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Switching Power Supplies

Week 6

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Buck Converter

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Buck Converter

A buck converter is a voltage step down and current step up converter. The simplest way to reduce the voltage of a DC supply is to use a linear regulator (such as a 7805), but linear regulators waste energy as they operate by dissipating excess power as heat.

Step-Down (Buck) Converter

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• low-pass filter: to reduce output voltage fluctuations

• diode is reversed biased during ON period, input provides energy to the load and to the inductor

• energy is transferred to the load from the inductor during switch OFF period

• in the steady-state, average inductor voltage is zero

• in the steady-state, average capacitor current is zero

• converts dc from one level to another

• the average output voltage is controlled by the ON-OFF switch

• pulse-width modulation (PWM) switching is employed

• lower average output voltage than the dc input voltage Vd depending on the duty ratio, D

• D=ton/Ts

• Average output:

Applications:• regulated switch mode dc

power supplies• dc motor drives

• Converts dc from one level to another• The average output voltage is controlled by the

ON-OFF switch• Pulse-width modulation (PWM) switching is

employed• Lower average output voltage than the dc input

voltage Vd depending on the duty ratio, D

• D=ton/Ts

• Average output:

Applications:• regulated switch mode dc power supplies• dc motor drives

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Step-Down (Buck) Converter: Continuous current conduction mode

• Inductor current iL flows continuously

• Average inductor voltage over a time period must be zero

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Buck converter is like a dc transformer where the turns ratio can be controlled electronically in a range of 0-1 by controlling D of the switch

Example…..For a buck converter, R=1 ohm, Vd=40 V, V0=5 V, fs=4 kHz. Find the duty ratio and “on” time of the switch.

Solution….

D = V0 /Vd = 5/40 = 0.125 = 12.5%

Ts = 1/fs = 0.25 ms = 250 ms

Ton = DTs = 31.25 ms

Toff = Ts – ton = 218.75 ms

When the switch is “on”: VL = Vd - V0 = 35 V

When the switch is “off”: VL = -V0 = - 5 V

I0 = IL = V0 / R = 5 A

Id = D I0 = 0.625 A

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Voltage and current waveforms in a buck converter

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Waveforms of the capacitor current and output voltage in

a buck converter

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Boost Converter

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Voltage and Current Waveforms in a Boost

Converter

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Voltage and Current Waveforms in a Boost Converter

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Impact of the inductor resistance on the voltage gain of a boost

converter

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Buck-boost Converter

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Voltage and Current Waveforms in a Buck-boost Converter

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uk Converter

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Waveforms of voltage and current in a uk converter

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SEPIC (a) and Zeta (b) Converters

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Voltage and Current Waveforms in SEPIC and Zeta Converters

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Voltage and Current Waveforms in SEPIC and Zeta Converters

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Vorperian’s Switch Model

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Averaged models of switched-mode dc-to-dc converters with Vorperian’s Switch Model: (a)

Buck, (b) Boost, (c) Buck-boost, (d) uk

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Waveforms of the output voltage and current in a buck converter: solid

lines – actual converter, dashed lines – averaged model

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Comparison of Converters

• Buck converter: step-down, has one switch, simple, high efficiency greater than 90%, provides one polarity output voltage and unidirectional output current

• Boost converter: step-down, has one switch, simple, high efficiency, provides one polarity output voltage and unidirectional output current, requires a larger filter capacitor and a larger inductor than those of a buck converter

• Buck-boost converter: step-up/step-down, has one switch, simple, high efficiency, provides output voltage polarity reversal

• Cuk converter: step-up/step-down, has one switch, simple, high efficiency, provides output voltage polarity reversal, additional capacitor and inductor needed

• Full-bridge converter: four-quadrant operation, has multiple switches, can be used in regenerative braking

Conclusions• In many industrial applications, it is

required to convert fixed dc voltage into variable dc voltage

• Various types of dc-to-dc converters• Operation of dc-to-dc converters • The step-down, step-up, buck-boost and

Cuk converters are only capable of transferring energy only in one direction

• A full-bridge converter is capable of a bidirectional power flow

• Like ac transformers, dc converters can be used to step-up or step-down a dc voltage source

• Applications: electric automobiles, trolley cars, marine hoists, mine haulers, etc.

• Also used in regenerative braking of dc motors to return energy back into the supply –energy savings for transportation systems with frequent stops