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restmushroomsElectronics - Devices

Oct 7, 2013 (3 years and 8 months ago)

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Switched Mode Power Supplies for DC to DC Conversion


Introduction

The widespread use of battery operated portable devices such as cell phones and laptops
has
created the need for D
C to
DC

conversion power supplies such as
s
witched mode power supplies.
Acc
ording to an
Electric Power Research Institute (
EPRI
)

study

[1]
, the amount of electricity that flowed
through switched mode power supplies in 2004 was “6% of the national electric bill

.

T
he prominence of
distributed energy sources such as solar photovolt
aic systems
,

which produce DC power has created a

demand
for
DC

to DC
converters
. Th
is paper briefly reviews the technology used in swit
ched mode
power supplies for DC to
DC conversion for transferring energy from input to output.

The paper will
focus on
the

use of switched mode
DC

to DC converters

to increase or decrease the output voltage as
compared to the input
voltage
and for providing impedance matching.


Commercial Applications


DC to DC converters
have a wide range of applications in

power supplie
s for devices
with batteries

such as cell phones and laptops
, connecting PV (photovoltaic) sources to the

electricity

grid
or a battery bank, in battery conne
ctions
in

hybrid electric cars
,

and in provi
ding the desired impedance
for Maximum Power Point T
ra
cking in solar PV modules.

Plug in hybrid cars accept AC power from the
grid and ut
ilize a combination of AC to DC and DC to
DC converters

to charge a battery
. In hybrid cars
such as the RX400h by Lexus
,

a boost

(step up)

dc
-
dc converter
is used “
to incre
ase the voltage for the
inverters to
650 V DC


[2].

In
batteries, switched mode DC to DC

converters are needed for
charging at the required
potential.

Solar hybrid cars are
powered

by
solar cells
.
T
he

DC

power obtained fr
om solar cells
is

used to
charg
e

a
battery onboard

the car.
The voltage obtained from the solar panel has to be scaled to the battery
voltage

using DC to DC converters
.

Solar photovoltaic systems

require

the

implementation of Maximum
Power Point Tracking (MPPT
) algorithms

in order to extrac
t the maximum power out o
f

a solar array.

MPPT

is achieved by


the insertion of a power converter between the PV array and load, which could
dynamically change the impedance of the circuit by using a control algorithm


[
3
]
.

Switched mode
DC to DC converter
s are available from manufacturers such as
Maxim

Integrated
Products

and
include the

MAX

15046 step down converter [
4
] and the MAX 668 step up converter [
5
].

The MAX 15046 has a unit price of

$ 3.2125
upon the purchase
of 2500

units [
6
]
,

whereas the MAX 66
8
has a unit price of

$2.65
upon the purchase of

5000

units [
7
]
.


Underlying Technology


Switched mode DC to DC

converters enable the transfer of
power from input to output
,

increasing or decreasing the output voltage

and providing the required impedance t
o the input.

The b
asic
circuit topology

of a switched mode DC to DC converter
includes
an inductor
-
capacitor (
LC
)

circuit with
a switch that is externally controlled

and a diode

to determine the direction of current flow
.
Operation of
the DC to DC convert
er

is

dependent on the transient nature
of the inductor

current

caused by flipping the
switch.

The switch is controlled by a PWM (Pulse Width Modulation) signal. Depending on the
frequency and the duty cycle of the PWM
, the desired level of impedance can
be achieved and the voltage
modified.
The ratio of output

voltage

to input voltage is
determined

by the duty cycle

of the PWM

signal
.

The different

circuit topologies for switched mode DC to DC converters

are buck, boost and
a
combination of buck and boost

converter
s
.

The b
uck

converter

is

used to step down the voltage

[
8
]
.
The
b
oost

converter

is

used to step up the voltage
[
8
]
.

A single circuit can

perform both buck and boost
function
s and in the case of a battery
can operate “
as a buck converter in the b
attery charge mode and as a
boost converter when the battery must supply the load (RL) or when the load energy demand is higher
than the energy generated


by an external source such as a solar cell [3
].


Switched mode DC to DC converters are characterized
by their range of input and output
voltages
, switc
hing fre
quencies
,

and conversion efficiency
.
The

step down

MAX
15046 accepts input
voltages of 18V to 36V

and

supplies
an output voltage

of
1.2V

[
4
].
The

MAX 15046 operates on a
switching frequency of 250

kHz

and has a
power
conversion

efficiency ranging from 74% to 82%

[4]
.


Implementation of
Technology


Switched mode power supplies achieve DC to DC conversion by the use of linear circuit elements
(
such as capacitors and inductors
)

and non
-
linear element
s
(
such as diodes
and MOSFET based switches
)
.
In order to increase
the
efficiency of conversion
,

a Schottky diode or a MOSF
ET as a rectifier
(synchronous
rectifier
)

is used
instead of a regular diode
[9].
Depending on

the circuit topology, a buck
converter,

a

boost

converter
,

or a combination of the two

may be created
.

The external PWM control is
obtained from a

control unit such as a microcontroller
.

In

a

DC to DC converter for a

Maximum Power
Point Tracker, the duty cycle of the PWM is varied to achieve th
e
required impedance level.

In order to
check performance, a

control system
is used to
constantly monitor output voltage and accordingly correct
the
PWM

signal provided to the DC to DC converter
.
A

parallel system
of

DC

to DC converters can be
used

to acco
mmodate multiple
power
sources

such as different arrays of solar cells
.

The combination of
low loss components such as MOSFET rectifiers, the PWM external control signal and continuous
m
onitoring of the
output

voltage

creates an
efficient
switched mode DC

to DC converter to

modify
the
output

voltage

and provide impedance matching
.

References

[1]

Electric Power Research Institute, “DC Power Production, Delivery and Utilization :
An EPRI
White Paper” June 2006. [Online]. Available:
http://www.epri.org

[Accessed: September 4, 2010]


[2]

R. Frank, “Power Conversion,”
Auto Electronics,
November 1, 2007. [Online]. Available:
http://autoelectronics.com/powertrain/hybrid_electric_vehicles/power_conversion
-
hybrid
-
electric
-
vehicles/index.html

[Accessed September 3
,

2010]


[3]

R. Gules, J.

De Pellegrin Pacheco, H.L. Hey and

J. Imhoff , "A Maximum Powe
r Point Tracking
System With Parallel Connection for PV Stand
-
Alone Applications,"
Industrial Electronics,
IEEE Transactions
, vol.55, no.7, pp.2674
-
2683, July 2008 [Online]. Available:
http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4497230&isnumber=4550662


[4]

Maxim Integrated Products, Appl. Note 4376, November 4, 2009
.



[
5
]


Maxim Integrated Products, “MAX668, MAX669
:
1.8V to 28V Input, PWM Step
-
Up
Controllers
in µMAX” MAX668, MAX669 datasheet,
October, 2002 [Revised September 2009].


[
6
]


Digi
-
key, “Digi
-
key
MAX15046BAEE

,
” digikey.com
,

September, 2010. [Online]. Available:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=MAX15046BAEE%2BT
-
ND

[Accessed September 6, 2010].


[
7
]

Digi
-
key, “Digi
-
key MAX668EUB
-
T
-
ND

,
” digikey.com
,

September, 2010. [Online]. Available:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=MAX668EUB
-
T
-
ND&quantity=1&cref=&em1=Minum
um%20Quantity%20Not%20Met&uq=6341938683990570
08

[Accessed September 6, 2010].



[8]

G.
Dzimano
, “Modeling of Photovoltaic Systems,” M. S. thesis, Ohio State University
,
Columbus, OH, 2008


[9]


E.

Seale
, “
Power switching circuits
,


solarbotics.net
,
Janu
ary 4, 2005
. [Online]. Available:
http://www.solarbotics.net/library/circuits/misc_switching.html

[Accessed: September 5, 2010].