AC Switched-Matrix Technique Opens New Possibilities for Offline LED Drivers

Texas Instruments, Inc. (TI) has introduced the industry's first floating switch architecture that simplifies the offline linear drive of LEDs in lamps, downlights and fixtures. The ac switched matrix technique features TI's TPS92411 floating MOSFET switch and is an innovative approach to producing low-ripple LED drive current without magnetic components. The architecture provides excellent compatibility with legacy wall dimmers and delivers high power factor and low total harmonic distortion (THD).

The TPS92411EVM-001 evaluation board module implements a 120Vac, 12W, phase-dimmable light engine with high power factor and low LED current ripple on a single-sided metal core printed circuit board (MCPCB). The included LED drive circuit is comprised of a high-voltage linear regulator and TPS92411 floating switches arranged to toggle three segments of a high-voltage LED string at eight discrete points during an ac half-cycle. Unlike traditional switched mode power supply (SMPS) converters, the TPS92411EVM-001 uses no magnetic components or high speed switching for power processing.

Flyback, buck and boost converters are common SMPS topologies in LED lamps today. These circuits operate at high frequencies and require inductive components to turn mains AC current into the constant DC current necessary to drive LEDs. Selection of inductors and transformers for power transfer is complicated, often requiring expensive custom components. In addition, an electromagnetic interference (EMI) filter consisting of four to ten passive components is required to prevent the SMPS circuit from harming other electronic equipment. If phase dimming is required, multiple design iterations may be necessary to optimize dimmer compatibility and prevent LED flicker.

The floating switch architecture with TI's TPS92411 overcomes these challenges. It requires no inductive components for power transfer, eliminating cost and bulk and simplifying design. The TPS92411 includes a 100-V, 2-Ohm floating MOSFET switch with 350 mA of current capability. The LED drive performance is comparable to traditional flyback, buck and boost converters: Greater than 0.95 power factor. Less than 15 percent THD. And low LED current ripple results in efficient LED utilization as compared with "tapped linear" drivers.

The TPS92411 features a low-frequency slew-rate controlled switch action produces very little EMI noise. Unlike designs using traditional converters, the very low-cost floating switch architecture requires no inductive components. This solution works well with both TRIAC and reverse phase wall dimmers. And it supports lighting designs up to 70 W. Available now in volume from TI and its authorized distributors, the TPS92411 is offered in a 5-pin SOT-23 package priced at US$0.23 each in 1,000-unit quantities. An evaluation module is offered at $75.00. The TPS92411 in an 8-pin PowerPad(TM) SOIC package will be available in the first quarter of 2014.