Design Of Current-mode Digital Controller For Buck Converter

In recent years,the market scale of the portable devices like smart-phones and tablets has been increased rapidly.However,the performance of the electronic device has a direct relationship with the performance and quality of the power supply.Therefore,the performance requirements of the power management module have been more advanced.Compared with thevoltage mode single loop control scheme,the current mode double loop control scheme has the advantages such as rapid dynamic response performanceand high stability accuracy.Meanwhile,the traditional RC inductance current detection scheme has the disadvantages that the passive devices are bulky.Besides,the digital controller cannot be controlled by analog quantities.Above all,there are challenges to apply the current mode digital control scheme in the switching power supply.In the thesis,a novel current mode double loop digital control scheme for Buck DC-DC converter is introduced.In the control scheme,inductance current sampling and the corresponding fixed off-time control loop are proposed for higher integration density.The inductance current sampling scheme is designed based on the topology of Buck DC-DC converter.Thus,the time constant of the inductance current detector should be larger than the switching period of the converter.And,the advantage oftheproposed scheme is that the inductance current detector can be completely digital controlled,which is immune to the variation of the main topology parameters.Besides,the compensator in the outer voltage control loop is designed based on the transfer function of the current mode controlled Buck converter,and the compensator generates the reference current.Then,the fixed off-time control module compares the inductance current detected with the reference current,generating a pulse signal to control the turn-on and turn-off of the converter,and realizes the purpose of stabilizing the output voltage.Furthermore,at the start-up stage,a soft-start module and a digital frequency locked loop in the outer control loop are introduced to solve the problem that the switching frequency in the pulse control scheme may change.Finally,simulink simulation and test verification are carried out.In the thesis,a test platform based on FPGA is set up to verify the current mode digital controlled Buck DC-DC converter.It is verified by the experimental results that with 5 V DC input voltage,100mA rated load current,the operating frequency of the current mode digital controlled converter,the output voltage and the steady state error are 1MHz,1.8V and 1.944%,respectively.When the load current varies from 0mA to 500mA,the transient response time is about 138pμs.In general,the proposed current mode digital controlled Buck DC-DC converter has the advantage over the voltage mode controlled one,and meets the requirements of the portable devices.