We use cookies to give you the best experience possible. By continuing we’ll assume you’re on board with our cookie policy

COT Controlled Buck Converter Essay Sample

essay
The whole doc is available only for registered users OPEN DOC

Get Full Essay

Get access to this section to get all help you need with your essay and educational issues.

Get Access

COT Controlled Buck Converter Essay Sample

The effect of equivalent series resistance (ESR) of an output capacitor on the performance of a constant -on-time(COT)-controlled buck converter is studied and a pulse bursting phenomenon is revealed. It indicates that the ESR of the output capacitor is one of the key factors causing pulse bursting phenomenon in COT-controlled buck converters, and the critical ESR is derived via. It is pointed out that , when the ESR of the output capacitor is smaller than the critical ESR ,pulse bursting phenomenon occurs, otherwise it disappears. Simulation results are provided to verify the theoretical analysis results.

INTRODUCTION
The control techniques of switching dc-dc converters can be generally divided into the following three ways. 1) Constant –frequency control ,such as pulsewidth –modulation-based voltage-mode control and current- mode control 2) Variable-frequency control ,such as output-voltage-ripple-based control like constant-on-time (COT) control or constant-off-time control 3) Hybrid control

To improve the transient performance and to simplify the controller design of switching dc-dc converters, COT control has attracted much attention recently.COT control is simple to implement and easy to design, without an error amplifier and its corresponding compensation network.

In this paper , pulse bursting phenomenon of a COT-controlled buck converter is revealed via both time and s-domain analysis, the ESR of the output capacitor for the pulse bursting phenomenon is obtained. Simulation results are provided to verify the theoretical analysis.

PULSE BURSTING PHENOMENON
A) Review of COT Control

COT controller consisting of a comparator, an Reset-Set(RS) trigger, and an ON timer. From the wave forms, once the output voltage Vo decreases to the reference voltage Vref , the RS trigger will be triggered to turn on the power switch S to increase Vo, after the preset COT interval Ton , the RS trigger will be reset to turn off S, and it will be turned on again when Vo decreases to Vref to initiate the next switching period.

B) Pulse Bursting Phenomenon Analysis

At the end of the COT interval that is at the time instant when S is just turned off, Vo should be higher than Vref to maintain normal operation .Otherwise ,after a very short off time caused by the reset signal of RS trigger ,will follow consecutively until Vo increases to higher than Vref. In this case , pulse bursting phenomenon will occur, with large inductor current and output voltage ripples. The output voltage variation of a buck converter is composed of voltage variation across the output capacitor and its ESR resistor. The voltage variation across the ESR resistor is in proportion to the inductor current. That is the voltage across the ESR resistor will increase during the time interval when S is turned on, which helps to make the COT –controlled buck converter operate in normal operation. The voltage across the output capacitor may increase or decrease when S is turned on, depending on the relationship between the inductor current and load current. When the inductor current is lower than the load current , although S is on , the output capacitor will be discharged , which makes its voltage decrease. Thus when the output voltage variation is dominated by the output capacitor voltage variation, Vo may be lower than Vref during the time interval when S is turned on. In this case , normal operation cannot be ensured, and pulse bursting phenomenon will occur.

C) Critical ESR For Pulse Bursting Phenomenon
When the voltage VESR across the ESR resistor is large enough to dominate the output voltage variation , the COT-controlled buck converter operates normally without pulse bursting phenomenon. When VESR is not large enough to dominate the output voltage variation ,the pulse bursting phenomenon occurs . After some COT intervals ,S is turned off for a relatively long time , resulting in large inductor current and output voltage variation. Between the adjacent COT intervals there is a very short off time caused by the reset signals of the RS trigger. If the slope of – VESR is less than or equal to that of Vc_ripple at the time instant when S is just turned off, normal operation can be ensured, and no pulse bursting phenomenon occurs. Otherwise , the buck converter will operate abnormally and pulse bursting phenomenon will occur. The slopes of –VESR and Vc_ripple at the time instant when S is just turned on can be obtained as

d(-VESR)dt|t=nTs= -dRESRiL-Iodt|t=nTs
= -RESRVin-VoL
dVc_rippledt|t=nTs= iL-Io/C|t=nTs
= -∆IL2C
= -Vin-VoTON2LC
respectively , where ∆IL is the peak –to-peak value of the inductor current in steady state. Equating the above two equations , the critical ESR RESRC can be obtained as
RESRC= TON/2C
To ensure normal operation ,RESR must be greater than RESRC. Otherwise , the COT –controlled buck converter will work abnormally, and pulse bursting will occur. From the above equation ,we can further have

ℷ=RESR≥TON2
That is once TON is designed ,the time constant ℷ of the output capacitor must be designed to be greater than TON/2

COT controlled buck converter a) Circuit block diagram b) main operation wave forms

Waveforms under two cases a) with large RESRand b) with small RESR SIMULATION AND EXPERIMENTAL RESULTS

MATLAB/ SIMULINK MODEL for COT –Controlled Buck converter

Time-domain simulation results of the COT-controlled buck converter with RESR=5mΩ

Time-domain simulation results of the COT-controlled buck converter with RESR=2Ω

Variable| Signification| Value|
Vin| Input voltage| 15V|
VO| Output voltage| 5V|
L| Inductane| 66.7mH|
C| Capacitance| 2.498mF|
R| Load resistance| 5Ω|
Ton| Constant on-time| 6.67ms|

PARAMETERS OF THE COT- CONTROLLED BUCK CONVERTER

ANALYSIS AND DESIGN
Using Vin=15V and VO=5V
Vo =kVS ,From this equation k=1/3 ,ie TONT=1/3
Here T is taken as 20ms ,then TON=6.67ms
Using the formula, C=Vsk1-k8∆VcLf2 C=2.498Mf
L=Vsk1-k∆IfL=66.7mH
RESRC= TON/2C RESRC=1.33Ω

Here the simulation results are shown with RESR=5mΩ and 2Ω One of the equivalent resistance greater than critical value and the other is less than RESRC Limitation: Here the MATLAB/SIMULINK Model does not contain flip flop , comparator, reference voltage as shown in the circuit block diagram, because of the connection difficulties in MATLAB. So the pulse bursting is not completely visible in the simulated wave forms. Based on the reference paper the simulated waveform is

a) With 5mΩb) With 20mΩ
CONCLUSION
In this paper, a pulse bursting phenomenon ,which is a complex nonlinear phenomenon ,in a COT –controlled buck converter has been revealed . It is indicated that the ESR of the output capacitor has great effect on the control performance of the COT-controlled buck converter .The investigation results presented in this paper provide a useful guideline for the design of COT-controlled buck converters,eg: ceramic capacitors are usually not appropriate as output filter capacitor they have very small ESR.

REFERENCES
1] E.Figueres ,G.Garcera,J.M.Benavent,M.Pascual,and J.A.Martinez,”Adaptive two- loop voltage mode control of dc-dc switching converters,”IEEE Trans.Ind.Electron.,vol.53,no.1,pp.239-253,Feb.2006. 2]Y-S Lai and C-A Yeh,” Predictive digital- controlled converter with peak-current-mode control and leading edge modulation,” IEEE Trans.Ind.Electron.,vol.56,no.6,pp.1854-1863,jun 2009. 3]Analysis of pulse bursting phenomenon in COT controlled buck converter, IEEE Trans.Ind.Electron,vol58 ,No 12,.dec 2011

We can write a custom essay

According to Your Specific Requirements

Order an essay

You May Also Find These Documents Helpful

American Mosquito Control Association

The American Mosquito Control Association states that there are over 3,000 known species of mosquitoes in the world. The most common and could be health threatening includes the Culex, Anopheles, and Aedes genera. (Mosquito World, 2017) According to an article by Orkin, mosquito eggs hatch into larvae within 24 to 48 hours. Their growth rate is naturally dependent on other factors such as temperature, moisture,...

Amendment of the United States Constitution

The Second Amendment of the United States Constitution states the rights to give citizens of America the right to bear arms. This amendment was adopted with the rest of the other amendments, in the Bill of Rights. With that being said, there are many people out there that takes that amendment literally, and feel as if they should be able to own and have any...

Reasons Why Buyers Now Control the Sale

The focused powers that face the J Crew are solid however they are not equipped for curbing the endeavors of the organization. In light of the delineation of the watchmen five model, the J Crew business would be liable to the member\'s forces. The provisions control is the advancement of the costs to go higher because of the impact of the providers. In connection to...

The modern management theory

Management is the control and organizing of an organization. It is a very dynamic discipline that always changes. People have been trying to develop methods to allay managers and many of those ideas and concepts come from the early management theories. Despite that though the role of the manager has changed a lot through the years and it is not the same as it was...

Components of Internal Controls

The components of internal controls include the following aspects: control activities, risk assessment, information and communication, control environment and monitoring activities. Control Environment The control environment is a bunch of forms that give the premise for carrying out inner control in the substance. It is the state of mind of the executives and representatives in the firm towards the inside controls. A great control environment...

Get Access To The Full Essay
icon
300+
Materials Daily
icon
100,000+ Subjects
2000+ Topics
icon
Free Plagiarism
Checker
icon
All Materials
are Cataloged Well

Sorry, but copying text is forbidden on this website. If you need this or any other sample, we can send it to you via email.

By clicking "SEND", you agree to our terms of service and privacy policy. We'll occasionally send you account related and promo emails.

Your Answer Is Very Helpful For Us
Thank You A Lot!

logo

Emma Taylor

online

Hi there!
Would you like to get such a paper?
How about getting a customized one?

Couldn't Find What You Looking For?

Get access to our huge knowledge base which is continuously updated

Next Update Will Be About:
14 : 59 : 59
Become a Member