Specifications

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Lighting

LED lighting

132 Maxim Industrial Solutions

electrical infrastructure (i.e.,

cabling, transformers, dimmers,

and sockets), a significant

advantage for LED technology.

Fitting an LED lamp into the existing

infrastructure challenges the

designer in two principal ways:

1. The form factor. Retrofit lamps

must fit in the form factor of the

previous light source.

2. Electrical compatibility.

Retrofit lamps must work

correctly and without light

flicker in the existing electrical

infrastructure.

We shall discuss each challenge in turn.

Fitting the existing form factor

The existing form factor imposes

both a physical limitation (i.e.,

the driver board has to be small

enough) and a thermal limitation

on a retrofit lamp. These limitations

pose challenges for the design of a

replacement lamp (e.g., PAR, R, and

A form factors), challenges that are

particularly hard to overcome for

smaller form factors like MR16

and GU10.

While size is important for a retrofit,

thermal limitation is often more

critical. LEDs emit only visible light;

they do not irradiate energy at

infrared wavelengths like other tech-

nologies. Thus while LEDs are more

energy efficient than incandescent or

halogen lamps, they dissipate much

more heat through thermal conduction

in the lamp.

Thermal dissipation is also the main

limiting factor for the amount of light

that a lamp can produce. Today’s

LED technology in retrofit lamps can

barely achieve a level of brightness

that is acceptable for the mainstream

market. Pushing the limits of bright-

ness and, consequently, thermal

design are essential for designing a

commercially successful product.

A corollary issue to the thermal dissi-

pation is the lifetime of the driver

board. To emit more light, the lamp

must work at a fairly high tempera-

ture (+80°C to +100°C). At these

temperatures, the lifetime of the

driver board can limit the operation

of the whole lamp. Electrolytic capac-

itors are, in particular, the biggest

challenge. Since they dry quickly at

those temperatures, the operation

of those capacitors is limited to no

more than a few thousand hours, and

this becomes the limiting factor for

the whole lamp. Since longevity is

a major selling point for LED lamps,

managing the relatively short lifetime

of electrolytic capacitors is a major

issue for the lamp designer.

Maxim has developed unique LED

solutions for both 120VAC/230VAC

input and 12VAC input retrofit

lamps. These LED solutions do not

require electrolytic capacitors on

the board. This extends the lifetime

of the lamp from typically less than

10,000 hours to up to 90,000 hours.

Not having electrolytic capacitors

also reduces the size of the solution,

so the driver board fits the small

retrofit form factor.

Matching the electrical

infrastructure

Retrofit LED lamps must work

correctly in infrastructures that

include cut-angle (triac or trailing-

edge) dimmers and electronic

transformers.

Working off the 120VAC/230VAC line,

the lamp can be preceded by a triac

dimmer. Triac dimmers are designed

to work well with incandescent and

halogen lamps, which are perfectly

resistive loads. With LED retrofit

lamps, however, the LED driver is

generally a very nonlinear and not

purely resistive load; its input bridge

rectifier typically draws brief, high-

intensity peaks of current when the

AC input voltage is at its positive and

negative peaks. This LED behavior

does not allow the triac dimmer to

work properly, because it provides

neither the needed start current

nor the hold current. As a result, the

dimmer does not start properly or

turns off while operating, and the

LED lamp light flickers in an unac-

ceptable way.

The electrical infrastructure is even

more complicated for 12VAC input

lamps, because an electronic trans-

former and trailing-edge dimmer

can be connected at the lamp’s input.

Again, a 12VAC input lamp driver that

uses the traditional bridge rectifier

and DC-DC converter topology

flickers because of incompatibility

with the transformer and dimmer.

Maxim’s LED solutions for 120VAC/

230VAC and 12VAC input lamps use a

single-stage conversion. By shaping

the input current so that the light

does not flicker even when dimmed,

these solutions are compatible with

triac and trailing-edge dimmers

and electronic transformers. No