Patent Application
20120268013
Embodiments of the invention generally relate to LED lights and, in particular, to multi-fixture LED power supplies.
An LED light fixture is a physically distinct housing that contains one or more LEDs arranged in one or more “strings” (i.e., series connections). A room may be lit by multiple LED fixtures arranged at different points on (e.g.) the room's ceiling, and each fixture may have its own power supply to power the one or more strings therein. Each string in a fixture, even if designed to be identical to the other strings, may have different voltage and current requirements from the other strings in the fixture due to, e.g., manufacturing variations or dynamic noise. A power supply serving each string, therefore, must be account for the worst-case string, even if the other strings behave nominally. For example, if the LED strings are designed to require 20 Vdc, the power supply may be required to output 24 Vdc to account for variations in the current/voltage requirements of the strings. Every string that runs at an operating point other than the worst-case (24 Vdc) will, however, waste the additional voltage range, known as “headroom,” as heat. In many cases, every string runs at its nominal operating point (20 Vdc), and the entire headroom is wasted.
Some prior-art LED fixtures use a local power supply (i.e., one per fixture) and a technique called dynamic-headroom control to partially address this problem. Because the LED load is known and fixed (i.e., the fixture is manufactured with a certain number of LEDs/strings, and this number does not change) and because each string is directly connected to the local power supply, the power supply may adjust its voltage to reduce the amount of wasteful headroom. For example, the power supply may lower its output voltage until it senses that a string has reached its minimum operating voltage. These prior-art techniques are, however, dependent the power supply designer's a priori knowledge of the size and type of LED load and on direct control/monitoring of each string.
Having a single power supply serving multiple fixtures may be more economical than having a separate power supply for each LED fixture in a room. In this case, the power supply distributes a single power bus to a plurality of fixtures. The savings gained from sharing the power supply, however, may be lost to power wasted to unnecessary headroom applied to the fixtures. Prior-art dynamic-headroom techniques cannot be applied to the multi-fixture LED power supply at least because (i) the multi-fixture LED power supply cannot predict what kind or how many fixtures it will be required to power and (ii) the multi-fixture LED power supply cannot directly monitor or control each fixture (or each string in a fixture). Thus, a need exists for a way to dynamically adjust the headroom in a multi-fixture LED power supply.
A dynamic-headroom power supply delivers its output voltage to multiple light fixtures. The fixtures and the power supply communicate so that the power supply raises its output voltage just enough so that each LED string is supplied with enough current, but no further. In other words, the power supply dynamically adjusts its voltage headroom to its minimum operable level.
In one aspect, a method dynamically controls headroom of a voltage supplied to a plurality of LED light fixtures. An adjustable voltage is supplied to the plurality of LED light fixtures, and the voltage is increased from zero. A current drawn by at least one of the plurality of LED light fixtures is monitored. The voltage increase is ceased when the current reaches a maximum value.
In various embodiments, the monitored current is drawn by one, some, or all of the plurality of LED light fixtures. The current may be monitored at a location remote to each of the plurality of LED light fixtures. A signal from the plurality of LED light fixtures indicating that all of the plurality of LED light fixtures have reached a maximum current value may be received. A dimming-level signal may be sent to each of the plurality of LED light fixtures. The maximum value of the current drawn by at least one of the plurality of LED light fixtures may be decreased in accordance with the dimming-level signal. Monitoring the current may include monitoring a signal, and the monitored signal may be piggybacked on a power signal, transmitted on a signal bus, or transmitted wirelessly. A reduction in a voltage required to maintain current flow in at least one of the plurality of LED light fixtures may be detected after initial power-up of the system, and the DC voltage of the power bus may be adjusted downward in accordance with the detected reduction. A sudden step change in the total current may be detected, and the DC voltage may be re-adjusted accordingly. The adjustments to the DC voltage may be optimized to deliver just enough voltage on the bus to achieve a desired lighting effect in all of the plurality of LED light fixtures.
In another aspect, a system dynamically controls headroom of a voltage supplied to a plurality of LED light fixtures. An adjustable AC/DC power supply supplies an adjustable DC voltage to a power bus connected to the plurality of LED light fixtures. A sensing circuit monitors a current of at least one of the plurality of LED light fixtures. A control circuit adjusts the DC voltage in accordance with an output of the sensor.
In various embodiments, the sensing circuit includes a current sensor for detecting a maximum current drawn by all of the LED light fixtures and/or a sensor for detecting maximum current drawn by one of the plurality of LED light fixtures. The control circuit may increase the DC voltage until the maximum current drawn by all of the LED light fixtures has been detected and/or until the signal has been received, and may adjust the DC voltage in accordance with the dimming-level signal. A dimming-control circuit may send a dimming-level signal to the plurality of LED light fixtures. The sensing circuit may monitor the current of each of the plurality of LED light fixtures and/or detect, after initial power-up of the system, a reduction in a voltage required to maintain current flow in at least one of the plurality of LED light fixtures; the control circuit may adjust the DC voltage of the power bus downward in accordance with the detected reduction. The sensing circuit may detect a sudden step change in the total current; the control circuit may re-adjust the DC voltage accordingly. The adjustments to the DC voltage may be optimized to deliver just enough voltage on the bus to achieve a desired lighting effect in all of the plurality of LED light fixtures.
In another aspect, a system dynamically controls headroom of a voltage supplied to a plurality of LED light fixture. An adjustable AC/DC power supply supplies an adjustable DC voltage to a power bus connected to the plurality of LED light fixtures. A control circuit induces small changes to the DC voltage on the power bus. A sensing circuit measures correspondingly induced changes in a current of at least one of the plurality of LED light fixtures. The control circuit compares the changes to the DC voltage to the induced changes in the current, thereby determining an operating point of the system, and the control circuit adjusts the DC voltage, if necessary, to position the system at a desired operating point.
In various embodiments, the sensing circuit includes a current sensor for detecting a maximum current drawn by all of the LED light fixtures and/or a sensor for detecting maximum current drawn by one of the plurality of LED light fixtures. The control circuit may increase the DC voltage until the maximum current drawn by all of the LED light fixtures has been detected and/or increase the DC voltage until the signal has been received. A dimming-control circuit send a dimming-level signal to the plurality of LED light fixtures. The control circuit may adjust the DC voltage in accordance with the dimming-level signal.
These and other objects, along with advantages and features of the present invention herein disclosed, will become more apparent through reference to the following description, the accompanying drawings, and the claims. Furthermore, it is to be understood that the features of the various embodiments described herein are not mutually exclusive and can exist in various combinations and permutations.
In the drawings, like reference characters generally refer to the same parts throughout the different views. In the following description, various embodiments of the present invention are described with reference to the following drawings, in which:
Each fixture 104 contains one or more LEDs 114 arranged in one or more strings 116. Each string 116 may include a linear current regulator 118 to control the flow of current through the associated string 116. The current regulator 118 may be any regulator known in the art, such as a sensing resistor and a current-control transistor, for regulating current in LEDs. The regulator 118 monitors the current in the string 116 and prevents it from increasing to a level that might damage the LEDs 114. In the illustrated embodiment, each fixture 104 has a single string 116 of LEDs 114, but a fixture 104 may have any number of strings 116.
In general, the current regulators 118 ensure that the LEDs 114 receive a constant current from the power bus 112, despite any variations in voltage on the power bus 112. The voltage on the power bus 112 must be above a minimum level, however, for the current regulators 118 to function. Due to processing and other variations in the LEDs 114 and/or current regulators 118, however, this required minimum voltage level may vary between strings 116. Any difference between the voltage of the power bus 112 and the required minimum voltage of a string 116 is headroom voltage. The voltage of the power bus 112 may be safely lowered to the level of the string 116 having the greatest minimum required voltage; decreasing the voltage of the power bus 112 past that point will cause at least that string 116 to cease functioning.
The power-supply unit 102 may further include a sensing circuit 120 for sensing a characteristic of the fixtures 104 and a control circuit 122 for adjusting the voltage on the power bus 112 in accordance with the sensed characteristic. The sensing circuit 120 may be a conventional current sensor for sensing the current drawn by one or more of the fixtures 104 and may be implemented using, for example, a digital circuit to sample and measure the current or an analog circuit to compare the current to a reference. As explained further below, the sensing circuit 120 may be disposed in the fixtures 104 and communicate with the power-supply unit 102 via the power bus 112 or signal bus 126; in this embodiment, the power-supply unit 102 includes a signal receiver for receiving communication signals from the fixtures 104.
The control circuit 122 may be an analog, digital, or mixed signal circuit, and may be implemented using an ASIC, microcontroller, low-power processor, or any other circuit known in the art. The control circuit 122 may further include a memory or other volatile or non-volatile storage device. The rectifier 108, switch-mode power supply 110, sensing circuit 120, and/or control circuit 122 may communicate via digital or analog signals sent over an internal bus 124. The power-supply unit 102 may communicate with the LED lighting fixtures 104 via digital or analog signals sent on a dedicated command bus 126 and/or via signals 128 superimposed or piggybacked on the power bus 112 (e.g., an analog data waveform may be combined with the DC voltage on the power bus 112 to transmit data). In other embodiments, the communication may be sent wirelessly or by any other method known in the art. In each case, the signaling may be bi-directional; the power-supply unit 102 sends instructions to the fixtures 104 to modify their behavior (e.g., to dim or brighten the LEDs 114), and the fixtures 104 may send signals to the power-supply unit 102 that carry information regarding a characteristic of the fixtures 104, as explained in greater detail below.
One way of operating the power-supply unit 102 to dynamically adjust the headroom on the power bus 112 supplying power to the fixtures 104 is illustrated in
In one embodiment, the monitored characteristic is the total current drawn by the fixtures 104 over the power bus 112. The sensing circuit 120 may monitor this current at the power-supply unit 102 without receiving further feedback from the fixtures 104. In this embodiment, the command bus 126/128 may be unidirectional; the fixtures 104 need not explicitly send signals back to the power-supply unit 102.
The sensing circuit 120 senses this leveling out of the current Iload using techniques known in the art, such as by sampling the current Iload drawn over the power bus 112 and comparing the last few samples for differences. Once the differences diminish below a threshold value for a minimum period of time, leveling out is deemed to have occurred. The leveling out of the current Iload may be detected within a tolerance such as, for example, a change of less than 1, 5, or 10 milliamps over the course of 10-100 milliseconds. Once the sensing circuit 120 detects the leveling out of the current Iload, the voltage Vsupply used 308 (i.e., Vmax) at that point is saved in, for example, the memory in the control circuit 112. Once saved, the voltage Vsupply may be held at (i.e., locked down at) the value Vmax during further operation of the circuit 100. In various embodiments, a new Vmax value may be computed at a future point in time to account for time-based changes to the fixtures 104 and/or power-supply unit 102; the recomputation may occur at regular intervals or in response to a detected change in the circuit 100. For example, a sudden, unexpected drop in Iload may indicate that one of the fixtures 104 has stopped drawing current, and Vsupply may be increased accordingly to try to re-enable the failing fixture 104.
In another embodiment, the control circuit 122 increases Vsupply while the sensing circuit 120 monitors the command bus 126 (and/or signals 128 on the power bus 112). In this embodiment, each fixture 104 includes a sensing circuit for detecting when that fixture 104 (and/or a string 116 in the fixture 104) reaches a maximum drawn current level and outputs a signal indicative of this condition onto the command bus 126/112/128. The power-supply unit 102 includes a signal receiver capable of detecting the first such signal and ceases increasing Vsupply when it is received. It may be assumed that the variation in characteristics between the various fixtures 104 and/or strings 116 is small enough such that, by the time Vsupply is high enough for one fixture 104 to have reached its maximum current, Vsupply is at least high enough for the remainder of the fixtures 104 to have at least turned on. In another embodiment, the sensing circuit 120 monitors the command bus 126 and/or signals 128 on the power bus 112 for an indication that all of the fixtures 104 (and/or all of the LEDs in each fixture 104) have reached a maximum current value.
An example of this relationship is illustrated in
As shown in
In one embodiment, the power-supply unit 102 sends a dimming-level signal to the plurality of fixtures 104. The dimming-level signal may be sent on the command bus 126 or be piggybacked on the power bus 112 (or wirelessly as noted above). Each fixture 104 may receive the dimming-level signal and use it to modify a dimming level of the LEDs 114 incorporated therein. The power-supply unit may modify the voltage on the power bus 112 in accordance with the modified dimming level—either to increase it to account for a greater current drawn by brighter LEDs 114 or to lower it to save power when the LEDs 114 are dimmed. In one embodiment, the voltage on the power bus 112 varies in order to help achieve a desired dimming level. The fixtures 104 may send a signal back to the power-supply unit 102 when the voltage on the power bus 112 has reached its new operating point by, for example, detecting a maximum current level of one or more of the fixtures 104.
In one embodiment, the LED fixtures 104 require a greater voltage (e.g., 20 V) on the power bus 112 upon initial power-up of the system 100 to achieve the minimum current required for the current regulators 118 to function. Once the LEDs in the fixtures 104 warm up, however, their minimum required voltage may decrease, and the LEDs may reach a steady-state operating point at which their minimum required voltage stabilizes at a value lower than its initial value. The sensing circuit 120 may detect this lower voltage requirement, and the control circuit 122 may lower the voltage on the power bus 112 accordingly (to, e.g., 15 V, 10 V, or lower).
The lower required voltage may be discovered by making small variations in the voltage on the power bus 112. The control circuit 122 varies the voltage and the sensing circuit 120 examines the current drawn by one or more of the fixtures 104 for a corresponding change in current. In one embodiment, the voltage variation is small (e.g., too small to produce a change in the brightness in the LEDs detectable by the human eye). If the voltage is lowered and no corresponding change in current is detected, the lowered voltage is adopted as the new voltage on the power bus 112. In other words, referring to
In another embodiment, a fixture 104 may be removed from or added to the system 100 (either inadvertently or by design), causing the rest of the fixtures 104 to be over- or under-powered, respectively. The sensing circuit 120 may detect a sudden step change in total current caused by such an event, and the control circuit may re-adjust the voltage on the power bus 112 accordingly. In one embodiment, to reduce the voltage of the power bus 112 may be adjusted to a point that achieves a desired lighting effect from all the fixtures (e.g., a brightness level) by monitoring the total current of all the fixtures.
Certain embodiments of the present invention were described above. It is, however, expressly noted that the present invention is not limited to those embodiments, but rather the intention is that additions and modifications to what was expressly described herein are also included within the scope of the invention. Moreover, it is to be understood that the features of the various embodiments described herein were not mutually exclusive and can exist in various combinations and permutations, even if such combinations or permutations were not made express herein, without departing from the spirit and scope of the invention. In fact, variations, modifications, and other implementations of what was described herein will occur to those of ordinary skill in the art without departing from the spirit and the scope of the invention. As such, the invention is not to be defined only by the preceding illustrative description.
