chap01.pdf

Introduction

1.1

INTRODUCTION TO POWER PROCESSING

The field of power electronics is concerned with the processing of electrical power using electronic

devices [1–7]. The key element is the switching converter, illustrated in Fig. 1.1. In general, a switching

converter contains power input and control input ports, and a power output port. The raw input power is

processed as specified by the control input, yielding the conditioned output power. One of several basic

functions can be performed [2]. In a dc–dc converter, the dc input voltage is converted to a dc output

voltage having a larger or smaller magnitude, possibly with opposite polarity or with isolation of the

input and output ground references. In an ac–dc rectifier, an ac input voltage is rectified, producing a dc

output voltage. The dc output voltage and/or ac input current waveform may be controlled. The inverse

process, dc–ac inversion, involves transforming a dc input voltage into an ac output voltage of controlla-

ble magnitude and frequency. Ac–ac cycloconversion involves converting an ac input voltage to a given

ac output voltage of controllable magnitude and frequency.

Control is invariably required. It is nearly always desired to produce a well-regulated output

Introduction

voltage, in the presence of variations in the input voltage and load current. As illustrated in Fig. 1.2, a

controller block is an integral part of any power processing system.

High efficiency is essential in any power processing application. The primary reason for this is

usually not the desire to save money on one’s electric bills, nor to conserve energy, in spite of the nobility

of such pursuits. Rather, high efficiency converters are necessary because construction of low-efficiency

converters, producing substantial output power, is impractical. The efficiency of a converter having out-

put power

and input power is

The power lost in the converter is

Equation (1.2) is plotted in Fig. 1.3. In a con-

verter that has an efficiency of 50%, power

is dissipated by the converter elements

and this is equal to the output power,

This power is converted into heat, which

must be removed from the converter. If the

output power is substantial, then so is the

loss power. This leads to a large and expen-

sive cooling system, it causes the electronic

elements within the converter to operate at

high temperature, and it reduces the system

reliability. Indeed, at high output powers, it

may be impossible to adequately cool the

converter elements using current technology.

Increasing the efficiency is the key

to obtaining higher output powers. For exam-

ple, if the converter efficiency is 90%, then

the converter loss power is equal to only 11%

1.1 Introduction to Power Processing

of the output power. Efficiency is a good measure of the success of a given converter technology. Figure

1.4 illustrates a converter that processes a large amount of power, with very high efficiency. Since very

little power is lost, the converter elements can be packaged with high density, leading to a converter of

small size and weight, and of low temperature rise.

How can we build a circuit that changes the voltage, yet dissipates negligible power? The vari-

ous conventional circuit elements are illustrated in Fig. 1.5. The available circuit elements fall broadly

into the classes of resistive elements, capacitive elements, magnetic devices including inductors and

transformers, semiconductor devices operated in the linear mode (for example, as class A or class B

amplifiers), and semiconductor devices operated in the switched mode (such as in logic devices where

transistors operate in either saturation or cutoff). In conventional signal processing applications, where

efficiency is not the primary concern, magnetic devices are usually avoided wherever possible, because

of their large size and the difficulty of incorporating them into integrated circuits. In contrast, capacitors

and magnetic devices are important elements of switching converters, because ideally they do not con-

sume power. It is the resistive element, as well as the linear-mode semiconductor device, that is avoided

[2]. Switched-mode semiconductor devices are also employed. When a semiconductor device operates in

the off state, its current is zero and hence its power dissipation is zero. When the semiconductor device

operates in the on (saturated) state, its voltage drop is small and hence its power dissipation is also small.

In either event, the power dissipated by the semiconductor device is low. So capacitive and inductive ele-

ments, as well as switched-mode semiconductor devices, are available for synthesis of high-efficiency

converters.

Let us now consider how to construct the simple dc-dc converter example illustrated in Fig. 1.6.

The input voltage is 100 V. It is desired to supply 50 V to an effective

current is 10 A.

Introductory circuits textbooks describe a low-efficiency method to perform the required func-

tion: the voltage divider circuit illustrated in Fig. 1.7(a). The dc–dc converter then consists simply of a

load, such that the dc load

Introduction

variable resistor, whose value is adjusted such that the required output voltage is obtained. The load cur-

rent flows through the variable resistor. For the specified voltage and current levels, the power dissi-

pated in the variable resistor equals the load power W. The source supplies power

W. Figure 1.7(b) illustrates a more practical implementation known as the linear series-pass

regulator. The variable resistor of Fig. 1.7(a) is replaced by a linear-mode power transistor, whose base

current is controlled by a feedback system such that the desired output voltage is obtained. The power

dissipated by the linear-mode transistor of Fig. 1.7(b) is approximately the same as the 500 W lost by the

variable resistor in Fig. 1.7(a). Series-pass linear regulators generally find modern application only at

low power levels of a few watts.

Figure 1.8 illustrates another approach. A single-pole double-throw (SPDT) switch is connected

as shown. The switch output voltage is equal to the converter input voltage when the switch is in

position 1, and is equal to zero when the switch is in position 2. The switch position is varied periodi-

cally, as illustrated in Fig. 1.9, such that is a rectangular waveform having frequency and period

The duty cycle D is defined as the fraction of time in which the switch occupies position 1.

Hence,

In practice, the SPDT switch is realized using switched-mode semiconductor devices,

1.1 Introduction to Power Processing

which are controlled such that the SPDT switching function is attained.

The switch changes the dc component of the voltage. Recall from Fourier analysis that the dc

component of a periodic waveform is equal to its average value. Hence, the dc component of

Thus, the switch changes the dc voltage, by a factor equal to the duty cycle D. To convert the input volt-

age into the desired output voltage of V = 50 V, a duty cycle of D = 0.5 is required.

Again, the power dissipated by the switch is ideally zero. When the switch contacts are closed,

then their voltage is zero and hence the power dissipation is zero. When the switch contacts are open,

then the current is zero and again the power dissipation is zero. So we have succeeded in changing the dc

voltage component, using a device that is ideally lossless.

In addition to the desired dc component the switch output voltage waveform also con-

tains undesirable harmonics of the switching frequency. In most applications, these harmonics must be

removed, such that the output voltage is essentially equal to the dc component A low-pass fil-

ter can be employed for this purpose. Figure 1.10 illustrates the introduction of a single-section L–C low-

pass filter. If the filter corner frequency is sufficiently less than the switching frequency then the fil-

ter essentially passes only the dc component of To the extent that the switch, inductor, and capacitor

elements are ideal, the efficiency of this dc–dc converter can approach 100%.

In Fig. 1.11, a control system is introduced for regulation of the output voltage. Since the output

voltage is a function of the switch duty cycle, a control system can be constructed that varies the duty

cycle to cause the output voltage to follow a given reference. Figure 1.11 also illustrates a typical way in

which the SPDT switch is realized using switched-mode semiconductor devices. The converter power

stage developed in Figs. 1.8 to 1.11 is called the buck converter, because it reduces the dc voltage.

Converters can be constructed that perform other power processing functions. For example, Fig.

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