The Design Of PWM In Switch Mode Power Supplies

Pulse Width Modulation (PWM) is the core part of switch mode power supplies(SMPS). This paper is a design in DaLian Lianshun Electronics Company. The circuit is a single chip MOS IC (Integrated Circuit). Design method is converse design. The initiative chip is FSDM0265RNB of FAIRCHILD. Because of the assigning of the work in the company, the work of this paper is to extract circuit, simulate the circuit and to discuss the produce process. In this paper, there is not switch device either the exterior circuits. Finally, the complete layout is provided.The input voltage of the SMPS which the PWM in is 25V-250V, and the output voltage can be modulated from 16V,12V,5.1V and 3.3V. The original chip layout has one metal layer, the substrate is P-type. It uses the Bi-COMS process. The minimal dimension is 1.5μm. The startup voltage of the PWM circuit is 13V, the internal work voltage is 4.5V, the work frequency during the 63kHz to 67kHz. Soft start time of this circuit is 15mS. When the voltage exceeds 19V and the temperature exceeds 140℃, the circuit start protection by itself.According to the function of the circuit, divide it into several parts as following: voltage reference circuit, oscillation circuit, bust mode circuit, frequent modulation circuit, startup circuit, soft start circuit, temperature protection circuit, under voltage lockout circuit, over load protection circuit, leading edge blanking circuit, over voltage protection circuit all of which are 11 parts. Every portion can receive the function and the parameter is consistent with what the technique special required. The whole circuit can be simulated by simply approximating. In the end, the right layout is fixed on. In the end, master the theory of high-voltage device and the IC production process.