| Switch-mode power supply has a very important position in power electronic technology. After several years of the development in power electronic technology, switch-mode power supply technology has been widely used in industrial, commercial, civilian, military and aerospace technology and some other fields. The core of Switch-mode power supply is to switch the electronic circuit, According to the different load conditions, Switch-mode power supply takes charge of the feedback control network to turn on or turn off the electronic circuit. In general, a switch-mode power converter contains two blocks. One is the power circuit which takes charge of transferring and transforming the power to the loads. It contains only the electronic switcher, inductor and capacitor. With different combination, we can acquire various type converters. The other is the complex control circuits which processes the control signals. With the development of VLSI, the integrated circuits have taken places of the discrete components to realize the control block.Now switch-mode power supply technology uses portable power equipment. For these devices, smaller size, lighter weight, longer battery life time is the main demand. As the battery occupy most of the equipment space and volume. In order to minimize the size, weight and extended battery life time, we need to improve power supply and management efficiency.This object of this paper is to research and design the PWM buck DC-DC converters with synchronous rectification technology, which is used to improve switching power supply efficiency.First of all, the history of switching power supply and technological development and the principle of the buck converter are introduced in this paper. The concept of the basic circuit topology, continuous inductor current mode (CCM mode) and inductor current intermittent mode (DCM mode) are given, and the same time inductor current intermittent and continuous critical conditions are derived. This paper analysis of the control mode: PWM control and PFM control, introduces the advantages and disadvantages of Current mode and voltage mode and gives the concept of a synchronous rectification and the merits of the synchronous rectification. For the power loss which is a critical factor to decide the conversion efficiency of the power. Losses in a switch-mode converter can be classified as: load dependent conduction losses (due to transistor on-resistance, diode forward voltage drop, inductor winding resistance, capacitor equivalent series resistance); frequency dependent switching losses (due to transistor and diode output capacitance charge and discharge, gate-drive losses, voltage/current overlap at switching transitions, inductor core losses, and controller frequency-dependent power consumption); fixed losses (due to controller standby current, and leakage currents of transistors, diodes, etc.)Then an analysis of the system of small-signal modeling is made, after that the power transfer function and system open-loop gain is given in this paper which also analysis of a small signal of stability and the peak current detection mode of the compensation,The forward design is done for the error amplifier circuit, comparator circuit, current detection circuit and synchronous rectification circuit.The system operates at 1 MHZ and the input voltage range from 2.8V to 5V, the output voltage steady at 2.5V, the output current is 1mA, the largest ripple voltage is 3.87mV when the input voltage is 5V and Power conversion efficiency up to 86%.Simulation results show that this system can work properly and meet the design requirement.
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