Ecosparks, some feedback here, it isn't the LEDs themselves that produces the EMI, nor is it capacitors in drivers, capacitors are used in conjunction with inductors for EMI suppression in fact. The problem is switchmode drivers. These can be found in all types of LED bulbs, sometimes even in units using many 5mm or similar 20mA LEDs. The type of LED has nothing at all to do with the driver type.
You seem to be confused as to what constitutes a driver. The driver is whatever electronic circuitry limits the current through the LEDs (and in doing so matches the supply voltage to the LED Vf). The driver is the circuit inside the MR16 lamp that limits the current, and is usually a tiny switchmode converter with constant current and therefore constant power output. The 240 volt AC to 12 volt transformer/power supply that is mounted in the roof space and drives either the original halogen or the replacement LED bulb is not the driver, it is simply a voltage converter. These don't usually cause problems as they have RF filtering built in. Some LED bulb/fitting setups do have off-line drivers which are both a voltage converter and current controller, but these are not common in domestic situations yet.
Most multi-LED units (with 20mA LEDs) do only use a simple driver system such as resistors for 12 volt use and a capacitor/resistor/rectifier bridge arrangement for mains use, and these won't produce electrical noise as there is no switching going on. In the case of the AC setup, the capacitor does most of the current limiting, the resistor is there to limit the initial inrush current to something the LEDs can tolerate, and the bridge is there for rectification of course.
What causes RFI in LED bulbs is when the driver is a switchmode type, without RF suppression. These can occur in both DC and mains (off-line) types. Topology of drivers varies a lot, but many mains powered bulbs are now using the supertex off-line driver ICs that work in conjunction with an inductor to do the current limiting (the HV9921/22/23 series). You can check these out at http://www.supertex.com/feature_LED_general.html
In MR16 (12/24 volt) bulbs, the drivers can be based on a large range of different miniature switchmode drivers, there are so many now I couldn't list them all here. These are meant to have RFI suppression components used in their designs but they often don't due to space limitations inside the bulbs themselves. The ironic thing is that most MR16 drivers use bulky electrolytic caps as their main rectifier filter caps, yet electros have a notoriously short lifespan, especially when running at high temps. The electros are usually the first thing that dies in these drivers and it in turn causes the driver IC to shut down or fail itself. Anyway, the space saved by using better caps like ceramic chip caps could be used for RFI suppression components in these bulbs. I guess it all comes down to price, but even most of the expensive LED bulbs have electros in their drivers and little or no RFI suppression.
LEDs are not made from silicon (or silicones, which are polymers containing silicon), they are made from more advanced materials like Gallium Nitride and Indium Gallium Nitride. The problem isn't so much in the materials though (although some of the chinese and korean LED chips could stand improvement) but rather how they are bonded inside the finished LED case. The bonding to the die cup can be variable and so the chip can't pass heat to the die cup/thermal slug, so they can run hot and fail early. Sometimes, the bonding is fine until the LED is soldered to with excessive heat, which can then compromise the die bonding. Also, the finished LEDs (ie the LED chip in its case etc) are often not well bonded to the metal PCB or whatever they are mounted on, so there can be multiple failure causes and there's quite a bit to it unfortunately...
The current crop of chinese LEDs are actually very good in efficacy terms, they are not far behind what Cree/Nichia have produced in recent years, they seem to be about a year behind best case technology.
But, if someone tells me they want halogen replacement bulbs and doesn't want to replace their fittings with well designed LED fittings or ES27 based can fittings, which can take LED, CFL etc bulbs (I usually recommend the ES27 fittings if you still want to stick with downlights), I do tend to recommend to people to buy Cree or Nichia based bulbs from companies that have independently tested their bulbs. But that's not always easy as they are often far more expensive than the competition, so it again boils down to up front cost.
Posted Tuesday 15 Jun 2010 @ 1:30:22 am from IP #