Driving High Power LEDs

High power LEDs, particularly “white” ones are becoming increasingly used in commercial products from cars and domestic lighting to street lighting and with efficiencies which can exceed that of compact fluorescent lamps (CFL). That efficiency is likely to improve as the technology develops but you must also pay attention to your drive circuitry, not simply the LED efficiency, if you are to maximize that efficiency. Also, an inefficient circuit will mean even more heat to dissipate on top of the LED heat.

A simple LED driver is a resistor! With a fixed supply voltage you can limit the current with a resistor although the current will vary with the LED temperature. How much the LED current varies depends on the LED and how much voltage is dropped across the resistor compared to the LED. However, for high power LEDs you need something better, particularly if you are looking for efficiency.

An alternative would be a constant current drive but that would not really be any better than a resistor – all you would be doing is swapping power dissipated in a resistor for power dissipated in another device such as a transistor. The only advantage is that you could run the circuit at a lower voltage than with a simple resistor limiter because a constant current source could run with a low dropout voltage whereas a resistor is providing the current setting method as well.

The simple example above, which could also have several LEDs in series, uses R1 to set the current. The LM317 will keep 1.2V across R1 and the remaining excess voltage will be dissipated across the LM317. Choose R1 to set your current. Be careful to calculate the power dissipated in the resistor and LM317.

As an example, if you were using something like the Cree CXA1850 which has a typical forward voltage at 85°C of 35V and a maximum at 25°C of 42V, you could run your circuitry from around 43.5V if you had a low enough dropout voltage. You would typically be dropping 8.5V across the constant current circuit so the efficiency would be 80% (8.5/42.5 is the proportion of the power being wasted). However, you also have to take account of the efficiency with which you generated the 43.5V in the first place.

The forward voltage is quoted at 25°C for the worst case because the LED forward voltage has a negative temperature coefficient – when the temperature increases the forward voltage drops. So, in order to ensure you have enough voltage to turn on the LED you need to consider the forward voltage at 25°C which is the worst case.

A better option for driving high power LEDs is to use one of the many ICs available. These follow similar principles to switching regulators but instead of trying to generate a constant voltage for the load they generate a constant current. So, they can be buck, boost or SEPIC type controllers depending on your power supply. You can also make an “offline” power supply using a device such as the Linear Technology LT3799 which will allow you to power the LEDs from a 110V or 220V AC power source, such as for a lamp to replace a conventional tungsten bulb or CFL. It also offers power factor correction and is dimmable using a TRIAC controller. The LM3447 from Texas Instruments is another IC for making and an offline LED controller with similar features to the LT3799. They aren’t simple, low parts count solutions though:

Simpler solutions exist if you are working with lower, DC, voltages such as for automotive applications. With automotive applications you do not have to deal with such high voltages and the power supply will already be DC.

The LM3404 or LM3404HV schematic would look like this

which is a lot simpler than an offline, power factor correcting schematic, and not much more complicated than a simple constant current schematic. With the LM3404HV you can power it from up to 75V and drive up to 1A. As with a lot of these ICs, it includes open LED detection, thermal shutdown and dimming ability. It has an integral MOSFET but devices with external MOSFETs are available for greater current drive. You should be able to get an efficiency approaching 90%. Some synchronous rectifier based ICs will get closer to 95% efficiency although some claim up to 98% efficiency but that is at a “sweet spot” at a very specific input voltage, LED voltage and current.

For AC mains power applications where you don’t need isolation there are simpler solutions such as the Fairchild FL7701 which seems to offer a neat, low cost, simple solution for mains powered LED based lamps. The online design tools seem to do a lot of the hard work for you and suggest an efficiency of over 92% driving a Cree CXA1850. It even predicts the emissions for you, based on the recommended filtering.