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AN10937_15: DescriptionDimmable LED based lamps; File Size385KB，13 Pages; ManufacturerNXP; Websitehttps://www.nxp.com

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Dimmable LED based lamps

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AN10937

Dimmable LED based lamps

Rev. 1 — 23 May 2011

Application note

Document information

Info

Keywords

Abstract

Content

LED, lighting, dimmable

This article discusses the changes in lighting technology, with specific

reference to developments in dimmable LEDs. It also explains these

different issues and provides examples of the possible compromises to be

considered.

NXP Semiconductors

AN10937

Dimmable LED based lamps

1. Introduction

In recent years, the lighting world has witnessed a new revolution with the appearance of

LED based lighting. In retail outlets, incandescent lamps now appear to belong to another

age, and the Compact Fluorescent Lamp (CFL) must compete with the new player in the

retrofit market. There are numerous reasons for the emergence of LED lighting

technology. LED technology is constantly developing and rapidly improving with time, and

LEDs are becoming far brighter and much more powerful. Added to their efficiency, these

features make them ideal for use in lighting. An incandescent lamp produces

approximately 10 lumens per Watt, but LEDs can now reach values of up to 100 lumens

per Watt. LEDs are already more efficient than the currently available CFL lamps seen in

the market today.

Ecological issues are also becoming increasingly relevant to the public. With a heightened

awareness of global warming and climate change, the idea of saving resources and

energy is becoming a major feature in many marketing strategies.

The concerted effort of manufacturers to decrease the fuel consumption of motor cars

over the last 10 years, and the end of the production and retailing of 100 W incandescent

lamps, perfectly illustrates this trend. Governments are also steering in this direction by

relaxing taxation and providing financial encouragement for ecologically friendly products,

generally referred to as “eco” products.

LEDs offer many other advantages including high dimmability, excellent lifetime and a

small form factor. They provide access to a world of new perspectives for lighting

applications in terms of form, color, and the creation of atmosphere.

However, the consumer LED market is presently made up of retrofit lamps, dictating the

need to address the following issues:

•

Special measures have to be taken to ensure compatibility with the existing housing

constraints, especially when dimmers are used

•

The form factor must be equivalent to an Edison type lamp

•

The heat created by the LEDs must be dissipated

AN10937

All information provided in this document is subject to legal disclaimers.

Application note

Rev. 1 — 23 May 2011

3 of 13

NXP Semiconductors

AN10937

Dimmable LED based lamps

2. Dimmer compatibility

Until now, control equipment for home lighting has been based on the incandescent lamp.

The most economical way to provide dimming functionality was to use AC mains

phase-cut dimmers. These dimmers were developed to be used with an incandescent

lamp that can be regarded as a resistive load.

The equivalent load of new electronic light sources such as CFL or LED lamps, is no

longer purely resistive. This makes a considerable difference in the way the dimmer

works. Using an unsuitable load can lead to unsatisfactory functioning of the system. It

can create some very unpleasant flickering of the light, or even damage the lamp or

dimmer. This can be a disappointment to customers, and after such an experience, a

negative opinion about the product can prevail. Moreover, dimmers are still quite

expensive, and customers cannot be expected to change their dimmers in order to suit

their new lamps. Consequently, dimmer compatibility is a very important feature of the

next generation of LED based products.

There are various types of phase-cut dimmers, but they all use the same principle i.e.

cutting a part of the mains sine wave during each period. This is achieved using a switch.

When the switch is conducting, a supply is delivered to the load (the lamp). When it is off,

no supply is delivered. The total energy delivered is modulated by adjusting the

conduction time.

There are two types of dimmers: leading edge and trailing edge types.

•

Leading edge dimmers

- the phase is cut at the beginning of the mains half period.

After a time corresponding to the dimming position, the switch is set to “on” and the

supply is delivered until the end of the half period. After crossing zero, the operation is

repeated, see

Figure 1

and

Figure 2.

•

Trailing edge dimmers

- the switch is set to “on” at the beginning of the half period,”

and closed after a time corresponding to the dimming position. It stays off until the end

of the half period. After crossing zero, the operation is repeated, see

Figure 3.and

Figure 4.

off

019aab537

off

019aab538

Fig 1.

Leading edge - dimmer

maximum on

Fig 2.

Leading edge - dimmer

minimum on

AN10937

All information provided in this document is subject to legal disclaimers.

Application note

Rev. 1 — 23 May 2011

4 of 13

NXP Semiconductors

AN10937

Dimmable LED based lamps

off

019aab539

019aab540

Fig 3.

Trailing edge - dimmer

maximum on

Fig 4.

Trailing edge - dimmer

minimum on

There are two technologies mainly used to achieve phase cutting:- triac switches and

transistor switches. Triac dimmers are always leading edge dimmers whereas transistor

dimmers can be either “leading” or “trailing” edge dimmers. The issue with triac dimmers

is that they require special conditions to work correctly. A triac is opened by triggering its

gate. Once the gate is triggered, the triac enters the conductive mode and it requires a

minimum current to maintain it. This current is referred to as the “latch current” and it must

be applied for some time for the triac to settle into the conductive mode.

Once the device is latched, a continuous “holding current” flows through the device. If this

current is removed or decreased, the triac switches off.

A LED based lamp has to sink the hold current needed for the dimmer to be dimmer

compatible. For example, if a 6 W LED lamp (approximately equivalent to 40 W

incandescent lamp) is used with a dimmer designed for a minimum load of 10 W, extra

circuitry is needed to provide sufficient hold current. In this situation, the efficiency of the

lamp decreases, but the energy savings will still be significant when compared with a

40 W incandescent lamp. Moreover, a LED lamp without this circuitry does not operate

correctly. There are variations in the hold currents of all dimmers.

To conclude, more additional losses will increase dimmer compatibility. The difficulty in

designing a dimmable application, is to find a compromise between efficiency and dimmer

compatibility.

The NXP SSL2101 (and derivative SSL2102) has two integrated bleeder switches

controlled by the IC. Two different bleeding currents can be set by connecting external

resistors. The IC offers good flexibility in the choice of current and this helps to optimize

dimmer compatibility. In the SSL2103 (the controller-only version of the SSL2101 and

SSL2102), the integrated switches are removed. They are replaced by low cost external

bipolar switches (still driven by the IC) for higher bleeding current levels. The internal

MOSFET, used to operate the converter, is also removed, and it is corrected externally to

enable a specific tuned solution.

AN10937

All information provided in this document is subject to legal disclaimers.

Application note

Rev. 1 — 23 May 2011

5 of 13

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