Current status and trends of LED lighting design

In the past, light-emitting diodes ( LEDs ) were used only as indicator lights. Power consumption was low, current was even lower, and heat generation was not a problem. However, times have changed. LED currents have increased from a few milliamps to amps; in some cases, LED power has jumped from a few milliwatts to more than 10W; and the heat generated by LEDs - well, it's safe to say that it has become a problem that cannot be ignored.

In the past, LEDs were so low-power that waste heat was very low, and they basically never burned out, said Rick Zarr, a technical expert at National Semiconductor . " But new LEDs generate a lot of heat, so if you don't control it correctly, it will have a big impact on their lifespan. An LED bulb could last 10, 20 or 30 years, but if you don't control it properly, it may be scrapped in two years. That's why project engineers now need to focus on LED drivers . At their simplest, drivers control the input current and input voltage, and then reconfigure them for the LED to use. In this respect, drivers are very similar to the ballasts that have been used in fluorescent lamps for decades.

On the other hand, though, drivers are becoming increasingly sophisticated, making them even more important in the new era of high-power, high-current, high-heat LEDs. Today’s drivers are capable of functions that were unthinkable a decade ago. For example, by sensing the current, they can estimate the LED’s brightness level and change it accordingly. They can compensate for changes such as heat or aging, and even use LEDs for lighting applications. And by using “heat foldback” schemes, they can prevent the LED from heating to the point where it would harm its lifespan.

Thanks to innovations like these, new generations of light-emitting diodes are finding a home in many unforeseen applications. Cars use LEDs as brake lights, "puddle lamps," and even headlights. Televisions use LEDs for backlighting; municipalities use them for streetlights; and consumer electronics manufacturers are using them in netbooks, tablets, and GPS systems .

"LEDs are getting better every year," said Peter Di Maso, marketing manager for lighting and power products at Texas Instruments . "These improvements are making LEDs viable for general lighting, which is why there is such a pressing need for better drivers."

Growing demand

Of course, not all forms of lighting require a driver. For example, incandescent light bulbs have long been optimized to work with the 110V power supply we commonly use, so no additional circuitry is needed to adapt the input current to the output.

However, light-emitting diodes are more complicated. "When using LEDs, you first need to understand the input power and how it varies," said Steve Bowling, applications manager for Microchip's 8-bit microcontroller products . "On the output side, engineers need to know their optical requirements and the lumen value they want to produce . In the end, the ultimate goal is to have the driver provide a constant current source to the LED."

Thanks to a phenomenon known as Haitz's Law, though, driving LEDs has become more complex and more necessary. Haitz's Law -- named after Roland Haitz, a retired scientist at Agilent Technologies -- states that the cost of an LED to produce one lumen falls by a factor of 10 every 10 years, while the brightness produced per package increases by a factor of 20. For LED users, Haitz's Law means increased demand, which in turn means the task of developing new technologies flows downstream.

"People want to put an LED in place and not have to think about it anymore," said National Semiconductor's Zarr. "They don't want to have to think about it, so the burden is shifted to the LED manufacturer, who then passes the pressure on to the driver manufacturer downstream."

Avoid fever

For driver manufacturers, the challenge is to add the necessary features in a small package. Driver chips are very small, typically measuring 10 × 10 mm or less. And as LED manufacturers incorporate LEDs into light bulb designs to replace incandescent bulbs, future driver chips will need to be made even smaller.

As they move to such applications, drivers need to be able to prevent LEDs from overheating. To do this, many suppliers tend to sell heat sinks and fans to customers to help dissipate heat. Although more and more LED users want to minimize the heat of the LED before it needs to be cooled, the solution to this problem starts with the driver.

Control dimmer

Experts say that over the past few years, component manufacturers have made drivers more powerful. TRIAC (triode-controlled alternating current) modulation circuits, long considered a problem for semiconductor lighting , can now be used with LEDs. Using LEDs in such circuits will enable many new applications for home, commercial and industrial lighting, experts say.

“When people do move to LED bulbs, they don’t want to change their infrastructure,” Zarr said. “They don’t want to have to take the dimmers off the wall. That’s why we need a solution for dimming applications.”

An example of a dimming solution is the National Semiconductor LM3445, which incorporates circuitry to read the dimming signal from a standard TRIAC dimmer and convert this information into a pulse-width modulated current to drive the LED.

Texas Instruments' TPS92010 solves a common power consumption problem in TRIAC-LED solutions. The TPS92010 integrates a circuit that does not generate power consumption during the use of the TRIAC.

"The current is only drawn when the voltage input to the light source is zero, so the power consumption is zero watts," said Di Maso of Texas Instruments, which offers an evaluation module to engineers who want to incorporate this feature into their bulb designs.

Increased intelligence

As driver manufacturers look to add new features, they are also incorporating microcontrollers into their products. Onboard intelligence makes microcontroller-based drivers the best candidates for implementing features such as dimming, as it enables them to monitor various functions and then make decisions accordingly.

“The driver can use the inputs from the system—in this case, the inputs from the TRIAC dimmer,” said Microchip’s Bowling. “It can read the line voltage. It can read the duty cycle of the TRIAC dimmer. It can monitor the operating voltage and temperature of the LEDs. Then, it makes decisions to keep the system operating at the selected brightness level.”

Microchip offers a family of 8-bit microcontrollers and 16-bit digital signal controllers for drive applications . The 8-bit microcontroller PIC16F785 integrates analog peripherals , including operational amplifiers, comparators and 12-channel 10-bit A/D conversion. "You can build blocks on the microprocessor," Bowling said. "You can use analog devices to adjust the drive and the microprocessor to facilitate this function."

Microchip's 16-bit dsPIC33GS digital signal controller family takes a step forward by integrating extremely high-speed A/D converters, enabling them to collect data in real time. As a result, the dsPIC system allows developers to expand into more advanced applications, including dimming, thermal protection and color control.

Likewise, Texas Instruments offers its Piccolo family of microcontrollers for LED driver applications. In a DC/DC LED development kit (see video about the kit), the technology uses an MCU to control a string of LED lights. The kit is targeted at customers who want to use LEDs in high-power applications without having to change bulbs as often. Applications include street lights, parking lots, and industrial lighting, where users don’t want to frequently replace burned-out bulbs.

“The Piccolo microcontroller can remotely command a specific LED string to a specific brightness level,” said Charlie Ice, marketing manager for C2000 microcontrollers at Texas Instruments. “It can sense the current and determine the brightness level, and it can compensate for the situation. If an LED has aged or overheated, it can also compensate for that.”

There is no “one-size-fits-all solution”

In almost all LED applications, experts point out that efficiency is the triggering factor. Compared with incandescent bulbs, many of which now use digital control to turn on and off at specific times to save energy, LEDs do not necessarily require intelligent control mechanisms. The efficiency of LEDs is so high, almost beyond any product, that it is almost unnecessary to implement intelligent control.

That’s why automakers have adopted LEDs in taillights, interior lighting, and headlights. In some cases, LEDs can last 300,000 miles, outliving the vehicle they’re in. As a result, automotive designers are eager to integrate LED modules into the body of the vehicle , knowing that the module may never have to be replaced. This, in turn, allows designers to be more creative, as they find it easier to integrate packaged modules.

For the same reason, designers are also using LEDs in GPS systems, netbooks, tablets and TV backlight systems. As brightness increases and costs fall, high-efficiency LEDs make more sense. "Of course, there are dynamic mechanisms for turning lighting systems on and off," said Zarr of National Instruments. "But the simplest mechanism is to replace the bulb with something more efficient in the first place."

That’s why drivers will always be an important part of LED solutions. Drivers keep LED bulbs cool and extend the life of the bulb. In addition, the lack of standards in the LED industry means that there will still be a need for a large number of driver solutions in the future, especially as new applications continue to grow.

“The LED industry is in an evolutionary phase right now,” said Microchip’s Bowling. “We are always in need of new solutions. There is no one-size-fits-all solution for all applications.”

“Thermal foldback is not designed to be used to dissipate heat for the purpose of protecting the bulb,” Zarr said. “If you have a situation where the LED is approaching its critical operating temperature, the driver will fold back the current and dim the bulb.”

Drivers will also give LED users other reasons to limit the current flowing through the device. For example, Analog Devices can use its ADP1650 driver to control the current in high-brightness LEDs that are often used in camera flashes .

“LEDs are easily controlled from a brightness standpoint by current,” said Jose Rodriguez, technical director of Analog Devices’ power management division. “So we set the current inside our drivers and control it very tightly.”