This patent is presumably innovative -- since the US Patent and Trademark Office issued it -- but I can't see where the potential usefulness of it is. Maybe readers can help.
The full, grandiose title of the patent is "Direct AC-to-DC Converter for Passive Component Minimization and Universal Operation of LED Arrays." I looked at this one because the topic of AC LEDs has been a frequent theme on this website, and I've written about them here as well. The patent was issued to Nick Langovsky and assigned to iLumisys (sic). As usual, we are going to look at: A) what problem the patent is addressing, B) how the invention works, and C) where we might expect to see LED lighting embodying this patent.
The problem
Direct AC drive is the wave of the future for LEDs. As the price of LEDs continues to plummet, the driver is becoming the biggest electronics cost in lighting. People are thinking about ways not to need a SMPS, but instead do some kind of direct drive, where the LEDs are driven more or less directly from the AC line.
Now in the old days (by which I mean a couple of years ago), people stacked up a long string of LEDs to get a forward voltage of, say, 120 V, and attached this string from hot to neutral on an AC line. Then to make sure that the LEDs weren't busted by reverse voltage when the line polarity changed each half-cycle, they put another identical string from neutral to hot. But this wasn't very good efficacy, because half of the LEDs were off during each half-cycle. Instead people realized that all you need is a bridge rectifier, and then the LEDs would all be on all the time.
The method
What this patent does may be thought of as a big step backwards. It too rectifies the AC line so that the LEDs are on all the time. But instead of using a diode bridge which costs a couple of cents, it makes a full-bridge out of four switches (see the figure). Not just MOSFET switches -- each switch is bidirectional, an N-channel MOSFET back-to-back with a second N-channel MOSFET!
Now, full bridges are fairly common in power supply world. They are used for very high power, when using a converter that only has one or two transistors would make those transistors too hot -- so you spread the heat out by having only one (diagonal) pair of transistors on at a time, and then the voltage is split between them.
This patent is still sort of direct drive -- there are no power inductors. The patent calls out that the switches' duty cycle can be modulated to affect how much current the LEDs get, and when during the line cycle that current is pulled from the line to optimize efficiency. So much for the good parts.
Likely applications
The bad parts, however, completely outweigh the good. Eight N-channel MOSFETs all rated at least 250 V. How can that possibly compare in cost with a diode bridge? While a diode bridge is on all the time, and this contraption can be controlled, you can accomplish the same duty cycle modulation with a single N-channel MOSFET in series with the LED string after the bridge. Plus, that single MOSFET would be ground-referenced so it's easy to drive. The upper two switches in this patent require some sort of floating gate drive to run them, which isn't cheap either.
Why was this patent applied for -- it costs a lot of money to get a patent -- and why did the Patent Office grant it? Well, the second question is easy. It's not the Patent Office's job to determine if a patent is cost-effective. It just determines whether it's novel. Since it was granted, I guess it is novel. What can the company do with this patent? There's no law that you have to use the technology you patent in your products. Maybe it looks good to the customers -- and investors -- if you have an issued patent. Of course it looks very good to an inventor to have a patent on his or her resume.
So that's my guess on why this patent was filed. I'd be delighted if someone can inform me if there is a good technology reason for it.
Related links
- Recent Patents: Modulating the Light Emission of a Lighting Device
- Recent Patents: Lumped Color
- Recent Patents: 'LED Lighting Lamp'
No comments:
Post a Comment