Autotuning of Digitally Controlled Buck Converters based on Relay Feedback W. Stefanutti, P. Mattavelli DIEGM University of Udine Italy [stefanutti,mattavelli]@uniud.it S. Saggini ST Microelectronics Industrial & Power Conversion Division Cornaredo (MI)-Italy stefano.saggini@st.com M. Ghioni DEI Politecnico of Milano Italy ghioni@elet.polimi.it Abstract— This paper proposes a simple autotuning technique for digitally controlled dc-dc synchronous buck converters. The proposed approach is based on the relay feedback method and introduces perturbations on the output voltage during converter soft-start. By using an iterative procedure, the tuning of PID parameters is obtained directly by including the controller in the relay feedback and by adjusting the controller parameters based on the specified phase margin and control loop bandwidth. A nice property of the proposed solution is that output voltage perturbations are introduced while maintaining the closed-loop control of the digitally controlled converters. The proposed algorithm is simple, requires small tuning times and it is compliant with the cost/complexity constraint of integrated digital ICs. Experimental investigation has been performed using discrete components, implementing the digital control in a Field Programmable Gate Array (FPGA). Simulation and experimental results of a 1.5V – 5 A synchronous buck converter confirm the effectiveness of the proposed solution. I. INTRODUCTION Integrated digital controllers for Switch-Mode Power Supplies (SMPS) are gaining growing interest, since it has been shown the feasibility of digital controller ICs specifically developed for high-frequency switching converters [1-4]. One very interesting potential benefit is the use of autotuning of controller parameters, so that the dynamic response can be set at the software level, independently of output capacitor filters, component variations and aging. In order to be an interesting solution, however, the autotuning process should satisfy two important requirements: 1) it should not affect converter operation under nominal condition and 2) it should be based on a simple and robust algorithm whose complexity does not require a significant increase of the silicon area of the IC controller. The first issue may be handled by performing controller autotuning during converter soft-start, where there are some degree of freedom for the introduction of output voltage perturbations. The second issue is much more challenging and requires the development of “ad-hoc” autotuning techniques specifically tailored for integrated digitally-controlled converters. Several auto-tuning techniques for classical regulator structures (such as Proportional-Integral-Derivative (PID)) have been widely discussed in literature [5-9]. The great effort was motivated by the fact that PID regulators are widely accepted in industrial applications and by the fact that the applications of microcontrollers and DSPs have been rapidly increased in power electronics/drives applications, mainly in the medium/high power range. Note, however, that most of the existing solutions are too complex for small-power dc-dc converters with integrated digital controllers due to the cost/complexity constraints existing in these applications. Some results of non-parametric methods for the on-line assessment of system dynamics in dc-dc converters are discussed in [10,11]; the methods reported in [10,11] are very interesting for converter transfer functions identifications, but they requires open-loop operation during the identification process and complex signal processing. This paper proposes a simple autotuning method for voltage-mode synchronous buck converters, applied as Point of Load converters (PoLs), by using the relay feedback [5]. By means of the relay control, small oscillations on the output voltage are generated during converter soft-start. Based on the measurements of the frequency and amplitude of the oscillations, the gain and phase of the converter transfer function at the oscillation frequencies are derived. Instead of estimating the converter transfer function at different frequencies, the paper proposes an iterative procedure, where the tuning of PID parameters is obtained directly by including the controller in the relay feedback and by adjusting the controller parameters based on the specified system phase margin and loop bandwidth. The proposed algorithm is simple and it requires small tuning times. Simulation and experimental results on a synchronous buck converter confirm the effectiveness and limitations of the proposed solutions. II. BASICS OF AUTOTUNING USING RELAY FEEDBACK The basic principle of relay feedback method can be explained following Fig. 1. The converter transfer function, which is going to be controlled, denoted with G(s), is regulated firstly using a relay (see Fig. 1a). Thus, an oscillation on signal y with period T u (or angular frequency ω u ) and amplitude a is generated (see Fig. 1b). The oscillation waveform is almost sinusoidal assuming that the low-pass filter action of G(s) filters the higher harmonics generated by the relay. The condition for the oscillation to be maintained is the following 1 ) ( ) ( − = ⋅ u j G a N ω (1)