Chapter 2 - Control Circuits

Abstract

This chapter describes various forms of controllers available from semiconductor manufacturers. The simplest form of control circuit is variable frequency/constant on-time or Pulse Frequency Modulation (PFM). Pulse width modulation (PWM) uses a constant frequency and varies the on-time of the switch. The chapter presents a graphical representation of the basics of a voltage mode PWM controller. The voltage divider is used with the error amplifier and reference voltage to generate a scaled error signal. The typical methods of setting up the error amplifier to control the output for a positive output supply and for a negative output supply is also presented. The error amplifier in PWM controllers usually has no internal compensation. PWM controllers bring the output of the error amplifier out to a pin so that poles and zeros can be added to the closed loop system to provide frequency compensation to the system. The chapter also presents the basic circuit of a current mode PWM controller in a boost converter. This circuit has two control loops. The outer loop measures the output voltage and provides an error signal to the inner loop. The inner loop compares the error signal and an analog of the inductor current to decide when to turn off the switch.