Constant temperature LED driver circuit

A constant temperature LED driver circuit for controlling the rate of lumen depreciation and therefore useful lifetime of an LED array by keeping the LEDs at a fixed temperature. A temperature sensing means, which measures the temperature of the LEDs, is coupled to a variable power source that drives the LED array. The current provided to the LED array is adjusted in response to the temperature sensing means to ensure that the LEDs remain at a constant temperature regardless of changes in ambient temperature.

CROSS-REFERENCE TO RELATED APPLICATIONS

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BACKGROUND OF THE INVENTION

The present invention is directed to an LED (light emitting diode) light source with a driving circuit to drive the LED light source. More particularly the present invention is directed to an LED light source with driving circuitry designed to maintain a constant temperature and therefore a constant useful life for the LED. The luminosity of the LED is adjusted to compensate for fluctuations in ambient temperature in order to prevent excessive depreciation in luminosity over time caused by overheating.

The amount of light produced by an electric light source decreases with time in a process known as lumen depreciation. For an LED, lumen depreciation is primarily caused by heat generated at the LED junction. The useful life of an LED is defined as the total time that the LED can be on before the luminosity decreases by a certain percentage, typically 40 percent. Therefore, the useful life of an LED is primarily temperature dependent.

In order to assign a useful lifetime to a particular LED, a maximum temperature must also be given. For example, an LED may have a lifetime of at least 20,000 hours if kept in an environment that remains below 30 degrees centigrade. This means the useful life may be significantly shorter if the LED is allowed to operate in an environment that regularly exceeds 30 degrees. Also, if kept in much lower temperatures, the LED may have a significantly longer lifetime.

In the prior art, LED circuits are well know with numerous examples and variations disclosed in U.S. Pat. No. 4,675,575. There is also an extensive variety of circuits specifically designed for driving LEDs with various functions and abilities. Several of these are disclosed in U.S. Pat. No. 7,116,294, and include methods for limiting current to the LEDs to prevent overheating. However, along with driving current, ambient temperature also plays an important role in LEDs overheating.

The method of using a temperature sensing means to control the luminous output of an LED is also known, as disclosed in U.S. Pat. Nos. 5,783,909 and 6,127,784. Both patents disclose the use of a temperature sensing means at an LED source to send feedback to a power supply. The average current supplied to the LED source is adjusted to maintain a constant LED luminosity by compensating for changes in luminosity resulting from changes in ambient temperature.

A circuit for maintaining a constant LED current is disclosed in U.S. Pat. No. 7,245,090 as well a method for determining the temperature of an LED based on the amount of current passing through it. However, like the above mentioned patents, the LED driver uses its temperature sensing capabilities to provide a constant luminous output rather than a constant temperature. This is important for many lighting applications where there is a minimum acceptable luminosity. In other applications such as household and decorative lighting, a constant luminosity may be less important than the longevity of the LED.

Therefore it would be desirable to have an LED light source with a driving circuit designed to maximize the useful life of the LED by sacrificing luminosity to compensate for high ambient temperature. Further, it would be desirable to have a LED light source that could reliably be assigned a useful lifetime that is independent of ambient temperature.

BRIEF SUMMARY OF THE INVENTION

The present invention is a Constant Temperature LED Driver Circuit designed to maintain a constant useful lifetime for an LED light source. The invention includes an LED or LED array, a driver circuit, and a temperature sensing means. The driver circuit provides enough current to cause the temperature of the LED array to reach a fixed ideal temperature. If the temperature sensor detects a temperature above the ideal value, the current is reduced. If the temperature drops below the ideal value the current is increased.

Therefore a general object of this invention is to provide an LED light source in which the temperature at the LED junction is held constant regardless of ambient temperature.

Another object of this invention is to maximize the useful lifetime of an LED by sacrificing luminosity in conditions that would otherwise cause an increase in lumen depreciation.

Yet another object of this invention is to provide an LED light source that maximizes luminous output so long as it can do so without sacrificing longevity.

Still another object of this invention is to provide an LED light source with a useful lifetime that can be predicted with reasonable accuracy.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the invention in more detail, inFIG. 1there is shown a simple block diagram for the general layout of the preferred embodiment of a constant temperature LED driver circuit. An AC (alternating current) or DC (direct current) power source10powers an LED driver12, which in turn drives an LED array14. The LED driver12accepts an input signal from a temperature sensing means16, which measures the temperature of the LED array14. The LED driver12uses this signal to adjust the average current provided to the LED array14.

Referring now toFIG. 2, there is shown a detailed diagram of the LED driver12ofFIG. 1. The LED driver12preferably includes an analog input20, an analog to digital converter22, memory storage24, a digital comparator26, and pulse generator28with a variable pulse width. The output30of the pulse generator28drives the LED array14.

As shown inFIG. 2, the analog input20is coupled to the output of the temperature sensing means and receives a voltage signal proportional to the temperature of the LED array14. The voltage signal is then passed through the analog to digital converter22to produce a digital temperature value. This measured temperature value is then sent to the comparator26and compared to an ideal temperature value that is preprogrammed into the LED driver's12memory storage24. The resulting signal from the comparator26is read by the pulse generator28.

The pulse generator28includes programming to increment the width of the output pulse when the comparator26signal indicates that the measured temperature is less than the ideal temperature and decrement the width out the output pulse otherwise. The pulse generator28operates with a fixed cycle time so the width of each pulse controls the average current to the LED array14. The cycle time is sufficiently short so that the LED array14pulses with a high enough frequency so that it appears to have a continuous luminous output.

Still referring toFIG. 2, the LED Driver12may also include a user input32allowing the user to access and set the ideal temperature value in the memory storage24.

Now referring toFIG. 3, as an alternative to the digital measured temperature and ideal temperature values, the signal to the analog input20from the temperature sensing means may be compared directly to a fixed reference voltage40using an analog voltage comparator42. The resulting signal from the analog voltage comparator42could then be used by the pulse generator28as previously described.

InFIG. 4there is shown a schematic diagram of the electronic circuitry of a constant temperature LED driver circuit powered by an AC power source50. The AC power source50is first converted to an appropriate DC source after passing through a transformer52and then a rectifier circuit54. The resulting current is used to power a LED driver12that drives an LED array14. The LED driver12receives an input voltage signal from a thermistor56placed in contact with or in close proximity to the LED array14.

The thermistor56is coupled to a positive voltage signal a one end and a voltage dividing resistor58at the other. The resistor58then shares a ground with the LED driver12. The LED driver12reads the voltage drop across the resistor58, which increases when the temperature of the LED array14increases and causes the resistance of the thermistor56to drop.

The circuit shown inFIG. 4may also be powered by a DC power source. In this case, the transformer52and rectifier circuit54are omitted and replaced with the power source.