A high efficiency transformerless step-up DC-DC converter with high voltage gain for LED backlighting applications

Single Boost converter has several limitations in driving a long LED string although it can have perfect current match between each LED. Extreme duty cycle and low efficiency are the two major drawbacks. In this paper, a high efficiency step-up DC-DC converter with high voltage gain for LED backlighting applications is proposed. Without using a transformer, a multi-modes charge-pump is cascaded with a Boost converter to achieve high gain. The charge-pump is unregulated and can switch between 1×, 2×, and 3× mode according to the output requirement. The LED current is regulated by controlling the duty cycle of the Boost converter. The charge pump is located at the low current side therefore can have very high efficiency. The Boost converter is located at the low voltage side and can operate at a relatively low duty cycle, thus the efficiency is improved. The proposed solution can achieve high voltage gain as well as high overall efficiency. The control will also be benefited by the two stage solution. Theoretical efficiency derivations are given in this paper. A prototype is built and the experiment shows a maximum 10.5% efficiency improvement when driving 11 LEDs from 2.7V input voltage.