Design And Implementation Of Current Sampling Circuit In High Efficiency DC-DC Converter

With the rapid development of information technology,such as 5G communication,mega data technology,fast charging technology,the requirement for chips in various devices is growing higher and higher.Among them,the demands for switching power supply chips are gradually turning to higher speed,larger current,muti-phase and so on.Compared with voltage mode control,current mode control has many advantages for switching power supply.The transient response is faster,the loop compensation is simpler,and it can be multiphase to extent output capacity with balanced current,etc.One of the difficulties in the design of current mode switching power supply is how to accurately sample current information without affecting efficiency.Therefore,semiconductor chip companies and scientific research intuitions have been researching in higher precision and lossless current sampling methods to further improve the conversion efficiency of switching power supply.This thesis proposes an innovative Gm-C lossless inductor current sampling technology with adaptively time-constant matching.Fixed-transconductance amplifier,and invariable sampling resistor are designed.Improvement in sampling accuracy can be achieved by adjusting the equivalent capacitance of the sampling capacitor.By comparing the zero-crossing time difference between the switching node voltage of converter and the Gm-C current sensing voltage,the control loop determines whether the Gm-C time constant matches the DCR time constant.The existing time difference will be first detected by a phase/frequency detector and the output signal of detector then controls the reversible counter to calibrate the equivalent capacitance of the sampling capacitor array,which helps to shrink the detecting time difference.The equivalent capacitance will not change when the time difference no longer exists or is small enough and that means the control loop works successfully and adjustment is finally finished Compared with previous work,the calibration process is smooth,and the sampling circuit can be modulated online when the converter runs in steady state,which means it can effectively adapt to the change of external conditions such as temperature,voltage and loading current.Based on the inductor current sampling technology mentioned above,a Four-Switch Buck-Boost converter chip is designed in this thesis.The input voltage range is 4.7V-60 V,the output voltage range is 1.2V-60 V,and the switching frequency range is 200 k Hz-700 k Hz.The wide switching frequency range is achieved by configuring external resistor.The Four-Switch Buck-Boost converter and inductor current sampling circuit is designed with 0.18μm BCD process.Simulation results show that the maximum load current of the converter is 5A,the peak efficiency is 96.4%.Compared with the series resistance sampling method,the current sampling circuit helps increase the efficiency of the Four-Switch Buck-Boost converter by 1.5%.Additionally,this innovative sampling method also effectively reduce the sampling error by 39.6%-55.2%.