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.