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
Revision history
Rev
v.1
Date
20110523
Description
first issue
Contact information
For more information, please visit:
http://www.nxp.com
For sales office addresses, please send an email to:
salesaddresses@nxp.com
AN10937
All information provided in this document is subject to legal disclaimers.
© NXP B.V. 2011. All rights reserved.
Application note
Rev. 1 — 23 May 2011
2 of 13
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.
© NXP B.V. 2011. All rights reserved.
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
i
on
t
i
off
on
on
t
off
on
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.
© NXP B.V. 2011. All rights reserved.
Application note
Rev. 1 — 23 May 2011
4 of 13
NXP Semiconductors
AN10937
Dimmable LED based lamps
on
i
off
off
t
i
on
off
off
t
on
on
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.
© NXP B.V. 2011. All rights reserved.
Application note
Rev. 1 — 23 May 2011
5 of 13
PCB设计
京公网安备 11010802033920号
Copyright © 2005-2026 EEWORLD.com.cn, Inc. All rights reserved
电子工程世界
