Design Of A Constant Frequency Valley Current Mode Buck Converter

With the wide application of portable electronic equipment,low-power converter is favored by people.Common 12 V power supply,such as vehicle power supply,can easily meet the power supply of many small electronic equipment.In order to apply it to 6 V power supply equipment,such as mobile power supply,toy electric vehicle,electronic scale,etc.a buck converter is needed to deal with the vehicle power supply voltage.Based on the 0.18 um BCD process,a constant frequency valley current mode buck converter is designed.The input voltage of the buck converter is 12 V,the output voltage is 6 V,and the load current is from 200 m A to 2 A.For some heavy-duty applications,in order to improve the conversion efficiency,a fixed working frequency of 200 k Hz is adopted;a double n-type power tube is used for synchronous rectification,and a dead zone circuit is added to prevent the main switch tube and synchronous switch tube from conducting at the same time.The sampling resistance is connected in series between the synchronous switch tube and the ground to sample the current signal when the inductance is discharged.In order to improve the transient response speed,valley current mode control is adopted.The sampling resistance is connected in series between the synchronous switch and the ground to sample the current signal when the inductance is discharged.The output signal of PWM comparator and clock signal respectively control the charging and discharging of the buck converter to adjust the duty cycle.The charging and discharging operations are controlled by the rising edge trigger,and then the on and off of n-type power tube are driven by the dead zone circuit and bootstrap circuit.Adding the soft start circuit can avoid the surge current when the chip starts,and the duty cycle increase slowly when the system starts to make the output voltage rise steadily.In addition,reentry current circuit,overvoltage and undervoltage detection circuits are added to ensure the safe operation of the converter.The output voltage error and load adjustment rate of the buck converter designed in this paper are all within 3%,the output voltage ripple is less than 50 m V,and the transient response time is not more than 200 us,and the conversion efficiency can reach over 96% in the target load range.The system works in CCM mode during heavy load and DCM mode or FCCM mode during light load.When the output voltage exceeds the preset range of 7.5% or the load is short circuited,the control circuit will be turned off to ensure the safety of the chip.