Research On DC Bias Measurement Of High Frequency Transformer Of DAB Converter

The proposal of the "dual carbon" goal will accelerate the process of zero carbonization of electricity.A new type of power system featuring "three highs"(a high penetration rate of renewable energy,a high proportion of power electronic equipment,and high-speed growth of DC load)is gradually taking shape.Power electronic equipment such as solid-state transformers and energy routers with bidirectional dual-active-bridge(DAB)isolation DC-DC converter topology as the core are important components of the new power system.The power grid’s demand for solid-state transformers is high reliability,high efficiency,and high power density.But the DC bias of the high-frequency transformer in DAB will seriously affect its efficiency and reliability.In response to this problem,this article has done the following:Combined with B-H curve and excitation current,the definition of DC bias is introduced.Four main factors that produce DC bias are analyzed: inconsistent driving signals;inconsistent characteristics of switching devices;uneven heat dissipation of switching devices;asymmetrical PCB layout.The reasons for the inconsistency of the driving signals are analyzed from the aspects of the PWM signal of DSP,the design of the driving circuit and the selection of the driving chip.And some suggestions for improvement are put forward.It is analyzed and pointed out that the inconsistency between the on-resistance and the threshold voltage of the device is the main factor affecting the DC bias.Analysis of the device data sheet shows that uneven heat dissipation of the switching device is also a potential factor leading to DC bias.A simple model is established to analyze the DC bias when the PCB layout is asymmetric,and a typical symmetrical PCB design style is given.A factor leading to DC bias,threshold instability of Si C MOSFETs,is proposed.The principle of threshold instability is analyzed,and an experimental platform is designed to verify it.The principle,structure and circuit of the experimental platform are introduced in detail.The on-resistance and switching characteristics of the device after aging were analyzed.And the influence of threshold instability on the device itself was quantitatively analyzed through the experimental results.A simple model is established to analyze the effects of on-resistance and duty cycle on DC bias,and the magnitude of DC bias caused by threshold instability is quantitatively analyzed by combining the experimental data of threshold shift.The DC bias tolerance of the transformer of the experimental platform is calculated,and the comparison with the DC bias generated by the threshold shift draws the conclusion that the threshold shift has a great influence on the DC bias.The voltage and current waveforms of DAB after the occurrence of DC bias are analyzed,and the conclusion is drawn that DC bias will affect the reliability of DAB.Through experiments,the influence of DC bias on the entire DAB loss is analyzed from four aspects: transformer loss,switching loss,backflow power and conduction loss.It is proved that the DC bias will seriously reduce the DAB efficiency after the transformer is saturated,and it is concluded that the DC bias must be eliminated.Based on the principle of fluxgate,a sensor that can measure small DC current mixed in high frequency AC current is designed.The principle of fluxgate current sensor is introduced,the limitation that it cannot be used at high frequency is verified by analysis and experiment.The principle of current transformer is proposed to overcome this limitation.A model is established to analyze the influence of the proposed method and high-frequency current on the fluxgate,and the fluxgate sensor is redesigned according to the conclusions drawn from the model analysis.Taking the DAB experimental platform as the sensor design target,the sensor design process is introduced and the specific considerations in sensor design are analyzed.Finally,after passing the calibration test,it is used to eliminate the DC bias of the DAB experimental platform.The experimental results show that,compared with ordinary current sensors,this sensor can measure small DC currents in large high-frequency AC currents,and the use of this sensor for feedback control can almost completely eliminate DC bias.Based on the principle of AC magnetic field cancellation,a sensor that can measure tiny DC currents in high-frequency AC currents is designed.The basic principle of AC magnetic field cancellation is introduced,and it is pointed out that the challenge of magnetic field cancellation is the phase deviation.Three reasons for phase deviation are analyzed: insufficient current transformer performance,insufficient op amp performance and op amp driving inductive loads.The high-frequency and low-frequency models of the current transformer are established,and the design rules of the current transformer are obtained by analyzing the phase-frequency characteristics derived from the model.The basic rules of op amp selection and the circuit and principle of driving inductive load without phase difference are introduced,and the effectiveness of the proposed method is verified by experiments.The design of the sensor’s EMI suppression circuit and low-pass filter circuit is introduced.The sensor is calibrated and used for DC bias cancellation on the DAB experimental platform.Compared with ordinary current sensors,using this sensor for feedback control can reduce the DC bias to the level of several milliamps.The advantages and disadvantages of the two proposed sensors were compared in terms of cost,design complexity and volume,which provided a basis for the selection of the method.