Patent Application
20170019965
The subject matter herein relates generally to an electrolytic capacitor management system for lighting applications.
A conventional power supply for an LED lamp takes power from an input line at one voltage (typically 12V AC 50/60 Hz) and converts it to a higher DC voltage (e.g., 30 V DC) to power the LEDs. The temporal characteristics of the power signal directly impact the quality of the light generated by the LED. Thus, the power supply also regulates the current to the LEDs to provide consistent lighting output.
Due to the zero crossings of the AC signal, which occur at twice the AC frequency, the power supplied to the LED is momentarily at zero. This leads to what is referred to as systematic flicker, which although may not be directly observable, nonetheless leads to perceptible degradation in the quality of the light generated by the LED. During these very low voltage points of the AC input or when the AC input is interrupted by a phase-cut dimmer, it is desirable to continue to provide power to the LEDs to prevent stroboscopic flicker.
In addition, noise and other disturbances in the electric power signal also degrade the performance of sourced LEDs. Thus, it is desirable to mitigate any noise or other power line disturbances in the power signal.
In order to alleviate both systematic flicker, power line disturbances and noise, an energy storage device such as a capacitor may be introduced between the power source and the LED. The energy storage device acts as a buffer and is designed to have enough capacity to continue to power the LED while the AC signal crosses zero. In general, the higher the voltage established on the energy storage device, the more immune the power supply is to systematic flicker and power line disturbances. Preferably, this solution utilizes a two-stage approach comprising a first stage introduced before the energy storage device and a second stage introduced after the energy storage device.
The first stage may be a voltage converter, which functions to fill the energy storage device. This converter allows for optimized input power draw from the line (high power factor (“P.F.”) for example). Because boost converters have significantly better P.F. than buck converters, they are used almost exclusively as the first power conversion stage in a two-stage arrangement. The intermediate DC voltage on the storage capacitor (output of the first stage) must be approximately twice the input RMS voltage for the boost converter to have high P.F.
The second stage may also be a voltage converter, which functions to draw energy from the energy storage device to drive an LED. The second stage allows for a highly uniform low or zero-ripple output to the LEDs. The second stage is typically a buck stage, which functions to reduce the voltage level at the storage capacitor down to the level of the LED with output current regulation as the main operating mode.
In this arrangement, the higher the intermediate voltage, the smaller the required storage capacitance to hold the LEDs up through the dropout periods. However, as this voltage is increased, each converter becomes less efficient. In very small lamps such as the MR16, this leads to a very challenging tradeoff between efficiency, cost, and lamp size. Typical efficiencies for boost and buck converters with 3:1 transformation ratios might be ˜87%. The net efficiency of this combination is thus ˜75%, a significant reduction.
With a buck stage, the input voltage must be higher than the output. Generally speaking, in the prior art the nominal voltage at which this capacitor operates is a fixed parameter such as 45 Volts. In some conventional power supplies, the intermediate capacitor voltage can vary but usually does so as a function of the type of power grid to which it is connected. For example, some power supplies allow the intermediate capacitor voltage to be 240 VDC when the input voltage is 120 VAC, and allow the capacitor voltage to rise to 380 VDC when the input voltage is 230 VAC. Most prior art two-stage power supplies fix the capacitor voltage (in this example) to the higher of the two (380 VDC) to allow the device to operate from either input voltage. (It is not permissible in this example for the input voltage to be 230 VAC while the output voltage is 240 VDC.)
Buck stage 106 draws energy from capacitor to power LED 108. Buck stage 106 may further comprise inductor 134(2), diode 132(2) and switch 136(2).
The input power of boost stage 104 is controlled by capacitor voltage control system 102 so that under typical operating conditions, the capacitor voltage (average, peak or some other measure) is held constant. The lowest undulation of the capacitor voltage must always be higher than the forward voltage of LED 108 in order to maintain the flicker-free output condition.
Eventually capacitor 112 ages and its capacitance is insufficient to prevent output ripple or possibly severe flicker. Also, there is typically a design margin required on the set-point of the capacitor voltage (perhaps 25% higher than the LED voltage), which can significantly reduce the efficiency.
Applicant has identified significant shortcomings in the conventional driver 100 as depicted in
The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is not intended to identify key/critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
The disclosed invention permits both the efficiency of the light emitting diode (LED) to be maximized, while monitoring capacitor life. In addition, the invention allows reasonable action to be taken at the inevitable end of capacitor life to ensure acceptable lamp performance following the capacitor's failure. In one embodiment, the invention comprises both a monitoring and control system to dynamically regulate the voltage of the capacitor. The regulation configuration operates the capacitor at minimum possible voltage to maximize the efficiency, to compensate for component variations and dimming signal variations, while maintaining flicker-free LED output.
For example, in one embodiment, a power supply for powering the LED comprises: (a) a capacitor; (b) a first voltage converter electrically coupled to an input voltage source and the capacitor; (c) a second voltage converter electrically coupled to the LED and the capacitor; and (d) a voltage control system, wherein the voltage control system controls a voltage established on the capacitor based upon a comparison of a voltage established on a cathode of the LED with a reference voltage source.
Converter 210(a) performs AC to DC conversion as well as voltage conversion of a received AC electromagnetic signal from power supply 110. In particular, converter 210(a) receives as input an alternating current (“AC”) electromagnetic signal from power supply 110 at a first voltage and generates as output a direct current (“DC”) electromagnetic signal at a second voltage (not shown in
The operation of dynamic power supply 214 via converter 210(a), energy storage device 212, converter 210(b), detector 220 and voltage control system 102 to eliminate periodic flicker in LED 108 output will now be described. Cathode (not labeled in
Voltage control system 102 operates to dynamically control a voltage established on energy storage device 212 based upon a control signal generated by detector 220 such that energy storage device 212 operates at a minimum possible voltage to compensate for component variations and dimming signal variations while maintaining flicker-free operation of LED 108.
As shown in
The control configuration depicted in
Likewise, LED 108 operated under cool conditions will not age capacitor 112 very quickly. Voltage control system 102 operates based upon true capacitor life rather than a conventional simple temperature-compensated elapsed-time measurement. Alternatively, as a longer-life capacitor is substituted for the original (for example, if the manufacturer makes a process improvement) voltage control operation shown in
Thus, according to one embodiment, an optimum capacitor voltage is established regardless of the forward voltage variations of LED 108 or an LED array. A conventional method would tend to make assumptions about LED voltage or implement awkward and error-prone high-side op-amp-based measurement circuits.
Another benefit of the operation of voltage control system 102 shown in
Detector 220 may further comprise comparator 204 and reference voltage source 206. Detector may generate an output signal (not shown in
According to one embodiment, the aforementioned measurement by the comparator at the cathode of the LED may be performed at the anode instead provided that the positions of inductor 134(2), diode 132(2), switch 136(2) and LED 108 are permuted is a specific way. This permutation is in fact commonly effected in power supplies and LED drivers and will be understood by skilled practitioners in the art. Thus, although the embodiments described herein refer to measurement at the cathode, it will be understood that in any of these embodiments, measurement may be performed at the anode of the LED instead.
According to one embodiment, voltage control system 102 comprises a micro-controller, CPU or other processing unit capable of executing programmatic instructions. However, all-analog implementations of the invention are possible and would be apparent to anyone skilled in the art.
According to one embodiment, voltage control system 102 operates to dynamically determine and set a minimum permissible voltage on capacitor 112 such that capacitor 112 operates at a minimum possible voltage to compensate for component variations and dimming signal variations while maintaining flicker-free operation of LED 108. In particular, according to one embodiment voltage control system 102 operates to allow the input of the buck converter 106 (the minimum capacitor voltage) to be controlled to be just above the instantaneous operating voltage of LED 108. According to one embodiment, voltage control system 102 operates to perform a continual monitoring and adjusting of capacitor 112 voltage utilizing an operation scheme such as that shown in
According to one embodiment, voltage control system 102 operates as a linear feedback control system which monitors the output signal generated by comparator 204 and produces a control output (not shown in
As will be further described with respect to
According to one embodiment, voltage control system 102 may function to determine whether capacitor 112 has reached its end-of-life and if so disable two-stage operation by disabling buck converter 106. According to one embodiment, an end-of-life condition may be detected when the minimum allowable capacitor 112 voltage signal can no longer be inhibited by increasing the voltage. When this condition persists for a short but sustained period of time, capacitor 112 is determined to have reached it end of life. This may be accomplished by determining whether the voltage on capacitor 112 can be reduced (as with a fresh capacitor) or whether the voltage needs to be increased beyond a threshold (as would be the case with a nearly exhausted capacitor). Once capacitor 112 has reached the end of its useful life, switch 136(2) on buck stage 106 may permanently closed such that voltage control system 102 is disabled. In this way the lamp is made to revert to single-stage operation the single stage simply draws a fixed average current or power level from the power source.
In the absence of methodologies described, typical efficiencies of a two stage LED driver might be 75%. Utilizing techniques of the dynamic power supply described herein, this efficiency is increased to 83%. Further systematic optimization of embodiments may further raise the efficiency, for instance to 85%, 90%. [Comment: we have not demonstrated it but that's ok; we don't do it for cost and size reasons. In another application (say automotive) where there is more room for optimized parts, it could easily be done. At another job with a prior-art version of this circuit I demonstrated an end-to-end efficiency of 95%.]
In contrast, embodiments of the invention described herein achieve less than 1% flicker.
In addition, embodiments of the invention may be optimized by considering various metrics of stroboscopic flicker. This includes percent flicker (as discussed above), flicker index, modulation depth, Stroboscopic effect Visibility Measure (SVM) and others. In an embodiment, a selected metric for flicker (or a combination of metrics) is chosen and a criterion is set for a maximum value for the metric. According to one embodiment, a design process is employed to maximize electrical efficiency while meeting the desired criterion. This design method relates to designing a two-stage driver according to embodiments of the invention described herein. In some embodiments, an optimization is performed to maintain a predetermined flicker value upon dimming of the LED (for instance, at 10% dimming 1% dimming and so on).
Embodiments of the invention can be employed in a variety of systems employing light-emitting sources. This includes lighting systems (such as lamps and fixtures), display and IT systems (such as computer screens, phone screens etc.), automotive applications and so on. The light-emitting sources may be light-emitting diodes (LEDs) as described herein; they may also be laser diodes or other light sources.
Some embodiments utilizing light-emitting sources include a plurality of light-emitting sources. In some cases, the light-emitting sources are distributed among several electrical strings, which can be driven with independent electrical powers. In some embodiments, the electrical power feeding each string can be varied (for instance over time according to a predetermined schedule, or following the input from a control system which may be controlled by a user or by an external stimulus). In some embodiments, the various strings may emit different light spectra (having different chromaticity, CCT, color rendition properties, and so on). In some embodiments, the electrical signal delivered by the two-stage driver is configured to obtain a predetermined flicker value, or operate the light sources at a selected efficiency.
Previous embodiments are described in the context of applications to LED drivers. However, embodiments of the invention can be used in other systems to drive a variety of electrical and electronic devices. In general, embodiments of the invention can provide various advantages: increased efficiency (by operating the device in a desirable voltage range), reduced transient effects (by reducing waveform variations sent to the device), increased lifetime (by operating the device in a desirable voltage range). Devices whose properties (efficiency, lifetime, etc.) are dependent on the input voltage or power can thus benefit from the techniques described herein. The techniques described herein achieved reduced heating of the circuitry. This allows for the life extensions of components, lower operating temperatures, etc. Any multi-stage power conversion device which must operate in a thermally stressed environment could benefit. Examples may include industrial motor drives, automotive drive train power converters, military equipment operating in hot areas.
Finally, it should be noted that there are alternative ways of implementing the embodiments disclosed herein. Accordingly, the present embodiments are to be considered as illustrative and not restrictive, and the claims are not to be limited to the details given herein, but may be modified within the scope and equivalents thereof.
This application claims the benefit of U.S. Provisional Application No. 62/191,831, filed Jul. 13, 2015, the entire disclosure of which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 62191831 | Jul 2015 | US |
