电气工程及其自动化专业英语第三章.ppt
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1、Chapter 3Power Electronic Technology,Section 2 The DC-DC Converters,Text,New Words and Expressions,Exercises,End,Transition of part of speech,Section 2 The DC-DC Converters,Introduction The DC-DC converter converts a DC power source to another DC source with different terminal specifications.The DC-
2、DC converters change DC to AC first and than change AC back to DC.The DC source is often the uncontrolled DC voltage with ripple from AC to DC rectifier.The DC-DC converters are widely used in the switch-mode power supplies and DC motor drive applications.Some of these converters,especially in power
3、 supplies,have an isolation transformer.,Section 2 The DC-DC Converters,The DC-DC converters are also used as interfaces between the DC systems of different voltage levels.The step-down and step-up voltage converters are the two basic converter topologies.These are referred as the Buck and Boost con
4、verters respectively.It must,however,be kept in mind that a step-down voltage converter is also a step-up current converter and vice versa because the input power must equal the output power.The Buck-Boost converter does both stepping up and down,Section 2 The DC-DC Converters,action.The UK converte
5、r is a dual of the Buck-Boost converter.All these converters have single,two and four quadrant variations in topologies.The voltage source driven DC-DC converts are more popular than the current driven converters,hence,this section will concentrate on the voltage driven converters.All converts use e
6、ffective filtering on both the input and the output to reduce the ac components from going outside the converters.,Section 2 The DC-DC Converters,Buck Converter The Buck converter is a voltage step-down and current step-up converter.The topology is shown in Fig.3-2.The two-position switch is synthes
7、ized from a switch and a diode.The switch is turned on for a time periodically at a rate 1/Ts.,Fig.3-2 Buck converter topology,Section 2 The DC-DC Converters,A prominent application of the Buck converter is a DC regulated power supply in which the output voltage is regulated against the variations i
8、n the load resistance and the input voltages.These power supplies are used in computers and portable instruments in the medical and communication field.There are two modes of Buck converter operations in which the output current waveform is either continuous or discontinuous in time over a switching
9、 period.,Section 2 The DC-DC Converters,Continuous conduction mode(CCM)The ideal current sink in the output represents infinitely large inductor in series with the load resistance.This is a hypothetical circuit but its discussion provides a good understanding of the Buck converter operation.The stea
10、dy-state operation consists of two circuit modes,shown in Fig.3-3.,Fig.3-3 Circuit modes in steady-state operation,Section 2 The DC-DC Converters,As the switch is turned on in mode l,the diode is reverse biased and the current flows through inductor into the voltage sink.After a time,the switch is t
11、urned off.The inductor current then freewheels through the diode as shown in the circuit of mode 2.The second mode is terminated at TS when the switch is turned on again.The current and voltage waveforms are shown in Fig.3-4.The average output voltage is,Section 2 The DC-DC Converters,or,(3-5),Fig.3
12、-4 The current and voltage waveforms,Section 2 The DC-DC Converters,D is called the duty ratio.The duty ratio D is also the turn ratio of the equivalent DC transformer.The turn ratio of the equivalent dc transformer can be controlled electronically in a range of 01.The DC output voltage is a linear
13、function of the on-time of the switch,the pulse width,and the switching frequency(inverse function of the switching period TS).The output voltage control using pulse width is known as the PWM control.The output voltage can also be controlled by switching frequency.The PWM control is more popular.,Se
14、ction 2 The DC-DC Converters,If the duty ratio D is made a linear function of uC,a control voltage,(3-6),The output voltage is then a linear function of the control voltage.This is also the principle of switch-mode linear amplifier.The gain of this amplifier is determined by the input dc source volt
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