Fundamental Concepts of Switch Mode Power Supply (SMPS) Control Theory
Introduction
Every Switch Mode Power Supply (SMPS) is a feedback system and care must be used in designing a stable system. By definition, a stable system is a system that is always able to reach a steady state condition independently of the input and output disturbances (load change and input-voltage change).
SMPS: Analog System (Buck Converter)
Figure 1 demonstrates an analog implementation of a buck converter. It is easy to locate the power stage which includes the switches, the freewheeling diode, the inductor, and the output capacitors. The compensator is the network that reads the output voltage (op amp, resistors, and capacitors). The Pulse Width Modulation (PWM) generator consists of a number of building blocks that are needed to implement a PWM generator in analog (sawtooth generator, comparator, flip-flops).
Figure 1
SMPS: Forward and Feedback Paths
The forward and feedback paths of a power converter are the basic elements of SMPS control theory. It is quite easy to identify the forward path of a buck converter and the feedback path that closes the loop (see Figure 2). Feedback systems are critical considerations when designing a stable power converter system. In the next module, we'll focus on a type of power converter that utilizes digital feedback control loops.
Figure 2
SMPS: Digital Control
dsPIC® Digital Signal Controller (DSC) offers full Digital Signal Processing (DSP) capability. Since the digital control approach operates on the equations and numbers that represent the most up-to-date values of the controlled variables, real-time data, such as current and voltage in the power converter control loop, need to be measured periodically. The DSP features of dsPIC33F allow efficient control of the power converter system. Figure 3 shows the additional blocks of the DSP engine in the power converter design that uses digital control.
Figure 3
