LECTURE 1 (Ch. 1) INTRODUCTION ECE 452 Power Electronics

Application of Power Electronics In early days, control of the electric power was achieved with electric machinery. Power electronics have revolutionized the concept of power control for power conversion and for control of electrical motor drives. Power electronics combine power, electronics, and control.

Control deals with the steady-state and dynamic characteristics of closed-loop system. Power deals with static and rotating power equipment. Electronics deals with the solid-state devices and circuits for signal processing to meet the desired control objectives.

Therefore, power electronics is defined as the applications of solid-state electronics for control and conversion of electric power. Power electronics is based on switching of the power semiconductor devices. It covers a variety of switching circuits.

History of Power Electronics The history of power electronics began with introduction of the mercury arc rectifiers in 1900. Devices which were based on the mercury arc valve technology were used until 1950. The first electronic revolution began in 1948 with the invention of the silicon transistor at Bell Labs.

Most of today's advanced electronic technologies are based on the transistor concept. The next breakthrough was invention of Thyristor (SCR) in 1956, which is a PNPN triggering transistor.  The second revolution began in 1958 with development of the commercial thyristor by GE.

That was the beginning of a new era of power electronics.

Power Semiconductor Devices Since the first thyristor was developed in 1957, there have been tremendous advances in the power semiconductor devices. Until 1970, the conventional thyristors had been exclusively used for power control applications. Since 1970 many types of power semiconductor devices were developed.

Control Characteristics The power semiconductor devices can be operated as switches by applying a control signals to gate.

Power semiconductor switching devices can be classified on the basis of: Uncontrolled turn on and off (diodes) Controlled turn on and uncontrolled turn off (SCR) Controlled turn on and off (BJT, MOSFET, GTO, IGBT) Continuous gate signal requirement (BJT, MOSFET, IGBT)

MOSFET, GTO) Pulse gate requirement (SCR, GTO) Bipolar voltage-withstanding capability (SCR, GTO) Unipolar voltage withstanding capability (BJT, MOSFET, GTO) Bidirectional current capability (TRIAC) Unidirectional current capability (SCR, GTO, BJT, MOSFET, DIODE)

Characteristics and Specification of Switches There are many types of power switching devices. Each has its own advantages and disadvantages for an application.

Ideal Switches In the on-state: carry high forward current, low forward voltage drop, and low resistance In the off-state: withstand a high voltage, low leakage current, and high resistance During turn-on and turn-off process instantaneously turn on and off

Low gate power for turn on and off Controllable turn on and off Turn on and off require a small pulse High dv/dt & di/dt Low thermal impedance

Sustain any fault current (i2t) Equal current sharing for parallel operation Low price

Characteristics of Practical Devices During the turn-on and turn-off process a practical device requires: a finite delay time  rise time  storage time fall time

Types of Power Electronic CKTs For control of electric power or power conditioning, the conversion of electric power from one form to another is necessary. Switching characteristics of the power devices permit this conversion.

Power electronics circuits can be classified into six types: Diode rectifiers Ac-dc converters (controlled rectifier) Ac-ac converters (ac voltage controllers) Dc-dc converters (dc choppers) Dc-ac converters (inverters) Static switches

Design of Power Electronics Equipment The design is divided into four parts: Design of power circuits Protection of power devices Determination of control strategy Design of logic and gating circuits

In the chapters that follow, we will describe various types of power electronic circuits. In analysis, the power devices are assumed to be ideal switches. The effect of circuit resistance and source inductance is ignored. Ignoring these parameters will simplify the design steps, but it is very useful to understand operation of the circuit and establish the control strategy.

Determining the RMS Value The RMS value of current should be known for determination of conduction losses and current rating of the device. The RMS value of a current waveform is: Also:

Peripheral Effects Operations of power converters are mainly based on the switching of power semiconductor devices. As a result, converters introduce current and voltage harmonics into the supply system and on the output of the converters.  

These can cause problems of distortion of the output voltage, harmonic generation into the supply system, and interference with the communication and signaling circuits. Therefore, it is normally necessary to introduce filters on the input and output of a converter system to reduce the harmonic level.  

The following figure shows the block diagram of a generalized power converter.