Research And Design Of Boost Converter Based On Adaptive Off-time

Boost converter is popular for its boosting characteristic and its application in more complex applications.The most common control modes for traditional Boost circuits are peak current mode and voltage mode.And the peak current mode has a more rapid linear transient response and a simpler voltage loop compensation with respect to the voltage mode,but there are subharmonic oscillations in applications where the duty cycle is greater than 0.5.The constant off-time(CFT)control mode can guarantee loop stability under various duty cycle applications with no slope compensation circuit required,and a simpler current sampling circuit can be applied,and it can also effectively compensate the right half plane zero of Boost converter.However,the CFT control mode has a problem of frequency variation.Under different duty cycle applications,the frequency changes with duty cycle due to the fixed off time,which is not conducive to the design of EMI filters.Based on the CFT control mode,this paper designs a Boost converter with adaptive off-time(AFT)control mode.The design of the AFT timer is the key to determining the fixed frequency capability of the AFT control mode.The traditional AFT timer monitors the change of the duty cycle by sampling the information of the input voltage and the output voltage,so that the off-time changes with duty cycle and achieves a constant frequency.In this paper,the application parameters are used to quantitatively analyze the problem of large frequency variation under the influence of load current.A new type of AFT timer is designed.by sampling the true duty cycle information,and the influence of load current on the frequency is basically eliminated.And the detailed circuit design rules for the AFT timer and the factors that may cause frequency changes are described.In this thesis,according to Boost's start-up characteristics,linear precharging circuit is designed to combine linear start-up and switch start-up,which makes the soft-start more smooth and effectively prevents the generation of inrush current.Ultra-low-power power selection and substrate selection circuit is designed,and the simulation shows that the static power consumption is less than 1.5?A and can normally complete the power switching and substrate switching.Built-in soft-start timer is designed and used for the short-circuit protection of Boost output.Finally,based on the 0.5?m CMOS process,the whole simulation is verified for the application of the input voltage of 2.4V to 4.2V,the output voltage of 5V and the peak current of less than 3A.In the case of transient response,the system is in a variable-frequency operation state,and the frequency returns to the constant-frequency state under steady-state conditions,indicating that the AFT control mode has both the transient characteristics of the CFT control mode and the fixed-frequency characteristics of the peak current mode.Under the influence of only load current variation,the range of the frequency variation of the new AFT control mode is only 1/10 of that of the conventional structure.In other cases,the maximum frequency change rate is only 1.5%.The startup process of the three typical input voltages uses 1A constant current charging,and its smooth with normal internal power supply switching and no surge current.The overall simulation results shows that the design achieves the desired goal.