| With the rapid development of the science and technology,the electronic devices are also facing new trends.The requirements for the electronic devices,especially the core components,are also increasing.High-precision,high conversion rate,stability,and portable electronic devices are also required,which have become the basic requirements of customers.At present,the traditional PWM controller cannot meet these requirements.Therefore,research on switching power supply is particularly important.Through the study of the working principle of various switching power supply current topology,the modulation mode and control mode,combined with the current mainstream switching power supply topology,focused on the DC-DC converter control mode,this dissertation proposed a PWM/PFM automatically switching and modulated Buck-type DC-DC to replace the conventional PWM-controlled power supply.This improves the power supply efficiency of the switching power supply during light loads and the overall system performance.The chip can be used in switch adapters and battery chargers,open switch power supplies and other portable power devices.First of all,this dissertation analyzes and studies the basic structures,basic principles and control mode of the switching power supply,and proposes a Buck DC-DC with PWM/PFM automatic switching modulation.This mode uses the constant frequency constant width pulse controlling signal to control the power switch on/off.The PFM modulation chosen in this dissertation is the pulse cross-cycle modulation.When the output voltage is less than the reference voltage,a constant clock signal is used as the control signal to turn on the power NMOS switch to make it work;if the output voltage exceeds the reference voltage value,no control signal is issued during this cycle,and the power switch tube does not work..The PFM modulation mode is started and the modulation voltage function is achieved by detecting the numbers of synchronous periodic pulses.The efficiency of the PFM modulation mode is very high,and it is almost independent of the load at light loads.In the case of constant pulse widths,there is a choice of ignoring some of the duty cycle to control the power conversion.The advantages are shown as following: high efficiency at light loads,low operating frequency,good frequency characteristics,and effective reduction of switching losses.Despite the large output ripple and weak input voltage adjustment capability,it is still widely used in the small power supplies.And moreover,most of the traditional switching power supply adopts the voltage control mode to modulate,but the voltage control mode is simple and easy to design in the switching control system.However,it is a closed-loop,single-loop control network.When the input voltage changes,it cannot perform the feedback in real time.It can only wait until the output voltage changes,that is,after completing a cycle,the change signal can be transmitted through the feedback network.Voltage modulation,so in the transient process,the output voltage of the single-loop system is disturbed by the outside,which is not conducive to the stability of the output voltage.In order to improve this situation and improve the stable output of the power supply voltage,a dual-loop control mode is proposed,i.e.,a current feedback is added on the basis of voltage control,so as to achieve a double guarantee.In order to obtain a simplified model of a dual-loop control system including an average current regulation switching control method in the power supply system,a current controller,a current compensation network,and a power stage are modeled and simulated by the software MathCAD,respectively.The approximate function of the closed loop transfer function of the current loop improves the phase margin of the system.The switching power supply system will detect the circuit signal in real time through the detection circuit and adopt the corresponding modulation mode,thereby improving the power conversion efficiency of the system.Through the whole modeling and analysis of the whole DC-DC converter under the average current control,the approximate function is obtained.The phase margin is 54.022°,and the phase margin of the original function is 53.595°.The difference between the two is 0.427°,which all meets the engineering requirements.This brings us a more in-depth understanding of the DC-DC system.In addition,the switching power supply system topology and circuit design principles are combined to analyze and design the switching power supply system.Based on the characteristics of the development of modern electronic products,this dissertation proposes a method of using two PWM/PFM control modes to automatically switch the load in accordance with the requirements of high switching power,miniaturization,and low power consumption of the switching power supply.The conversion efficiency has been greatly optimized.Using the appropriate power supply frequency also reduces the area of the chip from one hand.In the design analysis process,the structural principle and working principle of the switching power supply are introduced in detail.Several important modules of the internal circuit of the chip have been designed and simulated,and simulations have been performed to meet the engineering requirements.In this dissertation,the CMOS process is used,the design simulation software is cadence,and the process library is CSMC’s 0.5μm CSMC.According to the project parameters,the overall system circuit is comprehensively simulated,and the experimental data are analyzed from the switching power supply’s conversion efficiency and stability.The normal completion of the switching power supply function has reached the basic requirements of the project. |
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