Figure 5.1 shows a circuit diagram of a buck converter with a fixed switching frequency voltage-mode PWM controller. The feedback
circuit consists of an error amplifier, a PWM generator, a sawtooth wave generator and a reference voltage. Consider the small-signal component of vfb and vref around the steady-state operating point as Vfb ÆVfb + DVfb (5.1) Vref ÆVref + DVref (5.2)where Vfb and Vref represent steady-state terms and ∆Vfb and ∆Vrefare small-signal terms. By those variations, suppose the following small changes occur: Va Æ Va + DVa (5.3) VPWM Æ VPWM + DVPWM (5.4) D Æ D + DD (5.5)where Va, VPWM, and D represent steady-state terms of va, vPWM, and the duty cycle, respectively. Similarly, ∆Va, ∆VPWM, and ∆Drepresent small-signal terms of va, vPWM, and the duty cycle, respectively.