Design And Research Of Buck Converter Based On PWM/PFM Dual Mode Modulation

The rapid development of microelectronics technology has stimulated the research boom of integrated power management chips.DC/DC switching power supply have been widely used for their high efficiency,wide input voltage and stable output.The control circuit is the core design part of the DC/DC switching power supply,and it determines the performance.While the control circuit has various types and different performances.For example,the electromagnetic interference(EMI)problem can be avoided if pulse width modulation(PWM)control is used since its switching frequency is fixed,but the efficiency at light-load is greatly reduced.Although the pulse frequency modulation(PFM)could ensure high efficiency at light-load,the switching frequency changes frequently,which easily causes EMI problems.The converter with PWM and PFM dual mode modulation not only make its switching frequency fixed but also improve the efficiency at light-load.This work focuses on the buck converter.Besides,the stability conditions and two compensation methods of the single-loop adaptive on-time(AOT)controlled buck converter are studied.The main research contents and conclusive results in this work are as follows:1.The working principle and structure of several control technologies are studied.The switching frequency estimation model of single-loop AOT control,inductor direct current resistance sampling compensation(IDSC)control,current-type AOT control and currenttype adaptive off-time(AFT)control at light and heavy load are analyzed and derived separately.The three control,single-loop AOT control,IDSC control and current-type AOT control all belong to AOT control,which is a technology based on valley of ripple.The on time and switching frequency estimation model are identical.It’s worth nothing that the AFT control and the AOT control both make the switching frequency fix at heavy load,while the change of the switching frequency is converse at light load.2.A loop gain mathematical model of the single-loop AOT controlled buck converter is established using the state space averaging method.The factors and principle affecting stability are analyzed.The mathematical model between critical lower-bound ESR value and the input voltage is established,and the results indicate that critical ESR and input voltage are negatively correlated.Based on the state space averaging method,the loop gain of the IDSC control is analyzed and derived.The results indicate that the phase margin of the IDSC controlled buck converter is significantly improved.3.Based on Simscape-powerlib,the system-level physical architecture model of the AOT controlled buck converter is established using Simulink simulation tool.Using the Virtuoso simulation tool,a circuit-level physical model of the AOT controlled buck converter is established based on the Eastern 0.18μm BCD process library.The system-level and circuitlevel time-domain simulation results indicate that the pseudo constant frequency is achieved and the on-time can adaptively adjust under continuous conduction mode(CCM).The mode is changed to discontinuous conduction mode(DCM),the frequency is automatically reduced when load current is quite small,and high efficiency is achieved during full load.Simulation results show that the IDSC control technology can solve the instability problem caused by insufficient output capacitor ESR.Besides,the performance of switching frequency,conversion efficiency,output voltage ripple,inductor current ripple,minimum value of output voltage,etc.is compared in system level and circuit level.The results indicate that the circuit-level and the system-level physical model are highly consistent in all performance.4.The circuit models of current-type AOT controlled and current-type AFT controlled buck converters are established respectively,and the circuit-level time domain simulations are carried out.The results indicate that these two circuits can work,and the design goals are achieved.The performance characteristics of three dual-loop controlled buck converters with optimal compensation parameters are compared under the same circuit parameters.It shows that most of the performances of the IDSC controlled buck converter and current-type AOT controlled buck converters are consistent,while the performance of IDSC controlled buck converter,such as transient response time,is obviously superior than that of current-type AOT controlled buck converter.For current-mode AFT controlled buck converters,it’s not conductive to the conversion efficiency under light load due to the increased switching frequency.Simulation results indicate that the IDSC control can endure an ultra-high current load of 10 A,which is impossible for the other control technologies.In addition,the IDSC controlled buck converter requires only a simple RC compensation network to achieve stability,while the other control technologies require a current sampling circuit and an error amplifier with a type II compensation network.The IDSC controlled buck converter designed in this work is simple in structure and can be stable without ESR.The input voltage range is 3.3~6.1 V,the output voltage range is 1~3.93 V,the load current range is 5 m A~9.99 A,the maximum conversion efficiency is up to 96.43%,the switching frequency deviation rate is-0.27%~4.5%,the load regulation rate is 0.19%,the linear adjustment rate is 0.12%,the transient response speed is fast,the overshoot is small,and the conversation efficiency is high at light load.The research work in this paper provide references for the development and application of buck converter with high performance.