Digital Current Mode Control

In this chapter we begin the discussion of digital control techniques for switching power converters. In the previous chapter, we have introduced the topology and operation of the half-bridge VSI and designed an analog PI current controller for this switching converter. Referring to that discussion, the first part of this chapter is dedicated to the derivation of a digital PI current controller resembling, as closely as possible, its analog counterpart. We will see how, by using proper discretization techniques, the continuous time design can be turned into a discrete time design, preserving, as much as possible, the closed loop properties of the former. It is important to underline from the beginning that the continuous time design followed by some discretization procedure is not the only design strategy we can adopt. Discrete time design is also possible, although its application is somewhat less common: as we will explain, its typical implementations rely on the use of state feedback and pole placement techniques. The second part of the chapter will describe in detail a remarkable example of discrete time design and, in doing so, it will also show how the synthesis of regulators that have no analog counterpart whatsoever can be implemented. This is the case of the predictive or dead-beat current controller.