Design Of Key Circuits Of Forward Converter For Peak Current Mode

With the continuous development of integrated circuit technology and the increasing power density,the switching power supply module is developing towards low voltage and high current.As a member of DC-DC switching power supply,the forward converter has the advantages of high ratio voltage conversion,current isolation,simple structure,wide input voltage range,and large output current.It has important research value and becomes the focus of this thesis.This thesis mainly studies and designs the low-end active clamp forward converter.Firstly,the peak current mode control is analyzed and selected,and then the system architecture of the forward converter is determined.The parameters of the circuit including transformer,active clamp circuit,start-up and auxiliary power supply are designed.Then the operation of the low-end active clamp forward converter is divided into ten stages,and each stage is analyzed in detail.Secondly,after classifying different modeling methods,the state space averaging method is used to model the power level and the sampling data method is used to model the controller to complete the derivation of the closed-loop transfer function of the whole system.Then the zero-pole distribution of the system is analyzed,and the system loop compensation scheme is given.The simulation results have a 62.4°phase margin under the worst loop stability.Due to the leading edge oscillation spike during sampling,an adaptive leading edge blanking circuit is designed to filter out the spike part.Through oscillation time blanking and oscillation amplitude blanking,the optimal blanking time is tracked to realize real-time adaptive leading edge blanking.Due to the sub-harmonic oscillation problem of the peak current mode control in the case of large duty cycle,an adaptive slope compensation circuit is designed to adapt to the working conditions of different input voltages and steady-state duty cycles.Based on the small signal modeling of the system,the main pole,the resonant complex pole,and the half-switching frequency complex pole are separated,and the quality factor of the complex pole is extracted.The purpose of the designed circuit is to control the quality factor within a reasonable range,which can simultaneously track the rising slope of the sampling signal and the adaptive adjustment of the slope compensation to the duty cycle.The proposed circuit can suppress the amplitude-frequency spike of the resonant complex pole and the half-switching frequency complex pole without overcompensation.Based on the 0.18μm BCD process,the construction and simulation of all module circuits are completed.Then the layout of the designed chip is drawn,with a total area of1600μm×2000μm.Subsequently,Spectre and Hspice are used to complete the simulation verification of the forward converter system.Finally,the chip is packaged and tested.In the full load range,the output voltage can be stabilized at the design value of 3.3 V,which meets the design requirements.When the load is 15 A,the measured output voltage ripple is 70 m V,and the ratio is 2.1%.The peak efficiency is obtained at V_IN=36 V,I_load=8 A,with 92.86%.