IES20100687A2 - An LED-based light fitting - Google Patents

Abstract

A light fitting contains at least one LED and an accessory electrically connected in series with the LED(s) and powered by the current passing through the LED(s). In the preferred embodiments the LEDs are high power LEDs. The accessory may comprise a cooling fan for the LEDs. <Figure 1>

Description

BACKGROUND Until recently, LEDs were low power/low voltage devices mainly used as indicators and not for illumination. In the last ten years, however, some very powerful new LEDs have entered the market, rated of the order of 1W-10W, intended 15 for high efficiency lighting. Even more recently (in the last 3 years) high power LEDs have appeared that operate at about 50V-250V, not at <5V as in the past. An advantage of LEDs operating at 50V-250V is that it becomes possible to put several in series and run them directly off the power line, without the need for special electronic drivers and transformers for low voltage operation as was the case in the prior technology. This raises the prospect of LED based lamps that are similar to normal incandescent bulbs, i.e. with no electronics on board. Their operating currents are very low, of the order of 20mA.

These LEDs, rated at several Watts, become quite hot in use and it is necessary to cool them, e.g. by heat sinks or fans. Expensive substrates, based on aluminium, have also been proposed as a method to help spread out heat at the iirafa fall/M 0 O......-........' j IE 1 0 06 87 In order to make high voltage LEDs work at standard domestic line voltages, one needs to add series resistor (s) . The function of the series resistors is simply to absorb excess voltage. If the line is 230V, and, for example, one has 3 x 55V LED diodes (for a total of 165V), then one has a further 230V-165V = 65V to absorb. This excess voltage is highly variable on account of the utility voltage possibly varying between 207V and 253V (i.e. the official limits, 230V +/-10¾). The way to deal with this excess is to add the series resistors. These add further losses, though, and they lower overall efficiency. They also contribute to the general heat management problem.

It should also be borne in mind that space is usually very limited in light fittings, meaning that there are few cooling options. Big heat sinks are often not practical, and fans can be bulky, especially because they normally need a power supply to run them. Often, in fact, there may not be space for a conventional power supply.

Because the LEDs become hot in use, the number that can be placed in the same fitting is limited unless some effective cooling measures are taken. This limits the total brightness that can be obtained, and therefore the general take-up of this technology for other than aesthetic lighting purposes.

At the moment, these LEDs are quite expensive, so they have mainly been used in high-end lighting applications.

However, prices will fall, meaning that these LEDs will find use in lower cost applications and the cooling problem will need to be solved cheaply.

IE 1 0 06 87 Miniature cooling fans are used inside computers to cool specific integrated circuits. These can have ratings as low as 200mW to 2W. They have a very long life, measured in terms of tens of thousands of hours. The typical diameter of the fan is 40mm, with a unit volume cost of less than several euro, but there are even smaller ones, with 25mm being another standard offering. The typical running current of a cooling fan of this nature can be as low as 20mA.

These fans operate typically at nominal standard voltages such as 5, 12, 24, 36 or 48 Volts DC. The applied voltage can be varied below the nominal level, resulting in a lowering of the fan speed and a commensurate lowering of cooling capacity. It is really the current that they absorb, rather than the applied voltage, per se, that causes them to function.

Since such fans operate at a low voltage, in order to place a fan within the body of a high voltage LED-based light source, conventionally a dedicated power supply would need to be provided within the body of the fitting in order to power the fan. As previously mentioned, however, finding the space for such a power supply is a major problem. In addition, the heat of the LEDs would not make that objective any easier.

Where cooling fans are used to cool LEDs, as far as we are aware the fan is independently controlled in a low voltage circuit, or the fan is operating at 230V. For example, LED light fittings for indoor lighting, and which incorporate a cooling fan in the lamp housing, are marketed under the IE 1 0 06 8 7 name Sunon by Sunonwealth Electric Machine Industry Co. Ltd (see http://www.sunon.com/led/led_e.htm).

However, in the Sunon LED light fittings it appears that the fan is operated in the conventional way, controlled at low voltage, and that LEDs are of the normal low voltage variety.

It is an object of the invention to provide an improved, preferably high power, LED light fitting incorporating a low power cooling fan or other low power accessory.

SUMMARY OF THE INVENTION According to the present invention there is provided a light fitting containing at least one LED, the fitting further containing an accessory electrically connected in series with the LED(s) and powered by the current passing through the LED(s).

Preferably the at least one LED is a high power LED.

Preferably the accessory comprises a cooling fan for the LED(s).

Where the current passing through the LED(s) is DC, the cooling fan is connected in series with the LED(s).

On the other hand, where the current passing through the LED(s) is AC, the cooling fan is powered by a rectifier connected in series with the LED(s).

IE 10007 In preferred embodiments the fan generates a tacho signal while it is rotating, and the fitting comprises a circuit responsive to the tacho signal for diverting current through an impedance in series with the LED(s) in the absence of the tacho signal, whereby the LED is dimmed.

The embodiment takes advantage of the fact that the operating current in high power LEDs is of the same order of magnitude as the current that can operate a low power cooling fan or other low power accessory. Thus, the cooling fan or other accessory can be placed in the same circuit as the LED(s), rather than in parallel as would typically be the case.

BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a circuit diagram of a basic embodiment of the invention.

Figures 2 to 6 are circuit diagrams of more sophisticated 25 embodiments of the invention.

In the various figures the same references have been used for the same or equivalent components.

In Figure 1 a plurality {in this case four) of high power LEDs are connected across a high voltage DC supply V in series with a low power cooling fan 10 and a resistor R1. The value of the resistor R1 is selected according to the IE 1 0 06 87 magnitude of the supply voltage V and the operating voltages of the LEDs and fan 10 such that these operate within their current and voltage ratings. The LEDs, fan. 10 and resistor R1 are all contained within the same light fitting. The structural aspects of the fitting are not shown, but the electrical components of this embodiment, as well as those of the embodiments which follow, may be contained in a housing generally similar to used for the Sunon light fittings referred to above, or in other embodiments the fitting can have a form factor compatible with J-type linear tungsten halogen lamps.

The fan power supply (i.e. the voltage across the terminals Pl, P2) is clamped by a zener diode ZD1 at the fan's operating voltage for protection of the fan and its power supply, and a capacitor Cl in parallel with the fan 10 provides protection against voltage spikes. However, although the fan 10 would typically be rated at a nominal 5V, 12V, 24V, 36V or 48V, the circuit would preferably be arranged so that the actual voltage on the fan is less than the nominal under normal conditions. For example, the fan might be rated at 80mA, but used in a circuit where the normal (LED) current was 60mA. So, the fan would be running at less than rated speed, and there would also be scope for the speed to increase were the applied voltage increased or were one LED to fail short, the increased current not exceeding the current rating of the fan.

If desired, additional strings of series-connected LEDs can be connected in parallel with that shown, in the manner shown in Figure 4.

IE 1 0 0 6 8 7 In Figure 1 the power supply Pl, P2 is shown driving the cooling fan 10. However, the power supply could be used to drive any other low power accessory having a current rating of the same order of magnitude as the LEDs, and this is true of all the embodiments herein.

Figure 2 shows an embodiment for use with an AC supply. The AC supply is converted to DC in a bridge rectifier 12 and the converted DC is applied to a plurality of channels 14, 16 and 18 connected mutually in parallel. Each channel contains a plurality of LEDs and resistors R2, a positive temperature coefficient (PTC) device, and a switch SI, all connected in series. The channels 16-18 are connected in series with a fan 10 across the rectified DC provided by the bridge rectifier 12.

It can be seen the circuit of Figure 2 provides an impedance drop power supply Pl, P2 for the LED 10, the impedance in a typical impedance drop power supply being replaced by the bank of high voltage LEDs. Note that not all LEDs need to feed the power supply. In the embodiment above, each channel can be selectively connected in series with the fan using the switch Si. The series resistors R2 are used to compensate for line voltage variations. The PTC devices are low impedance components which, when they become hot, switch to a high impedance state. Their function is to protect the circuit from overheating in case of a fault in the cooling fan.

This represents a very economical, compact and elegant solution to the problem of cooling high power, high voltage LEDs. One obtains a free power supply for the fan or other accessory. There is no need for an additional IE 1 0 06 87 dedicated power supply, so the problems of space and high temperature in a small space are avoided.

Figure 3 shows a modification of the circuit of Figure 2, adapted for use with LEDs that are rated for use with AC current, as distinct from those operating with DC current as in the preceding circuits.

Figure 3 differs from Figure 2 in that each individual DC10 rated LED in Figure 2 is replaced in Figure 3 by a respective AC-rated LED, and in that the bridge rectifier at the AC power supply is replaced by a bridge rectifier at the power supply terminals Pl, P2. Each AC-rated LED can be implemented either as a pair of discrete DC devices connected back-to-back in parallel as shown, or as a single AC device.

Figure 4 shows an embodiment based on a switched mode power supply (SMPS). In Figure 4 the AC supply is converted to DC in a bridge rectifier 12 and the converted DC is applied to a plurality of channels 22, 24 and 26 connected mutually in parallel. Each channel contains a plurality of LEDs and a resistor R3 connected in series. The channels 22-26 are connected in series with the fan 10, and the whole circuit forms part of the SMPS.

This embodiment uses an SMPS control chip 28 (such as the LNK302 marketed by Power Integrations of San Jose, California) to generate a constant current, feeding the bank of LEDs with the fan 10 in series. Only a small balancing series resistor R3 is required in each channel, to ensure all LEDs share current equally, but the losses of the series resistors R2 present in the impedance drop power IE 1 0 06 87 supply embodiment of Figure 2 are absent, allowing for a more efficient solution, as well as the ability to add more LEDs into a smaller area.

Like most SMPSs, the current is limited. This has other benefits, especially in the area of safety. If an LED fails short, the current, being fixed, guarantees that there is no overheating, and that the light still works, even if a bit more dimly.

Although the above has described embodiments wherein a low power cooling fan 10 is placed in series with the LEDs,.the circuit could be used to drive any other low power accessory having a current rating of the same order of magnitude as the LEDs, such as: * Light dimmer (controllable by a sequence of user switching cycles, for example) * Remote control circuit (e.g. infra red) * PIR (passive infra red) circuit for sensing presence * Energy saving mode circuit (lower brightness) * Additional circuitry for protection against component failure These features are not feasible in small light fittings at the moment, since it is difficult to provide a local power supply, due to heat and space considerations, and they run hot. With this invention, it becomes practical to incorporate them in a small light fitting, since the main obstacle, a separate power supply, is not necessary.

Electronic circuits, especially those that are subjected to the mains voltage, need to be safe. Safety compliance is IE 1 0 06 87 determined according to various regulations but, in general, it needs to be demonstrated that in the case of at least a single component failure, the hazards of electrical shock, overtemperature and mechanical hazard should not be present. In the case of a light fitting, mechanical hazard can be largely discounted, but there certainly is potential for electrical shock and overtemperature. Electrical shock protection can be guaranteed by the use of adequate and appropriately situated insulation material, but overtemperatures are more difficult to deal with, especially in the case of fault.

The potential faults that could occur in a light fitting using high power LEDs are, most typically, a short of the semiconductor material that forms the basis of the LED, or a failure of the cooling fan, if one exists. One needs to consider other component faults as well. For example, if one had three or four LEDs in series across the mains, along with series resistors, and one LED failed short, the resulting current would be higher, resulting in the LEDs and the resistors absorbing more power and ultimately becoming even hotter. The temperature may not always exceed limits, but good engineering and safety practice would dictate that the design should ensure that that is always the case.

One solution is the one used in Figures 2 and 3, i.e. the use of a PTC (positive temperature coefficient) device placed on the same substrate as the LED. If the temperature of the LED substrate increases too much, the series impedance of the circuit increases due to PTC action, reducing the current and consequently the heating effect.

IE 1 0 06 87 However, PTCs can be expensive, and they are not really very precise. The same function can alternatively be implemented, possibly more economically, using a high voltage transistor. The embodiment of Figure 5, which is a modification of Figure 1, shows an implementation.

It should be pointed out, first of all, the small fans 10 contemplated for use in embodiments of the invention have a third terminal 30 which reports the speed of the fan as a tacho signal in a square wave format 32. If the fan 10 turns the tacho signal 32 is present, otherwise the tacho signal is absent. This helps monitoring electronics to know if cooling is actually taking place or not.

In this embodiment, if the fan 10 fails the square wave tacho signal will disappear. Also, the circuit contains an NTC (negative temperature coefficient) device, which is smaller and cheaper, as well as being more accurate than a PTC, and this is placed on the same substrate as the LEDs.

The circuit works as follows. While the tacho signal 32 is present it is rectified and smoothed by Dl/Cl and applied to the gate of a high voltage N-channel transistor QI.

This holds the latter ON, which in turn holds ON a second high voltage P-channel transistor Q2. This shorts out a dimming resistor R6 and ensures that the maximum current runs through the LEDs.

In the case of a fault due to the fan 10 not turning, where tacho signal 32 stops, or due to an overtemperature sensed by the NTC, the transistor Ql turns OFF, turning OFF transistor Q2 as well. This means that the LED current has IE 1 0 0 6 8 7 to flow through the dimming resistor R6, resulting in dimming of the LEDs. The overtemperature condition is averted.

Note that the NTC has a gradual effect. As the LED substrate heats up, the NTC along with the tacho signal 32 from the fan ensure that the current drops gradually, rather than abruptly. Note that the circuit of Figure 5 could be modified to use only N-channel transistors.

The fact that the dimming resistor R6 can control the brightness of the LEDs raises the following possibilities: * When a failure occurs the LEDs could be dimmed rather than switched off, giving the user a chance to replace the unit without being left totally in the dark in the. meantime. This is not a feature of incandescent or compact fluorescent lights.

* Dimming becomes a potential operating mode of such a light (e.g. dimmed most of the time, but bright when activated by an IR sensor).

A further embodiment is shown in Figure 6. This is similar to the embodiment of Figure 2 except that only two channels 22, 24 are used in this case. The main difference is that a series impedance has been added in the form of a capacitance C4 (this could be a single capacitor as shown or two or more capacitors in parallel).

As previously stated, the reason for a series impedance is to limit the current when the mains voltage (which is largely unregulated) goes high. Since the LEDs are fixed voltage devices, any surplus voltage (i.e. more than the W? 1 0 Ο 6 8 7 voltage that the LEDs have been designed to work at, which necessarily has to be the rated lower limit of mains voltage) has to be absorbed by a series impedance of some sort. Resistors can be used, and they are inexpensive, but they do suffer the problem of being lossy, i.e. they generate heat and add to system inefficiency. They reduce the efficiency rating of the fitting and they make the cooling problem more challenging.

A series resistor is also problematic in the case of an LED failing short. If such an event happens, the voltage drop that the resistor needs to support rises very substantially, and along with it the power it needs to absorb, as a square function of the voltage. This will normally be excessive for the resistor, meaning that its own temperature and that of the substrate onto which it. is soldered may exceed allowable limits of safety and good design practice.

Therefore, instead of using a resistor as the series impedance, a capacitor is used. It has the benefit of not being lossy, and therefore it generates no heat. It also has the benefit of representing a safer solution: if an LED fails short, the capacitor, not being a power absorbing component, does not heat up, and as long as it is rated at a sufficient voltage, it will not fail. Note that (smaller) resistors R3 are still required for the twin functions of limiting temporary mains voltage spikes and in particular to balance the current between several branches of LEDs when there is more than one branch.

A second difference relates to the method of protecting against overtemperature. In Figure 5 a transistor, which IE 10 0 6 8 7 would normally be conducting the LED current, is switched off in the case that an overtemperature were registered.

In the Figure 6'embodiment, a device Ul whose function is to detect a temperature transition on a circuit board is used, for example, an MCP1910HT-E/CH from Microchip Technologies Inc (www.microchip.com). This device has a logical output, which in the embodiment shown, simply switches off the transistor QI if and when the circuit board substrate temperature rises above an acceptable limit, such as could occur in the case of a fault.

Various modifications of the above embodiments are possible. For example: - an inductor could be added in series with the entire circuit to improve the power factor. This would help especially when there is a large number of lamp units on the same circuit. - the temperature limiting device Ul could be replaced by a microcontroller, together with a low cost temperature sensor device, that does the same function as Ul but which can also perform other functions, e.g. controlled and gradual dimming, stabilisation of light power level to compensate for mains variations, nightlight function (cycle power briefly at turn on, causing dimmed operation). - thermal protection could also be implemented using a standard thermal fuse or ETC located on the LED substrate, and connected in series with the entire circuit.

IE 1 0 06 87 - an overcurrent activating fuse could be added in series with the circuit in order to protect against a total short circuit. - a transorb (transient voltage suppression diode) could be added into the circuit in order to protect it from voltage surges. - a light sensor could be added that could detect daylight and, in conjunction with the microcontroller, automatically switch off the (or any) LED lamp unit in daylight.

The foregoing embodiments have used high power LEDs.

However, the invention is equally applicable to light fittings using low power LEDs connected in.series in .one or more channels, provided there are sufficient number of the low power LEDs that the LEDs and the fan (or other low power accessory) operate within their current and voltage ratings.

The invention is not limited to the embodiments described herein which may be modified or varied without departing from the scope of the invention.

Claims (5)

Claims

1. A light fitting containing at least one LED, the fitting further containing an accessory electrically connected in series with the LED(s) and powered by the current passing through the LED(s).

2. A light fitting as claimed in claim 1, wherein the at least one LED is a high power LED.

3. A light fitting as claimed in claim 1, wherein the accessory comprises a cooling fan for the LED(s).

4. A light fitting as claimed in claim 1, wherein the current passing through the LED(s) is DC, and the cooling fan is connected in series with the LED(s). 5. A light fitting as claimed in claim 1, wherein the current passing through the LED(s) is AC, and the cooling fan is powered by a rectifier connected in series with the LED(S). 6. A light fitting as claimed in claim 3, wherein the fan generates a tacho signal while it is rotating, and the fitting comprises a circuit responsive to the tacho signal for diverting current through an impedance in series with the LED(s) in the absence of the tacho signal, whereby the LED is dimmed. 7. A light fitting as claimed in claim 1, wherein a plurality of LEDs are connected in series with the accessory, and when all the LEDs are operative the current through the LEDs and accessory is less than the current IE 1 Ο ο 6 8 7 rating of the accessory so that if one LED fails short the increased current does not exceed the current rating of the accessory.

5. 8. A light fitting as claimed in claim 1, wherein there are a plurality of LEDs arranged in a plurality of channels connected in parallel, the channels being connected in series with the accessory.