The second issue is less obvious, but much more crucial to the lifetime
of the luminaire. At elevated temperatures, the passive components in
the LED driver, including electrolytic capacitors, will see reduced
Aluminum electrolytic capacitors offer an optimal combination of size,
capacitance and cost for applications such as power supplies and LED
drivers. Solid-state lighting applications require cost-effective
components that can handle rugged lighting operating environments. When
they gained popularity, electrolytic capacitors were mainly used in
open-air power supplies where their operating temperatures normally did
not exceed 60°C. When encapsulated power supplies gained popularity, the
electrolytic capacitor manufacturers created high-temperature-rated
devices, capable of operating up to 105°C. But, the guaranteed lifetime
at 105°C was only on the order of 2,000 hours. For some power-supply
applications, this is fine, but for solid state lighting, with the
promise of nearly 50,000 hours of operating life, this falls way short.
However, with careful thermal management, 50,000 hours can be achieved.
Figure 2 shows a typical lifetime curve based the operating
temperature of a high-temperature-rated electrolytic capacitor. The
relationship between temperature and operating life is nonlinear, where
for every 10°C reduction in temperature, the lifetime of the capacitor
doubles. An average expected lifetime of 5,000 hours at 105°C ambient
temperature for a typical electrolytic capacitor will increase to 40,000
hours at 75°C. The solid state lighting market needs to maintain the
ambient temperature of these components down to a level where the
capacitor can operate within the maximum expected lifetime of the
Click on image to enlarge.
Figure 2. Temperature characteristics of a typical electrolytic capacitor vs. ambient temperature.
A configurable thermal-protection circuit can allow designers to
establish a lower maximum cut-off point so that the IC ensures all
components stay below their maximum operating temperature and the
expected operating life of 30,000 to 50,000 hours can be assured. But,
if the output still shuts down when that level is reached, the maximum
potential is not reached either. The ability to reduce the power
dissipation as a function of temperature while maintaining the output
active allows for full protection and longevity for the light bulb
without ever losing light output.
The iW3626 (Figure 3
) is an example of an LED driver
that incorporates an intelligent thermal-management circuit that folds
back the output current supplied to the LEDs as a function of the
internal temperature of the IC. Figure 4
output current profile vs. the internal temperature of the iW3626. As
the temperature reaches a programmable maximum threshold, the output
current will fold back, reducing the output current to the LEDs and
subsequently reducing the heat generated by the LED. This keeps in check
the thermal conditions to which the rest of the components are
Click on image to enlarge.
Figure 3. iW3626 – Single-stage LED driver with integrated intelligent thermal management.
Click on image to enlarge.
Figure 4. Thermal derating curve of the iW3626. Hysteresis of 10°C and thermal shutdown at 150°C for absolute protection.
The iW3626 is only one example of intelligent thermal management
enabling long-term energy savings. It is based on an internal
temperature sensor and is ideal for luminaires such as the GU10 type,
which has a very small enclosure containing the driver, transformer and
all discrete components. But, not all applications in solid-state
lighting are in such confined spaces. Lighting ballasts for LED-based
fluorescent retrofit bulbs also face similar overheating potential and
require similar intelligent thermal management. Several LED drivers
designed for commercial applications offer the ability to remotely
monitor temperature via an NTC resistor and give precise control of the
temperature of either the LEDs themselves or any sensitive external
component that needs the most protection. The NTC device can be placed
directly at the thermally sensitive node and the thermal protection
circuit guarantees that the temperature at that point never exceeds the
maximum programmed level, while maintaining light output.
SSL designers need to carefully consider thermal management and the
impact that temperature has on the entire LED driver circuit and
luminaire operating life. Otherwise, the promise of nearly 50,000 hours
of operation in solid-state lighting doesn't stand a chance. The
designer identifies the weakest link and designs around it. Advances in
LED driver technology make designing around the weakest link a painless
Hubie Notohamiprodjo is the director of marketing
for solid-state lighting products at iWatt Inc. He has over 28 years of
experience in the lighting industry, along with extensive global
experience in the power-management market. Prior to iWatt, Hubie served
as director of marketing for AC-DC/Lighting products at Monolithic Power
Systems and General Manager of Marvel Semiconductor's Power Management
Division. He also held power management positions at Siemens, Motorola
and Micrel Semiconductor and was the founder of energy-saving lighting
systems company, Lumina International. Hubie earned his BSc degree in
Electrical Engineering from the University of Nebraska-Lincoln and holds
more than 21 patents.