INTRODUCTION POWER ELECTRONICS

Power Electronic Devices Diode Thyristor (also called as Silicon Controlled Rectifier, SCR) Gate Turn Off Thyristor (G.T.O) TRIAC Bipolar Junction Transistor (BJT) Metal Oxide Field Effect Transistor (MOSFET) Insulated Gate Bipolar Junction Transistor (IGBT)

Power Electronic Devices Properties Types and Characteristics Ideal Switch Losses Ratings Device Comparison Worked Examples

Power semiconductor devices (Power switches) Power switches operates in two states: Fully on. i.e. switch closed. Conducting state Fully off , i.e. switch opened. Blocking state Power switch never operates in linear mode. Switch ON(closed) Switch Off(opened) Power Semiconductor Devices are categorised into three groups: Uncontrolled: Diode : Semi-controlled: Thyristor (SCR). Fully controlled: Power transistors: e.g. BJT, MOSFET, IGBT, GTO, IGCT

Diode PN Junction Diode Conducts during forward bias (ON) About 0.7v drop when it conducts, carries maximum current Available with various current ratings(up to 1000A!) Doesn’t conduct during reverse bias(OFF), minor leakage current Entire voltage will drop across diode (up to 5000V!)

Forward and reverse biasing

Diode construction and VI curve

Diode Characteristics

Diode in different package

Thyristor / Silicon Controlled Rectifier(SCR) Diode with control terminal for conduction PNPN Structure having Gate terminal at P What is the role of Gate terminal?

SCR Characteristics

SCR Characteristics

HOLDING CURRENT LATCHING CURRENT If the anode current is reduced below the critical holding current value, the thyristor cannot maintain the current through it and reverts to its off state usually is associated with turn off the device. LATCHING CURRENT After the SCR has switched on, there is a minimum current required to sustain conduction. This current is called the latching current. associated with turn on and is usually greater than holding current.

Factors Causing Turn ON of SCR Forward voltage VAK > VBO. This should be avoided since it may permanently damage the device. Rise in device temperature can cause unwanted turn ON and hence should be avoided by cooling the device. By injecting positive gate current IG until IA = IL where IL is the latching value. This is the preferred method of turning ON the device. Forward dvAK / dt > rated value causes undesirable turn ON and should prevented by connecting a snubber circuit across the SCR. Light radiation of specific wavelength incident on junctions of SCR turns ON the device. LASCR’s are turned ON by this method.

To turn ON SCR Apply forward voltage to anode/cathode Apply current to gate (+ve gate/cathode) resultant anode/cathode current to exceed latching current(!) of device, SCR remains ON Gate current can be removed stays ON provided anode to cathode forward current is more than holding current(!) of device)

To turn OFF SCR Anode/cathode current reduced to less than holding current by forced commutation (dc system) by natural commutation (ac system) Blocks in reverse direction up to breakdown voltage. 5kV, 4kA

Gate Characteristics of SCR

Thyristor Gating Requirement To turn on a thyristor positively in the shortest time, it is required to have a gate current with a fast rise time up to the maximum permitted value This rise time is achieved by pulse techniques It is also referred as firing pulses

Gate Turn-Off Thyristor A gate-turn-off thyristor (GTO) like an SCR can be turned ON by applying a positive gate signal. It can be turned OFF by a negative gate signal.

TRIAC (AC Switch)

TRIAC-Modes of Operation

Bipolar Junction Transistors (BJT) E Base Control terminal Collector/Emitter Power terminals B

Characteristics of Ideal Switch ON – conducting with low voltage drop (saturation Region) OFF – blocking current against applied voltage (Cut-off Region) For BJT ON – apply base current OFF – remove base current 1kV, 400A, Switching frequency – 5kHz

Characteristics of BJT

Safe Operating Area Peak current Boundary Power Boundary Avalanche voltage breakdown boundary

Avalanche breakdown A transistor has a maximum collector-to-emitter voltage Vce(sat) that it can withstand, above which avalanche breakdown will occur. Avalanche breakdown need not necessarily result in permanent damage Secondary Breakdown It is a destructive phenomenon that results from the current flow to a small portion of the base, producing localized hot spots. If the energy in these hot spots is sufficient the excessive localized heating may damage the transistor permanently. Peak Current It is the maximum permissible collector current Maximum Power or Temperature Maximum permissible collector power or permissible junction temperature

Device Losses Forward conduction Switching Loss forward voltage drop x conduction current Leakage current during blocking state Losses in gate or base circuit path Switching Loss energy dissipated during turn-on and turn-off significant at high frequency

Tutorial from text book-EX 1.2 Pg 33 During turn-on and turn-off of a power transistor supplying a resistive load the voltage and current variations are as shown (idealised linear changes with time). Show that, for each, the switching energy loss is given by (VI/6) T, where V is the off-state voltage, I the on-state current, and T is the switching time. Determine the mean power loss due to switching at a frequency of 10kHz. If the transistor is operating with a duty cycle of 0.5, that is equal off and on times, determine the conduction loss if the collector-emitter voltage is 2.1 V at 200 A. Hence determine the total device loss.

MOSFET Drain Structure Gate Source Symbol

MOSFET is a voltage control device MOSFETs are of two types Depletion MOSFETs Enhancement MOSFETs 500V, 200A and very high switching frequency up to 1MHz It has high input impedance.(No loading of source,less current drawn and low input power dissipation) Internal(dynamic) resistance between drain and source during on state Rds(on) limits power handling capacity of MOSFET

N-channel Depletion MOSFET N-channel Enhancement MOSFET

MOSFET Characteristics Apply gate voltage to switch ON Remove gate voltage to switch OFF Match demand current with gate voltage. ,

Advantages Disadvantages No Power switching losses No secondary breakdown Much higher switching speed Better reliability and thermal stability Higher peak current handling capability Disadvantages The only disadvantage is higher conduction drop –typically 4.5V

Insulated Gate Bipolar Transistor(IGBT) IGBT is a voltage controlled device. It has high input impedance like a MOSFET and low on-state conduction losses like a BJT.

With collector and gate voltage positive with respect to emitter the device is in forward blocking mode. When gate to emitter voltage becomes greater than the threshold voltage of IGBT, a n-channel is formed in the P-region. Now device is in forward conducting state In this state p+ substrate injects holes into the epitaxial n- layer. Ratings 3.3kV, 1.2kA and typical switching frequency is 20-50kHz

Device Comparisons Ideally Thyristor unlimited voltage and current ratings instant turn-on and turn-off times zero leakage current zero conduction and switching loss zero gate or base power requirement ability to withstand current overloads and voltage transient easy to protect against spurious turn-on and fault conditions low cost and ease of assembly Thyristor highest ratings, robust, low conduction loss, slow, difficult to turn off in dc systems. Used in electricity supply systems - WHY ?

Ratings Ratings normally expressed as Voltage Current Temperature Time forward, reverse, repetitive, non-repetitive, rate of rise Current forward, leakage, rms or mean (waveshape), rate of rise. Temperature losses, cooling strategies Time current and voltage rates, switching speed Example Transistor Safe Operating Area (SOA)

Five Comparison Between MOSFET and BJT Voltage and Current Ratings The voltage and current ratings of MOSFET family is lesser than the BJT. It is 500V & 200A for MOSFET and 1kV & 400A for BJT. Speed of Operation Typical frequency of MOSFET is greater than 100kHz and for BJT it is less than 100kHz. Thus MOSFET is used for high frequency applications as dc-dc converter and switch mode power supplies

Conduction Loss, Switching Loss The conduction loss of BJT is lower than that of MOSFET, but the switching loss MOSFET is lower than BJT, thus making it good for high frequency application. The conduction loss of BJT is less due to low on state voltage drop (1-2V) Robustness Both the devices can operate with junction temperature upto 125 o c. Both the devices cannot withstand reverse voltage without additional protection and the MOSFET can be damaged by static charge handling. Cost The BJT tends to be cheaper for similar ratings. However, the cost of drive electronics is cheaper for MOSFET

Switches Comparison Thyristor GTO BJT MOSFET IGBT Availability Early 60s Mid 80s Late 70s Early 80s Late 80s Voltage Ratings 5kV 1kV 500V 3.3kV Current Ratings 4kA 5kA 400A 200A 1.2kA Switching Frequencies low 2kHz 5kHz 1MHz 100kHz On-state voltage drop 2V 2-3V 1-2V I*Rds(on) 4.5V Drive Circuit Simple Very difficult Difficult Very simple Comments Cannot turn-off using gate signals King in very high power Phasing out in new products Good performance in high frequency Best overall performance

Power Electronics Devices: Summary

Power Electronics Devices: Summary Figure 4Power Devices, Symbol, Characteristics Courtesy:‘Power Electronics’; M.H Rashid, Prentice Hall,2003

Types of Power Electronics Converters AC to DC: RECTIFIER AC input DC output AC to AC: CYCLO- CONVERTER DC input AC output DC to AC: INVERTER DC input DC output DC to DC: CHOPPER

Applications Static applications involves non-rotating or moving mechanical components. Examples: DC Power supply, Un-interruptible power supply, Power generation and transmission (HVDC), Electroplating, Welding, Heating, Cooling, Electronic ballast Drive applications contains moving or rotating components such as motors. Electric trains, Electric vehicles, Air-conditioning System, Pumps, Compressor, Conveyer Belt (Factory automation).

Applications Figure 5 Applications of power devices Courtesy:‘Power Electronics’; M.H Rashid, Prentice Hall,2003

Applications-HVDC TRANSMISSION

Applications-AC DC DRIVES

Applications-STANDBY INVERTER

Applications-RENEWABLE ENERGY

Applications-ELECTRIC VEHICLE

Applications-VARIOUS OTHER APPLICATIONS