Digital Control of PWM Converters

Angel V. Peterchev, Jinwen Xiao, and Jianhui Zhang
(Professor Seth R. Sanders)

Digital control has drawn increased attention to the area of pulse-width modulation (PWM) converters. Digital controllers (Figure 1) are attractive for their low power dissipation, immunity to analog component variations, compatibility with digital systems, and ability to implement sophisticated control schemes [1].

In this project, we have addressed system issues that are unique to digital control, such as the impact of the signal quantization in the feedback control loop. The quantization of the output voltage (Vout) in digital controllers can result in periodic oscillations of Vout (limit cycling) at frequencies lower than the PWM switching frequency, producing possibly undesirable output noise and electro-magnetic interference (EMI). We have developed a set of conditions necessary for the elimination of limit cycles in digital controllers [2]. This project further deals with the quantization resolution of digital controllers. In particular, we have analyzed and successfully used controlled digital dither to increase the effective resolution of digital PWM (DPWM) modules, while minimizing the dither ripple incurred on the regulated output voltage [2]. Also, we explore the use of a very low resolution analog-to-digital converter (ADC) module in the controller, its implementation, and the resulting control issues [3]. Finally, we have demonstrated an implementation of the above-mentioned techniques in a prototype digitally controlled PWM converter.

We are currently aiming to develop online power optimization techniques for a digital PWM controller. The idea is to minimize the power dissipation of the converter by dynamically adjusting parameters such as the synchronous rectification dead time [4] and the current sharing in multi-phase converters. The ability of the digital controller to implement complex computation algorithms offers a major advantage for this application.

Figure 1: Block diagram of a digitally controlled PWM converter

A. M. Wu, J. Xiao, D. Markovic, and S. R. Sanders, "Digital PWM Control: Application in Voltage Regulation Modules," Proc. IEEE Power Electronics Specialists Conf., Charleston, SC, June 1999.
A. V. Peterchev and S. R. Sanders, “Quantization Resolution and Limit Cycling in Digitally Controlled PWM Converters,” Proc. IEEE Power Electronics Specialists Conf., Vancouver, Canada, June 2001.
J. Xiao, A. V. Peterchev, and S. R. Sanders, “Architecture and IC Implementation of a Digital VRM Controller,” Proc. IEEE Power Electronics Specialists Conf., Vancouver, Canada, June 2001.
B. Acker, C. R. Sullivan, and S. R. Sanders, "Synchronous Rectification with Adaptive Timing Control," Proc. IEEE Power Electronics Specialists Conf., Atlanta, GA, June 1995.

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