Overview of Power Semiconductor Switches Presently available power

Overview of Power Semiconductor Switches Presently available power semiconductor switches can be divided into three groups according to their degree of controllability: – Diodes: ON and OFF states controlled by power circuits – Thyristors: latched on by a control signal but turned OFF by the power circuit – Controllable switches: turned ON and OFF by control signals Copyright © 2003 by John Wiley & Sons, Inc. Chapter 2 Power Semiconductor Switches: An Overview 2 -1

Diodes • On and off states controlled by the power circuit • Forward biased – conduction • Reverse biased – small leakage current flow until break down voltage reached Copyright © 2003 by John Wiley & Sons, Inc. Chapter 2 Power Semiconductor Switches: An Overview 2 -2

Thyristors • Semi-controlled device • Latches ON by a gate-current pulse if forward biased • Turns-off if current tries to reverse Copyright © 2003 by John Wiley & Sons, Inc. Chapter 2 Power Semiconductor Switches: An Overview 2 -3

Thyristor in a Simple Circuit • For successful turn-off, reverse voltage required Copyright ©

Generic Switch Symbol • • • Idealized switch symbol When on, current can flow only in the direction of the arrow Instantaneous switching from one state to the other Conduct large current with zero voltage drop in on-state Block large forward and reverse voltages with zero current flow when off • Infinite voltage and current handling capabilities Copyright © 2003 by John Wiley & Sons, Inc. Chapter 2 Power Semiconductor Switches: An Overview 2 -5

Bipolar Junction Transistors (BJT) • BJT is a current-controlled device • A sufficiently large base current will turn the device ON • Base current must be supplied continuously to keep it in the ON state • Used commonly in the past • Copyright Now used in specific applications, replaced by MOSFETs and IGBTs © 2003 Chapter 2 Power Semiconductor by John Wiley & Sons, Inc. Switches: An Overview 2 -6

Various Configurations of BJTs • dc gain is in the order of 5 -10 of one BJT • To achieve larger current gain, these devices are sometimes connected in the above configurations. Copyright © 2003 by John Wiley & Sons, Inc. Chapter 2 Power Semiconductor Switches: An Overview 2 -7

MOSFETs • MOSFET is a voltage-controlled device • Easy to control by the gate – continuous application of v. GS required to keep the device in the ON state • Faster switching speed (in the nanosecond range) than BJTs • Switching loss is lower compared to BJTs Copyright © 2003 by John Wiley & Sons, Inc. Chapter 2 Power Semiconductor Switches: An Overview 2 -8

Gate-Turn-Off Thyristors (GTO) • GTO as an ON/OFF switch – Once forward biased GTO can be turned ON by a gate pulse – GTO will stay ON – However, can be turned off by applying a negative gate-cathode voltage • Used at very high power levels • Require elaborate gate control circuitry Copyright © 2003 by John Wiley & Sons, Inc. Chapter 2 Power Semiconductor Switches: An Overview 2 -9

IGBT • High impedance gate – requires small amount of energy to switch the device • Current rating: ~1700 A • Voltage rating: 2~3 k. V Copyright © 2003 by John Wiley & Sons, Inc. Chapter 2 Power Semiconductor Switches: An Overview 2 -10

Comparison of Controllable Switches Copyright © 2003 by John Wiley & Sons, Inc. Chapter

Review of Basic Electrical and Magnetic Circuit Concepts Copyright © 2003 by John Wiley

Sinusoidal Steady State Copyright © 2003 by John Wiley & Sons, Inc. Chapter 2

Three-Phase Circuit Copyright © 2003 by John Wiley & Sons, Inc. Chapter 2 Power

Steady State in Power Electronics • Voltage produced by an inverter in an ac motor drive • Often line currents drawn from the utility by the power electronic circuits are highly distorted as shown in b Copyright © 2003 by John Wiley & Sons, Inc. Chapter 2 Power Semiconductor Switches: An Overview 2 -15

Fourier Analysis Copyright © 2003 by John Wiley & Sons, Inc. Chapter 2 Power

Phasor Representation Copyright © 2003 by John Wiley & Sons, Inc. Chapter 2 Power

Response of L and C Copyright © 2003 by John Wiley & Sons, Inc.

Inductor Voltage and Current in Steady State In steady-state, the average inductor voltage (over one time period) must be zero. Copyright © 2003 by John Wiley & Sons, Inc. Chapter 2 Power Semiconductor Switches: An Overview 2 -19

Capacitor Voltage and Current in Steady State In steady-state, the average capacitor current (over one time period) must be zero. Copyright © 2003 by John Wiley & Sons, Inc. Chapter 2 Power Semiconductor Switches: An Overview 2 -20