New structure for offline LED drive design eliminates inductors

November 05, 2013 // By Graham Prophet
A floating switch, AC switched matrix technique will replace flyback, buck and boost converters, according to Texas Instruments.

Texas Instruments claims a first for its floating switch architecture that will simplify 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 a new 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).

Flyback, buck and boost converters are commonly used in LED lamps today, operating at high frequencies and requiring inductive components; 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 optimise dimmer compatibility and prevent LED flicker.

TI says that its floating switch architecture in the TPS92411 overcomes these challenges. It requires no inductive components for power transfer, eliminating cost and bulk and simplifying design. Its features include;

- 100-V, 2- Ω floating MOSFET switch with 350 mA current capability

- LED drive performance comparable to traditional flyback, buck and boost converters:

Greater than 0.95 power factor.

Less than 15% total harmonic distortion.

Low LED current ripple resulting in efficient LED utilisation as compared with "tapped linear" drivers.

- 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.

- Works well with both TRIAC and reverse phase wall dimmers.

- Supports lighting designs up to 70 W.

TPS92411 comes in a 5-pin SOT-23 package at $0.23 (1,000) An evaluation module costs $75.00. The TPS92411 in an 8-pin PowerPad™ SOIC package will be available in the first quarter of 2014.

Texas Instruments; www.ti.com/tps92411-pr

A video is at; www.ti.com/pwr-pss-lpp-tps92411-pr-v-en