Research On Cellular MHz Isolated DC/DC Technologies With High Step-down Ratio

Power architecture with higher efficiency and higher density is urgently demanded due to the rapid development in areas such as 5G base stations,data centers,and phased array radars.To meet the increasing current requirement of various integrated circuit(IC)chips with low voltage,adopting the high voltage dc architecture(HVDC)concept and then integrating the isolated high-voltage-to-low-voltage DC/DC bus converter into the motherboard is a preferred option,which can reduce the bus-bar loss and achieve higher efficiency.To achieve the miniaturization and integration of power modules,high efficiency and high power density are especially required for the isolated DC/DC converters with a high step-down ratio.To reduce the volume of the passive components and improve the power density,a higher switching frequency needs to be adopted.However,the switching loss,driving loss of the devices as well as the eddy current loss of the transformers will increase rapidly under higher frequency,which become the bottleneck in the improvement of converter performance.To solve these problems,cellular topology with good soft-switching characteristic is adopted in this dissertation,where high-frequency low-loss driving strategy and optimization method for the highfrequency magnetic component are focused.The main contents are as follows:Series resonant converter(SRC)has been widely used in high-efficiency high power density applications for its good soft-switching characteristics.By replacing the single high-voltage SRC with multiple series-parallel connected low-voltage SRCs,where every low-voltage circuit is regarded as a circuit cell,the power module is made to be cellular,and the loss caused by the magnetizing current for ZVS at high frequency can be further reduced.However,a large number of MOSFETs are needed in the cellular MHz isolated SRC with a high step-down ratio.The conventional commercial voltage drive ICs will lead to serious driving loss.And the existing resonant drive schemes are mostly designed for PWM circuits or resonant circuits with single switch,which cannot meet the needs of cellular SRC topology.To solve these problems,a low-loss resonant drive transformer is proposed for the high-frequency cellular SRC,which can not only reduce the driving loss,but also provide multiple isolated symmetrical drive voltages.Therefore,the footprint of the drive circuit can also be reduced.Besides,considering the working characteristics of the series resonant converter,the optimization methodology of the parameters for the resonant drive circuit is also proposed.Finally,the efficiency and power density of the converter using the proposed resonant driver are both improved significantly.In the MHz isolated DC/DC converter with a high step-down ratio,the volume and loss of the high-frequency transformer usually occupy the largest proportion of the whole power module,which become the bottleneck in the improvement of efficiency and power density.To reduce the winding loss in low-voltage and high-current applications,the UI core with double magnetic legs is usually used to replace the EI core with a single window to achieve more parallel windings in fewer cores or PCB layers.However,limited by the loss density characteristics of magnetic materials,the thickness of the transformer core is still much larger than that of the semiconductor devices even after the frequency is increased to 1MHz,which results in a large wasted space.On the other hand,the influence of eddy current and parasitic parameters of windings are more serious at MHz,which results in a large loss.To solve the problem of wasted space caused by the core thickness,the magnetic flux distribution model in the core is established and a multi-window EI core transformer with I core extended in multiple directions is proposed,which can reduce the core thickness and improve the power density significantly without increasing the core loss.To reduce the eddy current loss at MHz frequency,the optimizing method of the winding terminals is proposed to fully cancel the vertical magnetic field.Better current sharing among windings can be achieved and winding loss can be reduced.Finally,based on the proposed optimization method of magnetic core and windings,a cellular MHz isolated DC/DC prototype with high step-down ratio is designed for verification,which achieves a breakthrough in efficiency and power density.To cope with the future demand for more stringent lightweight and miniaturization of power modules,the size of the power transformer in isolated DC/DC converters needs to be further reduced and the limitation of the power density need to be explored.In conventional planar transformer designs,the extension direction of the planar winding is perpendicular to the flux direction,which leads to a larger perimeter at the outside of the winding and a larger winding impedance.On the other hand,the transformer area increased rapidly by a square factor as the winding width increases.By modeling and analysis,these structural limitations of planar transformers are revealed and a transformer structure with 3D PCB winding is proposed,where the winding extension direction is made parallel to the flux direction.In this way,the impedance can be reduced efficiently with winding width extension.At the same time,the devices can be directly integrated into the windings surrounding the core.The contradiction in space utilization between core thickness and device thickness is overcome and the space utilization of the converter can be improved significantly.