Research On Digital Control Strategy Optimization Of MHz High-frequency Buck-Boost LLC Converter

In the fields of solar inverter,data centre and communication system,there is an urgent need for DC/DC converters with wide input voltage range and high power density.Buck-Boost LLC converters are focused due to their high efficiency,high power density and wide input voltage range.However,these converters still suffer from the problem of dynamic and stable performance,which restricts their application and development.Based on the MHz high-frequency Buck-Boost LLC converters,optimizing the dynamic performance is the first step.Firstly,the small signal model of Buck-Boost LLC is established and the transfer function of output-duty is deduced.According to the transfer function,the digital compensation method is designed to guarantee a fast response.Secondly,in order to reduce the large digital delay,HRTIM is used to trigger ADC,improving the speed and accuracy of ADC sampling.In addition,the calculation time of the main function and the interrupt response function is adjusted to decrease the program period.Finally,when the load changes greatly from heavy to light,nonlinear control strategy is chosen.It can reduce the input power and output capacitance energy rapidly,which stabilizes the output in a short time.In short,the dynamic response time and the overshoot in the full load range can be greatly shortened.The control strategy of phase P needs to be changed to improve stability performance.At first,the resolution of HRTIM is improved to avoid limit cycle oscillation when the precision of HRTIM and ADC is not matched.Next,the calculation of duty and phase are improved.The input voltage is replaced to avoid the jitter caused by sampling.What’s more,based on the traditional decoupling control algorithm,fitting + serial calculation is proposed to control the phase.It can avoid the influence of duty and phase on the Buck-Boost inductor current at the same time.Besides,SBILUT is introduced to fit the optimal phase,making a tradeoff among the accuracy,calculation time and memory consumption.Based on these methods,the jitter of the Buck-Boost inductor current and output voltage can be reduced.Therefore,the stability of the converter can be improved.Based on the above analysis,a 300V-420 V input,24V/5A-30 A output Buck-Boost LLC prototype is built to verify the proposed control method.When the load changes between full load and half load,the dynamic response time varies from 1.2ms to 1.5ms with the control scheme in this paper.Compared with the traditional control method,the response time is reduced by about 5 to 10 times.Additionally,the overshoot of the output voltage is less than 10% of the aimed voltage.The control strategy of stability proposed in this paper can improve the jitter of the Buck-Boost inductor current by about 40%.Also,it ensures the stable output and operation of the converter in the working area.