Research And Design Of Voltage Sampling Circuit Used In Primary Side Regulation Flyback Converter

New electronic equipment have an increasing demanding on power supply, switching power supply is widely used in portable devices for its high efficiency, low cost and high integration. Flyback converter is widely used in low power AC-DC applications for its low cost and good input-output isolation. Compared with the conventional flyback converter primary side regulation flyback needs fewer components and has a simpler structure which makes it possible to reduce size and cost, it is favored by many switching power supply design engineers.The major work is to research the principle of primary side regulation flyback converter and the principle of voltage sampling, and design a voltage sampling circuit which can reduce the error of sampling caused by secondary side rectifier diode’s forward voltage drop, a latch mode over voltage protection circuit is also designed to prevent a too high sampling voltage.The basic theory of liner regulator and switching mode regulator is introduced at first, the topologies of common converter is analyzed. Differences between continuous conduction mode (CCM) and discontinuous conduction mode (DCM) is analyzed and compared. The working principle of flyback converter is analyzed, principle of voltage sampling in traditional flyback converter and primary side regulation flyback is compared, advantages and disadvantages of each sampling method is summarized.is analyzed. Influence on feedback voltage of secondary side rectifier diode is analyzed; voltage sampling circuit is divided into three parts which are sampling signal generating circuit, sampling and zero current detection circuit and over voltage latch circuit.Then circuit in sub-block is designed, the voltage is sampled when the current which flows by the rectifier diode reaches zero, which will reduce the error of sampling caused by the forward voltage drop by rectifier diode, it is guaranteed by sampling signal generating circuit. It is compensated by the temperature compensation circuit, which can reduce the rectifier diode’s negative temperature coefficient effect on the feedback voltage. Zero current detection circuit is capable of detecting the position of resonant state and provides a reference signal for sampling signal generating circuit. In order to avoid damage to electronic device caused by high voltage, sampling circuit has an over voltage circuit witch has latch mode witch can shut down the chip when the sampling voltage is too high.At last, the circuit is simulated, it is designed and simulated with 0.5μm BICMOS process and simulation results show that the forward voltage drop is -42.6mV when sampling, influence on sampling voltage is decreased. Sampling signal generating circuit can generate sample signal when the current of rectifier diode reaches zero. Sampling and zero current detection circuit can detect the zero current point and sample the voltage. When sampled voltage continues above 2.8V, latch circuit can shut down PWM signal and reset when VDD is lower than 2.5V. Key parameters of the circuit are verified by simulation results.