Key Technologies Study For High-Efficiency Active-Clamp Forward Converter Systems

With the development of the electronic industry,power supply technology is constantly updated.Switching power supplies are widely used in various electronic devices due to their significant advantages in efficiency.Isolated switching power supplies are commonly used in AC-DC,server power supplies and telecommunication power supplies due to their excellent performance in terms of safety.This thesis focuses on the optimization of the conversion efficiency of forward converters,including the following aspects: 1)the active-clamp technique is used to reset the transformer so as to recycle the energy stored in the leakage inductance of the isolation transformer,and to reduce the voltage stress across the main power transistor.2)Each source of power loss of the converter is analyzed,and modeled.Off-chip components are selected accordingly.3)A current sensing amplifier is designed in the PWM controller chip to reduce the current sense resistance required,thus reducing power consumption.The experiment results show that the peak efficiency of the designed active-clamp forward converter is 92.86%.In addition,due to the high output current of the designed active-clamp forward converter,a cable voltage drop compensation circuit is designed in the PWM controller chip for remote load application to compensate for the voltage drop on the cable connecting the converter to the load.Thus,the load regulation is effectively reduced and the output accuracy is improved.Simulation results show that the overall load regulation of the converter with the proposed cable voltage-drop compensation is only 0.1 m V/A in the load current range of 0～30 A.Starting from the operating principle of forward converters and transformer reset techniques,this thesis explains the principle of the active-clamp reset technique and briefly introduces the two most frequently used fixed-frequency control methods of active-clamp converters,followed by the loss analysis of the designed converter.Components of the converter are selected based on the input voltage range,output voltage,load current,loss analysis and other parameters of the converter.Its power stage,current sensing and voltage feedback circuits are designed.On this basis,the small signal model of the converter power stage is derived and a type-II compensation network is designed.Based on the above work,a PWM controller chip with peak current mode control for active-clamp converters implemented in 0.18 μm BCD process is developed.The principle,design and simulation results of the current sensing amplifier,cable voltagedrop compensation circuit,oscillator with programmable maximum duty cycle clamp and programmable deadtime/overlap driver are thoroughly explained.The layout floorplan is presented and simulation verification is carried out.The designed converter has been tested experimentally and the experiment results show that the designed converter is stable in the range of input voltage 36～78 V and load current range 0～30 A.The overshoot and undershoot during load transient of the converter between 5 A and 25 A is small and meets the design requirements.