Dimmable power supply

Various embodiments of a dimmable power supply are disclosed herein. For example, some embodiments provide a dimmable power supply including an output driver, a variable pulse generator and a load current detector. The output driver has a power input, a control input and a load path. The variable pulse generator includes a control input and a pulse output, with the pulse output connected to the output driver control input. The variable pulse generator is adapted to vary a pulse width at the pulse output based on a signal at the control input. The load current detector has an input connected to the output driver load path and an output connected to the variable pulse generator control input. The load current detector has a time constant adapted to substantially filter out a change in a load current at a frequency of pulses at the variable pulse generator pulse output.

BACKGROUND

Electricity is generated and distributed in alternating current (AC) form, wherein the voltage varies sinusoidally between a positive and a negative value. However, many electrical devices require a direct current (DC) supply of electricity having a constant voltage level, or at least a supply that remains positive even if the level is allowed to vary to some extent. For example, light emitting diodes (LEDs) and similar devices such as organic light emitting diodes (OLEDs) are being increasingly considered for use as light sources in residential, commercial and municipal applications. However, in general, unlike incandescent light sources, LEDs and OLEDs cannot be powered directly from an AC power supply unless, for example, the LEDs are configured in some back to back formation. Electrical current flows through an individual LED easily in only one direction, and if a negative voltage which exceeds the reverse breakdown voltage of the LED is applied, the LED can be damaged or destroyed. Furthermore, the standard, nominal residential voltage level is typically something like 120 V or 240 V, both of which are higher than may be desired for a high efficiency LED light. Some conversion of the available power may therefore be necessary or highly desired with loads such as an LED light.

In one type of commonly used power supply for loads such as an LED, an incoming AC voltage is connected to the load only during certain portions of the sinusoidal waveform. For example, a fraction of each half cycle of the waveform may be used by connecting the incoming AC voltage to the load each time the incoming voltage rises to a predetermined level or reaches a predetermined phase and by disconnecting the incoming AC voltage from the load each time the incoming voltage again falls to zero. In this manner, a positive but reduced voltage may be provided to the load. This type of conversion scheme is often controlled so that a constant current is provided to the load even if the incoming AC voltage varies. However, if this type of power supply with current control is used in an LED light fixture or lamp, a conventional dimmer is often ineffective. For many LED power supplies, the power supply will attempt to maintain the constant current through the LED despite a drop in the incoming voltage by increasing the on-time during each cycle of the incoming AC wave.

SUMMARY

Various embodiments of a dimmable power supply are disclosed herein. For example, some embodiments provide a dimmable power supply including an output driver, a variable pulse generator and a load current detector. The output driver has a power input, a control input and a load path. The variable pulse generator includes a control input and a pulse output, with the pulse output connected to the output driver control input. The variable pulse generator is adapted to vary a pulse width at the pulse output based on a signal at the control input. The load current detector has an input connected to the output driver load path and an output connected to the variable pulse generator control input. The load current detector has a time constant adapted to substantially filter out a change in a load current at a frequency of pulses at the variable pulse generator pulse output.

In an embodiment of the dimmable power supply, the load current detector includes a comparator having a first input connected to the load path, a second input connected to a reference current source, and an output connected to the variable pulse generator control input.

In an embodiment of the dimmable power supply, the output driver also includes a current sense resistor in the load path. The first input of the comparator is connected through a low pass filter to the load path at a node of the current sense resistor. The time constant of the load current detector is based at least in part on the low pass filter.

In an embodiment of the dimmable power supply, the first input of the comparator is a non-inverting input and the second input of the comparator is an inverting input. The load current detector also includes a low pass filter connected in a negative feedback loop between the comparator output and the second input of the comparator.

In an embodiment of the dimmable power supply, the reference current source includes a voltage divider connected between the power input of the output driver and a ground. The reference current source has an output connected to the second input of the load current detector.

In an embodiment of the dimmable power supply, the voltage divider includes at least one upper resistor connected at a first end to the power input of the output driver, a transistor having an input connected to a second end of the at least one upper resistor and having an output connected to the reference current source output, and at least one lower resistor connected at a first end to a control input of the transistor and at a second end to the ground.

An embodiment of the dimmable power supply also includes a level shifter connected between the load current detector output and the variable pulse generator control input.

In an embodiment of the dimmable power supply, the level shifter comprises an optocoupler.

In an embodiment of the dimmable power supply, the output driver includes an inductor connected at a first node to a local ground and a switch connected between a second node of the inductor and a ground. The switch has a control input connected to the pulse output of the variable pulse generator. The output driver also includes a diode connected between the power input of the output driver and the second node of the inductor. The load path is located between the power input of the output driver and the first node of the inductor.

In an embodiment of the dimmable power supply, the output driver also includes a capacitor connected in parallel with at least a portion of the load path.

In an embodiment of the dimmable power supply, the load current detector includes at least one low pass filter that is referenced to the local ground.

In an embodiment of the dimmable power supply, the output driver also includes a current sensor connected between the switch and the ground. The variable pulse generator is adapted to reduce the pulse width when the current sensor detects a current level exceeding a threshold level.

In an embodiment of the dimmable power supply, the variable pulse generator includes a current limit switch connected to the current sensor. The current limit switch is adapted to reduce the pulse width in an inverse proportion to a temperature of the current limit switch.

An embodiment of the dimmable power supply includes an overvoltage limiter connected to the load current detector output. The overvoltage limiter is adapted to reduce the pulse width when a voltage level at the load current detector output exceeds a threshold level.

An embodiment of the dimmable power supply includes an internal dimming device connected to the load current detector. The load current detector and variable pulse generator are adapted to vary the pulse width based on an output of the internal dimming device.

In an embodiment of the dimmable power supply, the load current detector time constant is adapted to substantially keep the pulse width at the pulse output constant across an AC waveform at the power input of the output driver.

In an embodiment of the dimmable power supply, the output driver includes a transformer and a switch connected between the transformer and ground. The switch has a control input connected to the pulse output of the variable pulse generator. The output driver also includes a diode connected between the power input of the output driver and the transformer. The load path is located between the power input of the output driver and the transformer.

Other embodiments provide a method of dimmably supplying a load current including measuring a ratio between a reference current and a load current, producing pulses having a width that is inversely proportional to the ratio, and driving the load current with the pulses. The measuring is performed with a time constant that substantially filters out the pulses in the load current but substantially passes changes in the reference current.

An embodiment of the method of dimmably supplying a load current also includes generating the reference current based on an input voltage so that the reference current is directly proportional to the input voltage.

Other embodiments provide a power supply having an output driver with an inductor connected at a first node to a local ground, a diode connected between a power input and a second node of the inductor, a load path having a first node connected to the power input, a capacitor connected in parallel with the load path, and a load current sensor connected at a first end to the local ground and at a second end to a second node of the load path. The output driver also includes a switch having an input connected to the second node of the inductor and having an output driver control input, and a drive current sensor connected between an output of the switch and a ground. The power supply also includes a variable pulse generator having a control input and a pulse output. The pulse output is connected to the output driver control input. The variable pulse generator is adapted to vary a pulse width at the pulse output based on a signal at the control input. The variable pulse generator includes a current limit switch connected to the load current sensor. The current limit switch is adapted to reduce the pulse width in an inverse proportion to a temperature of the current limit switch. The variable pulse generator is adapted to reduce the pulse width when the drive current sensor detects a current level exceeding a threshold level. The power supply also includes a load current detector with a reference current source. The reference current source includes at least one upper resistor connected at a first end to the power input, a transistor having an input connected to a second end of the at least one upper resistor, and at least one lower resistor connected at a first end to a control input of the transistor and at a second end to the ground. The load current detector also includes a comparator having a non-inverting input connected to the second end of the load current sensor through a low pass filter and having an inverting input connected to an output of the reference current source transistor. The load current detector also includes a second low pass filter connected in a negative feedback loop between the comparator output and the inverting input. The load current detector has a time constant adapted to substantially filter out a change in a load current at a frequency on the order of a frequency of pulses at the variable pulse generator pulse output. The time constant of the load current detector is based at least in part on the low pass filter that is referenced to the local ground. The current detector is referenced to both the local ground and to the ground. The power supply also includes an optocoupler as a level shifter connected between an output of the comparator in the load current detector and the variable pulse generator control input. The power supply also includes an overvoltage limiter connected to the input of the level shifter. The overvoltage limiter is adapted to reduce the pulse width when a voltage level that appears across the load exceeds a second threshold level. The power supply also includes an internal dimming device connected to the load current detector. The load current detector and variable pulse generator are adapted to vary the pulse width based on an output of the internal dimming device.

This summary provides only a general outline of some particular embodiments. Many other objects, features, advantages and other embodiments will become more fully apparent from the following detailed description, the appended claims and the accompanying drawings.

DESCRIPTION

The drawings and description, in general, disclose various embodiments of a dimmable power supply for loads such as an LED or array of LEDs. The dimmable power supply may use either an AC or DC input, with a varying or constant voltage level. The current through the load from the dimmable power supply may be adjusted using conventional or other types of dimmers in the power supply line upstream from the dimmable power supply. Thus, the term “dimmable” is used herein to indicate that input voltage of the dimmable power supply may be varied to dim a load or otherwise reduce the load current, without the control system in the dimmable power supply opposing the resulting change to the load current and keeping the load current constant. Various embodiments of the dimmable power supply may, in addition to being externally dimmable, be internally dimmable by including dimming elements within the dimmable power supply. In these embodiments, the load current may be adjusted by controlling the input voltage of the dimmable power supply using an external dimmer and by controlling the internal dimming elements within the dimmable power supply. Internal dimming can be implemented and accomplished by, for example, among others, on/off using pulse width modulation (PWM) at appropriate frequencies, 0 to 10 V, the use of resistors including variable resistor(s), encoders, analog and/or digital resistors, or any other type of analog, digital or a mixture of analog and digital.

Referring now toFIG. 1, a block diagram of an embodiment of a dimmable power supply10is shown. In this embodiment, the dimmable power supply10is powered by an AC input12, for example by a 50 or 60 Hz sinusoidal waveform of 120 V or 240 V RMS such as that supplied to residences by municipal electric power companies. It is important to note, however, that the dimmable power supply10is not limited to any particular power input. Furthermore, the voltage applied to the AC input12may be externally controlled, such as in an external dimmer (not shown) that reduces the voltage. The AC input12is connected to a rectifier14to rectify and invert any negative voltage component from the AC input12. Although the rectifier14may filter and smooth the power output16if desired to produce a DC signal, this is not necessary and the power output16may be a series of rectified half sinusoidal waves at a frequency double that at the AC input12, for example 120 Hz. A variable pulse generator20is powered by the power output16from the AC input12and rectifier14to generate a train of pulses at an output22. The variable pulse generator20may comprise any device or circuit now known or that may be developed in the future to generate a train of pulses of any desired shape. For example, the variable pulse generator20may comprise devices such as comparators, amplifiers, oscillators, counters, frequency generators, etc.

The pulse width of the train of pulses is controlled by a load current detector24with a time constant based on a current level through a load26. Various implementations of pulse width control including pulse width modulation (PWM) by frequency, analog and/or digital control may be used to realize the pulse width control. Other features such as soft start, delayed start, instant on operation, etc. may also be included if deemed desirable, needed, and/or useful. An output driver30produces a current32through the load26, with the current level adjusted by the pulse width at the output22of the variable pulse generator20. The current32through the load26is monitored by the load current detector24. The current monitoring performed by the load current detector24is done with a time constant that includes information about voltage changes at the power output16of the rectifier14slower than or on the order of a waveform cycle at the power output16, but not faster changes at the power output16or voltage changes at the output22of the variable pulse generator20. The control signal34from the load current detector24to the variable pulse generator20thus varies with slower changes in the power output16of the rectifier14, but not with the incoming rectified AC waveform or with changes at the output22of the variable pulse generator20due to the pulses themselves. In one particular embodiment, the load current detector24includes one or more low pass filters to implement the time constant used in the load current detection. The time constant may be established by a number of suitable devices and circuits, and the dimmable power supply10is not limited to any particular device or circuit. For example, the time constant may be established using RC circuits arranged in the load current detector24to form low pass filters, or with other types of passive or active filtering circuits. The load26may be any desired type of load, such as a light emitting diode (LED) or an array of LEDs arranged in any configuration. For example, an array of LEDs may be connected in series or in parallel or in any desired combination of the two. The load26may also be an organic light emitting diode (OLED) in any desired quantity and configuration. The load26may also be a combination of different devices if desired, and is not limited to the examples set forth herein. Hereinafter, the term LED is used generically to refer to all types of LEDs including OLEDs and is to be interpreted as a non-limiting example of a load.

Referring now toFIG. 2, some embodiments of the dimmable power supply10may also include an internal dimmer40adapted to adjustably reduce the current32through the load26by narrowing the pulse width at the output22of the variable pulse generator20. This may be accomplished in a number of ways, for example by adjusting a reference voltage or current in the load current detector24that is based on the power output16from the rectifier14. The internal dimmer40may also adjust the level of a feedback voltage or current from the load26to narrow the pulse width and reduce the load current. The internal dimmer can also be based on pulse width modulation (PWM) and related methods, techniques and technologies.

Some embodiments of the dimmable power supply10may include current overload protection and/or thermal protection50, as illustrated inFIG. 3. As an example, the current overload protection50measures the current through the dimmable power supply10and narrows or turns off the pulses at the output22of the variable pulse generator20if the current exceeds a threshold value. The current detection for the current overload protection50may be adapted as desired to measure instantaneous current, average current, or any other measurement desired and at any desired location in the dimmable power supply10. Thermal protection50may also be included to narrow or turn off the pulses at the output22of the variable pulse generator20if the temperature in the dimmable power supply10becomes excessive, thereby reducing the power through the dimmable power supply10and allowing the dimmable power supply10to cool. The thermal protection may also be designed and implemented such that at a prescribed temperature, the pulses are turned off which effectively disables the power supply and turns off the output to the load. The temperature sensor can be any type of temperature sensitive element including semiconductors such as diodes, transistors, etc. and/or thermocouples, thermistors, bimetallic elements and switches, etc.

Elements of the various embodiments disclosed herein may be included or omitted as desired. For example, in the block diagram ofFIG. 4, a dimmable power supply10is disclosed that includes both the internal dimmer40and the current overload protection the thermal protection50.

As discussed above, the dimmable power supply10may be powered by any suitable power source, such as the AC input12and rectifier14ofFIG. 1, or a DC input60as illustrated inFIG. 5. Time constants in the dimmable power supply10are adapted to produce pulses in the output22of the variable pulse generator20having a constant width across the input voltage waveform from a rectified AC input12, thereby maintaining a good power factor, while still being able to compensate for slower changes in the input voltage to provide a constant load current.

Referring now toFIG. 6, the dimmable power supply10will be described in more detail. In the diagram ofFIG. 6, the load26is shown inside the output driver30for convenience in setting forth the connections in the diagram. An AC input12is shown, and is connected to the dimmable power supply10in this embodiment through a fuse70and an electromagnetic interference (EMI) filter72. The fuse70may be any device suitable to protect the dimmable power supply10from overvoltage or overcurrent conditions, such as a traditional meltable fuse or other device (e.g., a small low power surface mount resistor), a breaker, etc. The EMI filter72may be any device suitable to prevent EMI from passing into or out of the dimmable power supply10, such as a coil, inductor, capacitor and/or any combination of these, or, also in general, a filter, etc. The AC input12is rectified in a rectifier14as discussed above. In other embodiments, the dimmable power supply10may use a DC input as discussed above. In this embodiment, the dimmable power supply10may generally be divided into a high side portion including the load current detector24and a low side portion including the variable pulse generator20, with the output driver30spanning or including the high and low side. In this case, a level shifter74may be employed between the load current detector24in the high side and the variable pulse generator20in the low side to communicate the control signal76to the variable pulse generator20. The variable pulse generator20and load current detector24are both powered by the power output16of the rectifier14, for example through resistors80and82, respectively. The high side, including the load current detector24, floats at a high potential under the voltage of the input voltage16and above the circuit ground84. A local ground86is thus established and used as a reference voltage by the load current detector24.

A reference current source90supplies a reference current signal92to the load current detector24, and a current sensor such as a resistor94provides a load current signal96to the load current detector24. The reference current source90may use the circuit ground84as illustrated inFIG. 6, or the local ground86, or both, or some other reference voltage level as desired. The load current detector24compares the reference current signal92with the load current signal96using a time constant to effectively average out and disregard current fluctuations due to any waveform at the input voltage16and pulses from the variable pulse generator20, and generates the control signal76to the variable pulse generator20. The variable pulse generator20adjusts the pulse width of a train of pulses at the pulse output100of the variable pulse generator20based on the level shifted control signal102from the load current detector24. The level shifter74shifts the control signal76from the load current detector24which is referenced to the local ground86in the load current detector24to a level shifted control signal102that is referenced to the circuit ground84for use in the variable pulse generator20. The level shifter74may comprise any suitable device for shifting the voltage of the control signal76, such as an opto-isolator or opto-coupler, resistor, transformer, etc.

The pulse output100from the variable pulse generator20drives a switch104such as a field effect transistor (FET) in the output driver30. When a pulse from the variable pulse generator20is active, the switch104is turned on, drawing current from the input voltage16, through the load path106(and an optional capacitor110connected in parallel with the load26), through the load current sense resistor94, an inductor112in the output driver30, the switch104, and a current sense resistor114to the circuit ground84. When the pulse from the variable pulse generator20is off, the switch104is turned off, blocking the current from the input voltage16to the circuit ground84. The inductor112resists the current change and recirculates current through a diode116in the output driver30, through the load path106and load current sense resistor94and back to the inductor112. The load path106is thus supplied with current alternately through the switch104when the pulse from the variable pulse generator20is on and with current driven by the inductor112when the pulse is off. The pulses from the variable pulse generator20have a relatively much higher frequency than variations in the input voltage16, such as for example 30 kHz or 100 kHz as compared to the 100 Hz or 120 Hz that may appear on the input voltage16from the rectified AC input12. Note that any suitable frequency for the pulses from the variable pulse generator20may be selected as desired, with the time constant in the load current detector24being selected accordingly to disregard load current changes due to the pulses from the variable pulse generator20while tracking changes on the input voltage16that are slower than or on the order of the waveform on the input voltage16. Changes in the current through the load26due to the pulses from the variable pulse generator20may be smoothed in the optional capacitor110, or may be ignored if the load is such that high frequency changes are acceptable. For example, if the load26is an LED or array of LEDs, any flicker that may occur due to pulses at many thousands of cycles per second will not be visible to the eye. In the embodiment ofFIG. 6, a current overload protection50is included in the variable pulse generator20and is based on a current measurement signal120by the current sense resistor114connected in series with the switch104. If the current through the switch104and the current sense resistor114exceeds a threshold value set in the current overload protection50, the pulse width at the pulse output100of the variable pulse generator20will be reduced or eliminated. The present invention is shown implemented in the discontinuous mode; however with appropriate modifications operation under continuous or critical conduction modes can also be realized.

Referring now toFIG. 7, a schematic of one embodiment of the dimmable power supply10will be described. In this embodiment, an AC input12is used, with a resistor included as a fuse70, and a diode bridge as a rectifier14. Some smoothing of the input voltage16may be provided by a capacitor122, although it is not necessary as described above. A variable pulse generator20is used to provide a stream of pulses at the pulse output100. As described above, the variable pulse generator20may be embodied in any suitable device or circuit for generating a stream of pulses. Those pulses may have any suitable shape, such as substantially square pulses, semi-sinusoidal, triangular, etc. although square or rectangular are the most common in driving field effect transistors. The frequency of the pulses may also be set at any desired level, such as 30 kHz or 100 kHz, that enable the load current detector24to disregard changes in a load current due to the pulses input waveform and also realize a very high power factor approaching unity. The width of the pulses is controlled by the load current detector24, although a maximum width may be established if desired. For example, in one embodiment, the maximum pulse width is set at about one tenth of a pulse cycle. This may be interpreted from one point of view as a 10 percent duty cycle at maximum pulse width. However, the dimmable power supply10is not limited to any particular maximum pulse width.

The variable pulse generator20is powered from the input voltage16by any suitable means. Because a wide range of known methods of reducing or regulating a voltage are known, the power supply for the variable pulse generator20from the input voltage16is not shown inFIG. 7. For example, a voltage divider or a voltage regulator may be used to drop the voltage from the input voltage16down to a useable level for the variable pulse generator20.

In one particular embodiment illustrated inFIG. 7, the load current detector24includes an operational amplifier (op-amp)150acting as an error amplifier to compare a reference current152and a load current154. The op-amp150may be embodied by any device suitable for comparing the reference current152and load current154, including active devices and passive devices. The op-amp150is referred to herein generically as a comparator, and the term comparator should be interpreted as including and encompassing any device, including active and passive devices, for comparing the reference current152and load current154. The reference current152may be supplied by a transistor such as bipolar junction transistor (BJT)156connected in series with resistor160to the input voltage16. A resistor162and a resistor164are connected in series between the input voltage16and the circuit ground84, forming a voltage divider with a central node166connected to the base170of the BJT156. The BJT156and resistor160act as a constant current source that is varied by the voltage on the central node166of the voltage divider162and164, which is in turn dependent on the input voltage16. A capacitor172may be connected between the input voltage16and the central node166to form a time constant for voltage changes at the central node166. The dimmable power supply10thus responds to the average voltage of input voltage16rather than the instantaneous voltage. In one particular embodiment, the local ground86floats at about 10 V below the input voltage16at a level established by the load26. A capacitor174may be connected between the input voltage16and the local ground86to smooth the voltage powering the load current detector24if desired. A Zener diode176may also be connected between the input voltage16and the central node166to set a maximum load current154by clamping the reference current that BJT156can provide to resistor190. In other embodiments, the load current detector24may have its current reference derived by a simple resistive voltage divider, with suitable AC input voltage sensing, level shifting, and maximum clamp, rather than BJT156.

The load current154(meaning, in this embodiment, the current through the load26and through the capacitor110connected in parallel with the load26) is measured using the load current sense resistor94. The capacitor110can be configured to either be connected through the sense resistor94or bypass the sense resistor94. The current measurement180is provided to an input of the error amplifier150, in this case, to the non-inverting input182. A time constant is applied to the current measurement180using any suitable device, such as the RC lowpass filter made up of the series resistor184and the shunt capacitor186to the local ground86connected at the non-inverting input182of the error amplifier150. As discussed above, any suitable device for establishing the desired time constant may be used such that the load current detector24disregards rapid variations in the load current154due to the pulses from the variable pulse generator20and any regular waveform of the input voltage16. The load current detector24thus substantially filters out changes in the load current154due to the pulses, averaging the load current154such that the load current detector output200is substantially unchanged by individual pulses at the variable pulse generator output100.

The reference current152is measured using a sense resistor190connected between the BJT156and the local ground86, and is provided to another input of the error amplifier150, in this case, the inverting input192. The error amplifier150is connected as a difference amplifier with negative feedback, amplifying the difference between the load current154and the reference current152. An input resistor194is connected in series with the inverting input192and a feedback resistor196is connected between the output200of the error amplifier150and the inverting input192. A capacitor202is connected in series with the feedback resistor196between the output200of the error amplifier150and the inverting input192and an output resistor204is connected in series with the output200of the error amplifier150to further establish a time constant in the load current detector24. Again, the load current detector24may be implemented in any suitable manner to measure the difference of the load current154and reference current152, with a time constant being included in the load current detector24such that changes in the load current154due to pulses are disregarded while variations in the input voltage16other than any regular waveform of the input voltage16are tracked.

The output200from the error amplifier150is connected to the level shifter74, in this case, an opto-isolator, through the output resistor204to shift the output200from a signal that is referenced to the local ground86to a signal206that is referenced to the circuit ground84or to another internal reference point in the variable pulse generator20. A Zener diode210and series resistor212may be connected between the input voltage16and the input208of the level shifter74for overvoltage protection. If the voltage across load26rises excessively, the Zener diode210will conduct, turn on the level shifter74and reduce the pulse width or stop the pulses from the variable pulse generator20. There are thus two parallel control paths, the error amplifier150to the level shifter74and the overvoltage protection Zener diode210to the level shifter74.

The error amplifier150operates in an analog mode. During operation, as the load current154rises above the reference current152, the voltage at the output200of the error amplifier150increases, causing the variable pulse generator20to reduce the pulse width or stop the pulses from the variable pulse generator20. As the output200of the error amplifier150rises, the pulse width becomes narrower and narrower until the pulses are stopped altogether from the variable pulse generator20. The error amplifier150produces an output proportional to the difference between the average load current154and the reference current152, where the reference current152is proportional to the average input voltage16.

As discussed above, pulses from the variable pulse generator20turn on the switch104, in this case a power FET via a resistor214to the gate of the FET104. This allows current154to flow through the load26and capacitor110, through the load current sense resistor94, the inductor112, the switch104and current sense resistor114to circuit ground84. In between pulses, the switch104is turned off, and the energy stored in the inductor112when the switch104was on is released to resist the change in current. The current from the inductor112then flows through the diode116and back through the load26and load current sense resistor94to the inductor112. Because of the time constant in the load current detector24, the load current154monitored by the load current detector24is an average of the current through the switch104during pulses and the current through the diode116between pulses.

The current through the dimmable power supply10is monitored by the current sense resistor114, with a current feedback signal216returning to the variable pulse generator20. If the current exceeds a threshold value, the pulse width is reduced or the pulses are turned off in the variable pulse generator20. Generally, current sense resistors94and114may have low resistance values in order to sense the currents without substantial power loss. Thermal protection may also be included in the variable pulse generator20, narrowing or turning off the pulses if the temperature climbs or if it reaches a threshold value, as desired. Thermal protection may be provided in the variable pulse generator20in any suitable manner, such as using active temperature monitoring, or integrated in the overcurrent protection by gating a BJT or other such suitable devices, switches and/or transistors with the current feedback signal216, where, for example, the BJT exhibits negative temperature coefficient behavior. In this case, the BJT would be easier to turn on as it heats, making it naturally start to narrow the pulses.

In one particular embodiment the load current detector24turns on the output200to narrow or turn off the pulses from the variable pulse generator20, that is, the pulse width is inversely proportional to the load current detector output200. In other embodiments, this control system may be inverted so that the pulse width is directly proportional to the load current detector output200. In these embodiments, the load current detector24is turned on to widen the pulses.

In applications where it is useful or desired to have isolation between the load and the input voltage source, a transformer can be used in place of the inductor. The transformer can be of essentially any type including toroidal, C or E cores, or other core types and, in general, should be designed for low loss. The transformer can have a single primary and a single secondary coil or the transformer can have either multiple primaries and/or secondaries or both.FIG. 8illustrates one embodiment using a transformer in the flyback mode of operation to realize a highly efficient circuit with very high power factor approaching unity and with isolation between the AC input and the LED output. Such an embodiment can also readily support internal dimming as illustrated inFIG. 9.

Referring now toFIG. 8, a non-dimming power supply300with a transformer302will be described. An AC input304is shown, and is connected to the dimmable power supply300in this embodiment through a fuse306and an electromagnetic interference (EMI) filter308. As in previously described embodiments, the fuse306may be any device suitable to protect the dimmable power supply300from overvoltage or overcurrent conditions. The AC input304is rectified in a rectifier310. In other embodiments, the dimmable power supply300may use a DC input. The dimmable power supply300may generally be divided into a high side portion including the load current detector312and a low side portion including the variable pulse generator314. The high side portion is connected to one side of the transformer302, such as the secondary winding, and the low side portion is connected to the other side of the transformer302, such as the primary winding. A level shifter316is employed between the load current detector312in the high side and the variable pulse generator314in the low side to communicate the control signal320to the variable pulse generator314. The high side has a node that may be considered a power input322for the output driver, although the power for the power input322is derived in this embodiment from the transformer302. The load326receives power from the power input322. The load current detector312is also powered from the power input322through a resistor330, and a reference current328for the load current detector312is generated by a voltage divider having resistors332and334connected in series between the power input322and a high side or local ground336. The variable pulse generator314is powered from a low side input voltage340through a resistor342, and a switch344driven by pulses from the variable pulse generator314turns on and off current through the transformer302. The power supply voltage to the load current detector312may be regulated in any suitable manner, and the reference current input328may be stabilized as desired. For example, a voltage divider with a clamping Zener diode may be used as in previous embodiments, a precision current source may be used in place of the resistor332in the voltage divider, a bandgap reference source may be used, etc. Note that it is important in dimmable embodiments for the input voltage340to be a factor in the reference current input328such that this input328is clamped at some maximum value as the input voltage340rises, yet is allowed to fall as input voltage340drops (suitably filtered to reject the AC line frequency).

In the high side, as current flows through the load326, a load current sense resistor346provides a load current feedback signal350to the load current detector312. The load current detector312compares the reference current signal328with the load current signal350using a time constant to effectively average out and disregard current fluctuations due to any waveform at the power input322and pulses from the variable pulse generator314through the transformer302, and generates the control signal320to the variable pulse generator314. The variable pulse generator314adjusts the pulse width of a train of pulses at the pulse output352of the variable pulse generator314based on the level shifted control signal320from the load current detector312. The level shifter316shifts the control signal320from the load current detector312which is referenced to the local ground336by the load current detector312to a level shifted control signal that is referenced to the circuit ground354for use by the variable pulse generator314. The level shifter316may comprise any suitable device for shifting the voltage of the control signal320between isolated circuit sections, such as an opto-isolator, opto-coupler, resistor, transformer, etc.

The pulse output352from the variable pulse generator314drives the switch344, allowing current to flow through the transformer302and powering the high side portion of the dimmable power supply300. As in some other embodiments, any suitable frequency for the pulses from the variable pulse generator314may be selected, with the time constant in the load current detector312being selected to disregard load current changes due to the pulses from the variable pulse generator312while tracking changes on the input voltage322that are slower than or on the order of the waveform on the input voltage322. Changes in the current through the load326due to the pulses from the variable pulse generator314may be smoothed in the optional capacitor356, or may be ignored if the load is such that high frequency changes are acceptable. Current overload protection360may be included in the variable pulse generator314based on a current measurement signal362by a current sense resistor364connected in series with the switch344. If the current through the switch344and the current sense resistor364exceeds a threshold value set in the current overload protection360, the pulse width at the pulse output352of the variable pulse generator314will be reduced or eliminated. A line capacitor370may be included between the input voltage340and circuit ground354to smooth the rectified input waveform if desired. A snubber circuit372may be included in parallel, for example, with the switch344if desired to suppress transient voltages in the low side circuit. It is important to note that the dimmable power supply300is not limited to the flyback mode configuration illustrated inFIG. 8, and that a transformer or inductor based dimmable power supply300may be arranged in any desired topology.

Referring now toFIG. 9, the power supply300with a transformer302may be adapted for dimmability by providing level-shifted feedback from the AC input voltage340to the load current detector312. The level shifter318may comprise any suitable device as with other level shifters (e.g.,316). The level-shifted feedback enables the load current detector312to sense the AC input voltage340so that it can provide a control signal320that is proportional to the dimmed AC input voltage340.

Referring now toFIG. 10, the dimmable power supply300may also include an internal dimmer380, for example, to adjustably attenuate any of a number of reference or feedback currents. In the embodiment ofFIG. 9, the dimmable power supply300is placed to adjustable control the level of the reference current328. The reference current328generated by the internal dimmer380may be based on the input voltage340in the low side or primary side of the dimmable power supply300via a feedback signal380through the transformer302. Diode382may be included to ensure that current on the internal dimmer380flows only in one direction, and capacitor384may be added to introduce a time constant on the internal dimmer380. For example, referring toFIGS. 7 and 10simultaneously, if the high side of the dimmable power supply300ofFIG. 9were configured similar to that of the dimmable power supply10ofFIG. 7, the bottom of resistor164may be connected to the internal dimmer380rather than to the circuit ground84. Note also that diode390may not be needed if the dimmable power supply300is not configured for operation in flyback mode.

Turning now toFIG. 11, one embodiment of a method for dimmably supplying a load current is summarized. The method includes measuring a ratio between a reference current152and a load current154(block800), producing pulses having a width that is inversely proportional to the ratio (block802), and driving the load current with the pulses (block804. As described above, the measuring is performed with a time constant that substantially filters out the pulses in the load current154but substantially passes changes in the reference current152. Note, however, that a time constant is applied to the reference current152as well, thereby considering an average input voltage16rather than instantaneous. The time constant applied to the reference current152may be varied as desired, however, to maintain a high power factor the pulse width should be constant across an input waveform on the input voltage16. In some embodiments, the pulse width is kept substantially constant across a cycle of the input voltage waveform. Given the feedback and control of the dimmable power supply10and300, there may be changes in the pulse width across a cycle of an input waveform when the load current is being held constant despite noise on the input voltage, or when the load current is being varied by an external or internal dimmer. The statement that the pulse width will be kept substantially constant across a cycle of the input waveform does not preclude these changes to the pulse width that may occur partially or entirely across a cycle of the input waveform, but indicates in these embodiments that the pulse width is not substantially varied in direct response to the rising and falling input voltage due to the waveform itself, such as to the half sinusoidal peaks of a rectified AC waveform.

The dimmable power supply10disclosed herein provides an efficient way to power loads such as LEDs with a good power factor, while remaining dimmable by external or internal devices.

While illustrative embodiments have been described in detail herein, it is to be understood that the concepts disclosed herein may be otherwise variously embodied and employed. The configuration, arrangement and type of components in the various embodiments set forth herein are illustrative embodiments only and should not be viewed as limiting or as encompassing all possible variations that may be performed by one skilled in the art while remaining within the scope of the claimed invention.