GB2507308A - LED module driver - Google Patents

Abstract

An illumination device comprises an LED module together with a driver circuit configured to receive signals from an AC supply via an electronic transformer. The driver circuit comprises sequentially a full-wave rectifier, a reservoir capacitor C21 and a boost converter U21 such that the illumination device draws current from the AC supply during only part of the AC cycle, thereby overcoming problems of intermittent and unstable operation common with these transformers.

Description

DESCRIPTION

TECHNICAL FIELD

The technical field of this invention is solid-state lighting, and in particular the use of light-emitting diodes WEDs) as replacement bulbs that can operate with existing driver circuits to allow the replacement of incandescent bulbs in lighting fixtures and luminaires

BACKGROUND

The development of the technology of light-emitting diodes based on alloys of Gallium, Aluminium and Indium Nitrides has now reached the stage where white light sources based on these materials are more efficient that almost any other light sources. In particular, white light, usually but not necessarily based on the use of a blue (450 to 470nm) LED together with a phosphor that is excited by the blue and has a broad emission centred at in the range 550 to 600nm. can now be produced with an efficiency greater than incandescent or fluorescent sources. The efficiency is generally described in teirns of the output light energy, expressed in lumens to take account of the physical response of the human eye, relative to the input electrical energy. Typically an incandescent lamp will have an efficiency of 15 lumenslW and a compact fluorescent 6Olumens 1W, in comparison to the best commercial white LEDs that are now achieving l3OlumensIW. and still improving.

This advantage in efficiency is supplemented by other major advantages. Unlike all fluorescent tubes the LED lights do not contain mercury, an environmental hazard, and do not require a ballast system to generate an electrical discharge and potentially cause electrical interference. The reliability is much better than that of other sources of light, with expected lifetimes of 50,000 hours or more, in comparison to the lifetimes of incandescent lights, which can be as low as 1000 hours, or compact fluorescent lights with lifetimes of typically 5000 hours. LED lights do not require a glass envelope and so in general are much more robust.

The technical development of LEDs has triggered a major effort to use these light sources to produce efficient light sources and luminaires. The main technical problems can be classified into three groups i. Thermal management. Dissipation in an LED is concentrated into a smaller volume than is the case in incandescent and in particular fluorescent tubes. The heat must be removed efficiently if it is not to cause toss of performance and degradation. The problems are magnified because a plastic (silicone or epoxy) encapsulation is usually used around the phosphor and the semiconductor chip for protection and to enhance light extraction. Heat can be removed efficiently from the active region of the semiconductor into the body of the LED lamp, but heat transfer from the luminaire to air by means of free convection is an inefficient process. and is usually the limiting factor in the operation of the light.

ii. Optical design. Almost all commercial white LEDs are packaged in such a way that they act as Lambertian emitters, with the intensity of the light varying as the cosine of the angle from the normal. For a focussed beam a collimating lens is therefore needed.

Moreover, a single LED can provide typically 200-300 lumens of light, in comparison to up to 1000 lumens from a typical 60W incandescent bulb or equivalent compact fluorescent light. A practical light must therefore contain a number of LEDs. Controlling the light and superposing the light from a number of LEDs involves careful optical design to ensure that there is no significant reduction in efficiency.

iii. Cost. The major limitation on the widespread adoption of solid state light sources is cost.

To some extent this is a reflection of the relative immaturity of the technology, but the high cost also results from the need to satisfy therma', electrical and optical design constraints.

The repbcement light bulb market is seen as the most promising route for the ear'y adoption of solid-state lighting, even though it does not necessarily build on the major strengths of LEDs. In particular, the attempt to provide a cost-effective "rctro-fit" solution introduces additional problems by requiring that LED light bulbs should be compatible with existing power supplies designed for incandescent bulbs.

For the sake of clarity and to avoid ambiguity the words "light" and "bulb" are taken to be equivalent to the phrase "light bulb" and can be used interchangeably. The word "luminaire" is used to refer to the complete light fitting that includes both the light bulb and the housing that contains it, including any means for reflecting the light and supporting the light bulb.

Generally, domestic incandescent light bu'bs are designed to be driven either by the full domestic AC mains voltage. typica'ly with a root mean square (rrns) value of 11 OV or 230- 250V, or by a lower voltage of typically 12V rrns. Bulbs designed to operate with one voltage will not operate successfully with another. By way of example, one popular class of domestic bulbs used in "downlighters" can be purchased either as GU1O bulbs for operation at 240Y or

I

as MR 16 buths for operation at I 2V, although other examples of thw-vohage buths are also known. Most GJJ1O and MR16 incandescent bulbs require input powers in the range 35W to 60W, and generate an output light energy in the range 400 to 600 lumens.

The power requirements of an equivalent LED light are much lower. Approximately 3 LEDs each with an efficacy of I O0lumensfW and operating at a dc current of 700mA are needed to provide a total light output equivalent to a single GU1O or MR16 bulb. The total power required by the LEDs is about 7W. The current drawn by the LEDs depends on the way in which they are connected, with a maximum cutTent of 2. 1A at a voltage of 3.2V. This power is significantly smaller than the power drawn by an incandescent bulb of equivalent light output. The exact value of the current depends on the form of the interconnection topology so that in this example the LEDs could also be driven with a total current of 700mA at a voltage of 9.6 V. In general LEDs will not operate efficiently from an AC voltage source and require a DC current source or a rectified AC signal. In contrast to incandescent lamps the response time of LEDs is very short so that if the rectified AC signal is not adequately smoothed the lamp may be perceived by some people to flicker.

The problem of designing a replacement LED bulb is therefore to transform an AC voltage supply used for incandescent bulbs to a current source operating at lower voltages, and do so at high efficiency and low cost. The challenges are different for GUI 0 and MR 16 buths because of the different supply voltages, with the MR 16 bulbs presenting particular difficulties because of the various possible commercial transformers that are already installed to reduce the normal mains supply voltage to 12V mis. Commercial electronic transformers (ETs) often have a minimum power rating of 20W and MRI6 LED lamps typically draw less than lOW. It is largely this power mismatch that causes problems when trying to use LED MR 16 lamps with ETs designed for incandescent bulbs.

Many commercial semiconductor companies have standard integrated circuits available that can be used to dnve LEDs in LED versions of GU1O bulbs. An example is the Infineon CL8001G integrated circuit which is driven by a rectified 240V rms supply, and includes capabilities for power correction at the input, with a variable constant current output. Quoted power correction factors of up to 98% and overall efficiencies of up to 80% are given for this circuit. Similar capabilities are included in the Fairchild FL7701 for lower currents. With the use of standard components these integrated circuits can be interfaced to the LEDs in the replacement bulb to produce the desired operating characteristics. Most of the ICs also include a dimming capability compatible with existing installed dimmers for incandescent dimmers, although there is not 100% compatibility.

The design of an LED replacement for a standard MR 16 bulb is less straightforward. A typical power supply for a low voltage incandescent MR16 bulb uses an electronic transformer designed to provide 35 to SOW of electrical power at a root mean square voltage of 12V, corresponding to a root mean square current in the range 3 to 4A. Electronic transformers operate by modulating the incoming AC supply at an ultrasonic frequency typically in the range 40-60kHz. The higher frequency signal can be transformed using smaller value components that are physically smaller and of lower cost, resulting in a transformer of smaller size, weight and cost when compared to conventional magnetic transformers.

Transformers used with MRI6 incandescent lamps are optimised to drive resistive loads at a minimum of 20W and in many cases take advantage of the minimum load requirement to implement the ultrasonic oscillator based on transformer feedback. This may not work if the actual toad is significantly tess than the design load.

The current drawn by an MRI6 incandescent lamp is significantly larger than the maximum total current of about 2A drawn by the equivalent LED replacement bulb. The relatively low current drawn by an LED retro-fit MR 16 will affect the performance of some existing electronic transformers which often do not function correctly when driving the higifly non-linear and relatively low power load offered by a typical LED replacement lamp. Furthermore there is a wide variety of different versions of electronic transformers in the existing infrastructure and the major challenge is to ensure that a given replacement LED bulb is compatible with all of them.

Some electronic transformers do in fact work down to low currents and will drive an LED MR16. As a result, some LED lamp manufacturers have taken the path of listing transformers that work satisfactorily with their LED lamps. Although a pragmatic approach, this is not a satisfactory one, not least because most customers will not know which transformer is installed in their Ughting circuit.

Some better but more expensive designs use an RC oscillator which runs independently of the value of the load and these designs normally work well with low wattage resistive linear loads. However, even for these electronic transformers the performance with the highly non-linear load presented by LED circuits is variable and not readily predictable.

Simple solutions that have been suggested to order to overcome the problem include the use of a single transformer to drive several LED lamps or the addition of a shunt resistor, thereby ensuring a minimum current at all times. The first is clearly not possible in the case of replacement lamps, and the second leads to an unacceptable loss of efficiency.

Alternative solutions have been developed to drive MR1Ô LED lamps from magnetic transformers and, as mentioned above, these can work with some electronic transformers. A number of the major semiconductor manufacturers now produce suitable integrated circuits such as the National Semiconductor LM3401. The operation of a circuit including this IC is illustrated in figure 1. The rectified AC voltage is modulated at the ultrasonic frequency of the electronic transformer and a smoothing capacitor shown as CII in figure Iserves to remove this oscillation. A typica' value of CII is 2RF.

The rectified and smoothed voltage is applied to the input of the control IC and also to a iS standard buck circuit comprising the following components; a MOSFET transistor Qi i, a "catch" diode Dl 2, an inductor LI I and a reservoir capacitor C 12. A typical value of Cl 2 is 2 RF, similar to thc valuc of C 11.

The gate of QI I is modubted (switched) at a frequency determined by the voltage that appears across the current sensing resistor Rl lwhich measures the average current passing through the chain of LEDs. In this simplified representation of a control IC the IC has the following connections; supply voltage Vin, current sensing CS, gate drive voltage Vg, current feedback pin CF, dimming input DIM (not connected in figure 1) and ground pin OND.

The use of a buck converter implies that the circuit can realistically drive typically three CaN-based LEDs in series. To provide the equivalent light output to an incandescent MRI6 bulb the circuit should provide approximately 700mA though each of the three LEDs. The resulting current drawn by the driver circuit from the electronic transformer is insufficient to ensure reliable operation of all electronic transformers.

The invention described in the present document addresses the problem of ensunng compatibility of an MR16 bulb with every electronic and magnetic transformer by ensuring that sufficient current is drawn from the transformer to maintain operational stability and efficiency during part of the AC cycle. The solution uses a reservoir capacitor together with a boost converter circuit. Because the circuit will not operate correcfly if the load voltage is less than the supp'y voltage the invention uses an LED module that requires an operating DC voltage higher than the peak value of 17V supplied by typical 12V (rms) electronic transformers. The circuit uses selected high-quality electrolytic capacitors that with good thermal design can give sufficiently long lifetimes.

The term LED module is taken to mean a series combination of a number of LEDs such that the DC voltage needed to produce a DC current equal to or greater than lOOmA is greater than the peak value of the AC supp'y voltage. A specific example of an LED module is the Cree MT-G module with the twelve LEDs connected in series and requiring a DC supply voltage of 33V to produce a current of lOOmA through the module, greater than the peak lO value of l7Y provided by a typical 12V AC supply voltage. Other series connections of different numbers of LEDs are of course also possible.

LED Module Driver Description

The operation of the driver circuit disclosed in this patent application will be explained in more detail by reference to the following figures.

1. Figure 1 provides a schematic representation of the existing art in which a standard "buck" converter is used to reduce the rectified AC input voltage in order to drive an LED chain.

2. Figure 2a) shows in schematic form the basic circuit described in this invention in which the rectified AC voltage is applied to a reservoir capacitor connected across the input of a standard "boost" converter. Figures 2b) and 2c) show schematically the way in which this boost circuit can be implemented using either an integrated transistor switch in tigure 2b), or by using an external FET switch as in figure 2c) 3. Figure 3 shows the current at the output of the bridge rectifier (3a) and 3b)) and at the input to the boost circuit (3c) and 3d)) for two different values of the capacitance of the reservoir capacitor. Figures 3a) and 3c) correspond to a value of the capacitance C2i equal to IOORF, and figures 3c) and 3d) correspond to a value of C2i equal to 4701ff.

4. Figure 4 shows one implementation of the complete driver circuit.

The basic circuit for the driver is illustrated in schematic form in figure 2a). An AC input signal is rectified and applied to a reservoir capacitor C2l connected across the input of a boost converter U21. The output of the boost converter TJ2l is connected across an LED module consisting of a string of LEDs connected in series where the number of LEDs is such that the voltage required to drive the LEDs is greater than the peak voltage of the rectified AC input. By appropriate choice of the value of the capacitor C2i the circuit can be adjusted to ensure that it only draws current from the transformer over part of the AC cycle and that the current drawn from the transformer over part of the cycle is larger than the minimum needed to ensure that the transformer operates in a stable manner without causing flicker or current spikes.

Figures 3a) and 3b) show the results of idealised circuit simulations for the current supplied by the bndge rectifier for two different values of the reservoir capacitor C21. Figure 3a) shows the rectifier current for a value of C21 of I OOjiF and figure 3b) shows the rectifier current for a value of C2 1 of 47OtF. This rectifier current is equal to the culTent supplied by the electronic transformer after rectification and smoothing to remove the modulation produced by the ultrasonic oscillator in the transformer. In this implementation. when a value of I OOpF is used there is a finite current for all times during the complete period of I Oms corresponding to the 100Hz frequency of the rectified AC signal. When the value of C2l is increased to 470tF the current flows wily for part of the cycle and consequently the current is high enough during that part of the cycle to ensure correct operation of the electronic transformer. Also by drawing maximum current at zero crossing the probability of the electronic transformer starting smoothly is maximised.

Figures 3c) and 3d) show the input culTent to the boost converter for the two values of C21.

As would be expected by those skilled in the art, for the larger value of C21 the peak current is significantly reduced.

it can be seen there are two key differences between the approaches indicated schematically in figures 1 and 2a). The first is that there is a large reservoir capacitor in figure 2a, b) and c).

where C21 is typically 470jW in comparison to a typical value for the smoothing capacitor Cl 1 of 2RF in figure 1. The second innovative feature is that the rectified AC voltage from the bridge rectifier is used to drive a boost converter and not a buck converter. In this invention, the combination of the use of a reservoir capacitor and a boost converter gives a current that flows only for part of the 100Hz cycle, and which has a magnitude sufficiently large over this part of the cycle to trigger reliable operation of the electronic transformer.

A further significant difference is that the buck converter in figure 1 can drive a series chain of at most three to four LEDs. but the boost converter in figure 2a) requires a series chain of at least 6 LEDs if the output voltage is to be greater than the maximum input voltage. The longer string has a further advantage in that it enables a higher conversion efficiency to be achieved. This means that not only is less energy required for a given light output, but also that there is tess heat to dissipate which reduces the operating temperature of the electrolytic capacitor and so increases its life.

Careful choice of the reservoir capacitor is essential to ensure long term reliability. The use of electrolytic capacitors is often regarded as problematic because of perceived poor reliability, but with improved electrolytic capacitor performance and improved characterisation data a suitable choice can be made. In general, the higher the quality of the capacitor the higher the cost and so a balance must be struck between cost and performance. The invention shown in figure 2 has a low component count, and so the cost of using the electrolytic capacitor can be justified on the basis of a cost-benefit analysis.

Alternatively, the large reservoir capacitor may be replaced by a combination of smaller capacitors in parallel in order to improve the ease of packaging the capacitors and other electrical components within the MRI6 or other replacement bulbs.

The lifetime of an electrolytic capacitor is in general an exponential function of temperature and in this application will depend on careful thermal design of the MR 16 replacement bulb.

Thermal simulations and measurements have shown that acceptable lifetimes can be achieved with present state-of-the-art LEDs, and of course as the efficiency of LEDs improves further the problem of thermal design becomes less critical.

The use of a boost converter implies that a significantly larger number of LEDs must be used in the series chain for correct and efficient operation. In figure 2a) the boost converter is shown schematically, but in figure 2b) a specific implementation is shown in which the inductor, diode and capacitor in the boost converter are shown explicitly, although the switch is assumed to be incorporated into the integrated circuit U2l. Figure 2c) shows an alternative implementation in which a transistor switch is shown explicitly outside the integrated circuit.

A more complete circuit description will now be given using as a specific example the Cree MT-C which is a module containing twelve LEDs where optionally all twelve LEDs can be connected in series to give a specified forward v&tage of approximately 36V at a current of 350rnA. In order to provide the equivalent light output to an incandescent MR 16 bulb the module is driven at 200mA. corresponding to a total input power to the LEDs of 6.7W.

Figure 4 shows the full circuit implementation of the driver using a boost converter. In this implementation the IC used in the boost converter is the Texas Instruments TPS6i i65 LED driver shown as 1J41, although it will be obvious to those skilled in the art that other integrated circuits could be used. This IC is an example of one that contains an integrated FET that is used for the boost converter. The bridge rectifier, consisting of four diodes D4lto D44, contains a small resistor R41 to limit the large initial current resulting from the use of the large reservoir capacitor C41 at the output of the bridge. Simulation and measurement show that this small resistor contributes a negligible amount to the thermal dissipation.

The components C42, C43. C44 and L41 are required to reduce the potential electromagnetic interference resulting from the high frequency operation of the boost converter. Capacitor C45 at the output serves the same function, and also acts as part of the boost converter.

Components resistor R42, diode D46 and capacitor C46 provide a permanent voltage supply topins5and6ofthelCU4l.

The inductor L42 is the storage inductor which together with the flywheel diode D45, the capacitor C45 and the FET in the IC U4l forms the boost converter. The resistor R43 sets the average LED current by supplying a current sensing voltage to pin 1 of the IC. Pin 4 of the IC is the switch which shorts the inductor L42 to ground during operation of the boost converter.

pin 3 is the ground connection and pin 2 is used for loop compensation by connecting capacitor C47 to ground. If required pin 5 can be used for dimming and also allows a PWM signal to be used for control of the LED current.

Although the operation of the circuit has been described in terms of one specific implementation of the boost converter, it will be obvious that a similar circuit could be achieved in other ways while maintaining the essential innovative feature of using a reservoir capacitor together with a boost converter.

Claims (10)

1. Claims 1. An illumination device comprising an LED module with a driver circuit configured to receive signals from an AC power source and where the driver also comprises sequentially a) a full-wave rectifier and b) a reservoir capacitor and c) a boost converter such that the device draws current from the AC supply during only part of the AC cycle.
2. 2. An illumination device according to claim 1 where the LED module consists of at least 6 LEDs connected in series.
3. 3. An illumination device according to claim 1 where the LED module consists of 12 LEDs connected in series.
4. 4. An illumination device according to claims 1 and 2 where the AC supply voltage is nominally 12V.
5. 5. An illumination device according to claims 1 and 2 where the AC supply voltage is nominally 24V.
6. 6. An illumination device according to claim lwhere the boost converter comprises an integrated circuit together with at least one additional inductor, diode and capacitor and switching transistor external to the IC.
7. 7. An illumination device according to claim I where the boost converter comprises an integrated circuit that contains an integrated switching transistor together with at least one additional inductor, diode and capacitor external to the IC.
8. 8. An illumination device according to claim I where the device additionally contains components to suppress electromagnetic interference.
9. 9. An illumination device according to claim 1 where the reservoir capacitor C comprises a parallel combination of two or more smaller capacitors.
10. 10. An illumination device according to claim 1 where additional electrical connections and electronic components are induded to implement a dimming function in the boost converter.