One of the “quests” I have been on lately, is to find an optimal solution for driving Light Emitting Diodes. I want a solution that balances three key needs; efficiency, flexibility and cost. Efficiency and cost have a direct linear relationship it seems – the more efficient something is, the more it will cost. Flexibility seems to have an inverse relationship with efficiency – the more flexible a solution is the less efficient.
For example, take these three basic “led driver” prototype circuits:
Starting off, the plain ‘ol resistor. The only strength this solution has is cost. It is neither flexible nor efficient. That is not to say an optimally designed solution will not see decent efficiency using a plain resistor, the designer needs to closely match the LEDs forward voltage to that of the supply. Now as long as the supply voltage never changes and the attributes of the LEDs never change, the resistor will do its job, limiting current. However, if the designer has limited control over the supply voltage, the resistor’s efficiency is down the drain. Because a resistor is a passive component, it cannot react at all to any changes – therefore the designer needs to specify a resistor for the maximum anticipated supply voltage, which results in sub-optimal light output during periods when the supply voltage is less than maximum. Low efficiency equates to wasted power and excessive heat. In summary, the plain ‘ol resistor is the easiest ‘driver’ to build, using simple arithmetic, the value and capacity of the resistor can be calculated; R = (Vsupply – Vleds) / Ileds and P = Ileds ^ 2 * R.
Next, we have the humble linear regulator. I chose the LM317, which is widely available and very inexpensive. The strengths of linear regulation are two fold. First, linear regulation provides a wide degree of flexibility. Secondly, linear regulation provides a very low cost. Two out of three is not bad, but the last one is the kicker. Linear regulation is terribly inefficient. A linear regulator configured for constant current mode is going to consume (dissipate) almost as much power as the load it is regulating. This means a 5 watt LED load is going to have the regulator dissipating an additional 5 watts. So using linear regulation in your design, the power source must deliver 10 watts to give yeild 5 watts of power for the lights, and this is an optimistic 50% efficiency – National Semiconductor gives the LM317 something like 43% efficiency! In summary, the linear regulator is marginally more complex than using a simple resistor, and about as easy to design. A single equation gets us the value of Rs, the current sense resistor; Rs = 1.25 / I.
Next we have the compound, pre-made group of devices that I call switchmode drivers. The switchmode driver is a ‘black box’, in its simplest form, offering four leads, two inputs, two outputs. Drivers are available in two primary configurations; step up (boost topology) and step down (buck topology). Both configurations excel in efficiency. While efficiency alone is a good enough reason for some designs, such as battery powered applications, the switchmode drivers also offer a limited degree of flexibility. The negative aspect of a switchmode driver is cost. Prices for switchmode drivers start in the neighborhood of $10 and top out around $60. Flexibility in switchmode drivers is usually in the form of allowable ranges. For example, a step down driver may allow an input range Vout + 2V to 36V. That same driver may also have a range on the output, for example 3V to 28V. The biggest drawback to the switchmode driver is the lack of adjustable drive current. In general, a switchmode driver has to be purchased from the factory with a pre-programmed drive current. While this is fine for most designs, it is not optimal. In summary, the switchmode driver offers excellent efficiency, often times better than 85%, but it offers this efficiency with great cost and limited flexibility. There is no math required to design using a switchmode driver, you just have to pick one that matches your supply voltage and LED current needs.
The last driver I will briefly touch on, saving the details for next time, is the home-brew or DIY switchmode driver. In kit form, this driver satisfies all three of my design goals. Decently low cost (less than $10 in parts), high efficiency (more than 80%) and good flexibility (I control the programming). Of course, there are some drawbacks; some of the parts are hard to find, there is a LOT of math involved and due to high frequencies and currents, careful circuit layout needs to be observed.
Another technology I will mention, but have not researched much (it is truly bleeding edge in the industry) is something called SEPIC. The acronym stands for Single Ended Primary Inductor Converter. It is a technology pioneered by Maxim and combines the abilities of both a step up and step down regulator into a single design. The ‘old fashioned’ buck-boost regulator of yesteryear do their voodoo by internally generating very high voltages, and using the positive rail as a low side reference for the load … that design has some very hard limits and is also rather inefficient. SEPIC hopes to solve these problems with a radically different and more complex design. SEPIC offers a power supply that with ultimate in flexibility – there need be no correlation between input voltage and output voltage. Want to run 15 volts of LEDs off a 3.7 volt lithium battery, no problem. Want to run 11 volts of LEDs off an automotive supply that varies between 11 and 15 volts, no problem.
I have more to write on this subject – my next post on this subject will sum up my experience in designing switchmode power supplies and share some designs the reader may find useful.