Quasi Resonant DC Link Converters - Analysis and Design for a battery Charger Application

Abstract: Environmental aspects have during the last years made electric vehicles an interesting competitor to the present internal combustion engine driven vehicles. For a broad introduction of pure electric vehicles, a battery charging infrastructure is deemed necessary. However, to build and maintain such an infrastructure is costly. Active power line conditioning capabilities could be included in the battery charger, making the infrastructure advantageous from the distribution network point of view. Another option is that the battery charger could be able to support the grid with peak power during periods of stability problems or emergency situations. This means that energy is borrowed from the batteries of vehicles connected to a charger. The price for energy supplied by the batteries is likely to be several times higher than the normal electric energy price, due to the wear costs of the batteries. Therefore, the battery charger losses also represents a high cost. It is often stated that resonant converters have a high efficiency compared to hard switched. Since carrier based pulse width modulation is employed, quasi resonant DC link converters are of interest. Four of the most promising quasi resonant DC link topologies reported in the literature are compared. A fair comparison is obtained by designing them to meet certain common design criteria, in this case the duration of the zero voltage interval and the maximum output voltage time derivative. The derivation of the design expressions are given, and also the simulation results, by means of efficiency. A 10 kW battery charger, equipped with one of the quasi resonant DC links investigated is implemented. A hard switched battery charger with the same rating is also tested to compare the measured efficiency with the simulated. Both the simulations and measurements shows that the efficiency decrease for quasi resonant battery chargers compared to the hard switched case. Furthermore, low frequency harmonics appear in the battery charger input and output currents. However, full control of the output voltage derivatives is obtained.