Unlike incandescent, halogen, metal halide, and fluorescent lights, LEDs are quite sensitive to elevated temperature. High temperatures reduce the light output of an LED as well as its lifetime. While temperatures above roughly 100° C. can shorten the lifetime of an LED, when combined with high operating currents such elevated temperatures are particularly damaging to the LED. While it is possible that high ambient operating temperatures may result from the local environment external to the fixture containing the LED, typically excessive operating temperatures are due to improperly designed or assembled light fixtures or failure of an LED cooling system (e.g., a fan). Currently LEDs are approximately 20% efficient, i.e., 20% of the incoming electrical energy is converted to light; the remaining 80% of the electrical energy is dissipated in the form of heat. If not removed from the vicinity of the LED, this dissipated heat can lead to deleteriously high LED operating temperatures and, consequently, shortened lifetimes.
In view of the foregoing, there is a need for systems and techniques for controlling the operating current of an LED in response to its operating temperature in order to prevent deleterious overheating. However, methods of controlling LED operating current are complicated by the fact that many LED-based illumination devices are dimmable; moreover, multiple different dimming techniques may be utilized. For example, two common dimming techniques are phase dimming and 0-10 V dimming, each of which controls the light output of the LED-based device in a different manner.
In phase dimming, a dimming controller such as a semiconductor-controlled rectifier (SCR) or a bidirectional triode thyristor (triac) reduces the power input by truncating the leading or trailing edge of the sinusoidally varying line voltage waveform. Driver electronics (typically designed to supply constant current to the LED) connected in series to the phase dimmer sense the duration of conduction, i.e., the phase angle, of the abbreviated waveform and reduce the current to the LED in response, resulting in decreased light output from the LED.
In 0-10 V (or similarly, 1-10 V) dimming, the 0-10 V analog power signal is connected to the two wires of a specially designed dimming LED driver. The dimmer, located remotely, produces a voltage signal that is accepted by the driver and is used via appropriate circuit elements to reduce the output current. In general, these analog dimmers have circuit elements that maintain electrical isolation from line voltage for safety.
Other types of dimming schemes such as Digitally Addressable Light Interface (DALI) or DMX512 also exist. DMX512 dimmers generate a digital signal that is de-multiplexed to a 0-10 V signal. DALI is a specialized bi-directional digital interface control that uses purpose-designed LED drivers.
The use of different techniques for dimming LED-based illumination systems complicates attempts to thermally protect the LEDs themselves, as current control for thermal protection may have different impacts on the light output of the system depending on the dimming system. Furthermore, current control may be incompatible with non-dimmable LED drivers, which are designed to supply a constant current level regardless of variations in input voltage. Thus, there is a need for systems and techniques of providing thermal fold-back protection to LED-based lighting systems and that are compatible with various types of dimming solutions.