It was only a couple of years ago that I started using LED light bulbs. I was at the store one day, I noticed LED bulbs that were much cheaper than I expected (thanks to some sort of rebate from the energy company), and I bought them.
I still prefer incandescent bulbs during the winter, but it’s clear that LEDs will become an increasingly dominant lighting technology. The improved efficiency compared to incandescent bulbs is impressive, and I for one am satisfied with the quality of the light. In fact, when I think about the major advantages of LED bulbs, I start to wonder, “What’s the catch?” There is a catch, but it’s not something that consumers notice. Rather, it’s something that electrical engineers notice: LED lighting circuitry is complicated.
Driving LEDs from AC
An incandescent bulb requires some glass, some metal, and a tungsten filament. It doesn’t get much simpler than that. In contrast, an LED bulb must incorporate highly sophisticated circuitry in order to ensure effective LED control that is powered directly from mains voltage.
The following schematic shows an example of a power supply built around the UCC28910, which is a high-voltage flyback switcher from Texas Instruments intended for applications such as LED lighting and wall adapters.
Diagram taken from the UCC28910 datasheet.
The circuit actually looks much more simple than it really is—take a look at the next diagram, which shows you what’s going on inside the controller IC.
Diagram taken from the UCC28910 datasheet.
Primary Side vs. Secondary Side
One of the challenges involved in designing power supplies of this kind is the issue of output regulation. In the context of this article, output regulation refers primarily to output current regulation, because in LED applications we need to focus on providing constant current rather than constant voltage.
Safety requires that the power supply’s output be electrically isolated from its input. This is achieved by using a transformer, such as the one in the schematic shown above. This raises a question, though: How can the controller IC regulate the output if the output is electrically isolated from the IC and everything else to the left of the transformer?
The typical solution is to use an optocoupler. This overall scheme—achieving regulation via optically coupled feedback from the output side of the transformer—is called secondary side regulation (SSR).
The alternative to secondary side regulation is primary side regulation (PSR). With this technique, no secondary-side feedback is required. Somehow the controller derives adequate feedback information from the magnetic field associated with the transformer. I don’t claim to understand the details of this method, which appears to be quite sophisticated.
A thorough comparison of PSR and SSR would probably require an entire article (and I’m not the one to write it). The bottom line is that PSR is generally preferred, because it reduces board space and cost, both of which are critical factors in LED lighting and many other applications.
STMicro describes their new LED lighting controller as a “state-of-the-art” device that provides “greater energy savings” along with “convenience and simplicity.” I don’t doubt that this is a high-performance IC that offers improved functionality, but I think that this description came more from the marketing department than from the engineering department.
The HVLED001B supports both primary side and secondary side regulation. It converts AC to DC with high power factor, which is an increasingly important characteristic as LED lighting becomes more widespread, and it can deal with various types of failure conditions. The typical quiescent current is quite low—about 500 µA. I’ve never really thought about the quiescent current consumed by a light bulb, but if urban areas become populated with countless LED bulbs, I suppose all that quiescent current could add up.
As with the TI part discussed above, there is a lot of circuitry inside the HVLED001B:
Diagram taken from the HVLED001B datasheet.
The datasheet for the HVLED001B indicates that the part has “high efficiency” over “a wide voltage and current range.” This is an important specification—increased efficiency is one of the major selling points for LED bulbs.
However, I couldn’t find any specific efficiency information in the datasheet. The performance plots don’t involve efficiency, and a search for the word “efficiency” returns only the “high efficiency” statement included in the “Features” list on page one. The datasheet for the UCC28910, in contrast, has quite a bit of efficiency-related information.
Do you have any experience with designing AC to DC converters for LED applications? If so, it would be great to hear your thoughts on this new part from STMicro.