Low-Cost LED Driver Based on a Low-Dropout Current Regulator Combined With a Quasi-Resonant Buck Preregulator

The ease of control of light-emitting diodes (LEDs) allows the development of drivers with luminous flux control without significantly increasing the cost. The preferred control system to minimize light color deviation at low power levels is through pulsewidth control. However, IEEE PAR1789 recommendations indicate that the frequency of these pulses should be greater than 1.5 kHz to eliminate the adverse effects of light flicker. This strongly affects the design of LED drivers. Among other effects, it makes it more difficult to avoid the appearance of audible noise in the magnetic elements and can limit the ratio between the minimum and maximum power achievable without losing linearity. In this article, a new solution based on a quasi-resonant buck in series with a low-voltage-drop current regulator is presented. The quasi-resonant buck is used to minimize the headroom voltage of the dissipative current regulator, thus maximizing the efficiency. The operation of the resonant converter is analyzed using an energy-balance approach to model the circuit behavior during the resonant transitions. Based on this approach, a straightforward design procedure is proposed and applied to a design example. This example is used to build and test a low-cost experimental prototype based on the L6562A control integrated circuit. This article is accompanied by several LT Spice files demonstrating the basic operation of the proposed circuit.