Reducing Low-Frequency Ripple Using Alternative Output Capacitor Connection on Integrated Converters for LED Drivers

Abstract

In this paper, an alternative connection for the output capacitor on a buck-boost power control (PC) stage integrated with a buck-boost power factor correction (PFC) stage is explored. Although advantageous in several aspects, the converter's integration leads to a loss of independence between the stages. This aspect makes it impossible to adopt control techniques that act directly on the duty cycle, aiming to eliminate the low-frequency ripple in the output current without distorting the input. To comply with IEEE 1789–2015, the integrated converters use large capacitors to mitigate low-frequency ripple. The alternative connection provides current feedback to the bus capacitor, acting to reduce the output current and bus voltage low-frequency ripple. A dynamic model for the alternative connection capacitor in continuous conduction mode (CCM) is obtained and used to determine the output current modulation as a function of converter parameters. The proposed topology in CCM always leads to a smaller low-frequency ripple than a conventional counterpart at the same operating point. A numeric example designs the proposed converter in compliance with IEEE 1789–2015, and demonstrates that a conventional counterpart with same components and operating point is not able to comply the same recommendation.