0.3V). Practically, the diode characteristic consists of two regions, as shown in Fig. 2.1 -b; a forward bias region (ON state) where both vD and iD are positive and the current in this region increases exponentially with the increase in the voltage, and a reversed bias region (OFF state) where both vD and iD, are negative and very small leakage current (μA to mA) flows through the diode until the applied reverse voltage reaches the diode’s breakdown voltage limit VBR. Ideally, the diode is represented by a short circuit when forward biased and as an open circuit when reversed biased with the ideal characteristic shown in Fig. 2.1-c. Fig.2.1 Diode: a) symbol, b) characteristic, and c) ideal characteristic . Fig. 2.4 Thyristor:( a) symbol, (b) ideal characteristic , Fig.2.5 Thyristor gate circuit Fig. 2.6 Thyristor (SCR) characteristic Thyristors can only be turned on with three conditions: 1. The device must be forward biased, i.e., the anode should be more positive than the cathode. 2. A positive gate current (Ig) should be applied at the gate. 3. the current through the thyristor should be more than the latching current. Once conducting ,the anode current is LATCHED (continuously flowing). Important points for the SCR characteristic: • Latching Current: This is the minimum current required to turn on the SCR device and convert it from the Forward Blocking State to the ON State. • Holding Current: This is the minimum forward current flowing through the thyristor in the absence of the gate triggering pulse. • Forward Breakover Voltage: This is the forward voltage required to be applied across the thyristor to turn it ON without the gate signal application. • Max Reverse Voltage: This is the maximum reverse voltage to be applied across the thyristor before the reverse avalanche occurs. Ideally, SCRs are represented by a short circuit when operating within the conduction region and as an open circuit when operating within the blocking region. The ideal characteristic is shown in Fig. 2-b. It is also worth mentioning that once the SCR is triggered and turned ON the gate signal can be removed without turning it OFF. SCRs are turned OFF when reversing the terminal voltage and current. Turning on/off mechanism • Thyristor (SCR) Conduction • In reverse -biased mode, the SCR behaves like a diode. It conducts a small leakage current which is almost dependent of the voltage, but increases with temperature. When the peak reverse voltage is exceeded, avalanche breakdown occurs, and the large current will flow. • In the forward biased mode, with no gate current present (i.e. in the untriggered state, the device exhibits a leakage current. If the forward breakover voltage (Vbo) is exceeded, the SCR “self-triggers” into the conducting state and the voltage collapses to the normal forward volt- drop, typically 1.5-3V. The presence of any gate current ig will reduce the forward breakover voltage and will trigger the thyristor at any required instant (α), see Fig.2.7. Fig.2.8 Typical forced commutation circuit for a thyristor. Types of Thyristors 1.Phase controlled • rectifying line frequency voltage and current for ac and dc motor drives . • large voltage (up to 7 kV) and current (up to 5 kA) type1 capability . • low on-state voltage drop (1.5 to 3V). 2.Inverter grade • used in inverter and chopper • Quite fast. Can be turned-on using “force commutation” method. 3.Light activated • Similar to phase controlled, but triggered by pulse of light . • Normally very high power ratings. SCR ratings for voltage and current approach those of diodes. Devices for high- voltage dc (HVDC) conversion have been built with simultaneous 12 kV and 6 kA type 2 ratings. Figure 2.9 below shows types of thyristors in practice. Fig . 2.9 Power semiconductor switches (thyristors). nPNGP nA Triac as two back –to-back thyridstors plus one extra N- region Thyristor constructio n Triac Symbol MT2 MT2 G MT1 K MT1 G MT1 G MT2 Fig.2.10 The four possible mode of operation of triac are a. MT2 +, G + (both relative to MT1) Gate current flows into gate terminal. b. MT2+, G – Gate current opposite to (a). c. MT2- , G + Gate current as (a) d. MT2- , G – Gate current as (b) 11 F 0 B 7 F 0 B 7 F 0 B 7 Ratings: Voltage: Vak < 6kV, Current: Ia < 2 kA Frequency < 5 kHz.-G e drive design is very diffic lt. Need very larg reverse gate current to turn off. GTO normally requires snubbers. High power snubbers are expensive. In very high power region (> 6kV, > 2kA), devel 2. Metal Oxide Semiconductor Field Effect Transistors “MOSFETs” These are three terminal switches as shown in Fig. 2.13. Fig.2.13 MOSFET:symbol (n-channel) This is considered the fastest power switching device (200 kHz) for rating voltages < 500V, current IDS < 300A ,or at 100 kHz , < 1500 V , 300 A . MOSFET characteristics • Turning on and off is very simple. Only need to provide VGS = +15V to turn on and 0V to turn off. Gate drive circuit is simple. • Basically low voltage device. High voltage device are available up to 600V but with limited current. Can be paralleled quite easily for higher current capability. • Dominant in high frequency application (>100 kHz). Biggest application is in switched-mode power supplies. • On state loss relatively high. VDS > 3 V. Practically, MOSFET’s characteristic consists of three regions, as shown in Fig.2.13 : • Cut OFF region (OFF state) when VGS < VTh. • Linear region when VDS <VGS - VTh , and • Active region when VDS > VGS - VTh. Ideally, MOSFETs are represented by a short circuit when operating within the ON State and as an open circuit when operating within the OFF State. Fig.2.13 MOSFET characteristics Other Semiconductor Devices: These include; Bipolar Junction Transistors (BiTs), 3 - Insulated Gate Bipolar Transistor “IGBT” This is also a three terminal switch as shown in Fig. 2.14. Its operation modes and characteristics are almost similar to those for MOSFETs, shown in Fig. 2.13, except for the operating ranges. Fig.2.14: IGBT IGBT has a combination of BJT and MOSFET characteristics. Compromises include: • Gate behavior similar to MOSFET - easy to turn on and off. • Low losses like BJT due to low on-state Collector-Emitter voltage VCE = (2-3V). • Ratings: Voltage: VCE < 6000V, Current 2500A currently available. • Good switching capability (up to 100 kHz) for newer devices. • Typical application, IGBT is used at 20-50 kHz. • For very high power devices and applications, frequency is limited to several kHz - • Very popular in new products; practically replacing BJT in most new applications • .Snubberless” operation is possible. Most new IGBTs do not require snubber. Other switching devices: There are several other power switching device available such as: Diac , Static Induction Transistors (SITs), Static Induction Thyristors (SITHs), and MOS- Controlled Thyristors (MCT). MCTs (MOS Controlled Thyristor) An effort to combine the advantages of bipolar junction and field-effect structures has resulted in hybrid devices: ▲ The IGBT is an improvement over a BJT. ▲ The MCT is an improvement over a thyristor. MCT characteristics • MCT can be switched on or off by negative or positive gate voltage, respectively. • It has fast TURN-ON and then OFF times, with high-speed switching capability. • Low conduction losses, low switching losses, and high current density. • The gating requirements of an MCT are easier than those of the GTO since it needs smaller gate turn-off current due to its high gate input impedance. • Compared with the power MOSFET, it has higher current density and lower forward drop . It has great potential in high-power, high-voltage applications. A peak power of 1 MW can be switched off in 2 ns by a single MCT. Therefore, the MCT overcomes several of the limitations of the existing power devices and promises to be a better switch for the future. The MCT symbol and equivalent circuit are shown in Fig.2.15. The MCT characteristics are shown in Fig.2.16. (b)(a)