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 email@example.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. 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. 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. 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. 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 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. 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  Ned Mohan, Tore M. Undeland, William P. Robbins, “Power Electronics: Converters, Applications, and Design”, 3rd Edition, Wiley  B. J. Baliga, “Modern Power Devices”, New York: Wiley, 1987.  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  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  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   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.