LECTURE3 Switching Regulators

8/3/2019 LECTURE3 Switching Regulators

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Switching Regulators


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INTRODUCTION

Linear regulators have one major

disadvantage: a large part of the power is

dissipated by the regulator rather than

delivered to the load. A switching regulatoroffers improvements in system efficiency

by dissipating only small amounts of

energy in the regulator. By turning thepass transistor on and off instead of

regulating it in a linear fashion, efficiency

is improved.


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Switched-Mode Power Supply

SMPS can also be classified into four types

according to the input and output

waveforms, as follows. AC in, DC out: rectifier, off-line converter

DC in, DC out: voltage converter, or

current converter, or DC to DC converter

AC in, AC out: frequency changer,

cycloconverter

DC in, AC out: inverter


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SMPS compared with linear

PSUsThere are two main types of regulated power suppliesavailable: SMPS and Linear. The reasons forchoosing one type or the other can be summarizedas follows.

Size and weight Efficiency

Heat output or power dissipation

Complexity

Radio frequency interference Electronic noise at the output terminals

Acoustic noise

Power factor

Electronic noise at the input terminals


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Switching Power Supply: Block

Diagram


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review how inductors charge and

discharge


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review how inductors charge and

discharge


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Types of SMPS

Switched-mode power supplies can be classifiedaccording to the circuit topology.

Buck converter (single inductor; output voltage is

less than the input voltage) Boost converter (single inductor; output voltage

is greater than the input voltage)

buck-boost converter (single inductor; output

voltage can be more or less than the inputvoltage)

flyback converter (uses output transformer;allows multiple outputs and input-to-output

isolation)


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BUCK CONVERTER

The operation of the buck

converter is fairly simple,

with an inductor and two

switches (usually a

transistor and a diode)

that control the inductor. It

alternates between

connecting the inductor to

source voltage to storeenergy in the inductor and

discharging the inductor

into the load.


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BUCK CONVERTER (cont)


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BUCK CONVERTER (cont) When the switch pictured above is closed (On-state), the

voltage across the inductor is VL = Vi Vo. The currentthrough the inductor rises linearly. As the diode isreverse-biased by the voltage source V, no current flowsthrough it;

When the switch is opened (off state), the diode isforward biased. The voltage across the inductor is VL = VO (neglecting diode drop). The current IL decreases.

The energy stored in inductor L is

T

herefore, it can be seen that the energy stored in Lincreases during On-time (as IL increases) and thendecrease during the Off-state. L is used to transferenergy from the input to the output of the converter.

With VL equal to Vi Vo during the On-state and to Voduring the Off-state.


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BUCK REGULATOR CIRCUIT

converterbuckatvoltageinputV

T

tcycledutyDwhere

in

on

!

!!:DVVinout

!

Also called as STEP-DOWN SWITCHING RGULATOR.


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(cont)

When VOUT attempts to decrease, the ON-time of Q1

increases.


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(cont)

When VOUT attempts to increase, the ON-time of Q1

decreases.


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BOOST CONVERTER

A boost converter (step-up

converter) is a power

converter with an output dc

voltage greater than its input

dc voltage. It is a class of(SMPS) containing at least

two semiconductor switches

and at least one energy

storage element. Filters madeof inductor and capacitor

combinations are often added

to a converters output to

improve performance.


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BOOST CONVERTER (cont)


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BOOST CONVERTER (cont)

When a boost converter operates in continuous mode,the current through the inductor (IL) never falls to zero.

The output voltage can be calculated as follow, in thecase of an ideal converter (i.e using components with an

ideal behaviour) operating in steady conditions: A. During the On-state, the switch S is closed, causing

the current in the inductor to increase at a rate given by:

B. During the Off-state, the switch S is open, so theinductor current flows through the load. If we considerzero voltage drop in the diode, and a capacitor largeenough for its voltage to remain constant, the evolutionof IL is:


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BOOST REGULATOR CIRCUIT

Also called as STEP-UP SWITCHING RGULATOR.

)1( D

V

V inout

!


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BOOST REGULATOR CIRCUIT

(cont)

When Q1 is ON


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BOOST REGULATOR

CIRCU

IT(cont)

When Q1 is OFF


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BUCK-BOOST CONVERTER


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(cont)


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If the current through the inductor L nevers falls to zeroduring a commutation cycle, the converter is said tooperate in continuous mode.

From t=0 to D.T, the converter is in On-State, so the

switchS

is closed.T

he rate of change in the inductorcurrent (IL) is therefore given by:

During the Off-state, the switch S is open, so the

inductor current flows through the load. If we assumezero voltage drop in the diode (we consider an idealdiode), and a capacitor large enough for its voltage toremain constant, the evolution of IL is:


(cont)


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BUCK-BOOST REGULATOR

CIRCUIT

)1( D

DVV

in

out

!

Also called as INVERTING SWITCHING RGULATOR.


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CIRCU

IT(cont)

When Q1 is ON, D1 is reversed-biased


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CIRCU

IT(cont)

When Q1 is OFF, D1 is forward-biased

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LECTURE3 Switching Regulators