The Research And Design Of A PWM Mode DC/DC Buck Converter

With the rapid development of electronic technology, the performance requirement for the electronic instrument and equipment rises subsequently. Especially in switching power supply, the property, operability, volume and quality will be improved. Furthermore the conversion efficiency of the switching power supply is also the focus of the pursuit. The characteristic, the application and the development of the switching power are described in this thesis. And the essential theories the switching power are also introduced.A buck DC-DC converter chip with PWM controlling mode is designed. The voltage feedback control mode is adopted, and the chip operates under 40V working voltage and 45V limiting voltage. The input voltage of enable pin ranges from -0.3V to 40V. The normal working temperature ranges from 25℃to 125℃.The conversion efficiency can be up to 85%. This chip includes the start up circuit, the benchmark source circuit, the EA circuit, the protecting circuit for the short circuit, the oscillation circuit, the pulse width modulation circuit, the over-current protection circuit, the over-temperature protection, the buffer circuit, the logic control circuit and the output driver circuit. The theory and the parameters of these circuits are in detail discussed. Pspice software is applied to simulate these functional modules. Then the topological structure of the chip is established and the function is simulated. The simulation results show the design has satisfied the performance indexes. The layout is designed with L-EDIT software based on the UTC, 7μm, 40V bipolar process. In addition, some essential layout units are elaborated.The chip is packaged with SOP8 style. The spot test and the final test for the chip are accomplished. The test results show that the output voltage has about 50mV ripple wave, and the quiescent current is about 3mA, the chip can realize short circuit drop frequency function, operating frequency reduces from 81KHZ to 23KHZ, when output voltage is less than 0.5V. Good regulation rate of the load is about 0.2% when output load step changes from 0.2A to 2A. The efficiency of the chip can reach more than 85%.