Power Electronics

David Allan Bradley

• Publisher: Routledge
• Year: 2017
• ISBN:
  9781351423168
• Edition: 2
• Topic: Tecnologia e ingegneria
• Subtopic: Ingegneria elettronica e telecomunicazioni

Since its inception, the Tutorial Guides in Electronic Engineering series has met with great success among both instructors and students. Designed for first and second year undergraduate courses, each text provides a concise list of objectives at the beginning of each chapter, key definitions and formulas highlighted in margin notes, and references to other texts in the series.This volume introduces the subject of power electronics. Giving relatively little consideration to device physics, the author first discusses the major power electronic devices and their characteristics, then focuses on the systems aspects of power electronics and on the range and diversity of applications. Several case studies, covering topics from high-voltage DC transmission to the development of a controller for domestic appliances, help place the material into a practical context. Each chapter also includes a number of worked examples for reinforcement, which are in turn supported by copious illustrations and end-of-chapter exercises.

1

Power semiconductors

Objectives

□ To introduce the major power electronic devices.

□ To define their operating regimes and modes of operation.

□ To establish their ratings.

□ To consider losses and heat transfer properties.

□ To examine means of protection.

An intrinsic semiconductor is defined as being a material having a resistivity which lies between that of insulators and conductors and which decreases with increasing temperature. The principal semiconductor material used for power electronic devices is silicon, a member of Group IV of the periodic table elements which means it has four electrons in its outer orbit.

If an element of Group V, such as phosphorus, with five electrons in its outer orbit is added to the silicon, each phosphorus atom forms a covalent bond within the silicon lattice, leaving a loosely bound electron. The presence of these additional electrons greatly increases the conductivity of the silicon and a material doped in this way is referred to as an n-type semiconductor.

By introducing an element from Group III as impurity, a vacant bonding location or hole is introduced into the lattice. This hole may be considered to be mobile as it can be filled by an adjacent electron, which in its turn leaves a hole behind. Holes can be thought of as carriers of positive charge and a semiconductor doped by a Group III impurity is referred to as a p-type semiconductor.

The extra, mobile electrons introduced by doping into the n-type material and the equivalent holes in the p-type material are referred to as the majority carriers. In an n-type material there is a small population of holes and in a p-type material a small population of electrons.

These are called the minority carriers.

Diode

The semiconductor junction diode shown in Fig. 1.1 is the simplest semiconductor device used in power electronics. With no external applied voltage the redistribution of charges in the region of the junction between the p-type and n-type materials results in an equilibrium condition in which a potential barrier is established across a narrow region depleted of charge carriers on each side of the junction. This equilibrium may be disturbed by the application of an external applied voltage of either polarity.

Fig. 1.1 The diode.

Ghandi, S.K. (1977).

Semiconductor Power Devices.

Wiley Interscience.

Sparkes, J.

Semiconductor Devices. (1987).

Van Nostrand Reinhold.

Fig. 1.2 Diagrammatic representation of the diode static characteristic. (Note: forward and reverse voltage scales are unequal. The forward voltage drop is of the order of 1 V while the reverse breakdown voltage varies from a few tens of volts to several thousand.)

The region over which the potential barrier exists is known as the depletion or transition layer.

If a reverse voltage – cathode positive with respect to anode – is applied, the electric field at the junction is reinforced, increasing the height of the potential barrier and increasing the energy required by a majority carrier to cross this barrier. The resulting small reverse leakage current shown in the diode static characteristic of Fig. 1.2 is due to the flow of minority carriers across the junction. The magnitude of the reverse leakage current increases with temperature because the number of minority carriers available increases with the temperature of the material.

The magnitude of the reverse leakage current can vary from a few picoamperes for an integrated circuit diode to a few milliamperes for a power diode capable of carrying several thousand amperes in the forward...