TABLE OF CONTENTS So far, we have discussed duty ratio control of PWM converters, in which the converter output is controlled by direct choice of the duty ratio d( t). We have therefore developed expressions and small-signal transfer functions that relate the converter waveforms and output voltage to the duty ratio.
Another control scheme, which finds wide application, is current programmed control [1 13], in which the converter output is controlled by choice of the peak transistor switch current peak( i s( t)). The control input signal is a current i c( t), and a simple control network switches the transistor on and off, such that the peak transistor current follows i c( t). The transistor duty cycle d( t) is not directly controlled, but depends on i c( t) as well as on the converter inductor currents, capacitor voltages, and power input voltage. Converters controlled via current programming are said to operate in the current programmed mode (CPM).
The block diagram of a simple current programmed controller is illustrated in Fig. 12.1. Control signal i c( t) and switch current i s( t) waveforms are given in Fig. 12.2. A clock pulse at the Set input of a latch initiates the switching period, causing the latch output Q to be high and turning on the transistor. While the transistor conducts, its current i s( t) is equal to the inductor current i
