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Week 5 DC-to-DC Converters 1

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Functional Block Diagram of DC-DC Converter System Unregulated dc voltage obtained by rectifying the line voltage, and therefore will fluctuate with line voltage magnitude Controlled dc output at a desired voltage level

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Control of DC-DC Converters In a dc-dc converter: Average output dc voltage must be controlled to equal a desired level. Utilizes one or more switches to transform dc from one level to another. The average output voltage is controlled by controlling the switch on and off durations (t on and t off ). Let’s consider the following switch-mode dc-dc converter: Average output dc voltage V o depends on t on and t off. Switching is done at a constant frequency with switching time period T s. This method is called pulse-width modulation (PWM) in which the duty ratio D is varied to control V o, where D=t on /T s

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Control of DC-DC Converters (cont’d) The switch control signal, which controls the on and off states of the switch, is generated by comparing a signal level control voltage v control with a repetitive waveform. The switching frequency is the frequency of the sawtooth waveform with a constant peak. The duty ratio D can be expressed as

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Static dc switch based on a fully controlled semiconductor power switch 5

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Voltage and current waveforms in a static dc switch 6

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SCR-based static dc switch with a resonant commutating circuit 7

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Sub-circuit of an SCR-based static dc switch 8

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Voltage and current waveforms in an SCR-based static dc switch 9

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Step-down chopper: (a) block diagram, (b) equivalent circuit for the dc output voltage and current 10

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Equivalent circuits of the first- quadrant chopper: (a) state 1 (b) state 0 11

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First-quadrant chopper 12

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Example waveforms of output voltage and current in a first- quadrant chopper 13

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Single cycle of the output voltage and current in a first- quadrant chopper 14

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Current ripple in a chopper as a function of the magnitude control ratio and switching frequency 15

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Second-quadrant chopper 16

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Equivalent circuits of a second- quadrant chopper: (a) state 1 (b) state 0 17

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Waveforms of output voltage and current in a second- quadrant chopper 18

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First-and-second-quadrant chopper 19

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Waveforms of output voltage and current in a first-and- second-quadrant chopper 20

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First-and-fourth-quadrant chopper 21

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Equivalent circuits of a first-and- fourth-quadrant chopper operating in the fourth-quadrant: (a) state 2 (b) state 0 22

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Waveforms of output voltage and current in a first-and- fourth-quadrant chopper 23

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Four-quadrant chopper 24

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Allowable ranges of the magnitude control ratio in step-down choppers 26

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Waveforms of the output voltage and input current in a step-up chopper (d = 0.75) 27

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Typical supply system of a chopper 28

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dc drive with a chopper with a braking resistor 29

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Chapter 630

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