buck converter

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INTRODUCTION INTRODUCTION

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Page 1: buck converter

INTRODUCTIONINTRODUCTION

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BUCK BUCK CONVERTERCONVERTER

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What is Buck Converter

Image of a prototype buck converter

A buck converter is a step-down DC to DC converter. Its design is similar to the step-up boost converter, and like the boost converter it is a switched-mode power supply that uses two switches (a transistor and a diode) and an inductor and a capacitor.

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The simplest way to reduce a DC voltage is to use a voltage divider circuit, but voltage

dividers waste energy, since they operate by bleeding off excess voltage as heat; also, output voltage isn't regulated (varies with

input voltage). A buck converter, on the other hand, can be remarkably efficient (easily up to

95% for integrated circuits) and self-regulating, making it useful for tasks such as converting the 12-24V typical battery voltage in a laptop down to the few volts needed by

the processor.

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CIRCUIT OPERATION

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The two circuit configurations of a Buck converter: On state, when the switch is closed, and Off-state, when the switch is open.

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The operation of the buck converter is The operation of the buck converter is fairly simple, with an fairly simple, with an inductor and two and two switches (usually a switches (usually a transistor and a and a

diode) that control the inductor. It ) that control the inductor. It alternates between connecting the alternates between connecting the inductor to source voltage to store inductor to source voltage to store

energy in the inductor and discharging energy in the inductor and discharging the inductor into the load.the inductor into the load.

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Naming conventions of the components, voltages and current of the Buck converter.

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MODES OF BUCK MODES OF BUCK CONVERTERCONVERTER

1.1. CONTINUOUS MODECONTINUOUS MODE

2.2. DISCONTINUOUS MODEDISCONTINUOUS MODE

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1.CONTINUOUS MODE1.CONTINUOUS MODE A Buck converter operates in A Buck converter operates in continuous mode if the current continuous mode if the current

through the inductor (IL) never falls through the inductor (IL) never falls to zero during the commutation to zero during the commutation

cycle. In this mode, the operating cycle. In this mode, the operating principle is described by the principle is described by the

chronogram in figure.chronogram in figure.

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Evolution of the voltages and currents with time in an Evolution of the voltages and currents with time in an ideal Buck converter operating in continuous mode. ideal Buck converter operating in continuous mode.

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2.DISCONTIONUOUS 2.DISCONTIONUOUS MODEMODE

In some cases, the amount of energy required In some cases, the amount of energy required by the load is small enough to be transferred in by the load is small enough to be transferred in

a time lower than the whole commutation a time lower than the whole commutation period. In this case, the current through the period. In this case, the current through the

inductor falls to zero during part of the period. inductor falls to zero during part of the period. The only difference in the principle described The only difference in the principle described

above is that the inductor is completely above is that the inductor is completely discharged at the end of the commutation cycle discharged at the end of the commutation cycle (see figure 5). This has, however, some effect (see figure 5). This has, however, some effect

on the previous equations.on the previous equations.

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Evolution of the voltages and currents with time in an Evolution of the voltages and currents with time in an ideal Buck converter operating in discontinuous mode.ideal Buck converter operating in discontinuous mode.

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Evolution of the Normalized output voltages with the normalized Evolution of the Normalized output voltages with the normalized output current. From discontinuous to continuous mode (and vice output current. From discontinuous to continuous mode (and vice

versa) versa)

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Evolution of the output voltage of a buck converter with the duty cycle Evolution of the output voltage of a buck converter with the duty cycle when the parasitic resistance of the inductor increases. when the parasitic resistance of the inductor increases. Non ideal circuitNon ideal circuit

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EFFECT OF NON-IDEALITY EFFECT OF NON-IDEALITY ON EFFICIENCYON EFFICIENCY

A simplified analysis of the buck converter, as described A simplified analysis of the buck converter, as described above, does not account for non-idealities of the circuit above, does not account for non-idealities of the circuit components nor does it account for the required control components nor does it account for the required control

circuitry. Power losses due to the control circuitry is circuitry. Power losses due to the control circuitry is usually insignificant when compared with the losses in usually insignificant when compared with the losses in

the power devices (switches, diodes, inductors, etc.) The the power devices (switches, diodes, inductors, etc.) The non-idealities of the power devices account for the bulk non-idealities of the power devices account for the bulk

of the power losses in the converter.of the power losses in the converter. Both static and dynamic power losses occur in any Both static and dynamic power losses occur in any

switching regulator. Static power losses include switching regulator. Static power losses include II22RR (conduction) losses in the wires or PCB traces, as well as (conduction) losses in the wires or PCB traces, as well as

in the switches and inductor, as in any electrical circuit. in the switches and inductor, as in any electrical circuit. Dynamic power losses occur as a result of switching, Dynamic power losses occur as a result of switching,

such as the charging and discharging of the switch gate, such as the charging and discharging of the switch gate, and are proportional to the switching frequency.and are proportional to the switching frequency.

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It is useful to begin by calculating the duty cycle for a non-ideal buck converter,

which is:

where: VSWITCH is the voltage drop on the

power switch, VSYNCHSW is the voltage drop on the synchronous switch or diode,

and VL is the voltage drop on the inductor.

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The voltage drops described above are all static power losses which are dependent primarily on DC current, and can therefore be easily calculated. For

a transistor in saturation or a diode drop, VSWITCH and VSYNCHSW may already be known, based on the properties of the selected device.

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SYNCHRONOUS SYNCHRONOUS RECTIFICATIONRECTIFICATION

Simplified schematic of a synchronous converter, in which D is replaced by a second switch, S2

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A synchronous buck converter is a modified version of the basic buck

converter circuit topology in which the diode, D, is replaced by a second switch, S2. This modification is a

tradeoff between increased cost and improved efficiency.

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MULTIPHASE BUCKMULTIPHASE BUCK

Schematic of a generic synchronous n-phase buck converter.

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CONCLUSIONCONCLUSION

Finally, the current can be measured at Finally, the current can be measured at the input. Voltage can be measured the input. Voltage can be measured losslessly, across the upper switch, or losslessly, across the upper switch, or using a power resistor, to approximate using a power resistor, to approximate the current being drawn. This approach the current being drawn. This approach is technically more challenging, since is technically more challenging, since switching noise cannot be easily filtered switching noise cannot be easily filtered out. However, it is less expensive than out. However, it is less expensive than emplacing a sense resistor for each emplacing a sense resistor for each phasephase

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