The Design Of Dcm AC-DC Flyback Converter

Switching power supply is developing rapidly in recently years with its small size, light weight and high efficiency. Switching power supply applications has spread to all aspects of life. Switching power supply is divided into two types of AC/DC and DC/DC conveter. Thanks to the soft-switch technology and process upgrading, DC/DC conveter development has gradually matured. Due to the relatively large size, and accompanied by internal pressure high-frequency, high-current switching operation, the AC/DC switching power supply needs for more power-optimized design.This paper presents a design of DCM peak current control mode AC/DC flyback converter, including the transformer design, RCD clamp circuit, the feedback loop design, compensation circuitry, and Vdd overvoltage/undervoltage, VS over design pressure, burst mode, OCP and other protective circuits.These circuits can effectively improves the AC/DC converter output voltage stability and loop security, the main contents of this paper and innovative achievements are summarized as follows:1. Based on the theory of linear control systems and flyback converters and Buck/Boost converters equivalence analysis, using an average of the current model, AC small signal model and micro-winding of approximation for modeling and derivation.the CPM-DCM mode flyback converter power level control and output function2. The design of a simple and effective frequency compensation converter feedback network and compensation network, and the feedback network and the transfer function of the compensation network have been deduced. Using pieces of pole-zero analysis of the loop Bode plot of the transfer function.Reasonable compensation circuit design to ensure the constant voltage mode converter output over the full load range have good stability and speed of response.3.Based on DCM model and the value of input and output voltage, converter transformer design has finished and the inductance of the transformer primary inductance value, the turns ratio of output of the primary and secondary side inductor inductor, and the urns ratio of primary side inductance and VS inductance. VS inductor design can not only detect the output voltage, but also achieve reducing frequency mode by VS sampled voltage in order to reduce the switching losses.The power MOS RCD clamp circuit is designed to prevent the damage to the power MOS caused by transformer leakage inductance voltage pulse.4. The Vdd overvoltage/undervoltage protection, VS overvoltage protection, burst mode which can guarantee protect circuit chips inner chip will not burn when encounter some unexpected situations. VDD voltage protection ensures turn off power MOS when the input VDD is higher then terimal. VDD undervoltage protection can be triggered when the converter is turned on to start the next hv start cycle when VS overvoltage protection trigger, the power MOS will shut off and may enter latch mode through the inner counter.Burst mode can greatly reducing static power of the conveter.