TABLE OF CONTENTS After the discussion of open-loop control in Chapter 9, we will carefully discuss the closed-loop control in this chapter.
Although all existing converters applied in industrial applications are stable, they have been working in variable state (stable but shifted) because of the interferences. Particularly, some loads, such as the motor inertia in motor drive systems, are naturally unstable elements. Closed-loop control is necessary to keep converters working in steady state to satisfy the industrial requirements.
Closed-loop control is applied in most industrial applications to keep converters working in steady state to satisfy the industrial requirements. Traditionally, the proportional-plus-integral (PI) control and proportional-plus-integral-plus-differential (PID) control are very popular in closed-loop control systems.
A PI controller can be constructed by analog form using operational amplifier (OA) as shown in Figure 10.1. The input signal is v in( t), which inputs into the OA via a resistor R 0, and the OA output signal is v O( t). The feedback circuit of the OA is an R C circuit. Its transfer function in time domain is:
Its transfer function in the s-domain is:
where p is the proportional transfer gain, p = R/ R 0, and ? is the integral time constant, ? i = RC.
The transfer function in the s-domain can be written in two items as:
where p i = p
