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بسم هللا الرحمن الرحيم The Islamic University of Gaza Faculty of Engineering Electrical Engineering Department POWER ELECTRONICS EELE 5450 — Fall 2009-2010 Instructor: Eng.Moayed N. EL Mobaied Lecture 29 Power switches Power Semiconductor Devices Power devices are the key elements of a power converter. The commonly used devices are: (1) Power Diode (2) Silicon-Controlled Rectifier (SCR) or Thyristor (3) Gate Turn-off Thyristor (GTO) (4) Power Bipolar Junction Transistor (Power BJT) (5) Power Metal-Oxide Field-Effect Transistor (Power MOSFET) (6) Insulated-Gate Bipolar Transistor (IGBT) (7) Gate controlled thyristors (IGCT). Bipolar Junction Transistor (BJT) C (collector) IC B (base) + VCE _ IB • Ratings: Voltage: VCE<1000, Current: IC<400A. Switching frequency up to 5kHz. Low on-state voltage: VCE(sat) : 2-3V • Low current gain (b<10). Need high base current to obtain reasonable IC . • E (emitter) Expensive and complex base drive circuit. Hence not popular in new products. Mosfet D (drain) ID G (gate) + VGS _ + VDS _ • Ratings: Voltage VDS<500V, current IDS<300A. Frequency f >100KHz. For some low power devices (few hundred watts) may go up to MHz range. • Turning on and off is very simple. – To turn on: VGS =+15V – To turn off: VGS =0 V and 0V to turn off. S (source) • Gate drive circuit is simple Insulated Gate Bipolar Transistor (IGBT) C (collector) IC G (gate) VGE + _ + VCE _ E (emitter) IGBT: symbol • Combination of BJT and MOSFET characteristics. – Gate behaviour similar to MOSFET - easy to turn on and off. – Low losses like BJT due to low on-state Collector-Emitter voltage (2-3V). • Ratings: Voltage: VCE<3.3kV, Current,: IC<1.2kA currently available. Latest: HVIGBT 4.5kV/1.2kA. • Switching frequency up to 100KHz. Typical applications: 20-50KHz. Gate turn-off thyristor (GTO) A (Anode) Ia + Vak _ I g K (Cathode) GTO: Symbol • Behave like normal thyristor, but can be turned off using gate signal • However turning off is difficult. Need very large reverse gate current (normally 1/5 of anode current). • Gate drive design is very difficult due to very large reverse gate current at turn off. • • Ratings: Highest power ratings switch: Voltage: Vak<5kV; Current: Ia<5kA. Frequency<5KHz. • Very stiff competition: Low end-from IGBT. High end from IGCT Insulated Gate-Commutated Thyristor (IGCT) • Among the latest Power Switches. • Conducts like normal thyristor (latching), but can be turned off using gate signal, similar to IGBT turn off; 20V is sufficent. Ia + Vak _ I g K (Cathode) IGCT • Power switch is integrated with the gate-drive unit. • Ratings: Voltage: Vak<6.5kV; Current: Ia<4kA. Frequency<1KHz. Currently 10kV device is being developed. • Very low on state voltage: 2.7V for 4kA device Power Switches: Power Ratings 1GW Thyristor 10MW GTO/IGCT 10MW 1MW IGBT 100kW 10kW MOSFET 1kW 100W 10Hz 1kHz 100kHz 1MHz 10MHz Switches comparisons (2003) Thy BJT FET GTO IGBT IGCT Availabilty Early 60s Late 70s Early 80s Mid 80s Late 80s Mid 90’s State of Tech. Mature Mature Mature/ improve Mature Rapid improve Voltage ratings 5kV 1kV 500V 5kV 3.3kV Rapid improvem ent 6.5kV Current ratings 4kA 400A 200A 5kA 1.2kA 4kA Switch Freq. na 5kHz 1MHz 2kHz 100kHz 1kHz On-state Voltage 2V 1-2V I* Rds (on) 2-3V 2-3V 3V Drive Circuit Simple Difficult Very simple Very difficult Very simple Simple Comm-ents Cannot turn off using gate signals Phasing out in new product Good performan ce in high freq. King in very high power Best overall performanc e. Replacing GTO Drivers and Snubbers Driver circuit (Base / gate) Interface between control (low power electronics) and (high power) switch. Functions: – amplifies control signal to a level required to drive power switch – provides electrical isolation between power switch and logic level Complexity of driver varies markedly among switches. MOSFET/IGBT drivers are simple but GTO drivers are very complicated and expensive. ELECTRICAL ISOLATION FOR DRIVERS Isolation is required to prevent damages on the high power switch to propagate back to low power electronics. Normally opto-coupler (shown below) or high frequency magnetic materials (as shown in the thyristor case) are used. Many standard driver chips have built-in isolation. For example TLP 250 from Toshiba, HP 3150 from HewlettPackard uses opto-coupling isolation. ELECTRICAL ISOLATION FOR DRIVERS Power semiconductor devices can be categorized into 3 types based on their control input requirements: a) Current-driven devices – BJTs, MDs, GTOs b) Voltage-driven devices – MOSFETs, IGBTs, MCTs c) Pulse-driven devices – SCRs, TRIACs CURRENT DRIVEN DEVICES (BJT) Power BJT devices have low current gain due to constructional consideration, leading current than would normally be expected for a given load or collector current. The main problem with this circuit is the slow turn-off time. ELECTRICALLY ISOLATED DRIVE CIRCUITS EXAMPLE : GATE DRIVE FOR THYRISTORS Pulse transformer is used for isolation. R1 is to limit the gate current Normally a pulse with length 10us with amplitude of 50mA is sufficient to turn-on the thyristors. It is quite common to fire the thyristors with successive pulses to ensure proper turn-on. It is not possible to turn-off a thyristor with the above circuit SIMPLE MOSFET GATE DRIVER Note: MOSFET requires VGS =+15V for turn on and 0V to turn off. LM311 is a simple amp with open collector output Q1. When B1 is high, Q1 conducts. VGS is pulled to ground. MOSFET is off. When B1 is low, Q1 will be off. VGS is pulled to VGG. If VGG is set to +15V, the MOSFET turns on. RCD SNUBBERS In general, snubbers are used for: – turn-on: to minimize large overcurrents through the device at turnon – turn-off: to minimize large overvoltages across the device during turnoff. – Stress reduction: to shape the device switching waveform such that the voltage and current associated with the device are not high Heatsink Heat Removal Mechanism Fin-type Heat Sink SCR (hokey-puck-type) on power pak kits Heat Removal Mechanism SCR (stud-type) on air-cooled kits Assembly of power converters Applications Static Application: DC Power Supply Static Application: DC Power Supply AC voltage AC LINE VOLTAGE F (1 or F 3 ) DIODE RECTIFIER FILTER DC-DC CONVERTER LOAD V control (derived from feedback circuit) Drive Application: Air-Conditioning System Power Source Power Electronics Converter Desired temperature Desired humidity System Controller Variable speed drive Motor Indoor temperature and humidity Air conditioner Temperature and humidity Building Cooling Indoor sensors Uninterruptible Power Supply (UPS) Powering Automotive System Motor Drive System Motor Drive System End of Lecture