Presentation on theme: "POWER ELECTRONICS Instructor: Eng.Moayed N. EL Mobaied The Islamic University of Gaza Faculty of Engineering Electrical Engineering Department بسم الله."— Presentation transcript:
POWER ELECTRONICS Instructor: Eng.Moayed N. EL Mobaied The Islamic University of Gaza Faculty of Engineering Electrical Engineering Department بسم الله الرحمن الرحيم EELE 5450 Fall 2009-2010 Lecture 29
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) + V CE _ ICIC IBIB C (collector) B (base) E (emitter) Ratings: Voltage: V CE <1000, Current: I C <400A. Switching frequency up to 5kHz. Low on-state voltage: V CE(sat) : 2-3V Low current gain ( ). Need high base current to obtain reasonable I C. Expensive and complex base drive circuit. Hence not popular in new products.
Mosfet + V DS _ IDID D (drain) G (gate) S (source) + V GS _ Ratings: Voltage V DS 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: V GS =+15V –To turn off: V GS =0 V and 0V to turn off. Gate drive circuit is simple
Insulated Gate Bipolar Transistor (IGBT) IGBT: symbol + V CE _ ICIC C (collector) G (gate) E (emitter) + V GE _ 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: V CE <3.3kV, Current,: I C <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) K (Cathode) + V ak _ IaIa GTO: Symbol IgIg 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: V ak <5kV; Current: I a <5kA. Frequency<5KHz. Very stiff competition: Low end-from IGBT. High end from IGCT
Insulated Gate-Commutated Thyristor (IGCT) K (Cathode) + V ak _ IaIa IgIg 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. Power switch is integrated with the gate-drive unit. 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
Power Switches: Power Ratings 10Hz1kHz 1MHz100kHz 10MHz 1kW 100kW 10kW 10MW 1MW 10MW 1GW 100W MOSFET IGBT GTO/IGCT Thyristor
Interface between control (low power electronics) and (high power) switch Interface between control (low power electronics) and (high power) switch. Functions: amplifies control signal to a level required to drive power switch – amplifies control signal to a level required to drive power switch provides electrical isolation between power switch and logic level – provides electrical isolation between power switch and logic level MOSFET/IGBT drivers simpleGTO drivers complicated and expensive Complexity of driver varies markedly among switches. MOSFET/IGBT drivers are simple but GTO drivers are very complicated and expensive. Driver circuit (Base / gate)
ELECTRICAL ISOLATION FOR DRIVERS required to prevent damages on the high power switch 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 Hewlett- Packard 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.
EXAMPLE : GATE DRIVE FOR THYRISTORS used for isolationlimit the gate current Pulse transformer is used for isolation. R 1 is to limit the gate current It is quite common to fire the thyristors with successive pulses to ensure proper turn-on 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. not possibleturn-off It is not possible to turn-off a thyristor with the above circuit
SIMPLE MOSFET GATE DRIVER MOSFET requires V GS =+15V for turn on and 0V to turn off. Note: MOSFET requires V GS =+15V for turn on and 0V to turn off. LM311 is a simple amp with open collector output Q 1. B 1 is high MOSFET is off When B 1 is high, Q 1 conducts. V GS is pulled to ground. MOSFET is off. B 1 is low If V GG is set to +15V, the MOSFET turns on. When B 1 is low, Q 1 will be off. V GS is pulled to V GG. If V GG 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 turn- on – turn-on: to minimize large overcurrents through the device at turn- on turn-off:to minimize large overvoltages across the device during turn- – turn-off: to minimize large overvoltages across the device during turn- off. off. Stress reductionto shape the device switching waveform such that the voltage and current associated with the device are not high simultaneously. – Stress reduction: to shape the device switching waveform such that the voltage and current associated with the device are not high simultaneously.