Design Article
Efficient method for interfacing TRIAC dimmers and LEDs
James Patterson, National Semiconductor Corp.
6/28/2011 10:55 AM EDT
There are more than 2 million installed TRIAC dimmers worldwide, and they can prove a challenge to the control circuitry of an LED light. With such a large base, backward compatibility is a must.
At a glance
The lighting industry has relied on the incandescent bulb for more than a century, and, over the last 50 years, the phase dimmer has become the main dimming control. Standard forward-phase, or TRIAC (triode-alternating-current), dimmers are notoriously difficult to interface with LED drivers. To make things worse, the performance of each dimmer varies greatly. Although there are newer, better reverse-phase dimmers available, the standard forward-phase dimmer is so common in the world’s electrical infrastructure that LED-lighting manufacturers simply cannot ignore it. As usual, backward compatibility is paramount.
Forward-phase dimmers
A standard forward-phase dimmer contains a TRIAC, a DIAC (diode-alternating-current), and an RC (resistance/capacitance) circuit (Figure 1). A potentiometer controls the resistance, and the resulting RC time constant controls the amount of delay before the TRIAC turns on, or the firing angle. The portion of time when the TRIAC is on is the conduction angle, θ. The resulting voltage waveform is a phase-cut sinusoid.
This type of dimming works well in incandescent bulbs, which are simple resistive loads. The time-averaged voltage across the filament’s resistance decreases as the conduction angle decreases, providing naturally smooth dimming.
The TRIAC also has a minimum holding-current requirement. The current flowing through the TRIAC must remain above this minimum level to ensure that it remains on throughout the entire conduction angle. The incandescent load easily satisfies this condition because of the load’s inherent power levels—40, 60, and 75W, for example.
Next: Compatibility with LEDs
At a glance
The lighting industry has relied on the incandescent bulb for more than a century, and, over the last 50 years, the phase dimmer has become the main dimming control. Standard forward-phase, or TRIAC (triode-alternating-current), dimmers are notoriously difficult to interface with LED drivers. To make things worse, the performance of each dimmer varies greatly. Although there are newer, better reverse-phase dimmers available, the standard forward-phase dimmer is so common in the world’s electrical infrastructure that LED-lighting manufacturers simply cannot ignore it. As usual, backward compatibility is paramount.
Forward-phase dimmers
A standard forward-phase dimmer contains a TRIAC, a DIAC (diode-alternating-current), and an RC (resistance/capacitance) circuit (Figure 1). A potentiometer controls the resistance, and the resulting RC time constant controls the amount of delay before the TRIAC turns on, or the firing angle. The portion of time when the TRIAC is on is the conduction angle, θ. The resulting voltage waveform is a phase-cut sinusoid.
This type of dimming works well in incandescent bulbs, which are simple resistive loads. The time-averaged voltage across the filament’s resistance decreases as the conduction angle decreases, providing naturally smooth dimming.
The TRIAC also has a minimum holding-current requirement. The current flowing through the TRIAC must remain above this minimum level to ensure that it remains on throughout the entire conduction angle. The incandescent load easily satisfies this condition because of the load’s inherent power levels—40, 60, and 75W, for example.
Next: Compatibility with LEDs
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Dr DSP
6/29/2011 12:17 AM EDT
This is a great overview and helps explain PFC is a real world design. Good job.
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EREBUS
6/29/2011 4:10 PM EDT
I agree with Dr DSP, a very interesting perspective for those of us who are not fully intimate with the evolving LED technology.
Thanks
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Tadeusz
6/29/2011 4:24 PM EDT
I find this article very interesting as it shows that apparently simple electronic schemes open an enormously area of problems and then of thinking and being creative. I will recommend it to my students as an excellent example of good engineering job.
Paul
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kendallcp
7/3/2011 10:42 AM EDT
Nice part, nice design trick, great article. I have a reservation about the whole thing, though, which is nothing to do with this device. It's that we really should try to wean ourselves off the triac-based phase cut dimmer. It's a noisy harmonic-spewing monster that turns a high power-factor load - whether it's a resistive filament or a carefully designed PFC'd LED lamp - into about the worst-PF load you could conceive of. And the utility companies have got to deliver all that harmonic current to you. All this alt.energy stuff like solar is only going to make things worse for the utilities. Inverters are regulated to have a pretty clean sinewave output and don't supply the harmonics. So the current the utility sees is only going to keep getting dirtier, unless we impose tough PF constraints on everything that connects to the grid.
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Greggg
7/13/2011 1:02 PM EDT
The problem with that line of thought is the size of the existing install base, and of course the reluctance of consumers to change to LEDs, let alone the existing dimming infrastructure.
Cheers,
Gregg
LightBased Technologies
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GREAT-Terry
7/5/2011 12:30 PM EDT
Interesting sampling scheme. Can this scheme support very low duty dimming? What is the limit of this scheme?
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rajuchaluva
8/31/2011 1:41 AM EDT
Nice information! Thanks.
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