Performance and scalability of isolated DC-DC converter topologies in low voltage, high current applications

摘要

Fuel cells are a promising alternative for clean and efficient energy production. A fuel cell isprobably the most demanding of all distributed generation power sources. It resembles a solarcell in many ways, but sets strict limits to current ripple, common mode voltages and loadvariations. The typically low output voltage from the fuel cell stack needs to be boosted to ahigher voltage level for grid interfacing. Due to the high electrical efficiency of the fuel cell,there is a need for high efficiency power converters, and in the case of low voltage, high currentand galvanic isolation, the implementation of such converters is not a trivial task.This thesis presents galvanically isolated DC-DC converter topologies that have favorablecharacteristics for fuel cell usage and reviews the topologies from the viewpoint of electricalefficiency and cost efficiency. The focus is on evaluating the design issues when considering asingle converter module having large current stresses.The dominating loss mechanism in low voltage, high current applications is conduction losses.In the case of MOSFETs, the conduction losses can be efficiently reduced by paralleling, but inthe case of diodes, the effectiveness of paralleling depends strongly on the semiconductormaterial, diode parameters and output configuration. The transformer winding losses can be amajor source of losses if the windings are not optimized according to the topology and theoperating conditions. Transformer prototyping can be expensive and time consuming, and thusit is preferable to utilize various calculation methods during the design process in order toevaluate the performance of the transformer. This thesis reviews calculation methods for solidwire, litz wire and copper foil winding losses, and in order to evaluate the applicability of themethods, the calculations are compared against measurements and FEM simulations. Byselecting a proper calculation method for each winding type, the winding losses can bepredicted quite accurately before actually constructing the transformer. The transformer leakageinductance, the amount of which can also be calculated with reasonable accuracy, has asignificant impact on the semiconductor switching losses. Therefore, the leakage inductanceeffects should also be taken into account when considering the overall efficiency of theconverter.It is demonstrated in this thesis that although there are some distinctive differences in the lossdistributions between the converter topologies, the differences in the overall efficiency canremain within a range of a few percentage points. However, the optimization effort required inorder to achieve the high efficiencies is quite different in each topology. In the presence of practical constraints such as manufacturing complexity or cost, the question of topologyselection can become crucial.