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PWM Dc-to-Dc Power Conversion

This introductory chapter presents an overview of the pulsewidth modulated (PWM) dc-to-dc power conversion. We discuss basic principles and unique natures of dc-to-dc power conversion circuits, along with the concept of the PWM technique. This chapter also presents features and issues of dc power distribution systems for modern electronic equipment and systems. Finally, this chapter outlines the contents of forthcoming chapters.

1.1 PWM Dc-to-Dc Power Conversion

The PWM dc-to-dc power conversion is generally described as the process of changing the voltage level of a dc source using the PWM technique. However, a more definitive and precise description is necessary to comprehend natures and features of the PWM dc-to-dc power conversion circuit.

1.1.1 Dc-to-Dc Power Conversion

To formulate an accurate description of the dc-to-dc power conversion, this section presents two different approaches to energizing an electric bulb using a dc voltage sourced from a battery. It is presumed that the electric bulb requires a strict 12 V for operation, while the battery voltage is varied between 18 and 30 V depending on the charging status and operational conditions. Figure 1.1 shows the first approach where a variable resistor and controller are employed between the battery and electric bulb. The controller is assumed to only draw a negligible current.

In Figure 1.1, the controller adjusts the resistance of the variable resistor to meet the following relationship

where is the voltage across the electric bulb, denotes the resistance of the bulb, and is the battery voltage that varies between . Figure 1.1 certainly fulfills the goal of providing a fixed dc voltage from a variable voltage source; however, it has one critical problem that renders this approach impractical.

The variable resistor is accompanied by an ohmic power loss

where is the input power drawn from the battery, is the output power delivered to the electric bulb, and is the current flowing from the battery to the electric bulb. The power loss is given by the product of the load current and the difference between the battery voltage and bulb voltage. This power loss easily becomes significant. For example, when an electric bulb which consumes a power at voltage level is connected to a battery, the power loss is as large as . This loss is even larger than the power consumed in the bulb, .

Figure 1.1 Conventional approach to lighting electric bulb.

The power loss is always transformed into heat and the resulting heat must be removed using an appropriate cooling system. The cooling system usually employs bulky heat sinks and noisy fans, consequently increasing the size and weight of the entire system. Accordingly, Figure 1.1 cannot be used for applications where the dimension and weight should be limited, which is usually the case for most modern electronic equipment and systems.

Figure 1.2 shows an alternative approach where a switch network and LC filter are inserted between the battery and electric bulb. The switch network periodically changes its connection. Within each switching period , the switch network maintains position a for and position p for the remaining part of the switching period, . This switch network is called the single-pole double-throw (SPDT) switch because it contains one pole that is always connected to one of the two contacts, the throw a and the throw p. With the switching action of the SPDT switch, the battery voltage is transformed into a rectangular waveform at the output of the SPDT switch, in Figure 1.2. The rectangular waveform is then applied to the LC filter. The LC filter alters the rectangular waveform into a smoothly-filtered continuous voltage waveform, in Figure 1.2.

Figure 1.2 Dc-to-dc power conversion applied to power electric bulb. (a) Circuit diagram. (b) Input and output waveforms of LC filter.

If the LC filter provides sufficient filtering, the output voltage nearly becomes a dc waveform corresponding to the average value of

To maintain at the presence of the battery voltage variation, the controller adjusts the ratio to . With a fixed , the controller changes to meet the condition

For example, with a battery voltage and switching period , the controller generates to produce . If the battery voltage is increased to , the controller reduces to to regulate at 12 : .

Although Figures 1.1 and 1.2 both achieve the same goal, a crucial difference exists between them. Figure 1.2 presumes a lossless operation because the SPDT switch and reactive components in the LC filter do not consume any power. The lossless operation eliminates all the problems associated with the power loss. Because no heat management is required, the circuit could be packaged with a smaller size and lighter weight, thereby making it fully compatible with modern electronic systems.

A more definitive description of the dc-to-dc power conversion is now established as the process of changing the voltage level of a dc source, while eliminating or minimizing power loss. In this perspective, Figure 1.2 is a typical example of the dc-to-dc power conversion circuit, while the conventional circuit illustrated in Figure 1.1 is not classified so.

1.1.2 PWM Technique

The concept of PWM technique could be envisaged from the operation of Figure 1.2, where the ratio to of the SPDT switch is adjusted to keep the output voltage constant. By changing the ratio, the pulsewidth of the rectangular voltage waveform is adaptively modulated so that the output voltage remains constant despite the input voltage variation. This control scheme is called the PWM technique and the dc-to-dc conversion circuit based on the PWM scheme is known as the PWM dc-to-dc converter. The PWM dc-to-dc converter is widely adapted to modern industrial and consumer electronics, thereby becoming the most prevailing dc-to-dc power conversion circuit.

1.2 Standalone Dc-to-Dc Power Conversion System

The basic concept illustrated in Figure 1.2 is generalized into dc-to-dc power conversion systems, whose block diagram representation is shown in Figure 1.3. The system consists of the dc source, dc-to-dc converter, and load. The dc source provides an arbitrary dc voltage to the dc-to-dc converter. The dc-to-dc converter then converts the level of the given dc voltage into the value required by the load and delivers it to the load. The load is an application system that operates with a fixed dc voltage and eventually consumes electrical power. This section presents characteristic features of the dc source, dc-to-dc converter, and load.

Figure 1.3 Standalone dc-to-dc power conversion system.

1.2.1 Dc Source with Non-ideal Characteristics

The practical dc source falls short of the characteristics of an ideal voltage source in many aspects. First, the voltage level of the dc source could vary with time, as is the case with batteries, fuel cells, and other standalone dc sources. The change in the voltage could occur either gradually or abruptly, depending on the characteristics and conditions of the dc source.

Second, a rectified ac source is often used as a substitute for the dc source. For this case, the rectified ac source usually contains a considerable amount of ac components, known as ac ripple. In addition, the output of the rectified ac source could be corrupted with various noises. Accordingly, the dc source represents an arbitrary non-ideal source whose voltage could be varied, polluted with ac ripple and noises, and switched from one value to another.

1.2.2 Dc-to-Dc Converter as Voltage Source

The dc-to-dc converter receives an arbitrary voltage from the non-ideal source and is required to provide a fixed dc voltage for the load. Thus, in addition to altering the voltage level, the dc-to-dc converter should have the capacity of maintaining its output voltage constant at the presence of the variation, ac ripple component, and abrupt change in the input voltage. Ideally, the dc-to-dc converter should function as an ideal voltage source, powered from a non-ideal voltage source and programmed to produce the required dc voltage for the load, regardless of the condition of the voltage source.

Although a practical dc-to-dc converter is more complicated in structure and operation than the circuit in Figure 1.2, the converter is still grouped into two functional blocks: the power stage and controller. The power stage alters the level of the input voltage into a desired value using various active and passive circuit components, while the controller provides the necessary signals for the power stage to execute its function.

Dc-to-dc converters come with numerous variations in their power stage configuration and each dc-to-dc converter is named differently after its power stage structure. Despite the wide diversity in structure, all the power stages employ the common electronic components to perform the dc-to-dc power conversion. The power stage utilizes semiconductor devices to implement the function of the SPDT switch, energy storage components to perform...