LED power-supply detection and control

A circuit detects the type of a power supply driving an LED by analyzing a signal received from the power supply. The circuit controls a behavior of the LED, such as its reaction to a dimmer or to thermal conditions, based on the determined type.

TECHNICAL FIELD

Embodiments of the invention generally relate to LED light sources and, in particular, to powering LED light sources using different types of power supplies.

BACKGROUND

LED light sources (i.e., LED lamps or, more familiarly, LED “light bulbs”) provide an energy-efficient alternative to traditional types of light sources, but typically require specialized circuitry to properly power the LED(s) within the light source. As used herein, the terms LED light sources, lamps, and/or bulbs refer to systems that include LED driver and support circuitry (the “LED module”) as well as the actual LED(s). For LED light sources to gain wide acceptance in place of traditional light sources, their support circuitry must be compatible with as many types of existing lighting systems as possible. For example, incandescent bulbs may be connected directly to an AC mains voltage, halogen-light systems may use magnetic or electronic transformers to provide 12 or 24 VAC to a halogen bulb, and other light sources may be powered by a DC current or voltage. Furthermore, AC mains voltages may vary country-by-country (60 Hz in the United States, for example, and 50 Hz in Europe).

Current LED light sources are compatible with only a subset of the above types of lighting system configurations and, even when they are compatible, they may not provide a user experience similar to that of a traditional bulb. For example, an LED replacement bulb may not respond to a dimmer control in a manner similar to the response of a traditional bulb. One of the difficulties in designing, in particular, halogen-replacement LED light sources is compatibility with the two kinds of transformers (i.e., magnetic and electronic) that may have been originally used to power a halogen bulb. A magnetic transformer consists of a pair of coupled inductors that step an input voltage up or down based on the number of windings of each inductor, while an electronic transformer is a complex electrical circuit that produces a high-frequency (i.e., 100 kHz or greater) AC voltage that approximates the low-frequency (60 Hz) output of a magnetic transformer.FIG. 1is a graph100of an output102of an electronic transformer; the envelope104of the output102approximates a low-frequency signal, such as one produced by a magnetic transformer.FIG. 2is a graph200of another type of output202produced by an electronic transformer. In this example, the output202does not maintain consistent polarity relative to a virtual ground204within a half 60 Hz period206. Thus, magnetic and electronic transformers behave differently, and a circuit designed to work with one may not work with the other.

For example, while magnetic transformers produce a regular AC waveform for any level of load, electronic transformers have a minimum load requirement under which a portion of their pulse-train output is either intermittent or entirely cut off. The graph300shown inFIG. 3illustrates the output of an electronic transformer for a light load302and for no load304. In each case, portions306of the outputs are clipped—these portions306are herein referred to as under-load dead time (“ULDT”). LED modules may draw less power than permitted by transformers designed for halogen bulbs and, without further modification, may cause the transformer to operate in the ULDT regions306.

To avoid this problem, some LED light sources use a “bleeder” circuit that draws additional power from the halogen-light transformer so that it does not engage in the ULDT behavior. With a bleeder, any clipping can be assumed to be caused by the dimmer, not by the ULDT. Because the bleeder circuit does not produce light, however, it merely wastes power, and may not be compatible with a low-power application. Indeed, LED light sources are preferred over conventional lights in part for their smaller power requirement, and the use of a bleeder circuit runs contrary to this advantage. In addition, if the LED light source is also to be used with a magnetic transformer, the bleeder circuit is no longer necessary yet still consumes power.

Dimmer circuits are another area of incompatibility between magnetic and electronic transformers. Dimmer circuits typically operate by a method known as phase dimming, in which a portion of a dimmer-input waveform is cut off to produce a clipped version of the waveform. The graph400shown inFIG. 4illustrates a result402of dimming an output of a magnetic transformer by cutting off a leading-edge point404and a result406dimming an output of an electronic transformer by cutting off a trailing-edge point408. The duration (i.e., duty cycle) of the clipping corresponds to the level of dimming desired—more clipping produces a dimmer light. Accordingly, unlike the dimmer circuit for an incandescent light, where the clipped input waveform directly supplies power to the lamp (with the degree of clipping determining the amount of power supplied and, hence, the lamp's brightness), in an LED system the received input waveform may be used to power a regulated supply that, in turn, powers the LED. Thus, the input waveform may be analyzed to infer the dimmer setting and, based thereon, the output of the regulated LED power supply is adjusted to provide the intended dimming level.

One implementation of a magnetic-transformer dimmer circuit measures the amount of time the input waveform is at or near the zero crossing410and produces a control signal that is a proportional function of this time. The control signal, in turn, adjusts the power provided to the LED. Because the output of a magnetic transformer (such as the output402) is at or near a zero crossing410only at the beginning or end of a half-cycle, this type of dimmer circuit produces the intended result. The output of electronic transformers (such as the output406), however, approaches zero many times during the non-clipped portion of the waveform due to its high-frequency pulse-train behavior. Zero-crossing detection schemes, therefore, must filter out these short-duration zero crossings while still be sensitive enough to react to small changes in the duration of the intended dimming level.

Because electronic transformers typically employ a ULDT-prevention circuit (e.g., a bleeder circuit), however, a simple zero-crossing-based dimming-detection method is not workable. If a dimmer circuit clips parts of the input waveform, the LED module reacts by reducing the power to the LEDs. In response, the electronic transformer reacts to the lighter load by clipping even more of the AC waveform, and the LED module interprets that as a request for further dimming and reduces LED power even more. The ULDT of the transformer then clips even more, and this cycle repeats until the light turns off entirely.

The use of a dimmer with an electronic transformer may cause yet another problem due to the ULDT behavior of the transformer. In one situation, the dimmer is adjusted to reduce the brightness of the LED light. The constant-current driver, in response, decreases the current drawn by the LED light, thereby decreasing the load of the transformer. As the load decreases below a certain required minimum value, the transformer engages in the ULDT behavior, decreasing the power supplied to the LED source. In response, the LED driver decreases the brightness of the light again, causing the transformer's load to decrease further; that causes the transformer to decrease its power output even more. This cycle eventually results in completely turning off the LED light.

Furthermore, electronic transformers are designed to power a resistive load, such as a halogen bulb, in a manner roughly equivalent to a magnetic transformer. LED light sources, however, present smaller, nonlinear loads to an electronic transformer and may lead to very different behavior. The brightness of a halogen bulb is roughly proportional to its input power; the nonlinear nature of LEDs, however, means that their brightness may not be proportional to their input power. Generally, LED light sources require constant-current drivers to provide a linear response. When a dimmer designed for a halogen bulb is used with an electronic transformer to power an LED source, therefore, the response may not be the linear, gradual response expected, but rather a nonlinear and/or abrupt brightening or darkening.

In addition, existing analog methods for thermal management of an LED involve to either a linear response or the response characteristics of a thermistor. While an analog thermal-management circuit may be configured to never exceed manufacturing limits, the linear/thermistor response is not likely to produce an ideal response (e.g., the LED may not always be as bright as it could otherwise be). Furthermore, prior-art techniques for merging thermal and dimming level parameters perform summation or multiplication; a drawback of these approaches is that an end user could dim a hot lamp but, as the lamp cools in response to the dimming, the thermal limit of the lamp increases and the summation or multiplication of the dimming level and the thermal limit results in the light growing brighter than the desired level.

Therefore, there is a need for a power-efficient, supply-agnostic LED light source capable of replacing different types of existing bulbs, regardless of the type of transformer and/or dimmer used to power and/or control the existing bulb.

SUMMARY

In general, embodiments of the current invention include systems and methods for controlling an LED driver circuit so that it operates regardless of the type of power source used. By analyzing the type of the power supply driving the LED, a control circuit is able to modify the behavior of the LED driver circuit to interface with the detected type of power supply. For example, a transformer output waveform may be analyzed to detect its frequency components. The existence of high-frequency components suggests, for example, that the transformer is electronic, and the lack of high-frequency components indicates the presence a magnetic transformer.

Accordingly, in one aspect, a circuit for modifying a behavior of an LED driver in accordance with a detected power supply type includes an analyzer and a generator. The analyzer determines the type of the power supply based at least in part on a power signal received from the power supply. The generator generates a control signal, based at least in part on the determined type of the power supply, for controlling the behavior of the LED driver.

In various embodiments, the type of the power supply includes a DC power supply, a magnetic-transformer power supply, or an electronic-transformer power supply and/or a manufacturer or a model of the power supply. The analyzer may include digital logic. The behavior of the LED driver may include a voltage or current output level. An input/output port may communicate with at least one of the analyzer and the generator. The analyzer may include a frequency analyzer for determining a frequency of the power signal. A dimmer control circuit may dim an output of the LED driver by modifying the control signal in accordance with a dimmer setting.

A bleeder control circuit may maintain the power supply in an operating region by selectively engaging a bleeder circuit to increase a load of the power supply. A thermal control circuit may reduce an output of the LED driver by modifying the control signal in accordance with an over-temperature condition. The generated control signal may include a voltage control signal, a current control signal, or a pulse-width-modulated control signal.

In general, in another aspect, a method modifies a behavior of an LED driver circuit in accordance with a detected a power supply type. The type of the power supply is determined based at least in part on analyzing a power signal received from the power supply. The behavior of the LED driver is controlled based at least in part on the determined type of power supply.

In various embodiments, determining the type of the power supply includes detecting a frequency of the power supply signal. The frequency may be detected in less than one second or in less than one-tenth of a second. Modifying the behavior may include modifying an output current or voltage level. A load of the power supply may be detected, and determining the type of the power supply may further include pairing the detected frequency with the detected load. The load of the power supply may be changed using the control signal and measuring the frequency of the power supply signal at the changed load. A country or a region supplying AC mains power to the power supply may be detected. Generating the control signal may include generating at least one of a voltage control signal, current control signal, or a pulse-width-modulated control 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 may exist in various combinations and permutations.

DETAILED DESCRIPTION

FIG. 5illustrates a block diagram500of various embodiments of the present invention. A transformer502receives a transformer input signal504and provides a transformed output signal506. The transformer502may be a magnetic transformer or an electronic transformer, and the output signal506may be a low-frequency (i.e. less than or equal to approximately 120 Hz) AC signal or a high-frequency (e.g., greater than approximately 120 Hz) AC signal, respectively. The transformer502may be, for example, a 5:1 or a 10:1 transformer providing a stepped-down 60 Hz output signal506(or output signal envelope, if the transformer502is an electronic transformer). The transformer output signal506is received by an LED module508, which converts the transformer output signal506into a signal suitable for powering one or more LEDs510. In accordance with embodiments of the invention, and as explained in more detail below, the LED module508detects the type of the transformer502and alters its behavior accordingly to provide a consistent power supply to the LEDs510.

In various embodiments, the transformer input signal504may be an AC mains signal512, or it may be received from a dimmer circuit514. The dimmer circuit may be, for example, a wall dimmer circuit or a lamp-mounted dimmer circuit. A conventional heat sink516may be used to cool portions of the LED module508. The LED module508and LEDs510may be part of an LED assembly (also known as an LED lamp or LED “bulb”)518, which may include aesthetic and/or functional elements such as lenses520and a cover522.

The LED module508may include a rigid member suitable for mounting the LEDs510, lenses520, and/or cover520. The rigid member may be (or include) a printed-circuit board, upon which one or more circuit components may be mounted. The circuit components may include passive components (e.g., capacitors, resistors, inductors, fuses, and the like), basic semiconductor components (e.g., diodes and transistors), and/or integrated-circuit chips (e.g., analog, digital, or mixed-signal chips, processors, microcontrollers, application-specific integrated circuits, field-programmable gate arrays, etc.). The circuit components included in the LED module508combine to adapt the transformer output signal506into a signal suitable for lighting the LEDs520.

A block diagram of one such LED module circuit600is illustrated inFIG. 6. The transformer output signal506is received as an input signal Vin. One or more fuses602may be used to protect the circuitry of the LED module600from over-voltage or over-current conditions in the input signal Vin. One fuse may be used on one polarity of the input signal Vin, or two fuses may be used (one for each polarity), as shown in the figure. In one embodiment, the fuses are 1.75-amp fuses.

A rectifier bridge604is used to rectify the input signal Vin. The rectifier bridge604may be, for example, a full-wave or half-wave rectifier, and may use diodes or other one-way devices to rectify the input signal Vin. The current invention is not limited to any particular type of rectifier bridge, however, or any type of components used therein. As one of skill in the art will understand, any bridge604capable of modifying the AC-like input signal Vinin to a more DC-like output signal606is compatible with the current invention.

A regulator IC608receives the rectifier output606and converts it into a regulated output610. In one embodiment, the regulated output610is a constant-current signal calibrated to drive the LEDs612at a current level within their tolerance limits. In other embodiments, the regulated output610is a regulated voltage supply, and may be used with a ballast (e.g., a resistive, reactive, and/or electronic ballast) to limit the current through the LEDs612.

A DC-to-DC converter may be used to modify the regulated output610. In one embodiment, as shown inFIG. 6, a boost regulator614is used to increase the voltage or current level of the regulated output610. In other embodiments, a buck converter or boost-buck converter may be used. The DC-to-DC converter614may be incorporated into the regulator IC608or may be a separate component; in some embodiments, no DC-to-DC converter614may be present at all.

A processor616is used, in accordance with embodiments of the current invention, to modify the behavior of the regulator IC608based at least in part on a received signal618from the bridge604. In other embodiments, the signal618is connected directly to the input voltage Vinof the LED module600. The processor616may be a microprocessor, microcontroller, application-specific integrated circuit, field-programmable grid array, or any other type of digital-logic or mixed-signal circuit. The processor616may be selected to be low-cost, low-power, for its durability, and/or for its longevity. An input/output link620allows the processor616to send and receive control and/or data signals to and/or from the regulator IC608. As described in more detail below, a thermal monitoring module622may be used to monitor a thermal property of one or more LEDs612. The processor616may also be used to track the runtime of the LEDs612or other components and to track a current or historical power level applied to the LEDs612or other components. In one embodiment, the processor616may be used to predict the lifetime of the LEDs612given such inputs as runtime, power level, and estimated lifetime of the LEDs612. This and other information and/or commands may be accessed via an input/output port626, which may be a serial port, parallel port, JTAG port, network interface, or any other input/output port architecture as known in the art.

The operation of the processor616is described in greater detail with reference toFIG. 7. An analyzer702receives the signal618via an input bus704. When the system powers on and the input signal618becomes non-zero, the analyzer702begins analyzing the signal618. In one embodiment, the analyzer702examines one or more frequency components of the input signal618. If no significant frequency components exist (i.e., the power level of any frequency components is less than approximately 5% of a total power level of the signal), the analyzer determines that the input signal618is a DC signal. If one or more frequency components exist and are less than or equal to approximately 120 Hz, the analyzer determines that the input signal618is derived from the output of a magnetic transformer. For example, a magnetic transformer supplied by an AC mains voltage outputs a signal having a frequency of 60 Hz; the processor616receives the signal and the analyzer detects that its frequency is less than 120 Hz and concludes that the signal was generated by a magnetic transformer. If one or more frequency components of the input signal618are greater than approximately 120 Hz, the analyzer702concludes that the signal618was generated by an electronic transformer. In this case, the frequency of the signal618may be significantly higher than 120 Hz (e.g., 50 or 100 kHz).

The analyzer702may employ any frequency detection scheme known in the art to detect the frequency of the input signal618. For example, the frequency detector may be an analog-based circuit, such as a phase-frequency detector, or it may be a digital circuit that samples the input signal618and processes the sampled digital data to determine the frequency. In one embodiment, the analyzer702detects a load condition presented by the regulator IC608. For example, the analyzer702may receive a signal representing a current operating point of the regulator IC608and determine its input load; alternatively, the regulator IC608may directly report its input load. In another embodiment, the analyzer702may send a control signal to the regulator IC608requesting that it configure itself to present a particular input load. In one embodiment, the processor616may use a dimming control signal, as explained further below, to vary the load.

The analyzer702may correlate a determined input load with the frequency detected at that load to derive further information about the transformer502. For example, the manufacturer and/or model of the transformer502, and in particular an electronic transformer, may be detected from this information. The analyzer702may include a storage device714, which may be a read-only memory, flash memory, look-up table, or any other storage device, and contain data on devices, frequencies, and loads. Addressing the storage device with the one or more load-frequency data points may result in a determination of the type of the transformer502. The storage device714may contain discrete values or expected ranges for the data stored therein; in one embodiment, detected load and frequency information may be matched to stored values or ranges; in another embodiment, the closest matching stored values or ranges are selected.

The analyzer702may also determine, from the input signal618, different AC mains standards used in different countries or regions. For example, the United States uses an AC mains having a frequency of 60 Hz, while Europe has an AC mains of 50 Hz. The analyzer702may report this result to the generator704, which in turn generates an appropriate control signal for the regulator IC608. The regulator IC608may include a circuit for adjusting its behavior based on a detected country or region. Thus, the LED module600may be country- or region-agnostic.

The analysis carried out by the analyzer702make take place upon system power-up, and duration of the analysis may be less than one second (e.g., enough time to observe at least 60 cycles of standard AC mains input voltage). In other embodiments, the duration of the analysis is less than one-tenth of a second (e.g., enough time to observe at least five cycles of AC mains input voltage). This span of time is short enough to be imperceptible, or nearly imperceptible, to a user. The analysis may also be carried out at other times during the operation of the LED module; for example, when the input supply voltage or frequency changes by a given threshold, or after a given amount of time has elapsed.

Once the type of power supply/transformer is determined, a generator circuit706generates a control signal in accordance with the detected type of transformer and sends the control signal to the regulator IC608, via an input/output bus708, through the input/output link620. The regulator IC608may be capable of operating in a first mode that accepts a DC input voltage Vin, a second mode that accepts a low-frequency (≤120 Hz) input voltage Vin, and a third mode that accepts a high-frequency (>120 Hz) input voltage Vin. The generator circuit706, based on the determination of the analyzer702, instructs the regulator IC608to enter the first, second, or third mode. Thus, the LED module600is compatible with a wide variety of input voltages and transformer types.

The processor616may also include a dimmer control circuit710, a bleeder control circuit712, and/or a thermal control circuit716. The operation of these circuits is explained in greater detail below.

Dimmer Control

The analyzer702and generator706may modify their control of the regulator IC608based on the absence or presence of a dimmer and, if a dimmer is present, an amount of dimming. A dimmer present in the upstream circuits may be detected by observing the input voltage618for, e.g., clipping, as discussed above with reference toFIG. 4. Typically, a dimmer designed to work with a magnetic transformer clips the leading edges of an input signal, and a dimmer designed to work with an electronic transformer clips the trailing edges of an input signal. The analyzer702may detect leading- or trailing-edge dimming on signals output by either type of transformer, however, by first detecting the type of transformer, as described above, and examining both the leading and trailing edges of the input signal.

Once the presence and/or type of dimming have been detected, the generator706and/or a dimmer control circuit710generate a control signal for the regulator IC608based on the detected dimming. The dimmer circuit710may include a duty-cycle estimator718for estimating a duty cycle of the input signal618. The duty-cycle estimator may include any method of duty cycle estimation known in the art; in one embodiment, the duty-cycle estimator includes a zero-crossing detector for detecting zero crossings of the input signal618and deriving the duty cycle therefrom. As discussed above, the input signal618may include high-frequency components if it is generated by an electronic transformer; in this case, a filter may be used to remove the high-frequency zero crossings. For example, the filter may remove any consecutive crossings that occur during a time period smaller than a predetermined threshold (e.g., less than one millisecond). The filter may be an analog filter or may be implemented in digital logic in the dimmer control circuit710.

In one embodiment, the dimmer control circuit710derives a level of intended dimming from the input voltage618and translates the intended dimming level to the output control signal620. The amount of dimming in the output control signal620may vary depending on the type of transformer used to power the LED module600.

For example, if a magnetic transformer502is used, the amount of clipping detected in the input signal618(i.e., the duty cycle of the signal) may vary from no clipping (i.e., approximately 100% duty cycle) to full clipping (i.e., approximately 0% duty cycle). An electronic transformer502, on the other hand, requires a minimum amount of load to avoid the under-load dead time condition discussed above, and so may not support a lower dimming range near 0% duty cycle. In addition, some dimmer circuits (e.g., a 10%-90% dimmer circuit) consume power and thus prevent downstream circuits from receiving the full power available to the dimmer.

In one embodiment, the dimmer control circuit710determines a maximum setting of the upstream dimmer514(i.e., a setting that causes the least amount of dimming). The maximum dimmer setting may be determined by direct measurement of the input signal618. For example, the signal618may be observed for a period of time and the maximum dimmer setting may equal the maximum observed voltage, current, or duty cycle of the input signal618. In one embodiment, the input signal618is continually monitored, and if it achieves a power level higher than the current maximum dimmer level, the maximum dimmer level is updated with the newly observed level of the input signal618.

Alternatively or in addition, the maximum setting of the upstream dimmer514may be derived based on the detected type of the upstream transformer502. In one embodiment, magnetic and electronic transformers502have similar maximum dimmer settings. In other embodiments, an electronic transformer502has a lower maximum dimmer setting than a magnetic transformer502.

Similarly, the dimmer control circuit710determines a minimum setting of the upstream dimmer514(i.e., a setting that causes the most amount of dimming). Like the maximum dimmer setting, the minimum setting may be derived from the detected type of the transformer514and/or may be directly observed by monitoring the input signal618. The analyzer702and/or dimmer control circuit710may determine the manufacturer and model of the electronic transformer514, as described above, by observing a frequency of the input signal618under one or more load conditions, and may base the minimum dimmer setting at least in part on the detected manufacturer and model. For example, a minimum load value for a given model of transformer may be known, and the dimmer control circuit710may base the minimum dimmer setting on the minimum load value.

Once the full range of dimmer settings of the input signal618is derived or detected, the available range of dimmer input values is mapped or translated into a range of control values for the regulator IC608. In one embodiment, the dimmer control circuit710selects control values to provide a user with the greatest range of dimming settings. For example, if a 10%-90% dimmer is used, the range of values for the input signal618never approaches 0% or 100%, and thus, in other dimmer control circuits, the LEDs612would never be fully on or fully off. In the present invention, however, the dimmer control circuit710recognizes the 90% value of the input signal618as the maximum dimmer setting and outputs a control signal to the regulator IC608instructing it to power the LEDs612to full brightness. Similarly, the dimmer control circuit710translates the 10% minimum value of the input signal618to a value producing fully-off LEDs612. In other words, in general, the dimmer control circuit710maps an available range of dimming of the input signal618(in this example, 10%-90%) onto a full 0%-100% output dimming range for controlling the regulator IC608.

In one embodiment, as the upstream dimmer514is adjusted to a point somewhere between its minimum and maximum values, the dimmer control circuit710varies the control signal620to the regulator IC608proportionately. In other embodiments, the dimmer control circuit710may vary the control signal620linearly or logarithmically, or according to some other function dictated by the behavior of the overall circuit, as the upstream dimmer514is adjusted. Thus, the dimmer control circuit710may remove any inconsistencies or nonlinearities in the control of the upstream dimmer514. In addition, as discussed above, the dimmer control circuit710may adjust the control signal620to avoid flickering of the LEDs612due to an under-load dead time condition. In one embodiment, the dimmer control circuit710may minimize or eliminate flickering, yet still allow the dimmer514to completely shut off the LEDs612, by transitioning the LEDs quickly from their lowest non-flickering state to an off state as the dimmer514is fully engaged.

The generator706and/or dimmer control circuit710may output any type of control signal appropriate for the regulator IC608. For example, the regulator IC may accept a voltage control signal, a current control signal, and/or a pulse-width modulation control signal. In one embodiment, the generator706sends, over the bus620, a voltage, current, and/or pulse-width modulated signal that is directly mixed or used with the output signal610of the regulator IC608. In other embodiments, the generator706outputs digital or analog control signals appropriate for the type of control (e.g., current, voltage, or pulse-width modulation), and the regulator IC608modifies its behavior in accordance with the control signals. The regulator IC608may implement dimming by reducing a current or voltage to the LEDs612, within the tolerances of operation for the LEDs612, and/or by changing a duty cycle of the signal powering the LEDs612using, for example, pulse-width modulation.

In computing and generating the control signal620for the regulator IC608, the generator706and/or dimmer control circuit710may also take into account a consistent end-user experience. For example, magnetic and electronic dimming setups produce different duty cycles at the top and bottom of the dimming ranges, so a proportionate level of dimming may be computed differently for each setup. Thus, for example, if a setting of the dimmer514produces 50% dimming when using a magnetic transformer502, that same setting produces 50% dimming when using an electronic transformer502.

Bleeder Control

As described above, a bleeder circuit may be used to prevent an electronic transformer from falling into an ULDT condition. But, as further described above, bleeder circuits may be inefficient when used with an electronic transformer and both inefficient and unnecessary when used with a magnetic transformer. In embodiments of the current invention, however, once the analyzer702has determined the type of transformer502attached, a bleeder control circuit712controls when and if the bleeder circuit draws power. For example, for DC supplies and/or magnetic transformers, the bleeder is not turned on and therefore does not consume power. For electronic transformers, while a bleeder may sometimes be necessary, it may not be needed to run every cycle.

The bleeder may be needed during a cycle only when the processor616is trying to determine the amount of phase clipping produced by a dimmer514. For example, a user may change a setting on the dimmer514so that the LEDs612become dimmer, and as a result the electronic transformer may be at risk for entering an ULDT condition. A phase-clip estimator720and/or the analyzer702may detect some of the clipping caused by the dimmer514, but some of the clipping may be caused by ULDT; the phase-clip estimator720and/or analyzer702may not be able to initially tell one from the other. Thus, in one embodiment, when the analyzer702detects a change in a clipping level of the input signal618, but before the generator706makes a corresponding change in the control signal620, the bleeder control circuit712engages the bleeder. While the bleeder is engaged, any changes in the clipping level of the input signal618are a result only of action on the dimmer514, and the analyzer702and/or dimmer control circuit710react accordingly. The delay caused by engaging the bleeder may last only a few cycles of the input signal618, and thus the lag between changing a setting of the dimmer514and detecting a corresponding change in the brightness of the LEDs612is not perceived by the user.

In one embodiment, the phase-clip estimator720monitors preceding cycles of the input signal618and predict at what point in the cycle ULDT-based clipping would start (if no bleeder were engaged). For example, referring back toFIG. 3, ULDT-based clipping306for a light load302may occur only in the latter half of a cycle; during the rest of the cycle, the bleeder is engaged and drawing power, but is not required. Thus, the processor616may engage the bleeder load during only those times it is needed—slightly before (e.g., approximately 100 μs before) the clipping begins and shortly after (e.g., approximately 100 microseconds after) the clipping ends.

Thus, depending on the amount of ULDT-based clipping, the bleeder may draw current for only a few hundred microseconds per cycle, which corresponds to a duty cycle of less than 0.5%. In this embodiment, a bleeder designed to draw several watts incurs an average load of only a few tens of milliwatts. Therefore, selectively using the bleeder allows for highly accurate assessment of the desired dimming level with almost no power penalty.

In one embodiment, the bleeder control circuit712engages the bleeder whenever the electronic transformer502approaches an ULDT condition and thus prevents any distortion of the transformer output signal506caused thereby. In another embodiment, the bleeder control circuit712engages the bleeder circuit less frequently, thereby saving further power. In this embodiment, while the bleeder control circuit712prevents premature cutoff of the electronic transformer502, its less-frequent engaging of the bleeder circuit allows temporary transient effects (e.g., “clicks”) to appear on the output506of the transformer502. The analyzer702, however, may detect and filter out these clicks by instructing the generator706not to respond to them.

Thermal Control

The processor616, having power control over the regulator IC608, may perform thermal management of the LEDs612. LED lifetime and lumen maintenance is linked to the temperature and power at which the LEDs612are operated; proper thermal management of the LEDs612may thus extend the life, and maintain the brightness, of the LEDs612. In one embodiment, the processor616accepts an input624from a temperature sensor622. The storage device714may contain maintenance data (e.g., lumen maintenance data) for the LEDs612, and a thermal control circuit716may receive the temperature sensor input624and access maintenance data corresponding to a current thermal operating point of the LEDs612. The thermal control circuit716may then calculate the safest operating point for the brightest LEDs612and instruct the generator706to increase or decrease the LED control signal accordingly.

The thermal control circuit716may also be used in conjunction with the dimmer control circuit710. A desired dimming level may be merged with thermal management requirements, producing a single brightness-level setting. In one embodiment, the two parameters are computed independently (in the digital domain by, e.g., the thermal control circuit716and/or the dimmer control circuit710) and only the lesser of the two is used to set the brightness level. Thus, embodiments of the current invention avoid the case in which a user dims a hot lamp—i.e., the lamp brightness is affected by both thermal limiting and by the dimmer—later to find that, as the lamp cools, the brightness level increases. In one embodiment, the thermal control circuit716“normalizes” 100% brightness to the value defined by the sensed temperature and instructs the dimmer control circuit710to dim from that standard.

Some or all of the above circuits may be used in a manner illustrated in a flowchart800shown inFIG. 8. The processor616is powered on (Step802), using its own power supply or a power supply shared with one of the other components in the LED module600. The processor616is initialized (Step804) using techniques known in the art, such as by setting or resetting control registers to known values. The processor616may wait to receive acknowledgement signals from other components on the LED module600before leaving initialization mode.

The processor616inspects the incoming rectified AC waveform618(Step806) by observing a few cycles of it. As described above, the analyzer702may detect a frequency of the input signal618and determine the type of power source (Step808) based thereon. If the supply is a magnetic transformer, the processor616measures the zero-crossing duty cycle (Step810) of the input waveform (i.e., the processor616detects the point where the input waveform crosses zero and computes the duty cycle of the waveform based thereon). If the supply is an electronic transformer, the processor616tracks the waveform618and syncs to the zero crossing (Step812). In other words, the processor616determines which zero crossings are the result of the high-frequency electronic transformer output and which zero crossings are the result of the transformer output envelop changing polarity; the processor616disregards the former and tracks the latter. In one embodiment, the processor616engages a bleeder load just prior to a detected zero crossing (Step814) in order to prevent a potential ULDT condition from influencing the duty cycle computation. The duty cycle is then measured (Step816) and the bleeder load is disengaged (Step818).

At this point, whether the power supply is a DC supply or a magnetic or electronic transformer, the processor616computes a desired brightness level based on a dimmer (Step820), if a dimmer is present. Furthermore, if desired, a temperature of the LEDs may be measured (Step822). Based on the measured temperature and LED manufacturing data, the processor616computes a maximum allowable power for the LED (Step824). The dimmer level and thermal level are analyzed to compute a net brightness level; in one embodiment, the lesser of the two is selected (Step826). The brightness of the LED is then set with the computed brightness level (Step828). Periodically, or when a change in the input signal618is detected, the power supply type may be checked (Step830), the duty cycle of the input, dimming level, and temperature are re-measured and a new LED brightness is set.