Research On Key Technologies Of Isolated DC-DC Converters

As the society develops into a high-speed,intelligent information age,humankind’s demand for energy is growing increasingly day by day.As one of the main energy sources in production and life,electric energy plays an indispensable role in new energy vehicles,5G communications,intelligent AI and other fields.Among them,the isolated DC-DC power supply is widely used in high and low voltage transfer modules,system primary power supplies,medical equipments and other fields due to its galvanic isolation.However,because its topology contains a transformer which is needed to sample the output voltage information from the secondary side,the system is quite complex and difficult to design.And due to the late start of the domestic semiconductor industry,products related to isolated power supplies lag behind foreign products in terms of control technology,performance,maturity,etc.This dissertation takes two isolated power supplies,flyback converter and forward converter,as the research object,and almost complementary various control modes,such as:RCD and active clamping,voltage mode and current mode control,DCM and CCM mode,asynchronous and synchronous rectification,primary side regulation and optocoupler feedback,traditional and planar transformers and etc are adopted.The content of this dissertation covers most of the technical solutions for isolated DC-DC power supplies.The main content and innovation points are explained as follows.For the flyback converter system,this dissertation has proposed a system startup time sequence model based on iterative recursion algorithm to analysis the non-steady state of the startup process in the time domain.For the system in the steady state,through large signal analysis of the loop control,the small signal model in the steady state is obtained in the frequency domain.And a type II compensation optimal parameter solution strategy based on genetic algorithm is proposed.For the design of flyback converter control chip,this dissertation proposes an adaptive start-up technology without external capacitor to solve the positive feedback phenomenon of flyback converter during the start-up process.By avoiding the use of extra pins with large off-chip capacitors,the speed adaptive adjustment under light load and heavy load startup is achieved,while the startup overshoot of the output voltage is reduced and the startup time is shortened.At the same time,the primary-side feedback technology is used in this dissertation to replace the traditional optocoupler feedback,and a knee-point voltage sampling circuit has been proposed.Based on the structure of the bandpass filter,the delay matching between the signal path and the sampling path is optimized to improve the sampling accuracy of the knee voltage under different duty cycles to more than 99%.In view of the phenomenon that the flyback converter system cannot be unloaded,this dissertation adopts PWM,PFM and DPWM modes to control the loop under heavy load,light load and utra light load respectively,finally reducing the dummy load power consumption to8m A.By analyzing the continuity problems existing in the mode switching process,a multi-mode seamless switching control method has been proposed in this dissertation.By designing corresponding logic control,adaptive pseudo-pulse width locking and other circuits,the normal performance of the system under multi-mode switching is finally guaranteed.Finally,the flyback converter control chip designed in this dissertation is fabricated in 0.18μm BCD process and the chip area is 0.75×1.5mm².The chip is in QFN package,and a corresponding PCB test board has been designed.The IC controller has passed functional testing and standard temperature and humidity cycle testing.The test results show that the output voltage of the system is stable at 32V within the full load range.The start-up time,output overshoot voltage and output ripple under heavy load and utra light load conditions are respectively 35ms,4V,980m V@V_IN=28V,I_O=0.8A and 48ms,0.2V,104m V@V_IN=28V,I_O=0.008A.And the results calculated by the startup time series model fit the actual simulation and test startup output voltage curves very well.The system load regulation rate reaches 0.98%@I_O=0.8-0.008A,the linear regulation rate reaches0.29%@V_IN=20-32V,and a peak efficiency of 88.52%is achieved.For the forward converter system,this dissertation establishes a complete small-signal model of the current mode control loop.At the same time,for the subharmonic oscillation phenomenon that occurs when the duty cycle is greater than 50%under peak current mode control,the relationship between the corresponding compensation amount and the stability boundary conditions is derived through mathematical derivation.A complete active clamp forward converter system topology was built based on Simplis software.A suitable compensation scheme was obtained through simulation calculations.For the primary-side active clamp control and secondary-side self-driven synchronous rectification control,a drive logic control circuit with dual drive control,high and low sides selectable,and adjustable overlap/dead time have been proposed in this dissertation.At the same time,corresponding state switching logic control has been designed for various protection functions of the forward converter,such as over-temperature protection,under-voltage protection,over-current protection,etc.,and an off-chip adjustable cycle-by-cycle over-current protection is proposed by the slow and fast restart functions when the system overcurrent occurs.Aiming at the power transistors drain voltage of the current mode controlled forward converter which is affected by the input voltage and duty cycle,an adaptive maximum duty cycle limiting circuit has been proposed in this dissertation so that the maximum duty cycle limit changes with the input voltage.Thereby,the phenomenon of large voltage and large duty cycle causing the drain voltage of the power transistors to exceed its withstand voltage has been avoided.In the system-level design of the forward converter,a planar transformer has been uesed to replace the traditional transformer,which not only reduces the system volume,but also shortens the connection path under high current,thus making the system efficiency improved.Finally,the IC controller designed in this dissertation is fabricated in 0.18μm BCD process and the chip area is 1.6×2mm².The chip is in CSOP package,and the corresponding PCB test board and planar transformer are designed.The system volume is 39cm³.The test results show that the system output voltage is always stable at 3.3V within the full load of 30A and the full input voltage range of 36-78V.The load current changes from 5A to 25A within 120μs,and the overshoot and undershoot are both less than 200m V,which shows a good transient response performance and load regulation.At the same time,functions such as adaptive maximum duty cycle limit,cycle-by-cycle overcurrent protection,and undervoltage soft shutdown are implemented.This dissertation conducts a detailed analysis and verification of flyback converters and forward converters from the aspects of system modeling,chip-level design,system-level design,and actual testing.Based on the current technical solutions,new circuits and control methods are proposed,which will promote the future development of isolated DC-DC power supplies.