Institute of Technology Bahir Dar University Institute of Technology School of Computing and Electrical Engineering Eeng-5181 : Power Electronics and Electric Drives Nov, 2017

1. Introduction to power electronics & electrical Drive Power Electronics is the art of converting electrical energy from one form to another in an efficient, clean, compact, and robust manner for convenient utilization. Exmple : A passenger lift in a modern building equipped with a Variable-Voltage-Variable-Speed induction-machine drive offers a comfortable ride and stops exactly at the floor level.

Interdisciplinary Nature of Power Electronics

Power Electronics involves on the study of: Power semiconductor devices - their physics, characteristics, drive requirements and their protection for optimum utilization of their capacities, Power converter topologies involving them, Control strategies of the converters, Digital, analogue and microelectronics involved,

Rotating and static electrical devices, Capacitive and magnetic energy storage elements, Rotating and static electrical devices, Quality of waveforms generated, Electro Magnetic and Radio Frequency Interference, Thermal Management

Application of Power Electronics

Power Electronics verses Linear Electronics In Power Electronics all devices are operated in the switching mode either 'FULLY-ON' or 'FULLY-OFF' states. The linear amplifier concentrates on fidelity in signal amplification, requiring semiconductor devices to operate strictly in the linear (active) zone. Power semiconductor devices are operating at ‘CUT OFF’ or ‘SATURATION’ region.

Typical Bipolar transistor based (a) linear (common emitter) (voltage) amplifier stage and (b) switching (power) amplifier

Operating zones for operating a Bipolar Junction Transistor as a linear and a switching amplifier

Review of power semiconductor devices & their characteristics power semiconductor devices includes on the study of: power diode, Thyristor (SCR, Triac, GTO, MCT) BJT, MOSFET, IJBT.

Power diodes play an important role in power electronics circuits They are mainly used as: Uncontrolled rectifiers , Electro plating (coat a metal object by electrolytic deposition with another metal), Battery charging, Welding, Power supply (dc or ac), Variable frequency drives, Feed back and freewheeling functions.

Construction and Characteristics Power Diodes Power diodes are usually constructed using silicon because silicon diodes can operate at higher current and at higher junction temperatures than germanium diodes. Conventional diode is two layer of p-n junction device. But, a typical Power diode has P-I-N structure (additional or drift layer) in between p-n layer. This is to improve high power holding capacity with minimum power loss.

Construction of Power Diodes

voltage-current characteristics of Power Diodes Ideal condition: Forward conduction: Resistanceless Reverse blocking: Lossless Switching ON/OFF time: Instantaneous a. Ideal b. Practical

Types of Power diode Line-frequency diodes:- These are PIN diodes with general-purpose rectifier type applications High speed /or fast recovery/:-used for high frequency switching of power convertors. Schottky diode:-have low on state voltage and very small recovery time, typically nanoseconds

Thyristor: The thyristor is the general name given to a family of power semi conductor device(SCR, Triac, GTO, MCT) Silicon controlled rectifier (SCR): SCR is basically a four-layer three-junction PNPN device. It has three terminals: anode(A), cathode(K), and gate(G).

(a) (b) (c)

voltage-current characteristics of SCR

Latching Current:-This is the minimum anode current required to maintain the thyristor in the on-state immediately thyristor has been turned on and the get signal has been removed. Holding current:- This is the minimum anode current required to maintain the thyristor in the on-state.

Turn-ON Method of Thyristor Forward voltage triggering Thermal triggering Radiation triggering dv/dt- triggering Get trigering Turn-OFF Method of Thyristor Natural commutation Forced commutation

Example1: The latching current of an SCR circuit below is 50mA, the duration of the firing pulse is 50µs. Will the SCR get fired? The answer will be ‘NO’

Example2: If the latching current of an SCR circuit below is 4mA, obtain the minimum width of the gating pulse required to properly turn on the SCR.

TRIAC: The Triac is a member of the thyristor family. But unlike a thyristor which conducts only in one direction a triac can conduct in both directions. Thus a triac is similar to two back to back (anti parallel) connected thyristosr but with only three terminals The triac turns off only when the current through the main terminals become zero

It is extensively used in: residential lamp dimmers, heater control and for speed control of small single phase series and induction motors

(a) anti parallel connected thyristosr (a) Electrical symbol Exit

Steady state V-I characteristics of Triac

GTO (Gate Turn-Off Thyrstor) Gate Turn-Off Thyrstor(GTO) as the name indicates, is basically a thyristor type device that can be turned on by small positive gate current pulse. But in addition, has the capability of being turned off by a negative gate current pulse. Has slow switching speeds Used at very high power levels

A GTO : (a) Electrical Symbol, (b) practical v-I characteristics, (c) idealized v-I characteristics

The power MOSFET Unlike a diode, turn on and turn off of a transistor are controllable. Types of transistors used in power electronics circuits include MOSFET, BJT, IGBT. MOSFET is a Voltage controlled device Its construction includs a parasitic (body) diode. It is a three terminal device where the voltage on the gate terminal controls the flow of current between the output terminals, Source and Drain.

Circuit symbol of MOSFET

V-I characteristic of MOSFET

IGBT The IGBT has the high input impedance and high-speed characteristics of a MOSFET with low saturation voltage of a bipolar transistor. The IGBT is turned on by applying a positive voltage between the gate and emitter And turned off by making the gate signal zero or slightly negative. It has a much lower voltage drop than a MOSFET of similar ratings.

The IGBT cell has a parasitic p-n-p-n thyristor structure embedded into it The constituent p-n-p &, n-p-n transistor and the driver MOSFET are shown in its structure Important resistances in the current flow path are also indicated.

Parasitic thyristor in an IGBT cell. Schematic structure Exact equivalent circuit Approximate equivalent circuit Circuit symbol EXIT

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