Design Of The Key Circuits For Primary Side Regulated Flyback Converter

Being supposed to work normally under extraordinary conditions like extreme temperatures and strong radiation,applications in aerospace and other domains keep placing higher demands on a safe and stable operation of power supply system.As such,by introducing transformers in the topological structure,Flyback which realizes electrical isolation in both input and output has become a remarkably safe product in the switching power system.Among them,the primary side regulation is adopted in Flyback,which conserves the excellent characteristics of the Flyback topology,and avoids the use of optical coupler that is sensitive to environmental factors such as temperature and irradiation:it realizes closed-loop control feedbacks by its auxiliary winding information.Such particularity makes it possible to install a radiation hardened power supply at the system level,and has a far-reaching significance in the military field,especially in aerospace.This thesis starts with the parameter selection of the power stages device of primary side regulated Flyback.Firstly,the primary and secondary side inductance of the transformer was designed according to the the maximum duty cycle of switching rated voltage parameter design system under discontinuous conduction mode.Then,based on the applying conditions of the power supply system,the RCD snubber parameter was designed to absorb the leakage inductance energy of the transformer.The average switching period was used to complete the modeling analysis of primary side regulated Flyback.In line with the system minor signal model,the type II compensating network parameter was designed.The analysis and calculation show that the designed system has good closed-loop stability and a phase margin of about 88°under rated application environment with an output voltage of 32V and a full work load of 30W.In the control chip design of the primary side regulated Flyback,this thesis proposes a DCM adaptive start-up technology applied to the primary side regulation,which determines the discharge capacity of the current output voltage to the secondary side inductance cycle by cycle,and then controls switching duty cycle to achieve adaptive start-up of the Flyback.The BCD process of 0.18?m was applied.DCM self-adaptive start-up circuit,a knee voltage sampling circuit,a load current detection circuit and an oscillator whose frequency can be controled by applied voltage were all designed.In addition,the overall layout of the chip,with an area of 2.6mm~2,was completed and has passed DRC and ERC check,as well as LVS verification.The simulation results of different process corner revealed that the primary side regulated Flyback designed in this thesis is able to realize DCM self-adaptive start-up under all voltage input and all work load output.In particular,the maximum start-up time is about 40ms under light load output and about 80ms under heavy load output.The maximum system overshoot is 13.172%,the maximum output voltage ripple rate is 0.275%and the sampling error of the output voltage is within±5%.