AU2006203675B8 - Current regulation and reliability of led array - Google Patents

Abstract

An electrical circuit for powering a plurality of LEDs, the electrical circuit 5 comprising: a series combination of at least two LED's, an alternative current path in parallel with each LED; a current limiting device; a current regulating device; a current sensing device; and a control circuit that takes its input from the current sensing device and provides input to the current regulating device. The circuit is particularly useful for maintaining a string of diodes and 10 preverable the alternative current path is a zener diode connected across each LED so if the LED goes open circuit, current can still flow through th other LEDs and the current regulating device is a pulse with modulator whereby the average current can be maintained for the remaining LEDs is one or more short circuit. Y:\SPEC\Kossmann LED Aray 140806.wpd 15 August 2006

Description

Regulation 3.2 AUSTRALIA Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT APPLICANT: Christopher Kossmann NUMBER: FILING DATE: Invention Title: CURRENT REGULATION AND RELIABILITY OF LED ARRAYS The following statement is a full description of this invention, including the best method of performing it known to me: 2 IMPROVEMENTS RELATING TO THE RELIABILITY OF LED-ARRAY SIGNALS This invention relates to a new means for improving the reliability and 5 efficiency of a Light Emitting Diode (LED) array signal, and a method of same. Signals are widely used for the control of vehicular and pedestrian traffic. In these applications, it is desirable for the signal to have a high level of 10 reliability and a long service life, to ensure the on-going safety of the traffic that is being controlled and to minimise maintenance costs. The efficiency of the signal is also a consideration, as this will impact the on-going operating costs. 15 Signals that incorporate LED arrays are becoming more widespread, as they provide a number of advantages over conventional incandescent or quartz halogen lamp designs: * LEDs have a significantly longer operational life time than either incandescent or quartz halogen lamps. This means the LED array based 20 signalling lamp will last longer and require less frequent replacement, reducing maintenance costs. * LED array lighting provides redundancy in the event that a single LED fails, improving reliability. The single LED represents only a fraction of the total light output, so overall the signal remains functional. For 25 conventional incandescent or quartz halogen applications, failure of the single light bulb causes the whole signal to fail. * LED array lighting is much more energy efficient than either incandescent or quartz halogen equivalents, providing beneficial savings in operating costs and reducing energy wastage. 30 A typical LED array signal might consist of a quantity of parallel "strings" of LEDs. Each string consists of a number of LEDs connected in series Y:\SPEC\Kossmann LED Anay 140806.wpd 15 August 2006 3 together with an impedance, such as a resistor, and energised from a power source. The function of the series impedance is to reduce the sensitivity of the LED light output to variations in the supply voltage. The output light intensity of an LED is a function of the drive current and without the series 5 impedance, small variations in supply voltage may result in large variations in drive current, and hence LED light output. It is desirable to maximise the number of LEDs connected in series in each string, as this allows the overall current to the LED array to be reduced, thus 10 reducing losses and improving efficiency. One problem with the approach of having multiple LEDs in series is that should a single LED in the series string fail to an open circuit, the rest of the LEDs in that string will extinguish. This limits the number of LEDs that can 15 be connected together in a string, as failure of the entire string due to the failure of a single LED may result in the signal becoming inoperable. So the design of existing LED based signals involves a trade-off between efficiency (using a large number of LEDs per string) and reliability (using a small number of LEDs per string) 20 An additional problem exists for the case where one LED in the string fails to a short circuit. Here, the decrease in overall impedance of the string due to the LED failure may cause a significant increase in the current through the remaining LEDs, such that the operating current may be outside the range for 25 reliable long-life. An LED array that provides a means of ensuring continued operation in the event of a single LED failure, and that enables an increased number of LEDs to be connected in series in a string would offer significant improvements in 30 operational life-time and efficiency over the prior art. This is the objective of the present invention. Y:\SPEC\Kassmann LED Array 140806.wpd 15 August 2006 4 The invention provides an electrical circuit for powering a plurality of LEDs such that if one of said plurality of LEDs fails, the remaining one or more of said plurality of LEDs will remain functional, said electrical circuit being characterized by: 5 a series combination of at least two LEDs; an alternative current path in parallel with each of said LEDs; a current limiting device; a current regulating device; a current sensing device; and 10 a control circuit, the input to said control circuit being from said current sensing device, said control circuit providing an input in the form of a control signal to said current regulating device, said control signal being adapted to operate said current regulating device.. 15 The current limiting device may take the form of a resistor, selected so as to provide the most significant contribution to the overall circuit impedance. This feature has the advantage that current in the circuit, and thus the intensity of the light output from the LEDs, is less sensitive to variations in the input voltage of the energising source. 20 The alternative current path in parallel with each LED may take the form of a voltage clamping device. This feature has the advantage that, in the event of open circuit failure of the LED, current will flow via the alternative parallel path and the overall circuit will remain functional. 25 The current sensing device provides a means of monitoring the instantaneous current in the circuit. Output from the current sensing device provides input to the control circuit, the output of which provides input to the current regulating device. This feature has the advantage that in the event 30 that there is a short circuit failure of any LED, the current sensing device will detect the instantaneous change in current and then operate the current regulating device to control the current to within operating limits. Y:\Alfred\Ioln C\Solux LED Array Amcndedwpd 7 April 2011 5 More than two LEDs may be connected in series. The number of LEDs connected in series may be significantly more than two. This feature has the advantage that the total current required to energise the signal can be 5 reduced, improving the efficiency. These features of the present invention enable the design of an LED array that is more tolerant to failure of any individual LED, has a longer operational life and which can be made more efficient than the prior art, 10 For a better understanding of the present invention and to show how it may be carried into effect, reference shall now be made, by way of example only, to the accompanying drawing, in which: 15 FIGURE 1 shows an embodiment of the present invention. Figure 1 shows two LEDs (1, 2) connected in series. In parallel with each LED is a zener diode (3, 4). The clamping voltage of the zener diodes (3, 4) is chosen to be greater than the forward voltage drop of the LEDs (1, 2). In 20 series with the two LEDs (1, 2) is a current limiting device (5), current regulating device (6) and a current sensing device (7). The current regulating device preferably consists of a pulse width modulating device such as a transistor switch. A control circuit (8) takes input from the current sensing device (7) and provides a control signal (9) as input to the current regulating 25 device (6). Under normal operating conditions, the forward voltage drop across the LED (1) is less than the operating voltage of zener diode (3) and so the majority of current flows from anode to cathode of the LED (1), A similar situation exists 30 with LED (2) and zener diode (4). Y:\Alfrcd\John C\Solux LED Arrmy Amcnded.wpd 7 April 2011 6 In the event that the LED (1) should fail to an open circuit, the voltage across the zener diode (3) increases rapidly and approaches, within the limit, the level of the energising source. At the point where the voltage reaches the clamping level of the zener diode (3), it current will begin to conduct and 5 current will flow from cathode to anode, bypassing the failed LED (1). In this way, the continuity of the overall circuit is restored and the LED (2) will continue to receive current and thus continue to provide light output. If LED (2) should fail to an open circuit, zener diode (4) would act in the same 10 way as described for zener diode (3) in the example above, LED (2) would continue to receive current and thus continue too provide light output. Current limiting resistor (5) connects to the cathode of LED (2). It acts to limit the total current through the circuit to within the operating limits of LEDs (1,2) 15 If there is a short circuit failure of either LED (1,2), then the current flowing through the circuit will increase. The current sensing device (7) monitors the instantaneous current in the circuit. It provides as output an input to the control circuit (8). The control circuit (8) compares this signal to an internal 20 reference representing the acceptable average current through the circuit and provides control signal (9) to operate the current regulation device (8). In this way, the control circuit (9) is able to pulse width modulate the current regulation device (8) from the off state to the on-state and then back to the off state so as to control the average current through the circuit. 25 The speed of operation of this control loop is sufficient so as to be able to detect the instantaneous short circuit failure of either LED (1, 2) whilst at the same time adapting the current flow if there is an open circuit failure of either LED (1,2). 30 Whilst in the drawing, there has been described an array of two LEDs, it is to be understood that the circuit will provide satisfactory control for very much Y:\Alfted\John C\Solux LED Array Amended.wpd 7 April 2011 7 larger arrays and, generally is adapted to cope with the failure of more than one LED, either by open circuit or short circuit, to permit the other LEDs in the array to continue to operate under conditions which will maintain the operating conditions to maximise their life. 5 Whilst there has been described herein one particular circuit arrangement which will provide the required control of the current flow on failure of one or more LEDs, it is to be understood that variations may be made in this whilst providing the essential aspects of the invention, an alternative circuit if one or 10 more LEDs fails open circuit, and current limiting means if one or more LEDs fail through becoming open circuit. Y:\Alfredohn C\Solux LED Array Amended.wpd 7 April 2011

Claims (10)

1. An electrical circuit for powering a plurality of LEDs such that if one of 5 said plurality of LEDs fails, the remaining one or more of said plurality of LEDs will remain functional, said electrical circuit being characterized by: a series combination of at least two LEDs; an alternative current path in parallel with each of said LEDs; 10 a current limiting device; a current regulating device; a current sensing device; and a control circuit, the input to said control circuit being from said current sensing device, said control circuit providing an input in the form of a 15 control signal to said current regulating device, said control signal being adapted to operate said current regulating device.

2, An electrical circuit as claimed in claim 1, characterized in that said alternative current path comprises a voltage clamping device which, 20 under normal operation when one of said plurality of LEDs is functional carries little current but if said one of said plurality of LEDs fails to open circuit, provides the alternative current path.

3. An electrical circuit as claimed in claim 2, characterized in that said 25 voltage clamping device is a zener diode connected in parallel with each of said plurality of LEDs.

4. An electrical circuit as claimed in any preceding claim, characterized in that said current limiting device is an impedance. 30

5. An electrical circuit as claimed in claim 4, characterized in that said current limiting device is a resistance. Y\AlfediJohn C\Solux LED Array Amended.wpd 7 April 2011 9

6. An electrical circuit as claimed in any preceding claim, characterized in that said current regulating device is a pulse width modulating device whereby the average current passing through the circuit can be 5 controlled.

7. An electrical circuit as claimed in claim 6, characterized in that said pulse width modulating device is a transistor switch. 10

8. An electrical circuit as claimed in claim 6, characterized in that said current sensing device provides an input to said control circuit which, in turn, provides an output to effect the control of said pulse width modulating device to maintain a required average current through said circuit. 15

9. An electrical circuit as claimed in claim 8, characterized in that said control circuit includes reference means which act as a comparator with the output from said current sensing device to provide the required output to effect operation of said pulse width modulator. 20

10. An electrical circuit for powering a plurality of LEDs such that if one of said LEDs fails, the remaining one or more of said LEDs will continue to produce light when powered, substantially as hereinbefore described in relation to the accompanying drawings. 25 DATED this 7 April 2011 A TATLOCK & ASSOCIATES 30 Patent Attorneys for the Applicant: SOLUX PTY. LTD. YAMfred\lohn C\Solux LED Array Anmded.wpd 7 April 20 11