PWM-based sliding mode control schemes for DC/DC power converters

Abstract

Pulse-width modulated (PWM) DC/DC power converters operating at a high switching frequency are a class of systems having cyclically varying structures. Such systems are inherently nonlinear as the control function involves varying the relative durations of the constituent structures [1]. Existing design practice employs predominantly voltage mode and current-mode control strategies that are based on linear small-signal techniques, and the resulting performances in terms of output regulation, transient response, and stability are therefore adequate only if the converters operate within a narrow range of parameter variation [2]. For applications involving power sources and loads that are nonlinear and more widely varied, the use of linear controllers is expected to produce sub-optimal control performances and may even fail to meet the desired specifications when the condition of operation deviates significantly from the usual small-signal condition [3]. Sliding mode (SM) control is a nonlinear control method that is particularly well suited for DC/DC power converters working with power sources and loads that vary widely and nonlinearly [4], [5]. The inherent operation of SM control is a natural strategy for controlling systems having a discontinuous switching characteristic. When applied to DC/DC power converters, highly robust and versatile SM systems that give fast and consistently stable control performance will be resulted [6], [7]. Furthermore, among all available nonlinear control techniques, the SM control is arguably by far most practical for power converters, due to its simple implementation, ease of design, and low cost. In particular, fixed-frequency PWM-based SM controllers, which are quasisliding mode (QSM) controllers that work with close operational resemblance to ideal SM controllers but without the complexity of using variable switching frequency, are found to be most suited for practical applications [7]. Specifically, operating power converters at a constant frequency achieves simpler filter design and keeps the power converter size small. Furthermore, fixed-frequency PWM-based SM controllers can be easily implemented in either digital form by using low-end inexpensive microcontrollers (MCU), digital signal processors (DSP) or field-programmable gate arrays (FPGA), or in analogue form using a few simple discrete components. Nevertheless, the implementation of SM control in analogue form does present a few advantages over the digital form in terms of its ability to achieve a faster transient response especially for converters switch at very high frequency and the absence of issues related to discretization. For commercial applications, the PWM-based SM controllers can be fabricated as analogue integrated circuit (IC) controller chips at a very competitive cost relative to existing PWM controllers. We will begin this chapter with a brief account of the principle of SM control and PWM-based SM control for the DC/DC converters. The key idea of connecting the duty ratio control in conventional PWM control with an equivalent SM control will be covered. Then, the derivation of PWM-based SM control schemes for DC/DC converters will be illustrated. The mathematics behind the modelling and the inherent control features will be discussed. A detailed exposition of how such kind of controllers can be practically engineered to suit specific purposes in controlling power converters will be provided. The practical aspects of the implementation are also described.