New Technologies to Improve the Transient Response of Buck Converters

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Meyer, Eric David
Power Electronics , DC/DC Conversion , Non-Linear Control , Digital Control
As the speed and power demands on Buck converters continue to increase, it has become time to replace the linearly-controlled conventional Buck converter. Digital circuits, such as microprocessors, are requiring higher dynamic currents, at lower voltages, than ever before. Traditionally, such Buck converters have been controlled by linear voltage-mode or current-mode control methods. While these controllers offer such advantages as fixed switching frequencies and zero steady-state error, their reaction speed is inherently limited by their bandwidth which is a fraction of the converter switching frequency. Therefore, to improve the transient response of a Buck converter in a practical manner, four novel ideas are presented in this thesis. The first contribution is an analog “charge balance controller”. The control method utilizes the concept of capacitor charge balance to achieve a near-optimal transient response for Buck converters undergoing a rapid load change. Unlike previous work, the proposed controller does not require expensive and/or slow analog multipliers/dividers. In addition, the nominal inductance value is not required by the proposed controller. Simulation and experimental results demonstrate a significant improvement in transient performance over that of a linear voltage-mode controller. For low duty cycle applications, the unloading transient performance of a Buck converter tends to be poor when compared to the corresponding loading transient performance. Therefore, the second contribution is an auxiliary circuit and an analog auxiliary controller which drastically improves the performance of a Buck converter undergoing an unloading transient. Significant overshoot reduction was observed over that of a linearly-controlled conventional Buck converter. The third contribution is a digital implementation of the aforementioned “charge balance control” concept. Through digital implementation the control law is extended to include load-line regulation. Unlike previous work, large lookup tables are not required to perform complex mathematical functions, thus the number of required gates is significantly reduced. The final contribution is a digital implementation of the “charge balance controller” capable of operating with the previously-mentioned auxiliary circuit. This complete solution is capable of improving the voltage deviation caused by loading and unloading transients. In addition, the combined auxiliary circuit and control law is extended to load-line regulation applications.
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