A bright idea - LED floating switch architecture

TI’s TPS92411 LED floating switch architecture recently won a 2014 ACE (Annual Creativity in Electronics) Award in the LED Lighting category.  It’s garnered thousands of sample requests and we’ve sent more than 850 evaluation boards to customers.  But what makes this product so special and how was it developed? Here’s my perspective on the design and team effort that led to the award.

The TPS92411 provides comparable performance to popular switch mode power supply (SMPS) LED drivers with less design effort than conventional AC/DC LED drivers.  In a typical LED lighting product, the cost is evenly divided between the drive electronics, where TI’s TPS92411 fits, the LEDs and the mechanical enclosure.

You can think of these SMPS electronics as being similar to a wall adapter for a tablet.  In the case of an LED light, the electronics convert AC into DC current. Unlike SMPS approaches, TI’s TPS92411 solution does not use transformers or inductors as found in common flyback or high voltage buck circuits used in lighting.  Additionally, flyback or buck converters require an elaborate electromagnetic interference filter and that typically adds one or two more inductors.  The TPS92411 and competing tapped “AC” linear drive techniques eliminate the need for these magnetic parts, which helps lower costs and simplifies design.

TI’s AC LED driver circuit realizes low ripple DC current that makes better use of LEDs while producing a more pleasing (and safe) light source.  For manufacturers, there is an added cost savings of using the TI AC LED driver solution versus “tapped” linears because a typical TI circuit needs fewer LEDs to produce the same amount of light.  External third parties have independently tested the TPS92411 against “AC” competition and found its results were superior.

There were challenges to overcome.  Due to LED process limitations, manufacturers guarantee the amount of light an LED will produce based upon a fixed DC current flowing through it.  The voltage drop across an LED at a given DC current is variable depending upon the manufacturers’ process.  LED drivers regulate current and not voltage so as not to damage the LEDs.  In a traditional SMPS AC to DC driver, the variable AC is “buffered” by reactive components to reduce the line frequency (50-60Hz) ripple content and produce a stable DC current for powering the LEDs. 

Competing tapped “AC linear” LED drivers take a simple approach.  They work by using three to five constant current sinks which divide a long, series-connected string of LED string into “taps”.

Internal control logic senses the sinusoidal AC voltage and determines when each tap should turn on/off and draw current directly from the rectified mains through the LEDs on a tap.   When the AC voltage rises greater than the LEDs connected to the first tap, its current sink turns “on” illuminating this section of LEDs.  As the AC voltage rises further and above the voltage of the first plus second tap points, the first tap current sink turns “off” and the second one turns “on.”  Likewise for the third tap.  At this point the full LED string is illuminated.  The sequence reverses as the AC voltage decreases from its peak back to zero.

One major drawback here is 100% current ripple in the LEDs.  When the line is less than the first tap LED voltage, the entire string is off which means 100-120Hz, 100% current (and light) ripple are present.  Some people have shown physiological sensitivity to low frequency, high current ripple light sources.  In addition, low frequency ripple may present hazards around machinery (see DOE fact sheet) limiting where LED fixtures based on AC drive can be installed.

A second problem with tapped AC linear drivers is LED utilization.  The LEDs connected to the second and third taps are “on” less time than the LEDs in the first tap.  The result is more LEDs are needed to produce the same amount of light compared to a true constant current SMPS LED driver.

Fortunately, the TPS92411 overcomes these problems.  While the TI AC direct drive idea is similar – match LED stacks as closely as possible to the alternating line voltage – the TI approach utilizes energy storage capacitors across each “tap” to provide current during the portion of the AC cycle when the tap voltage is greater than the line voltage.  If the capacitors are sufficiently sized, low ripple DC current – similar to that of a SMPS LED driver – flows continuously through the LED stings.  Low percent flicker and flicker index values can be achieved.  LED utilization is high and predictable because the LED current is basically DC.

Announced today, the TPS92410 is a 450V linear current controller that makes it even easier for lighting engineers to create a design with the TPS92411 floating switch architecture.  The TPS92410 includes some added driver and LED circuit protection features needed for industrial lighting and includes an analog dimming pin with LED shutoff.  The dim pin is ideal for “intelligent” lighting applications where a conventional microcontroller or RF micro is used to control the lighting intensity.  Simply put, the TPS92410 further enhances the capabilities of the TPS92411.  Try it out today and let me know how it works for you?

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