Switching Power Supplies Week 6 1. Buck Converter 2.
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Transcript of Switching Power Supplies Week 6 1. Buck Converter 2.
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Switching Power Supplies
Week 6
2
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
Assuming a lossless circuit
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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