Dynamic characteristics of hysteretic PWM control operated in double switching frequency of clock signal

A hysteretic PWM controller is one of attractive control methods to realize the fast transient response for the rapid change of the load current and input voltage of switching power converters. The performance of this control method depends on the time constant of the RC network which generates a ripple voltage for the PWM circuit in the feedback path and has a feature of trade off between the higher switching frequency and better dynamic characteristics when applying it to dc-to-dc converters. To overcome this trade off, a robust control technique to increase the switching frequency has been proposed by expanding the hysteresis window voltage at the beginning of switching actions. This method is effective to make the switching frequency high but it still operates as a self-oscillating circuit. To solve these problems, this paper proposes a novel hysteretic PWM controller that consists of two comparators with hysteretic characteristics and OR gates. The proposed method operates in double switching frequency of the clock signal for synchronization with keeping the advantage of the excellent transient performance. In addition, this paper presents quantitative analysis for proper design of the control circuit. The presented analysis introduces a step change on the reference voltage at the input of hysteretic comparator and transfer functions of the hysteretic PWM controller with or without synchronization were revealed. Applying the proposed controller to the buck converter, it is confirmed that both theoretical results and measurements of the frequency responses agree well and the proposed method provides excellent transient performance with reduced ripple voltage at the higher switching frequency with synchronization.