What do you have to do to make an LED-based light bulb behave just like the real, inefficient, thing?

Beyond getting enough light, the first big battle is working with the huge installed base of dimmers, all designed to work with incandescent loads.

Domestically, dimmers are connected in series with the light bulb and work by conduct for a variable part of the mains cycle.

When the dimmer’s triac or mosfet is off, the dimmer continues to need a load to maintain its timing circuits.

“Dimming is one of the great problems,” Jacques le Berre, business development director at NXP told EW. “LED lamps with equivalent light output take only a tenth of the current.”

“Most dimmers are designed to work with 20-60W loads. A wattage sufficient to keep dimmer happy is needed. You need an additional 1.5-3W depending on the operation of the dimmer,” he said. 
 

Bright and efficient, if a little expensive, LED light bulbs seem an ideal replacement for incandescent light bulbs.

Bleed vs RAIS
The classic solution is a bleeder circuit – a resistor that keeps some current running through the dimmer at all times.

In general, to cut waste a mosfet and control circuit shut off the resistor when the LED is drawing sufficient power.

Both the SSL210x range of mains LED drivers from NXP and the LM3445 from National Semiconductor take the bleeder circuit approach.

An alternative, claimed to be more efficient than the bleeder approach, is the RAIS (resonant asymmetric inductive supply) topology from Elight on the Isle of Man, Although little of this technology has been revealed.

Dimmers are also designed to give a pleasing action when used with incandescent bulbs, producing a fairly linear rise in apparent brightness as they are operated.

See also: Cree: 160 lm/W LEDs soon

Smoothing
As LEDs have dramatically different voltage/intensity curves than filaments so some driver chips, including the two above, include circuits to compensate and achieve a smooth response for the user.

“The SSL2101 dims to better than 1% if the dimmer works that far,” NXP applications engineer Victor Zwanenberg told EW. “The circuit will actually give users the possibility of 0.1%, although our customers sometimes do not want to dim this close to the ‘off’ state.”

Although on-chip mosfets do the switching, the resistors associated with the LED driving and bleeder circuits need to be optimised for the application, using the lamp makers’ knowledge of likely applications.

“There is no standard for dimmers, some are very good and some are very poor,” said Zwanenberg. “One of our customers tested over 400 dimmer types.”

Recognising a desire for further designer customisation, NXP recently introduced the SSL2103 version of its chip without integrated switches to allow customers to select their own mosfets or bipolar transistors.