Ballast for light emitting diode light sources

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. §119(e) and 37 C.F.R. §1.78 of U.S. Provisional Application No. 60/894,295, filed Mar. 12, 2007 and entitled “Lighting Fixture.” U.S. Provisional Application No. 60/894,295 includes exemplary systems and methods and is incorporated by reference in its entirety.

This application claims the benefit under 35 U.S.C. §119(e) and 37 C.F.R. §1.78 of U.S. Provisional Application No. 60/909,458, entitled “Ballast for Light Emitting Diode Light Sources,” inventor John L. Melanson, and filed on Apr. 1, 2007 describes exemplary methods and systems and is incorporated by reference in its entirety.

U.S. patent application Ser. No. 11/926,864, entitled “Color Variations in a Dimmable Lighting Device with Stable Color Temperature Light Sources,” inventor John L. Melanson, and filed on Mar. 31, 2007 describes exemplary methods and systems and is incorporated by reference in its entirety.

U.S. Provisional Application No. 60/909,457, entitled “Multi-Function Duty Cycle Modifier,” inventors John L. Melanson and John Paulos, and filed on Mar. 31, 2007 describes exemplary methods and systems and is incorporated by reference in its entirety. Referred to herein as Melanson I.

U.S. patent application Ser. No. 12/047,258, entitled “Multi-Function Duty Cycle Modifier,” inventors John L. Melanson and John Paulos, and filed on Mar. 12, 2008 describes exemplary methods and systems and is incorporated by reference in its entirety. Referred to herein as Melanson II.

U.S. patent application Ser. No. 11/695,024, entitled “Lighting System with Lighting Dimmer Output Mapping,” inventors John L. Melanson and John Paulos, and filed on Mar. 31, 2007 describes exemplary methods and systems and is incorporated by reference in its entirety. Referred to herein as Melanson III.

U.S. patent application Ser. No. 11/864,366, entitled “Time-Based Control of a System having Integration Response,” inventor John L. Melanson, and filed on Sep. 28, 2007 describes exemplary methods and systems and is incorporated by reference in its entirety. Referred to herein as Melanson IV.

U.S. patent application Ser. No. 11/967,269, entitled “Power Control System Using a Nonlinear Delta-Sigma Modulator with Nonlinear Power Conversion Process Modeling,” inventor John L. Melanson, and filed on Dec. 31, 2007 describes exemplary methods and systems and is incorporated by reference in its entirety. Referred to herein as Melanson V.

U.S. patent application Ser. No. 11/967,275, entitled “Programmable Power Control System,” inventor John L. Melanson, and filed on Dec. 31, 2007 describes exemplary methods and systems and is incorporated by reference in its entirety. Referred to herein as Melanson VI.

U.S. patent application Ser. No. 12/047,262, entitled “Power Control System for Voltage Regulated Light Sources,” inventor John L. Melanson, and filed on Mar. 12, 2007 describes exemplary methods and systems and is incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates in general to the field of electronics and lighting, and more specifically to a system and method for providing ballast for light sources, such as light emitting diode light sources.

2. Description of the Related Art

Commercially practical incandescent light bulbs have been available for over 100 years. However, other light sources show promise as commercially viable alternatives to the incandescent light bulb. Light Emitting Diodes (“LEDs”) are becoming particularly attractive as main stream light sources in part because of energy savings through high efficiency light output, long life, and environmental incentives such as the reduction of mercury.

LEDs are semiconductor devices and are best driven by direct current. The brightness of the LED varies in direct proportion to the current flowing through the LED. Thus, increasing current supplied to an LED increases the brightness of the LED and decreasing current supplied to the LED dims the LED.

Dimming a light source saves energy when operating a light source and also allows a user to adjust the brightness of the light source to a desired level. Many facilities, such as homes and buildings, include light source dimming circuits (referred to herein as “dimmers”).

FIG. 1 depicts a lighting circuit 100 with a conventional dimmer 102 for dimming incandescent light source 104 in response to inputs to variable resistor 106. The dimmer 102, light source 104, and voltage source 108 are connected in series. Voltage source 108 supplies alternating current at line voltage V_line. The line voltage V_line can vary depending upon geographic location. The line voltage V_line is typically 120 Vac with a typical frequency of 60 Hz or 230 Vac with a typical frequency of 50 Hz. Instead of diverting energy from the light source 104 into a resistor, dimmer 102 switches the light source 104 off and on many times every second to reduce the total amount of energy provided to light source 104. A user can select the resistance of variable resistor 106 and, thus, adjust the charge time of capacitor 110. A second, fixed resistor 112 provides a minimum resistance when the variable resistor 106 is set to 0 ohms. When capacitor 110 charges to a voltage greater than a trigger voltage of diac 114, the diac 114 conducts and the gate of triac 116 charges. The resulting voltage at the gate of triac 116 and across bias resistor 118 causes the triac 116 to conduct. When the current I passes through zero, the triac 116 becomes nonconductive, i.e. turns ‘off’. When the triac 116 is nonconductive, the dimmer output voltage V_DIM is 0 V. When triac 116 conducts, the dimmer output voltage V_DIM equals the line voltage V_line. The charge time of capacitor 110 required to charge capacitor 110 to a voltage sufficient to trigger diac 114 depends upon the value of current I. The value of current I depends upon the resistance of variable resistor 106 and resistor 112. Thus, adjusting the resistance of variable resistor 106 adjusts the phase angle of dimmer output voltage V_DIM. Adjusting the phase angle of dimmer output voltage V_DIM is equivalent to adjusting the phase angle of dimmer output voltage V_DIM. Adjusting the phase angle of dimmer output voltage V_DIM adjusts the average power to light source 104, which adjusts the intensity of light source 104. The term “phase angle” is also commonly referred to as a “phase delay”. Thus, adjusting the phase angle of dimmer output voltage V_DIM can also be referred to as adjusting the phase delay of dimmer output signal V_DIM. Dimmer 102 only modifies the leading edge of each half cycle of voltage V_line.

FIG. 2 depicts the dimmer output voltage V_DIM waveform of dimmer 102. The dimmer output voltage V_DIM fluctuates during each period from a positive voltage to a negative voltage. (The positive and negative voltages are characterized with respect to a reference to a direct current (DC) voltage level, such as a neutral or common voltage reference.) The period of each voltage sine wave 202.0 through 202.N is the same as 1/frequency of V_line (FIG. 1), where N is an integer. The dimmer output voltage V_DIM is a phase modulated dimmer signal. The dimmer 102 chops the voltage sine waves 202.0 through 202.N to alter the duty cycle of each sine wave 202.0 through 202.N. The dimmer 102 chops the positive half of sine wave 202.0 at time t₁ so that the positive portion of sine wave 202.0 is 0 V from time t₀ through time t₁ and has a positive voltage from time t₁ to time t₂. The difference in time between time t₀ at which a full cycle of sine wave 202 would have started but for the chopping and the time t₁ at which dimmer output voltage V_DIM is chopped introduces a phase delay α into sine wave 202. For example, for sine wave 202.0, α_202.0=sine wave chop time t₁-full cycle start time t₀ and α_202.N=t₄−t₃. The phase delay α is the same for the negative half of sine wave 202. Additionally, since the dimmer output voltage V_DIM is periodic, the phase delay can also be referred to as a phase angle. Each half cycle of sine wave 202 is 180 degrees, and the phase angle is 180 degrees minus the (phase delay/the half period) times 180 degrees. For example, the phase angle □_202.0 equals 180·[1−(t₁−t₀)/(t₂−t₀)] for sine wave 202.0 The “Sine wave 202” represents all sine waves 202.0 through 202.N. The light source 104 is, thus, turned ‘off’ from times t₀ through t₁ and turned ‘on’ from times t₁ through t₂. Dimmer 102 chops the negative half of sine wave 202.0 with the same timing as the positive half Equation [1] represents the duty cycle of dimmer 102:

Duty ⁢ ⁢ Cycle = ( t 2 - t 1 ) ( t 2 - t 0 ) . [ 1 ]

When the resistance of variable resistance 106 is increased, the duty cycle of dimmer 102 decreases. Between time t₂ and time t₃, the resistance of variable resistance 106 is increased, and, thus, dimmer 102 chops the full cycle 202.N at later times in the first half cycle 204.N and the second half cycle 206.N of the full cycle of voltage sine wave 202.N with respect to voltage sine wave 202.0. Dimmer 102 chops the first half cycle 204.N with the same timing as the second half cycle 206.N. So, the duty cycles of each half cycle of voltage sine wave 202.N are the same. Thus, the full duty cycle of dimmer 102 for voltage sine wave 202.N is:

Duty ⁢ ⁢ Cycle = ( t 5 - t 4 ) ( t 5 - t 3 ) . [ 2 ]

Since times (t₅−t₄)<(t₂−t₁), less average power is delivered to light source 104 by the sine wave 202.N than sine wave 202.0 of dimmer voltage V_DIM, and the intensity of light source 104 decreases at time t₃ relative to the intensity at time t₂.

The voltage and current fluctuations of conventional dimmer circuits, such as dimmer 102, can destroy LEDs. U.S. Pat. No. 7,102,902, filed Feb. 17, 2005, inventors Emery Brown and Lodhie Pervaiz, and entitled “Dimmer Circuit for LED”(referred to here as the “Brown Patent”) describes a circuit that supplies a specialized load to a conventional AC dimmer which, in turn, controls an LED device. The Brown Patent describes dimming the LED by adjusting the duty cycle of the voltage and current provided to the load and providing a minimum load to the dimmer to allow dimmer current to go to zero.

Exemplary modification of leading edges and trailing edges of dimmer signals is discussed in “Real-Time Illumination Stability Systems for Trailing-Edge (Reverse Phase Control) Dimmers” by Don Hausman, Lutron Electronics Co., Inc. of Coopersburg, Pa., U.S.A., Technical White Paper, December 2004, which is incorporated herein by reference.

Line voltage fluctuations adversely affect LEDs. Line voltage fluctuations can produce a disproportional change in drive current to the LED. Increases in drive current increase heat and, thus, reduce the useful life of the LED. Useful life can be defined, for example, when the light output of the LED declines by thirty percent or more.

SUMMARY OF THE INVENTION

In one embodiment of the present invention, a lighting system ballast includes a light source driver stage configured to deliver power to one or more light emitting diodes with an adjustable drive current. The lighting system also includes a sensor having one or more input terminals to receive a phase modulated dimmer signal, wherein a phase delay of the phase modulated dimmer signal indicates a dimming level and the sensor is configured to detect the dimming level and to generate a dimmer output signal representing a dimming level indicated by the dimmer signal. The lighting system also includes a controller coupled to the light source driver stage and the sensor. The controller includes at least one input terminal to receive the dimmer output signal from the sensor and to generate a drive control signal to cause a drive current supplied to each light emitting diode to dim each light emitting diode in response to the dimmer signal. The drive current is approximately constant for each dimming level throughout a cycle of the phase modulated dimmer signal.

In another embodiment of the present invention, a method of supplying a drive current to one or more light emitting diodes includes receiving a phase modulated dimmer signal wherein a phase delay of the phase modulated dimmer signal indicates a dimming level. The method further includes generating a dimmer output signal representing a dimming level indicated by the dimmer signal and generating a drive control signal to cause the drive current to respond to the dimmer output signal. The method also includes providing the drive control signal to a switch to vary conductivity of the switch and cause the drive current supplied by the light source driver stage to each of the light emitting diodes to be approximately constant through a cycle of the phase modulated dimming signal.

In a further embodiment of the present invention, a lighting system includes a light source ballast system. The light source ballast system includes a power factor converter and drive system to receive an alternating current, phase modulated dimmer input voltage, to convert the input voltage into a power factor corrected direct current (DC) output voltage, and to generate a light emitting diode drive current. A phase delay of the dimmer input voltage indicates a dimming level. The system also includes a ballast controller, coupled to the power factor converter and drive system, to generate one or more control signals for the power factor converter and drive system to control power factor correction and to adjust the drive current in conformance with the dimming level represented by the dimmer signal. The drive current is approximately constant throughout a cycle of the phase modulated input voltage for each dimming level and the level of the constant drive current varies in response to variations in the indicated dimming level.

In another embodiment of the present invention, a method of responding to a dimmer signal and supplying a constant voltage and variable drive current to a light source includes receiving an alternating current, phase modulated dimmer input voltage. A phase delay of the dimmer input voltage indicates a dimming level; generating one or more control signals for a power factor converter and drive system to control power factor correction and to vary the drive current in conformance with the dimming level represented by the dimmer signal. The drive current is approximately constant throughout a cycle of the phase modulated input signal for each dimming level and the level of the constant drive current is varied in response to variations in the indicated dimming level.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be better understood, and its numerous objects, features and advantages made apparent to those skilled in the art by referencing the accompanying drawings. The use of the same reference number throughout the several figures designates a like or similar element.

FIG. 1 (labeled prior art) depicts a lighting circuit with a conventional dimmer for dimming an incandescent light source.

FIG. 2 (labeled prior art) depicts a dimmer circuit output voltage waveform.

FIG. 3 depicts a lighting circuit with an electronic ballast for providing power to an LED light source.

FIG. 4A depicts a lighting circuit.

FIG. 4B depicts graphical relationships between a drive current and various dimming levels indicated by a dimmer signal.

FIG. 4C depicts a magnified view of the drive current of FIG. 4B.

FIG. 5A depicts a voltage sensor.

FIG. 5B depicts a specific embodiment of the voltage sensor in FIG. 5A.

FIG. 6 depicts a phase delay detector

FIG. 7 depicts another phase delay detector.

FIG. 8 depicts a phase delay detector.

FIGS. 9-11 depict graphical relationships between various control signals, sense signals, and drive currents.

FIG. 12 depicts an LED bank.

DETAILED DESCRIPTION

A lighting source ballast utilizes switching and control technology to convert an alternating current (AC) phase modulated dimmer input voltage into an approximately constant drive current to illuminate one or more light emitting diodes (LED(s)). In at least one embodiment, the state of the drive current conforms to a phase delay of the input voltage to facilitate, for example, dimming. The phase delay of the input voltage indicates a particular dimming level. The drive current varies for different dimming levels. However, the light source ballast controls drive current so that the drive current is approximately constant for each dimming level throughout the cycle of an input sine wave of the AC dimmer input voltage. In at least one embodiment, the ballast emulates a resistive load and, thus, the ballast has an approximately unity power factor. The switching frequency of one or more switches can be modified to spread the spectrum of electromagnetic radiation generated by the ballast.

FIG. 3 depicts a lighting circuit 300 with an electronic LED ballast 302. The electronic ballast 300 converts an alternating current (AC), phase modulated input voltage V_in into a drive current I_drive for light source 304. The phase modulated input signal V_in is a phase modulated dimmer signal that can be generated using any device that generates a phase modulated, AC input voltage. In at least one embodiment, the input voltage V_in is the dimmer output voltage V_DIM (FIGS. 1 and 2). In at least one embodiment, the light source 304 includes one or more individual LEDs or one or more groups of intraconnected LEDs. The AC-DC power factor converter 306 converts the phase modulated AC input voltage V_in into a voltage V_C1 across hold-up capacitor 312. In at least one embodiment, voltage V_C1 is an approximately constant value over time and, thus, approximates a DC voltage.

A line voltage sensor 308 samples the input voltage V_in over time. The line voltage sensor 308 samples the input voltage V_in at a frequency sufficient to detect the phase delay of input voltage V_in. By detecting the phase delay of input voltage V_in, the line voltage sensor 308 detects the dimming level indicated by input voltage V_in. The line voltage sensor 308 generates a dimmer output signal V_TH that represents the dimming level indicated by the phase delay of the input voltage V_in. It is possible, and straightforward, to dim the LED(s) 422 by turning the LEDs 422 off when the input voltage V_in is cutoff by a phase modulation dimmer. This cutting off unfortunately leads to a narrow dimming range and to flicker of the LED(s) 422 when dimmed. By keeping an approximately constant voltage V_C1 fed to a constant current driver, these problems can be eliminated.

The drive stage 310 and ballast controller 314 convert the detected phase delay of the input voltage V_in into a drive current I_drive for driving light source 304 and dimming the light source 304. The drive current I_drive conforms to a phase delay of the input voltage V_in and has an approximately constant value for each dimming level indicated by the input voltage V_in. In other words, variations of the phase angle of input voltage V_in indicates different dimming levels. The drive current I_drive also varies in response to variations in the phase angle of the input voltage V_in and has a respective, approximately constant value for each dimming level. Thus, since the dimming level indicated by the input voltage remains constant for at least one cycle of the input voltage V_in, the drive current remains approximately constant for each cycle of the input voltage V_in until the dimming level indicated by the input voltage V_in changes.

By conforming to the dimming level, in at least one embodiment, the drive current I_drive is directly proportional to the input voltage V_in. In another embodiment, the drive current I_drive conforms to the duty cycle of the input voltage V_in via a mapping function as, for example, described in Melanson III.

The drive stage 310 can also be referred to as a DC-DC ballast controller because the drive stage 310 converts the approximately DC voltage from the power factor converter 306 into a DC voltage and drive current for constant voltage light source 304. The drive stage 310 provides a link voltage V_C1 and drive current I_drive to voltage light source 304 so that the power supplied by drive stage 310 is responsive to the dimming level indicated by the input voltage V_in.

In at least one embodiment, the line voltage sensor 308 senses the dimming level indicated by the input voltage V_in by determining the duty cycle, phase angle, and/or phase delay of the input voltage V_in and generates a threshold voltage V_TH that conforms to the dimming level indicated by the input signal V_in. The duty cycle, phase angle, and/or phase delay represent the dimming level indicated by the input signal V_in.

The ballast controller 314 generates control signals to control power factor correction by the AC-DC power factor converter 306 and drive stage 310. In at least one embodiment, the ballast controller 314 generates control signals for the AC-DC power factor converter 306 in accordance with the exemplary systems and methods described in Melanson V and Melanson VI. In at least one embodiment, the ballast controller 314 generates a control signal to control the generation of drive current I_drive in accordance with the exemplary systems and methods described in Melanson IV.

As previously stated, the drive current I_drive conforms to a duty cycle of the input voltage V_in and has an approximately constant value for each dimming level indicated by the input voltage V_in. In at least one embodiment, the threshold voltage V_TH is directly proportional to the dimming level indicated by input signal V_in. In at least one embodiment, the threshold signal V_TH represents a mapping of the dimming level indicated by input voltage V_in to predetermined values different than the dimming levels indicated by the input voltage V_in. In at least one embodiment, the mapping maps measured light levels to perception based light levels as described in conjunction with the exemplary systems and methods of Melanson I and Melanson II.

FIG. 4A depicts a lighting circuit 400, which is one embodiment of lighting circuit 300. Lighting circuit 400 includes an AC-DC power factor converter 402 to convert an AC, phase modulated, input voltage dimmer signal V_DIM into an approximately constant light source drive current I_drive. The dimmer signal V_DIM can be generated using any phase modulation dimmer circuit, such as the dimmer 102 (FIG. 1). The AC-DC switching power converter 402 includes a switch 404 that turns ‘on’ (conducts) and turns ‘off’ (nonconductive) in response to a control signal C_S1 generated by ballast controller 406. When switch 404 is ‘on’, inductor 408 energizes with the current I_L1 from the full-bridge diode rectifier 410. When switch 404 is ‘off’, the inductor 408 drives current I_L1 through diode 412 to charge capacitor 409. The control signal C_S1 varies the duty cycle of switch 404 so that the direct current (DC) voltage on storage capacitor 409 averages to a desired value of raw DC voltage V_C1. In at least one embodiment, link voltage V_C1 has an average value in the range of 200 V to 400 V. Ballast controller 406 controls the duty cycle of switch 404 such that current I_L1 is proportional to the input voltage V_x(t). In at least one embodiment, switch 404 is an N-channel field effect transistor (FET), and control signal C_S1 is a gate voltage.

Ballast controller 406 controls the timing of switches 404 and 416 and, thus, controls the voltage V_C1 and current I_drive. The voltage V_C1 is controlled to remain approximately constant over time, and drive current I_drive is approximately constant for each dimming level indicated by the threshold voltage V_TH. The drive current I_drive is a drive current for LED(s) 422. The current I_drive is controlled so as to be responsive to dimmer signal V_DIM and to dim light source 418 in accordance with, for example, the dimming level indicated by dimmer signal V_DIM. In at least one embodiment, the current I_drive is controlled so that the intensity of the LEDs of light source 418 conforms to a mapping function as, for example, described in Melanson and Melanson II. In at least one embodiment, the light circuit includes an auxiliary power supply 420 to supply power to ballast controller 406. In at least one embodiment, switch 416 is also an N-channel FET, and control signal C_S2 is a gate voltage. In another embodiment, switch 416 is a bipolar junction transistor with control signal C_S2 providing current to the base of the transistor. In at least one embodiment, the switching frequencies of switch 404 and/or switch 416 are greater than an upper audio baseband frequency, e.g. greater than 25 kHz. Ballast controller 406 can generate control signal C_S2 in any of a variety of ways. Melanson IV describes an exemplary system and method for generating control signal C_S2.

Lighting source 418 includes one or more LED(s) 422. The LED(s) 422 can be any type of LED including white, amber, other colors, or any combination of LED colors. Additionally, the LED(s) 422 can be configured into any type of physical arrangement, such as linearly, circular, spiral, or any other physical arrangement. In at least one embodiment, each of LED(s) 422 is serially connected. Capacitor 424 is connected in parallel with LED(s) 422 and provides filtering to protect the LED(s) 422 from AC signals. Inductor 426 stores energy from LED current I_drive to maintain an approximately constant current I_drive, for a given dimming level indicated by the threshold voltage V_TH, when switch 416 conducts. Diode 428 prevents reverse current flow when the polarity of inductor 426 reverses during switching of switch 416. A typical switching frequency for this approximately constant-current I_drive would be 50 kHz-100 kHz. The frequency is chosen to optimize efficiency and minimize size of a switch 416. The ripple of the current I_drive (as shown in FIGS. 4B and 4C) at the switching frequency is selected to be about 10-20% at full intensity, again chosen for efficiency and size of switch 416. This remaining high-frequency ripple has minimal effect on the performance of the LEDs, and is herein referred to as “approximately constant current”. This is contrasted with pulse-width modulation at 120 Hz, which has detrimental effects on flicker and efficiency.

To reduce radio frequency interference, the timing of conductive transitions (i.e. “on” and “off” times) of switches 404 and 416 can be modified with respect to each other and also dithered to spread the spectrum of electromagnetic energy and, thus, minimize radio frequency interference. Dithering can be accomplished, for example, by modifying the “off” time of switch 404 with a random or pseudo random noise source. Inductor 430 and capacitor 432 provide filtering to smooth drive current I_L1 so that the average drive current I_L1 is sinusoid in phase with dimmer signal V_DIM. Thus, a power factor of unity or approximately unity is achieved.

The lighting system 400 includes a phase delay detector 434, an optional mapping system and filter 436, and a digital-to-analog converter (DAC) 438 to generate a threshold voltage V_TH. The threshold voltage V_TH is a dimmer output signal indicating a dimming level conforming to a dimming level indicated by the dimmer signal V_DIM. In at least one embodiment, the phase delay detector 434 detects the phase delay, duty cycle, and/or the corresponding phase angle of the dimmer signal V_DIM and generates a phase delay signal □_D. In at least one embodiment, the lighting system 400 includes a mapping system and filter 436 that uses a mapping function to map the dimming level indicated by phase delay signal □_D into different dimming levels. In at least one embodiment, the mapping function converts the dimming levels indicated by dimmer signal V_DIM (and, thus, indicated by phase delay signal □_D) into a digital dimming signal D_V—digital having values that map measured light levels to perception based light levels as described in conjunction with the exemplary systems and methods of Melanson I and Melanson II. DAC 438 converts the mapped dimmer signal D_V—digital into the threshold voltage V_TH. In at least one embodiment, the phase delay signal □_D is a digital signal. DAC 438 converts the digital phase delay signal □_D into an analog threshold voltage V_TH. In at least one embodiment, the threshold voltage V_TH is updated at least the same frequency as the frequency of dimmer signal V_DIM. This update is desirable, as the phase modulated dimmer signal V_DIM operates only over a narrow range (<10:1), whereas it is desirable to dim over a wide range, say 100:1.

The threshold voltage V_TH is compared by comparator 438 with voltage V_sense. Voltage V_sense is taken across sense resistor 440 and is proportional to drive current I_drive. In at least one embodiment, resistor 440 is 1 ohm. In another embodiment, the drive current I_drive is sensed by a magnetic current sensor in the proximity of current flowing through inductor 426. The comparator 438 provides an output voltage V_COMP as input data to ballast controller 406. The output voltage V_COMP is used by ballast controller 406 to control the conductivity of switch 416 and, thus, control drive current I_drive as, for example, described in Melanson IV. The conductivity of switch 416 is modulated by the control signal C_S2 provided by ballast controller 406 such that the average drive current I_drive is responsive to dimmer signal V_DIM. The drive current I_drive serves as the drive current for light source 418. Adjusting the drive current I_drive modifies the intensity of light source 418.

FIG. 4B depicts a graphical relationship 452 between an approximately constant LED drive current I_drive and a phase modulated dimmer signal V_DIM indicating a 50% dimming level and a graphical relationship 454 between an approximately constant LED drive current I_drive that is varied to respond to a phase modulated dimmer signal V_DIM indicating a 25% dimming level. The dimmer signal V_DIM 456 has phase delays α of T/4 representing a dimming level of 50%, where T is the period of a full cycle of dimmer signal V_DIM 456. In at least one embodiment, T is 1/60 (16.7 msec) or 1/50 (20 msec). Lighting system 400 generates an LED drive current I_drive 458 that is approximately constant during each cycle of dimmer signal V_DIM 406. The dimmer signal V_DIM 460 changes from the dimming level of 50% indicated by dimmer signal V_DIM 456 to a dimming level of 25%. The phase delay of dimmer signal V_DIM 460 is T/8. Lighting system 400 generates an LED drive current I_drive 462 that is approximately constant during each cycle of dimmer signal V_DIM 460. Thus, the LED drive current I_drive varies with variations of the dimming level indicated by dimmer signal V_DIM, and LED drive current I_drive remains approximately constant for each dimming level indicated by dimmer signal V_DIM. The LED drive current I_drive changes in response to other dimming levels indicated by dimmer signal V_DIM in the same manner as depicted in FIG. 4B.

FIG. 4C depicts a 1,000× magnified view 480 of LED drive currents I_drive 458 and 462. The LED drive currents I_drive 458 and 462 rise and fall in accordance with pulses of control signal CS₂. For each dimming level, the peak-to-peak values of the LED drive current I_drive is, for example, 100-200 mA, which is approximately constant for LED lighting applications. The average value of the LED drive current I_drive represents a DC offset of the LED drive current I_drive. The DC offset of the LED drive current I_drive represents the dimming level of the LEDs 418.

FIG. 5A depicts an exemplary voltage sensor 550, which is one embodiment of V_C1 sensor 414. The voltage sensor 550 is an analog-to-digital converter (ADC) 552 that generates a digital value V_S representing the voltage V_C1.

FIG. 5B represents an exemplary voltage sensor 500, which represents one embodiment of the ADC 552. Resistors 502 and 514 provide a voltage divider to compare a percentage X of voltage V_C1 to a reference voltage V_REF. The reference voltage V_REF is chosen to be the same percentage X as the desired value of voltage V_C1. In one embodiment, the desired value of voltage V_C1 is 250 V, and the reference voltage is 1 V. So the ratio of resistor 502 to resistor 514 is 250:1. In one embodiment, the value of resistor 502 is 2.5 Mohms, and the value of resistor 514 is 10 kohms. The comparator 504 provides an output voltage V_S. When voltage V_S is high, voltage V_C1 is high, so the switching frequency of switch 404 is decreased to decrease the voltage V_C1. When voltage V_S is low, voltage V_C1 is high, so the switching frequency of switch 404 is increased to increase the voltage V_C1.

FIG. 6 depicts a phase delay detector 600, which represents one embodiment of phase delay detector 434. The phase delay detector 600 includes an analog integrator 602 that integrates dimmer signal V_DIM during each cycle (full or half cycle) of dimmer signal V_DIM. The analog integrator 602 generates a current I corresponding to the duty cycle of dimmer signal V_DIM for each cycle of dimmer signal V_DIM. The current provided by the analog integrator 602 charges a capacitor 604 to a value representing the phase delay of dimmer signal V_DIM. The ADC 612 generates the phase delay signal □_D from the voltage across capacitor 604. The analog integrator 602 can be reset after each cycle of dimmer signal V_DIM by discharging capacitors 606 and 604. Switch 608 includes a control terminal to receive reset signal S_R. Switch 610 includes a control terminal to receive sample signal S_S. The charge on capacitor 604 is sampled by capacitor 606 when control signal S_S causes switch 610 to conduct. After sampling the charge on capacitor 604, reset signal S_R opens switch 608 to discharge and, thus, reset capacitor 604. In at least one embodiment, switches 608 and 610 are n-channel field effect transistors, and sample signal S_S and reset signal S_R have non-overlapping pulses. In at least one embodiment, each cycle of dimmer signal V_DIM can be detected by every other zero crossing of dimmer signal V_DIM.

FIG. 7 depicts a phase delay detector 700 that converts the dimmer input signal V_DIM into a digital dimmer signal value □_D. The digital data of dimmer signal value □_D represents the phase delay of dimmer signal V_DIM. The phase delay detector 700 determines the duty cycle of dimmer signal V_DIM by counting the number of cycles of clock signal f_clk that occur until the chopping point of dimmer signal V_DIM is detected by the duty cycle time converter 700.

FIG. 8 depicts a phase delay detector 800 that represents one embodiment of phase delay detector 800. Comparator 802 compares dimmer signal V_DIM against a known reference. The reference is generally the cycle cross-over point voltage of dimmer output voltage V_DIM, such as a neutral potential of a household AC voltage. The counter 804 counts the number of cycles of clock signal f_clk that occur until the comparator 802 indicates that the chopping point of dimmer signal V_DIM has been reached. Since the frequency of dimmer signal V_DIM and the frequency of clock signal f_clk is known, the phase delay can be determined from the count of cycles of clock signal f_clk that occur until the comparator 802 indicates that the chopping point of dimmer signal V_DIM. Likewise, the phase angle can also be determined by knowing the elapsed time from the beginning of a cycle of dimmer signal V_DIM until a chopping point of dimmer signal V_DIM is detected.

FIG. 9 depicts a graphical relationship 900 in time between the switch 404 control signal CS₁ and drive current I_L1. When control signal CS₁ causes switch 404 to conduct, drive current I_L1 ramps up. When control signal CS₁ causes switch 404 to turn “off”, drive current I_L1 ramps down. Thus, in this embodiment, the drive current I_L1 is controlled by pulse width modulation. The period of control signal CS₁ is a matter of design choice and is preferably chosen to avoid audio baseband frequencies.

FIG. 10 depicts a graphical relationship 1000 between the comparator voltage V_COMP, control signal C_S2, and current I₂ across sense resistor 440 (FIG. 4). When control signal C_S2 is high, switch 416 conducts, and drive current I_drive increases. When the comparator voltage V_COMP goes high, ballast controller 406 keeps control signal C_S2 high until the comparator voltage V_COMP goes low again. In another embodiment, the control signal C_S2 goes high when comparator voltage V_COMP goes high, and ballast controller 406 keeps control signal C_S2 high for twice as long as the amount of time that comparator voltage V_COMP is high. In this manner, the average drive current I_drive is responsive to the dimmer signal V_DIM, and, thus, the intensity of light source 418 is also responsive to dimmer signal V_DIM.

FIG. 11 depicts a graphical relationship 1100 between control signal C_S2 and drive current I_drive. The drive current I_drive ramps up when control signal C_S2 is high (i.e. causes switch 416 to conduct) and ramps down when control signal C_S2 is low (i.e. causes switch 416 to turn ‘off’). The average drive current I_drive tracks the dimmer signal V_DIM either directly or via a mapping function. The intensity of light source 418 is approximately directly proportional to the driving drive current _drive.

FIG. 12 depicts an LED bank 1200, which represents a substitution of light source 418. The raw DC voltage V_c2 is applied across the series connected LEDs 1202 and the LEDs 1204. Ballast controller 406 supplies control signals C_S2A and C_S2B to turn respective switches 1208 and 1210 ‘on’ (conductive) and ‘off’ (nonconductive). In at least one embodiment, switches 1208 and 1210 are n-channel field effect transistors (FETs). In this embodiment, ballast controller 406 provides the gate voltages to switches 1208 and 1210. The average values of the drive currents I_A and I_B control the respective intensity of LEDs 1202 and 1204. The diodes 1212 and 1214 permit current flow in only one direction. Inductors 1216 and 1218 and capacitors 1220 and 1222 regulate the voltage across the respective LEDs 1202 and LEDs 1204 and provide filtering. The voltage across resistors 1224 and 1226 is fed back to ballast controller 406 to allow ballast controller 406 to adjust the switching frequency of switches 1208 and 1210 and, thus, correlate drive currents I_A and I_B with the selected dimming level. The type, number, and arrangement of LEDs in LED bank 1200 is a matter of design choice and depends, for example, on the range of desired intensity and color temperatures of LED bank 1200.

Thus, a lighting source ballast utilizes switching and control technology to convert an alternating current (AC) phase modulated dimmer input voltage into an approximately constant drive current to illuminate one or more light emitting diodes (LED(s)). In at least one embodiment, the state of the drive current conforms to a phase delay of the input voltage to facilitate, for example, dimming. The phase delay of the input voltage indicates a particular dimming level. The drive current varies for different dimming levels. However, the light source ballast controls drive current so that the drive current is approximately constant for each dimming level. In at least one embodiment, the ballast emulates a resistive load and, thus, the ballast has an approximately unity power factor. The switching frequency of one or more switches can be modified to spread the spectrum of electromagnetic radiation generated by the ballast. The light sources can be configured in any of a variety of configurations, such as multiple LED banks. In at least one embodiment, the ballast emulates a resistive load and, thus, the ballast has an approximately unity power factor. Additionally, the switches can be timed to spread the spectrum of electromagnetic radiation generated by the ballast.

Although the present invention has been described in detail, it should be understood that various changes, substitutions and alterations can be made hereto without departing from the spirit and scope of the invention as defined by the appended claims.