WEB EXCLUSIVE: Advances in TRIAC dimmable drivers open way for lamp subsidies

Resonant asymmetric inductive supply (RAIS) technology provides LED lamps with a high degree of functionality across different dimmer types together with added driver efficiency.

May 30th, 2009
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One of the main drawbacks to public acceptance of energy-saving lamps for home applications has been their inability to work properly with dimmer switches. While some recent LED lamps have overcome this to a certain degree, they have not been able to work with all dimmers. This though looks set to change with a technology known as resonant asymmetric inductive supply (RAIS).

RAIS is the brainchild of a company called E-Light, based on the Isle of Man, UK, and brings with it impressive figures both for efficiency and power factor. In fact, it has become the first dimmable GU10 application to be submitted the UK’s Energy Saving Trust scheme for approval (see News). The company is now considering ways of taking products made with this technology to market.

What is key to the technology’s potential for being accepted into the market is its simplicity. Even the heatsink is smaller and cheaper because all it needs to do is cool the LED whereas with some technologies more than 30 per cent of the heatsink is being used to cool the power supply.

All this should make the resultant LED lamp one of the cheapest on the market. With the subsidies that will result from being Energy Saving Trust-approved, and through that eligible for use in the UK government’s carbon reduction programme, the cost barrier to LED lamps in the domestic market could at last be broken.

The company also believes that, because of the product, the US Energy Star and UK Energy Saving Trust may seriously consider raising the entry level for their schemes. This could make it harder for other companies to comply with these schemes.

Dimming

The RAIS technology is the circuitry that sits between the mains and the LED. What E-Light believes will be the key to the technology's acceptance is the way it naturally emulates an ordinary light bulb. You reduce the power into the bulb, and it dims proportionately. You increase the power and you increase the brightness.

To put this to the test, the company sent members of staff out to the main high street stores and bought every type of dimmer they could find. These were added to dimmers from other sources and grouped into four categories. The company found that RAIS technology works with all of them.

Figure 1
Other LED replacement lamp products, for example the Philips Master LED, seem to work well with most of the leading-edge dimmers that turn or slide with a knob but not with the increasingly popular touch or pushbutton leading-edge domestic dimmers or trailing-edge MOSFET dimmers.

The ability of RAIS to handle these dimmer technologies was verified in a recent report from the Lighting Association, a UK-based testing laboratory, which compared a RAIS lamp with that of a Philips Master LED (see Figure 1). Generally, the light output performance of the Philips Master LED and the lensing were of a very high standard, however the report said: “The RAIS lamps offer a higher degree of functionality across different dimmer types together with added driver efficiency over its competitor.” The full comparison is shown by the table in Figure 2.

Figure 2
This should make it a hit with the retail distribution network. The average consumer knows little about the difference between dimmer technologies and many are likely to be upset if they buy a “dimmable” LED lamp only to find when they get it home that it does not work with the dimmers in their house. Retailers will not thank the industry if they have to refund customers due to confusion over dimmer compatibility. Future requirements of the Energy Saving Trust will reflect this; if a product says it is dimmable on the packaging, then it will have to work satisfactorily with a wide and growing range of dimmer platforms.

RAIS is inherently compatible with dimmers because it continuously draws current in the same way as a conventional lamp, in other words it looks like a resistor to the mains. This also has an impact on the efficiency of a RAIS controlled LED lamp.

To explain, consider the alternative of a specialist IC such as the SSL2101 and LM3445EC. These use a buck converter and as such do not look like a resistor to the mains through the full cycle. They also require a bleeder circuit or holding current resistor between 3 and 5kΩ to ensure proper TRIAC operation. As the user dims down, this circuit becomes more prominent meaning that efficiency is then rapidly lost. Effectively, what it is doing is draining the power through the resistor, or in other words wasting the power.

On normal load, the input to output efficiency of an RAIS system is about 91 per cent compared with less than 74 per cent for a typical tested specialist IC based system. As the light is dimmed, the efficiency of the RAIS system will fall to around 63 per cent, but an IC system can easily drop to around 10 per cent because of this wasted power (see Figure 3).

Efficiency

Figure 3
Efficiency is one of the main criteria for achieving Energy Saving Trust approval. It insists that the power consumption of the LED lamp must be a maximum of 25 per cent of the lamp it is replacing. For example, a replacement for a 35W GU10 with a 400 lumen output must only consume a maximum of 8.75W from the mains. The driver efficiency is critical in achieving this. Also, low efficiency drivers dissipate more heat and thus need a larger heatsink. The smaller heatsink needed by the RAIS driver is one of the crucial factors in keeping it size closer to that of a conventional bulb.

Heat dissipation is thus a limiting factor on light output. If increasing the output increases the heat to be dissipated, then because of the larger heatsink needed this increases the size of the lamp. RAIS, on the other hand, can use the extra power it can produce because of its high efficiency to increase the output for the same size of lamp.

RAIS achieves this efficiency in a novel way. LED lighting typically involves LEDs connected in series. The resultant forward voltage may be in the region of 10 to 20V. In such cases, the ratio between the mains voltage and the voltage required to drive the load is between 10 and 20. With such a large ratio, conventional circuits used to drive LEDs become very inefficient because the switching will be operating at extremes of duty ratio with very short conduction times and high peak currents. This inevitably means that extra components, such as common mode chokes, need to be used.

It is therefore common to include a magnetic or piezoelectric (ceramic) transformer with an input-to-output ratio suitable to create a step down in voltage and a corresponding step up in current. This though introduces further efficiency loss, cost and bulk.

In contrast, the RAIS technology can drive high current, low voltage LED strings from a 240V AC mains supply without high peak currents, a transformer or common mode choke and still achieve the turn’s ratio.

Power Factor

There is also no need for power factor correction or a valley fill circuit as the RAIS topology has inherently near unity power factor. The quoted figure is actually 0.96, significantly higher than the 0.66 for an IC driven LED lamp.

The question of including power factor figures in LED lighting requirements has become a hot topic. Companies that are struggling with the technology to achieve a high power factor without a loss in efficiency are usually in the camp arguing against this. The Carbon Trust and the utilities are naturally in favour of a high power factor, but they are treading carefully because they don’t want to set unachievable limits as that could damage the whole move to energy saving lighting. However, if high power factors can be achieved with no significant difficulties, then there would appear to be no argument against them being made mandatory.

RAIS technology thus looks set to stir up the industry in a way not seen before because it achieves the high power factor simply without any associated costs or loss in efficiency. This is a win-win situation as the Energy Saving Trust would like a 0.9 power factor as a future requirement for domestic lighting and the Energy Star scheme in the USA has recently moved from having no requirement to one of 0.7 in its latest draft document. In both the UK and the USA, commercial lighting has a 0.9 requirement.

Constant current

Figure 4
The RAIS topology is also inherently constant-current so there is no need for a second stage, current sensing, feedback or short circuit protection. Most simple lamp drivers use a power transformer rather than constant current, which is not ideal for the LED or the output capacitors.

This unique property of the RAIS technology is based on a range of granted and pending patents. Basically, they say that at a particular frequency the output characteristic becomes constant current on a single stage power supply with no sensing. This results in the topology remaining at high efficiency throughout the full mains voltage cycle without feedback, unlike a conventional LC resonant circuit.

There are two main issues within the RAIS patents that contrast with patents for other LED technologies. The first is that RAIS does not charge an inductor or capacitor with a switch that then discharges into the load and as such does not generate large peak currents. The RAIS technology also lets the output diodes remain in conduction for longer allowing a reduction in the size of the output capacitor.

Because the output is constant current, the LED is protected from transient damage as is the now small output capacitor. This results in no need for the higher frequency components to be filtered out prior to providing DC power to an LED, and indeed they are not, yet the output flicker is reduced.

Size

Another aspect that should help RAIS technology make a breakthrough in domestic lighting is the small size of the circuitry. This can be shown by taking a GU10 fitting. While all GU10 fitting must be contained within a standard maximum envelope, this in itself does not guarantee that they will fit all fittings as not all fittings makers allow for this maximum. Thus, it is better to stay close to the actual size of a current GU10 lamp (see Figure 4).

According to the Lighting Association report: “The circuit size is smaller thereby enabling a lamp to be produced of conventional size. The RAIS lamp does mimic the outline dimensions of a conventional GU10 halogen lamp and therefore would be capable of retrofit applications.” Because RAIS is so efficient and single stage, the power supply is very small and the size of the heatsink can be reduced. A RAIS lamp in a GU10 fitting, for example, has the potential for the heatsink to be significantly shorter than for a lamp using specialist IC technology.

This will also scale up through lamp sizes and gives great scope for designers to realise the flexibility of LEDs in designs without worrying about the size of the power supply.

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