Phase-dimmable LED drivers and the holy grail of total harmonic distortion levels

The LED lighting industry is still very new and many engineers involved with LED driver design are still grappling with all of the aspects, such as options and trade-offs to consider when selecting a topology. My life over the last year and a half has pretty much revolved around this topic. To help lighting power supply designers out there select a driver topology or controller IC, I thought I would organize the information I have learned into an article for Electronic Products magazine.  Power supply designers considering a buck power factor correction (PFC) design for another application could find the information helpful as well.

We’re all familiar with boost and flyback PFC applications and of course, buck converters, but buck PFC's are something new. My colleague, Montu, and I wanted to explain the basic operation as well as point out some trade-offs (there is always something). And when using the buck PFC topology for an LED driver, there is even another level of trade-offs to consider. Some aspects are not as intuitive as they seem, for example, you might try a buck PFC and later change the LED load to 120V, then wonder, gee, why am I getting such poor power factor? In this case, since the peak of the AC is 170V and the load is 120V, it does not leave much of the line cycle to provide any current.  The highest power factor is achieved when the input current follows the line cycle for the entire line cycle. This would inherently limit the maximum power factor that can be achieved. We came up with several examples in our article to help folks quickly understand the basics when starting a buck PFC design.

TRIAC-dimmable buck PFC LED drive

TRIAC-dimmable buck PFC LED drive

Of course there is always an “aha” moment for us who think we’ve seen it all.  In the course of writing this paper and testing out the examples, we found that while a common goal for power factor is 0.9, it is amazing how non-sinusoidal your input current can be and still achieve 0.9 power factor.  On the flip side, it was amazing how minor a change in input current shape can affect total harmonic distortion (THD).  Even slight deviations from a sine around zero cross dropped our THD from 9% to 13%, missing the holy grail of THD levels, <10%.  Go figure -

Buck PFC LED driver waveforms. Conditions: 23.5-V LED stack, 380-mA output current, 14.1% THD.

Conditions: 23.5-V LED stack, 380-mA output current, 14.1% THD

So, we invite you to give our article “Low-cost, phase-dimmable LED drivers” at Electronic Products a read and let us know your thoughts.

Additional resource: How to reduce total harmonic distortion to below 10%.

For more power management solutions, visit the Power House blog.