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Posted on 29 June 2019

Triac Dimmers for LED Lamps

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The basic triac dimmer circuit is so simple that no DC supply is needed

Dimming LED lamps with triac based dimmers has been widely discussed and various techniques have been proposed to enable smooth and flicker free dimming. Unfortunately these all add complexity and cost to the LED driver and at the same time reduce efficiency. Engineers in the industry are currently developing solutions to best meet the tradeoff between these factors and the required dimming performance.

By Peter B. Green, Manager LED Systems and Applications, International Rectifier

 

A better solution to the problem might be to re-design the dimmer itself to enable smooth and flicker free dimming with simple and efficient LED driver circuits.

However, at present marketing factors rather than technical justifications dictate that LED lamps need to be compatible with established dimmer technology that has been in use for several decades.

The ubiquitous triac based phase cut dimmer was originally designed to operate with resistive incandescent lamps that draw current in the range 100mA to 1A. The triac is fired by means of a simple time delayed trigger circuit and continues to conduct until the current falls below the holding current, which will happen close to the end of the AC line cycle. Adjusting the firing angle controls the RMS current supplied to the lamp and the light output. This is known as leading edge dimming.

When CFL light bulb replacements were introduced in the 1980s they were expensive as LED based equivalents are now. One of the major disadvantages was that they could not be dimmed and to attempt to do so would very probably damage the CFL or the dimmer.

Triac dimming waveform

Despite numerous attempts to produce dimmable CFL designs only limited performance could be achieved with the inevitable cost and efficiency penalties. Although LEDs are easier to dim than gas discharge lamps they consume even less current than CFLs to produce the required light output, which often does not provide sufficient load for a dimmer. Without sufficient current dimmers fire erratically sometimes several times during a single cycle resulting in severe flickering because the current drops below the triac holding current prematurely. To avoid this there is no alternative but to consume additional current by means of a “bleed” circuit to maintain stable dimmer operation. This must necessarily result in wasted power. Smart techniques are being developed to minimize this however the problem can never be totally eliminated. Bleed circuit losses are greater in 220VAC systems than in 120VAC systems meaning that dissipative stabilization techniques may not even be practical in many parts of the world restricting LED lamp use to certain dimmers with low holding currents only.

It is very possible that new dimmers will gradually replace existing triac based products on the market since government legislation is set to phase out incandescent lamps in the next few years. This initiative is motivated by the desire for energy efficiency and clearly wasteful bleed circuits go against this goal. It has been suggested that since a lot of power savings have already been made by converting to LED and a single LED light may only consume 10W, the small amount of power wasted in each lamp could be tolerated.

This may be the case today, however such justifications fall apart´when considering lighting as a load consisting of millions of lamps rather than considering only one individual lamp. The bottom line is that Lumens per Watt (luminous efficacy) must be maximized for the total lighting load connected to the power grid. For this reason replacing outdated triac dimming technology can potentially eliminate a significant quantity of wasted power.

At first glance the design of a replacement dimmer might seem quite straightforward. However there are several challenges in producing a low cost circuit that meets the performance requirements. One major problem in designing a replacement dimmer is that the triac dimmer generally requires only a two wire in and out connection like a switch with no neutral connection. The triac based circuit when operating with a resistive incandescent lamp is well suited to operate in this way, however this is not ideal when designing alternatives. The neutral connection is a problem since the existing light switch wiring in most homes does not provide a neutral cable only a hot, load and ground.

Any dimmer design requires a bi-directional switch. At present most dimmers are rated for operation up to several Amps, however this is only necessary because of the relatively high current required for incandescent lamps. A dimmer for LED lighting could be rated to a lower current which would enable two high voltage MOSFET devices or one MOSFET and a diode bridge to form the switching element without requiring large expensive devices and maintaining lower power loss. A co-packaged dual MOSFET may be a cheaper alternative. Perhaps new Silicon or GaN bi-directional switches may be developed specifically for this application considering the huge potential volumes required.

A triac dimmer circuit

Dimmer designs exist that get around the neutral connection issue by using the ground connection in place of the neutral, however this is bad practice and risks tripping GFIs connected in the circuit. The solution therefore should be to devise a more innovative 2 wire dimming solution perhaps with improved power semiconductor devices.

A dimmer without a triac

The erratic firing of the triac based dimmer when connected to a reactive load such as an LED driver or CFL can be solved by using power switches where both switch on and switch off are controlled such as MOSFETs. High performance dimmers are already on the market that use this method but most of them require an additional neutral connection in order to provide a source for the low voltage DC supply needed to run the control circuitry. The basic triac dimmer circuit is so simple that no DC supply is needed because the triac switch off is not controllable. Without a neutral connection it is a challenge to obtain a DC supply over the full dimming range while keeping the cost low and the efficiency high. These factors will mean that new LED friendly dimmers are likely to cost more than existing triac products at least in the short term probably creating a barrier to adoption for consumers that don't consider long term benefits.

Unlike incandescent dimmers, dimmers for LED lights need not dim all the way to full voltage. That is not to say that the light output does not need to be adjustable over the full range but instead that maximum light output could occur at close to full output voltage. This would mean that under all dimming conditions the dimmer switching element would be open for some period during the AC line cycle. It may be possible to derive a low voltage DC supply from the open switch voltage. In theory this would enable a design to be made without needing a neutral connection. The dimmer control circuit should require only a small current in the milli-Amp range which should be possible to supply without significant losses.

In conclusion the best solution to the LED dimming would be to change over to a different dimmer design that allows controlled switching for stable phase cut control. This would be more efficient and cost effective than making LED lights compatible with old technology. LED friendly dimming is best achieved by having an additional neutral connection to the dimmer which may require wiring modifications in many locations. A two wire dimmer with no neutral could be designed to get around this but there are significant design and cost factors to overcome.

 

 

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