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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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BUCK REGULATOR CIRCUIT
(cont)
When VOUT attempts to decrease, the ON-time of Q1
increases.
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BUCK REGULATOR CIRCUIT
(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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BUCK-BOOST CONVERTER
(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:
BUCK-BOOST CONVERTER
(cont)
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BUCK-BOOST REGULATOR
CIRCUIT
)1( D
DVV
in
out
!
Also called as INVERTING SWITCHING RGULATOR.
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BUCK-BOOST REGULATOR
CIRCU
IT(cont)
When Q1 is ON, D1 is reversed-biased
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BUCK-BOOST REGULATOR
CIRCU
IT(cont)
When Q1 is OFF, D1 is forward-biased