Design And Development Of A High-performance AC/DC Converter Control Chip

With the rapid development of science and technology,a variety of electrical appliances constantly appear to meet people's needs while electrical appliances can't be separated from the power supply.Because of the advantages of small size and high efficiency,switching power supply has gradually replaced linear power supply.For example,AC/DC converter,which is responsible for the conversion of alternating current to direct current,is widely used.Flyback AC/DC converter is widely used in a number of low power applications because of its simplicity and low cost.Traditionally,the output voltage and current of converter are regulated by utilizing the secondary side regulation with an opto-coupler and a secondary-error amplifier.While secondary side regulation flyback converter has the characteristics of high accuracy and response speed,it will cause undesired power losses at standby conditions and suffers from the current transfer ratio degradation due to temperature rise.Compared with secondary side regulation method,the primary side regulation method eliminates the optocoupler,saves space on PCB and reduces cost as well as standby power consumption.In this work,the control chip of primary side regulation flyback AC/DC converter is designed and developed based on Nuvoton 0.35 um BCD process.The main research works and innovations of this article are as follows:1.The working waveforms of the primary side regulation flyback AC/DC converter are analyzed,which will help to derive the optimal sampling time when using the auxiliary winding for output voltage detection.Furthermore,the working principle of the key module of the knee point voltage detection is described.And the whole output voltage detection control circuit is accomplished.2.The principle of constant current output control is discussed.Using a reference voltage and the demagnetization time,the peak current threshold generator is proposed and the simplified circuit is used to figure out the working principle.The whole control circuit is accomplished and different constant output currents of the converter can be obtained by adjusting the off-chip devices,such as the sampling resistor and the transformer turns ratio.3.The working principle of the bandgap reference circuit is discussed and a bandgap reference module with trimming is designed.Temperature coefficient of the output is 22 ppm/? in a temperature range from-25? to 125?.The power supply rejection ratio is over 40 d B under 100 KHz.Besides,the range of trimming is-4.55% to +9.21%.And the relation of the working clock to the electromagnetic interference of the system is analyzed,an oscillator with frequency jitter is proposed to decrease the electromagnetic interference,while the output clock is range of 62 KHz to 69 KHz at high frequency working status.The chip was designed and simulated by Cadence and Spetre.The first version of the chip has been fabricated and tested.The constant voltage control works but needs to be improved.As for the constant current control,the constant output current precision is within 5.4% when the input voltage changes from 100 V to 320 V and while the load changes from 1.6? to 4.5?,the output current changes by 4.1%.The improvement and optimization of the chip is in the next version.