Mains voltage LEDs?

[MarkS]

My mind was wandering last night while I was trying to go to sleep, as it typically does, and I came upon the weird thought as to why there are no mains voltage rated LEDs. Most LED home lightbulbs use inefficient capacitor droppers and this is the primary failure point. My initial thought was that it's an issue of putting that much voltage and current across a doped piece of silicon, but then remembered rectifier diodes, SCRs, thyristors and the like. Clearly, it's "possible". Is there a technical reason as to why this doesn't exist, or does it exist and I'm just unaware?


[edit] A quick Digi-key search and I found some! But, they're obsolete. 

[thm_w]

There are various 3V, 6V, 12V etc. LEDs with matching number of dies in them. You take 10 or 20 of those and put them in series.
This allows you to spread out the heat easier.

But yes, COB LEDs with 30+ die inside also exist. Its just that you'll get a lot of 60Hz flicker from them if run directly.

You can find a lot of good variations from big clive

[tom66]

There are definitely mains voltage LEDs.  I'm sure Clive has done a teardown on one that used linear regulation of a ~250V strip from the 320V rectified capacitor supply.  And I don't mean that video above, but a COB LED that is all the required LED die in series.

[TimFox]

My mind was wandering last night while I was trying to go to sleep, as it typically does, and I came upon the weird thought as to why there are no mains voltage rated LEDs. Most LED home lightbulbs use inefficient capacitor droppers and this is the primary failure point. My initial thought was that it's an issue of putting that much voltage and current across a doped piece of silicon, but then remembered rectifier diodes, SCRs, thyristors and the like. Clearly, it's "possible". Is there a technical reason as to why this doesn't exist, or does it exist and I'm just unaware?


[edit] A quick Digi-key search and I found some! But, they're obsolete. 

Rectifier diodes, thyristors, transistors, etc. can tolerate relatively high voltages across reverse-biased PN junctions.
However, forward-biased junctions operate at relatively low voltages: roughly 0.2 or 0.3 V for Ge or Schottky Si diodes, 0.6 to 1 V for PN Si diodes, > 2 V for LEDs (depending on color).
To emit light, an LED must be forward-biased.

[MarkS]

Rectifier diodes, thyristors, transistors, etc. can tolerate relatively high voltages across reverse-biased PN junctions.
However, forward-biased junctions operate at relatively low voltages: roughly 0.2 or 0.3 V for Ge or Schottky Si diodes, 0.6 to 1 V for PN Si diodes, > 2 V for LEDs (depending on color).
To emit light, an LED must be forward-biased.

Thank you! I knew it was something like that, but couldn't remember exactly what.

[IanB]

The new design of "filament" LED bulbs use long strings of LED chips in series so that the voltage adds up to just a few volts short of the rectified mains voltage. They then use a current regulator that only has to drop a few volts to keep the right current flowing through the LED strings. These LED filaments could be considered "mains voltage LEDs".

[tooki]

Note also that the forward voltage of LEDs correlates to the color, with some variance for the LED chemistry. The longer the wavelength, the lower the voltage. IR LEDs are often 1.2-1.5V, red 1.7-2, lime green 2-2.2V, emerald green 2.7-3V, blue 3.2-3.4V, 3.4-3.6V for UV-A, all the way to 6V for UV-C. (White LEDs are based on blue LEDs.)

So you can’t actually make a high voltage LED. Their voltage is innate. It’s not like an incandescent lamp where you can design it to natively use whatever voltage you want. So when you see “high voltage” LEDs, it’s actually one or more LED dies, either in large arrays, or with some circuitry inside.

The other reason LED bulbs need driver circuits is because they need DC, not AC, and to smooth it so they don’t flicker.

[coppercone2]

I use some of those filament lamps, I think they look good when you put them in a shrowded holder

[Marco]

You can find a lot of good variations from big clive

AFAICS the majority of cheap bulbs are just rectifier, capacitor, led-string and current-limiter now.

[NiHaoMike]

Most LED home lightbulbs use inefficient capacitor droppers and this is the primary failure point.

Capacitive droppers are very efficient. They should also be very reliable if the components used are good quality and properly rated. The problem is manufacturers cutting every last cent they can get away with in order to get a bit ahead of the competition.

[coppercone2]

I think the problem is overloads. When I had a overload that broke a bunch of things I measured a bunch of caps in like air conditioner and stuff, they were all severely reduced in capacitance. There is just not enough protection in houses and the grid. It's such a flimsy structure that I don't think its durable enough for the rough power grid. Still LED light seems to be doing better then CFL for now.

[BadeBhaiya]

Maybe slightly related, I have seen a new kind of one of these mains LED boards.

These LED boards control the current though the LED string by using an in-series inductor and varying the switching frequency of the rectified mains though it. The switching part was an SO-8 and the reference current was set using a resistor

I don't remember the exact part number, but it was a Chinese one. Also I might be wrong and I just may have been unaware of their existence, and they are not new at all

[Marco]

These LED boards control the current though the LED string by using an in-series inductor and varying the switching frequency of the rectified mains though it. The switching part was an SO-8 and the reference current was set using a resistor

Probably some offline buck converter, without an electrolytic capacitor that's still going to flicker though.

I wonder why almost no one uses offline boost converters with LED strings. Just operate at poor efficiency and power factor low on the mains voltage. Minimal flicker, no electrolytic if you don't mind terrible PF.

[tom66]

Probably some offline buck converter, without an electrolytic capacitor that's still going to flicker though.

I wonder why almost no one uses offline boost converters with LED strings. Just operate at poor efficiency and power factor low on the mains voltage. Minimal flicker, no electrolytic if you don't mind terrible PF.

It's probably cheaper to use a proper filter cap and a buck converter.   The LED bulbs we have don't flicker.

[Marco]

Thanks to the electrolytic it doesn't flicker, without the electrolytic you could increase reliability though. I doubt boost would increase cost over buck.

TI has been making the TPS92561 for a while, but the topology seems relatively rare.