Encyc - Article in Wiley Encyclopedia of Electrical and...

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Article in Wiley Encyclopedia of Electrical and Electronics Engineering DC-DC Power Converters Robert W. Erickson Department of Electrical and Computer Engineering University of Colorado Boulder, CO 80309-0425 Dc-dc power converters are employed in a variety of applications, including power supplies for personal computers, office equipment, spacecraft power systems, laptop computers, and telecommunications equipment, as well as dc motor drives. The input to a dc-dc converter is an unregulated dc voltage Vg. The converter produces a regulated output voltage V, having a magnitude (and possibly polarity) that differs from Vg. For example, in a computer off-line power supply, the 120 V or 240 V ac utility voltage is rectified, producing a dc voltage of approximately 170 V or 340 V, respectively. A dc-dc converter then reduces the voltage to the regulated 5 V or 3.3 V required by the processor ICs. High efficiency is invariably required, since cooling of inefficient power converters is difficult and expensive. The ideal dc-dc converter exhibits 100% efficiency; in practice, efficiencies of 70% to 95% are typically obtained. This is achieved using switched-mode , or chopper , circuits whose elements dissipate negligible power. Pulse-width modulation (PWM) allows control and regulation of the total output voltage. This approach is also employed in applications involving alternating current, including high-efficiency dc-ac power converters (inverters and power amplifiers), ac-ac power converters, and some ac-dc power converters (low-harmonic rectifiers). A basic dc-dc converter circuit known as the buck converter is illustrated in Fig. 1. A single-pole double-throw (SPDT) switch is connected to the dc input voltage V g as shown. The switch output voltage + V g L C R 1 2 + V Dc input Switch network Low-pass filter Load Dc output (a) + v s ( t ) v s ( t ) V g DT s (1 – D ) T s 0 t Switch position: 12 1 V s = DV g (b) Figure 1. The buck converter consists of a switch network that reduces the dc component of voltage, and a low-pass filter that removes the high-frequency switching harmonics: (a) schematic, (b) switch voltage waveform.
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v s ( t ) is equal to V g when the switch is in position 1, and is equal to zero when the switch is in position 2. The switch position is varies periodically, such that v s ( t ) is a rectangular waveform having period T s and duty cycle D . The duty cycle is equal to the fraction of time that the switch is connected in position 1, and hence 0 ± D ± 1. The switching frequency f s is equal to 1/ T s . In practice, the SPDT switch is realized using semiconductor devices such as diodes, power MOSFETs, IGBTs, BJTs, or thyristors. Typical switching frequencies lie in the range 1 kHz to 1 MHz, depending on the speed of the semiconductor devices. The switch network changes the dc component of the voltage. By Fourier analysis, the dc component of a waveform is given by its average value. The average value of v s ( t ) is given by V s = 1 T s v s ( t ) dt 0 T s = DV g (1) The integral is equal to the area under the waveform, or the height V g multiplied by the time DT s . It can be
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Encyc - Article in Wiley Encyclopedia of Electrical and...

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