International Journal of Applied Engineering Research, ISSN 0973-4562 Vol.7 No.11 (2012)
© Research India Publications;
http://www.ripublication.com/ijaer.htm
“Comparison between Asynchronous and Synchronous Buck Converter Topology”
# Er.Vishal Mehta
## Er.Pradeep Malik
Assistant Professor
Surya School of Engineering and Technology.
Panjab Technical university, Patiala, India
vishal.2jan@gmail.com
Abstract:
SMPSs are widely used in DC/DC conversions, where the
input is a DC. voltage that can be, for example, a battery or
fuel cell voltage. In such power conversions, DC/DC
converters operate at relatively high switching frequencies,
and this enables the use of small inductive components which
improve the dynamic behaviour and reduce the size of the
converter.This paper basically deals with the comparison of
synchronous and Asynchronous Buck topology for mobile
applications.In this paper power stage parameters are
calculated and we use orcad capture for design and simulation
is done in p spice/cadence with the help of Analog behavioral
Modeling.
Keywords: Dc-Dc Converter, Buck converter, MOSFET
INTRODUCTION
There are many different types of DC-DC converter, each of
which tends to be more suitable for some types of application
than for others. For convenience they can be classified into
various groups, however. For example some converters are
only suitable for stepping down the voltage, while others are
only suitable for stepping it up; a third group can be used for
either. Another important distinction is between converters
which offer full dielectric isolation between their input and
output circuits, and those which don’t. Needless to say this can
be very important for some applications, although it may not
be important in many others.[4][5]
A. Non-isolating converters
The non-isolating type of converter is generally used where
the voltage needs to be stepped up or down by a relatively
small ratio (say less than 4:1), and there is no problem with the
the output and input having no dielectric isolation. Examples
are 24V/12V voltage reducers, 5V/3V reducers and 1.5V/5V
step-up converters. There are five main types of converter in
this non-isolating group, usually called the buck, boost, buckboost, Cuk and charge-pump converters. The buck converter
is used for voltage step-down/reduction, while the boost
converter is used for voltage step-up. The buck-boost and Cuk
converters can be used for either step-down or step-up, but are
essentially voltage polarity reversers or .inverters. as well.
(The Cuk converter is named after its originator, Slobodan
Cuk of Cal Tech university in California.)The charge-pump
converter is used for either voltage step-up or voltage
inversion, but only in relatively low power applications.[2]
B. Buck Converter-Step Down Converter
The most common power converter topology is the buck
power converter, sometimes called a step down power
converter. Power supply designers choose the buck power
converter because the output voltage is always less than the
input voltage in the same polarity and is not isolated from the
input. The buck regulator circuit is a switching regulator. It
uses an inductor and a capacitor as energy storage elements so
that energy can be transferred from the input to the output in
discrete packets. The advantage of using switching regulators
is that they offer higher efficiency than linear regulators. The
one disadvantage is noise or ripple, the ripple will need to be
minimized through careful component selection.[6]
TOPOLOGY DESIGN
Buck Converter with Voltage mode control is shown in
Fig.1This Design process Comprises the power stage
,compensator and pulse width Modulator and we assume
converter is working in continuous conduction mode.[1]
A. Synchronous Buck Topology-
Fig.1.Synchronous buck Topology
A requirement of the design is to have high current slew rate
to increase switching speed of microprocessor from one state
to the other but this causes voltage drop spikes at the processor
power supply. To achieve high current slew rate the inductor
Lo should be as small as possible. This in turn while achieving
faster transient response will cause the output voltage ripple to
increase. The input current for a buck power converter is
discontinuous due to the power switch, the current pulses from
0 to max every switching cycle. The output current for a buck
power converter is continuous because the output current is
supplied by the output inductor/capacitor combination; the
output capacitor never supplies the entire load current for
continuous inductor current mode operation.
International Journal of Applied Engineering Research, ISSN 0973-4562 Vol.7 No.11 (2012)
© Research India Publications;
http://www.ripublication.com/ijaer.htm
B. Asynchronous Topology-
Fig2.Circuit Diagram[2]
ExplanationIt is the desire of all designers of power supplies, whether they
are switching or not, for accurate and tight regulation of the
output voltage(s). To accomplish regulation we need to add a
feedback loop. The feedback loop can cause an otherwise
stable system to become unstable. Even though the transfer
function of the original converter might not contain any right
hand poles but after feedback it is possible that right hand
poles may be introduced. Also we need to introduce a high DC
gain. But with high gain again comes the possibility of
instability.[7]
C. Design Equations Of Simple Buck Converter1. Duty Ratio
DESIGN EXAMPLE
Here we discuss an example of buck topology, So for
microprocessor and mobile loads, some of the parameters are
predefined for synchronous Buck converter.
D=v (in)/v(out)
2. Inductor SelectionL =[Vin-Vout].D/ fsw] / Iripple
2.1 Power LossPcu=(I load)2 *Esr
Parameter
Magnitude
Input V
Output V
L
C
Resr
Rdcr
fsw
3v
1.5v
10uH
5uF
5mOhm
2mOhm
330khz
3. Capacitor SelectionTable-1.Parameters for Buck
∆V=∆I.[ESR+∆T/C+ESL/∆T]
*Assume the Converter is working in CCM mode
3.1 Power Loss
P Ripple =[Iripple]2*ESR
A. P-Spice Equivalent of Simple synchronous BuckL1
R1
M1
IRF151
10u
5m
4. MosfetV2
1
C1
2
4.1 P conduction-(ID) *Rds(Hot)*D
4.2 Pswitching=[∆V.ID/2].(Ton+Toff).Fsw+coss.V2..Fsw
5. Switching FrequencyFor the On-state:
For the Off-state:
D Design Process: This flowchart basically represent the
basic process which is to simulate the circuit
V1
3
V
M2
IRF151
TD = 0
TF = 100n
PW = 1.54u
PER = 2.857u
V1 = 0
TR = 100n
V2 = 5v
V3
2
0
TD = 0
PW = 1.314u
TF = 100n
PER = 2.857u
V1 = 5v
TR = 100n
V2 = 0v
0
Fig 3.Simple Pspice Synchronous equivalent
5u
r
2
International Journal of Applied Engineering Research, ISSN 0973-4562 Vol.7 No.11 (2012)
© Research India Publications;
http://www.ripublication.com/ijaer.htm
A.1 P-Spice Equivalent of Synchronous Buck with
compensatorThis Diagram basically represent Synchronous buck topology
with LM111 IC used as PWM modulator and concept of ETable is used for calculation of difference between o/p voltage
and reference voltage.IRF 150 are used as power and
synchronous switch..
L1
M1
IRF150
R2
5m
10uH
V2
I
0Vdc
R3
2m
M2
IRF150
3.3v
I
I1 = 100mA I1
I2 = 300mA
TD = 8m
R1 TR = 0.1u
10 TF = 0.1u
PW = 0.5m
PER = 1m
V1
c
4.7uf
0
+
-
E1
E
0
GAIN = 3
4
1
7
6
0
V6
15
V-
G
-
U2
LM111
3
OUT
B/S
5 8
+
B V+
V5
2
0
0
V4
TD = 0
TF = 10nE2
ETABLE
PW = 10n
V3
1.65v
R5
C1
487
1n
R4
+NI +TUO
-NI -TUO
+
-
0
V7
V1 = 0
V2 = 10
TD = 0
TR = 10n
TF = 10n
PW = 1.3
PER = 2.857u
PER = 10u
V1 = 0
V(%IN+, %IN-)
TR = 10u-20n
V2 = 3v
R7
15k
R6
165k
C2
7.87k
Fig6. Output Voltage
B.3 Inductor RipplesThis diagram represent the ripples across inductor which stor
energy as ½ LI².
3.2nF
15
C3
0
120pf
0
Fig 4 Pspice equivalent of Synchronous buck
B. I/P and O/P Waveforms- Here are the simulation
diagrams which are observed with the predefined
parameters as shown in Table1.These simulations are
carried out with run time of1000us and Transient sweep
analysis is observed with no initial values saved.
B.1 Input- 3v
This is input Voltage which is to be converted into 1.5 V
Fig7. Output ripples
B.7 Comparison between Asynchronous and Synchronous
topology.
Property
Asynchronous Synchronous
Peak rush-in Voltage
2.9 v
2.1v
1.2A
0.8A
Peak Rush-in Current
Steady state Time
O/p Ripple
50us
60us
Synchronous
voltage
Fig5. Input Voltage
B.2 Output Voltage- This is the output of Buck
converter which is 1.5 v used for mobile and
microprocessor loads.
switch
1%(p-p)
2%(p-p)
Rds
0.5 v
0.7 v
0.02 ohm
Table2.
0.004 ohm
International Journal of Applied Engineering Research, ISSN 0973-4562 Vol.7 No.11 (2012)
© Research India Publications;
http://www.ripublication.com/ijaer.htm
CONCLUSION
This paper has presented the various parameters which must
analytically specified before designing. This paper also
describe that how to design a synchronous buck converter and
by adding the compensation loop system becomes stable. This
paper present comparison between synchronous and
Asynchronous Topology with input voltage of 3.3 v
ACKNOWLEDGMENT
I Would like to thanks UIET, Panjab University for providing
P-spice tools and support.
REFERENCES
[1]
Ned Mohan, Tore M. Undeland, William P. Robbins,
“Power Electronics: Converters, Applications, and
Design”, 3rd Edition, Wiley
[2]
B. J. Baliga, “Modern Power Devices”, New York:
Wiley, 1987.
[3]
Chin Chang, “Robust Control of DC-DC Converters:
The Buck Converter”, Power Electronics Specialists
Conference, 1995. 26th Annual IEEE Volume 2, Issue
, 18-22 Jun 1995 Page(s):1094 - 1097 vol.2
[4]
Mika Sippola and Raimo Sepponen, “DC/DC onverter
technology for distributed telecom and microprocessor
power systems – a literature review”, Helsinki
University of Technology Applied Electronics
Laboratory, Series E: Electronic Publications E 3,
2002
[5]
Chang, C., “Mixed Voltage/Current Mode Control of
PWM Synchronous Buck Converter”, Power
Electronics and Motion Control Conference, 2004.
IPEMC 2004. The 4th International, Publication Date:
14-16 Aug. 2004, Volume: 3, On page(s): 1136- 1139
Vol.3
[6]
[7]
http://powerelectronics.com
http://powerelectronics.com/mag/power_buck
converter losses/index.html
AUTHOR PROFILEVishal Mehta received his B.Tech (Hons.)
degree in ECE from MMEC College,
Kurukshetra University in 2008 and
completed
his
M.Tech.
degree
in
Microelectronics from Panjab University
,Chandigarh (2010-2012),. He was a,
lecturer, with Department of ECE in College
affiliated with Punjab Technical University in (20082009).Now, He is working as Assistant professor (ECE) in
Surya school of Engineering and Technology, Patiala.His
research interests include VLSI design ,Control System, Chip
fabrication, Embedded system, Microelectronic Packaging and
Digital circuit design.