Increasing LED bulb lifespan improves solid-state lighting
Light-emitting diode (LED) bulbs offer the potential of up to 50,000 hours of operating lifetime, or nearly 25 years of typical usage. This is a 50x improvement over incandescent-equivalent technology. With demand for LED lighting growing rapidly, a key issue that could hold the industry back is if solid-state lighting (SSL) bulbs do not achieve the promise of long operating life. The obvious design considerations for solid-state lighting are efficiency and cost. But, thermal management is just as vital as any other design criteria, because too much heat can impact operating life, not to mention bulb safety. The energy savings of solid-state lighting gains the most over the full operating lifetime potential of the actual luminaire. While the LEDs offer the promise of this long lifetime, additional components required in the LED driver circuit can dramatically decrease luminaire operating life if intelligent thermal management is not implemented.
It is easy to overlook some important thermal aspects of LED design — issues that can result in potentially catastrophic luminaire failures. An LED bulb can be used in an enclosed lighting fixture or a fixture that is open to normal air circulation. The thermal conditions in these two cases are radically different, but the bulb in both instances has the same physical and electrical design. The temperature inside a closed lighting fixture can rise quickly to levels above 60°C, which subsequently causes the temperature inside the light bulb to exceed 90°C. In open-air fixtures, the temperature inside the bulb itself can be as much as 30°C lower than its closed fixture counterpart.
An LED-based bulb with no thermal protection whatsoever used under conditions where there is near zero air flow could result in a thermal runaway condition. Figure 1 shows the construction of a typical A19 retrofit LED bulb and the confined space in which the driver circuit needs to operate. This tight space exacerbates the temperature issues. Early examples of poorly designed LED luminaires include devices that failed after 1,000 hours, just like the incandescent bulbs they were intended to replace, and even a design where the bulb itself experienced thermal runaway, melting the casing and posing a potential fire risk. The end result was a costly recall of a large number of bulbs. These early models did not take into account the importance of thermal design on the overall quality of the LED bulb. A simple solution is to integrate a basic thermal shutdown circuit, something that is already very common in IC technology.
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Figure 1. Typical construction of an LED-based solid-state lighting luminaire.
Most LED drivers used in solid-state lighting contain a straightforward thermal protection circuit. Most power-management ICs employ a simple thermal shutdown function where the output of the regulator shuts down to protect itself when a maximum temperature is reached. This does protect the main IC, but when applied to an LED lighting circuit, it presents two critical problems. First, the output of the LED driver shuts down completely, eliminating the light. The output doesn’t turn on again until the thermal event clears and the temperature of the IC drops below the hysteresis point in the thermal shutdown circuit. Next issue
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 operating lifetimes.
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 overall bulb.
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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 shows the 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 subjected.
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Figure 3. iW3626 – Single-stage LED driver with integrated intelligent thermal management.
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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 process.
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.
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