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How to efficiently interface TRIAC dimmers and LEDs

Posted: 11 Jul 2011     Print Version  Bookmark and Share

Keywords:TRIAC dimmers  LED drivers  DIAC 

The lighting industry has been dependent on the incandescent bulb for more than a century, and, over the last 50 years, the phase dimmer has become the primary dimming control. Standard forward-phase, or triode-alternating-current (TRIAC), 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 diode-alternating-current (DIAC), and an resistance/capacitance (RC) 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.

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Figure 1: A standard forward-phase dimmer contains a TRIAC, a DIAC and an RC circuit.

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.

Compatibility with LEDs
Unfortunately, solid-state lighting lacks the benefits of the phase-dimming approach. An LED is a semiconductor device; controlling light output is accomplished by regulating its forward current. High-brightness LEDs, which can conduct hundreds of milliamps to amps of current, almost always use a switching converter to maintain system efficiency.

A standard switching converter regulates its output regardless of the average input voltage, meaning that the phase-chopped waveform that the phase dimmer provides must first be decoded. The decoded information can be used to control the reference for output regulation. Although this task is relatively simple for power-electronics designers, many complexities hide under the surface.

An obvious difference is that the load is no longer purely resistive. Instead, the converter looks like a reactive load to the phase dimmer due to both capacitive and inductive components within the circuit. This condition causes a standard converter to have problems with the fast rising edge of the phasechopped voltage. Designers usually will employ standard RC-damping methods to reduce the problematic ringing that this rising edge induces. However, this approach always involves extra power loss.

An even larger problem comes from an unexpected source. The efficacy of modern LEDs is far superior to that of incandescent bulbs, which waste more than 75% of their light output in the infrared spectrum as heat. LEDs, on the other hand, provide most of their light output in the visible spectrum. The newest high-brightness LEDs are five to six times more efficient than comparable incandescent lights, meaning that a current LED replacement for a 60W bulb or fixture could be as low as 10 to 12W. This power savings is great for the consumer but not for the phase dimmer, which has the minimum holding-current requirement.

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