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Improved Models for DC-DC Converters

Johansson, Bengt LU (2003)
Abstract
To obtain high performance control of a dc-dc converter, a good model

of the converter is needed. It is suitable to consider the load to be included in

the converter model since it usually affects the dynamics. The load is often

the most variable part of this system. If the load current and the output

voltage are measured there are good possibilities to obtain a good model of

the load on-line. Adaptive control can then be applied to improve the control.

In peak current-mode control, the output voltage and the inductor

current are measured and utilized by the controller. This thesis analyses some

properties that can be obtained if the load current is also measured... (More)
To obtain high performance control of a dc-dc converter, a good model

of the converter is needed. It is suitable to consider the load to be included in

the converter model since it usually affects the dynamics. The load is often

the most variable part of this system. If the load current and the output

voltage are measured there are good possibilities to obtain a good model of

the load on-line. Adaptive control can then be applied to improve the control.

In peak current-mode control, the output voltage and the inductor

current are measured and utilized by the controller. This thesis analyses some

properties that can be obtained if the load current is also measured and

utilized for control. Accurate expressions for the control-to-output transfer

function, the output impedance, and the audio susceptibility are derived for

the buck, boost, and buck-boost converters operated in continuous

conduction mode in the case where the load is a linear resistor. If the

measured load current is utilized properly by the controller, the output

impedance becomes low and the control-to-output transfer function becomes

almost invariant for different loads. The use of load current acts as a

feedforward term if the load is a current source. However, if the load is a

resistor the load current is influenced by changes in the output voltage and

the stability is affected. Therefore, the use of load current is not a feedforward

action in this case. Instead it can be seen as gain scheduling, which can be

considered a special case of adaptive control.

In the thesis it is also shown that the two published models for currentmode

control, Ridley (1991) and Tan and Middlebrook (1995), give accurate

expressions for the control-to-output transfer function and the output

impedance but not for the audio susceptibility. A novel model for the audio

susceptibility is presented and it is used to improve the two published models.

Most of the results in the thesis are validated by comparing the frequency

responses predicted by the expressions and switched large-signal simulation

models. (Less)
Please use this url to cite or link to this publication:
author
supervisor
organization
publishing date
type
Thesis
publication status
published
subject
pages
353 pages
publisher
Department of Industrial Electrical Engineering and Automation, Lund Institute of Technology
ISBN
91-88934-29-2
language
English
LU publication?
yes
id
68a19aa2-2b22-499f-8d21-910c3bcaf81a (old id 587910)
alternative location
http://www.iea.lth.se/publications/Theses/LTH-IEA-1037.pdf
date added to LUP
2016-04-04 11:11:58
date last changed
2018-11-21 21:03:17
@misc{68a19aa2-2b22-499f-8d21-910c3bcaf81a,
  abstract     = {{To obtain high performance control of a dc-dc converter, a good model<br/><br>
of the converter is needed. It is suitable to consider the load to be included in<br/><br>
the converter model since it usually affects the dynamics. The load is often<br/><br>
the most variable part of this system. If the load current and the output<br/><br>
voltage are measured there are good possibilities to obtain a good model of<br/><br>
the load on-line. Adaptive control can then be applied to improve the control.<br/><br>
In peak current-mode control, the output voltage and the inductor<br/><br>
current are measured and utilized by the controller. This thesis analyses some<br/><br>
properties that can be obtained if the load current is also measured and<br/><br>
utilized for control. Accurate expressions for the control-to-output transfer<br/><br>
function, the output impedance, and the audio susceptibility are derived for<br/><br>
the buck, boost, and buck-boost converters operated in continuous<br/><br>
conduction mode in the case where the load is a linear resistor. If the<br/><br>
measured load current is utilized properly by the controller, the output<br/><br>
impedance becomes low and the control-to-output transfer function becomes<br/><br>
almost invariant for different loads. The use of load current acts as a<br/><br>
feedforward term if the load is a current source. However, if the load is a<br/><br>
resistor the load current is influenced by changes in the output voltage and<br/><br>
the stability is affected. Therefore, the use of load current is not a feedforward<br/><br>
action in this case. Instead it can be seen as gain scheduling, which can be<br/><br>
considered a special case of adaptive control.<br/><br>
In the thesis it is also shown that the two published models for currentmode<br/><br>
control, Ridley (1991) and Tan and Middlebrook (1995), give accurate<br/><br>
expressions for the control-to-output transfer function and the output<br/><br>
impedance but not for the audio susceptibility. A novel model for the audio<br/><br>
susceptibility is presented and it is used to improve the two published models.<br/><br>
Most of the results in the thesis are validated by comparing the frequency<br/><br>
responses predicted by the expressions and switched large-signal simulation<br/><br>
models.}},
  author       = {{Johansson, Bengt}},
  isbn         = {{91-88934-29-2}},
  language     = {{eng}},
  note         = {{Licentiate Thesis}},
  publisher    = {{Department of Industrial Electrical Engineering and Automation, Lund Institute of Technology}},
  title        = {{Improved Models for DC-DC Converters}},
  url          = {{https://lup.lub.lu.se/search/files/5717458/587911.pdf}},
  year         = {{2003}},
}