Research On Active-clamp Flyback Based Control Strategy And Drive Circuit

In recent years,fast charging technology has received widespread attention in the market as one of the solutions to the lack of battery capacity and limited range performance of mobile devices.Flyback converter has become the first peak for fast charging of low power off-line applications in major companies because of its simple topology,low electromagnetic interference and low cost.However,traditional flyback converters cannot effectively use the leakage energy inside the topology,as a result,flyback converter has lower conversion efficiency and higher heat generation in the power supply compared to other switching power supply topologies overall.In addition,the power silicon devices in the Flyback converter topology generate higher losses under high frequency switching due to their larger Q_G and Q_OSS,further limiting the popularity of flyback converters in consumer products.Therefore,designing a high-frequency and high-efficiency flyback control strategy and drive system has become the key to the development of fast charging technology.In this paper,we focus on the control circuit and the driver circuit to improve the efficiency of the Flyback converter.First of all,for the control circuit,based on the operating principle of Flyback,the operating rules of Flyback in CCM,DCM and CRM are elaborated,and the leakage inductor,which is the main energy loss component in the topology,is introduced.After comparing common clamping methods,active clamping techniques that can achieve zero-voltage switching is applied to handle the leakage energy,and three operating modes are designed to control the converter efficiently through different load conditions in accordance with the application requirements.The control strategy is verified in Simplis under different load conditions,and the maximum power output of the system is 45W,with a constant output voltage of 20V and a load current of up to 2.25A.Based on the full-load control strategy,a bandgap reference circuit is designed to provide a reference voltage,an LDO module to provide a stable voltage supply internally,and a high-bandwidth comparator module to complete the pulse count in adaptive burst mode.The system sub-module is simulated based on the 0.18μm BCD process,all the desired function is achieved.Secondly,the efficiency loss caused by high-frequency switching of conventional Si FETs in Flyback converters can no longer be ignored,this paper first identifies the use of GaN devices with low on-resistance,small output capacitance,and more suitable for high-frequency applications to replace conventional Si FETs in order to adapt to the high-frequency application requirements of Flyback converters.In order to drive GaN power devices reasonably,the problems of three different GaN devices:Cascode cascade type,depletion type,and enhancement type in high frequency switching are discussed based on the understanding of GaN FETs switching characteristics,and the programmable dV/dt control driving strategy of this paper is proposed for these shortcomings.Through the 2X charge pump structure in the device conduction dV/dt and dI/dt phase shielding the role of the power supply to use external resistors for device gate charging swing rate control,the system EMI noise is reduced and the overall efficiency is improved.The chip designed in this paper uses 0.35μm HV CMOS process to complete the flow,through the double pulse circuit test,the results show that the driver chip can work at a high switching frequency of 2MHz,the propogation delay is only 11ns,the programming resistor can be completed on the slew rate of more than 5 times,achieving the optimization of EMI performance.