LLC-Resonant-Converter with Current-Doubler
Resonant converters are increasingly used in recent years because they convince by a relatively simple and compact buildup and a good efficiency. Such topologies are known for a long time, but these were successfully used only in the past years. The reason for this is that the required specifications of the available semiconductors and capacitors cannot be fulfilled 10 years ago.
One possible version of this is the LLC converter. This topology can be best used for fixed input and output voltages. This allows the converter to be operated with a fixed switching frequency, independently of the load. Efficiencies of greater 97% are easily achieved. For large output currents, we also encounter the limits of the LLC converter, as the secondary copper losses cannot be neglected anymore and thereby the required efficiency is difficult to reach. Furthermore, the thermal design of the transformer, rectifier as well as the placement of the output capacitor is becoming increasingly important.
One way to improve this behavior is the insertion of a Current-Doubler on the secondary side of the converter. This type of rectification is especially known in push-pull stages, full-bridge or half-bridge converter, which require a full bridge rectifier at the output. With an additional coupled inductor, the output current is doubled and the voltage halved which reduces the losses substantively on the output side.
In this work it had to be ascertained if the Current Doubler can also be used for the LLC converter. With the derived analytical description the influence of Current-Doubler on the behavior of the LLC converter could be described. The considerations obtained therefrom helps to characterize the converter and provides a design tool for the converter components. On a 400W demonstrator, the principle and advantages of the Current-Doubler could be shown. The verification of the simulation and measurement are in a very good agreement.
Project partner: Infineon Technologies AG, Munich (DE)