PFM/PWM 듀얼모드 회로의 모드 변환 불안정성에 관한 연구

Abstract

To obtain higher efficiency PFM/PWM dual-mode control is widely used for DC-DC converters. Pulse width modulation (PWM) control shows high conversion efficiency for heavy load range in DC-DC converter. On the other hand, pulse frequency modulation (PFM), according to its effective switching frequency variation proportion to the load, is adopted for high efficiency in light load condition. But some serious jitter in switch control signal is brought out during the mode transition in PFM/PWM dual-mode control. The jitter can cause system instability. In this thesis, a stable mode transition circuit based on Schmitt trigger is designed to solve the problem. At first, operating principle of PFM/PWM dual-mode circuit is presented . On this basis, the reason of jitter is clarified. It's because comparator is not stable when sensing current is compared with reference current. Subsequently, a stable mode transition circuit based on Schmitt trigger is designed. A detailed analysis, design method and its practical performances are illustrated. In order to verify the performance of the proposed circuit, the circuit has been fabricated by using XFAB CMOS process. Experimental results show that the mode transition instability has been solved well.