Switched mode power supplies (SMPS) do a very good job of controlling the input power to LED lights. So it’s not their performance that can cause anguish for LED lamp designers – its the need for bulky magnetics, which can be tricky to design and expensive to implement. In addition, SMPSs have trouble with legacy TRIAC and phase-cut dimmer switches, because these switches expect to see a purely resistive load, such as an incandescent light, rather than the SMPS’s wonky reactive load.
And perhaps the biggest problem of all for SMPS’s is that lamp designers don’t often have the design chops needed to tweak such complex electronic circuits.
Texas Instruments believes it has come up with the perfect solution for the problem of driving dimmable LED lighting over fixed voltage ranges with the floating switch architecture used in its new TPS92411. The TPS92411 replaces flyback, buck and boost converters’ SMPS architectures with a very simple AC switched matrix technique. (For more on the evolution of AC drivers read AC LEDs, no; AC drivers for LEDs, yes.)
This architecture requires no inductors for energy storage, and its non-isolated architecture does away with an isolation transformer, making the design magnetics-free.
Another advantage of eliminating magnetics is that since magnetics generally require through-hole components, and thru-hole is difficult to implement on metal-core boards, the elimination of magnetics frees the pc board designer to use the metal-core board technology generally required by LEDs, and move towards all surface-mounted devices – a cheaper high-volume production method.
Here’s a simplified schematic of how the switches work. Each bank of LEDs has differing numbers of LEDs, depending on the forward voltage needed for the bank of LEDs.
The current regulator indicated at the base is a linear circuit that uses about a dozen common (non-magnetic) components. (The overall efficiency of the driver is about 84%, in line with common non-isolated SMPS drivers.)
Note that there is a diode at the top of each bank of LEDs. These diodes allow the switches to close without discharging the capacitors, which allows continuous current flow in the LEDs. Thus the switches aren’t cutting out the LEDs, but rather switching out which bank is seeing the rectified voltage, and which bank is running off its storage capacitor. When all switches are closed (at zero volts), all LEDs will be powered by their storage capacitors. When all switches are open (which will occur at the peak 170V), the current flows through all diodes and LEDs. The three switches will open and close throughout the rectified cycle based on their set points, switching each bank’s capacitor in or out. Because the switches are floating, they require no communication. The switch trip points are set by two external resistors.
This architecture also plays nicely with legacy dimmer switches, which see the LED/switch load as purely resistive, similar to an incandescent light.
What about the size of the capacitors needed to support the LEDs when their switch is closed? In a typical 12W design, (shown below) the three capacitors will be 68uF over the 80V stack, 135 uF over the 40V stack, and 270uF over the 20V stack. If you want to play with these values yourself, you can download the TPS92411 design calculator spreadsheet tool here: 120V version.
TPS92411 Reference Design
Each 92411 chip has a 100-V, 2-Ohm floating MOSFET switch with 350 mA of current capability. It supports lighting designs of up to 70W, and provides >.95 power factor with less than 20 percent of total harmonic distortion. Its low-frequency, slew-rate controlled switch action produces very little EMI noise.
Above is a 12W reference design using Cree XLamp ML-E 10V LEDs. You can see the relatively small caps used.
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 SOIC package will be available in the first quarter of 2014.