Design And Optimization Of Output Intrinsically Safe Quasi-resonant Flyback Converter

At present, the application of electrical equipment used in the coal industry isbecoming increasingly widely. Safety performance of power supply, which undertakesthe key task of all electrical equipment, is extremely important. Output intrinsicallysafe power supply should meet the small volume, high efficiency requirements, aswell as high reliability and service life. Spark and thermal effect is two ignitionsource of explosive gas, the research of intrinsically safe power supply is mainlyconcentrated in the former currently, while the thermal effect caused by powerdevices is rarely studied. In order to improve efficiency and reliability, controltemperature and reduce the volume of power supply, output intrinsically safequasi-resonant flyback converter is designed and optimized this article.Firstly, quasi-resonant flyback converter and back-end cut-off protection isselected based on the design requirements of output intrinsically safe switching powersupply. The principle of quasi-resonant flyback converter and the energy transferprocess is analysed, the output short circuit discharge energy of natural dischargemode and cut-off discharge mode is derived respectively. The moment when theMOSFET cuts off is the most dangerous. The output short circuit discharge energy ofcut-off discharge mode is lower than natural discharge mode and increases when loadresistance reduces. Compared with the minimum output short circuit discharge energy,the non-explosion evaluation of output intrinsic safety performance is obtained.Following, the output intrinsically safe quasi-resonant flyback converter isdesigned, including power circuit, control circuit and back-end cut-off protectioncircuit. Using small signal analysis method to design feedback loop compensator incontrol circuit, system has good dynamic and steady-state performance. The cut-offprotection circuit can detect overvoltage, overcurrent fault, cut off the output quicklyand realize self-recovery when fault disappears. The converter achieved outputintrinsic safety occording to the non-explosion evaluation method.Then, the cut-off protection circuit built and simulated in the Saber simulationplatform based on the designed parameters. The prototypes of quasi-resonant flybackconverter and cut-off protection circuit are set up. Experiments show that prototypesperformance meets the design specifications, which verify the correctness of thetheoretical analysis and design.Finally, according to the output intrinsically safe switching power supply working environment, the thermal path and the requirements of thermal design isanalyzed, the main power consumption of designed quasi-resonant flyback converteris calculated. Set the environment temperature to extreme temperature （50℃）, andsimulate the state thermal in finite element thermal analysis software ANSYS Icepak.The results of changing the devices layout, the internal medium and the structure ofthe flameproof enclosure are analyzed and comparaed based on the thermal analysis.Finally balance the requirements between heat dissipation, volume and cost, choosethe program changing the device layout, filling with thermal-conduction andflame-retardant rubber and1/3smaller size as an optimal thermal design solution.