AU2015101059A4 - A Light Emitting Diode (LED) Lamp - Google Patents

Abstract

A Light Emitting Diode (LED) lamp, the LED lamp includes: a string of connected LEDs configured to receive current from a driver; a temperature sensor configured to sense a temperature of the LED lamp; a switch connected in parallel with one or more of the LEDs in the string of LEDs configured to selectively bypass said current from the driver to said one or more of the LEDs, wherein the switch is further configured to bypass said current to the one or more of the LEDs upon receipt of a signal indicative of a turn OFF temperature threshold value being sensed by the temperature sensor. <filename> Li I coI cc: 0 Ln~ Ln LI) U( w wl ~~"M- \! / U a), F-f

Description

1 A Light Emitting Diode (LED) lamp Technical Field [0001] The present invention relates to a Light Emitting Diode (LED) lamp with thermal management and a method of thermally managing a LED lamp. [0002] In particular, but not exclusively, the LED lamp includes a switch adjacent to a temperature sensor for bypassing current to one or more LEDs in a string of LEDs of the LED lamp upon receipt of a signal indicative of a turn OFF temperature threshold value being sensed by the temperature sensor. Background of Invention [0003] Light Emitting Diode (LED) lamps are widely used in many lighting applications; for example, they are used in downlight lighting applications. The LEDs housed in the lamps, however, generate heat during operation. Indeed, most of the energy that is put into an LED that does not result in light being emitted from the LED is transformed to heat. Accordingly, LED lamps typically require some type of thermal management to cool the LEDs as operation at elevated temperatures reduces the output of the LEDs and shortens their serviceable lifetime. In extreme cases, the operation of LEDs at elevated temperatures can cause catastrophic failure of the LEDs, resulting in the LED lamp ceasing to operate. [0004] In one existing method of thermal management, each LED in a LED lamp is mounted on a circuit board that is attached to a heat sink. The heat sink cools the LEDs by dissipating heat from the LEDs into the surrounding atmosphere. In some cases, however, such as in elevated ambient temperature environments, the absolute operating temperature of the LEDs connected to the heat sink may still reach a level such that degradation of the LEDs accelerates rapidly. The same issue is present when the performance or design of the heat sink is compromised in some way. [0005] In another existing method of thermal management of an LED lamp, an external LED driver reduces power to the LEDs in the LED Lamp to reduce the amount of heat generated by the LED lamp. In this method, either the voltage or the current delivered to the LEDs in the LED lamp is reduced to reduce the operating temperature of the LEDs. In one example, a temperature sensing element is 2 disposed on the LED circuit board and is connected to a compatible LED driver, with or without electronic signal conditioning circuitry. The LED driver in this case reduces its output current in response to a signal from the temperature sensing element indicating that the LED lamp has reached its thermal operating design limit. Whilst being effective at managing the temperature of an LED lamp, this approach has several draw-backs including: the LED lamp and the driver becoming a matched pair so that use of generic equipment is not possible; and this approach has increased manufacture cost due to the requirement of additional components in the LED lamp. Another drawback is that driver cost for a LED lamp is also increased as the driver needs to have additional componentry that allows it to process and respond to a thermal feedback signal from the temperature sensing element of the LED lamp. Summary of Invention [0006] Accordingly, in one aspect of the invention, there is provided a Light Emitting Diode (LED) lamp, the LED lamp includes: a string of connected LEDs configured to receive current from a driver; a temperature sensor configured to sense a temperature of the LED lamp; a switch connected in parallel with one or more of the LEDs in the string of LEDs configured to selectively bypass said current from the driver to said one or more of the LEDs, wherein the switch is further configured to bypass said current to the one or more of the LEDs upon receipt of a signal indicative of a turn OFF temperature threshold value being sensed by the temperature sensor. [0007] In another aspect of the invention, there is provided a method of thermally managing an Light Emitting Diode (LED) lamp, including supplying current to a string of LEDs of the LED lamp; sensing a temperature of the LED lamp with a temperature sensor; and bypassing said current to one or more of the LEDs in the string of LEDs upon receipt of a signal indicative of a turn OFF temperature threshold value being sensed by the temperature sensor. [0008] It will be appreciated by those persons skilled in the art that the LEDs typically have a reasonably stable operating voltage. For example, a Phillips Lumileds Luxeon T TM LED has an operating voltage range that varies from 2.64 to 2.86V over the operating current range of 1 0OmA to 1 OOmA. Thus, in this example, 3 it is more effective to reduce LED lamp power by reducing the current supplied to the LEDs of the LED Lamp rather than the voltage. [0009] Preferably, the LED lamp further includes an LED circuit board and the string of connected LEDs is disposed on the LED circuit board, the temperature sensor is disposed on the LED circuit board, and the switch is disposed on the LED circuit board adjacent the temperature sensor. In an alternate embodiment, the driver is also disposed on the LED circuit board. In most embodiments, however, the driver is remote from the LED circuit board and the LED lamp, and each of the LEDs in the string receives current from the driver via wires (e.g. two wires: positive and negative). Different types of types of drivers can therefore be used with respect to the LED lamp. [0010] Indeed, for example, multiple ones of the LED lamps can be connected in series to a single driver remote from the LED lamps. If a temperature sensor of one of the LED lamps senses its temperature as overheating, current to one or more of the LEDS of the overheating LED lamp will be bypassed as per the above method. Thus, only the overheating LED lamp in the system of LED lamps would experience a reduction in brightness. Any other lamps connected to that same driver would continue to deliver full brightness. In an example of a prior art system of LED lamps with thermal management that are connected to a single driver, the system would reduce the brightness of all of the connected LED lamps in response to one LED lamp overheating. [0011] Also, with reference to the above mentioned prior art method of thermal management where an external LED driver reduces current and thus power to the LEDs to reduce the amount of heat generated by the LED lamp, typically long wire runs are used between the components of the LED lamp. These long wire runs can, for instance, introduce a small offset voltage to the temperature sense signal. This may cause the LED driver to consider that a small reduction in LED current is necessary, resulting in an unnecessary reduction in light output from the LED lamp. Also, there can be a thermal lag that occurs due to the physical separation of the temperature sensor and the LEDs. Under some circumstances this can result in oscillation of the current to the LEDs and therefore oscillation of the LED lamp brightness. These problems are alleviated using the above mentioned embodiment where the LED circuit board has all the components of the LED lamp disposed 4 thereon. Also, it will be appreciated by those persons skilled in the art that, with all the thermal management components being contained on the LED circuit board of the LED lamp, any voltage drop over the wires between the driver and the LED circuit board has no practical effect on the performance of the LED lamp. [0012] Thus, in this embodiment, the abovementioned LED circuit board provides thermal management for the LED lamp. It will be appreciated by those persons skilled in the art that the driver, configured to supply current to the string of LEDs in the LED lamp, is typically a constant-current device. Thus, the driver continues to deliver the nominal output current irrespective as to whether thermal management is active in the LED lamp or not. The only impact on the driver when the switch bypasses current to some of the LEDs in the string of LEDs is a reduction in output voltage. As the driver is a constant-current device, the voltage can float up or down as required with negligible impact on output current. [0013] Also, due to the thermal management of the LED lamp being self contained on the LED circuit board itself, no additional secondary circuit board is required inside the LED Lamp. The self-contained nature of the thermal management allows the LED circuit board to be re-used in other applications without any re engineering of the thermal management. [0014] Also, by removing the dependence between the driver and the LEDs themselves in the embodiment, the LED lamp cost can be lowered. Additional wire(s) between the driver and the LEDs in the LED lamp are not required. The driver does not require any additional circuitry in order to process and respond to a thermal feedback signal, and the LED lamp does not require any signal conditioning circuitry for the signal to the switch from the temperature sensor that is indicative of a turn OFF temperature threshold value being sensed. [0015] It will be appreciated by those persons skilled in the art that typical traditional temperature sensor thermal feedback systems are dependent upon additional wires to connect the temperature sensor to the driver and the LEDs to the driver. That is, it will be appreciated that one sensor may be used for a plurality of LED lamps and located, say, centrally to the plurality of lamps. Also, one driver may be used for a plurality of LEDs disposed in many spaced apart LED lamps. These 5 wires, however, are subject to voltage drop which increases linearly with the length of the wires. This means that, for instance, the LED driver may detect a thermal feedback signal of small magnitude even when the temperature sensor element in the LED Lamp is not attempting to send a signal back to the driver. This may result in a reduction in LED Lamp current and therefore a reduction in light output from the lamp. By providing thermal management on the LED circuit board itself, the operation of thermal management of the LED amp is independent of any voltage drop between the LED lamp and the driver and/or the sensor, making the thermal management independent of the length of wire between the driver, sensor and LED lamps. [0016] In another embodiment, the switch is further configured to supply current to the one or more of the LEDs upon receipt of a further signal indicative of a turn ON temperature threshold value being sensed by the temperature sensor. In the embodiment, the LED lamp can further include a hysteresis circuit connected to the temperature sensor providing a hysteresis temperature value so that the switch is configured to supply current to the one or more of the LEDs upon receipt of the signal indicative of the turn ON temperature threshold value minus the hysteresis temperature value being sensed by the temperature sensor. Preferably, the hysteresis circuit is also located on the LED circuit board. [0017] It will be appreciated by those persons skilled in the art that hysteresis of the turn ON and turn OFF switching temperature is employed to avoid oscillation of the LED string voltage, and therefore the LED Lamp brightness. To prevent oscillation, the thermal hysteresis temperature value has a magnitude that is larger than the temperature reduction wrought by operation of the switch. [0018] As described, traditional thermal management implementations can be subject to a degree of oscillation in output from the LED lamp. This can occur because of the thermal lag between the LED components and the temperature sensor. For example, in operation the temperature sensing signal can still be high for some time after the driver reduces the output current to the LEDs resulting in the driver reducing the output current further. As the LED lamp cools, the temperature sensor of the embodiment will reduce the magnitude of the turn ON temperature threshold value by a designated hysteresis temperature value to minimise the occurrence of oscillation in driver current and therefore in LED lamp output.

6 Brief Description of Drawings [0019] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: [0020] Figure 1 shows an LED lamp circuit according to an embodiment of the present invention; and [0021] Figure 2 shows an LED lamp circuit according to another embodiment of the present invention. Detailed Description [0022] Figure 1 shows an embodiment of a Light Emitting Diode (LED) lamp circuit 10 for a LED lamp including a string of connected LEDs 12A 12B that are connected in series to a driver in the form of a DC voltage source (not shown) that is configured to supply current to the string of LEDs 12A 12B. The LED lamp circuit 10 also includes a temperature sensor 16 configured to sense a temperature of the LED lamp and a switch 14 connected in parallel with one of the LEDs 12A in the string of LEDs 12A 12B configured to selectively bypass current from the driver to the LED 12A. The switch 14 is directly connected to the temperature sensor 16 and is configured to bypass current to the LED 12A upon receipt of a signal indicative of a turn OFF temperature threshold value being sensed by the temperature sensor 16. [0023] For example, the turn OFF temperature threshold value is designated at 600C. Thus, in use, the driver of the LED lamp circuit 10 will supply current to the connected LEDs 12A 12B in the string of LEDs whilst the temperature is sensed by the temperature sensor 16 as being below 600C. Once the temperature sensor 16 senses that 600C has been reached, a signal is sent to the switch 14 indicative of the turn OFF temperature threshold value being sensed. The switch 14 then bypasses current to the LED 12A to thermally manage the LED lamp in the form of reducing the heat generated by the LED lamp. That is, at temperatures below the designated turn OFF temperature threshold value, the switch 14 is open, with no impact on the operation of the LEDs 12A 12B. Once the signal indicative of the designated turn OFF temperature threshold value being sensed by the sensor 16 is received by the switch 14, it closes thus bypassing current to the LED 12A. Bypassing current to (i.e.

7 shunting) the LED 12A causes the LED 12A to extinguish due to a reduction in voltage which lowers the voltage of the LED string 12A 12B as a whole; thereby reducing the power consumed by the LED lamp and the heat that it generates. [0024] The switch 14 is further configured to also supply current back to the LED 12A upon receipt of a further signal indicative of a turn ON temperature threshold value being sensed by the temperature sensor 16. In one embodiment, the temperature sensor 16 is a variable temperature sensing element and the designated turn ON temperature threshold value is 550C. It will be appreciated that, in this case, the turn ON threshold value is designated at a temperature that is below the turn OFF threshold value to account for the heat generated by the LED lamp being instantly reduced by the act of bypassing current to the LED 12A by the switch 14 when the turn OFF threshold value was reached. It will also be appreciated that this designated reduction in turn ON temperature threshold value by, for instance, 50C minimises the oscillation in driver current and therefore minimises oscillation in the LED lamp output. [0025] In an embodiment, the LED lamp includes an LED circuit board (not shown) and the LED lamp circuit 10 shown in Figure 1 is disposed on the LED circuit board. That is, the LEDs 12A 12B, the temperature sensor 16, and the switch 14 are all disposed on the LED circuit board. The temperature sensor 16 and the switch 14 are therefore located adjacent each other on the board so as to minimise the use of long wires which can compromise the thermal management of the LED lamp as described above. Although not shown, the driver is connected to the LED circuit board in the embodiment via two wires, and each of the LEDs 12A 12B receives current from the driver via these wires. As the temperature sensor 16 and the switch 14 are disposed on the LED circuit board, any voltage drop over the wires between the driver and the LEDs has no practical effect on the performance of the LED lamp as previously mentioned. [0026] Figure 2 shows another embodiment of a Light Emitting Diode (LED) lamp circuit 20 for an LED lamp. The LED lamp circuit 20 also includes a string of connected LEDs 22A 22B 22C that are connected in series to a driver in the form of a DC voltage source (also not shown) that is configured to supply current to the string of LEDs 22A 22B 22C. The LED lamp circuit 20 also includes temperature sensors 26A 26B configured to sense temperatures of the LED lamp. The LED lamp circuit 20 also 8 includes switches 24A 24B connected in parallel with respective ones of the LEDs 22A 22B in the string of LEDs and are configured to selectively bypass current from the driver to these respective LEDs 22A 22B. The switches 24A 24B are directly connected to respective ones of the temperature sensors 26A 26B and are configured to bypass current to respective ones of the LEDs 22A 22B upon receipt of a signal indicative of a turn OFF temperature threshold value being sensed by the respective temperature sensors 26A 26B. Also, in an embodiment, the LED lamp includes an LED circuit board (not shown) and the LED lamp circuit 20 shown in Figure 2 is disposed on the LED circuit board. [0027] The switch 14 in Figure 1 and switches 24A 24B in Figure 2 are preferably some form of electronically controlled switching device, such as a relay or semiconductor switching device. For instance, the switch 14 is a MOSFET semiconductor device, which is suitable due to factors such as its small size, low drive requirement, low loss etc. In any case, the switch 14 provides the ability to bypass (shunt) the total supplied current to the one or more LEDs with comparatively small voltage drop to ensure that the power reduction equates to the LEDs' power being shunted. Hence, the operation of the switch 14 bypassing current to the LEDs ensures a maximum drop in temperature of the LED lamp. Also, in an embodiment, the LED lamp includes a heatsink and the operation of the switch 14 ensures a maximum drop in temperature of the heatsink of the LED lamp. [0028] In an example, the turn OFF temperature threshold value for temperature sensor 26A is designated at 600C. In addition, a further turn OFF temperature threshold value of 65 0C is designated for the temperature sensor 26B. Thus, in use, the driver of the LED lamp will supply current to the connected LEDs 22A 22B 22C in the string of LEDs whilst the temperature is sensed by each of the temperature sensors 26A 26B as being below 600C. Once the temperature sensor 26A senses that 600C has been reached, a signal is sent to the corresponding adjacent switch 24A indicative of the turn OFF temperature threshold value being sensed. The switch 24A then bypasses current to the LED 22A to thermally manage the LED lamp. [0029] In an example where the ambient temperature is high and the heat generated by the LED lamp continues to rise, the temperature sensor 26B senses that 650C has been reached. Here, a signal is sent from the temperature sensor 26B 9 to the corresponding switch 24B indicative of the second turn OFF temperature threshold value being sensed and the switch 24B then bypasses current to the LED 22B to further thermally manage the LED lamp. That is, the concept of power reduction through bypassing current to successive LEDs in the LED lamp through forward voltage reduction is scalable. In the embodiment shown in Figure 2, the LED lamp circuit 20 has multiple switches 24A 24B, each with its own temperature sensor 26A 26B connected thereto with designated temperature threshold values. In this way, if the reduction in power achieved by the operation of the first switch 24A is not sufficient to keep the LED lamp temperature at a safe level, subsequent switches 24B and so on can be activated to lower the LED string voltage further and thus lower the LED lamp temperature. [0030] In another embodiment, the switches 24A 24B of Figure 2 are connected to a single temperature sensor (not shown) capable of signalling, with additional circuitry, to each of the switches 24A 24B that the corresponding turn OFF temperature threshold values have been reached. [0031] In addition, the switches 24A 24B are also configured to supply current back to the LEDs 22A 22B upon receipt of further signals indicative of turn ON temperature threshold values being sensed by the corresponding temperature sensors 26A 26B. Also, in another embodiment, the switches 24A 24B could be connected to a single temperature sensor (not shown) capable of signalling, with additional circuitry, to each of the switches 24A 24B that the corresponding turn ON temperature threshold value has been reached. As described above, the temperature sensors 26A 26B can be variable temperature sensing elements and, for example, the designated turn ON temperature threshold values are 550C for the switch 26A and 50 0C for the switch 26B. The successive turn ON temperature threshold values of the temperature sensors 26A 26B enable power to the LED lamp to be progressively increased. [0032] Also, in an embodiment, the temperature sensors 16 26A 26B are binary temperature sensing elements. In this case, the LED lamp circuits 10 20 of Figures 1 and 2 include a hysteresis circuit to provide hysteresis temperature values to prevent oscillation of the LED lamp power and thus lamp brightness during operation of the LED lamps.

10 [0033] With respect to the embodiment shown in Figure 1, the designated turn ON temperature threshold value is the same as the turn OFF value of 600C as the temperature sensor is a binary sensing element. The hysteresis circuit is connected to the temperature sensor 16 and provides a designated hysteresis temperature value so that the switch 14 is configured to supply current to the LED 12A upon receipt of the signal indicative of the turn ON temperature threshold value minus the hysteresis temperature value being sensed by the temperature sensor 16. For example, the hysteresis temperature value is 5 0C so that the effective turn ON temperature threshold value for the switch 16 is 550C. As above, the exemplary hysteretic value of 50C is designated as it exceeds the anticipated drop in temperature of the LED lamp following turning OFF on the LED 12A as a result of operation of the switch 16. In this way the thermal management will operate once and then remain that way, assuming steady-state operating conditions, resulting in no oscillation of the LED lamp power and therefore LED lamp output for the hysteretic temperature range. [0034] With respect to the embodiment shown in Figure 2, a hysteresis circuit is connected to each of the temperature sensors 26A 26B and provides a hysteresis temperature value so that the switches 24A 24B are configured to supply current to the respective LED 22A 22B upon receipt of signals indicative of the turn ON temperature threshold values form the respective temperature sensors 26A 26B minus the hysteresis temperature value. The LED lamp therefore implements thermal management as a number of discrete steps in relation to the LED string 22A 22B 22C voltage, and therefore the LED Lamp power. That is, successive ones of the LEDs 22A 22B 22C will be turned OFF as the sensed temperature increases and successive ones of the LEDs 22A 22B 22C will be turned back ON as the senses temperature decreases again, taking into account the hysteresis temperature range where steady-state operation occurs. [0035] It will be understood that there may be other variations and modifications to the configurations describe here that are also within the scope of the present invention. [0036] The discussion of document, acts, material, devices, articles and the like is included in this specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters 11 formed part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claims of this application.

Claims (5)

1. A Light Emitting Diode (LED) lamp, the LED lamp includes: a string of connected LEDs configured to receive current from a driver; a temperature sensor configured to sense a temperature of the LED lamp; a switch connected in parallel with one or more of the LEDs in the string of LEDs configured to selectively bypass said current from the driver to said one or more of the LEDs, wherein the switch is further configured to bypass said current to the one or more of the LEDs upon receipt of a signal indicative of a turn OFF temperature threshold value being sensed by the temperature sensor.

2. A LED lamp as claimed in claim 1, further including an LED circuit board, wherein the string of connected LEDs is disposed on the LED circuit board, the temperature sensor is disposed on the LED circuit board, and the switch is disposed on the LED circuit board adjacent the temperature sensor.

3. A LED lamp as claimed in claim 1 or 2, wherein the switch is further configured to supply current to the one or more of the LEDs upon receipt of a further signal indicative of a turn ON temperature threshold value being sensed by the temperature sensor.

4. A LED lamp as claimed in claim 3, further including a hysteresis circuit connected to the temperature sensor providing a hysteresis temperature value so that the switch is configured to supply current to the one or more of the LEDs upon receipt of the signal indicative of the turn ON temperature threshold value minus the hysteresis temperature value being sensed by the temperature sensor.

5. A method of thermally managing an Light Emitting Diode (LED) lamp, including supplying current to a string of LEDs of the LED lamp; sensing a temperature of the LED lamp with a temperature sensor; and bypassing said current to one or more of the LEDs in the string of LEDs upon receipt of a signal indicative of a turn OFF temperature threshold value being sensed by the temperature sensor.