WO2002023956A2

WO2002023956A2 - Power supply for light emitting diodes

Info

Publication number: WO2002023956A2
Application number: PCT/US2001/028932
Authority: WO
Inventors: George W. Panagotacos; David G. Pelka, Ph.D.
Original assignee: Teledyne Lighting and Display Products Inc
Current assignee: Teledyne Lighting and Display Products Inc
Priority date: 2000-09-15
Filing date: 2001-09-13
Publication date: 2002-03-21
Anticipated expiration: 2003-03-15
Also published as: WO2002023956A3; AU2001291008A1

Abstract

A power supply for powering at least one light emitting diode (LED) with AC voltage comprises an AC input line, a step-down impedance, a rectifier, a low pass filter, and a current regulator. The AC voltage is carried through the AC input line to the step-down impedance, which attenuates the voltage. The rectifier subsequently rectifies the voltage while the lower pass filter removes ripple from the rectified voltage. The current regulator converts the rectified voltage into a substantially constant supply of DC current that is delivered to the LED.

Description

POWER SUPPLY FOR LIGHT EMITTING DIODES

Background of the Invention

The present invention relates to electrical power supplies, and more specifically to power supplies that provide a regulated current output for driving light emitting diodes. Various lighting applications require a string of series-connected light emitting diodes (LEDs) as a source of illumination. To emit light, the LEDs must receive direct current (DC) electrical power; in particular, a minimum amount of DC current needs to be supplied to each of the LEDs for light emission. These LEDs will emit more light with increasing amounts the current, but the lifetime of the LED is shortened when the LEDs are driven with excessive current. Although the LEDs may be powered with batteries, employing commercial or residential alternating current (AC) line voltage is often more practical, and thus power supplies that convert AC line voltage into a DC voltage are desirable for powering LEDs. The AC line voltage, however, is not perfectly stable, but rather fluctuates. For example, residential AC line voltage, nominally 115 AC volts (VAC) can vary between 110 to 120 VAC. Such variation presents a problem if, as a result, the current fed to the LEDs is less than the minimum current required to activate the LEDs or if the current exceeds the maximum, and thereby reduces the lifetime of the LED. Variation in the electrical characteristics between different LEDs also can cause the level of current to be too low or too high. When current is passed through an LED, the device is forward biased; in this mode of operation of the

LED, a voltage known as the forward voltage drop can be measured across the LED. This forward voltage drop differs somewhat between individual LEDs. When a series of such LEDs are strung together, e.g., 24 or 48, the small difference in forward voltage drop accumulates into a larger, more significant value. Variations in this cumulative voltage drop across a large number of LEDs present difficulties for an engineer who must design the power supply circuit so as to provide the LEDs with a current above the minimum and below the maximum current levels. For a power supply with a fixed impedance, differences in a load translate into different amounts of current sunk into that load. Thus, if the voltage drop across a group of LEDs connected in series is not the same, the current into the LEDs will likewise be dissimilar. If the disparity is extreme, the current may fall below the minimum, causing the LEDs to fade out, or the current may exceed the maximum, thereby damaging the LEDs, or at least shortening their lifetime.

Conventional supplies for powering LEDs are also large and expensive, yet inefficient. For example, many power supplies employ a transformer to step down the voltage from the AC line. A transformer, however, is large and heavy and adds to the cost of the supply.

Thus, there is a need for an efficient, compact, and inexpensive power supply for powering a string of series- connected LEDs. This power supply must provide a stable current that does not fall below the minimum required by the LED, thereby causing the LEDs to stop emitting light, and does not exceed the maximum current level for the LED, thereby damaging the LEDs or at least shortening their lifetime.

Summary of the Invention In one aspect of the invention, an apparatus comprises a light emitting diode and a power supply for powering the light emitting diode, wherein the power supply comprising a step-down impedance, a current regulator, a rectifier, and a low pass filter. In this power supply, the current regulator is electrically connected to the step-down impedance, while the rectifier is electrically connected between the step-down impedance and the current regulator. The low pass filter is electrically connected between the rectifier and the current regulator. Additionally, the current regulator has an output for electrical connection to the at least one light emitting diode. Another aspect of the invention comprises a power supply for providing electrical power to at least one light emitting diode. This power supply comprises a step-down impedance, a current regulator, a rectifier, and a low pass filter. The current regulator is electrically connected to the step-down impedance, the current regulator rectifier is electrically connected between the step-down impedance and the current regulator, and the low pass filter is electrically connected between the rectifier and the current regulator. The current regulator has an output for electrical connection to the at least one light emitting diode that outputs a current and voltage compatible with the light emitting diode.

In another aspect of the invention, a method of providing power to one or more light emitting diodes (LEDs) using an AC voltage includes attenuating the AC voltage. After attenuation, the AC voltage is rectified to produce a rectified voltage. Subsequent to the rectification, ripple is removed from the rectified voltage, and subsequent to this ripple removal, the rectified voltage is converted into a substantially constant supply of DC current. This DC current is applied to power the one or more light emitting diodes (LEDs).

Brief Description of the Drawings FIGURE 1 depicts a block diagram of a power supply that is a preferred embodiment of the present invention. FIGURE 2 is a schematic circuit diagram of a power supply that includes each of the elements in the block diagram of FIGURE 1.

FIGURES 3A-3F are plots on axes of time and voltage depicting waveforms corresponding to electrical signals at different stages in the circuit shown in FIGURE 2.

Detailed Description of the Preferred Embodiment

Embodiments of the invention will now be described with reference to the accompanying Figures, wherein like numerals refer to like elements throughout. The terminology used in the description presented herein is not intended to be interpreted in any limited or restrictive manner, simply because it is being utilized in conjunction with a detailed description of certain specific embodiments of the invention. Furthermore, embodiments of the invention may include several novel features, no single one of which is solely responsible for its desirable attributes or which is essential to practicing the inventions herein described. A block diagram of a power supply 10 that is a preferred embodiment of the present invention and that electrically connects a source of AC line voltage 12 with one or more light-emitting diodes (LEDs) 14 is shown in

FIGURE 1. The power supply 10 comprises a surge protector 16, a voltage attenuator 18, a rectifier 20, a low pass filter 22 that suppresses ripple, a charge dissipator 24, and a current regulator 26, each having an input and an output.

The source of AC line voltage 12 may be a residential or commercial AC electrical line. The input of the surge protector 16 receives the AC line voltage and offers protection for the circuitry in the remainder of the power supply 10 against spurious voltage spikes in the AC line. The AC line voltage after passing through the surge protector 16 is fed into the voltage attenuator 18 to lower the AC voltage. The output of the voltage attenuator 18 is directed to the rectifier 20, where full-wave rectification is performed, and then to the low pass filter 22, which reduces ripple in the rectified voltage. The output of the low pass filter 22 is coupled to the current regulator 26, which provides a regulated DC current to the LEDs 14. A charge dissipator 24, shown in FIGURE 1 between the low pass filter 22 and the current regulator 26, prevents dangerously rapid discharge of voltage within the power supply 10 when the LEDs 14 are disconnected.

As depicted in FIGURE 2, the power supply 10 has two input terminals 28 for connecting to the source of AC line voltage 12. These input terminals 28 must be capable of safely receiving the AC line voltage 12, which may be a 120 volt (V) residential line having an average RMS voltage of 115 volts or 240 volt commercial line having an average

RMS voltage of 230 volts. This line voltage may also be 220 V as is standard for residences in Europe. The surge protector 16 is shown in FIGURE 2 as a surge protection device 30 shunted across the input terminals 28. This surge protection device 30 may comprise any device capable of preventing voltages in excess of 150 VAC, preferably 135 VAC, from reaching other electrical components in the power supply 10. One example of such a device is a ZI\IR, P/N ERZ-vo5D241, manufactured by Panasonic.

Preferably, the voltage attenuator 18 comprises a capacitor 32, hereinafter referred to as the step-down capacitor, which is in electrical contact with one of the two input terminals 28. The step-down capacitor 32 is required to be a bipolar capacitor capable of receiving an AC input that oscillates between negative and positive voltages and preferably comprises one or more polyester or Mylar® polyester film capacitors. A capacitor or plurality of capacitors having a net capacitance in the range of between 0.1μF-10μF may be employed. The appropriate level of capacitance is a function of numerous factors, including but not limited to, the AC line voltage input to the power supply 10 (e.g., residential, commercial), the number of LEDs 14 in the array, and the electrical characteristics of the LEDs, in particular, the current drawn by and the voltage drop across each of the LEDs, as well as the arrangement of the LEDs (e.g., whether the LEDs are arranged in series or both series and parallel). Although the voltage attenuator 18 is shown in FIGURE 2 as comprising a capacitor, a resistor or any other element can be employed that provides a voltage drop from the input terminals 28 to the rectifier 20, but an element having an impedance substantially comprising an imaginary part is preferred, to avoid thermal heating. The impedance of a capacitor, for example, comprises solely of an imaginary part, known in the art as reactance, and does not heat up as AC current is passed through it. Unlike capacitors, resistors have a real impedance or resistance, and heat up when conducting current. Accordingly, capacitors are preferred to minimize temperature rise and loss of energy due to thermal dissipation.

Alternatively, an inductor can be shunted across the input terminals 28. An ideal inductor, like a capacitor, comprises solely of reactance. Real inductors, however, are formed from wire wound about a magnetic core. This wire is resistive, so that a conventional inductor will dissipate heat. Additionally, an inductor capable of providing a sufficient step-down voltage for the power supply 10 would be substantially larger than a capacitor suitable for the application described herein. For the foregoing reasons, a capacitor is therefore preferred. As shown in FIGURE 2, the rectifier 20 comprises a diode bridge 34, a type of full-wave rectifier that is well known in the art. This diode bridge 34 includes four diodes 36 connected together at four junctions 38a, 38b, 38c, 38d. The step-down capacitor 32 is affixed to one of the junctions 38a such that one of the two input terminals 28 is electrically connected to the bridge 34 through the capacitor. The oppositely situated junction 38b is in direct electrical contact with the other of the two input terminals 28. Conducting lines 40a, 40b extend from the remaining two junctions 38c and 38d, respectively, and lead to the low pass filter 22, also acting as the ripple suppression filter. These two conducting lines, 40a and 40b, are designated as plus ("+") and minus {"-") to indicate that the plus conducting line is at a higher potential than the minus conducting line, which may be grounded. The diodes 36 in the diode bridge 34 are conventional diodes such as IPJ4003 diodes, manufactured by International Rectifier Corporation, and capable of receiving a maximum (max) peak reverse voltage of 200 V. Although a diode bridge that provides full- wave rectification is shown in FIGURE 2, other full or half-wave rectifiers may be employed in accordance with the present invention. Preferably, however, the rectifier 20 contains little or no resistive components, so as to minimize heat dissipation.

As depicted in FIGURE 2, the ripple suppression filter 22 comprises a capacitor 42 shunted across the plus and minus conducting lines 40a, 40b, i.e., one end of the capacitor is electrically coupled to the plus electrically conducting line while the other end of the capacitor is electrically coupled to the minus electrically conducting line. This capacitor 42, hereinafter referred to as the ripple-suppression capacitor, may have a capacitance between 10 μF and 100 μF and preferably comprises tantalum (Ta). A suitable capacitance is determined at least in part by the AC line voltage and the electrical properties of the LEDs 14. Although the ripple suppression filter 22 shown comprises a capacitor shunted across the plus and minus conducting lines 40a, 40b, other filters capable of reducing oscillation can be employed as the ripple suppression filter 22.

The charge dissipator 24 depicted in FIGURE 2 comprises a resistor 44, also shunted across the plus and minus electrically conducting lines 40a, 40b and, similarly, across the ripple suppression filter 24. The resistor 44, hereinafter referred to as the charge-dissipation resistor, may comprise a conventional resistor well known in the art having a value of resistance between 10 kΩ and 100 kΩ. The appropriate resistor may be selected based on, among other factors, the AC line voltage and the discharge rate (time) required. Although the charge dissipator 24 shown comprises a resistor, other components arranged to safely dissipate charge released when the power supply 10 is disconnected from the array of LEDs 14, can be employed as the charge dissipator 22.

As shown in FIGURE 2, the current regulator 26 comprises a voltage regulator 48 in series with a current set-point resistor 50. This voltage regulator 48, a device adapted to supply a substantially constant voltage, may comprise a conventional voltage regulator or one yet to be devised. This voltage regulator 48 may, for example, comprise an LM317AT voltage regulator manufactured by National Semiconductor, adapted to supply an input-output voltage differential of +40 V maximum and a maximum current of 1.5 A. The voltage regulator 48 has an input terminal 52, an output terminal 54, and a reference terminal 56; the input terminal is electrically connected to the plus electrically conducting line 40a, and the output terminal is electrically connected to one end of the current set-point resistors 50. The other end of the current set-point resistor 50 is electrically connected both to a positive output lead 58 for the power supply 10 and to the reference terminal 56 of the voltage regulator 48. The resistance of the set- point resistor 50, which may range from 0.8 Ω to 1.2 kΩ, is determined by the voltage difference between the output and reference terminals 54 and 56 of the voltage regulator 48 along with the current to be drawn by the LEDs 14. Although a voltage regulator in series with a set-point resistor is shown, other current regulators, i.e., devices adapted to provide a substantially constant current to a given load, can be employed in accordance with the present invention. Preferably, this current supply provides a current within the range of between 1.0 mA and 1.0 A, more preferably between 25 and 50 mA.

Preferably, more than a single current set-point resistor 50 is included in the current regulator 26, as shown in FIGURE 2, which depicts a switch 60 electrically connected to the output terminal 54 of the voltage regulator 48.

This switch leads to the current set-point resistor 50 when set in a first position, and when set to a second position leads to another current set-point resistor 51. Although two such current set-point resistors 50 and 51 are shown, more than two resistors can be included in the current regulator 26 in accordance with the present invention. Alternatively, the current regulator may comprise only one single current set-point resistor 50, but a plurality of current set-point resistors, and a switch 60 that incorporates any one of the resistors in a continuous electrical path from the output terminal 54 of the voltage regulator 48 to the positive output lead 58 of the power supply 10 is preferred.

As shown in FIGURE 2, a capacitor 61 is electrically connected in parallel across the plus and minus contacting lines 40a, 40b. This capacitor 61 prevents the voltage regulator 48 from oscillating. A zener diode 62 and resistor 64 are electrically connected in parallel across the input and output terminals 52, 54 of the voltage regulator 48. This zener diode 62 may comprise a conventional zener, e.g., a 39 V zener, available from Diodes, Inc. Selection of the appropriate zener diode 62 and resistor 64 depends on the electrical characteristics of the voltage regulator 48, namely, on the maximum and minimum voltage that can be sustained between the input terminal 52 and the output terminal 54 without interrupting the constant delivery of proper current levels to the LED's or without damaging the voltage regulator. The string of LEDs 14 shown in FIGURE 2 has two ends, one end being electrically connected to the positive output lead 58 for the power supply 10, while the other end of the string of LEDs 14 is electrically connected to a negative output lead 66 of the power supply. The positive output lead 58 is in electrical contact with one end of the set-point resistors 50, 51, as discussed above, while the negative output lead 66 is in electrical contact with the minus conducting line 40b, which may be grounded. The power supply 10 is designed to accommodate a variety of different types of LEDs 14 having a wide range of center wavelengths and electrical properties. Significantly, different LEDs 14 may require different amounts of current, depending on their design as well as the brightness of the output emission desired. Current set-points for the LEDs 14 may be between about 1mA to about 1A, more preferably, between about 25 mA to about 50 mA. Example of suitable LEDs 14 include a TS AllnGaP LED, which emits red light having a wavelength of about 630 nm and an InGaN LED, which emits green light having a wavelength of about 526 nm. In addition to accommodating different types of LEDs, the power supply 10 can power arrays of various size from a single LED up to 60 LEDs for 115 VAC input power, and in various arrangements such as LEDs in series or both series and parallel. For example, the plurality of LEDs 14 may comprise two or more strings in parallel with each other, wherein each string includes a number of LEDs connected in series.

The power supply 10 delivers power to the array of LEDs 14 by converting AC line voltage into a DC regulated current that is delivered to the LEDs. As discussed above, the amount of current supplied to the LEDs 14 controls the level of emission output therefrom, higher current producing a more intense optical output. Since driving the LEDs 14 with too much current shortens their lifetime, a safe level of current, hereinafter designated l_SET._P0|_NT, which provides sufficiently bright illumination but that also ensures adequate lifetime of the LEDs, must be determined and consistently provided to the array of LEDs 14 when activated. By choosing the appropriate components for the power supply 10 shown in the FIGURE 2, a substantially stable DC current at the set-point, I_SET.POINT_< can be delivered to a plurality of LEDs 14. To power the LEDs 14 at the DC current set-point, the power supply 10 receives AC line voltage from a residential or commercial AC line at the input terminals 28, thereby causing a sinusoidal voltage to be coupled to the step-down capacitor 32 and to the diode bridge 34. Before being attenuated by the step-down capacitor 32, however, any voltage spikes in excess of 150 VAC are clamped by the surge protector 16, which prevents excessively high voltages from reaching the step-down capacitor and the remainder of the power supply circuitry. The AC line voltage input into the power supply 10 is shown in FIGURE 3A as having a peak voltage of V_AC LINE UMAX₎ and ^an RMS value of V_{AC LINE (RMS)}. The voltage reaching the diode bridge 34 is reduced by the voltage drop, V_CAP, across the step-down capacitor 32, the value of which is determined, in part, by the impedance of the capacitor. FIGURE 3B shows the AC voltage attenuated by the step-down capacitor 32 having a magnitude of V_{AC L},NE ₍MAX₎ - V.AP _(MAX|.

This oscillating voltage is applied to the diode bridge 34, which rectifies the sinusoidal voltage in a manner well known in the art. FIGURE 3C illustrates the effect of a rectifier circuit, such as the diode bridge 34, on a sinusoidal signal, like the signal across the two junctions 38a and 38b of the diode bridge. The rectified voltage exhibits a small voltage drop, V_BRraGE, introduced by the bridge. The extent of oscillation or ripple in the rectified voltage is greatly reduced by the ripple suppression capacitor 42. FIGURE 3D depicts the voltage across the plus and minus conducting lines 40a and 40b electrically connected to two of the junctions 38c and 38d of the diode bridge 34 and to the ripple suppression capacitor 42. FIGURE 3D illustrates both the rectification and suppression of ripple, as well as the reduction in the overall signal caused by the voltage drop V_CAP ,_MAX) and V_BraDGE introduced by the capacitor 32 and the diode bridge 34. The voltage at the plus conducting line 40a is applied to the input terminal 52 of the voltage regulator 48, which produces a voltage at the output terminal 54 that is a fixed increment above the voltage at the reference terminal 56 of the voltage regulator. For the LM317AT voltage regulator, this fixed increment is 1.25 V; accordingly, the voltage at the output terminal 54 is consistently 1.25 V lower than the voltage at the reference terminal 56. FIGURE 3E depicts this voltage at the output terminal 54 of the voltage regulator 48. The voltage at the output terminal 54 is reduced by a voltage drop across the set-point resistor 50, and this reduced voltage is applied to the reference terminal 56. In this manner, a voltage regulator similar to 48 is converted into the current regulator 26. That is, since the voltage at the output terminal 54 will be a fixed incremental voltage, (i.e., 1.25 V) below the reference terminal 56, for a given set-point resistor 50 the current through that resistor is known. Thus, by selecting different resistors 50, different set-point currents can be established. The switch 60 that connects the voltage regulator 48 with the plurality of set-point resistors 50, 51 in FIGURE 2 permits various set-point currents to be chosen, depending upon which resistor is employed. FIGURE 3F shows the current flowing though the positive and negative output leads 58, 66 and the load, i.e., the plurality of LEDs 14, electrically connected thereto.

The voltage regulator 48 essentially converts the voltage waveform depicted in FIGURE 3D into a substantially constant current shown in FIGURE 3F. The voltage regulator 48, such as the LM317AT, may, however, operate improperly if excessive voltage is applied across the input terminal 52 and the output terminal 54 of the voltage regulator. The zener diode 62 and resistor 64 in parallel therewith prevent excessive voltages from being applied across the input and output terminals 52 and 54 of the voltage regulator 48 and from possibly damaging that device. The charge dissipation resistor 44 in the power supply circuit 10 provides discharge of the ripple suppression capacitor 42 when the plurality of LEDs 14 is disconnected from the positive and negative output leads 58 and 66. Charge stored on the ripple suppression capacitor 42 during operation will require a known time to discharge when the

LEDs 14 are removed from the circuit. The resultant energy will have a time constant, τ, that is decreased by the introduction of the charge dissipation resistor 44 thereby increasing the discharge rate and minimizing the likelihood of stored energy which could cause a shock.

The circuit design shown in FIGURE 2 presents numerous advantages over prior-art power supplies. Power loss is minimized by limiting the number of resistors. Resistors convert an amount of energy equal to the potential energy drop across the resistor into thermal energy, energy that is lost, i.e., that is not used to power the LEDs 14. To limit the number of resistive elements in the power supply 10, the AC line voltage is preferably stepped down to a lower voltage with a capacitor, the step-down capacitor 32, instead of with a resistor. Unlike resistors, capacitors in AC circuits are reactive: the average power dissipated by a capacitor receiving a complete cycle of a sinusoidal current or voltage is close to zero. Similarly, the average power over a complete cycle is exactly zero for any purely reactive circuit, such as, e.g., an ideal capacitor or an ideal inductor, and for circuits operated at low frequency that draw low current and therefore resemble purely reactive circuits. Minimizing the resistive elements in the circuit also causes the circuit to be more reactive. The power supply 10 depicted in FIGURE 2 has a minimal number of resistive elements and is designed to be operated at 50 Hz or 60 Hz, and to draw low current from the AC line. Consequently, the amount of power dissipated thermally by the power supply 10 is less than conventional LED power supplies, providing numerous advantages over prior art designs. Since a small amount of energy is transformed into thermal energy and radiated away from the power supply, most of the energy coupled into the power supply 10 is efficiently transferred to the LEDs 14 as electrical power. Accordingly, less power is required to operate the power supply 10, thereby reducing operating costs. Light emitted by light emitting diodes 14 is less bright when the LEDs are exposed to heat, therefore by limiting the heat reaching the LEDs, the power supply 10 promotes efficient production of light from electrical power. Also, since the power supply 10 does not heat up significantly during operation, it can be incorporated in systems that are sensitive to temperature without the inclusion of large or heavy heat sinks. For example, the power supply 10 and accompanying LEDs 14 may be contained in plastic tubes that encase the LEDs. Additionally, since no large heat sinks are necessary, the power supply 10 may be compact. The power supply circuit 10 also provides longer lifetime for the LEDs 14, as the current regulator 26 ensures that that the current received by the LEDs never exceeds safe levels even with fluctuations in the AC line voltage. As discussed above, the power supply 10 delivers a relatively stable current to the LEDs 14. This current is established by selecting the appropriate set-point resistor 50. This current, the set-point current I_SET.POINT_< is determined by the selection of the set-point resistor 50 and does not vary with differences in the voltage drops across LEDs 14. Accordingly, a single circuit design can accommodate a plurality of LEDs 14 even if the manufacturing specifications, and more specifically, the voltage drop across the LEDs varies from one LED to another. This feature is advantageous as LEDs of the same model are not all alike; in particular, any two LEDs of the same model manufactured by the same manufacturer may provide different voltage drops when current is passed through them. Nevertheless, the power supply circuit 10 depicted in FIGURE 2 will deliver a fixed amount of current, i.e., I_SET.POINT> to the plurality of LEDs 14. Also, delivering a stable current to the LEDs ensures that their intensity does not vary with fluctuations in the AC power line amplitude, which are commonplace.

Another advantage, as discussed above, is that the power supply 10 is simple and compact. Since the power supply 10 does not employ a large and heavy transformer, nor any large or heavy heat sinks to dissipate thermal energy, the power supply can be compact and light. The power supply 10, may range in size from between 1 and 2 inches (in.) wide and between 1 and 2 in. long. Preferably, the power supply the power supply is about 1 in. wide and

2 in. long. Preferably, the power supply 10 is enclosed in a metal cage to isolate switching noise and electro-magnetic interference (EMI). A metal cage, known in the art as a Faraday cage, shields electronics external to the power supply 10 from signals generated by the power supply. Even without the Faraday cage, however, the power supply 10 generate negligible electro-magnetic interference since the power supply is not a switching power supply. The low- pass filter 22 also provides AC noise reduction. Additional features of the power supply 10 include its ability to provide extremely uniform intensity across the plurality of the LEDs 14 in the array that are connected in series. The power supply 10 can also be manufactured at a low cost by using standard electronic components. The power supply preferably costs less than $100 and more preferably less than $5.

Various embodiments of the present invention are illustrated by the following examples: Example 1

The power supply circuit 10 shown in FIGURE 2 can be employed to power a plurality of forty-eight (48) LEDs such as AllnGaP LEDs manufactured by Hewlett-Packard that emit red/amber light having a wavelength between about 615 and 590 nm. So as not to shorten the lifetime of the LEDs 14, which typically ranges between about five to ten years, while providing sufficient emission from the LEDs for certain lighting applications, the power supply 10 delivers 50 mA to each of the plurality of LEDs, which are arranged in series. Although each of the LEDs 14 are of the same model and manufacturer, any given LED in the string may have a voltage drop across it that may vary as much as

0.1 DC volts (VDC); in particular, the forward voltage for these LEDs ranges from about 2.3 to 2.4 VDC. Accordingly, for a string of 48 such LEDs 14, the total voltage drop across the LEDs can be as small as 110.4 VDC and as large as

115.2 VDC, the average being 112.8 VDC. In this example, the power supply circuit 10 includes an LM317AT voltage regulator, which provides a voltage at the output terminal 54 that is 1.25 V below the voltage at the reference terminal 56. The LM317AT voltage regulator will be damaged if a positive voltage of greater than +40 V or a negative voltage more negative than

-0.3 V is applied between the input terminal 52 and the output terminal 54. Additionally, the LM317AT voltage regulator will no longer provide a regulated voltage at the output terminal 54 if, when operating at room temperature (25°C), less than +3.75 V is applied between the input terminal 52 and the output terminal. The range of voltages that can therefore be applied between these input and output terminals 52 and 54 is between +3.75 V and +40 V.

Thus, in this example the power supply circuit 10 is designed to provide a proximate voltage drop between the input terminal 52 and the output terminal 54 of about 25 V.

To establish a 50 mA set-point current for the LEDs 14, a set-point resistor 50 of 25 Ω is employed. Since the voltage difference between the reference terminal 56 and the output terminal 54 is 1.25 V, a set-point resistor 50 of 25 Ω shunted across the reference terminal and the output terminal will ensure that 50 mA flows out the output terminal through the 25 Ω set-point resistor and to the LEDs 14.

In this example, the surge protection device 30 comprises a ZNR surge protector manufactured by Panasonic and the diode bridge 34 comprises a four-diode bridge manufactured by Microsemi Corporation. The ZNR surge protector limits the voltage coupled to the step-down capacitor to 135 VAC. The diode bridge 34 rectifies the AC voltage as is well known in the art and introduces a 2 V voltage drop, V_BR|_DGE, between the input AC voltage and the rectified voltage.

The power supply 10 is designed to deliver a substantially constant current to the string of 48 LEDs 14 despite the variation in AC line voltage and in the voltage drop across the 48 LEDs. As is well known, the AC line voltage varies in amplitude; for example, commercial line voltage varies between about 110 V and 120 V, a total of ± 5

V, on average being about 115 V. Also, as discussed above, the total voltage drop across the 48 LEDs 14 varies about

+ 2.4 V, i.e., between about 110.4 VDC and 115.2 VDC. The total variation introduced by variations in line voltage and in the voltage drop across the LEDs 14 is therefore about ± 7.4 V.

An appropriate value of the capacitance for the step-down capacitor 32 needs to be selected so as to maintain an operating voltage across the input terminal 52 and the output terminal 54 of the voltage regulator 48, corresponding to 25 V despite the fluctuations in the amplitude of the AC line voltage 12 and differences in voltage drop across the 48 LEDs 14. The voltage drops across the components in the power supply 10 are related by:

_\y _Ac LINE ~^~ CAP ) ^x J- V_BRIDGE — V_REGULATOR — v_LEDs = 0 For an average AC line voltage, V_{AC UNE}, of 115 V and an average voltage drop across the LEDs, V_LEDs, of 112.8 V, where the voltage drop across the bridge 34, V_BR|_DGE, is 2.0 V and the operating point for the voltage regulator 48, i.e., the voltage, V_REGULATαR, between the input terminal 52 and the output terminal 54 is 25 V, the voltage across the step-down capacitor 32, V_CAP, is determined to be 16.13 V. Since the line voltage fluctuates ± 5 volts and the voltage drop across the 48 LEDs can vary as much as ± 2.4 V, then the voltage across the step-down capacitor 32, V_CAP, is about 16.13 +

7.5 V. The capacitance required to establish the requisite step-down voltage can be determined from the equation

defining the impedance for a capacitor; i? = |Z| = where R is the resistance corresponding to the

2πfC magnitude of the impedance Z, f is the frequency of the AC voltage, and C is the capacitance of the capacitor. The voltage V_CAP across the step-down capacitor 32 must be largest when the line voltage is 5 V higher than the average line voltage, and the voltage drop across the 48 LEDs, V_LEDs, is 2.4 V below average, in which case the voltage drop across the capacitor must be 16.13 + 7.5 V or 23.65 V. Since 50 mA is delivered to the 48 LEDs 14, 50 mA is drawn through the input terminals 28 of the power supply 10 and through the step-down capacitor 32. The resistance of the step-down capacitor 32 can be computed from the Ohm's Law: V = IR . To establish a 23.65 V drop across the step-down capacitor 32 with a 50 mA current, a 473 Ω resistance is required. For 60 Hz AC line voltage, the capacitance of the step-down capacitor 32 is therefore preferably 5.6 μF.

As described above, the power supply 10 minimizes thermal or l²R loss, since the number of resistors is held to a minimum. As is well known, power is determined by the relationship P = IV where P is power, or with the following equation for resistors P = I²R , where R is resistance. Accordingly, the power into the power supply 10 is a product of the RMS value of the line voltage (115 V) and the current drawn by the power supply (50 mA), which corresponds to 5.75 W. Similarly, the power delivered to the plurality of LEDs 14 equals the average voltage drop across the LEDs (112.8 V) times the current through the LEDs (50 mA); this product equals 5.64 W. A comparison of the power into the power supply 10 (5.75 W) and into the plurality of LEDs 14 (5.64 W) yields a total efficiency for the power supply of 98%.

Also, as discussed above, the power supply 10 provides a substantially constant level of current to the 48 LEDs 14, despite fluctuations in the AC line voltage and variations in the voltage drop across the 48 LEDs. In Example

1, the set-point current is 50 mA, and the voltage regulator 48 will deliver this current provided the voltage V_REG between the input terminal 52 and the output terminal 54 is within the range of +3.75 VDC and +40.0 VDC. For example, when the AC voltage line is 110 V (RMS) and the voltage drop across the plurality of LEDs is 115.3 V, with a 23.65 V drop across the step-down capacitor 32 and a 2.0 V voltage drop across the diode bridge 34, results in a 14.69 V voltage across the input terminal 52 and the output terminal 54 of the voltage regulator 48, which satisfies the +3.75 VDC minimum requirement for proper operation of the voltage regulator. Similarly, in the case where the AC line voltage is 120 V (RMS) and the voltage drop across the 48 LEDs is 110.4 V, with the voltage drop across the step-down capacitor 32 of 23.65 V and the voltage drop across the diode bridge 32 of 2.0 V, the voltage V_REG across the input terminal 52 and the output terminal 54 is 33.95 V, which does not exceed the + 40.0 VDC limit for safe operation of the voltage regulator 48. In computing these values for the voltage drop across the voltage regulator 48, the RMS line voltages, i.e., of 110 V, and 120 V, are multiplied by V2 to convert the RMS voltage into peak voltage. To further protect the voltage regulator 14 from being subjected to voltages across the input and output terminals in excess of +40 V, the zener diode comprises a 39 V zener, P/N No. 1 474ADICT manufactured by Vishay and the resistor in parallel with the zener comprises a 10 kΩ resistor. This combination safeguards the voltage regulator 14 from excess voltages across the input and output terminals 52 and 54, upon discharge of the ripple suppression capacitor 42 initiated by electrically disconnecting any one of the 48 LEDs from the power supply 10. To prevent arcing of the ripple suppression capacitor 42, the charge dissipation resistor 44 is provided. This resistor 44 will permit the dissipation of 170 VDC or 120 y/2 , the peak voltage associated with the 120 V RMS AC line voltage and that is stored by the ripple suppression capacitor 42. The charge dissipation resistor 44 comprises a 50 kΩ resistor. If the charge dissipation resistor 44 is too large, heat will be generated as current is passed through this resistor upon discharge of the ripple suppression capacitor 42 as well as during normal operation of the power supply 10. As discussed above, excessive heating here may be incompatible with the packaging of the power supply 10 or of the surrounding environment.

Also, excessive ripple in the current delivered to the LEDs 14 may permanently damage them. For the standard LED 14, the ripple in the voltage received by the voltage regulator 48 must not exceed 10 V and preferably is between 4-6 V. Accordingly, the ripple suppression capacitor 42 comprises a 33 μF capacitor that reduces ripple in the rectified voltage output by the diode bridge 34 to between 4-6 V. The preferred capacitance of the ripple suppression capacitor 42 is proportional to the set-point current delivered to the LEDs 14, thus, when the set-point current is small, the capacitance is similarly reduced. Also, the oscillation-suppression capacitor 61 comprises a 0.1 μF capacitor, which prevents the voltage regulator 48 from oscillating and driving the LED 14 with currents outside the safe range of operation. The power supply 10 in Example 1 is designed to convert residential AC line voltage 12, corresponding to

115 VAC at 60 Hz, into a regulated 50 mA current that is delivered to 48 LEDs 14 in series, wherein each LED has a 2.3 - 2.4 V voltage drop. The power supply 10, however, can be designed to draw power from other AC sources, such as commercial AC line voltages or European AC line voltages, which operate at 220 V and 50 Hz. The power supply 10 may also be designed to accommodate more or less than 48 LEDs in series, or both series and parallel, having different voltage drops and being driven with different amounts of current. The electrical components, e.g., the step-down capacitor 32, the ripple suppression capacitor 42, the charge-suppression resistor 44, and the set-point resistors 50 and 51, will have respective capacitance and resistance values determined by the requirements of the power supply 10. These requirements include, but are not limited to, the AC line voltage, the set-point current, and the voltage drop across the plurality of light-emitting diodes 14. Example 2 The power supply circuit 10 of Example 1 may be modified to accommodate 24 red LEDs 14 in series, by simply altering the step-down capacitance from 5.6 μF to a 2.2 μF and the current regulator 48 will deliver the proper regulated DC current of 50 mA. To modify the set-point current, the set-point resistance is changed. As with the circuit in Example 1, the power supply 10 of Example 2 receives an AC line voltage of 115 VAC. The efficiency of this configuration is reduced as the number of LEDs 14 is smaller and therefore the amount of power transferred into light output is less. Similarly, if the number of LEDs 14 is increased, the amount of power transformed into light energy is greater and the efficiency is likewise higher.

The present invention may be embodied in other specific forms without departing from the essential characteristics as described herein. The embodiments described above are to be considered in all respects as illustrative only and not restrictive in any manner. The scope of any invention is, therefore, indicated by the following claims rather than the foregoing description. Any and all changes which come within the meaning and range of equivalency of the claims are to be considered in their scope.

Claims

WHAT IS CLAIMED IS:

1. An apparatus comprising a light emitting diode and a power supply for powering said light emitting diode, said power supply comprising: a step-down impedance; a current regulator electrically connected to the step-down impedance; a rectifier electrically connected between the step-down impedance and the current regulator; and a low pass filter electrically connected between the rectifier and the current regulator, wherein said current regulator has an output for electrical connection to said at least one light emitting diode.

2. The power supply of Claim 1, further comprising a surge protector in electrical contact with said step-down impedance.

3. The power supply of Claim 1 , wherein said reactive element comprises a bipolar capacitor.

4. The power supply of Claim 3, wherein said at least one light emitting diode comprises a plurality of light emitting diodes connected in series.

5. The power supply of Claim 4, wherein said plurality of light emitting diodes comprise at least two sets of serially connected light emitting diodes connected in parallel.

6. The apparatus of Claim 4, wherein said plurality of light emitting diodes comprises at least 12.

7. The apparatus of Claim 4, wherein said plurality of light emitting diodes comprises at least 24.

8. The apparatus of Claim 4, wherein said plurality of light emitting diodes comprises at least 48.

9. A power supply for providing electrical power to at least one light emitting diode, comprising: a step-down impedance; a current regulator electrically connected to said step-down impedance; a rectifier electrically connected between said step-down impedance and said current regulator; and a low pass filter electrically connected between said rectifier and said current regulator, wherein said current regulator has an output for electrical connection to said at least one light emitting diode that outputs a current and voltage compatible with said light emitting diode.

10. The power supply of Claim 9, further comprising a surge protector in electrical contact with said step-down impedance.

11. The power supply of Claim 9, wherein said step-down impedance comprises a reactive element.

12. The power supply of Claim 11, wherein said reactive element comprises a bipolar capacitor.

13. The power supply of Claim 12, wherein said bipolar capacitor comprises film.

14. The power supply of Claim 9, wherein said rectifier comprises a diode bridge.

15. The power supply of Claim 9, further comprising a charge dissipator electrically connected between said low pass filter and said current regulator.

16. The power supply of Claim 15, wherein said charge dissipator comprises a resistor.

17. The power supply of Claim 9, wherein said low pass filter comprises a capacitor.

18. The power supply of Claim 9, wherein said current regulator comprises: a voltage regulator having an input terminal, an output terminal, and a reference terminal electrically connected such that said output terminal is at a voltage offset from the voltage of said reference terminal by a substantially constant potential difference; and at least one set-point resistor electrically connecting said output terminal of said voltage regulator to said reference terminal of said voltage regulator.

19. The power supply of Claim 18, wherein said current regulator comprises at least two set-point resistors and a switch settable in a plurality of positions to electrically connect different set-point resistors to said output and reference terminals of said voltage regulator.

20. The power supply of Claim 9, wherein said current regulator comprises: a voltage regulator having an input terminal, an output terminal, and a reference terminal; and a resistor, wherein said output terminal is electrically connected to said reference terminal through said resistor.

21. The power supply of Claim 9, wherein said current output by said current regulator output comprises about 30 milliamps to 1 amp.

22. The power supply of Claim 9, wherein said voltage output at said current regulator output ranges between about 2 to 170 volts.

23. The power supply of Claim 9, wherein said at least one light emitting diode comprises a plurality of light emitting diodes connected in series.

24. The power supply of Claim 23, wherein said plurality of light emitting diodes comprise at least two sets of serially connected light emitting diodes connected in parallel.

25. A method of providing power to one or more light emitting diodes (LEDs) using an AC voltage comprising: attenuating said AC voltage; subsequent to said attenuating, rectifying said AC voltage to produce a rectified voltage; subsequent to said rectifying, removing ripple from said rectified voltage; and subsequent to said removing, converting said rectified voltage into a substantially constant supply of DC current and applying the DC current to power said one or more light emitting diodes (LEDs).

26. The method of Claim 25, wherein said step of attenuating comprises applying said AC voltage to a substantially reactive component.

27. The method of Claim 26, wherein said step of attenuating comprises applying said AC voltage to a capacitor.

28. The method of Claim 25, wherein said one or more LEDs comprises a plurality of LEDs connected in series.

29. The method of Claim 28, wherein said plurality of LEDs comprises at least 12.

30. The method of Claim 28, wherein said plurality of LEDs comprises at least 24.

31. The method of Claim 28, wherein said plurality of LEDs comprises at least 48.