(已打印)A Hysteretic Control Method for Multiphase Voltage Regulator

I. INTRODUCTIONator (VR), powering the nextgeneration microprocessor, faces stringent challenges such as higher current and lower voltage demand. Furthermore, the processor performs as a fast dynamic load with a high di/dt current slew rate. In order to deal with this fast dynamic load requirement, the speed of the VR, which is determined by the control, should be higher; otherwise, a huge number of output capacitors are necessary [1], which increase the cost and the size of the motherboard. Most VR controllers are based on the linear control theory, and the speed of the VR is determined by the control bandwidth, which is limited by the switching frequency. Therefore, with this type of controller, the switching frequency should be higher to power the faster dynamic load, which reduces the system efficiency [1]. However, with the nonlinear control method, the speed of the VR is not limited by the switching frequency.
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IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 24, NO. 12, DECEMBER 2009
A Hysteretic Control Method for Multiphase Voltage Regulator
Kisun Lee, Member, IEEE, Fred C. Lee, Fellow, IEEE, and Ming Xu, Senior Member, IEEE
A faster transient response can be achieved by using a relatively low switching frequency [2]. Yet, these nonlinear control methods cannot be directly used in multiphase VR applications, because it is very difficult to achieve the other necessary functions for steady-state operation, such as adaptive voltage positioning (AVP) control, current sharing capability, and interleaving among the phases, which are easily implemented with conventional linear control methods. There are two possible approaches for this issue. One, based on the conventional linear control method for VRs, is to implement the nonlinear function during the transient [3]–[6]. The other possible approach, based on a nonlinear control method like the hysteretic control method, is to implement the necessary functions during the steady state [7]–[12]. In this paper, based on the second approach, the challenges are discussed and a possible solution is proposed. Section II describes the hysteretic control method and its limitations for use in multiphase VRs. To enable multiphase operation, Section III proposes the hysteretic control method with a bandwidth-changing loop. The analysis, model, and design of the proposed control are discussed in Section IV. In Section V, the proposed control and analysis is verified by hardware-based experiments. II. REVIEW OF HYSTERETIC CONTROL METHODS FOR VOLTAGE REGULATORS Among the nonlinear control methods, the hysteretic control method is a good candidate for future VRs because of its fast dynamic response [2]. Many people have tried to realize the hysteretic control method for the multiphase VR [7]–[10]. First, the voltage mode hysteretic control method is suggested for multiphase VR application. In this control method, using the output voltage ripple, the total phase’s duty cycles are made, and those duty cycles are distributed to each phase to achieve the interleaving function [7], [8]. With this method, the interleaving function is perfectly achieved. However, this solution is not used in VRs for several reasons. In VR applications, the output voltage ripple is very small, and it is difficult to sense without the switching noise. Additionally, the load step-up transient is not good because each phase’s duty cycle cannot be overlapped, since they are also interleaved during the transient state. Furthermore, in order to achieve current sharing, additional circuitry such as a DSP is necessary, which makes the cost higher [8]. Current-mode hysteretic control method is used for multiphase VRs [9], [10]. Instead of the output voltage ripple, the inductor current ripple is used to make the duty cycle, which reduces the switching noise. Since it uses each phase’s current information, the current-sharing function is easily implemented.
Abstract—Obtaining a faster dynamic response is a challenge for future microprocessor voltage regulators (VR). The hysteretic control method is one of the fastest control methods, but it is difficult to use in multiphase VRs because of the difficulty in achieving a constant switching frequency and interleaving operation among the phases. To resolve this, a hysteretic control method with a bandwidth-changing loop is proposed. By slowly adjusting the hysteretic bandwidth through the phase-locked loop, the duty cycle is synchronized with an external fixed-frequency clock reference. By phase-shifting this external clock reference for each phase, the interleaving among the paralleled phases can be easily implemented. On the other hand, slow hysteretic bandwidth changing will make this controller perform as though it has conventional hysteretic control during the transient for the fast dynamic response. After the transient, each phase will be pulled back to interleaving status through its bandwidth adjustment. Analysis and design guidelines for the proposed control are provided, and they are verified with hardware. Index Terms—DC–DC power conversion, power electronics, transient response.