Research And Design Of An Oscillator For Current Mode Switching Converter

With the popularization of electronic products and the requirement of energy saving, power management IC is more and more widely applied. SMPS (switched-mode power supply) with high performance, high efficiency, small volume, low power and low cost becomes the research hotspot of power management IC. The switching frequency is decided by the output frequency of oscillator which is an indispensable module for the control system of SMPS. Based on the analysis and comparison of modulation and control modes of SMPS, an oscillator applied in the hybrid control current mode SMPS is presented. This novel oscillator with simple structure and stable frequency can generate low pulse signal whose central frequency is 750kHz under the typical condition. At the same time, slope compensation signal which is needed when the duty cycle is more than 50% in current mode SMPS is obtained by citing the slope source generated in the oscillating circuit. Also, through enable signal and the setting of the parameters of shaping circuit, the range of external synchronization can be controlled precisely.The basic units and working principle of the oscillator are studied. Meanwhile, based on the analysis of merits and demerits about common structure of oscillator on chip and the working process of conventional relaxation oscillator, and combined with the design requirements, the structure of the oscillator is presented in this thesis. The two-circles oscillating generation circuit is composed of constant current source charging/discharging circuit and RC charging/discharging circuit. In order to obtain the stable clock signal, the charging current, which is less sensitive to the supply voltage, is got by means of the self-biased structure. Simultaneous in the constant current generation circuit resistor compensation is used to reduce the relative temperature coefficient. The magnitude of the slope of compensation signal is got by studying the instability problem of system in current mode. By citing the slope source in the oscillating generation circuit and adjusting the magnitude of the slope, the essential slope compensation signal in the system is obtained. Combining the oscillating generation circuit presented in this thesis and through enable signal and the setting of the parameters of shaping circuit, the range of external synchronization is set between 500kHz and 1000kHz.The circuit is realized in UMC 0.6μm BiCMOS process. HSPICE simulation results show that the required functions of oscillator have be realized. And under the fluctuation of supply voltage, temperature and process corner, the error is acceptable between -12.2% and 9.88% . The design meets the requirements.